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CHAPTER I

INTRODUCTION

Metals and alloys have wide contemporary and historic applications as structural

material and are employed in various constructions such as bridges, pipelines, cooling towers,

railroad electrification works, highways, and modern home appliances. The nation building

sectors like the energy, infrastructure, plant infrastructure, machinery, modern electronic

circuits, defense, transport (land, water airways) all pick up viable growth with huge

consumption of metals. Whether these metals are present in the open air or desert or marine or

underground, they do corrode, but in different manners.

1.1 .GENERAL VIEW ON CORROSION

Corrosion is environmentally modulated degradation, a pervasive phenomenon that

affects all classes of materials. Corrosion refers not only the oxidization of iron but can also

be the degradation of polymers, ceramics, semiconductors, and so on. Corrosion is an

electrochemical reaction based on universal laws of nature. Most metals occur naturally in the

form of oxides and are usually chemically stable. When exposed to oxygen and other oxidizing

agents, the refined metal reacts with its environment and reverts to its natural oxide state with

the creation of holes or cracks.

1.2. DEFINITION OF CORROSION FROM THE PERSPECTIVE OF GREAT

CHEMIST

Corrosion is defined as the destruction or deterioration of a material because of

reaction with its environment (Fontana, 1986). Ulick R. Evans, the British scientist who is

considered the "Father of Corrosion Science", has said that "Corrosion is largely an

electrochemical phenomenon as it involves the transfer of electrons between a metal surface

and an aqueous electrolyte solution (Kruger, 2001). Kruger, 2001 stated that corrosion results

from the overwhelming tendency of metals to react electrochemically with oxygen, water, and

other substances in the aqueous environment.

According to Uhlig and Winston corrosion is a partial degeneration of metal from its

metal stable condition to stable condition of the mineral, accompanied by a decrease in the

free energy of the system (Uhlig, 1985). Corrosion is the exothermic chemical transformation

of a metal or metal alloy to a non-reactive covalent compound such as an oxide or silicate that

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is often similar or even identical to the mineral from which the metals were extracted. Thus,

corrosion has been called “extractive metallurgy in reverse” (Payer, 1980)

1.3. IMPORTANCE OF CORROSION STUDY

Corrosion have an effect on human safety, environment and industrial productivity,

economic competitiveness due to failures in plant infrastructure and machinery which are

usually costly to repair, costly in terms of lost or contaminated product. As modern electronic

circuitry is reduced to ever-smaller dimensions in order to increase memory and computational

density, new problems arise from environmental attack on circuits as their surface to volume

ratio increases. Defense systems will present new challenges as new materials are inserted

into defense platforms. For an instant ground vehicles designed for cold war battlefields are

being used in desert environments, where the degradation modes are different. Even our

historical artifacts are constantly undergoing degradation and must be maintained to preserve

them for present and future generations. Recent years have seen an increasing use of metal

prosthetic devices in the body, such as pins, plates, hip joints, pacemakers, and other implants.

New alloys and better techniques of implantation have been developed, but corrosion

continues to create problems. Standing examples include failures through broken connections

in pacemakers, inflammation caused by corrosion products in the tissue around implants, and

fracture of weight-bearing prosthetic devices etc. A great deal of the development of new

technology is held back by corrosion problems because materials are required to withstand

higher temperature, pressure, and more highly corrosive environments.

1.4.1. COST OF CORROSION

Pioneer report on cost of corrosion by Uhlig in 1949 led to subsequent studies by

others from different parts of various countries. A similar study on cost of corrosion reveals

that the annual cost was estimated to be 1 to 5 percent of the GDP of a nation (ACA, 2010).

Rajan Bahri, Trustee, NACE International India Section, told Business Line that the annual

corrosion cost was found to be much higher than any of the calamities the nation had faced

over the years (The Financial Express, 2007).

1.5. PREVENTION OF CORROSION

The widespread misconception is that nothing can be done about corrosion but is

preventable and can be brought under control. The first step towards corrosion prevention is

that designers need to know about the mechanisms of corrosion failure, where a

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comprehensive understanding of corrosion can lead directly to lower maintenance costs,

longer service lifetimes, and less risk of failure.

