The effect of elevated temperature refinery service on the chemistry of M23C6 carbides in 9Cr-1Mo steel

In a previous study the influence of sulphur-bearing refinery environment on the corrosion behaviour of commercial 9Cr-1Mo steel has been investigated [1]. It has been stated that the progress of high temperature sulphide corrosion can be followed by internal carburization of this steel due to a release of carbon from the sulphur-attacked carbides and diffusion of carbon into the steel interior, accompanied by formation of new carbides, or growth of the existing M23C6 carbides. In the study presenting this it has been revealed that at the rapid, intergranular progress of sulphide corrosion in the presence of hydrogen, the growth/formation of M23C6 carbides, which is most intensive near the corrosion-attacked surface, results in an increased chromium and molybdenum content close to this surface, and in increased Cr/Fe and Mo/Fe ratios in M23C6 carbides, which was established by means of EDS analyses. Just beneath the surface, at a depth of less than about 0.2 mm, a steep depletion of the steel in the alloy elements measured by EPM technique can prove the reduced Cr/Fe and Mo/Fe ratios in the carbides.