A CREVICE-FREE ELECTRODE ASSEMBLY FOR THE DETERMINATION OF REPRODUCIBLE BREAKDOWN POTENTIALS FOR STAINLESS-STEELS IN HALIDE ENVIRONMENTS

In the present study, a 316 wire loop electrode system has been shown to give good reproducible results in a variety of halide environments. An unusual feature of these results is the fact that under all experimental conditions studied, the Br- ion is found to be considerably (approx. 200 mV) more aggressive than the Cl- ion in terms of oxide stability. This trend is also maintained with steels 302 and 304L. The presence of SO(4)2- ions in the halide electrolyte does not alter this order, nor does chemical pre-treatment in acid solutions prior to pitting potential determinations. Pitting potentials measured for the 316 wire loop samples are considerably more noble than those measured for samples of 316 cut from bar or sheet material. It is tentatively suggested that this order of oxide stability may be due to the superior ability of Cl- to adsorb at flaws in the developing oxide film, the wire loop samples containing fewer such flaws.