The public are to be addressed to the importance of corrosion problems in relation to

material cost, reduced performance, reliability, and impact on the environment. Regular

repainting and occasional inspection of electrical and plumbing lines of some of the

infrastructure equipment is essential to reduce loss due to corrosion. Some chemical

processing plants, power generation plants, aircraft and marine equipment, required extensive

maintenance schedules. Corrosion inspection and monitoring may be used to determine the

condition of a system and to determine how well corrosion control and maintenance programs

are performing.

There are eight forms of corrosion described by Fontana 1986, based on the

appearance of the corroded metal. Atmospheric corrosion, the most common form of

corrosion can be prevented by selecting proper materials or using coatings or inhibitors, or by

cathodic protection.

Fretting was common in riveted joints on ships and other riveted structures where

cyclic loads were used, which has largely been eliminated through welded construction.

Another type of localized corrosion is selective leaching, i.e., dezincification or graphitic

corrosion, in which zinc or carbon in an alloy may be selectively leached out without

producing visible pits or cracks but mechanical properties are greatly reduced. Explosions in

riveted steam boilers were believed to be triggered by stress corrosion cracking called caustic

embitterment because of caustic deposits found adjacent to the cracks. Leaks that develop in

stainless steel heat exchangers and other stainless steel equipment used in the petrochemical

industries are believed to be due generally due to stress corrosion cracking. Remedial

measures such as reducing stress by increasing the thickness of the metal or changing the

alloy or eliminating chloride ions or oxygen will reduce loss due to stress corrosion.

The presence of microbes such as sulfate reducing, acid-producing, general aerobic, and

anaerobic caused microbial corrosion which increased chloride concentration in the pits of

internal natural gas pipe led to rupture and fire near Carlsbad New Mexico on Aug 19, 2000,

which required use of anti microbial inhibitor for corrosion control.

A primary cause for corrosion of underground pipelines is due to galvanic corrosion. All

metals have different natural electrical potentials. Where two metals with different potentials

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are connected to each other in a common environment, current will flow causing corrosion to

occur. The coupling of steel to a different metal, such as copper, will cause a corrosion circuit

to be established. Underground pipeline corrosion can be prevented by avoiding use of

dissimilar metals, or underground gas and water pipelines being oxygen deprived can be

given cathodic protection by using a sacrificial anode such as zinc or aluminum which will

corrode and protects it from corrosion.

Paint is the most common coating used to slow the rate of atmospheric corrosion. The

most widely used organic coatings are paints. Other organic coatings include lacquers, waxes,

and greases .Many other materials, such as plastics, ceramics, rubbers, and even electroplated

metals, can be used as protective coatings. Coatings act as an artificial barrier to the corrosive

environment on a metal surface. Zinc coatings, however, act in the opposite manner because it

has a greater tendency to corrode than the metal to which it is applied, anodically protects it.

Other measures used to control electrochemical corrosion processes are the development of

corrosion resistant alloys which produce more effective protective films that resist breakdown

and repassivate rapidly. By connecting a metal of higher potential to a metallic structure it is

possible to create an electrochemical cell in which the metal with lower potential becomes a

cathode and is protected.

This technique is called cathodic protection. Galvanic cathodic protection uses anodes which

have a natural potential higher than that of the structure being protected. Magnesium and zinc

are used to protect the buried steel and aluminium and zinc for marine steel structures and

underground pipe lines. It is also possible to use an external power source to impress current

on a relatively inert material such as cast iron, graphite or mixed metal oxide anodes. This

method is called impressed current cathodic protection.

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1.5.1: CORROSION INHIBITORS

Corrosion inhibitor is a chemical compound which when a small quantity is added to

the fluid phase, protects the metal surface. The use of corrosion inhibitor for metallic

protection can be traced to the last half of the century (Sastri, 1998). The use of chemical

inhibitors is a very practical and most economical method of combating corrosion. Corrosion

inhibitors are extensively used in petroleum oil piping, cooling water systems, electroplating,

and concrete. In general an inhibitor forms a protective film in situ by the reaction with the

corroding surface. As a result the rate of the anodic and /or cathodic corrosion reactions are

retarded.

Some inhibitors condition the corrosive environment by removing oxygen.

For example oxygen scavengers, when added to water, either alone or with inhibitor retards

corrosion by removing oxygen from water systems in the pH range 6.5-8.5.

