The corrosion resistance of nitrogen bearing martensitic stainless steels

It has been mentioned that nitrogen enhances the corrosion resistance of the martensitic stainless steels. In this work, by means of electrochemical and mass loss tests, the corrosion resistance of three experimental martensitic stainless steels, with partial substitution of carbon by nitrogen were investigated. Martensitic stainless steels type AISl 410 and AISl 420 were also tested, for comparison, and all steels were tested in both hardened and tempered conditions. The resistance to general corrosion in 0.5 M H2SO4 was evaluated by mass loss tests and by potentiodynamic polarization tests, the last through the critical current density and the passive current density. The resistance to pitting corrosion was evaluated by potentiodynamic polarization tests in 0.01 M NaCl + 0.01 M Na2SO4, through the pitting potential. The results of both general corrosion and pitting corrosion tests show that the nitrogen, when present in the solid solution condition, improves the corrosion resistance. it was observed that with the partial substitution of carbon by nitrogen, the passive current densities were lower, in 0.5 M H2SO4, and the pitting potentials were higher, in the 0.01 M NaCl + 0.01 M Na2SO4 solution. The results are discussed basically through the different chemical composition of the steels and the microstructure observed in each sample.