chapter ii review of literatures -...

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Review of Literature

CHAPTER II

REVIEW OF LITERATURES

The catastrophic failures due to corrosion impart economic inflation as well

as human loss. Among the various types of corrosion, the general corrosion is the

prominent one in which whole of the metal surface is exposed, gets attacked and

may lead to failure in engineering sense. But this crash is usually avoided by the

application of suitable control measures. Our present study is designed with the clear

knowledge obtained from the detailed literature survey on the works underway in

the anticorrosive measures. Thus the main objective of this study is to protect the

metallic substrates against corrosion using conducting polymers and rare earth metal

oxide coating, rather than investing capita in maintenance and replacement of

corroded parts.

Chromate conversion coatings have been widely used in industry for a long

time to improve the corrosion resistance. These coatings provide anodic protection

to metals against corrosive environment. Even though the presence of chromium

makes the coating significant in many applications, they are toxic and the use of

these coating may also expedite carcinogenic effects [9]. Thus, the recent studies

proved the copolymer coatings and rare earth metal coatings would be a promising

alternative to these conversion coatings.

The potential of conducting polymer coatings for corrosion protection is a

topic of current controversy. In general, the efficacy of conducting polymers very

much depends on how they are applied and on the conditions of the corrosion

experiment, depending on the exact conditions, a conducting polymer may have

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excellent protection capability or may lead to a disastrously enhanced corrosive

attack. Hence a keen research and observation is a must to propose a specific

polymer for anticorrosion purpose. Thus a wide collection of literatures reported

were discussed here to get a thorough vision on the use of homopolymers,

copolymers and bilayer as well as multilayer coatings towards corrosion protection.

The use of conducting polymers as coatings for protecting the metals and

alloys against corrosion has received focused attention for many years [45-47].

Furthermore, the main advantage of conducting polymers, is that they act both as

physical and electronic barriers, improving the protection afforded by other

materials that simply acts as physical barriers alone. In this context, several

methodologies have been proposed for the application of conducting polymer

coatings [48] either as sole or as combination.

ƒ Conducting polymers as a primer

This method is based on the electrochemical deposition of the conducting

polymers directly on the metal surface. Electropolymerization was considered to be

an easy and convenient method for this coating/electrodeposition. Investigations on

the protection imparted by these coatings to metals, mainly iron and steel have been

renewed [49,50]. Furthermore, improvements have been achieved by adding a

relatively small concentration of anticorrosive pigments.

ƒ Role as a primer coating with conventional topcoats

Conducting polymers combined with conventional topcoats or

electrophoretic coatings made of epoxy and polyurethane resins used in industries

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provides better corrosion protection than simple top coat and some primer-top coat

systems containing inorganic corrosion inhibitors [51-55].

ƒ Copolymer coatings as blends

The blends of conducting polymers with conventional resins are also an

alternative used for corrosion protection. For example, polymer coatings blended

with polyurethane or alkyd resins would provide better performance against

corrosion than the polymer coatings [56, 57].

ƒ As an anticorrosive additive to the paint formulation

In this type of application, the inorganic anticorrosive inhibitors usually

employed are replaced by the conducting polymers of lower concentrations and thus

the results were found to have better performance than ordinary paint formulations

[58-65]. There are also many report published on the corrosion protection

performance of epoxies, polyacrylamide derivatives, polyurethanes, polyesteramides

and metal matrix composite polymer coatings [66-72]. performance

Organic and metal-organic polymer coatings possess many beneficial

properties such as good thermal stability, chemical resistance, protective efficiency,

gloss, catalytic and biological activity, electric as well as magnetic properties.

For many decades these coatings were chemically synthesized and utilized for

anticorrosive performances. However there are many more disadvantages with these

chemical syntheses and the electrochemical synthesis is generally preferred as a

simple as well as most convenient method for synthesizing these types of

anticorrosive coatings on metallic surfaces and thus a prominent literature survey on

electrochemical synthesis are discussed here.

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4

2.1 HOMOPOLYMER FOR CORROSION PROTECTION

Organic polymers such as polyindole [73,74], polyaniline [75-83],

polypyrrole [84-90], polythiophene [91-93], etc are used for anticorrosive

performances, as inhibitors or as coatings synthesized by conventional and

electrochemical routes. Many homopolymers were reported to reduce the corrosion

of iron, steel, and other oxidizable metallic materials.

Polyindole has received a significant amount of attention in the past several

years and may be a good candidate for applications in various areas, such as

electronics, electrocatalysis, anode materials in batteries, anticorrosion coatings and

pharmacology. The electrodeposition of polyindole films has mainly been performed

in neutral solvent such as acetonitrile, CH2Cl2, ClO - or BF4

-. Many studies

underwent in the past few decades, to attain a clear understanding of the

electropolymerization of indole as well as substituted indoles in aprotic medium or

in aqueous acidic medium. Mostly they have explained as protonic and electronic

exchanges between the polymer and the electrolytic solution [94].

Tourillon and Garnier [95] were the first to report the electrochemically

synthesized polyindole and used subsequently for a variety of applications. Since

then a wide collection of reports were found for the use of polyindole in the field of

corrosion as inhibitors, coatings on different metallic substrates. Jingkun et al., [73]

electrochemically synthesized polyindole films in boron trifluoride diethyl etherate.

