Corrosion control conductive polymer coatings

Kinlen, RJ., C.R. Graham, and Y. Ding. 2004. Corrosion protection of aluminum alloys by controlled release of inhibitors from inherently conductive polymer coatings. Polym Prepr Am Chem Soc Div Polym Chem 45 (2) 146. 

Soluble conducting polymers can be solvent cast to form coatings. The addition of appropriate substituents to the polymer backbone or to the dopant ion can impart the necessary solubility to the polymer. For example, alkyl or alkoxy groups appended to the polymer backbone yield polypyrroles [117,118], polythiophenes [118], polyanilines [119,120], and poly(p-phenylenevinylenes) [97] that are soluble in common organic solvents. Alternatively, the attachment of ionizable functionalities (such as alkyl sulfonates or carboxylates) to the polymer backbone can impart water solubility to the polymer, and this approach has been used to form water-soluble polypyrroles [121], polythiophenes [122], and polyanilines [123]. These latter polymers are often referred to as self-doped polymers as the anionic dopant is covalently attached to the polymer backbone [9]. For use as a corrosion control coating, these water-soluble polymers must be cross-linked [124] or otherwise rendered insoluble. 

An electrically conductive polymer composite of polypyrrole and poly(ethyl methacrylate) has been prepared by an emulsion polymerization procedure [ 142]. In this case, the relation between conductivity and the polypyrrole content of the composite exhibited a percolation behavior, with conductivities as high as 6-7 S/ctn. Such composites might be amenable to melt processing for coating formation. Composite films consisting of polypyrrole or poly(N-ethylaniline) filler dispersed in a polyimide matrix have been described for potential use as corrosion control coatings for the A1 alloy AA 2024-T3 [143]. 

Gelling, V.J., et al. 2001. Electroactive-conducting polymers for corrosion control 4. Studies of poly(3-octyl pyrrole) and poly(3-octadecyl pyrrole) on aluminum 2024-T3 alloy. Prog Org Coat 43 (1-3) 149. 

Visual observations and spectroscopic investigations on the various aluminum alloys coated with doped polyaniline and doped polypyrrole indicate that there is no long-range electrode polarization phenomenon like that observed on mild steel-coated samples. The corrosion protection of aluminum using conducting polymers is affected by the composition of the aluminum alloy. Corrosion inhibition is controlled solely by anodic protection. Doped polyaniline coatings provide better corro-... 

Conducting polymers can be prepared by chemical or electrochemical techniques. Electrochemical synthesis provides easier routes when compared with chemical synthesis and allows control over film formation, especially relevant if polymers are required as thin films deposited on the surface of metallic substrates. However, electrochemically synthesized polymers are usually more porous, a feature that requires consideration when a barrier effect is necessary. Another important aspect in the corrosion field is that the application of potential/current necessary to promote electropolymerization may accelerate dissolution (corrosion) of the metal. In some cases, an oxide pre-layer is deposited between the metal and the polymer to promote adhesion and hinder metal dissolution during the electropolymerization process (Tallman et al., 2002 Spinks et al., 2002). Alternatively, the application of layered coatings based on different conducting polymers can be a strategy to overcome the problem of metal dissolution. In the work of Lacroix et al. (2000), a layer of PPy was firstly deposited on zinc and mild steel in neutral conditions, followed by deposition of PANi in an acidic medium, because the direct deposition of PANi on those metallic substrates was not possible in an acidic medium, causing dissolution of the metal. 

SECM has also been recently used to investigate the growth of conducting polymers deposited onto metal surfaces both chemically [73] and electrochemically [10] from solution. Our own interest in conducting polymers stems from their potential as alternatives to chromate conversion coatings for corrosion control of active metals and metal alloys. Specifically, we are investigating the use of hydroxybenzene sulfonates as electrochemical mediators for the electrodeposition of polypyrrole... 

Controlled release of corrosion inhibitors maybe involved in new-generation corrosion protection coatings based on conducting polymers. Considerable evidence indicates that conducting polymers provide beneficial protection to many metals in a corrosive environment. However, the mechanism by which this protection is afforded is not well understood at the present time. Many studies since the mid 1980s have shown that a coating of PAn, PPy or PTh can inhibit the corrosion rate of mild steel, stainless steel, aluminium ... 

Several additional physical processes potentially exist that can create somewhat more complicated equivalent circuit diagrams, as is evident for example from discussions in Chapter 10 on conductive polymer films. For instance, additional macrodefect corrosion and diffusion effects may develop between the coating and the surface, with an additional low frequency relaxation becoming visible in the Nyquist plot. The interface between a pocket of solution and the bare metal is modeled as a double-layer capacitance in parallel with a kinetically controlled charge-transfer resistance R, which can also often include the diffusion element associated with corrosion products in series with R. If the diffusion element represents a finite diffusion, an additional Rpiipp I element appears and a third relaxation at low frequencies,... 

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