Influence of uv light on the passive behaviour of SS316—effect of prior illumination

Abstract

The influence of uv light (300 nm) on the nucleation of meta-stable pits on type 316 stainless steel in a neutral 0.5 mol dm −3 NaCl solution using current-time measurements is described. A significant increase in the induction periods and a decrease in the rate of pit nucleation were observed for specimens pre-passivated under illumination conditions, indicating that illumination leads to a modification of the passive film that persists even after irradiation is removed. This increased resistance to pitting attack is explained in terms of the semiconducting nature of the passive film and the Point Defect Model (PDM) for the growth and breakdown of passive films. It is proposed that generation of electron-hole pairs leads to a quenching of the electric field strength and consequent modification of the vacancy structure, leading to a decrease in the flux of cation vacancies across the barrier layer. Good agreement was observed between the ratios of experimentally measured and theoretically calculated induction periods for specimens pre-passivated in the dark and in the light.