A reaction typical of a scavenger is indicated in the equations, 1.1 and1. 2.

2 Na2SO3 + O2 → 2 Na2SO4 (1.1)

N2H4 + O2 → N2 + 2H2O (1.2)

Some are anodic polarizer, by increasing the anodic potential resulting in a small

current flow, and thereby causes the corrosion potential to shift in the noble direction. The

most effective and widely used anodic inhibitors are oxidizing anions, such as nitrite,

chromate and nitrate, which can cause passivity of steel even in the absence of oxygen. Non-

oxidizing ions such as phosphate and molybdate require the presence of oxygen to cause

passivity in steel.

Cathodic inhibitor shifts the corrosion potential to more negative values. The cathodic

reaction is either hydrogen ion reduction to form hydrogen gas, or reduction of oxygen. Both

these reactions cause the environment immediately adjacent to the cathodes to become

alkaline. Therefore ions such as zinc, magnesium and calcium may be precipitated as oxides to

form a protective layer on the metal. These sometimes cause hydrogen blistering and increases

hydrogen embrittlement, especially in acid solutions

Mixed Inhibitors type of inhibitor controls both anodic and cathodic reactions; by

influencing both the anodic and cathodic potential .In general organic inhibitors affect the

entire surface of a corroding metal when present in sufficient concentration by forming a film

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on the metal surface. Their effectiveness depends on chemical composition, molecular structure

and their affinities for the metal surface. This film reduces or prevents corrosion of the metal.

The extent of adsorption also depends on the nature of the metal, the metal surface

condition, besides the mode of adsorption and the type of corrosive media. Mixed inhibitors

protect the metal in three possible ways:

• Physical adsorption,

• Chemisorptions

• Protective film forming inhibitor

Vapour-Phase Inhibitors are similar to the organic adsorption-type of inhibitors and

possess a very high vapour pressure and are used to inhibit atmospheric corrosion of metals.

VPC inhibitors such as dicyclohexylamine nitrite and sodium benzoate are usually effective in

closed vapour spaces such as shipping containers and boilers because they would be lost

rapidly through any leaks in the package or container.

1.5.2. PICKLING INHIBITORS

Pickling is a surface treatment used to remove impurities, such as stains, inorganic

contaminants, rust or scale from ferrous metals, copper, and aluminium alloys. A solution

called pickle liquor, which contains strong acids such as hydrochloric acid or Sulphuric

nitric acid, phosphoric acid and sulfamic acid are used to remove the surface impurities. These

acids are commonly used to descale or clean metals before fabrication or electroplating process or

periodic servicing of plants or oil well acidizing in drilling rocks to explore oil and natural

gas etc. These acids used not only remove scales but also dissolves the base metal. The

mineral acids, for example, HCl remove rust from iron articles according to the equation (1.3)

Fe2O3 + 6 HCl → 2FeCl3 + 3 H2O (1.3)

To minimize metal loss, as well as to reduce hydrogen fume formation, and acid fume

formations synthetic inorganic or organic chemicals or natural plant materials are added to the

pickling baths. These inhibitors form a foam blanket preventing heat loss and acid spray. Acid

consumption is also reduced with the use of inhibitors.

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1.6. .ELECTROCHEMICAL MECHANISM OF CORROSION

When a metallic surface exposed to an aqueous electrolyte, oxidation or metal

dissolution (anodic reaction) with the liberation of electrons, and a reduction (or cathodic

reaction) take place. As the crystals, or grains, of a metal are made up of unit cells repeated in

a three dimensional array may have some type of crystal defects which act as "sites” for

anodic and cathodic reaction and make up a "corrosion cell". There will be the flow of direct

current from one part of the metal surface to another which causes the loss of metal at the

point, whereas several cathodic reactions are possible depending on what reducible species

are present in the solution. Typical reactions are the reduction of dissolved oxygen gas, or the

reduction of the solvent (water) or hydrogen ions in the case of acid solution to produce

hydrogen gas (Bockris et al., 2000). The corresponding reactions are given in equation (1.3)

M Mn+aq +

ne

O2 + 4H+ + 4e 2H2O (Oxygen reducion in acidic solution)

1/2 O2 +H2O + 2e 2OH- (Oxygen reducion in neutal or basic solution)

