Mechanical properties and corrosion resistance of αN-rich layers produced by PIII on a super ferritic stainless steel

The properties of the nitrogen expanded ferrite or alpha(N) phase have been scarcely studied to date. This phase was obtained here on a super ferritic stainless steel (SFSS) by plasma immersion ion implantation (PIII), which is a suitable technique to produce near to surface modifications on the Fe-Cr alloys due to the control of energy delivered to the surface. The UNS S44400 SFSS, a special alloy developed for extreme corrosion environments, was nitrided by PIII at temperatures from 300 degrees C to 400 degrees C resulting in the alpha(N) phase as the main product of nitriding on the modified layer. The surface hardness measured by nanoindentation was investigated in the modified layers, reaching up to 3.5 times (the 400 degrees C condition) the hardness value presented by the untreated sample. The nitrided surfaces also presented improved corrosion resistance in comparison to the untreated sample, inferred by potentiodynamic polarization tests in NaCl electrolyte. This was evidenced by increased corrosion and pitting potentials (E-corr and E-p, respectively) and decreased corrosion and passive current densities (i(corr) and i(p), respectively). Moreover, through of the analysis in the polarization curves of the nitrided samples, a greater stability of the passive layer and repassivation processes were identified. The best corrosion resistance was obtained for the sample nitrided at 300 degrees C, which presented the greatest reduction in i(corr) and i(p) (6.6 mu A/cm(2) and 43 mu A/cm(2), respectively) and increase in E-corr and E-p (108 mV and 214 mV, respectively), when compared to the untreated sample. In both untreated and nitrided samples, the addition of Nb and Ti in this alloy to prevent intergranular corrosion led to the formation of N- Ti-Nb-Mo precipitates that acted as initiators of pitting corrosion.