STEEL PLATE WITH ORGANIC COATING HAVING IMPROVED CORROSION RESISTANCE

摘要

The improved corrosion-resistant steel plate with an organic coating comprises a steel plate having a zinc or zinc alloy plate layer which is overlaid with a chromate film which in turn is coated with an organic resin paint film. The organic resin coat comprises an epoxy resin having a dialkanolamine incorporated into its bisphenol A skeleton, an amino resin, and silica. The dialkanolamine combines with the amino resin to provide satisfactory curability at low temperatures; at the same time, it combines with the silica to provide a satisfactory film reinforcing effect. The organic coat formed of this composition can be effectively cured at low temperatures and yet it will neither dissolve nor soften upon swelling under the action of the alkali that is generated during cationic electrodeposition at the interface between the electrodeposited film and the organic coat. Thus, the organic coat has good paint adhesion, high corrosion resistance, as well as good workability and weldability and makes the steel plate suitable for use as an automotive part after painting. If the proportion of trivalent chromium in the chromate film and the amounts of zinc oxide and silica sol to be incorporated as additives are properly adjusted, the organic coating will exhibit satisfactory resistance to chromium dissolution and hence its paint adhesion and corrosion resistance are further improved to make the steel plate more suitable for use as an automotive part after painting.