In the analyses section of EIS, the comparison of tin-free steel and
tin-containing steel shows that the time for augment of tin-free steel's
Rp values is slightly later than that of tin-containing steel. The phenomenon
can be explained consulting to corrosion morphology shown in
Fig. 10. Fig. 10(c) shows that the rust layer of tin-free steel is porous
and loosened. However, the rust layer shown in Fig. 10(d) is more
compact than the rust layer shown in Fig. 10(c), which indicates that
the rust layer shown in Fig. 10(d) is protective. This phenomenon indicates
that tin-containing steel has shorter time to form protective rust
layer than tin-free steel. Fig. 10(e) and (f) show that the rust layer of
the tin-containing steel is more compact than that of tin-free steel,
which is corresponding to EIS results for 40 days immersion that tin containing
steel has higher Rp value than tin-free steel.