In situ investigation of local corrosion at interphase boundary under an electrochemical-atomic force microscope

Different phases in a multiphase alloy may act as micro-electrodes, resulting in local or internal galvanic corrosion. Using an electrochemical-atomic force microscope (EC-AFM), we investigated local aqueous corrosion of a high-Cr cast iron in spots at different distances from the primary carbide. Results of the study demonstrated that carbides in the cast iron may not act as effective cathodes due to their high electrical resistivity. Instead, the carbide/matrix interfacial mismatch makes the regions in vicinity of the interface highly anodic, which act as anodes while the regions away from the interface act as cathodes. As a result, the corrosion rate at the interface can be considerably larger than that of regions apart away from the interface. Efforts are made to elucidate relevant mechanisms.