Conducting Polymers for Corrosion Protection a Review

Conducting polymers for corrosion protection: a review

Pravin P. Deshpande, Niteen G. Jadhav,

Victoria J. Gelling, Dimitra Sazou

American Coatings Association 2014

Abstract Conducting polymers (CPs) such as polyan-iline (PANI), polypyrrole (PPy), and polythiophene(PTh) are used for the corrosion protection of metalsand metal alloys. Several groups have reported diverseviews about the corrosion protection by CPs and hencevarious mechanisms have been suggested to explainanticorrosion properties of CPs. These include anodicprotection, controlled inhibitor release as well asbarrier protection mechanisms. Different approacheshave been developed for the use of CPs in protectivecoatings (dopants, composites, blends). A judiciouschoice of synthesis parameters leads to an improve-ment in the anticorrosion properties of the coatingsprepared by CPs for metals and their alloys. Thisarticle is prepared as a review of the application of CPsfor corrosion protection of metal alloys.

Keywords Conducting polymers, Pigments,Corrosion protection, Surface coating technology

Introduction

Metals and their alloys are thermodynamically favoredto undergo corrosion events. Corrosion can be definedas the destruction or deterioration of a materialbecause of reaction with its environment.1 The inhibi-tion of corrosion of metals is a subject of theoreticalresearch but, more importantly, is of practical interestto multiple industries. The most common corrosionprevention technique is, perhaps, the application ofpaint or an organic coating on a metal substrate.Ideally, organic coatings would provide long-lastingcorrosion protection. However, this property dependson the coatings ability to protect an exposed metalsurface when defects appear during their servicelifetime. In recent years, the design and developmentof alternative organic coatings with self-healing abili-ties [i.e., shape memory materials and intrinsicallyconductive polymers (CPs)] have been considereduseful materials in the protection of metals againstcorrosion.214

The interest in CPs stems from the possibility offormulating CP-based smart coatings, which canprevent metallic corrosion even in defect areas wherebare metal surface is exposed to the corrosive envi-ronment. CPs may exist in different states (oxidation-conductive state/reduction-nonconductive state) andcan easily interchange between them under appropri-ate conditions. The CPs undergo redox processes andthereby allow for the binding and expelling counter-ions (dopants) in response to the variation of the metalsurface potential triggered by local electrochemicalreactions due to the corrosion. The dopants, which maybe inserted or expelled by the CP depending on thelocal corrosive conditions, are often inhibitors prevent-ing the local corrosion process upon release.15,16 This isone of the strategies that is suggested in exploiting theadvantages provided when a CP is used as a keyconstituent of a corrosion-resistant coating.

P. P. Deshpande (&)Department of Metallurgy and Materials Science, College ofEngineering, Shivajinagar, Pune 411005 MH, Indiae-mail: [email protected]

N. G. Jadhav, V. J. GellingDepartment of Coatings and Polymeric Materials, NorthDakota State University, 1735 NDSU Research Park Drive,NDSU Dept #2760, P.O. Box 6050, Fargo, ND 58108-6050,USA

N. G. Jadhave-mail: [email protected]; [email protected]

V. J. Gellinge-mail: [email protected]

D. SazouDepartment of Chemistry, Aristotle University ofThessaloniki, 54 124 Thessaloniki, Greecee-mail: [email protected]

J. Coat. Technol. Res., 11 (4) 473494, 2014

DOI 10.1007/s11998-014-9586-7

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The use of CPs for the corrosion protection of metalshas attracted great interest over the last 30 years.DeBerry2 has indicated the Polyaniline (PANI) inducedstabilization of the passive state for 400 series stainlesssteels in sulfuric acid solutions since 1985. The keyfeature of this process is that CPs are able to maintain thesurface potential of the substrate into the passive statewhere a protective oxide film is formed on the metalsubstrates. This is due to the fact that CPs-based coatingsare pinhole and defect tolerant in a manner similar tothat of the coatings based on the environmentallyhazardous hexavalent chromium. This is explained bythe fact that oxygen reduction within the CP layerreplenishes the CP charge consumed by metal oxidation.Along with the switching of the CP into the oxidationstate, the potential turns into the passive state of themetal preventing the corrosion process.

Despite the extended work devoted to the anticorro-sive properties of CPs there are still many issues to beresolved for CP-based coatings to fulfill physico-electro-chemical and mechanical requirements of high-perfor-mance anticorrosive coatings under widely varyingpractical conditions. Limitations of CPs as anticorrosivecoatings include: (i) irreversible consumption of thecharge stored in the CP which is capable of oxidizing thebase metal and resulting in the formation of a passiveoxide layer, (ii) porous structure and poor barrier effect,(iii) anion-exchange properties, (iv) poor adhesion tothe metal substrate. All the above-mentioned disadvan-tages are especially magnified under severe corrosionconditions as in the presence of chlorides, which mayreach the metal-substrate surface either due to the CP-layer permeability or to its anion-exchange properties ifchlorides replace CP doping anions. Then, chlorides mayinduce extended localized corrosion and the storedcharge might be irreversibly consumed during the redoxreactions of the metal |metal oxide |CP system.

One of the efficient strategies to eliminate the abovedisadvantages is to consider CP-based composite systemsusually comprising a CP in which different inorganicfillers such as metal oxides have been encapsulated. TheseCP-based composite materials combine the redox prop-erties and hence the self-healing feature of CP withqualities of inorganic materials. Thus, CP-based compos-ites have shown better mechanical and physicochemicalproperties improving the barrier effect, adhesion andperhaps hydrophobicity. The more these properties areimproved the better the metal is protected againstcorrosion. Furthermore, the design and development ofCP-based coating systems with commercial viability isexpected to be advanced by applying nanotechnology,17

which has received substantial attention recently. Nano-composite CP-based coatings seem to combine moreefficiently the properties of CPs and organic polymers tothat of inorganic materials.18 This paper reviews theapplication of CP-based coatings for corrosion protectionof metals and alloys emphasized in recent advances.

Corrosion basics

Corrosion is mostly caused by an electrochemicalinhomogeneity in metal or its environment. When incontact with an electrolyte, metals corrode when areasof higher free energy or higher potential behave asanodes and those of lower free energy or lowerpotential behave as cathodes, thereby creating acorrosion cell. Metal ions are formed at the anodeand dissolve into the electrolyte. The electrons passthrough the metal to the adjacent cathode areas wherethey react with the environment. This flow of electronsfrom the anode to cathode and the associated chargetransfer through the electrolyte from the cathode tothe anode constitute the corrosion current. The corro-sion rate is, therefore, associated with the corrosioncurrent. The process of corrosion can be representedby the following reactions1,19:

(1) Anodic reaction The electrochemical reaction atthe anode or metal dissolution can be written as

M Mn ne:

The released electrons migrate to the cathode throughthe metal producing corrosion current.

(2) Cathodic reaction The nature of the reaction atthe cathode (in which the electrons released inanodic dissolution are consumed) depends uponthe nature of the environment. The most commoncathodic reactions that are encountered in corro-sion are as follows:

(A) Oxygen reduction (Acid solution)

O2 4H 4e 2H2O:

(B) Oxygen reduction (Neutral or basic solution)

O2 H2O 2e 2 OH :

(C) Hydrogen evolution

2H 2e H2:

(D) Metal ion reduction Metal ions present in solu-tion may be reduced:

Mn e Mn1:

This can occur only if there is a high concentration ofMn+ ions. In this reaction, the metal ion decreases itsvalence state by accepting an electron.


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(E) Metal deposition

Metal may be reduced from an ionic to a neutralmetallic state:

Mn e M:

Hydrogen evolution is common since acidic mediaare frequently encountered. Oxygen reduction is alsocommon since any aqueous solution in contact with airis capable of promoting this reaction. Metal ionreduction and metal deposition are less commonreactions. These partial reactions can be used tounderstand most of the corrosion processes; for exam-ple, corrosion of plain carbon steel in water (Fig. 1).

The corrosion occurs in two steps. In the first stage,regions covered by a water droplet are precluded fromsupply of oxygen and become anodic compared to theother areas that are freely exposed to air. Thus, plaincarbon steel, in contact with any electrolyte, low indissolved oxygen will be anodic with respect to plaincarbon steel, in contact with electrolyte, rich indissolved oxygen. A principal reason for this is thatin an area with water rich in oxygen the followingreaction will take place:

O2 2H2O 4e 4 OH :This is a cathodic reaction in the sense that it absorbselectrons. There will be a greater amount of oxygenavailable near the edge of the water droplet and thisarea will become cathodic with hydroxyl ions that areformed. The electrons will be supplied from anotherpart of metal where there is less oxygen and so suchareas act as anodes. At the anodic area under thecenter of the water droplet electrons will be generatedby the reaction:

Fe Fe2 2e:

The Fe2+ cations will move through the electrolytetoward the cathode and the (OH) anions will tend tomove toward the anode. Migration of the oxidativeproduct (Fe2+) from the anode and the reduction

product (OH) from the cathode occurs until theycombine to form ferrous hydroxide.

