Fig. 9 shows the surface morphology of corroded sample after
removing corrosion products. Tin-containing steel has lighter corrosion
degree than tin-free steel. Fig. 9(b) shows that some areas (Area A) of
the tin-containing steel exhibit a little more serious corrosion degree
than elsewhere, which may be because the rust layer can't cover the
whole corroded surface when the specimen was corroded for 18 days.
And the corrosion rate of the surface covered by rust layer is slower
than that of the surface without rust layer. The localized corrosion also
indicates that the rust layer formed on tin-containing steel is protective
[6]. Uniform corrosion is observed on the specimen immersed for
40 days, which could be attributed to that all corroded surface has
been covered by protective rust layer.