In this electrochemical synthesis, they have reported to have lower oxidation

potential for indole than that of the oxidation potential observed during

the electrochemical synthesis of indole in acetonitrile solution. But in their

study, the conductivity of polyindole reported to have only 0.1 Sm-1

, which need

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further investigations to attain a coating of better properties for anticorrosive

applications. These results are similar to that of the inference obtained by

Maarouf et al [74].

Eraldemir et al., [96] had chemically synthesized composites of polyindole

and poly(vinyl acetate) using FeCl3 as an oxidant agent in anhydrous media. The

compositions of the composite were altered by varying the indole monomer ratio

during preparation and cast on glass petri dishes. It is inferred that

polyindole/poly(vinyl acetate) composites coatings were thermally more stable than

the polyindole coatings and possessed increased conductivities depending on the

indole content in the composites.

Tuken and his co-workers studied the corrosion performance of

polypyrrole/polyindole coatings for mild steel corrosion protection in 3.5 % NaCl

solutions [97]. In this study, they synthesized the polypyrrole coating from oxalic

acid, as a primer on which polyindole was further coated on mild steel from

acetonitrile solutions towards corrosion protection performance. They have also

undergone the anticorrosive performance with the aid of electrochemically

synthesized polyindole on nickel coated mild steel [98].

Tuken in his another report, studied the polyindole for corrosion prevention

of copper using electrochemically synthesized polyindole coating on the

polypyrrole coated copper (as a primer). They have carried out all the corrosion

studies in 3.5% sodium chloride solution [99]. They have also investigated

polypyrrole/polythiophene obtained as in the previous study towards the

anticorrosion of copper [100].

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Sazou [101] has studied the dynamical behavior of the electrochemically

polymerized indole in acetonitrile-water mixture which greatly focuses both the

positive as well as negative features of the polyindole film. Thus polyindole being

low conductive in nature may result in more time consumption during the

electrochemical synthesis with porosity in the coating which may result in

delamination.

Dudukcu et al., [42, 43] have studied the inhibition performance of indole on

316L stainless steel corrosion in acidic and alkaline solutions of varied pH 4,8 and

10 in 0.3M NaCl. The study showed that indole has no important effect on the

corrosion of steel in alkaline solutions. The strong adsorptive interaction between

the hydroxide ions and the surface might not allow the molecule to be effective.

Controversially, the electrochemically synthesized polyindole on 304-stainless steel

in LiClO4-acetonitrile solution towards corrosion performance in NaCl solution

showed impact on the corrosion protection under different immerse conditions.

Polypyrrole, another heterocyclic polymer finds a huge application as

corrosion protective coatings on different metals at different exposed environment.

Polypyrrole has been a subject of a great number of studies due to its relatively easy

preparation from aqueous solutions on inert (Pt, Au and glassy carbon) and active

(Cu, Ti, Fe and stainless steel) in aqueous and non-aqueous solutions (acetonitrile

and methanol) in case of metals undergo dissolution along with the presence of

various inorganic ions [84].

Hermas et al., [80] investigated the electrochemical deposition of polypyrrole

from sulphuric acid. They inferred the formation of a stable passive film on the

substrate during the electrochemical synthesis by aging in sulphuric acid and also


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during the anodic polarization condition. They also reported that the passive film

formed from aging in sulphuric acid as more protective than that of the passive film

formed during anodic polarization under the same aging conditions.

Wencheng Su and Jude O. Iroh demonstrated the protective performance of

polypyrrole coatings on low carbon stainless steel using the electrochemical process.

They have observed that the substituted polypyrrole coating show better protection

against corrosion than that of pure polypyrrole coatings on the low carbon steel

[102]. In recent studies there are reports evidenced for the corrosion protection of

stainless steel using separate polypyrrole electrodes in acid solutions.

The polypyrrole electrodes were formed chemically synthesized pressed with

different surface areas and investigated their protection performance in acid

solutions with the galvanic anodic protection.

Gonzalez et al., [103] have studied the electrosynthesis of polypyrrole films

on 316L stainless steel from neutral and alkaline solutions containing molybdate and

nitrate anions. This type polypyrrole coating with dopants seems adherent strongly

and could completely protect the substrate in sodium chloride solution at all

immersion times. The corrosion behavior of the coated electrodes was explained

with the combination of protective mechanisms from the anions, electroactivity of

the polymer, galvanic interaction of the metal and stable oxide film formed on metal

substrate.

Nguyen Thi Le et al., [104] carried out an electrodeposition procedure of

polypyrrole on iron using potassium tetraoxalate as anion to prepare pretreatment

films for protective coatings against iron corrosion. They studied the protection

performance increases in an exponential way on the synthesis charge and ascribed


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the impedance diagram response mainly to the polymer layer and not of the passive

layer. The observed evolutions attributed to interchange between the tetraoxalate ion

of the film and chloride ion of the electrolyte but the process was not diffusion

limited.

Lehr et al., [105] have investigated the electropolymerization of pyrrole on

the iron substrates in the presence of a surfactant say sodium bis(2-ethylhexyl)

sulfosuccinate (AOT) in a wide pH interval. They obtained better coatings in this

procedure adopted with surfactant which was suggested that the improved protection

could be due to the entrapped AOT within the polymer matrix and also without

substrate dissolution, thus attained corrosion protection.

