Metal dusting corrosion of ferritic steels with varying chromium content

The present study focuses on the metal dusting behavior of ferritic steels as a function of Cr content ranging from 1.25 to 20 wt.% Cr in carbon-supersaturated environments (CO-H₂) over the temperature range, 350 ∼ 600°C. With increasing Cr content the overall corrosion rate decreases and the corrosion becomes more localized. All low chromium ferritic steels (e.g. 1_1/4Cr-1/2Mo and 5Cr-1/2Mo) in an overall sense disintegrate by metastable surface M₃C formation and growth and its subsequent decomposition upon carbon deposition, in good agreement with the mechanism for pure iron. While the presence of a continuous M₃C layer is characteristic of general corrosion, such a layer is absent in ferritic steels having more than 5 pct chromium (e.g. 9Cr, 13Cr and 20Cr). 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 microstructure evolution as degradation proceeds.