2H+ +2e H2 (Hydrogen evolution from acid solution)

2H2O +2e H2 + 2OH- (Hydrogen evolution from neutral water)

s

1.7

Cathodic reaction

1.6

1.5

1.4

1.8

Anodic reaction

When corrosion occurs on a metal surface the extent of corrosion depends upon the

environment to which the metal is exposed, for an example, a piece of metal may have

differential oxygen concentration (as the oxygen concentration in the electrolyte varies from

place to place). In such situation region with high oxygen concentration becomes cathodic to

the region with lower concentration and promotes corrosion at the anodic sites. Concentration

cells may also be formed where there are differences in metal ion concentration. In this case

metal in contact with the more dilute solution will corrode.

Galvanic corrosion occurs at the contact point of two metals or alloys with different

electrode potentials. Direct coupling of copper to steel will cause the steel to corrode much

faster than normal. Another form of this is the coupling of rusty pipe to new, clean steel. The

natural difference in potential causes the new steel to corrode more rapidly than the old steel,

also depend on soil types and oxygen availability. When brass in contact with copper the

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brass becomes anodic and suffers the loss of its zinc atoms. Brass in contact with galvanized

steel is protected, while the zinc coating on the steel is first dissolved, leaving the steel open

to attack. The better combination is the large anode-small cathode area.

1.6. 1: RUSTING OF IRON

Rust is the very common form of mild steel corrosion which is an electrochemical

process. The loose porous rust, Fe(OH)3 slowly transforms into a crystallized form Fe2O3.H2O

(hydrous ferrous oxide, is the principal component of red-brown rust and is also the

composition of mineral hematite. It exists in two forms, namely, α-form hematite and β-form

used in recording tapes. With limited O2, magnetite is formed (Fe3O4) which is black and also

known as magnetite/lodestone.

1.7. POURBAIX DIAGRAM

Pourbaix diagram (potential pH diagram) is a graphical representation showing

regions of thermodynamic stability of species in metal-water electrolyte systems which

describes the relation between potential and pH by means of an electrochemical equilibrium

diagram. It plots potential against pH, a parameter also of great importance to corrosion

processes. Fig 1.1 Pourbaix diagram for the iron/water/dissolved oxygen system showing the

effect of potential in moving the system from a corrosive (active) region to a passive region.

Depending on the reactants and products of the assumed reactions, these straight lines will be

horizontal, vertical, or sloping. An horizontal line on the diagram represents a invariant

system, whereas sloping lines, each of which is related to a divariant or invariant system

depending on whether the parameter contains one component (concentration) or two

components (containing two concentrations). It often has been found if the concentration of

Fe 2+

in the solution is around 10~6

gram per liter represents no corrosion (Uhlig, 1985).

Fig. 1.1. Pourbaix diagram for iron-Water system

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1.8. ELECTROCHEMICAL KINETICS AND CORROSION RATES

1.8.1 CORROSION RATE

Corrosion rate of a metal depend upon the metallic properties such as its position in

galvanic series, hydrogen or oxygen over voltage, anodic to cathodic area, purity of metal

nature of the surface film, passivity of the metal, solubility of corrosion products and volatility

of corrosion products. As far as the environment is concerned high humidity of air, presence of

impurities like chlorine, sulphur dioxide, nitric oxide, suspended particles, low pH of the

solution, nature of ions present (high concentration of chloride ions), conductance of the

medium, differential oxygen concentration and rate of flow of stream all promote corrosion.

Corrosion rate of a metal is determined by weigh loss method and electrochemical methods

The weight loss of a corrosion coupon after exposure to a corrosive environment,

expressed as mils (thousandths of an inch) per year penetration. Corrosion rate is calculated

assuming uniform corrosion over the entire surface of the coupon. The corrosion penetration

rate is related to current density (i) based on Faraday’s law (Fontana, 1986):

k a i

n D CR =

(1.9)

CR = corrosion penetration rate,’ k’ = a constant,’ a’ is atomic weight of the metal, ‘n’ is the

number of electrons involved during electrochemical reaction D-density in g /cm3. In many cases,

it can be estimated from current versus voltage data. A log current versus potential over a range

of about one half volt can be applied. The voltage scan is centered on open circuit potential

(OCP), and then fit the measured data to a theoretical model of the corrosion process to find out

Icorr.

