the inhibition of copper corrosion in aqueous environments with
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QUEENSLAND UNIVERSITY OF TECHNOLOGY
SCHOOL OF PHYSICAL SCIENCES
THE INHIBITION OF COPPER CORROSION
IN AQUEOUS ENVIRONMENTS WITH
HETEROCYCLIC COMPOUNDS
Submitted by NGOC HUU HUYNH, School of Physical and Chemical Sciences, Queensland University of Technology in partial fulfilment of the requirements of the degree of Doctor of Philosophy
February 2004
Examination of PhD Thesis Memo
QUEENSLAND UNIVERSITY OF TECHNOLOGY
DOCTOR OF PHILOSOPHY THESIS EXAMINATION
CANDIDATE NAME: Ngoc Huu Huynh
FACULTY: Science
SCHOOL: Physical and Chemical Sciences
CENTRE: Science Research Centre
PRI NCl PAL SUPERVISOR: Dr Steven Bottle
ASSOCIATE S U PE RVI SO R: Dr Paul Schweinsberg
THESIS TITLE: The Inhibition of Copper Corrosion in Aqueous Environmnets with Heterocyclic Compounds
Under the requirements of Ph D regulations, Section 16, it is hereby certified that the thesis of the above-named candidate has been examined. On advice from the Principal Supervisor and Head of School, I recommend on behalf of the University that the thesis be accepted in fulfilment of the conditions for the award of the degree of Doctor of Philosophy.
. . . . . . . ..................... Professor R M&er Chair of Research Degrees Committee
KEYWORDS
Corrosion
Inhibition
Inhibitor
Copper
Alkyl esters of carboxybenzotriazole
Polarisation
Weight-loss
SERS
EIS
Molecular modelling
5
ABSTRACT
Benzotriazole (BTAH) has been used as a corrosion inhibitor for copper and copper-
based alloys for more than 40 years. It has been successfully employed for the
prevention of both atmospheric corrosion and particularly for the protection of
copper under immersed conditions. Whilst BTAH is an excellent inhibitor in alkaline
solution its efficiency drops off markedly as the pH decreases. It was hypothesized
that a possible way to increase surface adsorption and subsequent better inhibition
over a wide pH range might be through the preparation of derivatives, particularly
carboxybenzotriazoles and alkyl esters of these compounds.
In this work the following techniques: weight loss measurements, potentiodynamic
polarisation, SERS spectroscopy, electrochemical impedance spectroscopy and
coulometry were employed to investigate the inhibition efficiency of 4- and 5-
carboxybenzotriazole and their alkyl ester for copper corrosion. Molecular modelling
was also investigated as a tool for inhibitor design.
Studies on 4- and 5- carboxybenzotriazole (CBT) showed that the inhibition
efficiency for copper corrosion in aerated acidic sulphate solution of each isomer was
pH, concentration and time dependant. At lower pH the 5-isomer is the better
inhibitor and this behaviour continues at higher pH. The anti-tarnishing test showed
that whilst both isomers exhibited these properties, 5-CBT was once again the
superior inhibitor.
It was found that a commercial mixture of the octyl esters of 4- and 5-
carboxybenzotriazole inhibits copper corrosion in sulphate environments open to air.
The inhibition efficiency of the ester mixture at the lxlO-' M level (pH - 0) is 98%
which compares very favourably with that for BTAH (- 50%). With respect to other
alkyl esters of 4- and 5-carboxybezotriazole, hexyl, butyl and methyl, it was found
that all of these inlibited copper corrosion in sulphate environments open to air. In
each case the inhibition efficiency is concentration, pH and time dependent. Both
coupon tests and EIS measurements indicate that inhibition efficiency depends on the
length of the alkyl chain. At pH - 0 the inhibition efficiency decreased in the order
octyl >hexyl >butyl >methyl. At higher pH (- 8) the order is reversed. At the 1x104
M level (pH - 0) the inhibition efficiency of each of the alkyl esters is equal to or
better than that for BTAH. At higher pH (- 8) the inhibition efficiency in each case is
7
reduced in comparison to BTAH. but is still good enough for practical use ( 2 75%).
The inhibitive behaviour of the alkyl esters at low pH can be attributed to
chemisorption through an azole nitrogen of the protonated alkyl esters. The
hydrocarbon chain is also physically adsorbed and the increase in physical adsorption
as the chain is lengthened accounts for the improved inhibition efficiency.
Dry films formed by immersing copper in solutions of alkyl esters of
carboxybenzotriazole also inhibit copper corrosion in both strongly acidic (pH - 0)
and near neutral (pH - S) sulphate corrodents. The inhibition efficiency depends on
the solvents used to dissolve the esters, solution temperature and immersion time.
Aqueous coating solutions furnish the most protective films. Films formed by
CBTAH-BU, CBTAH-HE and CBTAH-OE are more protective than that formed by
BTAH. The inhibition efficiency of the alkyl ester film increases as the alkyl chain is
made longer.
Molecular modeling showed that the optimum crude binding energy (Eblnd)
between each protonated ester molecule and the surface varied linearly with the alkyl
chain length. The resulting linear correlation between IE% and E bind for compounds
that are structurally similar suggested that the crude binding energy of a single
molecule with copper may be used to predict the inhibition performance of other
compounds constituting a series.
8
LIST OF PUBLICATIONS
V. Otieno-Alego, N. Huynh, T. Notoya, S.E. Bottle, D.P. Schweinsberg ; Inhibitive
effect of 4- and 5-carboxybenzotriazole on copper corrosion in acidic sulphate and
hydrogen sulphide solutions, Corrosion Science 11 (1999) 685-697
N. Huynh, T. Notoya, S.E. Bottle, D.P. Schweinsberg ; Inhibitive action of the octyl
esters of 4- and 5- carboxybenzotriazole for copper corrosion in sulphate solution,
Corrosion Science 42 (2000) 259-274
N. Huynh, S.E. Bottle, T. Notoya, A. Trueman, B. Hinton, D.P. Schweinsberg;
Studies on alkyl esters of carboxybenzotriazole as inhibitors for 'copper, corrosion,
Corrosion Science 44 (2002) 1257-1276
N. Huynh, S.E. Bottle, T. Notoya; D.P. Schweinsberg; Inhibition of copper corrosion
by coatings of alkyl esters of carboxybenzotriazole, Corrosion Science 44 (2002)
2583-2596
J. Bartley, N. Huynh, S.E. Bottle, T. Notoya, D.P. Schweinsberg; Computer
simulation of the corrosion inhibitor of copper in acidic solution by alkyl esters of 5-
carboxybenzotriazole, Corrosion Science 44 (2003) 8 1-96
9
TABLE OF CONTENT
11
CHAPTER 3. INHIBITIVE EFFECT OF 4- AND 5- CARBOXY-
BENZOTRIAZOLE ON COPPER CORROSION IN ACIDIC SULPHATE
AND HYDROGEN SULPHIDE SOLUTION ................................... 85
CHAPTER 4. INHIBITIVE ACTION OF THE OCTYL ESTERS OF 4- AND
CHAPTER 5. STUDIES ON ALKYL ESTERS OF CARBOXY-
BENZOTRIAZOLE AS INHIBITORS FOR COPPER CORROSION ---- 119
CHAPTER 6. INHIBITION OF COPPER CORROSION BY COATINGS OF
ALKYL ESTERS OF CARBOXYBENZOTRIAZOLE ...................... 141
12
STATEMENT OF ORIGINAL AUTHORSHIP
The work contained in this thesis has not been previously submitted for a degree or
diploma at any other tertiary educational institution. To the best of my knowledge
and belief, the thesis contains no material previously published or written by another
person except where due reference is made.
Signed ~
Date
13
ACKNOWLEDGEMENTS
I would like to thank the following people and organisations for their support
throughout this research:
My supervisors Dr D.P. Schweinsberg and Dr Steve Bottle who provided the
invaluable vision and guidance throughout this work;
Dr T. Notoya , Hokkaido University, Japan, and Johoku R&D Co., Japan for the
supply of inhibitors;
Dr Harvey Flitt and Dr John Bartley for their work on computer simulation;
Dr Bruce Hinton and Dr Tony Trueman from Aeronautical and Maritime Research
Laboratory, Melbourne for their support in the EIS measurements
Dr Vincent Otieno-Alego for initial work on the carboxybenzotriazoles
My colleagues and friends from School of Physical Sciences, QUT
The Centre for Instrumental and Developmental Chemistry, QUT for their guidance
and financial support
And finally, my wife Phi and my children, Quoc and Minh for their love and
invaluable support throughout my work.
15
CHAPTER 1
INTRODUCTION AND LITERATURE REVIEW
17
1.1 Significance of corrosion
Corrosion is the destructive result of chemical reaction between a material and its
environment’. The most common form of corrosion (discussed in this thesis) is
aqueous metallic corrosion in which the material is a metal or metal alloy and the
environment (corrodent) is an aqueous solution. In daily life, such corrosion is
present in various forms: corroded nails, tools, reddish-orange spots in car bodies,
leaking hot-water tanks, murky potable water are common examples2.
The economic cost of corrosion is enormous, and has been estimated to be in the
range of 2-4% of an industrialised country’s gross national product?. In addition to
these direct costs, there are also indirect costs associated with plant shutdown, lower
efficiency of equipment, contamination and overdesign. Parts and labour to replace
corroded equipment are often minor compared to the loss of production while the
plant is non-operational. Thus leaks in pipelines and tanks result in loss of costly
products and these leaks can also pose a serious environmental problem.
Accumulation of undesirable corrosion products on heat exchanger tubing and
pipelines decreases the efficiency of heat transfer and reduces pumping capacity.
Soluble corrosion products can contaminate a system and decontamination is costly.
In the absence of adequate corrosion rate information (metal weight loss/ unit area /
unit time) overdesign (e.g., thicker tube wall) is required to ensure reasonable service
life. This leads to waste of resources and, for moving parts, greater power
requirements.
1.2. Common methods of corrosion prevention2
In most industrial situations it is virtually impossible to “prevent” corrosion. The
general strategy is to use measures that reduce the corrosion rate to an economically
sustainable level. The most important corrosion mitigation procedures are as follows:
1.2.1. Selection of materials and design
Select materials for the particular working environment (composition,
temperature, ‘velocity) taking into account mechanical and physical properties,
availabiiity, method of fabrication and overall cost of component or structure. It
must be decided whether or not an expensive corrosion-resistant alloy is more
economical than a cheaper metal that requires protection or possible periodic
replacement.
Avoid geometrical configurations that facilitate corrosive conditions such as
19
a) Features that trap dust. dirt and water;
b) Crevices and situations where deposits can form on a metal surface;
c) Designs that lead to erosion-corrosion or to cavitation damage in flowing
systems;
d) Designs with inaccessible areas that cannot be re-protected, e.g., by
maintenance painting;
e) Designs that lead to heterogeneity in the metal or in the environment.
Avoid metal-metal or metal-non metallic contacting materials that facilitate
corrosion such as
a) Bimetallic couple;
b) A metal in contact with absorbent materials that maintain constantly wet
conditions;
c) Contact with substances that gives off corrosive vapours.
Avoid stresses that lead to stress corrosion, corrosion fatigue or fretting
corrosion.
1.2.2. Changing interfacial potential
Protect metal cathodically by making the interfacial (metal/solution) potential
sufficiently negative by means of either (i) sacrificial anode or (ii) impressed
current, i.e. by cathodic protection.
Protect metal by making the interfacial potential sufficiently positive to cause
passivation (formation of a protective film on the metal). This method is confined
to metals that passivate in the corrodent under consideration).
*
1.2.3. Protective coatings
Form metal reaction products, e.g., anodic oxide films on Al, phosphate coatings
on steel, chromate films on light metals (Zn, Al, Cd);
Generate metallic coatings that form protective barriers (Ni, Cr) or protect the
substrate by sacrificial action (Zn, Al, Cd on steel);
Use inorganic coatings, e.g., vitreous enamel, glasses, ceramics;
Apply organic coatings, e.g., paint, plastics, greases.
1.2.4. Changing the environment
20
0 For aqueous corrosion, make the environment less aggressive by removing
constituents or modifying conditions that facilitate corrosion: decrease
temperature, decrease velocity, prevent access of water and moisture, remove
dissolved 0 2 , increase pH (for steel)
For atmospheric corrosion, dehumidify the air and remove solid particles. 0
1.2.5. Adding inhibitors
Corrosion inhibition means the reduction of the corrosion rate of the metal by the
addition of a chemical compound to the solution in contact with the metal3. In
general, an inhibitor forms a protective film in situ by reaction with the corroding
surface. As a result, the rate of the anodic and/or cathodic corrosion reactions are
retarded. Normally, only a small quantity of the inhibiting compound is needed to be
effective (e.g., -104M). Corrosion inhibition is reversible and a minimum
concentration of the inhibiting compound is required to maintain the inhibiting
surface film. Good circulation and the absence of any stagnant areas are also needed
to maintain inhibitor concentration. There is often a synergism between different
inhibitors and commercial formulations usually consist of mixtures. If two or more
alloys are present in the system, specially designed mixtures are required.
Inhibitors are used mostly in recirculating systems. In once-through systems the
consumption of inhibitors is usually too high to be commercially feasible. The
effectiveness of inhibitors depends on solution corrosivity, concentration and
temperature. Many are effective for more than one type of alloy, but an inhibitor for
one metal may be corrosive to others. Both inorganic and organic compounds are
employed and many inhibitors are toxic (chromate, arsenic, hydrazine) and their use
has been limited by environmental regulations. Nevertheless, inhibitors still play a
critical role in corrosion prevention.
Inhibitors are most commonly used in three types of environments:
1. Cooling waters in the near neutral (pH 5 to pH 9) range.
2. Pickling acid solutions for removal of dust and mill scale during the
production and fabrication of metal parts or post-service cleaning of these
parts.
3. Primary and secondary production of crude oil and subsequent processes.
21
1.3. The corrosion of copper and organic inhibitors for copper corrosion
Copper and its alloys exhibit high electrical and thermal conductivity. high
formability, machinability and strength and as a result of these favourable properties
are extensively used in potable water pipes, valves, heat exchanger tubes and tube
sheets, wire, screens, shafts, roofing, bearings, stills, tanks. and printed circuits.
These materials also have good corrosion resistance to de-aerated non-oxidizing
acids such as < 10% HC1 at < 75OC. <70% HF at <lOO°C, 60% HzSO4 at <lOO°C,
H ~ P O J and acetic acid at room temperatures. Copper is not resistant to HNO;, hot
aerated H2S04 and >10% HC1.
It is important to know the mechanism of copper corrosion in order to design,
select and use inhibitors that affect the corrosion rate. In the absence of C1- ions or
NH3, reference to the Pourbaix potential/pH diagram for copper4 shows that the
anodic dissolution of copper occurs (depending on the pH) according to the
following reactions:
cu + cu’ + e-
CU+ -+ CU*+ + e-
The cathodic reaction involves reduction of oxygen and the Pourbaix diagram for
copper shows that hydrogen evolution is not part of the dissolution process.
It is also important to have a good knowledge of the nature and composition of
any corrosion product layer in order to design, select and properly use an inhibitor.
Thus for 10-6 M dissolved ion activity at alkaline pH, cuprous oxide is formed
initially which on oxidation, gives a cupric oxide film which is not stable and
protective. In contrast in acidic solution the inhibitor could be expected to be
adsorbed on a clean copper ~ur face”~ .
Many organic compounds have been used for the inhibition of copper
corrosion and historically one of the most effective is benzotriazole (B TAH).
Because this thesis is concerned with the action of some BTAH derivatives, the
following review concentrates on the inhibiting action of benzotriazole towards
copper and related compounds. The action of other types of surface active organic
compounds is also outlined.
Table 1.1 summarised the main types of organic inhibitors that were studied as
inhibitors for copper corrosion. More details were outlined in sections 1.3.1 to
1.3.36.
