Finding of gray points on the surface of the Sn-Fe alloy layer

After removing tin coating of tinplates offered by some steel works, the authors discovered massive, highly disperse gray points outspreading along the rolling direction on the surface of the alloy layer. The morphology of the alloy layer was observed by scanning electron microscope (SEM), and many cavities of the alloy layer were found out in the gray points. After analyzing the composition of the alloy layer, it was found that the content of Fe in the gray points was more than that in the normal alloy layer. Moreover, the corrosion resistance of the alloy layer declined with the increase of gray points amount. In addition, the hot-humidity testing was carried out for some plates whose surface has many gray points. After 14 days, there were many rust points occurring in the edge of gray points and in the small gray points. The morphology of rust points was observed by atomic force microscope (AFM). The reason why rust points were generated in the edge of gray points and in small gray points was discussed.