Aqueous corrosion of tantalum- and carbon-implanted low carbon steel

Abstract

The effect of tantalum implantation (5 × 10 16–10 17 ions cm −2; 80–150 keV) on the electrochemical behaviour of a low carbon steel in an acetate buffer solution of pH 5.6 has been investigated. Samples had been irradiated in the 300 keV accelerator at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt, and in the Cockcroft-Walton implantation machine at the Atomic Energy Research Establishment (AERE), Harwell. Tantalum-implanted specimens produced at GSI yielded potentiokinetic polarization curves similar to those obtained earlier for chromium-implanted iron with respect to the hydrogen evolution kinetics, the critical current densities required for passivation and the passive current densities. All effects were more pronounced after implantation of higher doses of tantalum. Successive sweeps indicated the possibility of preferential (selective) dissolution of iron. Tantalum implantations carried out at AERE repeatedly resulted in specimens with differently coloured surfaces, which then showed a much stronger inhibition of anodic as well as cathodic reaction kinetics. Implantations with Ti + and W + ions gave similar results. Carbon analysis using Auger electron spectroscopy and the 12C(d, p) nuclear reaction method showed the presence of carbon layers a few tens of nanometres thick on these samples. Further experiments carried out in an attempt to explain the above phenomena were (a) pre-implantation surface contamination with organic compounds, (b) successive bombardment with Ta +, C + and O + ions, (c) implantation of tantalum in residual gas atmospheres containing organic compounds and CO 2, and (d) implantation of tantalum into carbon (or hydrocarbon) layers deposited onto steel. The results of these experiments illustrate the marked difference between the corrosion behaviour of samples enriched in bulk carbon and the corrosion behaviour of samples with surface layers containing carbon.