Study of local Na+ and Cl− distributions during the cut-edge corrosion of aluminum rich metal-coated steel by scanning vibrating electrode and micro-potentiometric techniques

Scanning vibrating electrode technique (SVET) and micro-potentiometric pH measurements were performed simultaneously to study the cut-edge corrosion of aluminum-rich metal-coated steel. Different areas of pH and anodic/cathodic activity were identified and the analyzed corrosion products were linked to the values of pH measured at regions of precipitation. A step further into the understanding of the corrosion mechanism in the cut-edge is based on the knowledge concerning the movement of the ions which take part in the corrosion process. Local and in situ concentration of the ions of the supporting electrolyte, 0.005M NaCl, were recorded over the cut-edge of metal-coated steel. In this frame, zinc, aluminum and zinc/aluminum metal-coated steel samples were studied by micro-potentiometry with Cl− and Na+ selective microelectrodes. Either Cl− or Na+ distribution was mapped simultaneously with monitoring current density by SVET in continuous immersion. Interesting though unexpected finding is the depletion of Cl− in anodic sites and its accumulation in cathodic sites. Along with this, an increase of Na+ concentration occurs in anodic sites and Na+ content decreases in cathodic sites over the steel. Such distribution is explained by formation of Cl− and OH− containing corrosion products and charge compensation effects. The described Cl− distribution was partly confirmed by ex situ analysis of corrosion products performed by SEM-EDX and Raman spectroscopy.