Corrosion at the urea injection in SCR-system during component test

The corrosion behaviour of the austenitic stainless steels 304L and 904L was investigated after component testing in diesel exhaust urea environment. Cross-sectional analyses and depth profiling of corrosion products were performed with Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The samples of both materials displayed high amounts of silicon oxide on their surfaces, though this did not appear to have an active role in the corrosion. The higher alloyed 904L exhibited only small signs of corrosion and the inner oxide below the silicon oxide was chromium rich. Substantial nitrogen uptake was detected, presumably due to the presence of NH3, cyanates and similar species. The lower alloyed 304L showed more severe corrosion. The oxide on the 304L samples mainly consisted of iron oxide apart from the silicon oxide present. The nitrogen uptake was less pronounced and possibly connected to presence of nitride precipitates. The most pronounced corrosion was detected at sites where deposits frequently form and decompose. Those deposits are a probable nitrogen source and may also form compounds enabling the breakdown of the oxide.