EFFECT OF PALLADIUM COATINGS ON THE CORROSION POTENTIAL OF STAINLESS-STEEL IN HIGH-TEMPERATURE WATER CONTAINING DISSOLVED HYDROGEN AND OXYGEN

The electrochemical behavior of untreated 316 (UNS(1) S31600) stainless steel and similar samples with thin coatings of palladium are compared in high-temperature water containing varying concentrations of hydrogen and oxygen. The differences in potentials are consistent with the hypothesis that the electrochemical recombination of hydrogen and oxygen on the surface controls the corrosion potential (mixed potential) of the sample. Palladium coatings as thin as 0.03-mu-m result in marked improvement in the response to hydrogen water chemistry (HWC) in lowering the corrosion potential below the critical value for protection against stress corrosion cracking (SCC) even at relatively low hydrogen injection rates and high residual oxygen concentrations. Because the diffusion coefficient of hydrogen is greater than that of oxygen, the protection potential can be achieved with hydrogen:oxygen ratios in the bulk water below the stoichiometric ratio of 2.0 required to form water. Constant extension rate tests (CERTs) clearly demonstrate that under these conditions, SCC can be readily suppressed. The behavior with palladium is representative of that to be anticipated from any of the platinum metals.