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 examining potentiodynamic polarization curves. Corrosion behavior was compared to previously reported sensitization values. The SS was heat treated at temperatures of 600°C to 1,000°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°C at a scan rate of 100 mV/min. Corrosion currents measured at the corrosion potential did not change in the same manner with increasing aging time as values for the degree of sensitization (DOS). This was believed to be caused by greater sensitivity to chromium depletion during potentiodynamic polarization testing compared to sensitization measured by electrochemical potentiokinetic reactivation (EPR) testing (i.e., the minimum chromium level in the depletion zone required to cause sensitization, ≈ 14 wt%, was lower than that required to degrade corrosion behavior as measured by potentiodynamic polarization testing). Polarization curves exhibited two oxidation peaks in the active region and two in the passive region. All were functions of aging time and temperature. Solution analyses as a function of applied potential and x-ray diffraction (XRD) analyses were used to help identify the matrix components that might be responsible for those peaks.