Carmel B. Breslin

The corrosion protection afforded by rare earth conversion coatings applied to magnesium

The corrosion protection afforded by cerium, lanthanum and praseodymium conversion coatings formed on pure magnesium and a magnesium alloy, WE43, has been studied using dc polarisation and ac impedance techniques. The coatings, which were formed by immersion in rare-earth salt containing solutions, reduced significantly the dissolution of magnesium in a pH 8.5 buffer solution. With continued immersion of the treated electrodes in the aggressive pH 8.5 solution, the coatings first appeared to become more protective, but after periods exceeding 60 min began to deteriorate. This is attributed to the formation of magnesium hydroxy corrosion products and mixed rare earth/magnesium oxide/hydroxide coatings, which on continued immersion became consumed by the formation of magnesium corrosion products.

Electrochemical studies on the stability and corrosion resistance of titanium-based implant materials

The corrosion susceptibility of Ti, Ti-6A1-4V and Ti-45Ni was studied in a buffered 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 H2O2 surface treatment procedure; however, this surface modification process had no beneficial effect on the rate of pit repassivation. Impedance spectra, recorded under open-circuit conditions, were modelled using a dual oxide film 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-buffered 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 ohm cm2. 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.

The electrochemical synthesis of polypyrrole at a copper electrode: corrosion protection properties

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.

The electrochemical deposition of polyaniline at pure aluminium: electrochemical activity and corrosion protection properties

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.

An electrochemical study of the formation of benzotriazole surface films on copper, zinc and a copper-zinc alloy

The electrochemical behaviour of Cu, Cu–37Zn and Zn in benzotriazole (BTA) containing chloride solutions was studied and compared using potentiodynamic, cyclic voltammetry and electrochemical impedance spectroscopy. The presence of BTA in the chloride-containing solutions gave rise to higher breakdown potentials, significantly higher polarisation resistances and inhibited the formation of CuCl2 and zinc-containing corrosion products. These effects were observed for pure Cu, Cu–Zn and to a somewhat lesser extent pure Zn. The electrochemical impedance data were consistent with the formation of a polymeric BTA-containing layer for all three systems.

Studies on the passivation of aluminium in chromate and molybdate solutions

Abstract Results on the electrochemical behaviour of aluminium in aggressive chloride solutions containing molybdate and dichromate anions are presented and discussed. An ennoblement in the pitting potential of aluminium was observed in dichromate-containing solutions and in non-acidic molybdate-containing solutions. It was proposed that reduction of the molybdate and dichromate species occurred at flawed regions in the passive film, but that the larger polymeric molybdate species, formed in acid solutions, could not be accommodated at the flawed regions. An oxidation peak, observed at −1200 to −1400 mV(SCE), in the molybdate-containing solution was attributed to the growth of a mixed aluminium-molybdenum passive layer. The complete passivation of an aluminium electrode, undergoing meta-stable pitting, was achieved by the introduction of dichromate anions into the test solution. This was attributed to a transformation process whereby chloride-containing aluminium hydroxy complexes were converted into hydrated aluminium oxides.

Photo-induced dissolution of zinc in alkaline solutions

The passive behaviour of zinc in an alkaline pH 13.0 solution under dark and illumination conditions has been studied using dc polarization and ac impedance techniques. Illumination with polychromatic light in the wavelength region 200–900 nm caused significant dissolution of anodically formed passive layers on zinc. This was evident from increased anodic currents, a ten-fold decrease in the charge transfer resistance and an increase in the capacitance of the passive layer. The photo-induced dissolution effect was only slightly evident under open-circuit conditions, where a precipitated or adsorbed layer forms on the electrode. These results are explained in terms of the n-type semi-conducting properties of ZnO.

The activation of aluminium by indium ions in chloride, bromide and iodide solutions

The corrosion and passive behaviour of aluminium in chloride, bromide and iodide solutions in the absence and presence of indium, as activator ions, was investigated using electrochemical techniques. Pit formation mechanisms for aluminium in chloride-, bromide- and iodide-containing solutions appeared to be similar with the caveat that, in the case of iodide anions, liberated iodine attacked the base metal leading to destabilization of the forming film. In addition, the formation of iodic and hypoiodous acids gave rise to the build-up of a highly aggressive pit solution which inhibited repassivation. The difference in the activity of the halide anions was evident also on activation of the aluminium surface by indium ions introduced into the working solution. Greater adsorption of bromide and iodide anions at flawed areas reduced the rate and efficiency of activator deposition at the aluminium surface, with the result that the rate at which the surface was activated was reduced. Activation appeared, also, to be more permanent in chloride-containing electrolytes.

The electropolymerization of pyrrole at a CuNi electrode: corrosion protection properties

Pyrrole was successfully electropolymerized at a copper–nickel electrode in a near-neutral sodium oxalate solution containing Cu2+ cations to generate a homogenous and adherent polypyrrole film. The presence of the Cu2+ ions increased both the rate of the electropolymerization reaction and the adherence of the polymer at the CuNi interface. In the absence of these cations, oxidation of the electrode occurred generating a nickel-rich layer that was not sufficiently conducting, under the electropolymerization conditions employed, to facilitate the electron-transfer reaction and the electropolymerization of pyrrole. These films remained stable and exhibited significant corrosion protection properties in acidified and neutral 0.1 mol dm−3 NaCl solutions even on polarization of the electrodes to high anodic potentials.

Activation of aluminium in halide solutions containing ‘activator ions’

Abstract The activation of 99.999% aluminium by the ‘activator’ elements—indium, mercury, gallium, zinc and tin—was studied using potentiostatic current-time techniques. These ions, dissolved in the test electrolyte, plate on to the aluminium surface giving rise to localized attack which was manifested by current fluctuations or surges. The induction period τ, prior to the onset of attack, was shown to depend on (i) passivation potential; (ii) halide type and concentration; (iii) quality of the passive film; (iv) temperature. The data obtained with mercury ions support the oxide-detachment mechanism advanced by Reboul et al. An alternative mechanism based on the surface-charge characteristics of the new-formed oxide is proposed for activation by indium and gallium species.