Anodic Polarization Behavior and Pitting Potentials of High Nitrogen Bearing 18Mn-18Cr Austenitic Steels

Anodic polarization curves in 5.0%H2SO4 and 1.0%HCl solutions, pitting potentials in 3.0%NaCl solution and critical pitting temperatures in 6.0%FeCl3 solution were measured in 18Mn-18Cr-0.5N(N5), -0.8N(N8) and -0.07N-0.5C(N0), and several related alloys. Here, chemical compositions of alloys and solutions are all shown in mass%. Anodic polarization curves of N0, N5, and N8 in 5.0%H2SO4 showed the three peaks in current density with a negative loop. The threshold Cr concentration to appear the negative loop is estimated to be about 19% on the basis of assumption of preferential Mn dissolusion, which is slightly lower than 22% in Fe-Cr binary alloys. The increase in N concentration in an alloy causes little influence on the anodic polarization curves in 5.0%H2SO4, but lowers the weight loss of samples dipped in 0.5%H2SO4. The localized corrosion resistances in chloride solutions are markedly enhanced: both of the critical pitting potential and pitting temperature are improved by increasing the N concentration in Fe-Mn-Cr alloys as well as in Fe-Cr-Ni-Mo alloys.