On the Corrosion Resistance of Porous Electroplated Zinc Coatings in Different Corrosive Media

TL;DR

This study investigates how porous electroplated zinc coatings resist corrosion in various media, revealing multiple relaxation processes and the influence of pore structure on corrosion mechanisms.

Contribution

It provides new insights into the electrochemical behavior of porous zinc coatings in different corrosive environments, highlighting the role of surface defects and pore-related dissolution processes.

Findings

Multiple relaxation processes depend on NaCl concentration and pH.

Charge transfer resistance remains stable across pH 5 to 10.

Dissolution occurs at the base of pores, affecting corrosion behavior.

Abstract

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.