1.8.2. The Butler Volmer equation:

In a corrosion system, there will be two opposing reactions, i.e., anodic and cathodic.

The Butler-Volmer equation describes how an electric current of an electrode depends on the

electrode potential considering that both a cathodic and anodic reactions occur on the same

electrode and is given in equation (eq.1.10)

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i =io { [O]o e-α nF(E-Eeq)

RT RT[R]o e(1- α ) nF(E-Eeq)

- [R]bulk [R]bulk

}1.10

io = exchange current density,[O]o= concentration of oxidant at the electrode surface

[O]bulk, concentration of oxidant at the bulk, [R]o, concentration of reductant at the electrode

surface, [R]bulk, concentration of reductant at bulk, F, Faradays constant T, temperature, gas

constant, reaction order, n, number of electrons involved E, Applied potential, Eeq,

Equilibrium potential and α: is charge transfer coefficient, dimensionless. The transfer

coefficient lies between 0 and 1 and is normally ~1/2. R stands for reduced species and O for

oxidized species.

When the two opposing reactions, namely anodic and cathodic in a corrosion system

are kinetically controlled, and there is no concentration gradient, obeying the conditions [O]o=

[O]bulk and [R]o=[R]bulk, then the different expression of Butler-Volmer equation is (1.11).It is

applicable when the polarization depends only on the charge transfer kinetics.

i =io { e 2.3 ( E - Eeq ) e -2.3 ( E - Eeq )- }βc βa

(1.11)

1.8.3. TAFEL EQUATION AND TAFEL PLOTS

When the system is perturbed by applying more negative potential (E-Eoc <0, then the

system is cathodically perturbed, one half of the expression disappears from Butler –Volmer

equation (eq.1.11) and reduces to (eq.1.12). Then,

i = io e( 2.3(E –Eeq /β) (eq.1.12)

This is otherwise known as Tafel equation and overvoltage (η ) =E-E0 and the exchange

current density (io) is thus related to corrosion current by Tafel equation (eq.1.13) as :

Solving the above equation (eq.1.22) η is :

η = RT

αn Flog i0

log i- αn F

RT

(1.13)

Or Tafel equation can also be written as:

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η = b log ia + (1.14)

The current values are taken for several over potential values making test electrode

more and more negative. The log values of these current values are plotted against the over

potential on one side. In the next step the test electrode is perturbed anodically. As done

earlier the current is measured for various over potential values and plotted against them on

the other side of the graph. Log i versus η (over potential) plot is called a Tafel Plot. The

Tafel Plot in Figure 1.2 was generated directly from the Butler-Volmer equation (eq.1.11). In

practice, many corrosion systems are kinetically controlled and thus obey eq.1.11.From Tafel

Extrapolation of linear portion of polarisation curves results in intersection at Ecorr and

exchange current density i0.

Fig.1.2 shows polarisation curve or Tafel plot for a corroding system.

Fig. 1.2 Tafel plot or polarisation curve for a corroding metal

1.8.4: LINEAR POLARISATION

The polarization resistance behaves like a resistor when the potential to be very close

to equilibrium potential (Eeq.) Near Eeq, the current versus voltage curve approximates a straight

line. The slope, ∆E/ ∆I is, therefore, called the Polarization Resistance, Rp. Rp value can be

combined with an estimate of the Beta coefficients to yield an estimate of the corrosion

current.

If we approximate the exponential terms in B-V Equation(eq.1.11) with the first two terms of

a power series expansion ,

e x =

x

1 !+

x 2

2 !+ ......

We get one form of the Stern-Geary Equation (eq.1.15)

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i corr = βa βc

2.3 ( βa + βc )

1

Rp (1.15)

In a Polarization Resistance experiment, a current versus voltage curve is recorded as

the cell voltage is swept over a small range of potential that is very near to Eeq (generally ± 10

mV). A numerical fit of the curve yields a value for the polarization resistance

(Rp). Polarization To use eq.1.15. Beta values (β) are required. These can be obtained from a

Tafel Plot.

1.8.5 : ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY:

To make an EIS measurement, a small amplitude signal, usually a voltage between 5 to

50 mV is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000 Hz.