2Fe 2H2O O2 2Fe2 4 OH 2Fe OH 2:

Ferrous hydroxide (Fe(OH)2) precipitates, due to itsinsolubility, from the oxygenated aqueous solution andsettles as deposit near the cathode area. In the secondstage, at the outer surface of the Fe(OH)2 layer, accessto dissolved oxygen converts the ferrous hydroxide toinsoluble and the hydrous ferric hydroxide as follows:

2Fe OH 2 1/2 O2 2H2O 2Fe OH 3:

Reddish brown ferric hydroxide, Fe(OH)3, is knownnormally as red rust which forms in the initial stages ofcorrosion and gradually changes to ferric oxy-hydrox-ide, FeOOH. Different forms of ferric oxy-hydroxidesare also termed as red rust. When Fe(OH)3 is dehy-drated it becomes ferric oxide:

2Fe OH 3 Fe2O3 3H2O:

In the case of limited oxygen supply, this reactionresults in the formation of black magnetite (Fe3O4).Once a rust deposit has formed on the steel surface, thearea under the porous deposit will become oxygendeficient and, hence, anodic compared to the baresteel. In addition, the rust layers are not protectivebecause they are permeable to air and water. There-fore, the dissolution of steel can continue, unobserved,below the layer of rust.

Thermodynamics of corrosion

Metals have a propensity to revert to their lowerenergy forms (metal oxides). This thermodynamictendency is responsible for corrosion of metals. Theextent to which a metal corrodes depends not onlyupon its electrode potential but also upon pH of theaqueous solution. The combined effect of the potentialand pH of the aqueous medium on the products ofcorrosion has been well summarized in Pourbaixdiagrams.1 Figure 2 shows a schematic Pourbaix dia-gram for iron.

As shown in the Pourbaix diagram (Fig. 2), iron and,therefore, steel (say at point A) can be protected fromaqueous corrosion by

(1) Use of inhibitors The electrolyte can be mademore alkaline by increasing its pH by addinginhibitors (from point A to 1),

(2) Anodic protection Changing the potential ofmetal in positive direction by applying externalvoltage (from point A to 2),

(3) Cathodic protection By applying potential innegative direction, for instance using zinc coatingon steel substrate (from point A to 3).

Corrosion current Air Water droplet

OH OHOH OHFe++

Fe++Fe++

RustRust

e eAnode

SteelCathode Cathode

Fig. 1: Electrochemical corrosion showing rusting of plaincarbon steel


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In addition to essentially pure zinc, as in theconventional electrodeposition or hot-dip galvanizing,the zinc-based coatings may consist of different zinc-based alloys with the aim of improving subsequentpaintability while conserving the capacity to impartgalvanic protection and, thereby, providing corrosionresistance. These alloys include galfan (Zn withapproximately 5% Al) and galvanneal (Zn withapproximately 10% Fe). However, the corrosion rateof coated zinc increases rapidly in humid air. Varioussurface treatments have been developed to improve itscorrosion resistance such as chromate conversioncoating, phosphate conversion coating and organicpainting, etc. For example, chromate conversion coat-ing is widely used because of its superior corrosionresistance, good paint adhesion, low cost, and a simpleapplication process. Chromate conversion coating isgenerally performed by immersing the zinc-coatedspecimens in a chromic acid solution, which leads tothe formation of a passive layer containing zinc oxideswith mixed trivalent and hexavalent chromium oxides.Unfortunately, the hexavalent chromium species pres-ent in the chromate solution and layer are toxic andcarcinogenic.

Coatings basics

While the variety in pretreatments and coating forma-tion is nearly limitless, examples of common tech-niques employed in corrosion control for steelsubstrates by paint coatings are detailed below.1921

Zinc-rich coatings

Zinc-rich coatings contain zinc particles and, henceprotect the steel by a sacrificial mechanism or by

cathodic protection. To have sufficient electrical con-tact with the steel, the coating must contain 8595% w/w of zinc. In addition, the resulting zinc corrosionproducts, which preferentially form upon exposure tothe environment, tend to block the pores in the coatingand improve the barrier properties of the coating.Therefore, there are two modes of corrosion protectionvia this technique: sacrificial followed by increasedbarrier properties as the zinc is consumed.

Impervious or barrier coatings

Impermeable barrier coatings tend to reduce bothwater and oxygen permeation to a sufficient extent sothat corrosion is precluded. The prevention of oxygenaccess to the metal stops the cathode reaction andcurrent transfer between the anodic and cathodic areasis prevented due to ionic transfer resistance.

Inhibitive coatings

Inhibitive coatings, generally used as primers, areobtained by adding pigments that release corrosion-inhibiting substances. The pigments react with themoisture absorbed in the coating and passivate thesteel. However, all effective inhibitors such as stron-tium chromates have harmful effects on the environ-ment. As the release of inhibitors is based on leaching,coatings need to be highly pigmented in order to insuresufficient leaching. Toxic inhibitors, therefore, areconstantly released into the environment even whenthey are not needed.

To sum up, paint coatings are essential in one way oranother to protect steel products from corrosion andthere is a need to replace conventional paint contentsby environmental friendly and nontoxic formulations.Recently, the use of conducting polymers has shownthis promise.4,6,7,9,10 The environmental friendly natureof conducting polymers along with their stability andredox properties makes them promising coating mate-rials for the corrosion protection of metal alloys.

Conducting polymers in coatings

Various CPs are available in commercial forms. Theseinclude PANI, polypyrrole (PPy), polythiophene (PTh)and a few more, such as polycarbazole.2224 Chemicalor electrochemical oxidation methods are used for thesynthesis of CPs.2529 Since the 1980s, CPs have foundtheir applications in the field of corrosion protectionfor metals and metal alloys. Their ease of synthesisboth via chemical and electrochemical methods cou-pled with electrical conductivity makes them evenmore attractive as anticorrosive materials.7,8,12

Several mechanisms of interaction are possible for theperformance of CPs on a variety of substrates.3,10,15,30

0 7pH

Pote

ntia

l, V

Immunity3

1

0

1

2

Corrosion

Corrosion

Passivity1

2

A

14

Fig. 2: Schematic Pourbaix diagram for iron or steel


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These mechanisms mainly depend on the type ofsubstrate used, use of specific conducting polymer, itschemical composition, physical properties, amount andtype of dopant used, amount of CP used, and its relativeproportion with other active ingredients in the coating.Surface preparation also has a large effect on thefunctional properties of the CPs. This becomes moreimportant when there are issues related to adhesion andgrowth of possible corrosion products underneath thecoating. The exposure environment and pH of local areaalso have a strong effect on the performance of thesepolymers as coatings. For electropolymerization, sur-face cleaning is very important. Poor surface prepara-tion compromises the consistency of the deposited layerresulting in surface defects giving rise to poor corrosionprotection. A CP can be used as a primer alone or as aprimer coating with conventional topcoat or it can beblended with conventional polymer coatings or can beused as anticorrosive additive in the coating formula-tion. All these approaches are explored in the followingdiscussion under the heading of various conductingpolymers.

Polyaniline

PANI can be employed for corrosion protection ofvarious metallic substrates. PANI and its substitutedderivatives are widely used in anticorrosion coatingsbecause they are easily synthesized chemically orelectrochemically, have an increased environmentalstability and different redox states allowing regulationof their properties. Emeraldine salt [ES, a protonatedform of emeraldine base (EB)] is electronically con-ductive and widely accepted as responsible for theactive corrosion control of the metal substrate. Depend-ing on the synthetic method and isolation procedure,PANI may contain localized/delocalized polarons andbipolarons in various proportions. The well-definedinsulating forms of PANI are the leucoemeraldine base(LB), EB, and pernigraniline base (PB) differentiatedfrom one another by the number of benzenoid (x) andquinoid (y) rings. The ES in which x = y is the result ofthe oxidation of LB or the protonation of EB.28,31,32

Protonation and doping by anions are possible by usingvarious acids such as sulfuric, phosphoric, phosphonic,hydrochloric, perchloric, oxalic, and sulfonic acid.Electrodeposition of PANI and its substituted deriva-tives on iron or mild steel was first carried out fromoxalic acid solutions5,3338 and other organic acids39

since the main problem arising when using active metalsfor the electrochemical polymerization of aniline in acidsolutions is the simultaneous anodic dissolution of thesupport metal. Passivation of the metal substrate isrequired before the growth of the PANI. It was shownthat the choice of the proper electrolyte, pH, andelectrochemical technique for the growth of a protectivepassive layer is crucial. Potentiostatic electropolymer-ization of aniline carried out in phosphoric acidsolutions (pH 4.5) in the presence of metalinic acid

leads to PANI films on mild steel that shift the opencircuit potential (OCP) of the metal substrate topositive values for relatively longer periods than thePANI formed from oxalic or sulfuric acid solutions.4042

Homogeneous strongly adherent PANI films on mildsteel were also obtained under galvanostatic conditionsat certain applied current values from 2 M LiClO4solutions43 as well as from acetonitrileLiClO4 solu-tions.44 Electropolymerization on various types ofstainless steel and other metal substrates where passiv-ation precedes easily the polymerization process wasreadily performed from different media of both organicand inorganic acids resulting in adherent PANI coatingswith sufficient protective properties in corrosive med-ia.2,4552

In practice, PANIcan be used for the protection ofconcrete steel bar reinforcement. Saravanan et al.53

have studied epoxy primer containing PANI for steelbars. In this study, it was found that the base metalrepassivates due to the presence of PANI in thecoating. Epoxy PANI coating showed a better perfor-mance against chloride attack, which is one of themajor causes of steel bar corrosion in concrete. Thiswas determined from accelerated corrosion studiessuch as anodic polarization, cathodic disbondment test,and salt spray test. Chemical resistance studies of thesecoatings showed better alkali resistance, giving muchmore protection against alkaline environment of con-crete. Time to cracking study of coated and uncoatedbars in concrete was performed. Maximum current wasreached within 4 days in case of uncoated bars whereasit took nearly about 50 days for ones coated with epoxyPANI primer. Authors claim that this extra protectionwas provided by repassivation induced by PANI.

