A method for direct dc measurement of the Volta potential is presented. High intensity synchrotron x-ray beams were used to locally irradiate the atmosphere adjacent to the metal surface and produce a conducting path between a sample and a reference probe. The direct measurements of potential in the ionized air could be made at probe heights of around 1 mm compared to less than 0.1 mm for the Kelvin probe. The measurements were similar to traditional Kelvin probe measurements, but had a poorer spatial resolution. In contrast to the Kelvin probe methods, the approach described allows observation of the current as a function of impressed voltage. Methods to improve the special resolution of the technique and applications to corrosion under coating will be presented.

An investigation has been carried out on stainless steel to determine the important parameters that related the changes in pH around pits to the current coming from the pits. Potentiodynamic measurements at 1 mV/s were made on Type 302 stainless steel in agar containing 1M NaCl and a wide range pH indicator. Many pits suddenly appeared at the pitting potential, as indicated by the red, low pH region around the pits. Simulations of the changes in pH were based on diffusion from a point current source. The results also were considered in terms of the effects of a minimum detectable thickness of pH change within the gel.