14021.pdf

Paper No. 14021

Electrochemical Corrosion Evaluation of a Novel Doped Sol-gel Coating for Low Carbon Steel

Substrate in Saline Medium

Ubong M. Eduok

Department of Chemistry

King Fahd University of Petroleum & Minerals Dhahran 31261, Saudi Arabia.

[email protected]

Rami Suleiman

Center of Research Excellence in Corrosion

King Fahd University of Petroleum & Minerals Dhahran 31261, Saudi Arabia

[email protected]

Mazen Khaled

Department of Chemistry

King Fahd University of Petroleum & Minerals Dhahran 31261, Saudi Arabia.

[email protected]

ABSTRACT

A novel effective, crack-free, hydrophobic, and fast-curing hybrid sol-gel coating was synthesized and

applied on mild steel panels in order to prevent them from corroding in aerated saline (3.5% NaCl)

solution. The performance of this functionalized coating, which was prepared via simple and well

known sol gel chemistry, was evaluated using Electrochemical Impedance Spectroscopy (EIS) and

Potentiodynamic Polarization (PDP) studies. The electrochemical results were complemented with a

Scanning Electron Microscopy (SEM) characterization. Results obtained showed excellent adhesion and

corrosion protection properties on steel surfaces against metal oxidation and other associated corrosion

reactions. The inhibition efficiency of the newly prepared sol-gel coating was dramatically improved

upon by encapsulating two corrosion inhibitors. This improvement was clear from observations of the

new impedance values, a surface analytical analysis and even visible inspections. This work is aimed at

contributing to the solutions of material degradation and failures in the Gulf region a view to reducing

the colossal economic cost and losses already incurred.

Keywords: Corrosion, sol-gel, steel, electrochemistry

1. INTRODUCTION

Sol-gel derived silicate films have been a major bridge between chemical science and material

protection application for the past decade. Various modifications in the reactants and variation of

reaction parameters have resulted in numerous hybrid coatings with different mechanical strengths and

mailto:[email protected]



Paper No. 14021 15th

Middle East Corrosion Conference & Exhibition of 11

February 2-5, 2014

Manama, Kingdom of Bahrain

chemical properties for various uses [1].Since these innovations focus on improving performance

properties such as interfacial adhesion, corrosion resistance, even thermal and optical characteristics [2]

are considered essential in the syntheses of these silica-based films.

Normal and functionalized organic–inorganic hybrid, inorganic oxide, and other organically modified

silica (ORMOSILS) coatings have been reported to have some of these properties satisfied [3-5]. Sol-gel

coatings made from different processes, inhibitors and process conditions for numerous applications

have been used reliably for different metals. Since they are colloids on the metal surfaces, some are

susceptible to shrinking and could flake off from the metals over time if the interfacial chemistry is not

understood and effectively addressed. The pores in these inorganic-organic polymer hybrid films are

usually filled with several materials and to display different functionalities.

Several researchers have reported the effects of Titanium dioxide [6], Graphene [7], Nanoparticles [8],

Alumina [9], Carbon Nanotube [10] in coatings, yet some predominant defects still abound ranging from

irregular surface adhesion to surface in homogenity. In fact, having enhanced corrosion protection

properties and retaining surface adhesion, synthesizing promising coatings for metal structure and

industrial engineering materials is an issue. To address these channels of failed application properties in

synthesis, in this work, organically modified polydimethylsiloxane (PDMS) is linked to the network of

the sol-gel formed. By bonding the resulting Sol-gel network with a group similar to the side chain in

the polymer, a new Si-O-Si bond is formed; with properties, be far, different from those of the additives

listed above. Complete reactivity and hydrolytic stability over an extended stirring time bring about

material with enhanced properties.

The effect of film hydrophobicity and the curing period is considered to improve protection and surface

adhesion. Improvement of the microstructures of the film by encapsulation of some transition metal-

based corrosion inhibitors into the coatings was done. The mechanism for corrosion inhibition by these

metals can be drawn from their ability to readily precipitate oxides and/or hydroxide complexes that can

further reduce the cathodic reaction at the metal-solution interface. The enhancement of the corrosion

resistance of the sol-gel system is then achieved and the passage of atmospheric oxygen, water, radical

and other corrosive ions is remarkably reduced without using the banned biocide Tributyltin (TBT) with

its obnoxious health and environmental concerns [11].

This study uses simple and available silane precursors for making doped sol-gel derived film for

corrosion protection of mild steel in saline (3.5% NaCl) solution. It explains the design of reliable flash

corrosion coating, encapsulated with commercially available zinc and molybdenum-based corrosion

inhibitors, for steel in marine and terrestrial applications.

