The corrosion resistance of austenitic stainless steel, alloyed with nitrogen

The results of corrosion investigations have shown that, even in a very aggressive medium, the austenitic nitrogen-enriched stainless steel NTR50, which was solution heat treated at 1150 degreesC/ quenched - i.e. steel with interstitially dissolved nitrogen in the austenitic base, passivizes directly at a low anodic current density. This is the not case for the comparative AISI 316L stainless steel, although the pitting potentials of both materials are similar. However, significant differences exist between the repassivation capability of nitrogen-enriched steel and that of the comparative AISI 316L steel, which can be attributed to the important role of the nitrogen. Investigations of the passive film confirmed that nitrogen, in the form of N3- and NH4+ ions, acts as an inhibitor of the corrosion process. This means that the time for the initiation of corrosion pits, which are usually the source of stress corrosion cracking, is extended. Due to rapid changes in the character of the electrolyte in the corrosion crack and the repulsing action of the N3- ions towards the aggressive chloride ions, the growth of the crack is either prevented or greatly retarded.