22
Type of inhibitors Benzotriazole and analogous compounds
Table 1.1. Types of compounds studied as inhibitors for copper corrosion
Corrosive environments 5% NaC1, HzS, NH,Cl, IN NaOH, cooling water,
NaC104
1.3.1 Benzotriazole and analogous compounds
As early . a s 1967, Cotton et aL5 studied corrosion inhibition of Cu by
benzotriazole and analogous compounds where one or two of the N atoms of
benzotriazole are substituted by C or the labile H atom is replaced by a CH3 group,
i.e., indazole, benzimidazole, indole, and methyl benzotriazole. Salt-spray testing
showed only, benzotriazole and indazole prevented staining and only the former
retained tarnish-resistant properties after the treated Cu surface was washed with
organic solvents. Chemisorption through an azole nitrogen resulted in an insoluble
complex. Substitutional studies in the benzene ring showed C1 or nitro-groups in the
5-position increased acidity of the labile H atom but did not affect inhibition effects;
naphthotriazole behaved similarly but 5-hydroxy indazole and 4-methyl
benzimidazole were less effective than the parent compounds. Generally 5-
membered heterocyclics, imidazole, 1,2,4-triazole, and pyrazole and their derivatives
showed little inhibitive effect. The surface film apparently forms a true metal surface
bond and the complex is polymeric.
Mansfeld6 studied the effect of benzotriazole on the corrosion of Cu in 5% NaC1.
Here benzotriazole is a good inhibitor. It is chemisorbed through N in the azole ring
on the surface and this prevents adsorption of 0 and formation of a prenucleation
layer which is the forerunner of oxide formation. In acid solution, film thickness
appears high7, but for a short exposure time this is a result of a porous salt film which
23
is wiped off easily. For longer exposure times, this underlying film becomes thicker
and cannot be wiped off.
Walker8 reviewed the use of BTAH as a corrosion inhibitor and the theory of its
mode of protection of Cu in aqueous systems. BTAH is a good inhibitor for Cu and
brass when added to many neutral and alkaline solutions and acts as a weak buffer.
Cu surfaces pretreated with a hot solution of BTAH had an increased resistance to
staining in the atmosphere and also to corrosion in many solutions of salts. The
inhibitor reduced the dissolution of Cu in water.
The corrosion of Cu by H2S was inhibited by dipping the metal into aqueous
solutions containing Zn2+, benzotriazole and H2SO:.
The oxidation behavior of Cu dipped in BTAH was compared with that of
untreated Cu in air at 400 OC Io. The oxidation rate of the dipped Cu decreased with
immersion time, benzotriazole concentration and solution temperature.
A packing for Cu is prepared by impregnating paper or plastics with a mixture of
4-methylbenzotriazole and 5-methylbenzotriazole' * . The packing protects Cu from
corrosion in atmospheres containing H2SO3 or H2S.
Reaction products of BTAH with oleic acid, linoleic acid dimer,
di(nonylpheny1)phosphonate or mono(nonylpheny1) phosphate were prepared and
used as copper corrosion inhibitors in lubricating oils12.
Walker13 studied the corrosion of Cu in acidic, neutral and alkaline solutions
containing triazole, benzotriazole and naphtotriazole. Triazole was a poor inhibitor,
while benzotriazole and naphthotriazole were better. Naphthotriazole gave the best
protection. When used as a pretreatment for Cu surfaces, naphthotriazole was also
the most effective.
The inhibitive effect of BTAH as a corrosion inhibitor for Cu in 0.2M acetic,
monochloroacetic, dichloroacetic and trichloroacetic acid solution was studied I 4 - l 5 .
The inhibitor action was satisfactory and afforded adequate protection at a
concentration of 200 ppm. In trichloroacetic acid, a maximum inhibition of 98% was
found.
2-mercaptobenzimidazole, benzotriazole, 2-methylbenzothiazole and indole were
effective inhibitors for corrosion of Cu in HNo3 solutionsI6. A Langmuir isotherm
was obeyed in each case.
Indole, 2-mercaptobenzothiazole, 2-mercaptobenzimidazo1e, benzotriazole,
benzimidazole and 2-mercaptobenzoxazole at 200 ppm in 1N NH&1 were effective
24
corrosion inhibitors for 99.6% Cu and brass” . The inhibitor effectiveness increased
with increasing inhibitor concentration. The ionic species formed insoluble
protective complex salts.
X-ray photoelectron spectroscopy’* showed that on Cu and Cu-Ni alloys
benzotriazole forms a Cu(I) surface complex, which oxidizes rapidly to a Cu(II)
species on removal from the liquid phase. Results suggested that Cu20 facilitates
formation of the surface film.
BTAH, 2-mercaptobenzothiazole, 2-mercaptobenzimidazole, benzimidazole and
indole were investigated as corrosion inhibitors for Cu in IN NaOH”. Their
performance was satisfactory.
Azoles and triazines were found to be useful corrosion inhibitors for Cu,
especially in contact with water2’.
The corrosion inhibition of Cu by 1,2,4-triazole in 0.2M NHJCl solutions was
studied by photoelectron spectroscopy2’ . Corrosion inhibition was effected by the
formation of a Cu(II) oxychloride-I complex which produced a weakly adherent
scale.
The inhibitive effect of BTAH and tolytriazole (TTA) as corrosion inhibitors for
Cu and brass in 3% NaCl solutions were investigated22. The protection afforded by
pretreatment was tested in chloride solutions in the presence and absence of BTAH
and/or TTA. TTA by itself was found to be equally as effective as BTAH.
BTAH, 2-mercaptobenzothiazole, and 2,5-dimercapto-1,3,4-thiadiazole were
added to motor oil TB-20 to prevent corrosion of Cu and bronze engine parts by S-
containing antiseizing additives 23. The corrosion inhibitors decreased corrosion of
the engine parts and also improved antiseizing and antioxidation properties of the
motor oil.
BTAH, 2-mercaptobenzothiazole, 2-mercaptobenzimidazole, benzimidazole, and
2-methylbenzothiazole were investigated as corrosion inhibitors for copper in
Na2S0~ and various acid solutions24. All the inhibitors were generally effective in
the Na2S04 solution. 2-Mercaptobenzothiazole was usually effective in the acid
solutions.
Cu foil was treated with a mixture of BTAH, aminosilane and EtOH for increased
corrosion resistance”.
Cu parts for engine cooling systems were immersed in a solution containing
corrosion inhibitor(s) such as mercaptobenzothiazole, BTAH and tolyltriazole26.
25
Under typical cooling water conditions BTAH and tolytriazole (TTA) exhibit
significantly different inhibition b e h a ~ i o r ~ ~ . TTA forms a much thinner but more
hydrophobic film than does BTAH. The inhibitory effect of BTAH and TTA may be
partly explained by the stabilisation of the +1 oxidation state of Cu in the film against
further oxidation via an electronic effect of the triazole.
Tolyltriazole and benzoisothiazolone or its lower alkyl-substituted derivatives
were used as corrosion inhibitors for copper2'. The compounds showed excellent
corrosion inhibiting properties in cooling and boiler-water systems of petroleum
refineries, chemical plants and air conditioning plants.
The inhibition efficiency of BTAH, 5-aminotetrazole monohydrate , 2,5-diphenyl-
3(4-~hlorophenyl)tetrazolium nitrate, tetrazine B, a- and P-naphthylamine,
phenylthiourea, xanthane hydride, 2-mercaptothiazoline, and cupferron was
monitored over time as a function of inhibitor structure, corrosive media
(concentration, pH), concentration of inhibitor and Cu2+ ions, and Cu shape2'. In
general, the inhibitor efficiency decreased with increasing Cu2' concentration.
The inhibition efficiency and mechanism of BTAH, tolyltriazole,
mercaptobenzothiazole Na salt (MBT), and 2-(5-aminopentyl)benzimidazole (ABI)
for the corrosion of Cu in drinking water and synthetic seawater were studied3'.
MBT indicated mostly anodic action compared to the preferentially cathodic
mechanism of the other three.
The adsorption by Cu of the corrosion inhibitors BTAH, 2-
mercaptobenzothiazole, 2-mercaptobenzimidazole and 2-mercaptobenzoxazole was
studied in both neutral and acid chloride solutions by electrochemical techniques and
surface enhanced Raman scattering (SERS) 31. The undissociated inhibitors and their
anions are adsorbed simultaneously, the surface concentration ratio depending on the
pH and electrode potential. At low pH, BTAH is adsorbed weakly and it is displaced
from surface sites by both C1- and the strongly adsorbed 2-mercaptobenzothiazole.
The spectroscopic results explain the low inhibition efficiency of BTAH in acid
solution.
A mixture of mercaptobenzothiazole (I) and tolyltriazole (11) was used as a
corrosion inhibitor for Cu and Cu alloys in aqueous media32. The mixture has a
synergistic effect in aerated water (containing 0.087 % NaC1, pH 7). No synergism
was found in seawater containing 3.0% NaC1.
26
The corrosion rate of a Cu electrode in deaerated 1 . O M HC1 by Fe(III) ions, in the
absence and presence of BTAH was evaluated through weight loss measurement
using a rotating disc electrode (RDE) 33.
BTAH, tolyltriazole (TTAH), mercaptobenzothiazole (MBT), and 2-(5-
penty1amino)-benzimidazole at concentration - 10 ppm effectively inhibit Cu
corrosion in aerated seawater 34. MBT loses effectiveness at - 7OoC and it may not
be the optimum inhibitor for closed cooling units. The thickness of BTAH and
TTAH films formed under cooling water conditions are similar, as are the structures
of BTAH, TTAH and MBT films.
Cu corrosion in a boiler condensate system operating at - 40 atm is inhibited by
feeding a triazole (e.g., benzotriazole, tolyltriazole, or their alkali metal salts) into the
steam header 35. The triazoles are optionally used in combination with neutralizing
or film-forming amines.
BTAH, 2,2'-iminodiethanol-~aprylic acid (1 :2) condensate, and 2-aminoethanol
show a synergistic effect in protecting steel, copper, brass, and silver sheets from
corrosion in aqueous NaCl and/or NalS04 solution36. The caprylic acid Can be
substituted by lauric and oleic acid.
A mixture containing 2-mercapto-benzotriazole, Na tripolyphosphate,
polyethoxylated propyleneglycol ether and alkylpolyglycol ether phosphate prevents
fouling of circuits with corrosive deposits and inhibits biological conversion of
absorbed NH3 to aggressive NO3- 37.
Triazole derivatives and other heterocyclics were studied as corrosion inhibitors
for copper". The inhibitive effect depends on inhibitor concentration, potential, pH,
and temperature. 3-Amino-5-alkyl- 1,2,4- triazoles were the best inhibitors in the
whole pH range since they can form molecular compound layers in neutral and
alkaline solutions but heteropolar-compound layers in acid solutions An
intermediate aliphatic chain length gives the best efficiency since it allows sufficient
solubility and an effective hydrophobicity of the inhibitor layer. The similar
heterocyclic compounds have much smaller efficiencies. Benzotriazole is
comparable in neutral and weak alkaline solutions but it is less effective in acid
solutions and fails at high temperatures.
The cathodic and anodic behavior of Cu in tap water at pH values of 5 , 7 and 9
was studied at 50 OC in the presence different concentrations of 2-aminothiazole
(ATZ) and 2-amino-4,6- dimethylpyrimidine (ADMP)39. The chemical nature of the
27
complex formed between Cu and ATZ or ADMP was found in acidic solution to be a
Cu(II) complex and to a lesser extent Cu(I) complex. The concentration of Cu(I)
complex seems to increase as the solution becomes more basic. Polarisation studies
of the inhibition process suggest that the surface of the Cu in both cases in largely
covered by the complex.
A comprehensive theory on the bonding mechanism of the benzotriazole family of
inhibitors to Cu in neutral. aqueous systems was proposed4’ based on a 7c-bonding.
sandwich structure. This model explains the stereochemistry of the surface film and
its affinity for CU’ in preference to CU’+ ions.
A mixture containing benzotriazole, dioctyl phthalate. and balance iso-Pr alcohol
was applied on a twisted Cu wire and then the wire was coated with polyethylene4’.
The obtained Cu wire showed excellent corrosion resistances in aqueous solution
containing 100 ppm Na2S.
Discoloration of twisted hard-drawn Cu .wires due to Cu corrosion on exposure to
water is prevented without impairing the adhesion of the insulation to the Cu by
coating with corrosion inhibitor solutions containing benzotriazole and phosphate
plasticizers before applying the i n s u l a t i ~ n ~ ~ .
SERS measurements were performed for Cu-benzotriazole and Cu-6-tolyltriazole
interfaces with NaOH-containing electrolyte^^^. The effect of the solution pH was
examined. The suppression of the SERS signals was discussed. The adsorption of
corrosion inhibitors on Cu was assumed.
Alkylbenzotriazoles having the C6-12 alkyl group are corrosion inhibitors for Cu
or Cu alloys in aqueous systems44. The preferred inhibitors are heptylbenzotriazole
and octylbenzotriazole. and form stable hydrophobic films in aqueous cooling
systems and similar applications.
Corrosion inhibitors benzimidazole (BIMH), benzotriazole (BTAH), 2-
mercaptobenzimidazole (SBIMH), and polybenzimidazole (PBIMH) on Cu surfaces
were examined by SERS 4s. The corrosion protection against air oxidation followed
this order: SBIMH > BTAH > PBIMH > BIMH. Oxidation of Cu in 3% HC1,
NaOH. and NaCl was studied. No oxide was found after 12 h of exposure to a salt
solution. Good inhibition was attributed to the heteroatom at the 2-position of the
imidazole ring.
Cu or Cu alloys in aqueous systems are inhibited from corrosion with C3-18
alkoxy alkoxybenzotriazoles, e.g., butoxybenzotriazole. pentyloxybenzotriazole,
28
and/or hexyl-oxybenzothriazole46.
(penty1oxy)-benzotriazole.
A 99% inhibition was achieved with 5-
Near-IR Fourier transform surface-enhanced Raman scattering (FT-SERS) was
used in the study of competitive adsorption of benzotriazole and tolyltriazole on Cu
electrodes". There is coadsorption and benzotriazole displaces tolyltriazole when its
concentration is increased.
Corrosion inhibitors for Cu or Cu-alloy parts in aqueous systems comprise C3-12
alkylbenzotriazole and tolyltriazole, benzotriazole, mercaptobenzotriazole, and/or 1 - phenyl-5-mercaptotetrazole 48,49.
Butylbenzotriazole and tetrasodium EDTA as chelating agent inhibits copper
corrosion in aqueous bath containing NaC1, NaZS04 and NaHC03 (pH 8.5)
Corrosion inhibitors suitable for the protection of Cu or Cu-alloy parts in aqueous
systems (circulating hard water at pH - 7.8) contain a polyphosphate and an azole at
50:l to 1:50 weight ratio ' I . The azole is selected from C2-12-alkyl- or
alkoxybenzotriazoles, tolyltriazole, benzotriazole (with optimal substitution),
mercaptobenzothiazole, and/or 1-phenyl-5-mercaptotetrazole (or its isomers and
salts). The polyphosphate is optionally phosphorylate polyol.
The pitting corrosion of Cu has been investigated in plain NaC104, 7-11 pH
range, and in solutions containing benzotriazole 5 2 . Relationships between the
breakdown potential, the diameter of pits, the spatial pit distribution, the solution pH
and the applied potential have been establisheds2.
The in-situ measurement of open circuit photovoltage and AES technique were
used to investigate the corrosion of Cu in 3% NaCl solution and the inhibition
efficiency of TTA53.
Triazoles, such as tolyltriazole, have become the industry standard corrosion
inhibitors for copper metallurgy found in cooling system heat exchangerss4.
