Metal-dusting corrosion of low-chromium steels

Metal dusting is an aggressive form of corrosive degradation that Fe-, Ni-and Co-base, high-temperature alloys undergo when subjected to environments supersaturated with carbon (a_c > 1). This corrosion process leads to the actual conversion of bulk metal to powder or dust. The present study focuses on the fundamental understanding of the corrosion of low chromium steels containing about 1.25-13 wt.% Cr in carbon-supersaturated environments (CO-H₂) over the temperature range, 650-1100°F (343-593°C). With increasing Cr content the overall corrosion rate decreases and the corrosion becomes more localized. All low-chromium steels in an overall sense disintegrate by metastable surface M₃C growth and its subsequent decomposition upon carbon deposition in good agreement with earlier research on the metal-dusting mechanism of pure iron. While the presence of a continuous surface layer of M₃C is characteristic of general corrosion, such a layer is absent in low-chromium steels having more than 5% chromium (e.g. 9Cr and 13Cr). High-resolution electron microscopy of such steels reveals, in addition to metal particles and a mixture of graphitic and amorphous carbon, stable carbide (M₇C₃) particles in the corrosion product. The mechanistic aspects of metal dusting are discussed with particular attention to stages of micro-structure degradation process of low-chromium steels.