Studying localized corrosion in stainless steels with surface-sensitive methods

Different surface-sensitive techniques have been used to investigate localized corrosion of stainless steels in environments containing thiosulfate, chlorides, and sulfates. Pitting was induced in laboratory by using a scratch technique and electrochemical polarization. The diameter of the pits was less than 0.2mm. The morphology of the pits and the composition of the corrosion product in the pits were studied. The pits on surfaces were investigated directly after the electrochemical tests, on cross sections, and on replicas. The surface-sensitive techniques used provided consistent results with valuable complementary information. The corrosion product in the pits contained high amounts of chromium, and sulfur as sulfides and sulfates. The pit deposit in the molybdenum-alloyed stainless steel contained additionally a high amount of molybdenum. Indications of a very thin chlorine-rich layer next to the parent metal was detected.