Performance of nitrogen ion-implanted supermartensitic stainless steel in chlorine- and hydrogen-rich environments

Modified supermartensitic stainless steel surfaces were investigated as protective means against deterioration in Cl-- and H+-rich media. Nitrogen plasma immersion ion implantation at the 300-400 degrees C range produced top nitride-rich layers (with mainly gamma'-Fe4N and epsilon-Fe2.3N, but also with alpha'(N), according to the treatment temperature) followed by underneath expanded martensite cases. The 400 degrees C nitrided sample presented the best performance in potentiodynamic polarization tests with NaCl electrolyte, featured by 4.3 times increase in the corrosion potential and the absence of pits, attributed to the thickest and continuous epsilon-phase containing nitride-rich layer. The hydrogen embrittlement was assessed through cathodic hydrogenation tests. Both reference and 400 degrees C nitrided surfaces disclosed the phenomenon of intensified plastic flow under normal and tangential loadings. A decrease in hardness, elastic modulus and scratch resistance featured a ductile-to-brittle transition on the nitrided surface, possibly due to improved hydrogen trapping by nitride species with subsequent effects in plasticity. In summary, while the nitride layer played an advantageous role in protecting SMSS from chlorine attack, it was susceptible against the hydrogen corrosion.