Treatise on conducting polymers for corrosion protection – Advanced approach
New modern technologies require new innovative materials. Corrosion control of metals is an importantactivity of technical, economical, environmental, and aesthetic importance. The use of oxides, such as chromates, is highlyeffective in corrosion protection. Scientists are looking at alternative materials to replace environmentally hazardous materials, e.g., chromates. One of the group of materials identified as corrosion inhibiting are conducting polymers (CPs). CPs have acquired more attentions in the last decades due to their environmentally benign nature and high effectiveness to protect steels against corrosion. The conducting polymer such as Polyaniline, Polypyrrole,Polythiophene, Polycarbazole, Polyindole etc. work as a strong oxidant to the steel, inducing the potential shift to the noble direction. The strongly oxidative conducting polymer facilitates the steel to be passivated. The review paper presented below attemptsto summarize extensive studies of Polyaniline, Polypyrrole and Polythiophene polymer and their anticorrosive properties. Several researchers have reported diverse views about corrosion protection by CPs and hence various mechanisms have been suggested to explain theiranticorrosion properties. These include anodic protection, controlled inhibitor release as well as barrier protection mechanisms. Different approaches have been developed for the use of CPs in protective coatings (dopants, composites, blends).
Keywords: Conducting polymers, corrosion, conductivity, Polyaniline, Polypyrrole, Polythiophene, nanocomposite, multilayer coatings
CORROSION can be defined as the destruction or deterioration of a material because of reaction with its environment(1). Corrosion control is an important subject of increasing interest to the modern metallic finishing industry. Surface modification of metallic substrates by organic or polymeric coatings is an essential approach for enhancing surface properties such as wear, oxidation, and corrosion. Various conventional techniques are utilized in depositing the desired materials onto the metallic substrate to achieve surface modifications with an objective or rendering better protection to the substrate Organic or polymeric coatings on metallic substrates provide an effective barrier between the metal and its environment and/or inhibit corrosion through the presence of chemicals. Traditionally, hexavalent chromium coating is the most effective way to inhibit the corrosion of metals especially steel alloys.The Cr6+ ions that exhibits superior corrosion inhibition are environmentally unsafe and of some health concerns posing biological and ecological hazards. Since the release of this federal mandate on the control of chromate containing compounds, much of the collaborative work has been done in the academic, industrial and governmental sectors to provide a suitable method for corrosion control of metal and metal alloys. One of the most studied alternatives is the use of conducting polymers for corrosion protection of steel alloys(2). Since their discovery in late 1970 by Heeger, MacDiarmid and Shirakawa(3-5) intrinsically conducting polymers, because of unique combination of physical and chemical properties,offer possibility of both chemical and electrochemical synthesis, beside distinct electronic properties. They have processing advantages of conventional polymers and potentially lower cost which has drawn the attention of scientists andengineers during the last few years and are greatly researched materials for corrosion protection(6,7).
The use of CPs for the corrosion protection of metals attracted great interest over the last 30 years. Interest has recently been focused on the possible use of CPs as either film-forming corrosion inhibitors or in protective coatings[8-11]. CPshave gained more attentions in different applications where electrical conductivity is needed. Batteries, transistors, sensors, electro chromic displays, light-emitting diodes, capacitors and corrosion protection coatings are potential application areas of conductive polymers[12-18].
Corrosion protection using conductive polymers was first proposed by MacDiarmid in 1985. The first studies devoted to examine the protection imparted by CP coating on active metals were published at the beginning of eighties[19, 20]. In these studies the protective behaviour of polyaniline (PANi) coatings electrodeposited on stainless steel was examined. The interest in using CPs as anti-corrosive coating has risen up with the time due to their effectiveness and environmentally friendly nature. The conductive polymers can interact with the steel and form a compact layer to inhibit a corrosion process. When conductive polymers are used as anti-corrosive coatings, some points have to be considered to inhibit corrosion effectively. These points include on how to apply conductive polymers on steels,electrical conductivity and environmental stability besides processability and ease of synthesis. Based on the aforementioned factors there are several conductive polymers including polyaniline, polypyrrole, polyacetylene, polythiophene, polyparaphenylene which could act effectively to inhibit corrosion.
Principle of corrosion
Corrosion phenomenon refers to the electrochemical or chemical reaction between metals or metal alloys with surroundings. Corrosion is a natural process and is a result of the inherent tendency of metals to revert to their more stable compounds, usually oxides. When metal comes in contact with electrolyte, it start corroding where areas of higher free energy or higher potential behave as anodes and those of lower free energy or lower potential behave as cathodes, and thereby creating a corrosion cell. Metal ions are formed at the anode and dissolve into the electrolyte. The electrons pass throug the metal to the adjacent cathode areas where they react with the surroundings. The most favorable surroundings may be atmospheric humidity, offshore saltspray, acid rains, synthetic solutions and natural waters. This flow of electrons from the anode to cathode and the associated charge transfer through the electrolyte from the cathode to the anode constitutes the corrosion current. The corrosion rate is, therefore, associated with the corrosion current. The process of corrosion constitutes following reactions as mentioned by Fontana(1):
Anodic Reaction: The electrochemical reaction at the anode or metal dissolution can be written as M === Mn+ + ne-
These released electrons migrate to the cathode through the metal producing corrosion current.
Cathodic Reaction: The nature of the reaction at the cathode depends upon the nature of the surroundings. The most common cathodic reactions that are encountered in corrosion are as follows:
Oxygen reduction (Acid Solution) O2 + 4H+ + 4e- === 2H2O
Oxygen reduction (Neutral or basic solution) O2+ H2O+2e- === 2(OH)–
Hydrogen evolution 2H+ + 2e- === H2
Metal ion reduction (Metal ions present in solution may be reduced) Mn+ + e- === M(n-1)+
This can occur only if there is a high concentration of Mn+ ions. In this reaction, the metal ion decreases its valence state by accepting an electron.
- Metal deposition Metal may be reduced from an ionic to a neutral metallic state: Mn+ + e- === M
Hydrogen evolution is common since acidic media are frequently encountered. Oxygen reduction is also common since any aqueous solution in contact with air is capable of promoting this reaction. These partial reactions can be used to understand most of the corrosion processes like corrosion of steel in water (Fig. 1). The extent to which a metal corrodes depends not only upon its electrode potential but also upon pH of the aqueous solution.