Tolyltriazole (TT) and butylbenzotriazole (BBT) - the most recent addition to the
triazole family - are excellent inhibitors of copper corrosion under a wide variety of
cooling water conditions. The common oxidizing biocide, chlorine, as made
available by the hypochlorite ion, may disrupt rhe triazole inhibitor film if applied in
sufficient concentration and for extended exposure times. Surface analysis studies,
supported by electrochemical data54, were used to examine the copper surface in an
effort to provide a more clear understanding of the triazole/chlorine interaction. The
triazole inhibitor film on copper is not a static barrier; it can be penetrated by water
29
and chlorine molecules, as well as by protons. chloride, and metal ions. Thus. a shot-
feed of chlorine could result in a brief corresponding increase in copper corrosion but
does not disrupt the triazole inhibitor film. However. longer exposures and higher
concentrations of hypochlorite can cause a weakening and ultimately a destruction of
the inhibitor film. resulting in a loss of copper surface protection. This is particularly
the case when there is no residual inhibitor available in the bulk water to repassivate
areas of the film affected by chlorine. This film disruption can be observed. by
surface analysis, from the decrease in intensity of both nitrogen (Nls) and large
cluster-ions. which are characteristic of the protective triazole film. Even after the
attack of chlorine on the triazole inhibitor film. nitrogen-containing species can still
be found on the copper surface. These triazole "remnants" may retard the corrosion
process but they are not a substitute for the original inhibitor film with its unique
protection properties. The BBT inhibitor film is somewhat more stable than that of
TT in the presence of chlorine, this advantage is of a short-term duration only.
Corrosion of Cu alloys in aqueous cooling systems can also be prevented by
adding mixed-isomer tolyltriazole (containing >45% 4-methylbenzotriazole, the
concentration of which is - 0.01-100 ppm), optionally with other corrosion
inhibitors". The 5-methylbenzotriazole isomer or is aerobically degraded in the
presence of bacteria. but the 4- isomer is stable. This mixed-isomer corrosion
inhibitor has proved suitable for service in a cooling-water tower.
The resistance of copper to pitting corrosion from aqueous solutions containing
chloride ions is greatly enhanced when mixtures of benzotriazole and potassium Et
xanthate are present in the bulk solutionj6. The existence of a synergistic effect for
copper corrosion inhibition is supported by electrochemical and surface analysis
data. This effect is attributed to the anodic formation of a continuous protective
chemisorbed layer on the corroding metallic surface in which CuzO oxide becomes
further stabilized. The critical limit at which chloride ion starts breaking the film
causing pitting was also determined.
A photoelectrochemical approach was applied to the characterisation of oxide
layers grown on copper, under open circuit corrosion conditions, in 0.5M NaCl
solutions, containing 1,2,3-benzotriazole or 6-tolyltriazole as corrosion inhibitors".
The influence of the inhibitors on the spectral response of a thin copper oxide layer is
shown. and interpreted with the support of a theoretical model.
30
A correlation between inhibiting efficiency of inhibitor, mainly azole derivatives,
and the photoelectrochemical behavior of the Cu(I) oxide layer was reviewed”. The
copper/benzotriazole system is one for which the contribution of the surface oxide
layer has been examined in conjunction with the effect of the inhibitor. The
mechanism seems to be relatively well resolved when the inhibiting molecular reacts
on a clean surface. It is much more complicated if the molecule has to react in
realistic practical conditions, i.e., for a Cu/Cu20/corrosive layer system.
The influence of azole derivatives, on the corrosion of copper in 3% NaCl over a
range of temperatures has been examined through steady state and weight loss
measurements5’. The results showed that the compounds are mixed type inhibitors.
In the anodic range, they act by forming a passive film on the copper surface, the
existence of the film being more evident for BTAH and diaminotriazole.
The efficiency of 2-amino-5-mercapto- 1,3,4,-thiadiazole (AMT) as a corrosion
inhibitor for copper in 5% (w/w) citric, sulphuric, and hydrochloric acids was studied
60. AMT showed greater inhibition efficiency than BTA in sulphuric and
hydrochloric acids. In citric acid, AMT and BTA show similar behaviour.
Traditional electrochemical tests and the contact electrical resistance technique
were used to investigate the effect on corrosion of 99.999 wt.% Cu by adding
benzotriazole and 1 -hydroxybenzotriazole (1 -0HBTAH) to H2S04/ Na2S04 solutions
at pH 1.7 61. This technique permitted growth of oxide and/or salt films as well as
adsorption of the organic inhibitors on the Cu surface to be evaluated. Formation of
(Cu20)2, CuS04.5H20, CuSCN, CuI, CuBr, and CuCl films on Cu electrodes was
followed in-situ in sulphate solutions at various pH values under low overpotentials.
The effects of pH, solution anion content, and/or amount of corrosion inhibitor on the
electrical resistance of the surface films formed on Cu electrodes were investigated.
BTAH acted as a more efficient corrosion inhibitor than 1-OHBTAH and reached an
inhibition efficiency of -90%, compared to that of 1-BTAH which reached a
maximum of -76% in 2x10” M solutions. It was possible to distinguish between
maximum resistance of the surface film found in solutions containing BTAH
associated with the adsorption of neutral inhibitor molecules and the sharp rise in
resistance attributable to Cu-I complex formation.
The cathodic and anodic behavior of copper in 30% ethylene glycol containing
SO:-, C1- and HCO3- were studied in the presence of 3-phenyl-l,2,4-triazolin-5-one
(PTR)62. The chemical nature of the complex formed between Cu and PTR was
31
found to be a Cu(II) complex as detected by XPS.
inhibition process suggested that the surface is largely covered by the complex.
Polarisation studies of the
The inhibitive action of 2-mercaptobenzo-thiazole (MBT) against the corrosion of
Cu, brass, and AI in 0.1 N AcOH, ClCH2C02H: Cl2CHCO2H. Cl3CC02H was
studied in 63. MBT was a fairly satisfactory inhibitor, affording adequate protection
to Cu and brass in these environments. MBT is predominantly an anodic inhibitor.
Patel@ reported the inhibiting action of MBT, 2-benzimidazolethiol (BI) and
sulfathiazole (ST) against corrosion of Cu in dichloroacetic acid solution. BI and
MBT are effective inhibitors with 80 and 77% inhibition respectively at 0.02%
concentration. The inhibiting action is due to chelate formation. Patel also studied the
protection of Cu against 3% NaCl by p-thiocresol (TC), BI and SI 65. TC gave 58%
inhibition at 0.001% concentration and 99% at 0.02%, BI gave 27% at 0.001%
concentration and 90% at 0.004%, and SI gave 32% at 0.001% concentration and
98% at 0.02%. Thus all are efficient inhibitors, owing to a film of insoluble chelate
with Cu ions formed at the metal surface.
The effectiveness of 2-amino-5-mercapto- 1,3,4-thiadiazole as a corrosion inhibitor
for Cu in acidic, neutral, and alkaline solutions was investigated66. The inhibitor
forms a polymeric complex layer on the Cu surface in solutions having different pH
values and completely inhibits corrosion. A Cu sheet with the polymeric complex
layer was subjected to different corrosive atmospheres, and the complex layer was
resistant to corrosion. Formation of a complex layer was confirmed by IR spectral
studies.
The impedance technique was used to study the adsorption properties of
mercaptobenzothiazole (MBT) in the corrosion inhibition of Cu in 1% aqueous
Na2S04 (pH - 6 ) 67. Stable adsorption of MBT was observed within the wide region
of potentials (* 0.3 V near the corrosion potential). The maximum degree of
coverage of the metal surface was at the MBT concentration - l ~ l O - ~ mol/L. High
adsorption energy was indicative of the chemisorption of inhibitor on Cu. The
inhibitive effect of MBT was mainly anodic.
Surface films formed by two structurally related corrosion inhibitors, 2-
mercapto benzothiazole and 2-mercaptobenzimidazole, on Cu were studied by XPS
and X-ray induced Auger spectroscopy6*. Under all conditions these corrosion
inhibitors form Cu' surface films of various stoichiometries. The film thickness were
closely related to the pH of treatment solutions, which reflects the stability of Cu20,
32
and the thickness of the original CuzO layer. A precipitation mechanism is suggested
for film formation.
The retarding effect of benzimidazole, 2-mercaptobenzimidazole, 2-methylbenz-
imidazole, 5,6-dimethylbenzimidazole and 2-mercaptobenzothiazole on the corrosion
of Cu in chlorinated water was investigated at 3OoC 69. The 5,6-
dimethylbenzimidazole gave 97% protection for 10 days.
Corrosion of Cu and 63/37 brass in 0.1N HOAc, CIH2CC02H, C12HCC02H and
C13CC02H at 3OoC was studied by the weight-loss method 70. 2-
Methylbenzothiazole was found to be a good inhibitor in C13CC02H.
Corrosion of copper in diluted acids in presence of chelating agents, 2-amino-5-
mercapto- 1,3,4-thiadiazole, 2-amino-4-benzothiazole and imidazoline-2-thione was
studiedat 30 C . 0 71
Imidazole compounds were used as corrosion inhibitors for preventing nest-form
corrosion of Cu materials (e.g., Cu pipe) caused by lubricants containing O-
containing organic compounds72.
Imidazole derivatives, which are secondary products in Vitamin B6 production,
are used as atmospheric corrosion inhibitors for copper in electronic devices industry
and in the printed circuit board p r o d ~ c t i o n ~ ~ .
Addition of 200 ppm benzoxazole to 0.1N trichloroacetic acid gave corrosion
inhibition of - 85 to 88% for immersed Cu and 63/37 brass, re~pect ively~~. Addition
of 2-mercaptobenzoxazole gave 100% protection for both metals.
1.3.2. Mercaptan
The relationship between the amounts of Bu2O and cetyl mercaptan was
investigated with respect to the corrosion inhibitive rate7j. Mercaptan is adsorbed on
Cu only when the ether, a proton-accepting substance, has removed the water
adsorbed on Cu by H-bonding. However, when excess of the ether was present in
the filming solution, the ether was preferentially adsorbed on the metal, and
mercaptan could not be adsorbed.
Other experiments were carried out using Bu2O as a proton-accepting substance
and dodecyl mercaptan as an i n h i b i t ~ r ~ ~ . Mercaptan acts as a filming agent only in
the presence of strongly proton-accepting substances.
1.3.3. Carbamate
33
Sodium diethylcarbamate was used to inhibit copper corrosion in various buffer
solutions of NaOH and citric acid (pH 4.3-5.9) 77.
A mixture of thiocarbamate and a benzotriazole prevents copper corrosion and
barnacle adhesion in a cooling system using sea water78.
The inhibition effect of di-ethyl dithiocarbamate, piperidine dithiocarbamate and
morpholine dithiocarbamate on copper in acids was in~est igated~~. Di-ethyl
dithiocarbamate, piperidine dithiocarbamate are more effective inhibitors than
morpholine dithiocarbamate in 0.1N H2S04, HC1 and HNO3.
The effect of sodium diethyldithiocarbamate on the corrosion of copper and brass
in seawater was evaluated The pronounced inhibition action of
diethyldithiocarbamate is attributed to metal-sulphur bond formation and it acts as an
anodic inhibitor.
The organic compound consisting of phenothiazinyl and/or the thiocarbamic
group is used for the prevention of corrosion of Cu by the 0-containing components
of lubricating oils". The inhibited lubricating oils are especially suitable for bending
fabrication of copper tubes for a i rconditioners.
1.3.4. Nitrobenzoates and nitronaphtolates
Dinitrobenzoates of hexamethyleneimine were studied as corrosion inhibitors for
copperg2. The protection of the metal is due primarily to anodic retardation. The 3,5-
dinitrobenzoate of hexamethyleneimine is a more effective inhibitor than oxidized
amines, p-nitrobenzoate, benzoate, and o-nitrobenzoate.
Nitrophenolates of primary amines (methyl- and trimethylammonium 2,4-
dinitrophenolates) are effective corrosion inhibitors for Cu and Zn in H20g3.
The effect of polar groups in naphthalene compounds on their protective
efficiency and protection mechanism was investigated with respect to the corrosion
of steel, Zn, and Cu in distilled H2OS4. The most efficient inhibition was obtained
with Na nitronaphtholates. The protection mechanism of Na nitronaphtholates is
based on a chemical adsorption and that of Na naphtholates on a physical adsorption.
1.3.5. p-Thiocresol
The corrosion inhibition of Cu in 0.5N HC1 and 0.5N HOAc by 2-
benzimidazolethiol was studied 'j. 2-benzimidazolethiol inhibited the corrosion in
34
both acids. The inhibitive action was attributed to chelate formation with the metal
ion. The inhibition is predominantly controlled by cathodic polarisation.
The inhibiting effect of p-thiocresol and thiobenzoic acid towards Cu corrosion in
0.5N HNO3 is due to the formation of chemisorbed 2-dimensional barriers of a
molecular thicknesss6.
Thiophenol and p-thiocresol were found to inhibit the corrosion of Cu by NH4C1
solutions7.
The inhibitive action of 2-mercaptobenzoic acid, phenylthiourea, and
thiosemicarbazide on the corrosion of Cu in HOAc solution was studieds8.
Thiosemicarbazide gave the best protection, which is due to the formation of an
insoluble Cu complex.
The protective action of thiosemicarbazide, Na diethyldithiocarbamide, and 2-
mercaptobenzothiazole for Cu in 3% NaCl solution was investigated8’. 2-
mercaptobenzothiazole was more effective than Na diethyldithiocarbamide and
thiosemicarbazide offered the least protection
1.3.6. Thioglycolic acid
The adsorbed layer of thioglycolic acid formed as a corrosion inhibitor on a Cu
surface in diluted HNo3 was studied by IR spectrometrygO.
1.3.7.2-Mercapto-2-thiazoline
The effect of 2-mercapto-2-thiazoline as a corrosion inhibitor for Cu in HOAc,
C~CHZCCO~H, C12HCC02H, and C13CC02Hy was studied’’. The protective power of
2-mercapto-2-thiazoline increased with increase in acidic character of the acid, that
is, C13CC02H > C12HCC02H > ClH$2CO2H > HOAc. The reaction is cathodically
controlled.
A mixture of polyethylene polyamines and 1,2-benzoisothiazolin-3-0ne is suitable
for corrosion prevention of cu pipes in aqueous systemg2.
1.3.8. Cyclohexylamine carbonate
Cyclohexylamine carbonate is a volatile corrosion inhibitor for Fe and Cu
surfaces 93. In the case of Cu, cyclohexylamine carbonate does not act as an inhibitor
in the liquid phase but on the contrary accelerates the corrosion.
35
1.3.9. Acetylenic alcohols
Acetylenic alcohols were effective as inhibitors for copper in simulated industrial
atmospheres containing SO2 and humid air94. Certain alcohols of this type, in
particular 1 -ethynylcyclohexanol protect Cu and brass satisfactorily in acid
atmospheres.
1.3.10. Pyridine derivatives
The inhibiting effect of pyridine derivatives on Cu corrosion in 0.1M K2S208 was
as follows: 2,6-lutidine > 2-picoline > pyridine and m-anisidine > p-phenetidine >
aniline where aniline is less effective than pyridine".
The inhibition of the corrosion of A1-4% Cu alloy (B26S) in HC1 solutions by
some N-heterocyclic compounds was in the order: 4-picoline < 3-picoline < 2-
picoline < pyridine < piperidine < acridine 96. The inhibitors function through
general adsorption, following the Langmuir adsorption isotherm.
1.3.1 1. Dihydroxybenzene
Dihydroxybenzenes such as catechol, hydroquinone and resorcinol inhibit the
corrosion of Al-4% Cu alloy (AI B 26S) in NaOH and
The effect of some 4-amino-4'-nitroazobenzene derivatives as corrosion inhibitors
for Cu in HN02 solution was investigated by using thermometric and polarization
techniques98. All the inhibitors follow the Frumkin adsorption isotherm.
1.3.12. Phenols
Corrosion inhibition of AI B26S by phenols in NaOH was studied99. The
inhibition efficiency increases as follows: PhOH < o-cresol < m-cresol < p-cresol.
The better inhibition qualities of the latter cresol may be traced to the absence of
steric hindrance.
A general corrosion study was made on the aminophenol-NaOH- Al/Cu alloy
system"'. The inhibitor efficiency increases in the order: p-aminophenol < m-amino-
phenol < PhOH < o-aminophenol. Chelate formation and the Freundlich adsorption
isotherm are suggested as the main factor in the corrosion inhibition mechanism.
