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

Abstract

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 at pH 3, 6, and 9, and in 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 oxides and of 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. © 2004 The Electrochemical Society. All rights reserved.