Effects of Glass Transition Temperatures of Films on the Corrosion Resistance after Forming of Pre-Coated Aluminum Sheets

Abstract

Aluminum sheets with superior surface functions have been increasingly used in electrical fields. In this paper, the effects of the glass transition temperatures of the films on the corrosion resistance of pre-coated aluminum sheets before and after forming were investigated. The corrosion resistance after forming of polyester type pre-coated aluminum sheets with three different glass transition temperatures were investigated by salt spray test. It was found that corrosion resistances of the film with glass transition temperatures higher than the forming temperature were low. In the cups with low corrosion resistance after forming, whitening phenomena were observed. This whitening behavior was also investigated by the stretching test. Results showed that whitening was only caused in areas where lubricants were applied, and microcracks were only observed in the areas where whitening occurred. In addition, whitening was observed only in films with high glass transition temperatures. As films with high glass transition temperatures are hard, excessive internal stress (cause of cracks) tend to concentrate on local area. It is considered that the adsorbed lubricant on the polyester films acts to decrease the mechanical strength of the films. As a result of the lubricant adsorption, the film can not withstand internal stress in stress-concentrated regions, which in turn causes micro-cracks to develop. It can be thought that the corrosion resistance was deteriorated by the sodium chloride solution that penetrated into these micro-cracks.