Scanning Kelvin probe analysis of the potential distribution under small drops of electrolyte

The scanning Kelvin probe has been used to map the potential distribution under small drops of NaCl placed on carbon steel, galvanized steel, stainless steel and an aluminum alloy. The concentration of NaCl has been varied and the effect of chromate as inhibitor has been evaluated. In the absence of chromate local anodes and cathodes were established in a short time. For carbon steel similar potential profiles were observed in NaCl and NaClO4. However for stainless steel a much larger potential decrease in the center of the drop occurred in 0.5 N NaCl than in 0.5 N NaClO4 and a marked effect of chloride concentration was observed. When dichromate was added to NaCl a very uniform potential distribution was found for stainless steel and Al 2024. The main driving force for development of local anodes arises from oxygen reduction in very thin films of electrolyte surrounding the NaCl drop