ROLE OF NITROGEN IN IMPROVING THE PASSIVE FILM STABILITY AND PITTING CORROSION-RESISTANCE OF AUSTENITIC STAINLESS-STEELS

Fitting corrosion studies carried out on types 304, 316 and 317 stainl ess steel (SS) alloys containing different amounts of nitrogen in a me dium containing 0.5M NaCl and 0.5M H2SO4 indicated that the addition o f nitrogen improved the pitting resistance, more so, in the case of Mo -containing alloys. X-ray photoelectron spectroscopy (XPS) analysis of the passive films indicated significant changes in the nature and com position of the passive films with nitrogen content. The dissolution s tudies at the active potential indicated the segregation of N and Mo a t the surface when analysed by secondary ion mass spectrometer (SIMS). A mechanism by which nitrogen improves the passive film stability and pitting corrosion resistance is proposed. In the presence of nitrogen , the growth of the stable pit is delayed as nitrogen stabilizes the r epassivated film during initial stages of pit formation. Nitrogen pres ent at the pit site along with Cr and Mo forms ammonium ions and other inhibiting compounds, and thus increases the pH of the solution insid e the pits and stabilizes the passive film against further pit growth. This widens the passive range in which pitting is less probable and i ncreases the resistance against pitting attack.