Fig. 1: Electrochemical corrosion showing rusting of Steel
Mode of corrosion protection
through coatings Considering that metal corrodes due to the reaction with aggressive species which are present in the electrolyte, an obvious method for corrosion protection would be to block the access of these species to the metal surface. This can be done either through alloying i.e. by adding additional element to the metal while melt production but this method is very expensive. Another way is to coat the metal surface with a layer of substances which will not corrode or corrode at small rates. Some of the coating methods are discussed in following section.
Zinc rich coatings
The application of zinc rich coatings on ferrous substrate is a very efficient method of corrosion protection. Zinc rich coatings contain zinc particles embedded in polymer matrix and these zinc particles protect the steel by cathodic protection. It is also a known fact that to achieve a long service life, zinc rich coatings are applied as a first coat to the metal substrate. Zinc offers three fold protection since it seals the underlying metal from contact with corrosive surroundings through the formation of zinc corrosion products that provide galvanic protection through zinc to zinc and zinc to metal contact. These helps repair minor damages in a coating forming a barrier to further electrochemical reaction.
Barrier coating system
Impermeable barrier coatings tend to reduce both water and oxygen permeation to a sufficient extent so that corrosion is precluded. The prevention of oxygen access to the metal stops the cathode reaction and current transfer between the anodic and cathodic areas is prevented due to ionic transfer resistance.
Conversion coatings are formed by a transformation of the metal surface due to chemical or electrochemical reaction. Conversion coatings are mainly used as a pretreatment for subsequently applied organic coatings. In this case, besides their function for corrosion protection they are supposed to increase the adhesion of the coatings applied subsequently. There are generally two kinds of conversion coatings used today viz. phosphate or chromate based coatings. The pigments react with the moisture absorbed in the coating which passivates the steel. However, these chromates containing systems are found to show adverse effect on human health, as the release of these inhibitors is based on leaching. Besides these, toxic inhibitors are constantly released into the environment. Being potentially carcinogenic they are now stepwise abolished, which has led to intense research for possible replacements. In summary, paint coatings are essential in one way or another toprotect steel products from corrosion and there is a need to replace conventional paint contents by environmental friendly and nontoxic formulations. The uses of CPs have shown this promise(6-9, 21-23) .
Conducting Polymers: What makes polymers conductive?
Conducting polymers are polymer materials with metallic and semiconductor characteristics, a combination of properties not exhibited by any other known material. A key property of a conductive polymer is the presence of conjugated double bonds along the backbone of the polymer. Since the electrons in a conjugated system are only loosely bound, electron flow may be possible. Every bond contains a localized“sigma” (σ) bond which forms a strong chemical bond. In addition, every double bond also contains a less strongly localized“pi” () bond which is weaker. These enable the electrons to be delocalized over the whole system and so be shared by many atoms. This means that the delocalized electrons may move around the whole system. However, conjugation is not enough to make the polymer material conductive. In addition, the polymer material needs to be doped for electron flow to occur. Doping is either the addition of electrons (reduction reaction)or the removal of electrons (oxidation reaction) from the polymer. Once doping has occurred, the electrons in the bonds are able to “jump” around the polymer chain. As the electrons are moving along the molecule, electric current occurs. For better conductivity the molecules must be well ordered and closely packed to limit the distance “jumped”by the electrons. The conductivity of conducting polymers can be tuned by chemical manipulation of the polymer backbone, by the nature of the dopant, by the degree of doping, and by blending with other polymers. As discussed earlier, Shirakawa, MacDiarmidand Heeger, and coworkers discovered that when silvery films of the semiconducting polymer, trans “polyacetylene,” (CH)xare exposed to chlorine, bromine, or iodine vapor, uptake of halogen occurs, and the conductivity increases noticeably (over 107 in the case of iodine). Also it has been reported that depending on the extent of halogenation, silvery or silvery black films, high conductivity at room [3-5] temperature can be obtained . Idealized structure of some of the CPs is given in Fig. 2.
Fig.2: Idealized structure of some of common Cps
Electronic conductivity of conducting polymers
An example of polyacetylene is taken to explain electronic conductivity of CPs, and illustrate the principles of conduction mechanism in conducting polymers, because of the simplicity of its structure. The polyacetylene chain consists of single and double bonds which are situated in alternative sequence. As shown in figure 3a and 3b the polyacetylene has a degenerated ground state. In other words there is more than one equi-energetic resonance structure. When both structures are present in a single polymeric chain, a defect results where the two structures meet (Fig. 4a). This defect is called a soliton and consists of a single unpaired electron, however the overall charge still equals zero. Hence the above soliton does not carry any charge and therefore it can be called as neutral. By controlled addition of p – doping anions which consume free electrons a positive soliton can be created (Fig. 4b). The n – doping of polyacetylene will result in negative soliton (Fig. 4c). Neutral and charged solitons are stable. Their stability is achieved by spreading the charge over the several monomer units. The presence of solitons in the chain influences the band structure of the polymer. Due to them a new energy level is created in the middle of the band gap which can accommodate zero, one or two electrons per soliton. When the doping level is sufficiently high the soliton states may interact between each other and form a soliton band. Solitons may arise as a result of thermal isomerization of cis – polyacetylene to the trans structure. Solitons may move along the polymer chain by successive alteration of neighboring single bonds. They can propagate as a wave which is comparable to a moving electron or hole in semiconductors. Solitons may exchange electrons between neighboring chains according to “intersoliton hoping” mechanism which is explained by(24)
Fig. 3: Degenerated states in polyacetylene with reversed order of alternating bonds
In case of heterocyclic polymers like polypyrrole, polythiophene, etc. they have non degenerated states. Hence, in such structures formation of two single solitons is not energetically favorable. Hence polarons defects are created and at sufficiently high doping levels they recombine into bipolarons. These polarons or bipolarons are responsible for conductivity in these heterocyclic polymers. The band structure of Polymer with solitons and bipolaron is shown in Fig. 5.
