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.
Pyrrole was successfully electropolymerized at a copper electrode in a near neutral sodium oxalate solution to generate a homogeneous and adherent polypyrrole film. The growth of these films was facilitated by the initial oxidation of the copper electrode in the oxalate solution to generate a copper oxalate pseudo-passive layer. This layer was sufficiently protective to inhibit further dissolution of the copper electrode and sufficiently conductive to enable the electropolymerization of pyrrole at the interface, and the generation of an adherent polypyrrole film. These films remained stable and conducting for periods exceeding eight days and exhibited significant corrosion protection properties in acidified and neutral 0.1 mol dm (3 NaCl solutions even on polarization to high anodic potentials. #
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.
Polyaniline coatings were electrodeposited from an oxalic acid solution onto iron and their electrochemical activity and corrosion protection properties studied as a function of pH. It was found that the coating (emeraldine salt) had a limited effect on the corrosion protection of iron in acidic solutions. However, in an alkaline borate solution, where the conducting polyaniline was converted to the emeraldine base, the coating had a clear beneficial effect on the local breakdown of iron by chloride anions; much higher pitting potentials were recorded following a 2 h immersion period for the polyaniline-coated substrate relative to the uncoated electrode. Relatively small anions, such as acetates, nitrates and borates, were transported readily across the polymer interface. However, the emeraldine base inhibited the transport of the much larger ethylenediamine tetraacetate (EDTA) species to the iron interface, preventing complexation of the iron by EDTA. D
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