Different dopants can be used in order to improveconductivity and corrosion performance of conductingpolymers. Poly(methylmethacrylate-co-acrylic acid)-doped PANI films were used on an aluminum alloyAA3104-H19 by immersion of the pretreated surfacesinto a saturated ethyl acetate solution of the polymer.54

In this study, it was found that even though theseprimers were porous, nonuniform and allowed electro-lyte permeation, they were found to provide bettercorrosion performance when compared with a com-mercial epoxy resin (ICI 640C692). The type of dopantand topcoat used has a strong influence on thecorrosion protection.55 The type of media used forthe study also has an influence on the corrosionperformance properties of the final film. Benzoate-doped PANI-containing vinyl coatings were able toprotect steel in neutral media better than in acidmedia.56 A passivating layer of iron oxide stabilized byPANI was suggested to provide improved protection.

Kalendova et al.57 studied the influence of inorganicpigments like zinc phosphate, calcium borate, and zincpowder along with PANI for corrosion protection ofmetals. They found a synergistic effect that was moreefficient in protecting metal than just using a singledefense mechanism. Loading of PANI in nano partic-ulate form at 2.5% w/w in water dispersible alkyds


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resulted in increased corrosion performance of thesewater-based alkyd paints for metal substrates.58 Incor-poration of PANI in chlorinated rubber (CR) bindersystem in 1.5% w/w resulted in increased corrosionprotection as compared to the same amount of zincphosphate in marine coatings.59 PANI powder synthe-sized chemically and used as pigment within a concen-tration range 1015% was found to increase thecorrosion resistance of mild steel in 3.5% NaCl by1015 times.60 Pigments of PANI-H3PO4 andMg3(Si4O10)(OH)2 in PVC added in metal zinc coat-ings were proven to slow down the corrosion of steeland improve the mechanical properties of the coat-ing.61 A duplex coating constituted by a ceriumconversion coating as adherence promoter and PANIused on AA6063 aluminum alloy were shown toexhibit efficient protective ability.62 The presence ofthe cerium conversion coating increasedthe adherenceof PANI. It is suggested that the corrosion protectionmechanism can be understood by considering thebarrier effect of the cerium coating and the anodicprotection caused by the PANI.

Incorporation of CPs in a conventional polymermatrix can lead to a useful increase in properties likeconductivity and corrosion performance. Though thisincrease may not always be significant, incorporation ofclay in a polymer matrix enhances the barrier, thermal,and anticorrosive properties of the coating. Due to theplatelet nature of clay the path of corrosion ion, water,and oxygen ion ingress lengthens, resulting in bettercorrosion resistance. Both PANI and clay have beenadded to the polymer matrix in order to have improvedproperties. PANI and hydrophilic as well as organo-philic montmorillonite (MMT) displayed better corro-sion performance as compared to the pure PANI-containing coatings. A nanocomposite of PANI andclinoptilolite (a natural zeolite mineral) was alsoprepared.63 Clinoptilolite was used in order to improvethe barrier properties of the nanocomposite. Theclinoptilolite proportion was varied from 1 to 5% w/w in the nanocomposite. It was found that in the acidicmedia, the corrosion current for 3% w/w of clinoptil-olite in the nanocomposite was lower than that of 1 and5% w/w of clinoptilolite in the nanocomposite. Thisstudy determined that optimum concentration of zeo-lite is necessary for the required performance of thecomposite.

The effect of nanostructured particles on the anti-corrosion properties of PANI/alkyd coatings wasshown by Riaz et al.64 in comparison with the anticor-rosion properties of poly(1-naphthylamine) (PNA)/alkyd coatings. Comparisons between nanosized PANIand PNA dispersed soya oil-based alkyd compositesapplied on mild steel revealed that the PNA/alkydcoatings have better anticorrosion properties than thePANI/alkyd coatings. A coating with different amountsof PANI and PNA was investigated. The higherprotective ability of PNA/alkyd coatings was assignedto the smaller particle size of PNA as compared withthat of PANI. The smaller particle size of PNA

enhanced the crosslinking of the PNA with the alkydmatrix providing a better barrier effect. A far superiorcorrosion resistance performance compared to that ofsingle PANI/alkyd was also found in the case of thePANI/ferrite nanocomposite applied on steel.65

Hybrid nanocomposite coatings containing dodecylbenzene sulfonic acid-doped PANI and ZnO as nano-particles dispersed in poly(vinyl acetate) (PVAc)appeared to also exhibit an improved barrier effect ascompared to the single-component coating.66 Corro-sion studies of steel plates dip-coated with theseformulations showed excellent corrosion resistance insaline water for long periods. The results wereexplained by considering an improvement in the threesimultaneously operating mechanisms during protec-tion: (i) barrier effect, (ii) formation of p-n junctionthat prevents easy charge transport, and (iii) redoxbehavior of PANI which by storing a high amount ofcharge induces a self-healing effect that prevents theevolution of active dissolution in possible surfacedefects by the passive oxide formation.

PANI has also been used for the corrosion protec-tion of substrates other than steel and aluminum. APANI-inorganic pigment (titanium dioxide) compositewas found to perform better than just PANI alone inacrylic resin binder on magnesium substrate.67 PANIwas incorporated in epoxy powder coating formulationwith 1, 3, and 5% w/w. Powder coatings are highlyregarded due to their zero volatile organic content(VOC). It was found that after curing the PANI leadsto a higher crosslink density and even after a scratchwas formed it imparted self-healing behavior to thecoatings.68 Glass flakes containing coatings are tradi-tionally used for prevention of corrosion in marineenvironments, but there is a problem of coating defectsand pinholes. The performance of these kinds ofcoatings was modified by incorporation of PANI intothese paints containing glass flakes.69 In this case,passivation due to the PANI was suggested as thecause of improved corrosion protection. In electro-chemical impedance measurements it was found thatresistance of PANI containing coatings remainedabout at 108109 X cm2 and in the case of coatingscontaining only glass flakes it decreased to 105 X cm2.In another study, incorporation of 0.120 parts ofPANI was performed in styrene butyl acrylate copoly-mer and the coating was applied on mild steelsubstrate. This study determined that the lower percentof PANI displayed better corrosion resistance ascompared with the higher percent of PANI in thecomposition on mild steel.70

There are several problems with the direct applica-tion of coatings of conducting polymers. Their appli-cation on active metals becomes difficult owing to thefact that conducting polymers are insoluble and non-fusible. Several approaches have been attempted inorder to overcome solubility problems including chem-ical modifications and attachment of well-designedfunctional groups, rendering solubility in a givenmedium. Dopant anions have been added in order to


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increase overall conductivity. Functionalization torender medium solubility has been attempted byseveral researchers in last decade. Surface functional-ization also leads to the alteration of several propertiesbesides solubility. Addition of sulfonate groups to thepolymer backbone of PANI altered ion exchangeproperties of the polymer, giving rise to a self-dopingeffect.71,72 An alkyl and alkoxy modification to con-ducting polymer backbone resulted in an improvedsolubility in commonly used solvents. Thus, severalforms of PANI and their modifications can beemployed for corrosion protection of metals and theiralloys.7,73

Recently, nanocasting was applied to preparedPANI-based advanced anticorrosion coatings from anatural Xanthosoma Sagittifolium with biomimeticsuperhydrophobic structures (SH-PANI).74 Thesecoatings exhibit water-repelling properties with a waterangle equal to 156 and good adhesion to the cold-rolled steel (CRS). Evaluation of the anticorrosionperformance of the SH-PANI-coated steel in 3.5%NaCl showed a better protective ability for the SH-PANI in comparison with the corresponding coatingwith a relatively flat surface. The improved anticorro-sion property of the SH-PANI coating was ascribed tothe synergistic effect of the hydrophobicity that repelsthe moisture and aggressive ions and the PANI abilityto passivate the steel via the formation of an oxide film.

Polypyrrole

In the recent past, several reports on the corrosionprotection of metals and their alloys by PPy have beenpublished. PPy has been added to epoxy polyamidecoatings in 1% w/w. An improvement in corrosionprotection was found as compared to the controlsample, but higher proportions did not improve theperformance. It was observed that the efficacy of CPsdepends on the method of application and the condi-tions under which the experiments were performed. Inthis study, authors claim that the conducting polymershave a similar electrochemical mechanism of protec-tion as that of hexavalent chromium.75

CPs can be synthesized with different anions ontheir backbone. Upon oxidation of CPs, dopant anionis introduced in the conjugated polymer. This dopantanion is released when this polymer undergoes reduc-tion. These anions can be released in the coatings ondemand functioning as smart coatings. Dopant ionsused in the synthesis of PPy have a strong influence onthe properties of the resultant PPy. Kowalski et al.76

prepared bi-layered PPy coatings on carbon steel. Inthis system, the inner layer was doped with molybdo-phosphate and phosphate ions whereas the outer layerwas doped with naphthalenedisulfonate (NDS) ions.Molybdophosphate ions incorporated in the inner layerstabilize the passive oxide film on the steel and theouter layer doped by the large size organic ions of NDSrestricts decomposition and release of molybdophos-

phate ions in the inner layer. This resulted in overallimprovement in corrosion protection.

