The differentia shows that there is a coprecipitation process of tin oxides
with iron oxides, which can fill the crack in the rust layer effectively as
shown in Fig. 10(f). These results are corresponding to the research
results of Nam N D [12] that interaction of Sn with other elements can
improve the formation of the continuous rust layer. The existence of
SnO, SnO2 and the coprecipitation process are the reason why more
uniform and compact rust layer was formed. Meanwhile, the interaction
of Sn with Cu may also has a positive effect on it [12]. Furthermore, it is
obvious that the coprecipitation process also make the tin-containing
steel have shorter immerse period than tin-free steel to form a protective
rust layer.