The inhibition of corrosion of AI-Cu alloy by p-substituted phenols was studied
'O1. At 2.0% inhibitor concentration in 0.1M NaOH, the efficiency of the inhibitors
increased in the order p-hydroxydiphenyl < p-aminophenol < p-bromophenol <
36
hydroquinone < p-chlorophenol < p-cresol< p-hydroxyacetophenone < p-
nitrophenol< PhOH.
Inhibition of corrosion of B26S (Al-4%Cu alloy) in solutions of NaOH by
polyhydric phenols was studiedIo2. At 1% concentration in 0.1M NaOH the
efficiency of the inhibitors increases in the order: phenol (83.3%) < hydroquinone <
catechol < resorcinol < pyrogallol and phloroglucinol (98.9%); i.e.: monohydric <
dihydric < trihydric phenols. Nearly 100% inhibition is achieved with 2% inhibitor
in 0.1 M NaOH. The increase in steady-state potentials and galvanostatic polarisation
data suggest that the inhibitors are of the mixed type.
Inhibition by nitrophenols of the corrosion of B26S (Al-4% Cu alloy) in solutions
of NaOH was studiedio3. At an inhibitor concentration of 0.1-2.0% in 0.1M NaOH,
the efficiency increases in the order: p-nitrophenol <m-nitrophenol 5 o-nitrophenol <
phenol. The inhibitors appear to function through adsorption, following the
Langmuir adsorption isotherm. Galvanostatic polarisation data and open-circuit
potentials suggest that all 4 compounds are mixed-type inhibitors.
The dissolution of Cu in W O 3 solution in the presence of resorcinol, 0-, p-
aminophenols, catechol, o-cresol, and salicylaldehyde as corrosion inhibitors was
studied Io4. The inhibitors appear to function through general adsorption following
the Langmuir adsorption isotherm. The inhibiting effect of the tested compounds is
due to the decomposition of HNOz formed and its interference with the cathodic
reaction.
1.3.13. Colloids
Colloidal corrosion inhibitors for copper-aluminium alloy (B26S) in aqueous
0.1M NaOH were studiedIo5. The inhibition efficiency was due to adsorption and the
efficiency increased in the order: gelatin < dextrin < glue < agar agar < acacia <
tragacanth.
B26S A1 alloy was also exposed to Cl-substituted acetic acids containing colloids
as inhibitorsIo6. The colloid inhibitor efficiency (1.5% in 0.1 N CI~CCOZH) increased
in the order dextrin (42%) < gelatin < agar-agar < acacia < glue (89%). In HCI the
efficiency of the inhibitor increases in the order: dextrin (40%) < gelatin (56%) <
agar-agar (64.3%) < acacia (77.5%) and glue (81%) Io7. All five substances are
effective mixed inhibitors.
37
1.3.14. Benzimidazole
Benzimidazole was investigated as a corrosion inhibitor for brass in
trichloroacetic acid solutionio8. Its efficiency increased with increasing C1 content of
acetic acid i.e., CC13COOH
> CHC12COOH > CH2ClCOOH > AcOH. The inhibitive effect of benzimidazole is
mainly attributed to its chelate-forming tendency with Cu ions.
Thiocarbonyl compounds were effective corrosion inhibitors in acidic media' 09.
The most effective compound was 2-mercaptobenzimidazole.
2-(4-thiazolyl)-benzimidazole was used as corrosion inhibitor in water containing
C1- (500ppm) and S042- (500 ppm) ' I o .
Cu and Cu alloys were coated with 5-Me benzimidazole 5-60 nm thick and this
was effective against oxidation and corrosion" I .
The structure of various imidazoles on the surfaces of Cu and Au mirrors was
studied by Fourier transform IR reflection-absorption spectroscopy in order to
elucidate the molecular mechanisms of corrosion inhibition of metal surfaces by
imidazoles"*.
Fourier-transform IR reflection-absorption spectroscopy and measurement of Cu
oxide formation showed that poly(N-vinyl-, or 4(5)-vinylimidazoles) are effective
antioxidants (better than benzotriazole and imidazoles) for Cu at elevated temps' 1 3 .
Below 25OoC, no major degradation of the coated films on Cu was observed.
1 .3.15. Hydrazines
2,4-dinitrophenylhydrazine functions as a corrosion inhibitor for Cu in HzS04 by
inhibiting the cathodic reaction ". Corrosion inhibitors, prepared by adding N2H4 to phytic acid or its salt, were used
in a coolant for Cu vessels used for rapid cooling of foodIi6. The N2H4 inhibits
growth of oxide film on the Cu in contact with the coolant.
The effect of tosylhydrazine (THy) and 4-nitrobenzoylhydrazine (4-NBHy) on the
corrosion of Pb, Al, and Cu in NaOH solution was studied"'. These inhibitors
facilitate the conversion of Cu(OH)2 to black CuO film and thus inhibit corrosion of
Cu in alkaline solutions.
A mixture containing N-alkyl or -arylisoquinolinium bromide, hydrazine hydrate,
DMF, and still residues (from scrubbing of coking gas with monoethanolamine) is
38
used as corrosion inhibitor for Cu, Cu-containing alloys- and austenitic stainless steel
in aqueous acidic media"'.
1.3.16. Anilines
The action of aniline and twelve derivatives as inhibitors for the corrosion of
B26S aluminum in H3P04 solutions was e~amined ' '~ . At 0.5% inhibitor
concentration in 0.033M H3P04, p-tbluidine was the most effective.
The effect of substituted anilines on corrosion of Cu in 2N HNO3 was studied by
using weight loss and polarisation techniques'20. The relative inhibition efficiency of
these compounds was determined by the nature of the substituent group and its
position in the aromatic ring, as well as its concentration. These substituted anilines
act mainly as cathodic inhibitors. Both diazotization (removal of HN02) and
adsorption characteristics of amines (or diazonium salts) on the metal surface play a
significant role in inhibition.
1.3.17. Cyclohexylamine and amine
Salts prepared by reacting carboxylic acids R-CONH-R1 CO2H (R = C6- 12 aryl or
alkaryl, R1 = C1-5 alkylene) with alkanolamine and/or cyclohexylamine are used as
indispensable components for corrosion inhibition of Fe, Cu, and their alloys'2'.
C2-8-alkanolamine mercaptobenzothiazolate or alkali metal
mercaptobenzothiazolate, C 1 -8-alkanolamine phosphate, and a surfactant were used
as corrosion inhibitor in a cooling circuit'22. The amount of deposits, steel corrosion
and brass corrosion were decreased by 80,70 and 65 %, respectively.
Hexamethylenetetramine (HMTA) is a corrosion inhibitor for Cu in a bromide
medium containing Fe(III) ion or oxygen as oxidants'23. The passive film was
characterized by fluorescence spectroscopy as being a Cu(1)-HMTA-Br-complex.
Alkali metal salts of 2-(NYN-dialkylamino)-4,6-dimercapto-l ,3,5-triazine are
suitable corrosion inhibitors for Cu in electricity-conducting cables'24.
The inhibitive effects of aminoacids, e.g. d-alanine (AL), 1 -phenylalanine (PAL),
1-cystine (CYS), and 1-methionine (MET) on the corrosion of copper in nitric acid
have been investigated"'. Inhibitive efficiencies were in the order: MET > CYS >
PAL > AL.
Amine series organic compounds are used for prevention of corrosion of Cu by
the 0-containing components of lubricating oils'26. The inhibitors have general
39
formulas R(NH2)n, R I R ~ ( N H ) ~ , and RlRzR;N, where R, R1, Rz, and R3 = alkyl, Ph,
naphthyl, or alkyl Ph group, n = 1-3, and m = 1-2. The inhibitors are added to
lubricating oils containing an 0-containing organic. The lubricating oils are
especially suitable for bending fabrication of copper tubes for air conditioners.
1.3.1 8. Piperazines
A mixture containing 10-30% N-acylpiperazines and/or N-
(acylaminoalky1)piperazines , 70-90% alkylimidazolines and alkylamidoamines is
effective corrosion inhibitor for copper in aqueous media at pH 7-1 1 12'.
1.3.19. Benzoxazole
Ag or Cu is protected from tarnishing with 2-mercaptobenzoxazole or 1 -phenyl-
lH-tetrazole-5-thiol, which may be included in a resin solution for forming a wear-
resistant coating'28.
1.3.20. Thiourea
Thiourea was a more effective inhibitor than Na benzoate for Cu corrosion in
0.2 5 -N NaCl solutions1 29.
Corrosion inhibitors for preventing nest-form corrosion of Cu materials (e.g., Cu
pipe) caused by lubricants containing 0-containing organic compounds are urea or
thiourea compounds having general formulas: RNHCONH:! (R=H, alkyl, Ph,
naphthyl, alkylphenyl, or acetyl), RlNHCONHR2 (Rl,Rz=alkyl, Ph, naphthyl,
alkylphenyl, or acetyl), RNHCSNH2 (R=H, alkyl, Ph, naphthyl, alkylphenyl, or
acetyl), and RlNHCSNHRz (RI ,Rz=same as above)13'.
Thiourea, allylthiourea and phenylthiourea have been investigated as corrosion
inhibitors for AI-4% Cu alloy in trichloroacetic acid'31. Substitution of amino H
atoms by phenyl or allyl groups affects the inhibitive action and suggests that the
resonance effect increases more with phenyl than with allyl groups.
1.3.2 1. Benzoic acid
The corrosion of Cu in HC104 solution containing various concentrations of
benzoic acid, p-toluic acid, p-nitrobenzoic acid, phthalic acid, and terephthalic acid
was studied132. These compounds inhibit corrosion effectively even in trace
concentrations.
40
Salts of amines and amino alcohols with nitrobenzoic and dinitrobenzoic acids are
effective inhibitors which practically suppress the atmospheric corrosion under
conditions of 100% relative humidity of the surrounding^'^^.
1.3.22. Aminopyrimidine
The protective coating produced during the corrosion of Cu in the presence of 2-
aminopyrimidine (I) was studied"'. The individual units form a 3-dimensional
macro-structure via chloride bridging and H bonding.
The mechanism of the corrosion inhibition of Cu by 2-mercaptopyrimidine and
other pyrimidine derivatives were disc~ssed'~'. These compounds formed a
complexed coating on Cu and this can be simulated by their reaction with CuCl in a
solution.
1.3.23. Triphenylmethane and dyes
The inhibition of the corrosion of Cu-A1 alloy (B26S) in HC1 solutions by
triphenylmethane, anthraquinoid, and acridine dyes was studied'36. At an inhibitor
concentration of 0.1% in 0.5 M HC1, the efficiency increases in the order: fuchsine
acid (32%) < acridine orange 5 fuchsine base (39%) < alizarin red S (46%) < methyl
violet 6B (50%) < malachite green (64%) < crystal violet (70%) < light green (80%).
At 0.5% inhibitor concentration in 0.033M H3PO4, the efficiency increases in the
order: methyl violet 6B (15%) <crystal violet <fuchsine acid <fuchsine base (36%)
<malachite green (68%) <victoria blue (75%) <light green <fast green (88%) 137. In
both media the inhibitors function through adsorption following the Langmuir
adsorption isotherm.
1.3.24. Xanthenes and azo dyes
The corrosion inhibition of AI-4% Cu alloy (B26S) by xanthene and azo dyes in
HC1 solutions was studied'38. At 0.1% inhibitor concentration in 0.5M HC1, the
efficiency of inhibitors increased in the order: methyl red (-40%) < dimethyl yellow
(34%) < catechol violet < eriochrome black T < rhodamine B < fast sulfon black F <
bromocresol purple < bromocresol green (88%) The inhibitors functioned by
adsorption following the Langmuir isotherm. In 5.0 M HC1 acid the inhibition
efficiencies are in the order: bromocresol purple (1 6.8%) <<fuchsine acid(94.4%) 5
41
bromocresol green I methyl red (-99%) <crystal violet I fuchsine base 5 dimethyl
yellow I light green I malachite green
The effect of 1 -ethyl-2-(o-hydroxystyryl)pyridinium iodide cyanine dye on the
corrosion of Cu in HN03 was studied'". The inhibition effect is more pronounced in
the case of Cu coated by a thin film of dye than that obtained by its addition to the
corrosion medium.
The inhibition efficiency of triphenylmethane dyes in the corrosion of B26S A1
alloy in 4 M HzSO4acid is in the order14': methyl violet < fuchsine base < victoria
blue < fuchsine acid < light green < crystal violet < malachite green < fast green. The
inhibitors function through general adsorption following the Langmuir adsorption
isotherm.
The effect of 2-(2-hydroxystyryl)quinolinium- 1 -Et iodide and 4-(2-
hydroxystyry1)quinolinium- 1 -Et iodide cyanine dyes on the corrosion behavior of
copper in nitric acid solution has been studied'42. The inhibition effect for two
cyanine dyes is more pronounced in case of the addition of dye to the corrosion
medium than that obtained for copper coated by the dye thin film previously treated
in dye solutions before immersion in the corrosion medium.
1.3.25. Toluidine
The corrosion of A1-Cu in HC1 and its inhibition by toluidines were st~died'~ ' .
The efficiency of the inhibitor (1 .O %) increases as: p-toluidine < o-toluidine <m-
toluidine in different concentrations (0.1N-0.3N) of HC1.
1.3.26. Boric acid
Boric acid and/or its salt prevent discoloration of Cu and Cu alloys in contact with
gas144. The boric acid is preferably metaboric acid. The inhibitors are not volatilized
and oxidized at 260-280 OC, maintain their effectiveness at temperatures, and are
useful for solar energy systems utilizing Cu in contact with water or water and
propylene glycol.
1.3.27. Quinine and strychnine
The effect quinine and strychnine on the corrosion of Cu in H2S04 was
investigated'"'. The maximum corrosion inhibition (99%) was observed with 1 O-3M
strychnine. The adsorption followed the Bockris-Swinkels adsorption isotherm.
42
1.3.28. Fatty acids
A corrosion inhibitor for steel and Cu parts of petroleum distillation columns
consists of a soap of a C18-20 fatty acid146. The soap also contains imidazoline or a
mixture of imidazoline, amide, amine, amido-imidazoline (all obtained from the
reaction of diethylenetriamine with C18 fatty acids). The corrosion of the steel and
Cu parts of the column was decreased by 60%.
Ethoxylated fatty acids from soybean oils having the general formula R-
COO(CH2-CH2-O),-H7 where R is mainly a mixture of C17H29, C17H31 and C17H33
and (n) is the number of ethylene oxide per mol (n=10, 16 and 30) have been used as
corrosion inhibitors for copper in 2 M ~ ~ 0 3 ' ~ ~ .
1.3.29. Oxalic acid
Raman spectroscopic studies of the interaction of oxalic acid and sodium oxalate
used as corrosion inhibitors for copper showed the formation of a salt complex of the
inhibitor molecules and the Cu ions'". This suggested that this chemisorbed surface
species produced the protective layer.
1.3.30. Haloacetic acids
The corrosion behavior of Cu in HNO3 containing chloro-, dichloro-, trichloro-,
bromo- and iodoacetic acids was studied'49. A considerable decrease in corrosion
rate is observed in the inhibited HNO3.
1.3.3 1 Carboxylic acids
1,2,3,4-Butanetetracarboxylic acids CO~RICH~CH(CO~R~)CH(CO~R~)CH~-
(C02R4) (Rl-4 = C1-18 alkyl) or their partial esters are used as heat-resistant
corrosion inhibitors for Cu and Cu alloys'''. The treated Cu showed a low tarnish
after 4 hr at 170 OC.
A corrosion inhibitor for Cu alloy pipes used for heat exchanger in desalination of
seawater contains ferrous sulphate and citric acid"'.