2. EXPERIMENTAL

2.1 Preparation of S-36 Q-panel sheets

The one-sided ground (0.032”×3”×5”) S-36 grade Q panel mild steel test substrate was effectively

sonicated (Vibra-Cell Sonics and Material INC., US) for 15 minutes in a solution of 70% acetone

(Sigma Aldrich, US) and 80% ethanol (Sigma-Aldrich, US), rinsed with doubly distilled water and

finally air-dried.



February 2-5, 2014


2.2 RUS synthesis, modification and application

The siliconized sol-gel coating was synthesized by blending Tetraorthosiliate (TEOS) and

(Trimethoxymethyl siloxane) TMMS(10:6) in 0.05 N HNO3 (Sigma Aldrich, US) induced hydrolysis-

condensation reactions with 5ml ethanol (Sigma Aldrich, US) as solvent at room temperature for 24 hrs.

The molecular structures of the two used silanes are depicted in Fig. 1. Docylmethylsiloxane–(hydroxyl

alkyleneoxypropyl)–methylsiloxane (DMHM) copolymer (Gelest, US) was added later (3 ml) in drop

wise manner with continuous stirring for one hour until a viscous gel resulted which formed a stable gel

phase network with the previously formed sol-gel. Stirring continued for 12 hours at room temperature,

the final colloid was indefinitely stable and labeled RUS.Moly-white®S101-ED(MOLY) and

Hfucophos Zapp® (ZAPP) were added (5%w/v), respectively, into 10 ml of the RUS coating and

sonicated for 15 min and labeled RUM and RUZ, respectively, being inhibitor-modified sol-gel (RUS)

mixtures. A neat 40 μm thickness single-layer coating of each prepared sample matrices (both modified

and unmodified) was evenly applied on the previously pre-clean steel panels using a K101 Rod Coating

applicator (UK) and the panels were allowed to cure at 50°C in oven then air-dried for 48 hours [12].

2.3 Scanning Electron Microscope (SEM) Analysis

Steel polishing was done with different grades of silicon carbide paper and diamond paste scrub prior to

SEM-EDX (JSM-5800LV Scanning Electron Microscope, JEON, Japan) analysis for the only the RUS-

coated steel before immersion in 3.5% NaCl test solution [13].

2.4 Electrochemical assay

A three-electrode conductivity cell of 100 ml capacity from Gamry was used with a Standard Calomel

Electrode (SCE) as reference electrode and graphite counter electrodes without mechanical stirring. The

cell was connected to a GAMRY 3000 (Gamry Instruments, US) corrosion measuring system.

2.4.1 Electrochemical Impedance Spectroscopy (EIS):

The electrochemical system response at a small amplitude voltage perturbation of 10 mV was

tested on a 1 cm2 area of the coated mild steel substrate over a frequency range of 10 kHz to 0.01

MHz at the end of 24-96 hr immersion periods in 3.5% NaCl solution and at corrosion potential

(Ecorr). EChem Analyst software was used to fit the experimental data and other simulations.

2.4.2 PotentiodynamicPolarization (PDP):

The linear Tafel fitting was done with GamryEChem Analyst software to extrapolate the necessary

polarization parameters for the potentiodynamic polarization at ±0.25V versus Open Circuit

Potential (OCP) at 0.001V s-1

scan rate. Each experiment was carried out in triplicate for the sake

of reproducibility and the results presented here are average values of the measured

electrochemical parameters for 24-96hr immersion periods.



February 2-5, 2014


3. RESULTS AND DISCUSSION

3.1 Surface analysis: Electron microscopy

The SEM image below (Figure 2) shows the surface of the well-bonded and crack-free RUS coating

alone (undoped) on mild steel (a) before and (b) after immersion in 3.5% NaCl for 2.5 weeks. No

prominent surface blisters or any other defects were observed for all the coatings before immersion,

though the surfaces were relatively uneven. The white spots are likely the uncross-linked molecules of

the gel which forms the bulk of the organic-inorganic polymer colloid. INCA software was used to

generate the Energy-dispersive X-Ray (EDX) elemental composition of the coated sample. Elemental

composition by X-Ray-assisted SEM mapping is an effective tool in accessing the composition of

surfaces without prior sampling and rigorous preparation steps. A large and prominent Si peak on the

EDX spectra along with peaks that correspond to C and O atoms prove that the hybrid RUS coating was

synthesized from the silane-based precursors. The molecules of the pre-cursors are made of Si, C,O and

H atoms (Figure 1). The surface began to degrade after the 2 weeks of immersion in the electrolyte

showing evidence of leaching at the interface (Figure 2b). The white patches of the image are crystals of

NaCl after immersion. Though RUS resisted corrosion of steel, this image shows a degradation of the

surface at extended immersion time (2.5 weeks) – a possible solution being the addition of MOLY and

ZAPP. However, the dull colored coated metal substrate exhibited a charging effect on imaging, a factor

which is always avoided by immobilizing a fine and thin monolayer of Gold (Au) before analysis. This

is the reason why Au is appearing on the EDX spectrum.

