ケルビンプローブによる金属電極の電位分布測定

抄録

In outdoor or indoor atmosphere, metallic corrosion proceeds electrochemically through thin water film on metal surfaces. The reaction rate, in this case, is expected to be larger than that in bulk solution because of accelerated mass transfer or diffusion. Yet little information has been available on the mass-transfer, current flow and electrode potential in water film corrosion because of experimental difficulties. For the measurement of electrode potential of metals covered with thin water film, the Kelvin technique was applied, in which a gold wire of 0.5mm diameter was used as the Kelvin probe for potential detection. The gold probe placed in air was oscillated sinusoidally with respect to metal surface using a piezo-actuator so that it generates AC current proportional to electrostatic potential difference between the probe and metal surfaces. It is thus possible to measure relative electrode potential without touching the electrolyte. A galvanic couple model comprising of iron and zinc was used for demonstrating potential transition at the interface. Experiments were carried out under dry and wet condition with variety of water film thickness ranging 100-3000μm, and either pure water or 5%NaCl solution. Under the pure water film of 100μm thick, potential on zinc was about -1.1V while that on iron was about -0.6V. The zone of potential transition was 3.0mm wide from the boundary into Fe surface. The width of potential transition was influenced by both water film thickness and salt concentration. The width of potential transition zone under the film of the NaCl solution was larger than that under pure water. This indicates that galvanic effect reaches further into Fe surface in salt solutions. Potential distribution at Zn/Fe boundary was explained in terms of “transmission line model”.