The role of nitrogen in the corrosion of iron and steels

The beneficial effect of nitrogen in steels in resistance against localized corrosion, pitting and crevice corrosion is well proven, but not yet fully understood. The results on dissolution of nitrogen steels in acids are controversial, but the dissolution of Fe-N alloys is enhanced compared to pure iron. Many surface analytical studies have been conducted in the recent years to find out about the state of nitrogen, and mainly the presence of N-delta- in the metal surface and NH3 or NH4+ in the passive layer are well established, and moreover, NH4+ is the species transferred into the electrolyte in a wide range of potentials. Formation of NH4- will somewhat buffer the acidification in pits. The possible role of N in the steps of pitting, initiation, growth and possible repassivation are discussed, and most important appears the favourable effect on passivation and repassivation, which is effective also for Fe-N alloys. Many authors assume formation of stable Cr- and Mo-nitrides in the steel surface upon dissolution, and retardation of pit growth by these nitrides, however, this mechanism would not apply for Fe-N alloys. Desorption of the aggressive anions induced by the segregated N-delta- just after the local failure of the passive layer is proposed to be the mechanism by which nitrogen favours the rapid repassivation of pits.