Fig. 4: Solitons in the Polyacetylene Chain a) Neutral, b) Positive and c) Negative
Conducting polymers in coatings for corrosion protection
CPs have been identified as novel corrosion inhibiting coatings for metals. CPs are defined as “polymers that conduct electric currents without the addition of conductive (inorganic) substances. ”Various CPs are available in commercial forms. These include polyaniline (PANi), polypyrrole (PPy), polythiophene (PTh) and a few more, such as polyindole, polycarbazole ( 25, 26). The electrical conductivity in these polymers is considered to be intermediate between semi-conductors and metals. The conductivity ranges from 1- 1000 S/cm2. Amongst all the CPs studied so far, polyaniline is the first to achieve commercial availability. Chemical or electrochemical oxidation methods are used for the synthesis of CPs. Since 1980s, CPs have found their applications in the field of corrosion protection for metals and metals alloys. The effective use of CPs for corrosion protection of metals can be carried out by different methods; like formulation of polymers with paints, by electrodeposition of conducting polymers onto metal surface and by direct addition of polymers in the corrosive solution as corrosion inhibitors. Their ease of synthesisboth via chemical and electrochemical methods coupled with electrical conductivity makes them evenmore attractive as anticorrosive materials (22, 27-28 ).
Fig. 5: Band structure of polymer with soliton states and bipolaron states
Coatings on the surface of metals by polymeric materials have been widely used in industries for the protection of these materials against corrosion. Several mechanisms of interaction are possible for performance of CPs on variety of substrates and these have been cited in literature (23, 29-33 ). These mechanisms are mainly dependent on several factors likethe type ofsubstrate used, use of specific conducting polymer, itschemical composition, physical properties, amount andtype of dopant used, amount of CP used, and its relative proportion with other active ingredients in the coating. Surface preparation also has a large effect on the functional properties of the Cps. This becomes more important when there are issues related to adhesion and growth of possible corrosion products underneath the coating. The exposure environment and pH of local area also have a strong effect on the performance of these polymers as coatings. For electro polymerization, surface cleaning is very important. Poor surface preparation compromises the consistency of the deposited layer resulting in surface defects giving rise to poor corrosion protection. A CP can be used as a primer alone(34) or as a primer coating with conventional topcoat or it can be blended with conventional polymer coatings or can be used as anticorrosive additive in the coating formulation(35) .
In this article different method of synthesis, protection mechanism and applications for corrosion protection of following CPs is extensively reviewed.
- Polyaniline (PANi) Conducting Coatings
- Polypyrrole (PPy) Conducting Coatings
- Polythiophene (PTh) Conducting Coatings
Polyaniline conducting coatings
Among the CPs, Polyaniline (PANi) began to be investigated extensively some decades ago and attracted interest as a conducting material because of its low cost, straightforward synthesis that proceeds with high yield, environmental stability, and electrical conductivity. It is characterised by a relatively wide potential stability, a reproducible synthesis and a well behaved electrochemistry showing different domains of conductivities, which refer to different oxidation states of the polymer.The generalised formula of the base form of PANi consists of alternating reduced and oxidized repeat unit (Figure 6). The terms 'leucoemeraldine', 'emeraldine' and 'pernigraniline' refer to the different oxidation states of the polymer where y = 1, 0.5, and 0, respectively. PANi can be rendered conducting through two independent ways: oxidation of the leucoemeraldine base, or protonation of the emeraldine base. Depending on the oxidation state and the degree of protonation, PANi can be either an insulator or a conductor ( 3 6 ) with different conductivity . Emeraldine base (EB), itself is nonconductive and it hasto be doped to become conductive. On doping, the por – charge carriers are inserted in the polymer molecules, bothchemically by Lewis acids and bases and electrochemically. The most popular way of doping polyaniline consists ofusing protonic acids and creating pcharge carriers. Different inorganic and organic protonic acids are used asdopants like hydrochloric acid, p – tolunesulphonic acid, camphor sulphonic acid, dodecyl benzene sulphonic acid, perchloric acid, oxalic acid, etc. These acids determine not only the conductivityof EB but also its stability and compatibility with apolymeric matrix. As mentioned before, PANiis shown to have good anticorrosion properties, also in controllingpitting corrosion resulting from the permeation and breakdown of the protective coating.In recent years, several methodologies have been proposed for the application of PANicoatings: (i) as a primer alone; (ii) as a primer coating with conventional topcoats or with otherCPs films; (iii) blended with conventional polymer coatings, such as epoxy or polyurethane;(iv) as an anticorrosive additive to the paint formulation; (v) as a matrix where nano particles or nano structures materials are incorporated.
Method of designing polyaniline coatings:
Fig.6 : Forms of PANi
The first anticorrosion coatings based on CPs were made of pure polymers, in doped or undoped form. Their ability to transport and store charge has been believed to be the principal reason for their reliability to anodically protect metals against high corrosion rates. However the use of CPs on active metals has been not common because of high positive potential required to form the polymers by electrochemical methods. For instance, to polymerize aniline potentials of the order of 1 V is needed. At these potentials, the most part of metals are corroded too rapidly to allow the formation of a CP on their surface. For this reason, the first CPs coatings were applied on inert metals, such as stainless steels, and it was only in the second half of the 1990s that PPy and PANi started to be grown on aluminum and iron electrodes. These polymers can be prepared by either electrochemical polymerization technique or chemical polymerization technique. The latter one is less costly than the former one. Table 1 illustrates the different methodology used for the synthesis of PANi on different substrates.
Mechanism of corrosion protection by PANi
The mechanism of corrosion protection due to PANi is not understood completely. There are four possible proposed hypotheses(17, 20, 21and 23):
Anodic Protection Mechanism: This mechanism suggests that PANi containing coatings may lead to the formation of protective layers of metal oxides on the metal surface thus preventing corrosion by shifting the (17, 20) potential towards the passive state(17, 20)
Controlled Inhibitor Releas e Mechanism: According to this mechanism, the oxidized and hence doped CP-based coating deposited on a base metal substrate may release the anion dopantupon reduction, which is driven by a defect on thecoating from coupling to the base metal. In thecase of doped PANi, the anions are released not only through reduction mechanism but also dueto a simple elimination of acid-dopant if it issoluble in water.