Lacaze and co-workers found that polypyrrole film could be electrodeposited

on an iron electrode, for anticorrosive performance. But their synthesis associated

with the metal dissolution of the substrate, whereas this metal dissolution on the

surface of the steel would hinder the electrochemical synthesis [106]. Herrasti and

his coworkers worked on the electrochemical and mechanical properties of

polypyrrole coatings on steel based on the influence of preparation methods.

They obtained a granular type of coating as a result of oligomer aggregation which

led to deactivation of the polymer and lack of crosslinking between the chains,

producing major gaps [107].

Bereket et al., [108] have studied the single layered polypyrrole coating

electrosynthesized from oxalic acid solution towards corrosion protection of mild

steel in 3.5 % NaCl solution as the electrolyte for the electrochemical

characterizations. They have also compared the improved performance of

multilayered polypyrrole/poly(5-amino-1-napthol) coatings than that of the single


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polypyrrole coating. A new explanation was given for the improved protection with

the increased immersion period, as auto-undoping property of polypyrrole coatings

in corrosive solution along with the barrier property for the bilayer coatings.

Beck et al., [86] investigated the different conditions for

electropolymerization of polypyrrole in oxalic acid based electrolytes along with an

“auto-dedoping mechanism”. In the same way in 2006 another work undergone by

Grgur et al., [109] also reported the kinetics involved in the protection of mild steel

corrosion using polypyrrole-oxalate coating in sulphuric acid solution. In these

reports they explained the role of dedoping which impart the decrease of

conductivity which in turn affects the corrosion rate in the acidic environment.

Su et al., [102] have polymerized polypyrrole and substituted

poly (N-methylpyrrole) using aqueous electrochemical polymerization method. In

this method they have adopted a combined synthesis and deposition of the coating in

a single step process. But in this way of coating the substituted polymer coating fails

to have good protection. They also estimated that a possibility for the oxalate

coatings failure always occurs at the coating/steel interface and thus least adhesive

with the metal, which makes this technique unfavorable in all corrosion

environments.

Cascalheira et al., [110] had extensively studied the electrochemical

synthesis of polypyrrole coatings on copper in salicylate aqueous solution under

galvanostatic control and potentiostatic control. They have attained the polypyrrole

coating on the copper substrate without noticeable metal dissolution and possess a

better adherence to the metal substrate. Unfortunately, adherence was lost if the


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pyrrole layer is not in its oxidized state, when the system is subjected to higher

potentials.

Polypyrrole protective coatings on magnesium alloys and on aluminium

alloys have been also investigated [111-113]. Akundy and Iroh investigated the

polypyrrole coatings on aluminium by electrodeposition using cyclic voltammetry

with oxalic acid as electrolyte [114]. They had undergone the electrochemical

synthesis with the effect of varied scan rates ranging from 750 mV/s to 50 mV/s, and

reported that the synthesis obtained at lower scan rates provided considerable

amount of polypyrrole deposition even in one cycle.

Metehan et al., [115] optimized the electrochemical polymerization

parameters of polypyrrole from aqueous electrolytes of sodium salicylate on Mg-Al

alloy (AZ91D) electrodes and studied their corrosion performance using peeling off

tests under 10 days immersion in Na2SO4. They have carried out the one step

synthesis in which both passivation combined polymerization so as to achieve a non

dissolution of the substrate.

Tuken et al., [116] studied the polypyrrole film coated on copper, modified

with ruthenium electroless deposition. In this study they have investigated the

modified coating for its synthetic parameters affected the electronic behavior, as

well as ionic mobility through the film which makes the electrochemical interaction

with the substrate totally different with the requisite for a protective performance.

Hammache et al., [117] studied the modified polypyrrole film using copper

microparticles using the cementation process. They have also estimated the

corrosion protection performance in highly corrosive solution (3 % NaCl + HCl) of


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pH 1.They worked out the polypyrrole coatings on iron from aqueous oxalic acid

solution at different current densities. Beck et al., [86] also studied the same type of

coatings from aqueous solutions of pyrrole and oxalic acid and constituted them to act

as physical barrier towards aggressive chemical reagents. But all these coatings

implying the dissolution and hence need improved thickness of the polypyrrole

coatings.

Various methods have been used for the formation of polyaniline layer on

active metals. Many reports on the electropolymerization of aniline using cyclic

voltammetry method were investigated on stainless steel, nickel, titanium, aluminium

and lead has been studied in aqueous solutions of Na2ClO4, H2C2O4 and H2SO4.

DeBerry was the first to introduce the use of electrodeposited polyaniline for

the corrosion inhibition of 410 stainless steel in 1985 [76]. Since then, most

literatures were found for the polyaniline in corrosion protection. Also Zhong et al.,

[118] investigated the passivation mechanism of doped polyaniline on 410 stainless

steel in deaerated H2SO4 solution. Even though the passivation seems to be

protective against corrosion, the localized dissolution of the passive layer if it would

accelerates the corrosion in acidic medium.

Ahmad et al., [119] have found that the chemically deposited emaraldine

base protect the stainless steel in acidic chloride environments. In the similar way

the chemical films were found to protect carbon steel in NaCl environment.

There are reports in which polyaniline was used a primer gives more corrosion

protection while combined with epoxy or polyurethane top coat rather than as single

polypyrrole top coat [120-124]. There are also few reports suggest a poor

performance for these primer/polyaniline coatings [125-127].


