THE PITTING OF STAINLESS-STEEL UNDER A RUST MEMBRANE AT VERY-LOW POTENTIALS

Stable pitting of Type 304L stainless steel has been observed repeatedly at potentials as low as -140 mV(SCE) in the presence of a rust membrane, produced by corroding an electroplated iron layer. The onset of stable pitting in the 'rusted' specimens coincided with the onset of metastable pitting events on normal stainless steel surfaces. The rust layer acts as a super-crevice, combining the anion-selective action proposed by Sato with a low ionic resistance, and prevents or greatly delays repassivation of metastable pits. Such a layer is especially effective at low chloride concentrations, where conventional crevice corrosion is absent or propagates at a very low rate owing to the high solution resistance to current flow out of the crevice. These results provide an explanation for freshwater microbial corrosion without invoking sulphate-reducing bacteria or other sources of aggressive corrodents.