Electrochemical synthesis of CPs is an often-usedtechnique for the deposition of CPs on the requiredsubstrates and their synthesis. Bereket et al.77 studiedthe corrosion performance of electrochemically synthe-sized PPy as well as a topcoat of poly(5-amino-1-naphthol) on PPy films by cyclic voltammetry technique.It was determined that as more layers of this combina-tion were deposited, an improvement in the corrosionprotection performance was achieved. In another study,Su and Iroh78 successively electrodeposited PPy andpoly(N-methylpyrrole) coatings on steel substrates fromaqueous oxalate solutions to reveal the mechanism ofelectrodeposition of PPy on metal substrate. It wasdetermined that a FeC2O42H2O coating was firstdeposited on the steel substrate resulting in its passiv-ation. But this formed FeC2O42H2O, which wasdecomposed again when the electropolymerizationpotential of pyrrole was reached. Many of the importantproperties of the resultant coating, such as adhesion,were found to depend on local pH and applied currentdensities. In this study, it was further demonstrated thatthe electropolymerized PPy coating provided bettercorrosion resistance as compared to the electropoly-merized poly(N-methylpyrrole) coatings.

Several researchers have attempted to synthesizeelectropolymerized PPy on different substrates otherthan steel and aluminum. Herrasti et al.79 electrode-posited PPy on the surface of copper. The depositedlayer had low porosity and homogenous surfacecharacteristics. This was the outcome of constantgrowth rate of these conducting polymer films. ThesePPy deposits, which were formed on copper showedexcellent corrosion resistance in 3% sodium chloridesolution. They also showed that a higher monomerconcentration of about 0.3 M should be used in orderto have good barrier layer characteristics and goodredox properties. Jiang et al. electrodeposited yellow-black colored PPy coatings on AZ91 magnesium alloyssubstrates in alkaline solutions. They found that thedifferent pretreatments have an effect on the charac-teristics of potentiodynamic curves thereby changingthe resultant growth rate.80 Tuken et al. attemptedelectropolymerization of PPy on the surface of brassand copper. They determined that these PPy films havebetter adhesion on the oxidized copper, resulting inbetter corrosion protection as compared to brass. Theyproposed a mechanism of protection by barrier effectsexhibited by these coatings.81

In another attempt, Attarzadeh et al. added saccha-rin as a third component in order to yield coatings withimproved and modified properties. They incorporatedsaccharin in the electrodeposition bath at concentra-tions of 0.25, 0.5, and 2.5 g/L for PPy coating of steel.PPy coatings obtained with an addition of 0.5 g/Lsaccharin to the electropolymerization bath showedgreater corrosion resistance and a more compact andsmoother surface. Further they proposed that thiscompactness and smoothness might be the reason for


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an improved corrosion performance of the resultantcoating.82

Hosseini et al. studied PPy, MMT, and epoxynanocomposite materials for their corrosion perfor-mance on aluminum substrate. It was found thatepoxy-PPy-MMT nanocomposite displayed better cor-rosion performance as compared to epoxy, epoxy-PPy,and epoxy-MMT coatings. So the properties of differ-ent materials can be combined together in order toyield a better performance.83 In another study, Tall-man et al. synthesized a nanocomposite by pyrrolepolymerization on alumina nanoparticles in the pre-sence of dihydroxybenzene. They used this material inthe coating as filler in less than 2% proportion. It wasfound that the presence of dihydroxybenzene im-proved the adhesion of the coatings to the aluminumalloy surface. This study significantly explains thechanges induced in the geometry and volume fractionof voids in the coatings with the help of electrochem-ical impedance spectroscopy (EIS). The voids have adirect influence on the aging and penetration ofcorrosive solution into the coating.84

Polythiophene

There have been few reports published on PTh for thecorrosion protection of metal substrates. It is possibleto synthesize a range of substituted polymers of PTh.Some of the derivatives of PTh perform very well ascompared to other conducting polymer derivatives.This ultimately depends upon the nature of theenvironment that these conducting polymers will beexposed to. With the proper application of voltage,PTh and its derivatives can be generated on otherconducting polymer substrates like PPy. The combina-tion of these two has resulted in better corrosionperformance.

Kousik et al. attempted to generate PTh coatings ona mild steel surface using acetonitrile as a medium.From AC impedance studies, it was determined thatPTh-coated mild steel was protected by a passivationmechanism caused by the redox activity of the PTh.Water uptake and delaminating area studies alsoconfirmed protective action of electropolymerizedPTh on the mild steel surface.85 Ocampo et al. havecompared several anticorrosive marine paints with theaddition of a PTh derivative. It was found that theaddition of a low concentration (0.2% w/w) of poly(3-decylthiophene-2,5-diyl) increased the corrosion resis-tance performance of epoxy-based paints.86

Rammelt et al. investigated the role of specialsurface treatment on the substrate. After the treatmentwith 2(3-thienyl) ethylphosphono acids, homogeneousand very adherent polymethylthiophene films wereformed on the surface of mild steel. These films ofpolymethylthiophene were ultrathin (thickness was ca.1 lm) and highly ordered. So they exhibited betterprotection with material savings by utilizing ultrathinfilms. These films were found to effectively separate

the electrochemical processes of oxygen reduction andiron dissolution in the surface region.87 This rationaleof adhesion promoter incorporation can be very easilyused for the preparation of different homogenouscoatings with improved corrosion protection.

Corrosion protection mechanisms of conductingpolymers

The mechanisms of corrosion protection due to con-ducting polymers are not understood completely.There are four possible proposed hypothe-ses2,3,6,9,10,15,17,48,88,89:

(1) Anodic protection mechanism: This mechanismsuggests that CP coatings may lead to the forma-tion of protective layers of metal oxides on themetal surface thus preventing corrosion.2,3,48,89

(2) Controlled inhibitor release mechanism: Accord-ing to this mechanism, the oxidized and hencedoped CP-based coating deposited on a basemetal substrate may release the anion dopantupon reduction, which is driven by a defect on thecoating from coupling to the base metal.15 In thecase of doped PANI, the anions are released notonly through reduction mechanism but also dueto a simple elimination of acid-dopant if it issoluble in water.

(3) It is believed that when a metal comes intocontact with a doped semiconductor or an elec-tronically conducting polymer, an electric field isgenerated that would limit the flow of electronsfrom the metal to an oxidizing species, thuspreventing or reducing corrosion rate.25

(4) Conducting polymer coatings form a dense, adher-ent, low porosity film and maintain a basic envi-ronment on a metal surface thus restricting accessof oxidants and preventing the oxidation of themetal surface.90,91 The less porous the CP layer thebetter is the barrier effect and the lower is thetransport rate of O2 and H2O into the polymer. Byincreasing the compactness and adherence to thesubstrate, the reaction site of the O2 reductionmoves from the M|CP-coating interface to the CP-coating|solution interface.92 The shift of the oxy-gen-reduction site on the polymer surface resultsin the decrease of the reduction products (i.e.,OH) across the M|CP-coating interface, prevent-ing disbondment and delamination of the coat-ing.15,93 On the other hand, O2 reduction isinvolved in the local reoxidation of the CP andconstant active role of the CP coating in case thatlocal pinholes or small-size defects are formed.Thus, increasing the barrier effect by processesthat inactivate CPs should be avoided. As far asthe CP is in its conductive form the OCP of themetal |CP-coating| solution system is in the passivestate. The site and the kinetics of the oxygen


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reduction are important for the long-term protec-tive properties of the coating. It is generallyobserved that dehydration of CP electrodepositedfilms on metals from aqueous medium improvesthe barrier effect.

Recent experimental evidence shows that protectionof the metallic substrate provided by aniline oligomers(AO) is mainly connected with the change of theoxygen-reduction kinetics from a four- to a two-electron path on the AO-covered mild steel. As aconsequence, the formed hydrogen peroxide mayoxidize during corrosion Fe2+ to Fe3+ resulting in theformation of Fe(OH)3 in the AO pores.

94 The forma-tion of Fe(OH)3 reduces the corrosion rate and mayexplain the self-healing ability of metal coatingsconsisting of AO and perhaps of thin CP films. ThinPANI films formed often electrochemically are con-sidered to be more likely AO than real PANI.28

Several researchers show the protective efficiency ofAO in chloride-containing solutions either depositedalone on low-carbon steel95 or incorporated into epoxythermosets,96 polyamide,97 polyimide,98100 and poly-urethane,101 deposited on CRS. The AO deposited oniron surfaces modifies the AO|iron oxide interfaceleading to a band bending in the oxide and a decreasein the substrate work function.102

Among the above mechanisms, the first two areconsidered the most important ones by which corrosioninhibition by CPs can be rationalized in terms of theactive role of CP layer. The other two mechanismsshould contribute simultaneously to a remarkabledegree in cases where either the anodic protection orthe controlled inhibitor release mechanism determinedthe corrosion inhibition for a specific metal |CP-coating| solution system. The anodic protection mech-anism dominates discussions related to the way bywhich CPs prevent general corrosion (mostly in halide-free solutions) in most of the metal substratesand alloys. In particular, this mechanism has beenexplained in more detail for the PANI-based coatingsas it is analyzed in Anodic protection mechanism:thermodynamics of passivation and the role of PANIsection. The controlled inhibitor release mechanismhas been suggested in cases where CP-based coatingsprevent localized corrosion in chloride-containingsolutions as it is described in the Controlled inhibitorrelease mechanism section.