It is believed that when a metal comes into contact with a doped semiconductor or an electronically conducting polymer, an electric field is generated that would limit the flow of electrons from the metal to an oxidizing species, thus preventing or reducing corrosion rate.
CP coatings form a dense, adherent, low porosity film and maintain a basic environment on a metal surface thus restricting access of oxidants and preventing the oxidation of the metal surface(56).The less porous the CP layer the better is the barrier effect and the lower is the transport rate of O2 and H2O into the polymer. By increasing the compactness and adherence to the substrate, the reaction site of the O2 reduction moves from the M|CP-coating interface to the CP coating|solution interface. The shift of the oxygenreduction site on the polymer surface results in the decrease of the reduction products (i.e.,OH-) across the M|CP coating interface, preventing disbondment and delamination of the coating. On the other hand, O2 reduction is involved in the local reoxidation of the CP and constant active role of the CP coating in case that local pinholes or small-size defects are formed. Thus, increasing the barrier effect by processes that inactivate CPs should be avoided. As far as the CP is in its conductive form the OCP of the metal |CP-coating| solution system is in the passive state. The site and the kinetics of the oxygen reduction are important for the long-term protective properties of the coating. It is generally observed that dehydration of CP electro deposited films on metals from aqueous medium improves the barrier effect.
In the recent years many papers have reported the use of CPs as materials for anti-corrosioncoatings, applied on both ferrous and non-ferrous metals, providing a potentially cheaper alternative to chromium and phosphate treatments and their pollution.
Applications of polyaniline conducting coatings in corrosion protection
In metal protection from deplorable corrosion: S Sitaram et al(21). and Wessling, B(32) have illustrated the active role of Polyaniline in corrosion protection. They have also pointed out the importance of the reversible emeraldine leucoemeraldine redox reaction and its intermediary role by the cathodic oxygen reduction and acceleration of the passive layer formation on steel. T. Schauer et al(34). has studied the mechanism of the protective action of polyaniline primers on steel. They did not find any reversible emeraldine <-> leucoemeraldine transitions Therefore it is believed that these processes do not contribute essentially to the corrosion protection by polyaniline. To extend this idea further, Abdolreza Mirmohseni et al(41). prepared Polyaniline coatings in different solutions and compared it with conventional polyvinyl chloride (PVC) polymer coating system. They observed that polyaniline coating in many cases could be preferred over a conventional polymer such as PVC which was being confirmed by electrochemical measurements in corrosive environments in 3.5% NaCl solution.
Incorporation of CPs in a conventional polymermatrix can lead to a useful increase in properties like conductivity and corrosion performance. Though this increase may not always be significant, incorporation of clay in a polymer matrix enhances the barrier, thermal, and anticorrosive properties of the coating. Due to the platelet nature of clay the path of corrosion ion, water, and oxygen ion ingress lengthens, resulting in better corrosion resistance. Both PANi and clay have been added to the polymer matrix in order to have improved properties. PANi and hydrophilic as well as organophilic montmorillonite (MMT) displayed better corrosion performance as compared to the pure PANi containing coatings (50).
Corrosion protection in steel reinforced concrete structures
Corrosion of reinforced concrete structures causes substantial financial losses. The direct cause of destruction is increase of volume of steel reinforcement as the result of corrosion product formation. This causes the increase of stress and cracking of concrete. This can be addressed by using PANi for the protection of concrete steel bar reinforcement. K. Darowicki et al(43). have studied the electric and electrochemical measurements of specially prepared conducting coatings and thereby evaluated their suitability as anodes in cathodic protection of reinforced concrete structures. Saravanan et al(30). have studied epoxy based coating system containing conducting polyaniline as one of the pigments for corrosion protection of mild steel bars reinforcement. In this study, it was found that the base metal undergoes repassivation due to the presence of polyaniline in the coating which is confirmed through electrochemical impedance spectroscopy study. Epoxy PANi coatings showed a better performance against chloride attack, which is one of the major causes of steel bar corrosion in concrete. This was determined from accelerated corrosion studiessuch as anodic polarization, cathodic disbondment test, and salt spray test which are given in Table. Chemical resistance studies of these coatings showed better alkali resistance, giving much more protection against alkaline environment of concrete. Time to cracking study of coated and uncoatedbars in concrete was performed and discloses that polyaniline containing coating system showed withstanding performance of more than 30 days for initiation of crack in concrete even in the presence of 3.5% NaCl solution. Authors claim that this extra protection was provided by repassivation induced by PANi.
Corrosion protection through doped polyaniline films
Different dopants can be used in order to improveconductivity and corrosion performance of conducting polymers. The type of dopant and topcoat used has a strong influence on the corrosion protection(43) . Poly (methylmethacrylateco- acrylic acid) -doped PANi films were used on an aluminum alloy AA3104- H19 by immersion of the pretreated surfaces into a saturated ethyl acetate solution of the polymer(44). In this study, it was found that even though these primers were porous, nonuniform and allowed electrolyte permeation, they were found to provide better corrosion performance when compared with a commercial epoxy resin (ICI 640C692). The type of media used for the study also has an influence on the corrosion performance properties of the final film. S. Sathiyanarayanan et al(45). have studied polyaniline (PANi) pigmented coating on steelby electrochemical impedance spectroscopy(EIS) in 3% NaCl and 0.1N HCl solutions.Studies have shown that polyaniline pigmented coating on steel is highly corrosion resistant in both neutral and acidic media. This coating is found to be highly protective in acidic media than neutral media. It also passivates the iron surface and enhances its protective property. It was also noted that Benzoate doped PANi-containing vinyl coatings were able to protect steel in neutral media better than in acid media(46) . A passivating layer of iron oxide stabilized by PANi was suggested to provide improved protection.
