Surface-Grinding-Induced Recrystallization and Metal Flow Causes Corrosion-Assisted Penetrating Attack of High-Mn-Low-CR Casting Steel in Humid Environments

This study examined the surface-grinding-induced microstructural modifications and corrosion attacks in a penetrating form of a high-Mn-low-Cr casting steel slab under humid environments. Various experimental and analytical findings from field-emission scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and electrochemical analyses revealed that the abrasive grinding process led to the formation of a surface deformed region, comprising a recrystallized fine grain layer and multiple streamlines. Corrosion initially occurs preferentially along the boundary areas where Cr(Mn) C particles are precipitated. Moreover, the corrosion products (Fe-based oxy/hydroxides) with a high volumetric expansion ratio detach readily from the surface deformed regions, facilitating the easy penetration of corrosive media. In contrast to conventional low-alloyed steels, which exhibit uniform corrosion behavior, corrosion-assisted penetrating attacks on ground high-Mn-low-Cr casting steel slabs occur more severely and frequently during the summer/dry season (i.e., relative humidity levels around 60% to 80%, rather than 100%) when a thin water film can form on the steel surface. Based on the result, effective technical strategies in terms of metallurgical and environmental aspects to mitigate the risk of corrosion-assisted penetrating attack of high-Mn-low-Cr casting steel were discussed.