Anodic protection mechanism: thermodynamics ofpassivation and the role of PANI

According to the Pourbaix diagram as shown schemat-ically in Fig. 1, passivation of steel is possible when itssurface potential and the pH of an aqueous mediumare sufficiently high.1 PANI coating due to its redoxnature could create such a passivation condition at thepaintsteel interface. The paint potential shifts thesteel surface potential toward the noble direction. The

hydroxide ions, which either penetrate through thepaint coating from the aqueous medium or areliberated by anion exchange would raise the pH atthe interface.

Schauer et al.103 proposed a passivation model usinga paint system containing epoxy as a topcoat andconducting PANI as a primer. Lee and coworkers104

explained excellent corrosion protection properties ofconducting PANI based on its redox properties andpassivation. Sathiyanarayanan et al. argued that passiv-ation due to iron oxide was responsible for corrosionprotection of steel in basic medium and passivation ofpinholes or defects occurred due to ESLeucoemeral-dine salt transformation in acidic medium. Passivationmodel described in this work is shown in Fig. 3.105

In another investigation, Sathiyanarayanan et al.observed that coatings containing 1% and 3% phos-phate-doped PANI and 3% chloride-doped PANIwere highly corrosion resistant. In this work, thehigher corrosion protection ability of phosphate-dopedPANI was assigned to the redox property of PANIalong with the formation of iron polyaniline complexand secondary layer of iron phosphate on steel.106

Controlled inhibitor release mechanism

Doping of CPs can be employed in different ways tocontrol the electrolytic environment adjacent to themetal surface when a scratch is formed. When a scratchexists on the CP-coated metal, there is a galvaniccoupling between the metal and the CP (Fig. 4). The

Steel

Electrons

Electrons

Fe2+ Fe2O3OH,

O2

ES PANI(Acid medium)

O2 + 2H2O

PANI-based paint

40H-

(Neutral medium)

O2 + 2H2O 40H-

(Neutral medium)PANI based paint

LS PANI

ES PANI(Acid medium)

LS PANI

Fig. 3: Passivation of steel by conducting PANI-basedpaint coating


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cathodic reaction involves the reduction of the CPassociated with the release of the doping anionswhereas the anodic reaction involves the oxidation ofthe metal. On the other hand, the O2 reduction occurssimultaneously on both the CP coating and metalsurfaces leading to the CP reoxidation and theproduction of OH, respectively. A self-healing pro-cess might be initiated depending on the nature of themetal and doping anions. In several metals (i.e., steels,aluminum, copper), oxide formation is initiated (ano-dic protection mechanism) or the doping anions act asinhibitors (i.e., phosphonic acid derivatives, molyb-dates). Several investigations have shown this effect inthe case of PANI90,107 and PPy layers.93,108

Furthermore, when the anion is released during thereduction of CP, an anion-exchange process may occurif the electrolyte contains chlorides and the CP isdoped by small-size anions. By this process, chloridesmay reach the metal surface and induce pittingcorrosion. The transport of chlorides into the CP canbe slowed down by using bulky doping anions. In thisway the transport of H2O into the CP should also beeliminated. Using the electrochemical quartz crystalmicrobalance (EQCM) to study the transport of ionthrough the PPy layer, Weidlich et al.109 found thatduring the oxidation-reduction transition of the PPy,the insertion or the expulsion of the doping ionsdepends on the size, mobility, and valence of anionsand cations involved in redox processes. Indeed, it wasshown that Nafion-PANI composite films deposited bya two-step process on stainless-steel 304 completely

inhibit the transport of chlorides through the film incomparison with the simple electrodeposited PANIfilms.110112 It seems that the sulfonate groups inNafion act as inner doping anions excluding an anion-exchange process between the composite film and thechloride-containing solution.

Other properties related to doping ions are the thick-ness, porosity, delamination, and hydrophobicity of theCP layer. Thus, while optimizing the inhibitor releaseprocess and the use of CPs as intelligent coatings anumber of effects might be taken into account for effectivecorrosion protection. For instance, it was observed thatthe electrochemical reduction of PPy doped with variousanions does not involve anion expulsion exclusively butcation incorporation is also possible. This may occurespecially when large defects exist in the coating and smallcations are present in the electrolyte. Using ScanningKelvin Probe (SKP) microscopy for the study of artificialdefects it was shown that delamination of the PPy isfacilitated in the presence of small cations in the electro-lyte since cation insertion becomes possible during thereduction of PPy.113 Cation injection and hence delami-nation becomes much slower when large cations such astetrabutylammonium cations are present during theelectrochemical reduction. It was suggested that incorpo-rated cations neutralized some negative charge in thepolymer, which will be immobilized. Then, cation trans-port within the polymer occurs through hopping resultingin the reduction of the PPy and hence increasing themobility of cations. A similar mechanism was found forPANI-based composite coatings and it is postulated that itis of a general validity for coatings where CP functions asan active pigment.10 These findings suggest that CP-basedcoatings with an extended percolation network will fail toprovide protection when relatively largedefects exist sincea fast breakdown of the whole coating is possible by rapidreduction of the CP induced by cation transport. Tooptimize the controlled release mechanism, a compromiseis needed to the extension of the CP percolation networkand the amount of the CP that should insure a sufficientelectric conduct with the metal substrate at the defect site.

According to the above brief survey regarding thecontrolled inhibitor release mechanism, the mainsuggested strategies aiming to take advantage ofdoping anions or to eliminate their unfavorable effectsuse: (i) anodic or cathodic inhibitor do-pants,16,90,93,107,108,114 (ii) dopants controlling the anio-nic-exchange property of the coating,110112,115 (iii)dopants that favor the coating hydrophobicity.116,117

For example, a better barrier effect was found when amore hydrophobic surface was achieved in the pre-sence of diisopropylsalicylates than salicylates for thePPy electrodeposition on steel.118

Recent developments

Several strategies have been followed to increase theanticorrosion performance of CPs. For instance, the

Cl

O2Metal-dopant (MAn) complex

APANI

Metal oxide

Mn+

ES EM

ne

EMESPANIMetal oxide

Mn+

Mn+

Fig. 4: Controlled inhibitor release mechanism for a metal,M coated by a CP layer such as PANI doped with an ion, A2,which acts as a corrosion inhibitor


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use of CPs in epoxy or acrylic blends as well as anappropriate pretreatment of the substrate metal sur-face before CPs deposition is considered necessary inmost cases and especially in chloride-containing solu-tions in order to prevent localized corrosion of themetal substrate. Furthermore, other specific strategieshave been used depending on the metal and thecorrosive environment, such as (i) copolymerization,(ii) use of multilayers of CPs, (iii) various dopants, (iv)nanostructured CPs, (v) composites and nanocompos-ites of CPs.

Copolymers of CPs

Copolymers of CPs have been attempted for thecorrosion protection of metals and their alloys. Struc-tural modification of the CP backbone by copolymer-ization influences various properties of the polymersuch as conductivity, porosity, adherence to thesubstrate and stability. In particular, in the case ofPPy the water uptaking property is one of the maindrawbacks in the use of PPy as protective coating.Therefore, the anticorrosion performance of severalPPy-based copolymers was investigated in corrosivemedia. Poly(N-methyl pyrrole) and its copolymer withpyrrole showed improved protective properties formild steel protection.119 A series of poly(pyrrole-co-N-methyl pyrrole) copolymers containing different ratiosof monomers electrodeposited on Cu showed anoptimum protective performance in 3.5% NaCl for8:2 ratio of pyrrole:N-methyl pyrrole monomers.120

The improved anticorrosion efficiency of this coatingwas attributed to the highest interaction between thecopolymer and the Cu substrate. This was supported bytheoretical calculations indicating that for the 8:2 ratiothe copolymer has the most linear structure and hencethe best interaction with the Cu. Interacting with theCu surface, the oxidizing form of the copolymer acts aselectron donor.

In a different approach, pyrrole was copolymerizedwith substituted anilines. For instance, ter-polymerpoly(pyrrole-co-o-anisidine-co-toluidine) was electro-chemically deposited on the surface of low-carbonsteel.121 This ter-polymer exhibited improved resis-tance to water permeation. The copolymer poly(pyr-role-co-o-toluidine) was synthesized electrochemicallyon mild steel only at low temperatures exhibiting thebest protective efficiency in 3.5% NaCl for a pyrrole-o-toluidine feed 8:2 ratio.122 PPy and poly(pyrrole-co-o-anisidine) were electrochemically synthesized on thesurface of 3102 aluminum alloy.123 This copolymercontained a hydrophobic methoxy group which wasresponsible for lower water permeation than pure PPycoatings and hence demonstrated better corrosionprotection.