Electrochemical. methods were used in a systematic study of the abilities of the
homologous straight chain mono-and a, a-dicarboxylates to inhibit corrosion of Cu
in aerated, mildly saline, and near-neutral aqueous solutions'j2. Performance of both
compound types is critically dependent upon their chain length, the metal, and the
43
number of carboxylate groups. For dicarboxylates, with the possible exception of
mild steel, longer chain lengths were found advantageous. This was not true for
monocarboxylates, which showed abrupt decreases in inhibitor ability outside the
optimal range. The dramatic variations in inhibitor efficiencies probably resulted
from competing reactions, such as adsorption and complexation at the metal
(hydroxide) surface and micelle formation.
1.3.32. Schiff bases
The inhibiting action of some Schiff bases on the corrosion of Cu and its alloys in
HC1 and H2S04 was studiedL5'. The Schiff bases were synthesised by reacting
salicylaldehyde with aliphatic or aromatic amines. The Schiff bases prepared from
aliphatic amines stimulate Cu corrosion, whereas those obtained from aromatic
amines inhibit the corrosion. In both acid solutions considered, N-2-
thiophenylsalicylidenimine was particularly efficient. The inhibiting action is mainly
exerted on the cathodic reduction reaction. The effect of the various Schiff bases is
explained on the basis of their stability and by considering the effect of the different
substituents in the aromatic rings.
1.3.33 Organic P compounds
Some triaryl and trialkyl phosphites were investigated as corrosion inhibitors for
cu in HNO~ so~ut ions '~~ .
1.3.34. Aldehydes
Mixtures of RCHO and RlNH2 , where R and R1 are Cl-19 alkyls, C5-12
cycloalkyl, C7-13 aralkyl, C6, C10, or C14 aryl or substituted aryl, or monocyclic
heterocyclic remainder with 3 5 ring C atoms; or R is optionally -(CH2),CHO, and
R1 is -(CH& NH2; n = 1-6; 157 were used as inhibitors for copper in lubricants,
hydraulic fluids, machining fluids, or cooling media
1.3.35. Benzoylhydrazide
The effect of benzoylhydrazide derivatives on the corrosion of Cu in 3M HN03
solutions was studied1j8. The derivatives are adsorbed on the Cu surface according to
the Langmuir adsorption isotherm.
44
1.3.36. Polymeric compounds
Cu or Cu alloy fins for automotive radiators are coated with the corrosion
inhibitor containing poly(hydroxystyrene) compound and polyepoxy compound'59.
1.4. Background to this work
As can be seen from the preceding literature review, many organic compounds
containing N, 0 and S have been studied as inhibitors for copper corrosion in various
environments. However, benzotriazole and its derivatives remain the most widely
used compounds in industrial applications (e.g. cooling waters) due to their low cost,
availability and high effectiveness particularly in near neutral and alkaline waters. In
general, introducing substituents into either the benzene or benzotriazole ring of
BTAH (mostly some active functional group or a long chain alkyl group) increases
inhibition efficiency. This leads to the formation of some type of protective layer on
the copper surface most probably by chemisorption through the lone pair of electrons
of a N atom and physical adsorption by the alkyl group.
The nature of benzotriazole and its function as an inhibitor for copper corrosion
has been widely studied but there still remain contradictory opinions as to the nature
of the interaction of benzotriazole with the copper surface. One of the disadvantages
of benzotriazole as an inhibitor for copper is its loss of efficiency in acidic
environments. The performance of benzotriazole in acidic solutions can be improved
by adding additives such as KI (synergistic effect with 1- ions), but this so far has
received little practical application in industry. Some authors have tried to modify the
BTAH molecule and it had been suggested that the steric hindrance of the
substituents on the benzene part of benzotriazole leads to an increase in the inhibition
efficiency of this compound. However, the inhibition mechanism in acidic solutions
still remain controversial.
The work in this thesis is concerned mainly with the study of the inhibitive
effect of derivatives of BTAH (4-and 5 -carboxybenzotriazole and their alkyl
derivatives) for copper corrosion in aqueous environments. The compounds were
manufactured commercially by Johoku Ltd, Japan, and preliminary tests showed that
particularly the long alkyl chain compounds had very good inhibition efficiencies. It
can be hypothesised that the size of the alkyl chain in the ester substituent can
essentially reinforce the interaction of the benzotriazole portion of the molecule with
the copper surface and hence the inhibitive effect. It can also be expected that these
45
novel compounds will retain the general advantages of benzotriazole (reasonable
cost, availability, solubility). The present work aims to the above hypothesis by
studying the inhibitive effect of a series of alkyl esters in an acidic environment. In
addition the corrosion kinetics and nature of the species formed by interaction with
the copper surface will also be examined. Understanding the function of these
compounds as inhibitors of copper will contribute to the elucidation of the behaviour
of benzotriazole derivatives in general, and this should also lay the foundation for the
synthesis of more effective inhibitors. Molecular modelling will also be employed as
an aid to understanding the nature of the molecular interaction with the copper
surface and for the prediction of the inhibition efficiencies of compounds that are
structurally similar.
1.5. Objective of this work
The detailed objectives of this are as follows: .
1. To evaluate the inhibition efficiency of 4-and 5-carboxybenzotriazole and a
series of their alkyl ester derivatives for copper corrosion in aqueous
environments by standard weight loss measurement at different temperatures, pH,
concentration of inhibitor and time of exposure.
2. To establish the effect on the kinetics of the corrosion reactions of each
compound by potentiodynamic polarisation.
3. To evaluate the interaction of each compound with the metal surface by
electrochemical impedance spectroscopy (EIS).
4. To examine the nature of the species adsorbed on the copper surface by surface
enhanced Raman scattering (SERS) spectroscopy.
5. To establish the inhibitive effects of a coating formed on copper by a series of
alkyl esters dissolved in different solvents. This will include:
+ Polarisation study of the coating in different test environments
*:* Study of the anti-tarnishing properties of each ester in a supplied
environment
*3 Investigation of the nature and orientation of the species adsorbed on the
metal
6. To investigate molecular modelling as a tool for inhibitor design.
46
1.6. An account of scientific papers contributing to the objective & aims of the
thesis
The diagram in Fig. 1.1 can be used as a useful illustrative tool for summarizing
the research of this thesis.
Study on inhibition efficiency of 4- and 5-carboxybenzotriazole for copper corrosion (Chapter 3)
Study on the octyl esters of carboxybenzotriazole (Chapter 4)
& Study on other esters with different alkyl chain length: methyl, butyl, hexyl (Chapter 5)
Study on inhibitive effect of the films
Computer simulation to design new -b inhibitor (Chapter 7)
Fig. 1.1. A summary of the progress linking the scientific papers
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136. J.D. Talati, D.K. Gandhi, Werkst. Korros. 33 (1982) 155
137. J.D. Talati, J.M. Daraji, J. Electrochem. Soc. India 35 (1986) 175
138. J.D. Talati, D.K. Gandhi, Indian J. Technol. 20 (1982) 312
139. J.D. Talati, D.K. Gandhi, Indian J. Technol. 23 (1985) 232
140. M.T. Makhlouf, Z.H. Khalil, J. Chem. Technol. Biotechnol. 38 (1987) 89
141. J.D. Talati, J.M. Daraji, J. Indian Chem. Soc. 65 (1988) 94
53
142. M.T. Makhlouf, Z.H. Khalil, Collect. Czech. Chem. Commun. 58 (1993)
2003
143. R.B. Patel, J.M. Pandya, K. Lal, Trans. SAEST (1982), 17(4), 321-4
144. Kurita Water Industries, Ltd.. Japan; Japanese Patent 82-5 1390 82033 1
145. N.C. Subramanyam, B.S. Sheshadri, S.M. Mayanna, Br. Corros. J. 19
(1984) 177
146. E. Cristoloveanu, T.I. Decean, A. Dumitm, V. Manescu, Romanian
Patent 82-106943 820318; CA 103:74665
147. S.M. El-Haleem, M. Abdallah, A.I. Mead, Ann. Univ. Ferrara, Sez. 5
Suppl. (1995), 10 (8th European Symposium on Corrosion Inhibitors,
1995, Vol. 2), 963-73
148. H. Jeziorowski; B. Moser, Chem. Phys. Lett. 120 (1985) 41
149. N.C. Subramanyam, S.M. Mayanna, Indian J. Technol. 24 (1986) 17
150. A. Maeda; Chiyoda Kagaku Kenkyusho, Japan; Japanese Patent 86-
115876 860519; CA 108:172180
15 1. A. Sakanishi, K. Ueda, M. Sakimura; Mitsubishi Heavy Industries, Ltd.,
Japan; Japanese Patent 86-29 1596 86 1209; CA 1 10:2793 1 152. G.T. Hefter, N.A. North, S.H. Tan, Corrosion 53 (1997) 657
153. I.H. Omar, F. Zucchi, G. Trabanelli, Surf. Coat. Technol. 29 (1986) 141
154. A.S. Fouda, S.A. Gomah, M.N. Moussa, Qatar Univ. Sci. J. (1992), 12,
64-8
155. L. Komora, E. Komorova, Czech Patent 84-8196 841029; CA
109:26436
156. C.M. Mustafa, S.M. Shahinoor Islam Dulal, Corrosion 52 (1996) 16
157. W. Neagle; Ciba-Geigy A.-G., Switz.;German Patent 87- 10 195 870429
158. M.M. El-Tagoury, G.E. Bekheit, Bull. Soc. Chim. Fr. (1991), (Nov.-
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891213; CA 116:260574
54
CHAPTER 2
THEORY OF CORRROSION AND EXPERIMENTAL METHODS EMPLOYED
55
56
2.1. Electrochemical nature of corrosion 1-10
In aqueous environments the majority of metallic corrosion processes are
electrochemical. The reaction at the less stable anodic sites (A) on metal M (e.g.,
where there are dislocations, imperfections) can be explained by a simplified
equation:
M(s) -+ M"+(aq) + ne- (s) (2.1)
The corresponding cathodic reaction occurs at the cathodic sites (C) at the
metal/solution interface:
R(aq) +ne-(s) -+ Rn- (aq) (2.2) The species R is an oxidising agent in solution that can receive electrons from the
metal . In the corrosion process, all the electrons produced by the anodic reaction move
through the remaining sound metal to the cathodic site and are accepted by the
oxidising agent. The overall reaction of the corrosion process can be given by:
M(s) + R(aq) -+ M"+ (aq) + R"(aq)
Metal M
ne- ec - 3 '
A
Water
R + Rn-
M 9 M"+
Fig 2.1 Metal dissolution in water containing an oxidising agent R
On the metal surface many short-circuited galvanic cells (as shown in Fig. 2.1) are
set up. The metal will continue to dissolve in the presence of excess oxidant and this
process is spontaneous (negative AG) as long as the equilibrium (reversible) potential
for M(s) I Mn+(aq) is more negative than that of R(aq) I Rn-(aq).
For copper the main anodic reaction is copper dissolution:
CU(S> -+ Cu2+(aq> + 2e (E? =+0.337V) (2.4) Several cathodic reactions can occur in aqueous solution corresponding to the
copper dissolution reaction. In acidic solutions containing oxygen the main cathodic
reaction is:
57
Oz(aq) + 4Hf(aq) + 4e-(s) -+ 2 HlO(1) (l? = +1.229V) (2.5) In basic environments where there is sufficient oxygen, the following cathodic
reaction predominates:
Oz(aq) +2Hz0(1) +4e-(s) -+ 4OH-(aq) (l? = +0.401V) (2.6)
2.2. Corrosion kinetics
Once it is established that corrosion will occur, the next practical step is to
determine how fast is the corrosion rate. This is the objective of corrosion kinetics.
Consider reduction at a cathodic site. The rate of reduction depends essentially on the
rate at which the oxidant, e.g., Hf is transported in solution up to the cathodic site
and also on the rate of electronation (similar consideration applies to the oxidation
process). Slow rate of electronation and transport can now have an effect on the
reversible potential : the cathode potential no longer remains at its equilibrium value.
It is said to be polarised and the change in potential is called the overpotential. More
specifically slow electronation gives rise to the activation overpotential whilst slow
transport results in a concentration overpotential. Both forms are additive.
2.2.1 ..Activation polarization and the Butler - Volmer equation
To predict the corrosion rate (mm yr-l) of a pure metal M by the electrochemical
method, it is necessary to determine the corrosion current density, icon:
dW Micorr corrosion rate = - = - dt nF
Where:
M = atomic mass of metal
n = number of electrons transferred in corrosion process
F = Faraday's constant
i = corrosion current density A m-2
(Kg mol-')
(96,450 C mol-')
The corrosion current density is not directly measurable because a corroding
metal does not show any net current flow. However, it may be possible to determine
the corrosion current density by using the Butler-Volmer equation or its
approximation the Tafel equation.
58
Suppose metal M is in equilibrium with a solution of its own ions (Mn+ (aq)) and
its reversible or equilibrium potential is then changed to some other value (e.g. by
connecting it to another electrode to form a galvanic cell or to a potentiostat). The
resultant net current density (anodic or cathodic) is given by the Butler-Volmer
equation:
where:
inet = net current density (I/area of corroding metal) (A m-2>
io= equilibrium exchange current density (A m-2>
a = transfer coefficient ( - 0.5)
77 = activation overpotential (V)
T = absolute temperature (K)
R = gas constant (8.314 J K ' mol)
n = number of electrons in process
The Butler-Volmer equation relates the net current density (the electrode is not at
equilibrium) to the change in potential experienced by the electrode. The equilibrium
exchange current density io is the current density in each direction when the electrode
reactions (for example (2.1) and (2.2)) are at equilibrium. The potential change is the
potential by which the electrode' is polarised away from the equilibrium potential and
its value depends on the reaction rate of electron transfer. The slower this step the
greater is 7. The transfer coefficient a is related to an energy barrier which the
reacting species must overcome for the electron transfer to occur.
The Butler-Volmer equation has simplified forms:
(a) When 77 is small (+ 5 mV):
Equation (2.8) reduces to:
i.e. ia
RT io F
q a = - (2.10)
59
relatively low [02]. If the cathodic charge transfer step is very fast (i.e. there is no
activation polarisation) the concentration of oxygen in the interfacial region adjacent
to the cathodic site (CI) will drop to a value which is less than that in the bulk solution
(cg). This is because in this situation the rate of diffusion of 0 2 from the bulk of the
solution is not fast enough to replace those molecules reduced. As a result of the drop
in [ 0 2 ] at the interface the overall rate of reduction process will be decreased. A
limiting condition can be reached when the concentration of the reducible species at
the interface is reduced to zero and under these conditions the rate of oxygen
reduction, expressed as the cathodic current density, becomes fixed at a maximum
achievable value, i.e., the limiting value. The relationship between the cathodic
current density i, and the concentration is given by:
(2.16) nFD ( ~ i -ce)
6 ic =
where i, = cathodic current density ( A m-2)' .
n = F =
D =
CI =
number of electrons involved in the reduction reaction
Faraday constant (96 487 C mol-I)
diffusion coefficient of the reducible species (m2 s- ' )
concentration of reducible species at the interface (cathodic site)
(mol m-3>
6 = distance from metal surface over which concentration gradient
occurs (m) For the limiting situation where CI = 0 equation (2.16) becomes:
(2.17) nFDcs 6
iL = -
where iL = limiting or maximum current density that can flow.
The slow rate of diffusion of the oxygen molecule will have an effect on the
cathode potential. The reversible electrode potential is calculated using the Nernst
equation, assuming that the activity of the electroactive species at the interface is the
same as for the bulk solution, i.e., at a distance well removed from the metal. This is
not the case under the conditions described above and the electrode will exhibit a
potential different to its reversible potential, i.e. it has become concentration
polarised. The difference in potential is called the concetration overpotential q,C .