Corrosion protection through polyaniline modified pigments
Kalendova´ et al(47). studied the influence of inorganic pigments like zinc phosphate, calcium borate, and zinc powder along with PANi for corrosion protection of metals. They found a synergistic effect that was more efficient in protecting metal than just using a single defense mechanism. Incorporation of PANi in chlorinated rubber (CR) binder system in 1.5% w/w resulted in increased corrosion protection as compared to the same amount of zinc phosphate in marine coatings(48). Pigments of PANi-H3PO4 and Mg3(Si4O10) (OH)2 in PVC added in metal zinc coatings were proven to slow down the corrosion of steel and improve the mechanical properties of the coating(49).
hybrid nanocomposite coatings for corrosion protection (54) Amir Mostafaei et al. in his research developed Conducting nanostructured nanocomposites using the chemical oxidative method in the presence of aniline and ZnO nanorods by ammonium peroxydisulfate (APS) as anoxidant in camphor sulfonic acid (CSA) medium. They have shown that using PANi and ZnO nanorods in the form of nanocomposite is found to increase the durability of the coating and anticorrosion performance of the substrate. The resultant conducting polymer-oxide nanocomposites coatings can either act as a physical barrier toward corrosive environment or as inhibitory pigments by shifting the corrosion potential of the metallic substrate to higher values and reduction of the corrosion rate. Finally, rod-like morphology of zinc oxide nanostructured materials beside flaky shaped structure of the nanocomposite can play as a building block in paint film and prevent or postpone penetration of water molecules in dried film.The comparison among pure epoxy and epoxy containing PANi and PANi–ZnO nanocomposites is shown in Figure 7.
Fig. 7: Scheme of formation of the passive layer at the interface of the substrate and paint film after immersion in 3.5% NaCl
It clarifies the role of PANi–ZnO nanocomposite which has an improved protection of steel in comparison with use of PANi and ZnO nanorods lonely in a coating system.
Recently, nanocasting was applied to prepared PANi-based advanced anticorrosion coatings from a natural Xanthosoma Sagittifolium with biomimetic superhydrophobic structures (SH-PANI)(55) .These coatings exhibit water-repelling properties with a water angle equal to 156° and good adhesion to the cold rolled steel (CRS). Evaluation of the anticorrosion performance of the SH-PANI-coated steel in 3.5% NaCl showed a better protective ability for the SHPANI in comparison with the corresponding coating with a relatively flat surface. The improved anticorrosion property of the SH-PANI coating was ascribed to the synergistic effect of the hydrophobicity that repels the moisture and aggressive ions and the PANi ability to passivate the steel via the formation of an oxide film.
Miscellaneous applications of Polyaniline in Corrosion Protection:
Hybrid nanocomposite coatings containing dodecylbenzene sulfonic acid-doped PANi and ZnO as nanoparticles dispersed in poly(vinyl acetate) (PVAc) appeared to also exhibit an improved barrier effect as compared to the single-component coating (6). Corrosion studies of steel plates dipcoated with these formulations showed excellent corrosion resistance in saline water for long periods. The results were explained by considering an improvement in the three simultaneously operating mechanisms during protection: (i) barrier effect, (ii) formation of p-n junction that prevents easy charge transport, and (iii) redox behavior of PANi which by storing a high amount of charge induces a self-healing effect that prevents the evolution of active dissolution in possible surface defects by the passive oxide formation. PANi has also been used for the corrosion protection of substrates other than steel and aluminum. APANiinorganic pigment (titanium dioxide) composite was found to perform better than just PANi alone in acrylic resin binder on magnesium substrate (51).PANiwas incorporated in epoxy powder coating formulationwith 1, 3, and 5% w/w. Powder coatings are highly regarded due to their zero volatile organic content (VOC). It was found that after curing the PANi leads to a higher crosslink density and even after a scratch was formed it imparted self-healing behavior to the coatings(52). Glass flakes containing coatings are traditionally used for prevention of corrosion in marine environments, but there is a problem of coating defects and pinholes.The performance of these kinds of coatings was modified by incorporation of PANi into these paints containing glass flakes(53). In this case; passivation due to the PANi was suggested as the cause of improved corrosion protection. In electrochemical impedance measurements it was found that resistance of PANi containing coatings remained about at 108–109Ω cm2 and in the case of coatings containing only glass flakes it decreased to 105Ω cm2. #### Polypyrrole conducting coatings Polypyrrole (PPy) is one of the most promising materials because of its good environmental stability, facile synthesis, its non-toxic properties and higher conductivity than many other conducting polymers. It can be easil prepared from aqueous and organic solvents by both chemical and electrochemical methods. PPy has some advantage with respect to other polymer coatings like ease of electrochemical synthesis on metal surface in a single step, this eliminates many time consuming and expensive processes. The PPy film keeps its high conductivity in a wide pH range, the thermal stability of the film is very high (it is said to be 150 °C in the air) and the mechanical properties of this coating are very good. The good electronic conductivity (δ >1 S cm−2) allows the electrochemical synthesis of a top coating on PPy which can enhance the protection efficiency. PPy is one of the most promising materials for multifunctionalized applications. PPy can often be used as biosensors, gas sensors, wires, microactuators, antielectrostatic coatings, solid electrolytic capacitor, electrochromic windows and displays, and packaging, polymeric batteries, electronic devices and functional membranes, etc. PPY coatings have an excellent thermal stability and are good candidate for use in carbon composites. PPy can be easily prepared by either an oxidatively chemical or electrochemical polymerization of pyrrole.