The EIS instrument records the real resistance and imaginary (capacitance)

components of the impedance response of the system. Depending upon the shape of the EIS

spectrum, a circuit model or circuit description code and initial circuit parameters are

assumed. Often, data obtained by EIS is expressed graphically in a Bode plot or a Nyquist

plot. EIS data was most commonly analyzed by researchers by fitting the data to an equivalent

circuit model. The elements in the model are common electrical elements such as resistors and

capacitors; for example, the solution resistance was substituted by resistor and the

electrochemical double layer by non ideal capacitor (constant phase element). An electrode |

electrolyte interface behaves like a capacitance called electrochemical double-layer capacitance Cdl.

The equivalent electrical circuit for the redox reaction taking account of the double-layer

capacitance is shown in Fig. (1.3) another analog circuit commonly used to model the

electrochemical double-layer is called a constant phase element.

Fig. 1.3. Equivalent circuit

Equivalent circuit or Randles Cell for a redox reaction without mass-transfer limitation

has been shown Fig 1.3. It includes a solution resistance, a double layer capacitor and a

charge transfer (or polarization resistance).The double layer capacitance is in parallel with the

charge transfer resistance. The double layer capacitance is proportional to the area of the

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substrate exposed to the electrolyte. A capacitor is formed when two conducting plates are

separated by a non-conducting media, called the dielectric. The value of the capacitance

depends on the size of the plates, the distance between the plates and the properties of the

dielectric. The relationship is:

0 rdl

AC

d

ε ε= (eq.1.16)

With,εo = electrical permittivity , εr = relative electrical permittivity

A = surface area of plate, d = distances between two plates, whereas the electrical permittivity is a

physical constant, the relative electrical permittivity depends on the material.

Nyquist diagram for a simple electrochemical one time constant system

It is obtained by plotting real components of impedance on the X-axis and imaginary

components of the impedance on the Y-axis is shown in Fig.1.4

Figure 1.4: Nyquist plot for a simple electrochemical one time constant system

Warburg impedance appears at low frequencies and since the diffusion is a slow process, but

it is unperturbed by high frequencies. Therefore, with decreasing frequencies, the contribution

from a diffusion process to the total impedance increases. The curve

(Nyquist plot) is no longer a semi circle, and is shown in Figure 1.5.

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Fig.1.5: Nyquist plot with Warburg impedance

1.9:. The importance of non electrochemical and electrochemical techniques:

Non electrochemical and electrochemical techniques are two corrosion monitoring

techniques applied widely to evaluate corrosion inhibition effect of an inhibitor. Traditionally

the non electrochemical techniques are extensively used through the industry due to their

simplicity, robustness and reliability. Corrosion is an electrochemical process; therefore

electrochemical techniques have been used in static and flowing conditions with and without

inhibitor to identify the corrosion rates of each inhibitor. Indeed, a number of electrochemical

techniques have been developed over the years especially for the measurement of corrosion

processes and are very well accepted. Some of them have been developed specifically for

corrosion measurement. Among these techniques are Polarization Resistance, Tafel Plots,

Cyclic etc.. Some of these techniques are quite similar and also very straightforward to

employ in a corrosion laboratory.

The main advantages of the electrochemical methods are short measuring times. Real-

time weight loss measurements need days and sometimes weeks to make a reliable

measurement of corrosion rate. Electrochemical instrumentation can make a corrosion rate

measurement in minutes or hours. They are sensitive that modern, well-designed

electrochemical instrumentation can measure extremely low corrosion rates and the results are

accurate.

1.9.1: Fourier Transform Infrared Spectroscopy: is a technique which is used to obtain an

infrared spectrum of absorption, emission, photoconductivity or Raman scattering of a solid,

liquid or gas. An FTIR spectrometer simultaneously collects spectral data in a wide spectral

range. This confers a significant advantage over a dispersive spectrometer which measures

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intensity over a narrow range of wavelengths at a time. FTIR becomes new applications of

infrared spectroscopy, a Fourier transform (a mathematical algorithm) is required to convert

the raw data into the actual spectrum. The response from a sample exposed to a pulse of

radiation is a signal consisting primarily of the characteristic frequencies for that sample

(Sastri, 1998).