Moreover, copolymers of aniline with its derivativescontaining sulfonic, carboxylic, and methoxy groupswere found to show also improved protection efficiencydue to the modification of polymer backbone that

affects adherence, compactness, porosity or the doping/dedoping process of the coating. Poly(aniline-co-amino-naphthol-sulfonic acid) nanowires coating wassynthesized electrochemically on the surface of iron.124

Increased corrosion resistance in comparison with aPANI coating was attributed to the strong adherenceof the copolymer due to the interaction of side groupsto the iron substrate as well as its superior adsorptionto the iron substrate. Poly(aniline-co-4-amino-3-hydro-xy-naphthalene-1-sulfonic acid) was synthesized bychemical oxidative polymerization. In this synthesis,4-amino-3-hydroxy-naphthalene-1-sulfonic acid was adopant. The corrosion-inhibiting properties of thecopolymer were studied on an iron substrate in 1 MHCl. It was found that as the concentration of theinhibitor dopant increased in the monomer feed, thecorrosion inhibition efficiency also increased.125 Elec-trochemical synthesis of poly(aniline-co-o-anisidine)thin films on copper by cyclic potential sweep deposi-tion at low and high scan rates was homogeneous andstrongly adherent on the substrate exhibiting goodcorrosion resistance.126,127 In an attempt to investigatethe effect of a hydrophilic group attached to the PANIchain on the anticorrosive performance of PANI-basedcoatings, poly(aniline-co-metanilic) copolymers wereprepared chemically and deposited on carbon steel.128

Though the copolymers provide less protection of thecoated carbon steel in 1 M H2SO4 than the PANIitself, it seems that the lower the sulfur content incopolymer the better is the protective ability. Thisbehavior is attributed to the hydrophilic character ofthe sulfonic group that does not favor the anticorrosionperformance of the coating. In another study, poly(ani-line-co-m-amino benzoic acid) was electropolymerizedon the surface of steel.129 It was found that theimproved corrosion protection ability of the synthe-sized copolymer was attributed to the better compact-ness of the resulting films.

Multilayers of CPs

Multilayer coatings of CPs have been reported forcorrosion protection of metals and their alloys. Thisapproach has been used in the case of anilineelectropolymerization on zinc and mild steel fromaqueous media and is a two-step process.91,130,131 Athin PPy film was first deposited on the metal substratefrom a neutral salicylate medium. The thin PPy layerbehaves as a noble metal layer slowing down thesubstrate corrosion rate. Then in a second step, thePANI was electrodeposited on the metal|PPy substratefrom an acidic medium. The PPy/PANI bilayers as wellas the composites PPy/PANI electrodeposited in a one-step process132,133 indicate superior anticorrosion per-formance in sodium chloride solutions than the plainPANI or PPy layers. A finding that points to themechanism by which the protection efficiency of thePPy/PANI bilayer is greatly improved is that whenPANI was first deposited on steel, instead of PPy, the


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resulting PANI/PPy has a lesser protective ability thanthe PPy/PANI bilayer.91 This shows that the underly-ing PPy film provides a better barrier effect andsufficient adherence to the top PANI-layer. It issuggested that the strip-shaped PANI layer formedwithin the matrix of PPy lowered the permeability ofthe PPy/PANI coating and therefore the transport ofelectrolyte, O2, and H2O through the coating. Thecomplex PPy/PANI can be characterized as compositeor bilayer since the PPy is first formed either in theone- or two-step (layer-by-layer) process as the oxida-tion potential of pyrrole is lower than that of aniline.

Bilayers of PANI and poly(N-methylaniline)(PNMA) were synthesized electrochemically on a mildsteel surface in layer-by-layer manner. The galvanicinteraction of CP bilayers was responsible for bettercorrosion resistance than the respective homopolymerson the mild steel substrate.134 On similar lines, bilayercoatings of PPy and PNMA were obtained on mildsteel by electrochemical polymerization.135 In this case,PPy was doped with the dodecylsulfate ion. ThePNMA/PPy coating exhibited the best anticorrosionperformance among PNMA/PPy coating, PPy homo-polymer coating, and the PPy/PNMA coating. Thereduction in anionic permeability due to the presenceof the dodecylsulfate ion was found to be the cause ofimproved corrosion resistance performance. In anotherattempt, multilayer thin films of poly(vinylsulfonicacid, sodium salt) (PVSS) and PANI were synthesizedusing a layer-by-layer technique on the surface of AA2024.136 Several layers of PVSS/PANI were synthe-sized and were tested against corrosion. It was foundthat optimum anticorrosion performance was obtainedat eight layers of PVSS/PANI on the surface of AA2024 in the chloride media. PPy and PANI bilayerswere electrochemically synthesized on carbon steel aswell as stainless steel 304.137,138

Multilayers of CPs can result in improved corrosionprotection. PPy electropolymerized on the surface of acopper layer deposited on aluminum imparted greatercorrosion resistance than just copper-deposited alumi-num and PPy-deposited aluminum.139 This improve-ment was found to be due to the enhanced barrierprotection from the presence of various multilayers ofCPs and metal.

Doped CPs for corrosion protection

As discussed earlier, different corrosion-inhibitingdopants can be incorporated in the CPs. Both electro-chemical polymerization and chemical oxidative poly-merization can be employed for the synthesis of dopedCPs. Doping anions may also form complexes orproducts that slow down corrosion rate by acting asphysical or chemical barriers.

PANI doped with tungstate was prepared viachemical oxidative polymerization and was dispersedin primer containing vinyl resin as a binder.140 Thiscoating was applied on a steel substrate and its

anticorrosion performance was evaluated using saltspray and EIS. The EIS measurements were carriedout in 3% NaCl solution. The OCP values were in thenoble direction, which indicated the passivation pro-vided by PANI doped with tungstate coating. An iron-tungstate complex was also observed on the surface ofthe iron substrate signifying more resistance to corro-sion. In the same manner, sulfonate-doped PANI wasprepared by chemical oxidative polymerization andwas incorporated in a vinyl resin-based coating.141 Theanticorrosion performance of the coating on an ironsubstrate in 0.1 N HCl and 3% NaCl was monitoredvia EIS. The coating containing sulfonate-doped PANIexhibited very high impedance after 100 days exposureto acid and neutral media whereas coatings withoutsulfonate-doped PANI showed anticorrosion perfor-mance up to 50 days in acidic media and 100 days inneutral media. The increased ability to protect againstcorrosion was attributed to the formation of a passiveoxide film of PANI-iron-sulfonate complex. Moreover,phosphate and chloride-doped PANI, synthesizedchemically, were used in epoxy-coal tar coatings forthe corrosion protection of steel.106 From EIS studies,it has been found that the resistance of coatingscontaining 1% and 3% phosphate-doped and 3%chloride-doped PANI was kept high for 90 days in3% NaCl solution. The resistance of these pigmentedcoatings was higher by two orders of magnitudecompared to that of conventional coal tar epoxycoatings. The anticorrosive efficiency of the coatingscontaining phosphate-doped PANI was higher. X-rayphotoelectron spectroscopy has shown that beneaththe coating the complex PANI-Fe along with the ironoxide was formed resulting in the enhancement of thesteel protection against localized corrosion.

Comparing the protective performance of severalepoxy resin (EP)-based coatings containing eitherPANI as EB, ES, sulfonated and fibers or zincphosphate and zinc chromate on mild steel in 3.5%NaCl shows that the addition of PANI results inimproved corrosion protection of steel. Only thecoating containing PANI-EB showed a lower efficiencythan the other PANI-containing coatings.142 CR-con-taining PANI showed improved protective propertiesas compared with a simple CR-based coating againstthe corrosion of mild steel in 3.5% NaCl.143

In another study, PANI was electropolymerized onthe surface of 430 stainless steel in nitric acidsolution.144 Its performance against corrosion wasevaluated in 3% NaCl solution by EIS and anodicpolarization. The catalytic effect of PANI was respon-sible for the formation of a dense oxide layer at thepolymer metal interface. Dense oxide film and PANIwere responsible for the improved corrosion protec-tion of the coating on the 430 stainless steel substrate.In same way, PANI-molybdate coating was preparedby electrochemical synthesis on a steel substrate viacyclic voltammetry.145 Potentiodynamic polarizationand EIS were employed for monitoring the corrosionprotection ability of the PANI-molybdate coating in


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1% NaCl solution. The passivating nature of PANIand the formation of an iron-molybdate complex wereresponsible for the improved corrosion performanceof the PANI-molybdate coating on the surface ofsteel.

PANI doped with hydrofluoric acid was synthesizedby chemical oxidative polymerization.146 This pigmentwas incorporated into an epoxy primer, which wasapplied to AZ91D magnesium alloy. X-ray photoelec-tron spectroscopy results displayed the formation ofmagnesium fluoride (MgF2) underneath the coating asit was exposed to 3.5% sodium chloride solution. It wasfound that the insoluble MgF2 was responsible for theimproved corrosion resistance by fluoride-doped PANIon AZ91D magnesium alloy substrate. On similarlines, methane sulfonic acid (MeSA)-doped PANI wassynthesized by chemical oxidative polymerization.147

PANI-MeSA was used as an anticorrosive pigment inultraviolet light curable polyester acrylate matrix andwas applied on the surface of galvanized steel. OCPmeasurements in 3% sodium chloride solution indi-cated a shift in positive direction due to the anodicennoblement of the substrate giving long-term corro-sion protection.

