1998 Volume 47 Issue 6 Pages 391-395
For the measurement of electrode potential of metals covered with thin solution film, the Kelvin technique was applied, in which a silver wire of 0.5mm diameter with a AgCl coating was used as the Kelvin probe for potential detection. The probe placed in air was oscillated sinusoidally with respect to metal surface. 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 pure water to 0.5%NaCl solution layer with 100 and 400μm thick. Under the solution films, potential on zinc was about -1.2V while that on iron was about -0.7V. Under the pure water film of 100μm thick, the zone of potential transition was 3-4mm wide from the boundary into Fe surface. The width of potential transition was influenced by both solution 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. The effective distance of corrosion protection by zinc is inversely proportional to a square root of ρ/δ, in which ρ is a resistivity of a solution and δ is a thickness of a solution film. Potential distribution at Zn/Fe boundary was explained in terms of“transmission line model”.