Corrosion Resistance of Composite Coating of Thermal-Sprayed Metal and Painting against Atmospheric Corrosion

摘要

The corrosion resistance of the composite coatings of thermal-sprayed Zn, Zn-Al alloys, and Al, with a simplified painting of polyurethane resin, exposed to the atmosphere for 12 years in Sapporo and Tokyo was evaluated using the electric insulation test, the SEM observation, and EPMA. Despite losing electric insulation, the simplified painting showed excellent resistivity compared to a conventional multilayer painting. The diffusion of iron through the sprayed Zn was larger than that through the sprayed Al, while the amounts of chloride and sulfur penetrating through the painting on the sprayed Al was larger than those on the sprayed Zn. This suggests that the thermal-sprayed Zn layer has a tendency of cation selective permeability. The protection at the scratched portion on the composite film seemed to be obtained by the cathodic protection in Zn and the plug effect with the oxide in Al. Corrosivity of the atmosphere caused by chloride and sulfur was larger at Sapporo than at Tokyo. This suggests that the influence of airborne salinity and pollution substances is severe at the coast of the Japan Sea, especially in a long period of the snow-falling season.