PANI doped with benzene-sulfonate (BS) and ligno-sulfonate (LS) was synthesized by chemical oxidativepolymerization and was incorporated into a CR binderin varying weight proportions.148 The coatings wereapplied on mild steel and the corrosion performancebehavior was monitored by EIS with salt spray expo-sure and immersion in sodium chloride solution. Below1% concentration of BS-doped PANI and 3% concen-tration of LS-doped PANI showed improved corrosionperformance. In this study, it was observed that LS wasnot covalently bonded to PANI, which was responsiblefor the different behavior than that of BS-doped PANI.PANI-LS/epoxy coatings were also employed forcorrosion protection of AA2024-T3.149 The perfor-mance of PANI-LS/epoxy blends was investigated in0.6 M NaCl during 30 days for different loadings inPANI-LS. A better protection efficiency was found foran optimum amount of PANI-LS (5 wt%) in epoxyblends. It appears that at these optimized concentra-tions of PANI-LS, a uniform distribution of PANIparticles results in a decrease of the permeability ofH2O and O2. The improved corrosion protection isexplained in terms of the anodic protection mechanismand the controlled inhibitor release mechanism. Athickened oxide film is formed due to the active redoxrole of PANI whereas the release of dopant (LS) at thedefect site prevents active dissolution of the substrateby the formation of an aluminum-sulfonate complex.Moreover, dodecylbenzenesulfonic acid-doped PANInanoparticles (1 wt%) blended epoxy ester (EPE)showed better corrosion protection of the carbon steelsubstrate in 3.5% NaCl compared to the simple EPEcoating.150 This behavior was also attributed to thereleased dopant anions that with the iron cationsprovide a secondary barrier layer, which passivates thecarbon steel.

In the case of benzoate-PANI coatings, galvanostat-ically synthesized on mild steel, it was suggested thatthe protection effect can be described by the switch-ing zone mechanism.151 According to this mechanismduring an initial period, the corrosion potential shifts tomore negative values determined cathodically by thededoping process of PANI-benzoate along with theoxygen reduction and anodically by the iron dissolu-tion. As most of the benzoates were released the PANIconductivity decreases. During a medium period, thecorrosion potential shifts to less negative values deter-mined by the cathodic oxygen reduction at the baremetal in the bottom of the PANI pores and the anodicpartial doping of PANI by chlorides and the initialperiod is repeated. It is suggested that during aprolonged period a thin oxide layer is formed nearthe PANI and thus the oxygen reduction is catalyzed.

PPy doped with corrosion-inhibiting ions have beenstudied for the corrosion protection of metals and theiralloys. Molybdate and nitrate doped PPy were elec-tropolymerized on the surface of 316L SS in neutraland alkaline media.152 It was found that the pittingpotential of the substrate was reduced. Improvedcorrosion protection was attributed to the electro-chemical activity of PPy, the formation of a passiveoxide layer, and the nature of dopants used in theelectrochemical synthesis of PPy on the substrate. Inanother study, electrochemical polymerization of PPywas carried out on AA 2024-T3 substrate using variousdopants, such as camphor sulfonic acid, phenylphos-phonic acid, para-toluene sulfonic acid, oxalic acid, andcerium nitrate salt. Different morphologies and varyingcorrosion behaviors of the PPy were observed as afunction of dopant anion.153

Nanostructured CPs

The application of nanotechnology in metal anticorro-sion technology has attracted a great interest recently,although it is still in its infancy. Nanostructured CPssuch as nanoparticle, nanofiber, nanowire, nanorod,nanotube can be prepared by different meth-ods.17,154158 They are characterized by specific phys-icochemical properties that are found to enhance theperformance of CP-based materials when used inseveral applications.159161 Among new properties arespecific light absorption, quantum tunneling effects,hydrophobicity, stability, catalytic, and magnetic prop-erties.154,162,163 In particular, nanostructured CPs alsoseem to be promising materials in preparation ofprotective coatings against metal corrosion.17,50,164,165

It is suggested that the anticorrosion property isimproved through either the enhancement of theadhesion of CP-based coatings to the metal substrateby using bifunctional compounds or the increase of theCP-coating hydrophobicity. Nanostructured CPs resultin compact passive layers at the metal substrate,increasing protection efficiency. Nanotechnology isexpected to have a great potential for corrosion control


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in the case of either simple CP layers or CP compositeswhere various compounds might be incorporated toobtain multifunctionalized nanocomposites.166

PANI nanostructures synthesized chemically164,165

or electrochemically50 indicate improvement of theanticorrosion performance of PANI-based coatings.Yao et al.164 studied the anticorrosion properties ofPANI nanofibers synthesized by interfacial polymeri-zation and applied to carbon steel as a suspension inethanol in comparison with the anticorrosion perfor-mance of aggregated PANI-coated carbon steel in 5%NaCl aqueous solution. On the basis of potentiody-namic polarization, measurements show that the PANInanofibers have better corrosion protection to carbonsteel than the aggregated PANI. Raman spectroscopyshows that PANI nanofibers induce the formation ofbetter passive oxide layers on carbon steel. This isattributed to the better dispersion stability of PANInanofibers in ethanol, which results in the formation ofa more compact coating layer on carbon steel surface.

Yang et al.165 synthesized different one-dimensionalPANI nanostructures in sulfuric acid solutions usingdifferent polymerization methods, namely conven-tional polymerization, interfacial polymerization, anddirect mixed reaction. The PANI products wereapplied to the mild steel surface as a suspension inN-methyl-2-pyrrolidinone (NMP) and their anticorro-sion properties were investigated in 3.5% NaCl solu-tion by potentiodynamic polarization curves.Comparative studies revealed that PANI nanofiberssynthesized by direct mixed reaction have greateruniform morphology and better protective propertiesthan the PANI synthesized by other methods.

Moreover, PANI nanofibers were prepared by directmixed reaction in four different inorganic acids,namely HCl, H2SO4, HNO3, H3PO4, and use incomposite coatings of PANI-epoxy resin by mechan-ical grinding.167 The type of inorganic acid influencesthe morphology of PANI nanofibers. The effect of thePANIs content on the anticorrosion property of theepoxy composite coatings mixed with different PANInanofibers doped by four different acids was investi-gated for Q235 steel in 3.5% NaCl solution. The bestprotective performance was obtained when the amountof PANI was around 0.5% and doped with H3PO4. Theeffect of the doping anions on the protective ability wasvaried in the order H3PO4 > H2SO4 > HNO3 > HCl.The superior protection in the case of H3PO4 wasexplained considering the controlled inhibitor releasemechanism15,16 by which the release of phosphatesinduces the formation of an excellent passive film onthe steel substrate.47 In the case of the compositecoatings mixed with H2SO4, HNO3-doped PANI betterprotection was attributed to the PANI excellent fibermorphology and network structure.

In another situation, by employing a miniemulsionpolymerization technique, conducting polymer nano-particles (CPNs) of polyanisidine, polytoluidine, andtheir copolymers (CCPNs) were synthesized.168 Barrierproperties along with ability of conducting polymers to

act as an oxidant to underneath steel surface improvedthe corrosion protection provided by coatings based onCPNs and CCPNs.

Composites and nanocomposites of CPs

Many researchers have exploited the possibility ofusing composite materials based on CPs as anticorro-sive coatings. Composites combine the functionalproperties of diverse materials imparting improvedproperties for corrosion protection of metals and theiralloys. A number of different metal, metalloid, metaloxide particles or nanoparticles as well as carbonnanomaterials such as carbon nanotubes and graphenecan be encapsulated into the matrix of a PANI toproduce PANI-based composite materials.29 To dem-onstrate the improved protection ability of compositesof PANI, on a mild steel substrate, coatings based on aPANI matrix filled with zinc and zinc nanoparticleswere obtained on iron by solution mixing method.169

The presence of nanoparticles of Zn in PANIimproved its conductivity and anticorrosion propertiesin comparison to the microsized particles of Zn inPANI composite coatings. This study showed theimpact of filler particles, their loadings, nature, mor-phology, and size on the final anticorrosion properties.Further investigation was done by in situ synthesis ofPANI, in the presence of zinc nanoparticles.170 Chem-ical oxidative polymerization was used for PANI-Znnanocomposite synthesis. This nanocomposite wasthen applied to an iron substrate by the solutioncasting method. The maximum conductivity of thePANI-Zn nanocomposite coating was obtained at5% w/w zinc loading. The conductivity was found tobe the function of the amount of zinc present in thecomposite. Zinc protects iron cathodically by sacrificialprotection. The corrosion products of Zn can fill thepores in the CPs resulting in more barrier protectionalong with the added electrochemical activity of CPs.OCP measurements displayed a positive shift indicat-ing better anticorrosion performance for PANI-Znnanocomposite coating against just PANI coating.

Moreover, recently the chemical oxidation of anilinemonomer with ammonium peroxydisulfate was used inthe presence of camphosulfonic (CSA) acid for thepreparation of PANI nanocomposites containing ZnOnanorods.171 TGA results showed that the decomposi-tion of the nanocomposite was less than that of purePANI indicating its potential application in anticorro-sive paints. The doping effect of ZnO nanorods wasobserved in PANI-ZnO nanocomposites. The interac-tion between PANI and ZnO nanorods is based on theformation of hydrogen bonding and electrostaticinteraction between CSA-capped ZnO nanorods.

