  The corrosion resistance of an electroplated (EP) Zn coating whose surface
was chemically etched to produce surface defects (pores) is investigated in
this work. Impedance and DC polarisation measururements were employed to study
the behaviour of such coating in various corrosive media (NaCl, NaOH and rain
water). Four different faradaic relaxation processes were clearly revealed in
different NaCl concentrations (from 0.1M to 1M). In the most concentrated
solutions at least three relaxation processes at low frequencies (LF) appeared
and were related to zinc deposition and dissolution. At lower concentrations
and depending on the pH, only one process was observed. The charge transfer
resistance (Rct) and the corrosion current (Icorr) were practically stable in
the pH range 5 to 10. In deaerated NaCl 0.1M, the EIS diagrams showed two
time-constants at very close frequencies. From the EIS diagrams the porous
nature of the coating was highlighted and showed that the dissolution
mechanisms occurred at the base of the pores.

Materials Science