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The ability to depress corrosion of steel or iron had been invstigated using

the polyaniline layer electrodeposited with Pt microcrystallites has been studied by

Kulesza et al [128]. These polyaniline layers had also been investigated along with

the Cu microcrystallites using cementation process towards corrosion protection in

aggressive medium.

Sathiyanarayanan et al., [129, 130] has profound the use of polyaniline

electropolymerised on stainless steel from sulphuric acid medium and also studied

the corrosion protection performance in different corrosive medium such as

1 M H2SO4. 1 M HCl, 1 M NaCl and found the polyaniline coating as protective in

acid medium and not in sodium chloride medium.

Kilmartin and co-workers [78] grown polyaniline and

poly(o-methoxyaniline) was polymerized on stainless steel (316 and 304 stainless

steel) and monitored their corrosion performance in acidic solutions such as

0.5 M H2SO4, 0.5 M HCl. They concluded that the film shows fluctuations of

potentials produced by dissolved O2 released from oxidized polyaniline which reach

the metal and leads to an increased corrosion along with pits.

Thompson et al., [131] obtained the corrosion inhibition of mild steel coated

with polyaniline that had been exposed to saline (3.5% NaCl) and acidic solutions. It

has been proved that the level of protection provided by doped polyaniline is more

significant for dilute acids conditions than that for neutral solution. Camalet had

investigated the electrochemical deposition of polyaniline in oxalate which shows a

passivation on the stainless steel substrate during deposition [132].


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Tuken et al., [133] have investigated the polyaniline on mild steel in

acetonitrile-LiClO4 for corrosion resistance, but reported to have metal dissolution in

potential region at which monomer oxidation happened and thus requires a need of

polyaniline primer coating to attain the anticorrosion property. Genies et al., [134]

and Stejskal et al., [135] have dealt the studies with protonated polyaniline obtained

by chemical oxidation of aniline in acid media.

Moraes et al., [79] have studied the corrosion protection performance of

polyaniline electrosynthesized from phosphate buffer solution at different pH values.

They have aimed to achieve a uniform film growth with the aid of the phosphate-

based buffer solution for passivating steels. This method of corrosion prevention is

carried out by many researchers. But the drawback, associated with the building up

of phosphate layer and passivation of steel is modeled with the formation of iron

oxalate layers as evidenced in mild steels [136] and this may hinder the formation of

polymer deposition.

Thus the polyaniline coatings were found to exhibit protective performance

against corrosion. But the most encountered problem in this type of conducting

polymer coating is the poor mechanical properties, which constitutes obstacle to

their processability. Syntheses of conducting polymers were shown to be effective

when they are obtained as copolymer blends, composites, bilayer or multilayer to

compensate the deficiencies of individual coatings such as poor mechanical and

physical properties.

Wessling [137] indicated that the protection brought by polyaniline depends

on the underlying passive layer that it stabilizes through a „quasipotentiostatic‟

effect. The passivation of stainless steel is achieved by partial coating of the


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electrode surface with PANI. It is considered that the polymer film passivates the SS

by simply holding the potential in the passive region, forming a Cr-enriched passive

film.

Kraljic et al., [138] obtained different results for protection behavior of

polyaniline against corrosion of 420 stainless steel in two different acidic solutions,

say, sulphuric acid and phosphoric acid. Andrea Kalendova et al., [139] synthesized

polyaniline by reaction in aqueous solution of ammonium peroxidisulfate and

phosphoric acid, and they also synthesized polyaniline epoxy-ester coatings with

different volume % of polyaniline as inhibitors on steel. They claimed this method

as propitious with no heavy metals harmful to the environment

Ganash et al., [140] discussed electrodeposited polyaniline films on stainless

steel made from two different acidic solution say, sulphuric acid and phosphoric acid

containing the aniline monomer. The type of counter anions was shown to

significantly affect the polymerization reaction and the formation of an underlying

oxide layer. They obtained as polyaniline sulphate layers are more protective.

Since polyaniline layers are porous, it was suggested that the protection

behavior depends on the quality of the induced oxide film and the amount of

polyamine used that determines the thickness of the coatings. Even though vast

works were done using polyaniline for anticorrosive applications these coatings lack

in adherence to the metal which would expedite corrosion.

Many have reported on the electrochemical synthesis of thiophene on metals

either as a single coating or as polymer matrix. Thiophenes have been extensively

used for varied applications due to its electronic, optical and corrosion protective


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properties [141]. Among the different synthesis methods such as chemical oxidative

couplings of thiophenes [142], cross couplings of Grignard reagent of

dihalothiophenes [143] and electrochemical polymerization, the synthesis of

thiophene using electrochemical polymerization method stands more better

[144, 145].

The electrodeposition provided a homogenous and compact coatings. But as

an individual layer, the electrodeposition needs higher oxidation potential which

may remain as a flaw for some metal substrates during synthesis. So, a suitable

electrolyte is necessary to apply polythiophene coating for anticorrosive

applications. Reyman et al., [146] studied the ultrasonication assisted

electrodeposition of polythiophene in an acetonitrile solution containing lithium per

chlorate as background electrolyte, resulted a good yield of polythiophene.

