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. Variations between the potentials measured at the outside and the center of the drop were observed for all systems except those in which dichromate was added to the drop solution. This is consistent with the establishment of local anodes and cathodes in the absence of chromate, where the main driving force for development of local anodes is the reduction of oxygen in very thin films of electrolyte surrounding the drop. For carbon steel similar potential profiles were observed under NaCl and NaClO4 drops, indicating that the nature of the anion is not important in the establishment of local cells on carbon steel. However, for stainless steel a much larger potential decrease in the center of the drop occurred in 0.5 M NaCl than in 0.5 M NaClO4 and a marked effect of chloride concentration and exposure time was observed. When dichromate was added to the NaCl drop a very uniform potential distribution was found. Similar results were obtained with Al2024.