CORROSION AND POLARIZATION BEHAVIOR OF SENSITIZED HIGH-NITROGEN STAINLESS-STEELS

Corrosion of a sensitized high-nitrogen stainless steel (SS, nominally Fe-19% Cr-5% Ni-5% Mn-3% Mo-0.024% C-0.69% N) was studied by examinin g potentiodynamic polarization curves. Corrosion behavior was compared to previously reported sensitization values. The SS was heat treated at temperatures of 600 degrees C to 1,000 degrees C for times up to 1, 000 h. Potentiodynamic polarization curves were generated in deaerated 1 N sulfuric acid (H2SO4) + 0.01 M potassium thiocyanate (KSCN) at 30 degrees C at a scan rate of 100 mV/min. Corrosion currents measured a t the corrosion potential did not change in the same manner with incre asing aging time as values for the degree of sensitization (DOS). This was believed to be caused by greater sensitivity to chromium depletio n during potentiodynamic polarization testing compared to sensitizatio n measured by electrochemical potentiokinetic reactivation (EPR) testi ng (i.e., the minimum chromium level in the depletion zone required to cause sensitization, approximate to 14 wt%, was lower than that requi red to degrade corrosion behavior as measured by potentiodynamic polar ization testing). Polarization curves exhibited two oxidation peaks in the active region and two in the passive region. All were functions o f aging time and temperature. Solution analyses as a function of appli ed potential and x-ray diffraction (XRD) analyses were used to help id entify the matrix components that might be responsible for those peaks .