1.9.2: Scanning Electron Microscope (SEM) is a type of electron microscope that images a

sample by scanning it with a high-energy beam of electrons in a raster scan pattern. For

conventional imaging in the SEM, the selected area must be electrically conductive, at least at

the surface, and electrically grounded to prevent the accumulation of electrostatic charge at

the surface and protect it while the cross section is being milled. They are therefore usually

coated with an ultra thin coating of electrically-conducting material, commonly gold and

platinum deposited on the sample (Bard, 1991). The electrons interact with the atoms that make

up the sample producing signals that contain information about the sample's surface topography,

composition, and other properties such as electrical conductivity. A raster scan, or raster

scanning, is the rectangular pattern of image capture and reconstruction in television and this is

also the pattern of image storage and transmission used in most computer bitmap image

systems.

1.10: Need for the study

The corrosion of iron and mild steel is of fundamental, academic and industrial

concern that has received considerable amount of attention. Mild steel is a material of

construction, from nuts to rocket launcher made of iron and is known for its strength, easy

fabrication and low cost. Though it has good resistance to corrosion, it loses resistance power

owing to electrochemical, thermodynamic, metallurgical, physical and chemical factors. The

destructive damage to equipment and structures caused by corrosion leads to sudden failure

and shut down of equipments. Loss due to corrosion is found to be very high. Though there

are several preventive methods, widely accepted method is the use of corrosion inhibitors.

Corrosion inhibitors are chemical compounds added to the corrosive medium to reduce the

rate of attack on the metal or alloy.

Aggressive acids, predominantly hydrochloric, sulphuric, nitric and phosphoric acid

are widely used for industrial and some specific treatments (e.g. chemical cleaning and

pickling) of mild steel and for oil well acidizing. In such industrial situations it is virtually

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impossible to prevent loss of metal. The general strategy is to use measures that reduce the

metal loss to an economically sustainable level. So corrosion inhibitors are used. The safety

and environmental issues of corrosion inhibitors arisen in industries have always been a

global concern. Inhibitors are often easy to apply and offer the advantage of in-situ

application without causing any significant disruption to the process. However, there are

several considerations such as cost, toxicity; availability and environmental friendliness of the

inhibitor are taken into account when choosing an inhibitor.

Inhibitors function by adsorption of ions or molecules onto metal surface. They reduce the

corrosion rate by increasing or decreasing the anodic and/or cathodic reaction or decreasing

the diffusion rate for reactants to the surface of the metal. From the mechanism of the action of

inhibitors in acid solutions it is found that the most effective and efficient acid inhibitors are

inorganic and organic compounds containing polar functions with hetero atom such as oxygen,

nitrogen, sulphur and phosphorus in their molecular structures (El Din. 1996, Bayol, 2007, Arab

(1993); Larabi (2004) and Noor, (2008)). At the same time, the application of oxidizing

anions such as chromate, nitrites and nitrates as corrosion inhibitors has been reduced greatly

in recent years due to disposal and environmental issues (Fontana 1986,and Eddy (2008)) and

some of organic compounds are found to be cost ineffective, toxic and unavailable and hence

have come under criticisms. Research activities in recent times are geared towards finding

many alternative corrosion inhibitors to reduce the jeopardizing effects of corrosion and

related problems on humans, animals and environment. Both naturally occurring and synthetic

inhibitors have gained wide acceptance in this regard. The inherent stability, presence of

multiple adsorption sites, eco-friendliness, availability, and low cost makes these inhibitors to

be a better substitute for unfriendly inorganic and organic acids inhibitors

(http://www.scitopics.com;). Parikh, 2004 ; Zucchi, 1985, Umoren et al., 2006, Saleh et al., 1984,

Eddy, 2008, Oguzie, 2007, Gunasekaran et al., 2004, Abdel-Gaber et al., 2006, El-Etre, 2008,

Orubite et al., 2004, Saratha et al., 2003 and Ehteram, 2008 have appraised and documented

natural plant material as effective corrosion inhibitors.. Moreover, corrosion inhibitors for

hydrochloric acid, sulfuric acid and phosphoric acid have been attracted increasing attention

due to their extended applications. The present study is focused to investigate the potential of

the leaves extracts of Abutilon indicum (ABI), Hyptis suaveolens (HPY) and Sida

rhombifolia(SR) in each acid, such as 1 M HCl, 0.5 M H2SO4 and 1 M H3PO4 as corrosion

inhibitors for mild steel in three acid media. The selected plants are considered to be weeds

