Influence of thermal oxides on pitting corrosion of stainless steel in chloride solutions

X-ray diffraction analysis, Auger electron spectroscopy, X-ray photoelectron spectrometry, optical micrography, and spot potential measurements have been used to characterise AISI 316L stainless steel specimens with oxide films formed by heating in air, either isothermally (at 200-1000 degrees C) or in a temperature gradient. The pitting corrosion propensities of such specimens were assessed by exposure to FeCl3 solution and the evaluation of nine topographical criteria. Testing was also performed on previously heated specimens after removal of the thermal oxides by pickling. It is demonstrated that thermal oxides facilitate the nucleation of corrosion pits but do not influence their propagation. The effect on nucleation is caused by stresses and lattice defects in oxide film and metal substrate. The maximum susceptibility to pitting liability occurs after heating at 600 degrees C. The removal of thermal oxides formed at 200-800 degrees C by pickling improves the pitting resistance but the initial stability is never restored because of residual stresses and lattice defects on the cleaned metal surface. Indeed, after heating at 1000 degrees C, pitting tendencies are increased by pickling.