SiO2 is among the promising nanoparticles em-ployed in metal anticorrosion coatings. HydrophobicPANI-SiO2 composites nanocomposites (HPSC) syn-thesized chemically and deposited on mild steel exhibitimproved protective efficiency as compared with


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PANI. This was explained by considering that theHPSC results in the formation of a passive oxide filmon mild steel and simultaneously prevents the chlorideion penetration into the coating due to the hydropho-bic character of the composite. The HPSC coating actsas a barrier between the metal and the corrosiveenvironment. Moreover, the presence of SiO2 nano-particles leads to the reinforcement of PANI and hencediminution of its degradation.18 The synergistic effectof the PANI redox catalytic ability and hydrophobicityproperties leading to advanced anticorrosion coatingswas also observed in the case of composites preparedfrom fluoro-substituted PANI incorporated with sils-esquioxane spheres.172 Casting of the as-preparedcomposite material onto a CRS electrode results in acoated-CRS with enhanced corrosion resistance insaline conditions.

Composites of PANI with multiwall carbon nano-tubes (MWCNT) used as paints on low-carbon steelwere found to have promising anticorrosion proper-ties.173 It seems that the PANI-MWCNT-based paintssignificantly decrease the corrosion rate of the steel in3.5% NaCl aqueous solution.

PPy and nanocomposites of PPy and zinc oxide(PPy-ZnO) were electrodeposited.174 The anticorro-sion performance of the coatings in 3.5% NaCl solutionwas monitored by Tafel polarization and EIS. The PPy-ZnO nanocomposite coating showed improved corro-sion resistance in comparison with the PPy coating.This improved performance of PPy-ZnO nanocompos-ite coating over the PPy coating was attributed to theZnO morphology. ZnO was in the form of nanorods inthe PPy-ZnO nanocomposite coating. So the morphol-ogy of the contributing element in nanocomposite ofCPs also affects the final corrosion performanceproperties of composite coatings.

In the presence of TiO2 nanoparticles, poly(N-methyl pyrrole) coating was synthesized on the surfaceof steel.175 This synthesis was carried out in thepresence of dodecyl benzene sulfonic acid and oxalicacid. The presence of TiO2 nanoparticles in thepoly(N-methyl pyrrole) coating improved the interac-tive surface area of poly(N-methyl pyrrole) with theions involved in the corrosion reaction. This resulted inthe decreased water uptake by the composite coatingas well as the increase in the pore resistance of theresultant coating. Along the same lines, PPy/Sn-dopedTiO2 nanocomposites were used as anticorrosive pig-ment in epoxy primer on steel substrate.176 Sn dopingof TiO2 nanoparticles improved the anticorrosionability of PPy/Sn-doped TiO2 nanocomposites in com-parison with PPy/TiO2 nanocomposites. In this re-search, it was observed that Sn doping increases theband gap of TiO2 in turn decreasing the charge transferin the final coating based on PPy/Sn-doped TiO2nanocomposites. PPy/Ni-doped TiO2 nanocompositeswere also synthesized for the corrosion protection ofsteel.177 An increase in the surface area due to thepresence of nanoparticles resulted in increased inter-action between the particles and the ions involved in

the corrosion reaction. Ni doping also decreased thesize of the final PPy/Ni-doped TiO2 nanoparticles incomparison with PPy/TiO2 nanoparticles, which fur-ther improved the corrosion resistance.

PPy/carboxylic acid functionalized single-walledcarbon nanotubes (CNT-CA), and PPy/poly-amino-benzene sulfonic acid functionalized single-walledcarbon nanotubes (CNT-PABS) were electrochemi-cally prepared.178 In this synthesis, CNT served as thedopant. Their anticorrosion performance on carbonsteel in 3.5% NaCl solution and their mechanicalproperties were compared with pure PPy. It was foundthat the PPy/CNT-CA and PPy/CNT-PABS compositecoatings showed more anodic potential and lowercorrosion current against uncoated steel and improvedmechanical reinforcement. Besides CNT-CA, multi-walled carbon nanotubes (MWCNT) also served asdopants in poly(3,4-ethylenedioxythiophene) nano-spheres (PEDOT-NSP/MWCNT) changing the intrin-sic anion-transport selectivity of PEDOT to a cation-transport selectivity.179 The PEDOT-NSP/MWCNTwas synthesized using microemulsion polymerizationand investigated as a corrosion inhibitor for aluminumin LiPF6. The anticorrosion effectiveness of PEDOT-NSP/MWCNT was attributed to a synergistic effectthat involves the cation exchange PEDOT and theanion-repulsive pristine MWCNT surface. Due to thiscooperative action, the transport of the PF6

anionstoward the aluminum surface is blocked and hencepitting corrosion induced by PF6

anions is prevented.Composites and nanocomposites of CPs can be

prepared by the inclusion of layered materials. Theplate-like shape of these materials lengthens the pathof corrosive ions toward the substrate resulting in delayin the onset of corrosion process. The flake structurecan reduce the permeability of the solute by a factor of10 after their inclusion in the coating.180 CP/MMTclays composites and nanocomposite have been usedfor the corrosion protection of metals and theiralloys.181185 PANI/MMT nanocomposites were pre-pared by blending PANI and MMT in epoxy resin.181

The specific morphology of the nanocomposite re-sulted in improved corrosion protection of Al 5000.

In another attempt, PANI/clay nanocomposite wasprepared by chemical oxidative polymerization meth-od.182 This nanocomposite was then incorporated in azinc-rich ethyl silicate resin. EIS measurements werecarried out in 3.5% NaCl solution and OCP wasmonitored for the corrosion evaluation. The barrierprotection and the passivation of the substrate due topresence of the composite pigment resulted in im-proved corrosion resistance of the coating containingPANI/clay nanocomposite in comparison with thepristine coating. Along similar lines, PANI/MMTnanocomposite coatings were prepared on aluminumalloy (AA 3004) by galvanostatic method.183 Thecorrosion performance properties of these coatingswere monitored in 3.5% NaCl solution by impedanceand potentiodynamic measurements. With the nano-composite on the surface of AA 3004, the corrosion


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current was found to be decreasing. The anticorrosionperformance of the PANI/MMT nanocomposite wasattributed to the barrier nature of MMT and redoxcatalytic nature of PANI. Molybdate-doped PPy/MMTnanocomposites were also found to improve thesteel.186 The protection was attributed to the increasedbarrier protection due to the MMT and dopant releasefrom PPy.

Conducting polymer-containing composite coatings(CPCC) is a new class of materials, which can resolveprocessibility issues related to CPs along with theirbulk synthesis. PPy/aluminum (Al) flake compositecoatings have been found to protect aluminum alloy(AA 2024-T3) against corrosion.187189 The presence ofPPy in intimate contact with aluminum flakes activatesthem toward anodic activity resulting in galvaniccoupling between composite and the substrate. Theeffect of PPy morphology formed on the surface of Alflake was studied for the corrosion protection of AA2024-T3.190 It was found that wire PPy formed on thesurface of Al flake resulted in PPy/Al flake compositewhich in turn provided sacrificial corrosion protectionto the underlying AA 2024-T3 substrate. Phosphateand nitrate doped PPy/Al flake composite pigment-based coatings were applied on the surface of AA2024-T3.191 Dopant release mechanism combined withgalvanic coupling helps to protect underlying substrate.PPy/micaceous iron oxide (MIOX) composite pig-ments were synthesized by chemical oxidative poly-merization.192 Redox properties of PPy combined withbarrier properties of MIOX provided prolonged cor-rosion protection.

Conclusions

CPs can be used in various forms for corrosionprotection of metals and metal alloys. They can beused as pigments, multilayers, and composites. Corro-sion-inhibitive dopants can be incorporated on thebackbone of CPs which can be released when CPs arereduced under particular conditions. Bulk synthesis ofCPs is possible by chemical oxidative polymerizationmethod. Electrochemical deposition of CP layersdirectly on the oxidizable surfaces of metals and alloysusing appropriate electrolytes in polymerization solu-tions is also possible. Anodic protection occurs as far asthe OCP of the M|CP system is kept within thepassivation state of the metal substrate. However, asimple CP coating seems unable to provide protectionfor a long period in aggressive environments wherelocalized breakdown of the CP coating usually occurs.Using suitable doping ions and an optimized amount ofCP in protective coatings, a self-healing process oper-ates as far as the CP is in its electroactive state. Themorphology of CPs influences the anticorrosion prop-erties of the CP-based coatings. Future investigationsexerting more control over the synthetic paths ofnanostructured CPs are expected to lead to improved

protective CP-based materials with a better perfor-mance. Numerous studies in recent times show thetremendous potential of CPs for the corrosion protec-tion of metals and their alloys, which is yet to be fullyexplored.

Acknowledgments The corresponding author wouldlike to thank Prof. S.T. Vagge, Head, Department ofMetallurgy and Material Science, College ofEngineering, Pune 411005 (MH) India for providingfacilities, and Prof. Anil Sahasrabudhe, Director,College of Engineering, Pune 411005 (MH), India forhis constant encouragement along with support fromAlumni Association of COEP for granting ADFFMeta Fellowship, AICTE and UGC, New Delhi Indiaand ISRO, Bangalore. Authors from North DakotaState University (NDSU) gratefully acknowledge thesupport from US Army Research Laboratory underGrant No. W911NF-09-2-0014, W911NF-10-2-0082,and W911NF-11-2-0027.

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Conducting Polymers for Corrosion Protection a Review