which may be due to
the relatively fast electrochemical corrosion rate between substrate
steel and steel covered by protective rust layer. For tin-free steel, Rp
values exhibit the lowest value when immersed for 26 days. In
general, tin-containing steel has higher impedance than tin-free
steel, and the Rp value of tin-containing steel is almost 34% higher
than that of tin-free steel when they have been immersed for
40 days.