The derivative of polythiophene, poly (3-alkyl thiophenes) is reported against

corrosion protection of 1018 carbon steel. The high oxidation potential of these

polymers is an obstacle and makes their utilization complicated. P3HT was efficient as

compared to P3OT in the sulphuric acid environment [147]. Wang et al., [148] studied

the corrosion protection of iron and steel by ultrathin poly(methylthiophene) film has

been investigated by electrochemical impedance spectroscopy.

Kousik et al., [149] have carried out the in situ electropolymerization of

polythiophene coatings on mild steel using acetonitrile as a medium. They claim that

the polythiophene coating protects against corrosion in acidic environment by means

of passivation. Also they have evaluated the stability of the coatings by determining

the amount of delamination while exposed to the electrolyte using the AC

impedance data.


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Tuken et al., [100] have keenly investigated the anticorrosive performance of

polythiophene coatings in combination with polypyrrole coating on mild steel and

copper. They observed that the polypyrrole coating acted as a primer on the

polythiophene coated substrates and the corrosion behavior of PPy/PTh-coated

copper and mild steel were investigated in 3.5 % NaCl solution. Moreover the

0.15 M LiClO4 containing acetonitrile medium provides a good

electropolymerization without metal dissolution. In further studies on polythiophene

coatings on nickel coated mild steel substrate, the barrier protection improved for the

substrate in 3.5 % NaCl environment.

Ocampo et al., [150] compared the resistance against marine corrosion of

several paints before and after being modified by adding a conducting polymer

derived from polythiophene. The selected paints, which were applied to the naval

steel St F111, are primers specially indicated for protection in marine environments.

And the physical properties of both the unmodified and the modified paints were

characterized using viscosity measurements and mechanical properties evaluations.

Results show that the addition of a low concentration of the conducting polymer

greatly improves the performance of the epoxy-based resin.

In general, considerably high anodic potentials are applied during

electrochemical synthesis for most homopolymer and this may leads to poorly adhered

and mostly degraded polymer film product. Among the above said heteropolymers,

furans have not been often used for anticorrosive performance due to its higher

oxidation potential which would lead to the dissolution of the metal or alloy used as

substrate and the irreversible oxidation results in poor quality films [151].


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Armelin et al., [152] compared the protection against corrosion imparted by

different conducting polymers when these materials are used as anticorrosive

additives in the formulation of conventional epoxy paints. In contrast, the use of

conducting polymers composites also reduces the efficacy of the coatings. Hence

selection of appropriate conducting polymers in epoxy paint formulation is still a

long way process. Also several attempts had been practiced using organosiloxanes

as adhesion primer layers on metal substrates. But these organic silanes are not easy

to handle and such surface treatments take time and might constitute a severe

drawback for anticorrosion coatings.

Moreover all these attempts with homopolymers provides a moderate

protection, put forth the need for a flaw free copolymer coating in any form against

corrosion to provide a combined effect towards the metal to be protected.

2.2 COPOLYMERS

In the topical years, copolymers replace homopolymers and are widely

investigated for anticorrosive applications. The copolymers proved to be effective

agents in protecting metallic materials against corrosion in corrosive environments

[153-156]. The electrochemical synthesis of copolymer coatings such as polyaniline

and polypyrrole on mild steel electrode has attracted much interest in recent year.

It is most expected that copolymers or bilayer of conducting polymers to exhibit

combined properties of each polymer [157].

Gopi et al., [158] have recently studied the corrosion protection performance

on low nickel stainless steel using the methacrylate based organic copolymer

coatings. These coatings were obtained using free radical solution copolymerization


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such as Poly(N-vinyl carbazole-co-glycidyl methacrylate) have been synthesized by

free radical solution polymerization technique from different mole ratios of N-vinyl

carbazole (N-Vc) and glycidyl methacrylate (GMA).

Tan et al., [159] have investigated the copolymer as well as bilayer coating

of the conducting polymers like polypyrrole and polyaniline towards the

anticorrosive performance of 304 type stainless steel and carbon steel in aqueous

sodium chloride solutions. Unur et al., [160] studied imparting new chemical and

physical properties to conducting polymer by adding an appropriate functional group

to a conventional polymer using a technique called living polymerization.

They synthesized thiophene capped polytetrahydrofuran conducting copolymers.

Yagan et al., [161] have studied the inhibition performance of mild steel

using copolymer coatings obtained from polyaniline and polypyrrole on mild

steel.They have reported that the bilayer coating acts as stable host matrix for that

mild steel substrate. They have reported the mechanism of corrosion inhibition is

obtained by the formation of iron oxalate complex at the metal/polymer interface.

This is notable that these phenomenons deal with the initial dissolution of the metal

during electrochemical synthesis [162-166].

Pritee Pawar et al., [162] have investigated the synthesis of poly(aniline-co-

o-toluidine) coatings and their corrosion protection performance on low carbon steel

using electrocopolymerisation using aniline and o-toluidine with sodium tartarate as

the supporting electrolyte. They have ascertained the formation of copolymer with

the mixture of monomers in the aqueous sodium tartarate solution and did a critical

comparison of the results of individual monomers with that of the results of

copolymers in 3 % NaCl solution by electrochemical characterizations.


