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The heat treatments of Ni-free SUS445 stainless steel under nitrogen and argon gas atmospheres were conducted at 1373-1473 K in the present study. The heat-treated samples were characterized by an X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GD-OES), and scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX). The corrosion behavior of the heat-treated samples was evaluated by linear sweep voltammetry (LSV) measurements in a 0.5 mol dm −3 H 2 SO 4 solution. The results revealed that the nitrided stainless steel at 1373 K presents the best corrosion resistance and the AlN, expanded austenite γ N , CrN and Cr 2 N phases appeared based on the XRD results. In contrast, the corrosion resistance of the heat-treated stainless steel under an argon atmosphere was decreased and there was no change in the microstructure.

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To improve the performance of stainless steel, we subjected solid state steel to a nitrogen absorption treatment. In the fabrication process, a commercially available high chromium ferritic stainless steel (Fe 22Cr 1Mo) was heat treated at 1423 K in a nitrogen atmosphere. The heat treatment transformed the ferric phase into the austenite phase. This process loaded over 1 mass of nitrogen into the steel material. Most of the added nitrogen formed a solid solution in the matrix, but a minor portion formed nitrides with the very small quantities of elements such as titanium and aluminum that pre existed in the steel. The nitrogen containing steels were then analyzed by pitting potential measurements and ferric chloride corrosion examination. The pitting corrosion resistance of Fe 22Cr 1Mo 1N exceeded that of conventional materials such as Fe 18Cr 12Ni and Fe 22Cr 1Mo. However in the ferric chloride corrosion tests, pits developed in Fe 22Cr 1Mo 1N at temperatures above 323 K. These pits were possibly initiated at the sites of minute nitride resulting from the nitrogen absorption process.