Effect of Low-Temperature Plasma Nitriding on Corrosion of 304L Stainless Steel in Sulfate and Chloride Solutions

Conventional plasma nitriding of stainless steels is carried out at about 550°C which leads to an increase of hardness, but also to a decrease of the corrosion resistance. The latter can be avoided by low-temperature nitriding at 380-450°C. Anodic behavior of low-temperature nitrided 304L steel was examined here in 0.1 M Na 2 SO 4 at pH 3, 6, and 9, and in 0.1 M Na 2 SO 4 + 0.4 M NaCl at pH 3. The corrosion resistance of nitrided steel was in the chloride-free solution at pH 3 lower than that of unnitrided steel, whereas in the near-neutral solutions the resistance was higher. In the chloride-containing solution, nitrided steel exhibited much higher resistance to pitting corrosion than unnitrided steel. Nitrogen accelerated anodic oxidation in the initial stages of polarization, and promoted the subsequent passivation. X-ray photoelectron spectrometry analysis indicated the presence of nitrogen-rich species and large amounts of hydrated Fe 3 + / Cr 3 + oxides and of Fe 3 O 4 / Fe 2 O 3 on corroded nitrided steel. It is proposed that the improved resistance of nitrided steel to pitting corrosion can be related to the enhanced anodic reactivity of nitrogen-containing steel which results in the formation of oxide deposits and also in an accumulation of protective nitrogen-rich species.