Role of Nitrogen Uptake During the Oxidation of 304L and 904L Austenitic Stainless Steels

Nitrogen uptake affects corrosion of stainless steel in diesel exhaust systems where urea is injected to reduce NOx. This was examined by exposing plasma nitrided 304L and 904L to humid air at 450–600 °C. The samples were characterized primarily by using XRD, AES and XPS. The as-nitrided samples had 10–20 at.% N in 3–40 μm layers dominated by expanded austenite supersaturated with N, or the S-phase. Nitrogen was detrimental, with oxide thicknesses about 70–100 times thicker than on as-received 304L and 2–5 times thicker on 904L. In the S-phase the Cr activity is reduced by short-range ordering with N, hampering formation of protective oxides. On 304L the fraction of thick oxides decreased significantly with increasing temperature, despite formation of CrN, since formation of bcc promotes Cr diffusion. For 904L the S-phase is more stable and the corrosion less severe due to its alloying content. At 500 °C formation of CrN enhanced the corrosion.