A Mechanistic Investigation of Polyaniline Corrosion Protection Using the Scanning Reference Electrode Technique

Abstract

The need for pinhole-free coatings (necessitating multiple coats) coupled with growing environmental concerns involving heavy metals has lead to a new coating strategy employing inherently conductive polymers as a key component. The first documented observations of corrosion protection of steel by polyaniline were reported in 1981 by Mengoli et al., 1 though legend has it that aniline still bottom residues were used as anticorrosion paints for rail cars in the early 1900s. 2 Since then a number of papers have been published on the corrosion protection of carbon steel, 3-7 stainless steel, 8,9 iron, 10 titanium, 11 copper, 12 and aluminum 13 with ICPs. McAndrew has recently published a review article on the subject. 14 It has been postulated that the ICP stabilizes the potential of the metal in a passive regime, maintaining a protective oxide layer on the metal. Oxygen reduction on the polymer film is thought to replenish the polymer charge consumed by metal dissolution, thereby stabilizing the potential of the exposed metal in the passive regime and minimizing the rate of metal dissolution ICP reduction/metal oxidation (1/n)M (1/m)ICP m (y/n)H 2 O r (1/n)M(OH) y (ny) (1/m)ICP o (y/n)H [1] ICP oxidation by molecular oxygen (m/4)O 2 (m/2)H 2 O ICP o r ICP m mOH [2] Thus, PANI in its various oxidation states (Scheme 1) serves to mediate the anodic current between the passivated surface and oxygen reduction on the ICP film. In an acidic environment, emeraldine salt (green color) may be reduced reversibly to a leuco base. In a basic environment, emeraldine base (blue color) may be reversibly reduced to the leuco base. What Eq. 1 and 2 do not take into account is the nature of the dopant ion. In the work cited above, in which doped PANI was used in anti-corrosion applications, the usual dopants were sulfonic acids. A major drawback to the use of sulfonic acid-doped PANIs is that the dopant level generally comprises over 10% of the weight of the coating. 15 In addition, sulfonic acid itself is corrosive to carbon steels with passiva-tion observed only at high concentrations approaching 30% w/w. 16 Very little is known about the relative anticorrosion effectiveness of PANI doped with organic acids other than sulfonic acids. Since phosphonates are widely used in applications for minimization of scale formation and metal corrosion in aqueous systems, 17-19 it seemed to us these dopants may possibly enhance the corrosion protection of PANI with respect to sulfonic acid dopants. This paper describes the application of the scanning reference electrode technique (SRET) to the study of polyaniline (PANI) coatings on carbon steel. Results are presented which demonstrate conductive PANI "passivates" pinhole defects in coatings on carbon steel. Standard epoxy systems, on the other hand, were found to exhibit both anodic and cathodic pinholes. It is also shown that phosphonic acid salts of PANI are more effective for corrosion protection than sul-fonic acid salts (i.e., galvanic activity, as measured by the SRET, decreases with time). A model is proposed which entails passivation of the metal surface through anodization of the metal by PANI and formation of an insoluble iron-dopant salt at the metal surface. SRET Background SRET is a noninvasive electrochemical method allowing the measurement of localized variation of potential (due to current flow) over the surface of an electrochemically active specimen. The technique relies on detecting extremely small potential variations which exist over the surface of active materials in an electrolyte. The SRET technique has been to investigate weld metal corrosion, 20 pit initiation , 21 and localized corrosion of stainless steels 22-24 and galvanized steels. 25 Several variants of the SRET techniques now exist. 26 We have employed a vibrating probe (EG&G) which allows for increased electrical sensitivity and enhanced system stability. 27,28 The SRET utilizes a very fine platinum/iridium tip that is scanned over the surface of the specimen using an x-y scanning head. The probe is held at a constant height above the specimen surface and oscillated at 80 Hz using a piezoelectric crystal. As the probe oscillates through potential gradient contours normal to current flux lines the change in potential is picked up by a lock-in amplifier. Since it is an ac technique, high sensitivities are achieved with a differential elec-trometer. The SRET is sufficiently stable and sensitive to allow for the Growing environmental concerns regarding the use of heavy metals in coating formulations have lead to a new coating strategy employing inherently conducting polymers (ICPs), such as polyaniline (PANI), as a key component. The principal potential advantage offered by the ICP coating technology is toleration of pinholes and minor scratches. This paper describes the application of the scanning reference electrode technique (SRET) to the study of PANI coatings on carbon steel. SRET results demonstrate that conductive PANI "passivates" pinhole defects in coatings on carbon steel. In addition, it is shown that phosphonic acid salts of PANI are more effective for corrosion protection than sulfonic acid salts. A model is proposed which entails passivation of the metal surface through anodization of the metal by PANI and formation of an insoluble iron-dopant salt at the metal surface.