Effect of Local Surface Microstructure and Composition on the Electrochemical Behavior of Hot Dip Aluminum-Silicon Coatings on Steel

The goal of this article was to understand how differences in the composition and the microstructure of aluminum and aluminum-silicon coatings on steel affect the macroscopic electrochemical behavior. In particular, the focus is on the influence of the amount of silicon present in the molten aluminum bath. The electrochemical behavior is studied using electrochemical polarization experiments and is linked to the microstructure, analyzed using scanning electron microscopy-energy-dispersive x-ray (SEM-EDX) analysis. Local Volta potential differences at the metal surface are studied using scanning Kelvin probe force microscopy (SKPFM). Microstructural variations result in differences in the electrochemical behavior of the surfaces when performing full potentiodynamic scans, starting in the cathodic region. These influences, however, are not revealed as such when performing anodic and cathodic half scans, starting at open-circuit potential (OCP). Especially when first polarizing at cathodic potentials in the region of hydrogen gas evolution, significant influence of the nature of the silicon present in the aluminum coating is observed: preferential dissolution of the aluminum matrix is occurring. On this turn, this preferential dissolution leads to a shift in OCP and affects the subsequent anodic behavior. Besides, the presence of silicon also has an influence on the corrosion current density. Local potential differences are observed between the matrix and the more cathodic precipitates in the coating.