The transpassive dissolution mechanism of highly alloyed stainless steels: II. Effect of pH and solution anion on the kinetics

The effect of pH and solution anion on the kinetics of transpassive dissolution of highly alloyed austenitic stainless steels (AISI 904L, 254SMO and 654SMO) was studied by a combination of electrochemical techniques. The experiments were performed in 0.5 M sulphate and 0.5 M chloride solutions, and in an equimolar mixture of the two. The transpassive dissolution was found to start at higher potentials in solutions with higher pH. The rate of transpassive dissolution was shown to decrease with increasing pH and to be the lowest in chloride solutions and the highest in sulphate electrolytes. The steady-state current vs. potential curves and the impedance spectra of the studied materials in the transpassive potential region were found to be consistent with a proposed kinetic model. The model describes the process as dissolution of Cr as Cr(VI) and Fe as Fe(III) through the anodic oxide film via parallel reaction paths. The kinetic parameters of the model in solutions with different pH values and different anions were determined. The role of pH and solution anion in the transpassive dissolution process is discussed in relation to changes induced by these parameters in the composition of the anodic passive film. The factors determining the efficiency of Fe as a secondary passivating agent are also considered.