On the photoinhibition of passivity breakdown on iron in chloride-containing solutions

The photoinhibition of passivity breakdown (PIPB) on iron in buffered chloride-containing solution ([Cl-] = 0.01 M) has been explored using monochromatic light (lambda = 325 nm, hv = 3.815 eV) over a wide range of power densities (0-1680 mW/cm(2)). Our studies have confirmed the observation of Schmuki and Bohni [Electrochim. Acta, 40, 775 (1995)] that irradiation of iron with high-intensity, super-bandgap light prior to the introduction of chloride and prior to inducing breakdown by sweeping the potential in the positive direction, renders the metal unsusceptible to the nucleation of pits on the surface. Furthermore, we have confirmed the persistence of the effect after cessation of irradiation. However, we find that the extent of PIPE is a strong function of the light power density (i.e., intensity) for power densities less than 600 mW/cm(2). At higher power densities, after irradiation for more than 20 min, iron is rendered immune to pitting attack. Part of the enhanced resistance can be attributed to aging of the passive film, as characterized under dark conditions, but photoinhibition becomes increasingly important for passivation times (under irradiation) greater than 30 min. Finally, we explore the relative merits of the two mechanisms that have been previously advanced for PIPE (i.e., photoquenching of the electric field/modification of the vacancy structure and modification of the electronic structure via increases in the bulk doping and surface state density), and we conclude that PIPB is consistent with the point defect model for passivity breakdown. (C) 1999 The Electrochemical Society. S0013-4651(98)01-059-3. All rights reserved.