The corrosion susceptibility of Ti, Ti}6Al}4V and Ti}45Ni was studied in a bu!ered saline solution using anodic polarisation and electrochemical impedance measurements. Pitting potentials as low as#250 mV(SCE) were recorded for Ti}45Ni and once initiated pits continued to propagate at potentials as low as !150 mV(SCE). It was possible to increase the pitting potential of Ti}45Ni to values greater than #800 mV(SCE) using a H O surface treatment procedure; however, this surface modi"cation process had no bene"cial e!ect on the rate of pit repassivation. Impedance spectra, recorded under open-circuit conditions, were modelled using a dual oxide "lm model; a porous outer layer and an inner barrier oxide layer. The nature of this porous outer layer was found to depend on the nature of the electrode material and the presence of phosphate anions in the saline-bu!ered solution. The porous layers formed on Ti}45Ni and Ti}6Al}4V in the presence of phosphate anions had low resistances typically between 10 and 70 cm . Much higher porous layer resistances were recorded for Ti and also for Ti}45Ni and Ti}6Al}4V in the absence of the phosphate anions.

Polyaniline films were electrodeposited at pure aluminium from a tosylic acid solution containing aniline. These polymer films exhibited similar characteristics as pure polyaniline electrosynthesized at an inert platinum electrode, when removed from their respective substrates and dissolved in NMP. Both polymers had similar molecular weights and similar UV-visible absorption spectra. However, the aluminium substrate had a considerable effect on the electrochemical activity of the films. The polyaniline films deposited at aluminium appeared to lose electroactivity and the electrochemical impedance data were governed by the oxidized aluminium substrate. This is consistent with a galvanic interaction between the polymer and the aluminium substrate, giving rise to oxidation of the aluminium and reduction of the polymer. The polyaniline deposits appeared to offer only a slight increase in the corrosion resistance of aluminium. Surface potential measurements, using a scanning vibrating probe, showed that attack initiated underneath the polymer under anodic polarization conditions, indicating that chloride anions diffuse across the polymer to react at the underlying aluminium substrate. #

Adherent and homogenous polyaniline and polypyrrole films were electropolymerized onto iron from aqueous oxalic acid and phosphoric acid solutions, respectively, while copolymers of polyaniline and polypyrrole were formed on aluminium from a tosylic acid solution. In all cases, the polymers exhibited good corrosion-protection properties. In the case of iron, the polypyrrolephosphate system showed more superior corrosion-protection properties than the corresponding polyaniline-oxalate system. This was attributed to the greater stability of the phosphate layer deposited at the iron electrode.

Adherent polypyrrole films were electropolymerized from a tosylic acid solution onto an aluminium substrate and used for the reduction of Cr(VI). The reduction of Cr(VI) to the trivalent state was found to proceed by pseudofirst-order kinetics with an activation energy barrier of 15.13 kJ mol )1 . This catalyst could be regenerated by reducing the composite at a sufficiently negative potential, in the absence of Cr(VI). However, it was found that the catalyst functioned also through a self-sustained regeneration mechanism. In this mechanism, Cr(VI) was reduced by PPy 0 to generate PPy þ , but the PPy þ was subsequently reduced to PPy 0 by oxidation of the aluminium substrate, thus regeneration of the catalytic PPy 0 surface giving rise to a synergistic action between the polypyrrole and the aluminium substrate.