The electrochemical behaviour of stainless steels following surface modification in cerium-containing solutions

The influence of cerium treatments on the anodic and cathodic polarisation behaviour of Type 316, 304 and 316L stainless steels is described. A decrease in the rate of the oxygen reduction reaction was observed on treating the stainless steels by immersion in cerium-containing solutions at elevated temperatures followed by polarization in the deep cathodic region. However, no inhibition of the oxygen reduction reaction was observed on simply treating the electrodes in the hot cerium-containing solutions. A more significant reduction in the rate of the cathodic reduction reaction was observed following the deposition of cerium oxides/hydroxide from cerium-containing solutions. This cathodic inhibition effect was found to diminish with continued immersion of the electrode in chloride-containing solutions. Reductions in the passive current densities, increases in the polarisation resistances and increases in the pitting potentials were observed for the stainless steels following treatment in boiling cerium solutions. Similar results were obtained on treating the electrodes in equivalent sodium-containing solutions. These apparent increases in the resistance to pitting attack were attributed to the dissolution of MnS inclusions during the boiling treatments giving rise to surfaces with lower population densities of pit nucleation sites. The reductions observed in the passive current densities were attributed to passivation by nitrate rather than anodic inhibition effects promoted by deposited cerium oxides/hydroxides. (C) 1997 Elsevier Science Ltd.