BEHAVIOR OF UNDER-FILM CORROSION ON ZN AND ZN-FE ALLOY COATING IN SALT SPRAY ENVIRONMENT

The observation and analysis have been carried out at under-film corrosion tip of Zn and Zn-Fe alloy coated steel sheets after salt spray test. It has been attempted to investigate the coating corrosion mechanism with electrochemical method and to elucidate the correlation between the durability of coating against chlorine penetration and the composition of Zn-Fe alloy coating. As a result, following five conclusions have been obtained. (1) Corrosion of coated steel sheets proceeds by coating-precedence corrosion mechanism. (2) Initial corrosion product of coating is chloride of zinc. (3) Durability of coating against chlorine penetration is mainly due to coating itself corrosion resistance. (4) Coating corrosion under paint proceeds by charge transfer controlled corrosion mechanism. (5) Coating corrosion is mainly due to contribution of corrosion current by way of micro circuit between coating and iron substrate.