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which require periodic cleaning and have never been exploited as a corrosion inhibitor in any

acid medium. Literature survey on phytochemical constituents of these plants from the study

carried by Sammia Yasmin et al., 2008, Tiwari et al., 1979 and Menaka Thounaojam et al.,

2009 revealed presence of active principles such as fatty acids, proteins, tannins, alkaloids

flavones etc. Hence these were screened for their use as mild steel corrosion inhibitors. The

leaves of the above said plant materials were collected from Gounder Thottam, Pollachi,

Coimbatore district, Tamil Nadu, India.

The profiles of plants taken for study, ABI, HYP and SR are furnished in plates 1.1, 1.2 and

1.3 respectively.

In order to compare these plants efficiencies with that of synthetic compound

p-(N, N-dimethylamino) benzylidene acetone (DDMBA) was synthesized and evaluated as

mild steel corrosion inhibitor in H3PO4.

Synergism is one of the most important phenomena in corrosion inhibition processes and

serves as a basis for all modern corrosion inhibition formulations. The amount of the

chemicals applied can be decreased, as well as the application of environmentally acceptable but

less effective compounds can be made better corrosion inhibitors in this way. It is an effective

means of improving the inhibitive force of inhibitors (Lebrini, 2007, Hosseini et al., 2003, Li et

al., 2008). A study has been taken to investigate synergistic effect between the synthetic

compound, DMABA and the cationic surfactant, Cetyl trimethyl ammonium bromide (CTAB) on

mild steel corrosion in 1 M H3PO4.

18

Plate 1.1: Plant profile of Abutilon indicum

Botanical name : Abutilon indicum (L.) Sweet

Family : Malvaceae

Common Name : India mallow

Tamil Name : Nallatutti,, Perundutti and tutti

Habit : shrub

Distribution : widespread as a tropical weed throughout India.

Uses : The whole plant is used for anti inflammatory, Stimulating effect and

in the treatment of piles

Part used : Leaves

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Plate 1.2: Plant profile of Hyptis suaveolens

Botanical name : Hyptis suaveolens

Family : Lamiaceae

Habit : Perennial herbs and shrubs,

Distribution : throughout India

Common Name : American mint and bush mint

Tamil name : Nitta, Ganga Tulasi and Vilaayati

Uses : Control insects and nematodes, anti oxidant, anti bacterial and anti

Fungal.

Part used : Leaves

20

Plate 1.3: Plant profile of Sida rhombifolia

Botanical name : Sida rhombifolia.

Family : Malvaceae

Habit : Perennial herbs and shrubs

Distribution : Tropical and sub-tropical regions.

Common name : Cuban jute, Adibala, Arrow leaf Sida, Cuban jute, broom weed and

Indian hemp.

Tamil name : Aruvamanai poondu, Chitramutti and kurnthatti

Uses : Treat diarrhea, relieve swelling, relieve headache, the mucilage is

used as an emollient, and the root is used to treat rheumatism.

Part used : Leaves

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1.11: Objectives of the present study

To evaluate the inhibiting effect of leaves extract of each plant, Abutilon indicum,

Hyptis suaveolens and Sida rhombifolia in 1M HCl, 0.5M H2SO4 and 1M H3PO4 on mild steel

corrosion in respective acid media.

♦ To evaluate the inhibiting effect of p-(N, N-dimethylamino)benzylidene acetone on mild

steel corrosion in 1M H3PO4.

♦ To study the synergistic effect of p-(N, N-dimethylamino) benzylidene acetone with

CTAB on mild steel corrosion in 1M H3PO4.

♦ To find the effects of concentration of the inhibitor, immersion period and temperature on the

inhibition efficiency.

♦ To calculate activation energy and thermodynamic parameters.

♦ To test the data in various adsorption isotherms.

♦ To find the suitable storage condition and period of storage of the extracts.

♦ To monitor the pickling bath.

♦ To analyze surface of mild steel specimens.

♦ To propose a probable mechanism of inhibition.

♦ To find the industrial applicability of the studied plant inhibitors