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Bereket et al., [163] reported the electrochemical synthesis of poly(aniline-

co-2-anisidine) films on stainless steel in a tetrabutylammonium perchlorate/

acetonitrile solution containing perchloric acid. They investigated the corrosion

properties of these copolymer coatings in a 0.5M hydrochloric acid (HCl) solution

and they found that PANI, poly(2-anisidine), and poly(aniline-co-2-anisidine) films

have a corrosion-protection effect for 304 stainless steel in an aggressive medium of

a 0.5 M HCl solution. However, the durability of the poly(aniline-2-anisidine) films

is limited to 3 h.

Ozyilmaz et al., [164, 165] investigated the anticorrosion performance of

poly(aniline-co-o-anisidine) copolymer coatings on mild steel , found to behave as a

significant physical barrier against the attack of mild steel corrosion. Along with his

coworkers, he also reported on the polyaniline coating electrochemically synthesized

on copper from a neutral oxalate solution and its corrosion protection performance.

Jui-Ming Yeh et al., [166] have investigated enhancement of corrosion

protection using clay nanocomposite materials using poly(o-ethoxyaniline) via the

formation of poly(o-ethoxyaniline)-clay nanocomposite material coating for flame

retardant purposes and thermal behaviors, which had been early initiated by Usuki

and his Toyota‟s research group [167].

Radhakrishnan et al., [168] have studied the polyaniline-nano-TiO2

composite coatings for smart corrosion resistance performance in order to attain both

physical and barrier effectiveness towards anticorrosion behavior in NaCl solution

as an electrolyte. They have reported these films to act as sacrificial electrode in the

corrosive environment and protect the underlying substrate. Many others have also

reported on the anticorrosive performance of PANI-TiO2 composites on steel


52

corrosion. These literatures immensely need filler or pigment materials to achieve a

complete protection against corrosion.

Hur et al., [169] have studied the anti-corrosive properties of the

homopolymer polyaniline, poly(2-toluidine) and the copolymer poly(aniline-co-2-

toluidine) coatings on 304-stainless steel in HCl medium. They have investigated the

processable form of polyaniline obtained from the modification of the polymer

backbone through the introduction of various functional groups such as alkoxy,

amino alkyl, aryl and sulfonyl groups.

Even then these highly conductive forms many conducting polymers

frequently present low solubility and are infusible, the ability of processing,

electrically conductive polymers into thin films, controlling film thickness and

creating uniform pin-hole free films is often extremely challenging and important.

Thus the role conductive polymers coating is to prevent the access of corrosive

species to substrate and to reduce the corrosion rate.

There were also reports evidenced for the synthesis of conducting

copolymers of thiophene capped poly(ethylene oxide) with pyrrole and thiophene in

water-sodium dodecyl sulphate and acetonitrile tetrafluoroborate solvent electrolyte

couples using constant potential electrolyses [170]. Unur et al., [160] reported the

thiophene capped polytetrahydrofuran conducting copolymer synthesis in the same

electrolytes as Yildiz. Also, Kabasakaloglu elucidated the homopolymerization of

furan and thiophene and studied their structure from biopolymer films, yet they are

not studied for their corrosion performance [151].


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Moreover conducting polymers when used as free standing films on metal

substrates for the anticorrosive performances, the mechanical properties and

degradation temperatures were reduced [171]. These conducting polymers have

more technological potential when they are coated in the form of composites or

multiple layers or bilayers for corrosion protection using rare earth oxide coatings to

attain improved mechanical strength.

2.3 BILAYER OR MULTILAYER COATINGS

Low nickel stainless steel finds a wide variety of applications in oil and gas

pipe industries where there are need to protect the pipelines made of low nickel

stainless steel from the acidic environments. Coatings are commonly believed to

protect metal surfaces from corrosion based on some combination of their barrier

properties and electrochemical properties [172]. In many studies on copolymer

coatings, mechanical properties are still a question which arise a need for any other

metal oxide coatings such as ceria to attain improved the mechanical strength of the

organic polymer coating. The discussed reports evidence the importance of bilayer

coatings in many corrosive environments.

Rovshan Hasanov and Semra Bilgic [173] have studied the monolayer and

bilayer coatings of polypyrrole and polyaniline in oxalic acid solution for the

anticorrosive performance on steel. They carried out all the electrochemical

charecterizations in 1 M sulphuric acid medium in which the bilayer coatings were

found to have better corrosion performance than that of the performance of the

individual polypyrrole and polyaniline coatings on steel.


54

Kowalski et al., [157] investigated the corrosion resistance of carbon steel

using a bi-layered polypyrrole film, whereas the inner layer was doped with the

Keggin structure anions for stabilizations of the passive oxide film at the metal-

polymer interface and the outer layer with organic anions. Thus they performed the

corrosion protection with the aid of the oxidized state of the polymer in 3.5 wt. %

NaCl aqueous solution. Gomes and Oliveira have studied the corrosion protection

performance of multilayer coating obtained using polyaniline and poly(vinylsulfonic

acid, sodium salt) using layer-by-layer technique in the chloride containing

environment [174].

Recently, Zeybek et al., [175] in his report evidenced the importance of

bilayer coatings of poly(N-methylaniline) and polypyrrole-dodecylsulfate using

electropolymerization using the potentiodynamic method on mild steel. They have

revealed that the bilayer coating of poly(N-methylaniline)/polypyrrole-

dodecylsulfate on mild steel using aqueous oxalic acid solution show corrosion

protection even at immersion times.

