Abstract
Corrosion resistance and porosity of electroplated bright tin coating on steel substrate were evaluated by salt spray test and modified ferricyanide test, and the results were compared with those of other tin coatings.
Effects of diffusion of brass substrate in the electrodeposited coating were studied by sulfur dioxide test, microsection observation, and X-ray diffraction method. The results were as follows:
1. Porosity of bright tin plating was lower and its corrosion resistance was higher as compared with other tin coatings.
2. At places, where steel was exposed in pores of electrodeposited tin, oxygen was expelled from the corrosive solution and electrochemical behavior might be reverse. It was considered that the dissolution of tin was very little; because it was anodic to steel which was made unattached.
3. Tin salts such as SnCl2 were converted to precipitated stannous oxychloride by absorbing liquor. Stannous oxychloride was insoluble in water and inhibited exposed steel from corrosion by sealing up capillary tin pores.
4. Diffusion rate of zinc from brass substrate into tin coating was very higher than that of copper. The both diffusion rates were higher as compared with other tin coatings. The reason of higher diffusion rates in electrodeposited bright tin would be due to its fine crystals and richness in grain boundary.
