The interaction between alloyed molybdenum and dissolved bromide in the pitting corrosion of stainless steels

Artificial pit electrodes of types 304 and 316 stainless steels (304SS and 316SS) have been used to determine transition potentials for salt-film formation (E-T) as a function of the anodic limiting current density (i(lim)), a procedure introduced by Laycock and Newman. Plots of E-T vs log ilim are normally straight lines, and when extrapolated to a current density typical of an initiating pit, provide a model of the pitting potential. NaBr and NaCl solutions were compared using this procedure. It was found that the slope of the E-T vs log ilim plot was higher for 316SS than 304SS in chloride solution, but higher for 304SS than 316SS in bromide solution. Thus, in bromide solutions, the anodic kinetics of 304SS and 316SS become the same at an anodic current density of several A/cm(2). This set of observations can explain the absence of the usual beneficial effect of alloyed Mo on the stable pitting potential in bromide solution. On the other hand, metastable pits, which grow more slowly than stable pits, do show an inhibiting effect of alloyed Mo, in bromide as well as in chloride. The literature on comparative effects of halides on aqueous molybdenum chemistry has been reviewed, but no definite conclusions can be drawn. It is considered unlikely that the possible breakup of large polymolybdates by bromide is the underlying reason for the kinetic effects observed. Complexation of a low oxidation state of Mo is more likely. (C) 2007 The Electrochemical Society.