Yano et al., [176] has studied the importance of bilayer coatings in order to

overcome the drawbacks of the individual coatings. They have studied the

poly(2-N-phenylamino-4,6-dimercapto-S-triazine) layer prepared electrochemically

first on iron surface and further coated with a polyaniline coating to achieve a better

a corrosion protection as the individual polyaniline film needs more effort to

enhance the ability for practical uses.

Several studies have reported that conducting polymer prevent metal

corrosion through their interaction with the metal on which they are coated and thus

stays adherent for an increased period of time. Basically electrochemical properties


55

are commonly associated with inhibitors chemistry only, but should more broadly be

considered as related to all components of coatings also. As substrate plays a main

role in the corrosion related mechanisms, a review on collection of literatures dealt

with stainless steel is required.

Ananda kumar et al., [177] have investigated the corrosion resistant behavior

of electrochemically synthesized polyaniline-nickel, nickel-polyaniline bilayer

coatings in NaCl solution. Among which, the metal-polyaniline coatings is reported

to have least corrosion resistance. But both the coatings obtained electrochemically

are seems to be homogeneous and uniform in morphology.

Wu et al., [178] have investigated the influence of bilayer period and

thickness ratio on the mechanical and tribological properties of CrSiN/TiAlN

multilayer coatings using magnetron sputtering system. But these coatings have not

tried for corrosion preventive measures. Cerium has a considerable interest as a

promising agent against corrosion [179-181] which is nevertheless studied as an

environmentally friendly alternative as it acts as mixed type inhibitor reducing both

cathodic and slightly anodic activities. But, the porous and „mud‟ cracking nature of

the ceria film investigated by Goh et al., [182] provoked the modification of ceria

film using conducting copolymer coating since they have high conductivity and

good thermal stability rather than a homopolymer [183].

Goh et al., has also established crystalline CeO2 films grown in aqueous

solutions on glass slides. They pretty much tried for thin coatings of CeO2 in order

to achieve minimal mud crack pattern and also reasoned the crack which happens

under tensile stress as due to the grain coalescence during the film growth.


56

Shankar Rao et al., [184] reported improvement in the electrochemical

corrosion behavior of 304 stainless steel in 3 wt. % Ce(NO3)3.6H2O solution where

the role of Ce ions is considered to impart the effect of temperature and electrode

potential on the formation of Ce-oxide.

Arurault et al., [185] reported on the formation of ceria coating on ferritic

stainless steel by an electrochemical process in a cerous nitrate solution. They have

given an elaborated study for the achievement of adherent ceria coating. But the

coatings exhibits mere cracks which may prone to more corrosion as well as

deterioration of the coating while introduced in the aqueous acidic environment.

They emphasized on evaluating immersion, time, current density and temperature

for reproducibility of ceria coating formation. Also with an increase of temperature,

there is an increase of active sites leads to deposition of the polarized Ce ions, which

may also render chance for the active sites to promote corrosion.

Ranjan Sen et al., [186] and Amadeh et al., [187] has studied the

co-electrodeposition of the nanosized ceria particle in nickel material using

surfactant like sodium lauryl sulphate (SLS) has been added in the electrolyte.

But this method of coating using surfactants often ended up with agglomeration that

decreases the hardness of the coated matrix. Also Virtanen et al., Breslin et al., and

Lu et al., had proposed an inhibition mechanism for the reaction of oxygen

reduction-a cathodic depolarization reaction during corrosion in NaCl solution

[188-190]. Also, Lu et al., have investigated the effect of cerium during surface

treatment of stainless steel is ascribed to the inhibition of the rate of reactions that

proceeds during the corrosion process [191,192].


57

Hassan et al., [193] also dealt the magnesium coating assisted cerium

treatment on steel substrates and studied the corrosion behavior of that system in

0.05M sodium chloride solution. They have reported that the improved corrosion

protection behavior was obtained for the magnesium treated cerium coating which

could be due to the attribution of cerium oxides and hydroxides that would have

merely provided a physical barrier performance.

Stoyanova also studied the important role of cerium oxides electrodeposited

on stainless steel in solutions of 0.1 M HNO3 and 0.1 M H2SO4. They ascertained

their investigations with a pronounced shift towards the positive direction which is

the protective zone of steel against corrosion. These studies also dealt on the self

passivation in the nitric acid medium [194].

They have also formed CeO2-Ce2O3 oxide layer on OC 404 stainless steel.

They have studied the modification of steel surface by cerium oxide films deposition

and thus the improved corrosion protection behavior of steel 0.1 N H2SO4, in which

a linear dependence between the stationary corrosion potential of the

CeO2/Ce2O3/SS system and cerium concentration was established. They have also

studied the modification of surface with enriched oxides of chromium and

aluminium.

Creus et al., [195] obtained thin cerium oxide coatings elaborated by

cathodic electrolytic deposition on mild steel using Ce(III) Chloride aqueous and

mixed water, ethyl alcohol solution with and without hydrogen peroxide. In this

study they involve an external agent (hydrogen peroxide) to increase the deposition

rate. They held up with non-reproducibility of the morphology and composition of

thin films which also associated with chloride ions during the deposition.


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However large defects or cracks induce an accelerated local degradation of the steel

substrates removing partially the oxide deposits.

