The Influence of Drying Process in Cyclic Corrosion Test on the Underfilm Corrosion of Galvanized Steel Sheets

The influence of the drying time ratio on the underfilm corrosion growth of zinc coated steel sheets at the leading edge of corrosion was investigated using salt spray/drying cyclic corrosion test.The temperature and relative humidity were fixed at 35°C and 60%.The dependence of the growth of underfilm corrosion on the zinc coating weight changes with drying time ratio. At high drying time ratio (93.8%) the underfilm corrosion rate decreases with an increase of zinc coating weight.At the leading edge of underfilm corrosion of galvanized steel, anodic dissolution of zinc coating layer occurs from its upper side. This area increases with a decrease of drying time ratio. For salt spray test, the largest zinc dissolution zone was observed.From these results, an underfilm corrosion mechanism of galvanized steel is thought to be as follows;There are two kinds of corrosion electric cell. At the leading edge of corrosion, there is an anode followed by a cathode. There is an electric corrosion cell between the scribe and the zinc coating surface under the paint by the scribe. The proportion of these cells changes with drying time ratio, which change the underfilm corrosion ratio and morphological feature of galvanized steel sheets.