Fig. 8: Electrochemical oxidation of neutral non-conducting Ppy
Method of designing polypyrrole coatings Ppy coating can be applied to substrates by the electro polymerization process, either potentiostatically or galvanostatically. The Electrodeposition conditions can be optimized to give smooth, adherent coatings on steel and iron. The parameters that have been shown to play a part in PPy film quantity and quality include: electrolyte composition and agitation, monomer concentration, current density, time temperature, type of solvent, water, quantity in the reaction cell, and the type and the size of the counter ion used. The electropolymerization was investigated from various non – aqueous and aqueous electrolytes and it was found, iron dissolution occurs under all test conditions, except nitrate solution. Smooth and adherent PPy coatings are produced on iron from aqueous nitrate solution. The methods involving Electrodeposition through organic solutions is quite easy and achievable. The organic solvents have however many disadvantages incomparison with water: they are toxic, highly flammable and more expensive, so for practical purposes aqueous solutions are preferred. Thus, Electrodeposition of CP onto active metal surface from water solutions became a challenge. The Electrochemical oxidation of neutral non conducting PPy formed through electropolymerization is shown in Figure 8(64) .The neutral PPy formed through electropolymerization technique with a conjugated chain does not possess any conductivity. To add the conductivity into this neutral PPy, further oxidation is required. When the anodic potential is applied to the electrode, an electron is removed from electrons in the conjugated bond, yielding a pair of a radical and a positive charge (or cation) in the PPy backbone. This situation is called radical-cation state or polaron state. When the two radicals in the PPy are combined, the sites of single and double bond are replaced with each other and two cations remain in the PPy, the situation of which is called bication state or bi-polaron state. The cation thus formed in the PPy can move through electron clouds, yielding electronic conductivity in the PPy backbone. Electrodeposition techniques of PPy on different substrates with different solutions have been tabulated in Table 3.
Mechanism of corrosion protection For the corrosion protection, two mechanisms have been proposed; one is the physical barrier effect, and the other is anodic protection.
Barrier Effect Protection: In the barrier effect, the polymer coating works as a barrier against the penetration of oxidants and aggressive anions, protecting the substrate metals. This effect is similar to paint coating which inhibits the substances from penetrating to the substrate steel. Anodic Protection: In the anodic protection, the conducting polymer with the strongly oxidative property works as an oxidant to the substrate steel, potential of which is shifted to that in the passive state. In solution at neutral pH, the corrosion potential (or open circuit potential in corrosion) of bare steel is located in the active potential region and the corrosion rate of the steel is usually relatively high. Owing to the coating of conducting polymer, the maximum current in the active-passive transition region was limited by the barrier effect, and then the potential can be easily shifted to the higher potential in the passive state by the strongly oxidative property of the conducting polymer. In the passive state, the corrosion rate of steel becomes much lower. It is assumed that both the barrier effect and the oxidative property induce the anodic protection. Finally, the potential of the substrate steel may be in agreement with a redox potential of the PPy layer and thus, depends on the degree of oxidation state of the PPy layer. The conductivity of the PPy layer affects the oxidative power which brings about the passive state. If the coating layer haslittle conductivity, the role of the coating as the oxidant is limited in the neighbourhood of the passive oxide. If the layer has enough high conductivity; however, the oxidant power of the whole layer is available and the power increases with the increase of the layer thickness. The oxidation degree and the conductivity are assumed to decline with longer exposure to environment. If oxidants in the environment reoxidize the degraded PPy layer, the oxidation degree and conductivity can be recovered. When the oxidant in the environment, typically oxygen gas in air, can recover the PPy layer, the duration to maintain the oxidative power of the PPy layer can be prolonged and the passive state of the steel underneath the PPy layer can be kept for a longer period.
Applications in corrosion protection
In metal protection from Deplorable Corrosion In the recent past, several reports on the corrosion protection of metals and their alloys by PPy have been published. PPy has been added to epoxy polyamide coatings in 1% w/w. An improvement in corrosion protection was found as compared to the control sample, but higher proportions did not improve the performance. It was observed that the efficacy of CPs depends on the method of application and the conditions under which the experiments were performed. In this study, authors claim that the conducting polymers have a similar electro chemical mechanism of protection as that of hexavalent chromium(12)
J. Reut et.al(33) confirmed the corrosion protective effect of PPy coatings on mild steel by producing a shift of corrosion potential to positive direction and reducing the reduction oxidation current. It was found that the mechanical and chemical treatments of the electrode surface allow improving this effect by creating an additional positive shift of corrosion potential.
Composite material for corrosion protection
Sundeep K. Dhawan et al(77). designed a corrosion resistant epoxy coatings embedded with polypyrrole/SiO2 composite by chemical oxidative polymerization of pyrrole using FeCl3 . 3 The synthesized polymer composite was loaded in epoxy resin to develop coatings for mild steel substrates using powder coating technique. They have added SiO particles purposely during 2 chemical oxidative polymerization of pyrrole to enhance the thermal stability and mechanical integrity of the composite. The schematic view of the synthesis is shown in Figure 9.
The epoxy coating developed on steel substrateis designated as EC and epoxy coatings with different wt% loadingof polymer composite. Impedance measurements were carried out by keeping the test specimens in 3.5% NaCl solution at open circuit potential conditions for 1 hour at room temperature. ore resistance (Rpore), coating capacitance (Cc) were measured and tabulated in Table 4.
The specimen Pcs3 evidenced the highest Rpore (6.4 × 107 Ω) among the test specimens exhibiting its superior barrier property. The coating capacitance (Cc), occur in the decreasing order as, EC, PCs1, PCs2, PCs4 and PCs3. The electrochemical measurements and salt spray tests suggest polymer composites significantly improve the corrosion resistance properties of conventional epoxy coatings. A maximum protection efficiency of 99.99%is achieved for epoxy coating with 3.0 wt% loading of polymer composite.
Fig. 9: Schematic diagram of the synthesis of polypyrrole/SiO2composite
Similar kinds of study were also performed by Gozen Bereket et al. in which the electrochemical synthesis of PPy was achieved by cyclic voltammetry technique from aqueous oxalic acid solution. Hosseini et al. (78) studied PPy, MMT, and epoxynanocomposite materials for their corrosion performanceon aluminum substrate. It was found that epoxy-PPy-MMT nanocomposite displayed better corrosionperformance as compared to epoxy, epoxy-PPy,and epoxy-MMT coatings. So the properties of differentmaterials can be combined together in order toyield a better performance.
Multilayer Coating System
T. Tüken et al.(79)have studied a multilayer coating system of polyphenol and PPy on mild steel. A multilayer coating was prepared on mild steel by electrosynthesis of a thin polyphenol film on top of electro synthesized poly pyrrolelayer. The cyclic voltammetry technique was used for the synthesis of both coatings. They have studied the system coated with PPy and multilayer coated steel using EIS technique. The results for Rp, Ecorr, and Corrosion Rate (CR) are tabulated in Table 5. From the results, it's clear that multilayer coating found to enhance the life of coating as in this case lowest corrosion rate was observed.
