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(NO), and several related alloys. Here, chemical compositio ns of alloys and solutions are all shown in mass%. Anodic polarization curves of NO, N5, and NS in 5.0%H2SO4 showed the three peaks in curre nt density with a negative loop. The threshold Cr concentration to app ear the negative loop is estimated to be about 19% on the basis of ass umption of preferential Mn dissolusion, which is slightly lower than 2 2% in Fe-Cr binary alloys. The increase in N concentration in an alloy causes little influence on the anodic polarization curves in 5.0%H2SO 4, but lowers the weight loss of samples dipped in 0.5%H2SO4. The loca lized corrosion resistances in chloride solutions are markedly enhance d: 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.