Pitting corrosion studies on nitrogen-bearing austenitic stainless steels

AISI types 304, 316 and 317 stainless steels (SS) containing various levels of nitrogen were evaluated for their pitting corrosion resistance in an acidic chloride medium containing 0.5M NaCl and 0.5M H2SO4. Potentiodynamic anodic polarisation were conducted at room temperature to determine the critical pitting potentials of all the alloys, and the pit protection potentials of type 316 SS alloys. The pitted specimens were examined by optical microscopy and SEM. The results indicated that as the nitrogen content increased the critical pitting potentials increased, and the critical current density at the active to passive transition decreased indicating an improvement in the pitting resistance. Increase in nitrogen content in type 316 SS raised the pit protection potentials, and the potential range in which no new pits will initiate was also found to increase. The relationship between the critical pitting potentials and the nitrogen content was found using a nitrogen equivalent for molybdenum. Fitting attack was observed at triple points, grain boundaries, inclusions and at inclusion/matrix interfaces.