3.2 Electrochemical evaluation results

Electrochemical evaluation results from AC and DC experiments were used to highlight the corrosion

protection effect of the various coatings on mild steel immersed in 3.5% NaCl solution.

3.2.1 Potentiodynamic polarization:

The kinetics of the mild steel electrodes coated with the doped and un-doped sol-gel derived

silicate films for the anodic and cathodic reactions in 3.5%NaClis shown as a current–potential (i-

E) relation in Fig. 3a, b. The plots show that the presence of the immersed coatings at all

immersion periods led to a reduction in the corrosion rate and lowered the current densities. The

passage of corrosive ions to the metal surface through coating was greatly impeded due to an

improved interfacial barrier and the extra gained strength of the coated surfaces in the presence of

the corrosion inhibitors. For all of the studied immersion periods, and durations shown in the work,

these doped coatings by far performed better than in their undoped coating on steel with MOLY

being the best inhibitor. Values of Ecorr were higher and nobler for superior coating, and a marked

decrease in the corrosion current densities was observed. The values of the potentiodynamic

polarization constants extrapolated from the i-E curves of each system after 24 hr and 96hr

immersion are shown in Table 1.



February 2-5, 2014


Table 1: Potentiodynamic polarization parameters for the inhibitor-doped hybrid sol-gel coating

on steel panels compared to the bare steel (uncoated) in 3.5%NaCl after 24 and 96 hours

immersion periods.

Coating

system Ecorr (mV/SCE) Icorr(μA)

%ɳ

24hr 96hr 24hr 96hr 24hr 96hr

BARE -788.0 -777.0 5.320 5.340 - -

RUS -544.0 -577.0 1.320 1.490 74.19 72.10

RUZ -519.0 -536.0 0.822 1.340 84.46 74.91

RUM -497.0 -527.0 0.476 0.900 90.86 83.15

The corrosion protection behavior of these coating systems in 3.5% NaCl solution increased in this

order: RUM>RUZ>RUS compared to the bare steel that enormously corroded after the immersion

period. An improved inhibiting behavior has been observed for the coating containing Moly-based

corrosion inhibitor compared to Zinc and the undoped inorganic-organic (RUS) sol-gel. Moly-

based inhibitors draw this ability from the basis that molybdenum ions readily precipitate stable

oxides and hydroxides that prevent cathodic reduction reactions [14] at the working electrode

surface in the electrolytic solution [15], and the charge transfer instituting behavior of this metal is

more than Zinc. The ability of the molybdenum ions to remain inside the coating substrate for

extended immersion periods in any electrolyte is another key factor in solution-based oxidation

impedance compared to Zinc and Cesium (Cs) which readily leach out from the coating surfaces.

Wang and Akid [14] have reported the effect of compounds of this metal compared to

molybdenum.

3.2.2 Electrochemical impedance analysis:

The behavior of the electrochemical system at a minute AC amplitude perturbation and scan rate in

the solution of the electrolyte shows the degree of solution impedance in the matrix. With this, EIS

was employed to further demonstrate the corrosion protection performance of the newly prepared

of the sol-gel on the metal. The degree of protection of each coating is marked by a remarkably

large Zmodule values compared to the bare steel. By far the best coating with the highest corrosion

resistance, being MOLY-doped, had a 40MΩ and 0.4MΩ values, respective for 24 hr and 96 hr

immersion period more than other hybrid silicate films. The electrochemical impedance-frequency

plots of inhibitor-doped coatings on Q panel steel compared to a bare steel (uncoated) in

3.5%NaCl after (a) 24hr and (b) 96hr immersion periods is shown in Fig.4. EIS confirms the

corrosion protection order previously shown in the DC evaluation with the undoped coating being

the least of the coating: RUM>RUZ>RUS. Enhanced photo-digital images of these coated systems

in the solution of the electrolyte show a similar trend of results as in the electrochemical

techniques (Fig. 5). However, since there is no physical appearance of corrosion on these coated

steel samples, collection of electrochemical data will continue with a view to understanding the

system. Similar comparison has been done by Suleiman et al. [12]. Future studies will attempt to

explain the interfacial electrochemistry of highly passivating metals relation to the microstructures

of these silicate films which can demonstrate surface adhesion, corrosion protection and

hydrophobicity in the presence of other corrosive molecules in aqueous solutions [16].