Zhitomirsky and Petric [196] in their research obtained CeO2 films on nickel

substrates via cathodic electrolytic and electrophoretic deposition and compared the

influence of deposition parameters as well as additives on deposition yield and

morphologies. They report that the cathodically electrodeposited coating is of

crystalline nature the deposition rate rate under control up to 300 micrometers.

Even though preferable depositions of fine particles were achieved and adherence

Zhitomirsky ended up with cracking problem in the coating along with the increased

thickness of the ceria coating.

Avramova et al., [187] found a new electrochemical method using a

non-aqueous electrolyte for depositions of ceria-alumina composite coatings on

stainless steel substrates. They have also tried with the solution of absolute univalent

alcohol with different content of alumina and cerium salts to obtain a dense and

crack free coatings but this method involves annealing at higher temperatures.

Stefanov et al., [198] used cathodic electrodeposition to produce CeO2-AlO3

films on stainless steel. The mixed oxide was obtained by successive deposition of

Al2O3and CeO2 on a stainless steel substrate, followed by annealing at elevated

temperatures.

Nikolova et al., [199] investigated the changes in the corrosion behavior of

OC4004 stainless steel with an electrochemically deposited Ce2O3-CeO2 film.

They dealt with the additional circumstance affecting the corrosion resistance of

stainless steels unfavorably, in catalytically active systems. When reduction process


59

takes place on the working surfaces of the converter of these systems, in which the

natural protective passive film on stainless steel may be reduced and thus the

corrosion resistance of stainless steel in the desired catalytic process would also

decrease.

Also Johnson et al., [200] have demonstrated the effect of glycerol as an

additive in its coating solution to obtain an enhanced corrosion resistance of the

coatings on aluminum alloy due to the formation of fine particle size during the

coating accompanies the increase of pH near the surface to be coated.

Kobayashi et al., [201] underwent the effect of SO42-

on the corrosion

behavior of cerium-based conversion coatings on galvanized steel. They found that

SO4-2

acts as a grain refiner as well as growth inhibitor, thus enhancing the corrosion

resistance.

Hamlaoui et al., [202] investigated on the electrodeposition of cerium oxide

based films on carbon steel and of the induced formation of carbonated green rusts

during cathodic electrodeposition from a relatively concentrated solution and also

suggest a precipitation mechanism in dominance that ended up with green rust.

Hamlaoui et al., [203] in his another study, revealed the cathodic

electrodeposition of cerium-based oxide on carbon steels from concentrated cerium

nitrate solutions under the effects of varied parameters such as concentration,

temperature, pH and additives to improve the behavior of the film against corrosion,

depending upon the applied current density during deposition. Even then, significant

network of cracks are found along with needle-like morphology [204, 205].


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Breslin et al., [189] have studied the influence of Ceria towards anticorrosive

performance on stainless steels. The advantage of this approach lies in the fact that

the natural physical and mechanical properties of the material are retained, while the

corrosion resistance is increased. These investigations prompted us to step up the

research for performing ceria coating on low nickel stainless steel and to investigate

all of its properties in each aspects such as hardness adherence, corrosion resistive

performance etc.

Switzer studied the preparation of ceria coatings on stainless steel and also

concluded in particular that the grain size varied from 6 to 16 nm with increased

bath temperature. Aruna et al., has undergone synthesis of electrodeposited Ni/ceria

nanocomposite coatings in the form of metal matrix coatings of nickel for the

improved performance of lubrication wear properties along with microhardness,

corrosion resistance. They convey that their results focused an enhanced

performance than that of simple coatings [206, 207].

Hughes et al., [208] along with his co-workers in his many studies underwent

the coating of alloys along with varied thermal treatments ranging between 90˚C and

100˚C followed by subsequent anodization. But the only benefit obtained is a

non-toxicity of ceria on the substrate rather than that of chromium treatments [209].

Subsequently Dabala et al., [210] studied treatments of alloys after immersion of

substrates in H2O2 solutions with pH adjustments, where they got ceria coatings that

possessed dried mud like pattern.

Thus the cracked morphology as evidenced by Goh and Creus, thus pave a

need for inventing a protective second layer that flourishes as a bilayer coating


61

illustrated by many researches and made us to step up with the ceria/copolymer

bilayer coatings to protect our low nickel stainless steel.

Thus the above evidenced literatures portrayed each method and their

characteristics features towards their anticorrosive application.

While considering the protection mechanisms, they are based on three actions

ƒ Displacement of the electrochemical interface

ƒ Ennobling and self-healing effects

ƒ Barrier effects offering non specific protection.

Among the three, displacement and barrier effects are ineffective when a

defect appears in a coating and the ennobling effect fails for larger defects in

coating even though there could be excellent properties. Furthermore, delamination

of the coating upon reduction of the conductive polymer as well as the subsequent

incorporation of water and electrolyte within the coating will cause failure.

Thus it is clear that a coating must be adhesive for anticorrosion purpose so

as to avoid O2 and electrolyte diffusion. Several strategies have been used to

improve the adhesion of copolymer on to the metal substrate. Hence bilayer coatings

composed of two types electrochemically deposited coatings have been observed to

provide better adherence by enhancing the adherence of the first layer to the metal

and fulfills the scope of our study with Ceria/Copolymer bilayer coating in sulfuric

acid environment.

chapter ii review of literatures -...

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