The comparison of the corrosion performanceof this multilayered coating and the single PPy coating showed that the multilayer coating could provide a much better protection for corrosion of MS than single PPy coating, for much longer periods. It was concluded that the very thin PPhe film produced on the surface and within the pores of the PPy coating lowered the permeability of the coating and therefore the mobility of the electrolyte solution within the coating decreased significantly. This event improved the barrier effect of the coating against the attack of corrosive agents present in the medium and the protection time was found to be much higher with respect to single PPy coating. In addition, the underlying PPy film was able to provide an effective anodic protection through catalysing the formation of protective ferric oxides. Polythiophene conducting polymers In recent years, electro polymerization of thiophene and its derivatives have attracted much attention and uniform polymer films could be produced on noble metal surfaces with striking electrical properties and stability compared to other conducting polymers(80-82) . The polythiophene possesses high electrical conductivity, environmental stability, large specific surface area, light weight and well-sized and hence can be usually employed in electrochromic display devices, electrochemical sensors and electrochemical capacitors, electronic devices, applications related with their conductive properties and their resistance to polymer oxidation. n general these materials exhibit not only high conductivity but also excellent stability in the oxidized state, which are expected to be useful properties within the field of corrosion protection. Furthermore, it should be noted that the preparation of simple PTh does not require complex chemical or electrochemical processes, this being a practical requisite for the eventual commercialization of this type of additive in the future(83,84).
Method of Designing Polythiophene Coatings
Three approaches to polymerization of thiophene have been reported in the literature: (1) electropolymerization, (2) metal-catalyzed coupling reactions, and (3) chemical oxidative (85) polymerization.Liu R C et al. have been designed and successfully prepared polythiophene by chemical Oxidative polymerization in the presence of phase transfer catalyst (PTC) cetyltrimethylammonium bromide (CTAB) using triethanolamine (TEA) and Ammonium persulfate (APS). During the course of the reaction, the solution changed from colorless to opaque, and finally to black.
L. Ai et al(86). have reported a facile template-free method by using two different solvents: deionized water and acetonitrile, combining with FeCl2 as catalyst and H2O2 as oxidant to synthesis poly-thiophene nano structures. The mechanism of chemical oxidation polymerization to prepare polythiophene microspheres and its preparation process are illustrated in Figure 10. They have successfully prepared the PTh microspheres with uniform particle size with narrow particle size distribution.
However, the high oxidation potential value of thiophene constitutes a substantial drawback for synthesis of PTh films on oxidizable metals(87) . Nevertheless, there have been few attempts taken to prepare polythiophene by electropolymerization method. G. Kousik et al(88). have described the method for electropolymerization of thiophene on mild steel surface and also discussed the electrochemical characterization for the same. They found that it is possible to generate PTh coatings on mild steel using acetonitrile as a medium and thereby obtained PTh coated mild steel shows passivation mechanism of protection which was proved using EIS technique.
T Tuken et al(89). have discussed the method of synthesis of PTh on PPy modified mild steel surface. They have coated the MS sample with a thin layer of PPy and this thin layer serve as a primer coating and this will prevent the mild steel dissolution at monomer oxidation and polymerization stage of thiophene. This PPy/PTh coating was found to have very low porosity and exhibited barrier property for extensive periods in 3.5% NaCl solution.
Applications of polythiophene in corrosion protection
In metal protection from deplorable Corrosion
G. Kousik et al. explained the use of PTh coating for corrosion protection of mild steel. They coated the mild steel at different thicknesses and those substrates were studied for corrosion protection in 3.5% NaCl solution using EIS technique. They have proved the protective action of electropolymerized PTh through water uptake and delamination area calculations Further to support the (89) action of PTh coatings, T Tuken et al evaluated PTh coating formed on PPy coated mild steel surfaces. Polarization resistance values were found out using EIS technique and thereby the corrosion performance was studied and some of the constants given in Table 6.
From this study it was concluded that PTh coated mild steel surfaces showed better corrosion performance which was concluded through the corrosion rate (CR in mm per year).
Figure 10 - Mechanism of chemical oxidative polymerization of polythiophene (left) and its experiment procedure (right)
Polythiophene and their derivatives as anticorrosive additives for paints As a part of a wide project devoted to develop new technological applications for simple PTh derivatives, the role of these CP as anticorrosive additive in marine paints were investigated by F. Liesa et al(90). . In general these materials exhibit notonly high conductivity but also excellent stability in the oxidizedstate, which are expected to be useful properties within the field of corrosion protection. C. Ocampo et al.(91) studied different commercial paints like Sigma EP Multiguard 7461(Sigma Coatings Ltd.), Interprime 198 (International Coatings Ltd.), Hempadur 45182 (Hempel) and NZ Primer S (Chugoku Marine Paints Ltd.)by incorporating poly (3-decylthiophene-2, 5-diyl) – regioregular conducting polymer (0.2%, w/w) and studied in corrosive environments. The best protection was provided by Sigma EP Multiguard without PTh derivative,no signal of degradation being detected after 30 days. Conversely, Hempadur 45182 films without conducting polymer start to corrode after only 3 days. The considerable improvement was observed in Hempadur 45182 sample when loaded with conducting polymer. Whereas this PTh derivative was not useful in rest of the primer systems. Figure 11shows the photographs of Hempadur 45182 films inNaCl solution. The conducting polymer changes the yellowish grey colour of the paint to violet colour, despite its low concentration. The improved protection can be easily madeout through the figure as corrosion effect did not appear in the coated surface after 30 days exposure. Similar study has been presented by E. Armelin et al(35). .They have found that organic coatings modified by the addition of a low concentration of CP impart better protection against corrosion to metallic substrates than unmodified paints. In particular, excellent results have been achieved when PTh derivatives are used as anticorrosive additives of epoxy paints.