61
The relationship between the cathodic current density i, and the concentration
overpotential (no activation polarisation) is given by:
i, =i, [ l-exp- '2'1 (2.18)
where ic =
iL =
F=
N =
R =
T =
r," =
cathodic current density ( A m-2)
limiting or maximum current density (A m-2)
Faraday constant (96487 C mol-')
number of electrons involved in the cathodic reaction
8.314 J K-' mol-'
temperature (K)
concentration overpotential for the cathodic reaction (E actual- E,,,)
Equation (2.19) can be arranged to give:
r," - - E l o g [ l - k ] nF (2.19)
2.2.3. Combined activation and concentration polarisation
Both activation and concentration polarisation may occur at an electrode and are
additive. For a single cathodic process equations (2.15) and (2.19) may be added to
give:
- 2.3RT log? i + --log[l 2.3RT - (2.20) nF 7 7 , O k l l - - -
aF 10
This equation can be rearranged to give the following approximate expression for
the cathodic current density
(2.21) . I , =
i, exp[- aqF 1 R T ] i, exp(-aqF 1 R T ]
1,' if.
or (2.22) i, i,
I , =- I, + i c
For anodic processes concentration polarisation is usually not important.
1 2 4 6 2.3. Polarisation methods and corrosion rate estimation ' ' I
62
Polarisation methods such as potentiodynamic polarisation, are often used for
laboratory corrosion testing. The technique has the potential to provide useful
information regarding the corrosion mechanisms, corrosion rate and susceptibility of
specific materials to corrosion in designated environments. Potentiodynamic
polarisation is a technique where the potential of the electrode is varied at a selected
rate by application of a current through the electrolyte. The polarisation can be
carried out by using a potentiostat. Three electrodes are required: the working
electrode (WE)(that is the metal/alloy under investigation), the reference electrode
(the potential of the WE is measured relative to this potential), and counter or
auxiliary electrode (that the majority of the current passes through). The counter
electrode is required to prevent any resistive potential drop (ohmic drop) across the
reference electrode.
The potential of a corroding metal (WE) is varied (polarised) from its equilibrium
value (Ecorr) firstly in the negative and then in the positive direction and the current
response to the applied potential is recorded. The voltage/current density data pairs
produced from the polarisation of the, WE can then be used to construct a polarisation
diagram similar to Fig. 2.3
+
E
-
log i
Fig. 2.3 Comparison of experimental and theoretical polarisation curves for reaction
M+2H+ + M2+ +H2
In Fig 2.3 typical experimental polarisation curves (with both the anodic and
cathodic reactions exhibiting activation polarisation only) are overlaid with the Tafel
lines for the dissolution of metal M in oxygen-free acid. At the corrosion potential
63
(E,,,,) there is, by definition, no net current flowing in the cell. At a more negative
potential from E,,, (less than approx. k 50 mV), the current flowing will consist of
both the anodic and cathodic components and the plot is not linear. Only at higher
overpotentials (negative or positive) are the experimental curves in agreement with
the linear Tafel relationships. For cases similar to the above the corrosion current
density (therefore the corrosion rate) can be estimated from the intersection of the
two Tafel lines, i.e., the linear portion of the experimental curves. In some cases one
or both of the reactions may not exhibit activation polarisation only, i.e. be under
activation control, and this makes the estimation of the corrosion current density
either more difficult or impossible. When only one reaction is under activation
control, the intersection of the linear Tafel region for this reaction with the corrosion
potential may suffice for the calculation of corrosion rate. If the cathodic reaction is
under complete diffusion control at E,,, the limiting current density will represent
the corrosion current density. If both anodic and cathodic curves are non-linear any
arbitrary point more positive than E,,, (e.g. 100 mV) can be used as an estimate of
the rate of anodic dissolution. In this study the accurate determination of corrosion
rate in the absence and presence of inhibitors required the use of coupons (see
section 2.4). Relative corrosion rates only were obtained from polarisation curves.
I Anodic inhibitor I Cathodic inhibitor
log i log i
Fig. 2.4 Effect of cathodic and anodic inhibitors on E,,,
A corrosion inhibitor is referred to as being either cathodic or anodic according to
the reaction that it retards. If it retards both reactions, it is called a mixed inhibitor.
64
This behaviour can be identified from the polarisation curves and the movement of
E,,, from its value in the uninhibited system (Fig. 2.4)
1,2,5,9 2.4. Weight loss method to measure corrosion rate and inhibition efficiency
The simplest and most accurate method of estimating the corrosion rate is weight
loss analysis. A weighed sample (coupon) of the metal or alloy under consideration
is introduced into the corrosive environment, and later removed after a reasonable
time interval. The coupon is then cleaned of all corrosion product(s) and is
reweighed. The weight loss can also be determined by the amount of metal dissolved
into the solution and as corrosion product using instrumental techniques such as
atomic absorption spectroscopy (AAS) or induced coupled plasma - atomic
emission spectroscopy (ICP-AES). The weight loss is converted to an average
corrosion rate (R) , as follows: ,
where:
?+'I = initial coupon weight (mg)
W2 = final coupon weight (mg)
A = coupon area (dm2)
D = exposure time (days)
R = corrosion rate (mg dm-2 day-')
The technique requires no complex equipment or procedures, merely an
appropriately shaped coupon, a carrier for the coupon (coupon holder, if necessary),
and a reliable means of removing corrosion product without disruption of the metal
substrate. The weight loss measurement is still the most widely used means of
determining corrosion loss, despite being the oldest method currently in use. Weight
loss determination has a number of attractive features that account for its sustained
popularity:
0
0
Simple - no sophisticated instrumentation is required to obtain a result.
Direct - a direct measurement is obtained, with no theoretical assumptions or
approximations.
65
Versatile - it is applicable to all corrosive environments, and gives
information on all forms of corrosion.
The method is commonly used as a calibration standard for other means of
corrosion monitoring, such as linear polarisation and electrical resistance. In
instances where metal wasted is slow and averaged data are acceptable, weight loss
monitoring is the preferred technique.
The percent inhibition efficiency (IE%) was calculated for the compond under
investigation according to the following equation:
where ws and wb are the coupon weight losses in solution with and without inhibitor.
The choice of technique for initial preparation of the coupon surface, and for
cleaning the coupon after use, is critical in obtaining useful data. In this study, copper
coupons (3cm x lcm x 0.02 cm) were cut from AR copper sheet, polished with
PI200 grade S i c abrasive paper, degreased with acetone, etched with HNO3 1: 1 and
washed thoroughly with distilled water. The coupons were then immersed -
immediately in the test solution. After the test, any corrosion product(s) on the
coupons were removed by HC1 (3:l). The solutions were combined and the
concentration of copper was determined by ICP-AES.
2.5. Electrochemical impedance spectroscopy - corrosion mechanism study and
estimation of polarisation resistance 1,2,10-14
Electrical resistance is the ability of a circuit element to resist the flow of
electrical current. Ohm’s law (Equation 2-26) defines resistance in terms of the ratio
between voltage E and current I:
E I
R = - (2.26)
Ohm’s law is limited to only one circuit element, i.e., the ideal resistor. An ideal
resistor has several simplifying properties:
It follows Ohm’s Law at all current and voltage levels
0 Its resistance value is independent of frequency
0 AC current and voltage signals through a resistor are in phase with each other
66
The real world, however, contains circuit elements that exhibit much more complex
behavior and in place of resistance we use impedance, which is a more general
circuit parameter. Impedance is also a measure of the ability of a circuit to resist the
flow of electrical current. Unlike resistance, it is not limited by the simplifying
properties listed above.
Electrochemical impedance is usually measured by applying an AC potential
(AEsinot) to an electrochemical cell and measuring the current through the cell.
Suppose one applies a sinusoidal potential excitation. The response to this potential
is an AC current signal (Akin(cut + q)), containing the excitation frequency and its
harmonics (2~0, 3 0 etc.). The impedance 2 has a magnitude AE/AI and phase q and
hence is a vector quantity.
Electrochemical impedance is normally measured using a small excitation signal.
This is done so that the cell's response is pseudo-linear. It is possible to express the
impedance as a complex function. The potential is described as,
E(t) = Eoexpuot) (2.27)
and the current response as,
Z(t) = Ioexp(j0o-j @) (2.28)
The impedance is then represented as a complex number,
E Z
2 = - = 2, exp(j 4) = 2, (cos 4 + j sin 4) ( 2-29)
AC impedance can provide kinetic and mechanistic information when applied to
the study of electrochemical systems. For this reason the technique (normally called
electrochemical impedance spectroscopy or EIS) is applied to an increasing extent
for the study of corrosion processes in solution. The following are some areas of
corrosion where AC impedance has been applied successfully:
0 Rate determination
0 Inhibitor performance
0 Coating performance
0 Passive layer characteristic
The advantages of AC impedance over DC techniques are:
67
AC impedance techniques use very small excitation amplitudes, usually in the
range of 5-10 mV peak-to-peak. Excitation amplitudes of this order perturb the
electrochemical test system to a minimum extent, thus reducing errors caused by
the measurement technique itself.
Since AC impedance measurements provide data on both electrode capacitance
and charge transfer kinetics, the technique can provide valuable mechanistic
information.
The AC impedance technique does not involve potential scan and hence can be
applied to low conductivity solutions where DC techniques are subject to serious
potential control errors.
Although AC impedance measurements offer a great deal of information,
sophisticated techniques are required to interpret the data and extract meaningful
results. In this study, a frequency response analyser (FRA) in conjunction with a
potentiostat allowed a cell to be stimulated with an AC signal and the response in
terms of the cell’potential and current was measured. Using the transfer function
facility of the FRA, the impedance or admittance can be determined directly. A plot
of impedance against frequency or the magnitude and phase against frequency allows
the establishment of the characteristics of the system
1
Figure 2.5 Nyquist plot with
impedance vector
Figure 2.6 Bode plot with one time
constant
There are two ways to present the EIS data. If the real part is plotted on the x-axis
and the imaginary part on the y-axis of a chart, we get a Nyquist plot (Figure 2.5). In
this plot the y-axis is negative and each point on the Nyquist plot is the impedance at
68
one frequency. On the Nyquist plot the impedance can be represented as a vector of
length 14. The angle between this vector and the x-axis is @, where @ = arg(2).
Another popular presentation method is the Bode plot (Fig. 2.6). The impedance is
plotted with log frequency on the x-axis and both the absolute value of the
impedance (14 =ZO ) and phase-shift on the y-axis. Unlike the Nyquist plot, the Bode
plot explicitly shows frequency information
EIS data is commonly analysed by fitting it to an equivalent electrical circuit
model. Most of the circuit elements in the model are common electrical elements
such as resistors, capacitors, and inductors. To be useful, the elements in the model
should have a basis in the physical electrochemistry of the system.
2.5.1 Physical electrochemistry and common equivalent circuit models
2.5.1.1 A purely capacitive coating
A metal covered with an undamaged coating generally has a very high impedance.
The equivalent circuit for such a situation is in Figure 2.7.
jAMCIIl-
Fig 2.7 Purely capacitive
coating
Fig 2.8 Randles cell schematic diagram
2.5.1.2. Randles cell
The Randles cell is one of the simplest and most common cell models. It includes
a solution resistance, a double layer capacitor and a charge transfer or polarisation
resistance. In addition to being a useful model in its own right, the Randles cell
model is often the starting point for other more complex models.
The equivalent circuit for the Randles cell is shown in Figure 2.8. The double
layer capacity is in parallel with the impedance due to the charge transfer reaction.
2.5.1.3. Mixed kinetic and diffusion control
69
Fig 2.9 Equivalent circuit with mixed kinetic and charge transfer control
This circuit (Fig 2.9) models a cell where polarisation is due to a combination of
kinetic and diffusion processes (W: Warburg impedance)
2.5.1.4. Coated metal
Most coatings degrade with time, resulting in more complex behavior. After a
certain amount of time, water penetrates into the coating and forms a new
liquid/metal interface under the coating. Corrosion phenomena can occur at this new
interface. The interpretation of impedance data from failed coatings can be very
complicated. A simple equivalent circuit was shown in Fig 2.10.
Fig 2.10 Equivalent circuit for a failed coating
2.5.2. Extracting model parameters from data
EIS data is generally analysed in terms of an equivalent circuit model. The analyst
tries to find a model whose impedance matches the measured data.
The type of electrical components in the model and their interconnections controls
the shape of the model's impedance spectrum. The model's parameters (i.e. the
resistance value of a resistor) controls the size of each feature in the spectrum. Both
these factors effect the degree to which the model's impedance spectrum matches a
measured EIS spectrum. In a physical model, each of the model's components is
70
postulated to come from a physical process in the electrochemical cell. The choice of
which physical model applies to a given cell is made from knowledge of the cell's
physical characteristics.
Modern EIS analysis uses a computer to find the model parameters that cause the
best agreement between a model's impedance spectrum and a measured spectrum.
For most EIS data analysis software, a non-linear least squares fitting (NLLS)
Levenberg-Marquardt algorithm is used.
NLLS starts with initial estimates for all the model's parameters. Starting from this
initial point, the algorithm makes changes in several or all of the parameter values
and evaluates the resulting fit. If the change improves the fit, the new parameter
value is accepted. If the change worsens the fit, the old parameter value is retained.
Next a different parameter value is changed and the test is repeated. Each trial with
new values is called an iteration. Iterations continue until the goodness of fit exceeds
an acceptance criterion, or until the number of iterations reaches a limit.
2.6. Coulometry"
A metal surface, when pre-conditioned in an inhibitor solution, can show some
anti-tarnishing property. The amount of corrosion product, and hence the
effectiveness of the inhibitor as an anti-tarnishing agent, can be determined by
coulometry . In this work, constant-current coulometry was used. The method employed an
electrolysis cell operated at constant current. A schematic circuit is shown in Figure
2.1 1
Fig. 2.1 1 Constant current source based on an operational amplifier
71
The current is supplied by the operational amplifier. The potential across the
resistor R must be equal to the battery potential A@, and the current through R must
be i = AWR. Since the input impedance of the operational amplifier is high, virtually
all the current through R flows through the cell, independent of the cell’s internal
resistance and potential. Thus, provided that R is much less than the input impedance
of the operational amplifier and i(Rce!l+R) is within the range of the operational
amplifier voltage, the circuit will supply a constant current. With the addition of a
reference electrode, the working electrode potential can be monitored. The current
has to be very small so that mass transport can keep up with consumption at the
electrode.
The copper electrode (working electrode) was exposed to a tarnishing
environment, Na2S solution. In the cell, the tarnished product (containing mainly
Cu2S and Cu20) was then reduced. On a ‘potential - time curve, several arrests will
be observed, each arrest corresponding to the reduction of a species. The reduction
time (or the amount of charge) for each potential arrest is proportional to the amount
of tarnish product formed on the surface of the working electrode. For the copper
electrode, pre-conditioned in a solution of inhibitor, the reduction time can show the
effectiveness of that inhibitor as an anti-tarnishing agent.
2.7. Surface-enhanced Raman scattering (SERS) techniqueI6
Surface enhanced Raman scattering is a very useful tool in the investigation of
adsorption-mediated corrosion inhibition, because it has high intensity and is easily
applicable to the in situ observation of the behaviour of inhibitors on metal surfaces
in corrosive media.
2.7.1. The Raman effect and normal Raman scattering.
When light is scattered from a molecule, most photons are elastically scattered.
The scattered photons have the same energy (frequency) and, therefore, wavelength,
as the incident photons. However, a small fraction of light (approximately 1 in 10’
photons) is scattered at optical frequencies different from, and usually lower than, the
frequency of the incident photons. The process leading to this inelastic scattering is
termed the Raman effect. Raman scattering can occur with a change in vibrational,
72
rotational or electronic energy of a molecule. Chemists are concerned primarily with
the vibrational Raman effect. The difference in energy between the incident photon
and the Raman scattered photon is equal to the energy of a vibration of the scattering
molecule. A plot of intensity of scattered light versus energy difference is a Raman
spectrum.
2.7.1.1. The scattering process.
The Raman effect arises when a photon is incident on a molecule and interacts
with the electric dipole of the molecule. It is a form of electronic (more accurately,
vibronic) spectroscopy, although the spectrum contains vibrational frequencies. In
classical terms, the interaction can be viewed as a perturbation of the molecule’s
electric field. In quantum mechanics the scattering is described as an excitation to a
virtual state lower in energy than a real electronic transition with nearly coincident
de-excitation and a change in vibrational energy. The scattering event occurs in 1 0-14
seconds or less. The virtual state description of scattering is shown in Figure 2.12(a).
