Preformed pits as initiation sites for stress corrosion of austenitic stainless steels
Abstract
A study has been made of the effect of preformed pits on the corrosion behaviour of a heat-treated AISI304 austenitic stainless steel under constant load equal to 80% sp(0.2), at polarization potentials in the imperfect passivity region and under open circuit conditions, exposed to aqueous solutions with diverse SO4/Cl ratios.
With polarization in the passive range and a ratio of 1/0.2, as in the first stage of stress corrosion, intergranular attack on the bottom of pits with critical dimensions has been observed. On the walls of the cracks there are characteristic signs of transgranular attack. An increase in temperature causes an increase in the rate of the phenomenon, without leading to the appearance of different forms of corrosion. With polarization in the active range the trend of the crack is always transgranular. Sometimes the crack walls appear to be marked with fine parallel pleats, parallel grooves and micropits. In some conditions - low temperature and very small pits - the formation of martensite, attacked in a highly selective manner, prevents stress corrosion.
