Microscale investigations of the metal-dusting corrosion mechanism on mild steel

The internal and external products from metal-dusting corrosion of a mild steel specimen have been investigated, with the intention of further exploring the corrosion reaction mechanism. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been employed. A section of a steel tube, which had been subjected to heavily corrosive attack under controlled conditions, was studied. Electron-transparent TEM samples from this specimen were observed in the cross-sectional and plan-view orientations of the corrosion interface. Deposit on the corroded surface contained amorphous carbon, graphitic crystalline carbon, and decomposition products. Supersaturated cementite is an intermediate reaction product and was present at the surface of the exposed alloy. Surface cementite was seen to decompose locally into the graphite, where graphite basal planes were oriented perpendicular to the cementite surface. Iron was transported through the graphite, and the mild steel disintegrated by continuous formation and disintegration of the surface cementite. The observations are discussed with reference to the prevailing model mechanism for metal-dusting corrosion.