RUZ RUM



February 2-5, 2014


4. CONCLUSIONS

The following conclusions were drawn from this study; that:

(1) The synthesized hydrid inorganic-organic sol-gel derived silicate films in this study showed an

excellent corrosion inhibition for steel based on the electrochemical and surface analytical results,

and these coatings demonstrated good surface hydrophobicity.

(2) The modified sol-gel derived silicate films demonstrated superior corrosion protecting ability

compared to the unmodified film, possibility from the oxide, hydroxide or complex precipitate

formation by molybdenum and zinc metal ions which prevented cathodic reduction reaction at the

steel working electrode.

(3) The MOLY-doped coating demonstrated the highest corrosion protection property at all conditions,

compared to the ZAPP-doped and unmodified coating. This improved property of MOLY-doped

coating could be determined from its hydrophobic and blister-free surface, and possibly, from the

ability of zinc to leach readily from the coatings. This studyis a contribution to the various solutions

to the problem of material degradation and failures with a view to reduce the colossal economic

cost and losses already incurred.

5. ACKNOWLEDGEMENTS

Authors would like to appreciate CORE-C and KFUPM for providing the necessary support for this

work.

6. REFERENCES

[1] Cohen, T, Starosvetsky, J., Cheruti, U. and Armon, R. (2010).Whole Cell Imprinting in Sol-Gel

Thin Films for Bacterial Recognition in Liquids: Macromolecular Fingerprinting, Int. J. Mol.

Sci., 11, 1236-1252

[2] Ivanoua D.K., Starykevich, M., Lisenkov, A.D., Zheludkevich, M.L., Xue, H.B., Lamaka, S.V.,

Ferreira, M.G.S. (2013). Plasma anodized ZE41 magnesium alloy sealed with hybrid epoxy-

silane coating. Corrosion Science, 73: 300–308

[3] Elshad Abdullayev and Yuri Lvov (2010).Clay nanotubes for corrosion inhibitor encapsulation:

release control with end stoppers. J. Mater. Chem., 20, 6681–6687

[4] Abdel Salam Hamdy, D.P. Butt, A.A. Ismail (2007). Electrochemical impedance studies of sol–

gel based ceramic coatings systems in 3.5% NaCl solution. Electrochimica Acta, 52: 3310–3316

[5] Andre´s Pepe, Pablo Galliano, Mario Aparicio, Alicia Duran, Silvia Cere (2006).Sol-gel coatings

on carbon steel: Electrochemical evaluation, Surface & Coatings Technology, 200, 3486 – 3491

[6] Zaharescua, M., Predoanaa, L., Baraua, A.Raps, D., Gammel, F., Rosero-Navarroc, N.C.,

Castroc, Y., Duránc, A., Aparicio, M.(2009). SiO2 based hybrid inorganic–organic films doped

with TiO2–CeO2 nanoparticles for corrosion protection of AA2024 and Mg-AZ31B alloys.

Corrosion Science, 51: 1998–2005

[7] Kousalya, AS., Kumar, A., Paul, R., Zemlyanov, D., Fishe, T.S. (2013). Graphene: An effective

oxidation barrier coating for liquid and two-phase cooling systems, Corrosion Science, 69: 5–10



February 2-5, 2014


[8] Sharmila, R., Selvakumar .N.,Jeyasubramanian, K. (2013). Evaluation of corrosion inhibition in

mild steel using cerium oxide nanoparticles. Materials Letters, 91: 78–80

[9] Gergelya,A., Pfeifer, E., Bertóti, I., Török, T., Kálmán, E. (2011). Corrosion protection of cold-

rolled steel by zinc-rich epoxy paint coatings loaded with nano-size alumina supported

polypyrrole, Corrosion Science, 53: 3486–3499

[10] Jeon, H.R., Park, J.H., Shon, M.Y. (2013). Corrosion protection by epoxy coating containing

multi-walled carbon nanotubes. Journal of Industrial and Engineering Chemistry, 19: 849–853

