Crevice corrosion behaviour of a duplex stainless steel

Preferential dissolution behaviour in corroded areas and solution pH inside the crevice in growing crevice corrosion of duplex stainless steels were investigated. Using potentiostatic method, potential was held at various values during the crevice corrosion test (up to 10 ⁴ min). 0.6 M NaCl aqueous solution was used in the test at 50 ± 2 °C. The current first decreased according to passivation of the specimen. After a certain incubation time, the current increased for initiation of crevice corrosion. Then crevice corrosion grew continuously with fluctuation behaviour of current until the test was finished. Preferential dissolution was observed at corroded areas in all the potentials tested. It was found that three regions could be distinguished from the fringe to the center of the crevice; they were the passive region, the preferential region of dissolution of austenitic phase, and that of the ferritic phase. After the test was finished, the specimen was immersed into liquid nitrogen immediately so that the solution inside the crevice was frozen. Then the solution pH inside the crevice was measured by pH test papers. The minimum pH value was lower than the depassivation pH of the specimen in the test environment. According to these results, including our previous study, it is concluded that potential gradient formed from the fringe to the center of the crevice. This result is described by the IR drop theory and a galvanic cell between inside and outside the crevice.