Passivity breakdown of Type 316L SS (UNS S31603) in the presence of
aggressive and inhibitive anions has been experimentally studied and the
results have been interpreted in terms of the Point Defect Model (PDM). By expanding
the PDM to include competitive adsorption of and into surface oxygen
vacancies at the passive film/solution interface, the critical breakdown potential ( ) has
been predicted to vary linearly with and with
Cl- -
NO3
Cl- -
NO3
Vc
log[Cl- ] ([ - ] [ - ])
log Cl / NO3 , which is found
experimentally. The slope of vs. is found to be unaffected by , thereby
yielding the same values for the polarizability of the film/solution interface, regardless of
the nitrate concentration. The critical breakdown potential increases weakly with
increasing nitrate concentration at low
Vc log[Cl- ] -
NO3
[ - ]
NO3 but, at a concentration of 0.06 M, Vc
increases sharply and pitting attack is no longer observed. The viability of the PDM for
accounting for passivity breakdown on Type 316 SS is explored by measuring the
voltage scan rate dependence of the critical breakdown potential, from which the critical
areal (two dimensional) concentration of condensed vacancies at the metal/barrier layer
interface can be derived. Good agreement between the value obtained from
experiment and those calculated from structural arguments demonstrate the validity of
the PDM.