[11] Heming Wang, Robert Akid (2007). A room temperature cured sol–gel anticorrosion pre-

treatment for Al 2024-T3 alloys. Corrosion Science, 49: 4491–4503

[12] Suleiman, R., Mizanurrahman, M., Alfaifi, N., El Ali, B., Akid, R.(2013). Corrosion resistance

properties of hybrid organic–inorganic epoxy–amino functionalized polysiloxane based coatings

on mild steel in 3.5%NaCl solution. Corrosion Engineering, Science and Technology, DOI

10.1179/1743278213Y.0000000093

[13] moren, S.A., Li, Y., Wang, F.H. (2010). Electrochemical study of corrosion inhibition and

adsorption behavior for pure iron by polyacryl amide in H2SO4: Synergistic effect of iodide ions,

Corrosion Science, 52: 1777–1786

[14] Heming Wang, Robert Akid (2007). A room temperature cured sol–gel anticorrosion pre-

treatment for Al 2024-T3 alloys. Corrosion Science, 49: 4491–4503

[15] S.A. Umoren, U.M. Eduok, M.M. Solomon, A.P. Udoh (2011). Corrosion inhibition by leaves

and stem extracts of Sidaacuta for mild steel in 1 M H2SO4 solutions investigated by chemical

and spectroscopic techniques. Arabian Journal of Chemistry, doi:10.1016/j.arabjc.2011.03.008

[16] Ubong Eduok, E. Inam, S. A. Umoren & I. A. Akpan (2013). Chemical and spectrophotometric

studies of naphthol dye as an inhibitor for aluminum alloy corrosion in binary alkaline medium.

Geosystem Engineering,16, 2: 146-155.

http://www.tandfonline.com/action/doSearch?action=runSearch&type=advanced&searchType=journal&result=true&prevSearch=%2Bauthorsfield%3A(Inam%2C+E)

http://www.tandfonline.com/action/doSearch?action=runSearch&type=advanced&searchType=journal&result=true&prevSearch=%2Bauthorsfield%3A(Umoren%2C+S+A)

http://www.tandfonline.com/action/doSearch?action=runSearch&type=advanced&searchType=journal&result=true&prevSearch=%2Bauthorsfield%3A(Akpan%2C+I+A)

http://www.tandfonline.com/loi/tges20?open=16#vol_16

http://www.tandfonline.com/toc/tges20/16/2



February 2-5, 2014


7. LIST OF FIGURES

Fig. 1: Molecular structures of the two silanols used in the synthesis of the base sol-gel (a)

Tetraethyloxysilicate (TEOS) and (b) Trimethoxymethylsilane (TMMS).

Fig. 2: A 100μm SEM image and EDX spectrum of the RUS-coated sample coated onto the mild steel

substrate (a) before and (b) after 2.5 weeks immersion in 3.5%NaCl showing morphology and elemental

composition, respectively.

(a) (b)

FeClFe

Cl FeCl

FeO

Si

1 2 3 4 5 6 7 8 9 10

keVFull Scale 5769 cts Cursor: 4.271 (27 cts)

Spectrum 1

(a)

(b)



February 2-5, 2014


Fig. 3: Potentiodynamic polarization plots of the inhibitor-doped coatings on Q panel steel compared to

a bare steel (uncoated) in 3.5%NaCl after (a) 24 hr and (b) 96 hr immersion periods

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

-0.30

-0.20

1E-10 1E-09 1E-08 0.00000010.000001 0.00001 0.0001 0.001 0.01

E (V

vs

SCE)

i (A)

BARE_24HR RUS_24HR RUZ_24HR RUM_24HR

-1.10

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

-0.30

-0.20

1E-10 1E-09 1E-08 0.00000010.000001 0.00001 0.0001 0.001 0.01

E (V

vs

SCE)

i (A)

BARE_96HR RUS_96HR RUZ_96HR RUM_96HR(b)

(a)



February 2-5, 2014


Fig.4: Electrochemical impedance plots of the inhibitor-doped coating systems on Q panels steel

compared to bare steel (uncoated) in 3.5%NaCl after (a) 24 hr and (b) 96 hr immersion periods.

1

10

100

1,000

10,000

100,000

1,000,000

10,000,000

100,000,000

0 1 100 10,000 1,000,000

Z mo

d

Freq. (Hz)


1

10

100

1,000

10,000

100,000

1,000,000

0 1 100 10,000 1,000,000

Z mo

d

Freq. (Hz)




February 2-5, 2014


Fig. 5: Photo-digital images of the bare and coated steel substrates with different doped sol gel systems

submerged in 3.5%NaCl at room temperature after (a) 24 hour (b) 96 hour immersion .

BARE RUS RUZ RUM

14021.pdf

Documents