The energy difference between the incident and scattered photons is represented
by the arrows of different lengths in Figure 2.12(a). -Numerically, the energy
difference between the initial and final vibrational levels, v , or Raman shift in wave
numbers (cm-’), is calculated through equation (2.30):
(2.30)
in which h incident and h scattered are the wave number (cm-*) of the incident and Raman
scattered photons, respectively. The vibrational energy is ultimately dissipated as
heat. Because of the low intensity of Raman scattering, the heat dissipation does not
cause a measurable temperature rise in a material.
At room temperature the thermal population of vibrational excited states is low,
although not zero. Therefore, the initial state is the ground state, and the scattered
photon will have lower energy (longer wavelength) than the exciting photon. This
Stokes shifted scatter is what is usually observed in Raman spectroscopy. Figure 2.12
(a) depicts Raman Stokes scattering.
A small fraction of the molecules are in vibrationally excited states. Raman
scattering from vibrationally excited molecules leaves the molecule in the ground
state. The scattered photon appears at higher energy, as shown in Figure 2.12(b).
73
Energy Incident
Photon
- - - - - - -
Incident Photon I
1
- - - - - - - -
Anti - S toke s Scatter
Inidd
I Fiad
Figure 2.12. Energy level diagram for Raman scattering; (a) Stokes Raman scattering
(b) anti-Stokes Raman scattering
This anti-Stokes-shifted Raman spectrum is always weaker than the Stokes-
shifted spectrum, but at room temperature it is strong enough to be useful for
vibrational frequencies less than about 1500 cm-l. The Stokes and anti-Stokes spectra
contain the same frequency information. The ratio of anti-Stokes to Stokes intensity
at any vibrational frequency is a measure of temperature. Anti-Stokes Raman
scattering is used for contactless thermometry. The anti-Stokes spectrum is also used
when the Stokes spectrum is not directly observable, for example because of poor
detector response or spectrograph efficiency.
2.7.1.2. Vibrational energies.
The energy of a vibrational mode depends on molecular structure and environment.
Atomic mass, bond order, molecular substituents, molecular geometry and hydrogen
bonding all effect the vibrational force constant which, in turn dictates the vibrational
energy. For example, the stretching frequency of a phosphorus-phosphorus bond
ranges from 460 to 610 to 775 cm-' for the single, double and triple bonded moieties,
respectively. Much effort has been devoted to estimation or measurement of force
constants. For small molecules, and even for some extended structures such as
peptides, reasonably accurate calculations of vibrational frequencies are possible
with commercially available software.
Vibrational Raman spectroscopy is not limited to intramolecular vibrations.
Crystal lattice vibrations and other motions of extended solids are Raman-active.
74
Their spectra are important in such fields as polymers and semiconductors. In the gas
phase, rotational structure is resolvable on vibrational transitions. The resulting
vibration/rotation spectra are widely used to study combustion and gas phase
reactions generally. Vibrational Raman spectroscopy in this broad sense is an
extraordinarily versatile probe into a wide range of phenomena ranging across
disciplines from physical biochemistry to materials science.
2.7.1.3. Raman selection rules and intensities.
A simple classical electromagnetic field description of Raman spectroscopy can
be used to explain many of the important features of Raman band intensities. The
dipole moment, P, induced in a molecule by an external electric field, E, is
proportional to the field as shown in equation 2.
P = a E (2.3 1)
The proportionality constant a is the polarizability of the molecule. The polarizability
measures the ease with which the electron cloud around a molecule can be distorted.
The induced dipole emits or scatters light at the optical frequency of the incident
light wave.
Raman scattering occurs because a molecular vibration can change the polarizability.
The change is described by the polarizability derivative, where S d S Q is the
normal coordinate of the vibration. The selection rule for a Raman-active vibration,
that there be a change in polarizability during the vibration, is given in equation
(2.3 2).
(2.32) aa - * o SQ
The Raman selection rule is analogous to the more familiar selection rule for an
infrared-active vibration, which states that there must be a net change in permanent
dipole moment during the vibration. From group theory it is straightforward to show
that if a molecule has a centre of symmetry, vibrations which are Raman-active will
be silent in the infrared, and vice versa.
Scattering intensity is proportional to the square of the induced dipole moment,
that is to the square of the polarizability derivative (8~1 l8Q)~
If a vibration does not greatly change the polarisability, then the polarisability
derivative will be near zero, and the intensity of the Raman band will be low. The
vibrations of a highly polar moiety, such as the 0 -H bond, are usually weak. An
75
external electric field can not induce a large change in the dipole moment and
stretching or bending the bond does not change this.
Typical strong Raman scatterers are moieties with distributed electron clouds,
such as carbon-carbon double bonds. The pi-electron cloud of the double bond is
easily distorted in an external electric field. Bending or stretching the bond changes
the distribution of electron density substantially, and causes a large change in
induced dipole moment.
Chemists generally prefer a quantum-mechanical approach to Raman scattering
theory, which relates scattering frequencies and intensities to vibrational and
electronic energy states of the molecule. The standard perturbation theory treatment
assumes that the frequency of the incident light is low compared to the frequency of
the first electronic excited state. The small changes in the ground state wave function
are described in terms of the sum of all possible excited vibronic states of the
molecule.
2.7.1.4. Polarisation effect.
Raman scatter is partially polarised, even for molecules in a gas or liquid, where
the individual molecules are randomly oriented. The effect is most easily seen with
an exciting source which is plane polarised. In isotropic media polarisation arises
because the induced electric dipole has components which vary spatially with respect
to the coordinates of the molecule. Raman scatter from totally symmetric vibrations
will be strongly polarised parallel to the plane of polarisation of the incident light.
The scattered intensity from non-totally symmetric vibrations is 3/4 as strong in the
plane perpendicular to the plane of polarisation of the incident light as in the plane
parallel to it.
The situation is more complicated in a crystalline material. In that case the
orientation of the crystal is fixed in the optical system. The polarisation components
depend on the orientation of the crystal axes with respect to the plane of polarisation
of the input light, as well as on the relative polarisation of the input and the observing
polariser.
2.7.2. Surface-enhanced Raman scattering
The Raman scattering from a compound (or ion) adsorbed on or even within a few
Angstroms of a structured metal surface can be 103-106 X greater than in solution.
This surface-enhanced Raman scattering is strongest on silver, but is also observable
76
on gold and copper as well. At practical excitation wavelengths, enhancement on
other metals is unimportant. Surface-enhanced Raman scattering (SERS) arises from
two mechanisms. The first is an enhanced electromagnetic field produced at the
surface of the metal. When the wavelength of the incident light is close to the plasma
wavelength of the metal, conduction electrons in the metal surface are excited into an
extended surface electronic excited state called a surface plasmon resonance.
Molecules adsorbed or in close proximity to the surface experience an exceptionally
large electromagnetic field. Thus if an alkyl group is close to the surface the
intensities of the C-H stretching mode are enhanced. However, in aqueous solution
these bands may be obscured by water molecules. Vibrational modes normal to the
surface are most strongly enhanced.
The second mode of enhancement is by the formation of a charge-transfer complex
between the surface and analyte molecule. The electronic transitions of many charge
transfer complexes are in the visible, so that resonance enhancement occurs.
Molecules with lone pair electrons or pi clouds show the strongest SERS. The
effect was first discovered with pyridine. Other aromatic nitrogen or oxygen
containing compounds, such as aromatic amines or phenols, are strongly SERS
active. The effect can also been seen with other electron-rich functionalities such as
carboxylic acids.
The intensity of the surface plasmon resonance is dependent on many factors
including the wavelength of the incident light and the morphology of the metal
surface. The wavelength should match the plasma wavelength of the metal. This is
about 382 nm for a 5 pm silver particle, but can be as high as 600 nm for larger
ellipsoidal silver particles. The plasma wavelength is to the red of 650 nm for copper
and gold, the other two metals which show SERS at wavelengths in the 350-1000 nm
region. The best morphology for surface plasmon resonance excitation is a small
(-4 00 nm) particle or an atomically rough surface.
SERS is used to study monolayers of materials adsorbed on metals, including
electrodes. Other vibrational spectroscopy methods, e.g., infrared reflection
measurements, have been employed for the identification of chemical species in the
protective film formed on metal surfaces in the reaction with the inhibitors and for
the determination of film thickness. Since most of the infrared measurements have
been performed ex situ, direct information about adsorption-mediated corrosion
inhibition has scarcely been obtained. Surface enhanced Raman scattering is more
77
useful than other vibrational spectroscopy methods in the investigation of adsorption-
mediated corrosion inhibition, because it has high sensitivity and easily applicable to
the in situ observation of the behaviour of inhibitors on metal surfaces in corrosive
media.
Benzotriazole (BTAH), a common inhibitor for copper and copper based alloys,
has been the subject of SERS observation. In this work, by means of SERS, the
relation between the molecular structure of compounds structurally related to BTAH
and their inhibitive action against the corrosion of copper has been investigated.
2.8. Molecular rn~delling'~
Molecular modeling can be used to study the adsorption of inhibitor species on
the metal surface. If a model of the interactions that control the adsorption can be set
up, a measure of the binding energy of the molecule with the metal can be calculated.
Comparison of the relative binding energy of a series of structurally similar ligands
may lead to the prediction of other related compounds with potentially more
effective inhibiting actions.
2.8.1. Empirical force field models - Molecular mechanics
Quantum mechanical theory permits the calculation of molecular energy and the
structures with lowest energy should be predominant according to Boltzmann
statistics. Ab initio quantum mechanics also permits calculation of inter-molecular
interaction energy. However, the massive amount of integrals to be calculated for
large molecules makes these methods very time-consuming and almost impossible in
some cases. Therefore, in our study empirical force fields models are used for faster
energy calculations. The molecules are assumed to consist of balls (the atoms)
connected by springs (the bonds). The internal energy, Epot is expressed in terms of a
set of molecular coordinates (bond length energy, bond angle energy, torsion energy,
van der Waals' energy of interaction, charge-charge energy of interaction, and
miscellaneous or cross energy terms). Thus for a consistent valence force field
(CVFF) Epot is given by
Epot = Ebonds + 2 Eangles Etorsions 4- E van der Waals + c Echarge c Ecrosst
78
The forces on the atoms enable solving Newton's laws of motion, which leads to
simulations of time dependent phenomena.
Some features of the molecular mechanics force field models are:
The force fields are primarily design to reproduce structural properties but they
can also be used to predict other properties.
The functional form and parameters are transferable. Transferability means that
the same set of parameters can be used to model a series of related molecules,
rather having to define a new set of parameters for each invidual molecules.
Force fields are empirical. The functional forms employed in molecular
mechanics force field are often a compromise between accuracy and
computational efficiency; the most accurate functional form may often be
unsatisfactory for efficient computation. As the performance of computers
increases, it becomes possible to incorporate more sophisticated models.
For a force field it is usually necessary to assign an atom type to each atom in the
system. The atom type is more than just the atomic number of an atom; it usually
contains information about its hybridization state and .sometimes the local
environment.
2.8.2. Energy minimisation and related methods for exploring of the energy surface
Except for the very simplest systems, the potential energy is usually a
complicated, 'multi-dimensional function of the coordinates. For a system of N atoms
the energy is a function of 3N-6 or 3N Cartesian coordinates. It is therefore
impossible to visualise the entire energy surface except for some simple cases where
the energy is just a function of one or two coordinates.
In molecular modelling we are especially interested in minimum points on the
energy surface. Minimum energy arrangements of the atoms correspond to stable
states of the system; any movement away from the minimum gives a configuration
with a higher energy. There may be a very large number of minima on the surface.
The minimum with the very lowest energy is known as the global energy minimum.
To identify those geometries of the system that correspond to minimum points on the
energy surface we used a minimisation algorithm. Vast amounts of literature on such
methods are available and are divided into two groups: those which use derivatives
of the energy with respect to the coordinates and those which do not. Derivatives can
79
be useful because they provide information about the shape of the energy surface
and, if used properly, they can significantly enhance the efficiency with which the
minimum is located.
There are many factors that must be taken into account when choosing the most
appropriate algorithm (or combination of algorithms) for a given problem; the ideal
minimisation algorithm is the one that provides the answer as quickly as possible
using the least amount of memory. No single minimisation method has yet proven to
be the best for all molecular modeling problems and so most software packages offer
a choice of methods.
Most minimisation algorithms can only go downhill on the energy surface and so
they can only locate the minimum that is nearest to the starting point. Fig 2.13 shows
a schematic energy surface and the minima that would be obtained starting from
three points A, B and C. To locate more than one minimum or to locate the global
energy minimum we therefore usually require a means of generating different
starting points, each of which is then minimised. Some specialised minimisation
methods can make uphill moves to seek out minima lower in energy than the nearest
ones, but no algorithm has yet proved capable of locating the global energy
minimum from and arbitrary starting position. The shape of the energy surface may
be important if one wishes to calculate the
x
5
I
Confornational parameter
Fig. 2.13 A schematic one-dimensional energy surface
relative populations of various minimum energy structures. For example, a deep and
narrow minimum may be less highly populated then a broad minimum that is higher
in energy as the vibrational energy levels will be more widely spaced in the deeper
minimum and so less accessible. For this reason, the global energy minimum may
not be the most populated minimum. In any case, the ‘active’ structure (e.g. the
80
biologically active conformation of a drug molecule) may not correspond to the
global minimum, or to the most highly populated conformation, or even to a
minimum energy structure at all.
The input to a minimisation program consists of a set of initial coordinates for the
system. The initial coordinates may come from a variety of sources. They may be
obtained from an experimental technique, such as X-ray crystallography or NMR. In
other cases a theoretical method is employed, such as a conformational search
algorithm. A combination of experimental and theoretical approaches may also be
used.
Energy minimisation is very widely used in molecular modelling and is an
integral part of techniques such as conformational search procedures. Energy
minimisation is also used to prepare a system for other types of calculations. For
examples, energy minimisation may be used prior to a molecular dynamics or Monte
Carlo simulation in order to relieve any unfavourable interactions in the initial
configuration of the system. This is especially recommended for simulations of
complex systems such as macromolecules or large molecule assemblies.
2.8.3. Conformational search and computer simulation methods
The objective of conformational search is to identify the preferred confirmation
of a molecule: those conformations that determine its behaviour. This usually
requires us to locate conformations that are at minimum points on the energy surface.
An important feature of methods for performing energy minimisation is they move to
minimum point that is closest to the starting structure. For this reason, it is necessary
to have a separate algorithm which generates the initial starting structure for
subsequent minimisation. Several techniques can be used for this purpose, in this
study only two methods were employed to explore the conformational space of
molecules: molecular dynamics and Monte Carlo methods.
2.8.3.1. Molecular dynamics method
In molecular dynamics, successive configurations of the system are generated by
integrating Newton's law of motion. The result is a trajectory that specifies how the
position and velocity of the particles in the system vary with time. A common
strategy is to perform the simulation at a very high temperature. The additional
kinetic energy enhances the ability of the system to explore the energy surface and
prevent the molecule getting stuck in a localised region of conformational space. A
81
molecular dynamics run is typically over 1 x 10-l' seconds and a large number of
structures with their attendant energies are generated over this period. Representative
structures are then sampled at random and used for subsequent minimisation.
2.8.3.2 Monte Carlo simulation method
In general, a Monte Carlo simulation generates configurations of a system by
making random changes to the position of the species present, together with their
orientations and conformations where appropriate. For conformational searching, a
random search can explore conformational space by changing either the atomic
Cartesan coordinate or the torsion angles of rotatable bonds. At each iteration, a
random change is made to the current conformation. The new structure is the refined
using energy minimisation. Each newly generated structure (after energy
minimisation) is accepted as the starting point for the next iteration if it is lower in
energy than the previous structure or if the Boltzmann factor of energy difference is
larger than a random number between 0 and 1. If not, the previous structure is
retained for the next iteration. The procedure continues until a given number of
iterations have been performed or until it is decided that no new conformation can be
found.
