IN-SITU X-RAY MICROPROBE STUDY OF SALT LAYERS DURING ANODIC-DISSOLUTION OF STAINLESS-STEEL IN CHLORIDE SOLUTION

Salt layers play an important role in many electrochemical dissolution processes. The composition of salt films formed on austenitic stainless steel have, for the first time, been determined using in situ energy dispersive x-ray fluorescence microanalysis during dissolution in a chloride solution. The electrode was the cross section of a nickel/chromium steel foil sandwiched between plastic sheets. The foil was electrochemically dissolved producing a pit 1.6 mm deep. The electrode configuration simulated localized corrosion with a one-dimensional diffusion geometry. X-ray fluorescence intensities of chromium K-alpha, iron K-alpha, and nickel K-beta energies were measured as the steel/solution interface traversed a 6 mu m, polychromatic, high-intensity x-ray beam. Qualitative detrminations were made of the composition of the salt layer and the composition of the saturated solution. Salt layer thickness was found to increase with increased applied potentials. The salt layer was found to be rich in iron and depleted in nickel and, particularly, chromium. The effective diffusion coefficients of the dissolved species were determined from the composition of the saturated solution at the interface. Nickel. showed the highest and chromium the lowest effective diffusion coefficient.