Epoxy–silicone– Polythiophene interpenetrating polymer network for corrosion protection of steel
Interpenetrating polymer networks (IPNs), a class of polymer alloys exhibits a number of advantageous properties such as corrosion resistance, chemical resistance and improved mechanical characteristics. IPN may be described as a combination of chemically dissimilar polymers in which the chains of the polymers are completely entangled with one another, the entanglement being rendered permanent by homo-cross linking of polymeric chains. S. Palraj et al(92). epoxy–silicone– polythiophene interpenetrating polymer network for corrosion protection of steel. In this study, IPN was prepared by mixing different amounts of silicone resin with a constant amount of epoxy and thiophene monomers with other additives. The best performing IPN was identified, characterized with respect to heat resistant and corrosion properties were studied. Electrochemical impedance spectroscopy (EIS) technique was used to find corrosion performance of the samples. Sample S2-IPN with polyamide (with 0.01250 moles of silicone resin, 0.569 thiophene and 0.346 mole liquid epoxy), the structure of the cross-linked IPN-Polyamide obtained which contributed to the compactness of the coating that made water and chloride ions transport difficult resulting in improvement in corrosion resistance. The charge transfer resistance and double layer capacitance of IPNpolyamide coatings in 3% NaCl for different duration of time is shown in Table. It can be easily made out from this table that in case of S2-IPN incorporated coatings the changes in charge transfer resistance and double layer capacitance are less with little deviation from their initial values. Therefore, corrosion resistance of the steel substrate can be improved using Pth derivative in Epoxy–silicone–polythiophene interpenetrating polymer network.
Fig. 11: On the left, photographs of the painted rectangular test pieces (scale bar: 1 cm): (a) initial sample, (b) and (c) samples without and with conducting polymer after exposure, respectively. In the middle, optical micrographs from the polymeric films (scale bar: 200μm) and scanning electron micrographs on the right (scale bar: 100 μm).
Polythiophene microspheres and their dispersion for waterborne corrosion protection coatings
L. Ai et al. have successfully developed a method for preparation of PTh microspheres. These microspheres were redispersed in distilled water to get the Polythiophene redispersion and these were further evaluated in waterborne epoxy resin. The electrochemical behavior of epoxy coatings containing polythiophene on mild steel was investigated in 3.5 wt% NaClsolution. From Tafel polarization method was used to find out anticorrosion properties of coatings and the results are given in Table 8.
The polythiophene-containing coatings with the lower corrosion current (2.30 × 10-8A/ cm2) showed the much better protective effect than the pure epoxy polyamide matrix.
Recent Developments in Conducting Polymers(93)
The use of CPs as corrosion protection layers is partly motivated by the desire to replace coatings that are hazardous to the environment and to human health. Since the equilibrium potentials of several CPs are positive relative to those of iron and aluminum, they should provide anodic protection effects comparable to those provided by chromate (VI) or similar inorganic systems. Several strategies have been followed to enhance the anticorrosion performance of CPs depending on the metal and corrosive environment, such as (i) copolymerization, (ii) use of multilayers of CPs, (iii) various dopants, (iv)nanostructured CPs, (v) composites and nanocomposites of Cps. The scientists have also given an emphasis to find out new CPs for corrosion protection. Polyindole, Polycarbazole, these are the some examples of new CPs which found emerging technology in CPs for corrosion protection.
Polyindole Conducting Coatings
Tuken et al(94) . have described the method of synthesis of Polyindole films over PPy precoated mild steel surface through electrochemical deposition in acetonitrile medium. The coating system generated showed significant anodic protection. To extend this finding, Tuken et al(95) . able to develop a coating on copper surface and proved the improved performance of corrosion protection of the coated copper surfaces.
- Polycarbazole (PCz) films on Mild (96) steel : In this study, effect of frequently used supporting electrolytes such as LP, SP and TBAP on the corrosion performance of PCz films deposited on 304-SS was explored with the idea of determining their contributions to the protection abilities. * Copolymers of CPs: Structural modification of the CP backbone by copolymerization influences various properties of the polymer such as conductivity, porosity, adherence to the substrate and stability. There are several literatures available where copolymers with PANi, PPy were synthesized and found to improve the final properties of the developed coating systems. * Various attempts have been tried to develop multilayered coatings of CPs, some of them have been already discussed previously. This is also prone as another emerging field of research, and scientists are focusing on this particular area.
Conductive polymers have extensively been found as a proper replacement of conventional coatings because of their good effectiveness and also environmentally friendly nature. The mechanisms by which conductive polymers can demonstrate anticorrosive behavior are:
(a) Conductive polymers lead to formation of an electric field on the metal surface, hindering the flow of electrons from metal to oxidant; (b) Conductive polymers could act as barrier coating by forming a dense, strong adherent and low-porosity film on the metal surface and (c) In presence of conductive polymers, the metal could form a protective oxide layer on its surface.
Methods commonly used for designing of conductive polymers are chemical methods and electrochemical methods, where chemical method is suitable for large quantities of compounds and electrochemical method is suitable for getting a high quality coating with control on its thickness. Electrochemical deposition of CP layers directly on the oxidizable surfaces of metals and alloys using appropriate electrolytes in polymerization solutions is also possible. CPs can be used in various forms for corrosion protection of metals and metal alloys. They can be used as pigments, multilayers, and composites. Corrosion-inhibitive dopants can be incorporated on the backbone of CPs which can be released when CPs are reduced under particular conditions. Using suitable doping ions and an optimized amount ofCP in protective coatings, a self-healing process operates as far as the CP is in its electroactive state. Themorphology of CPs influences the anticorrosion properties of the CP-based coatings. Future investigations exerting more control over the synthetic paths of nanostructured CPs are expected to lead to improved protective CP-based materials with a better performance. Numerous studies in recent times show the tremendous potential of CPs for the corrosion protection of metals and their alloys, which is yet to be fully explored.
The authors take this opportunity to thank Mr. Bhushan Pradhan & Dr. Shekhar Tambe for their valuable suggestions, ideas & inputs during the course of reviewing the discussion. The authors are also thankful toMr. Ramakant Chaudhari and librarian –Mr. Anand Kondalkar for their valuable support.
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Buddhabhushan P Salunkhe* and Shwetal P Rane Asian Paints Research and Technology Center, Turbhe – 400705. Queries and Responses: firstname.lastname@example.org
Shwetal P Rane
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