2.8.4. Application of molecular modelling in inhibition study
In aqueous metallic corrosion, to be effective an inhibitor must displace water
from the metal's surface, interact with anodic and cathodic sites to retard the
oxidation and reduction corrosion reactions, and prevent transportation of water and
corrosion-active species to the surface. Therefore, it is necessary to design a ligand
that adsorbs strongly with the metal. Molecular modelling has the potential to study
this adsorption and possibly identify structurally similar compounds worthy of
synthesis and subsequent corrosion testing. If a molecular mechanics model of the
interactions that control the adsorption of a minimised structure can be set up, a
measure of the binding energy of the molecule with the metal can be calculated.
Comparing the relative binding energy of a series of structurally similar ligand may
lead to the prediction of other related compounds with potentially more effective
inhibiting action.
82
1.
2.
3.
4.
5 .
6.
7.
8.
9.
REFERENCES
D.A. Jones, “ Principles and Prevention of Corrosion”, Prentice-Hall Inc., New
Jersey, 1996
V.S. Sastri, “Corrosion Inhibitors: Principles and Application”, John
Wiley&Sons, West Sussex, 1998
M. Pourbaix, .‘ Lectures on Electrochemical Corrosion”, Plenum Press, 1973
J.C. Scully, “The Fundamentals of Corrosion”, 3‘d edition, Pergamon Press, 1990
M.G. Fontana, “ Corrosion Engineering”, 3 rd edition, McGraw-Hill Book
Company, 1987.
H.H.Uhlig, “Corrosion and Corrosion Control”, 2 nd edition, John Wiley and
Sons Inc., 1971
P.W. Atkins, “Physical Chemistry”, 4 th edition, Oxford University Press, 1990
D. P. Schweinsberg, “ Dynamic Electrochemistry and Metallic Corrosion”, QUT
Press, 1992
C.C. Nathan, “Corrosion Inhibitors”, 2nd edition, NACA, Houston, Texas, 1974
10. I.L. Rozenfeld, “Corrosion Inhibitors”, IS‘ edition, McGraw-Hill, 1981 .
1 1. J.R. Macdonald, “Impedance Spectroscopy; Emphasizing Solid Materials and
Systems” , Wiley Interscience Publications, 1987.
12. A.J. Bard, L.R. Faulkner, “Electrochemical Methods; Fundamentals and
Applications, Wiley Interscience Publications, 1 980.
13. J.R. Scully, D.C. Silverrnan, and M.W. Kendig, editors, “Electrochemical
Impedance: Analysis and Interpretation”, ASTM, 1993.
14. C. Gabrielle, “Identification of Electrochemical Processes by Frequency
Response Analysis”, Solartron Instrumentation Group, 1980.
15. P.H. Reiger, “Electrochemistry”, 2nd edition, Chapman&Hall Inc., 1994
83
16. W. Suetaka, J.T. Yates, Jr. “ Surface Infrared and Raman Spectroscopy :
Methods and Applications”, Plenum Press, New York, 1995
17. A.R. Leach, “Molecular Modelling : Principles and Applications”, 2”d edition,
Pearson Education Ltd, 200 I
84
CHAPTER 3
INHIBITIVE EFFECT OF 4- AND 5-
CARBOXYBENZOTRIAZOLE ON COPPER CORROSION IN ACIDIC SULPHATE AND HYDROGEN SULPHIDE SOLUTION
V. Otieno-Alego“, N. Huynha, T. Notoyab, S.E. Bottlea, D.P. Schweinsberg”
Centre for Instrumental and Developmental Chemistry, Queensland University of a
Technology, Brisbane, Queeensland 4000, Australia
Graduate School of Engineering, Hokkaido University, Sapporo, 060, Japan
Current Address:Faculty of Science, University of Canberra, Canberra,ACT 26 16,
Australia
85
STATEMENT OF JOINT AUTHORSHIP
Title
acidic sulphate and hydrogen sulphide solution
Inhibitive effect of 4- and 5-carboxybenzotriazole on copper corrosion in
Authors V. Otieno-Alego, N. Huynh, T. Notoya, S.E. Bottle, D.P. Schweinsberg
V. Otieno-Alego
Developed experimental design and scientific method; conducted measurement;
analysed and interpreted data; wrote manuscript
N. HUYNH (Candidate)
Contributed to experimental design and scientific method; conducted measurements;
analysed and interpreted data; assisted with manuscript
N. Notoya
Contributed to data interpretation and manuscript
S.E. Bottle
Contributed to data interpretation and manuscript
D.P. Schweinsberg
Contributed to experimental design and scientific method; contributed to data
interpretation and manuscript
86
CHAPTER 4
INHIBITIVE ACTION OF THE OCTYL ESTERS OF 4- and 5-
CARBOXYBENZOTRIAZOLE FOR COPPER CORROSION IN
SULPHATE SOLUTIONS
N.HUYNH, S.E. BOTTLE, T. NOTOYA* and D.P. SCHWEINSBERG'
Centre for Instrumental and Developmental Chemistry, Queensland University of
Technology, Brisbane. Queensland 4000, Australia
*Graduate School of Engineering, Hokkaido University, Sapporo 060 Japan
' Corresponding author
101
STATEMENT OF JOINT OWNERSHIP
Title Inhibitive action of the octyl esters of 4- and 5-carboxybenzotriazole for copper
corrosion in sulphate solutions
Authors N. Huynh, S.E. Bottle, T. Notoya and D.P. Schweinsberg
N. HUYNH (Candidate)
Developed experimental design and scientific method; conducted measurements;
analysed and interpreted data; wrote manuscript
N. Notoya
Contributed to data interpretation and manuscript
. S.E. Bottle
. Contributed to data interpretation and manuscript
D.P. Schweinsberg
Contributed to experimental design and scientific method; contributed to data
interpretation and manuscript
102
CHAPTER 5
STUDIES OF ALKYL ESTERS OF
CARBOXYBENZOTRIAZOLE AS INHIBITORS FOR COPPER
CORROSION
N. Huynha, S.E. Bottlea, T. Notoyab
Schweinsberg”*
A. Trueman‘ B. Hinton‘ and D.P.
Centre for Instrumental and Developmental Chemistry, Queensland University of
Technology, Brisbane, Queensland 4000, Australia
. bGraduate School of Engineering, Hokkaido University, Sapporo 060 Japan
Aeronautical and Maritime Research Laboratory, Melbourne, Victoria, Australia
* Corresponding author
a
C
119
STATEMENT OF JOINT OWNERSHIP
Title
corrosion
Studies of alkyl esters of carboxybenzotriazole as inhibitors for copper
Authors N.Huynha, S.E. Bottle", T. Notoyab , A. Trueman' , B. Hinton' and D.P.
Sc hweinsberg"
N. HUYNH (Candidate)
Developed experimental design and scientific method; conducted measurements;
analysed and interpreted data; wrote manuscript
N. Notoya
Contributed to data interpretation and manuscript
S.E. Battle
Contributed to data interpretation and manuscript
D.P. Schweinsberg
Contributed to experimental design and scientific method; contributed to data
interpretation and manuscript
A. Trueman
Contribution to EIS measurement and interpretation
B .Hin t on
Contribution to EIS measurement and interpretation
120
CHAPTER 6
STUDY ON THE INHIBITIVE EFFECT OF THE FILMS OF
ALKYL ESTERS OF CARBOXYBENZOTRIAZOLE ON
COPPER CORROSION IN SULPHATE AND SULPHIDE
ENVIRONMENTS
N.Huynh, S.E. Bottle, T. Notoya" and D.P. Schweinsberg#
Centre for Instrumental and Developmental Chemistry, Queensland University of
Technology, Brisbane, Queensland 4000, Australia
*Graduate School of Engineering, Hokkaido University, Sapporo 060 Japan
# Corresponding author
STATEMENT OF JOINT OWNERSHIP
Title Study on the inhibitive effect of the films of alkyl esters of
carboxybenzotriazole on copper corrosion in sulphats and sulphide environments
Authors N. Huynh, S.E. Bottle. T. Notoya and D.P. Schweinsberg
N. HUYNH (Candidate)
Developed experimental design and scientific method; conducted measurements;
analysed and interpreted data; wrote manuscript
N. Notoya
Contributed to data interpretation and manuscript
S.E. Bottle
Contributed to data interpretation and manuscript
D.P. Schweinsberg
Contributed to experimental design and scientific method; contributed to data
interpretation and manuscript
142
CHAPTER 7
COMPUTER SIMULATION OF THE COROSION INHIBITION
OF COPPER IN ACIDIC SOLUTION BY ALKYL ESTERS OF 5-
CARBOXYBENZOTRIAZOLE
J. BARTLEY, N.HUYNH, S.E. BOTTLE, H. FLITT,T. NOTOYA* and D.P.
SCHWEJNSBERG#
Centre for Instrumental and Developmental Chemistry, Queensland University of
Technology, Brisbane, Queensland 4000, Australia
*Graduate School of Engineering, Hokkaido University, Sapporo 060 Japan
Corresponding author
.
#
157
STATEMENT OF JOINT OWNERSHIP
Title Computer simulation of the corrosion inhibition of copper in acidic solution
by alkyl esters of 5-carboxybenzotriazole
Authors J. Bartley, N.Huynh, S.E. Bottle. H. Flitt, T. Notoya and D.P.
Schweinsberg
J. Bartley
Developed scientific method; analysed and interpreted data; wrote manuscript
N. HUYNH (Candidate)
Contributed to experimental design and scientific method; conducted measurements;
analysed and interpreted data; assisted with manuscript
H. Flitt
Contributed to experimental design and scientific method; contributed to data
interpretation and wrote manuscript
N. Notoya
Contributed to data interpretation and manuscript
S.E. Bottle
Contributed to data interpretation and manuscript
D.P. Schweinsberg
Contributed to experimental design and scientific method; contributed to data
interpretation and wrote manuscript
158
CHAPTER 8
GENERAL CONCLUSIONS AND FUTURE WORK
175
176
8.1. General conclusions
The objective of this work was to study the inhibitive effect of soluble
benzotriazole (BTAH) derivatives for copper corrosion in aqueous environments.
Whilst BTAH is an excellent inhibitor in alkaline solution its efficiency drops off
markedly as the pH decreases. It was hypothesized that a possible way to increase
surface adsorption and subsequent better inhibition over a wide pH range might be
through the preparation of derivatives, particularly carboxybenzotriazoles and alkyl
esters of these compounds. To achieve this objective the following techniques:
weight loss measurements, potentiodynamic polarisation, SERS spectroscopy,
electrochemical impedance spectroscopy and coulometry were employed. Molecular
modelling was also investigated as a tool for inhibitor design.
Studies on individual isomer of 4- and 5- carboxybenzotriazole (CBT) showed
that the inhibition efficiency for copper corrosion in aerated acidic sulphate solution
of each isomer was pH, concentration and time dependant. At lower pH the 5-isomer
is the better inhibitor and this behaviour continues at higher pH where 4-CBT
promotes corrosion. The inhibition efficiency of the 5-isomer increased with
exposure time; this was attributed to the increase in the thickness of the underlying
oxide layer. The anti-tarnishing test showed that whilst both isomers exhibited these
properties, 5-CBT was once again the superior inhibitor.
It was found that a commercial mixture of the octyl esters of 4- and 5-
carboxybenzotriazole inhibits copper corrosion in sulphate environments open to air.
The IE% is concentration, pH and time dependent. The inhibition efficiency of the
ester mixture at the 1 x l 0-4 M level (pH - 0) is 98% which compares very favourably
with that for BTAH (- 50%). Although the inhibition efficiency of the ester mixture
decreased gradually as the pH was raised, an IE of 75% at pH - 8 indicates that the
177
mixture is suitable for practical use at the higher pH values. SERS indicates that at
low pH inhibition is due to chemisorption of 4- and 5-CBTAH-OE molecules on the
copper surface through an azole nitrogen. At high pH inhibition results from the
formation of a polymer complex.
The improved inhibition efficiency of the esters at low pH can be attributed to
chemisorption (as for BTAH) in conjunction with physical adsorption through the
alkyl group. In addition, increased shielding of the copper surface occurred by van
der Waals’ forces of attraction between adjacent octyl groups.
With respect to other alkyl esters of 4- and 5-~arboxybezotriazole, hexyl, butyl
and methyl, it was found that all of these inhibited copper corrosion in sulphate
environments open to air. In each case the inhibition efficiency is concentration, pH
and time dependent. Both coupon tests and EIS measurements indicate that inhibition
efficiency depends on the length of the alkyl chain. At pH - 0 the inhibition
efficiency decreased in the order octyl >hexyl >butyl >methyl. At higher pH (- 8) the
order is reversed. At the 1 ~ 1 O - ~ M level (pH - 0) the inhibition efficiency of each of
the alkyl esters is equal to or better than that for BTAH, with CBTAH-OE providing
98% inhibition. At higher pH (- 8) the inhibition efficiency in each case is reduced in
comparison to BTAH, but is still good enough for practical use (2 75%)
The inhibitive behaviour of the alkyl esters at low pH can be attributed to
chemisorption through an azole nitrogen of the protonated alkyl esters. The
hydrocarbon chain is also physically adsorbed and the increase in physical adsorption
as the chain is lengthened accounts for the improved inhibition efficiency. Langmuir
treatment of the data indicates that near the optimum concentration of each inhibitor
the adsorbed molecules are adsorbed as a unimolecular layer and molecular
interaction is minimal. At high pH inhibition results from the formation of disordered
polymeric films. These conclusions are supported by polarisation and SERS
measurements.
178
Dry films formed by immersing copper in solutions of alkyl esters of
carboxybenzotriazole also inhibit copper corrosion in both strongly acidic (pH - 0)
and near neutral (pH - 8) sulphate corrodents. The inhibition efficiency depends on
the solvents used to dissolve the esters, solution temperature and immersion time.
Aqueous coating solutions furnish the most protective films. Films formed by
CBTAH-BU, CBTAH-HE and CBTAH-OE are more protective than that formed by
BTAH. The inhibition efficiency of the alkyl ester film increases as the alkyl chain is
made longer. CBTAH-OE in particular gives excellent protection in corrodents of
low and near neutral pH and is also effective inhibitor for protection against
atmospheric corrosion.
EIS spectra show that the film formed by CBTAH-OE can be stable up to 3 days
in acidic sulphate solution (pH - 0) and up to 10 days in near neutral solution @H - 8).
SERS spectra of the dry films indicate that the ring system is close to the metal
surface and reinforces the hypothesis that the alkyl ester molecules interact with the
copper surface through an azole ring nitrogen lone pair of electrons, and also
experience physical adsorption with the copper through the hydrocarbon chain.
Molecular modelling incorporating molecular mechanics and molecular
dynamics is a useful tool to simulate the adsorption from acidic solution of a single
target molecule representative of each of the esters (5-CBTAHC-R) with a clean
copper (1 10) surface. The optimum crude binding energy (Ebind) between each
protonated ester molecule and the surface varied linearly with the alkyl chain length.
The resulting linear correlation between IE% and E bind for compounds that are
structurally similar suggested that the crude binding energy of a single molecule with
copper may be used to predict the inhibition performance of other compounds
constituting a series.
8.2. Future work.
This study has shown that the inhibitive effect of the compounds studied for
copper corrosion results from chemisorption though the azole ring and in the case of
the alkyl esters by physical adsorption of the alkyl chain on the metal surface. The
study provides a basis for designing other compounds which may have higher
179
inhibition efficiency over a wider range of pH. Some of the molecular structures that
may be promising candidates for copper corrosion inhibition are as follows:
Molecules with two or multiple azole rings to provide stronger chemisorption
with the copper surface.
Molecules with different alkyl chain lengths at different positions in the benzene
ring, which can enhance both the activity of an azole ring and the physical
adsorption of the alkyl chain.
By manipulating the molecular structure in these directions, the list of novel
inhibitors is potentially very large.
In addition to the techniques outlined in this thesis, the following considerations
relating to the evaluation of new compounds as corrosion inhibitors should be
established:
cost and ease of synthesis
solubility
toxicity
stability
180
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