Galvanic Corrosion of an Fe-Zn Couple in a NaCl Solution under High Oxygen Pressures and High Temperatures

Abstract

In some cases, it is known that zinc doesn’t work as a sacrificed anode for preventing iron from corrosion. Electrochemical characteristics of an iron-zinc couple were studied in a 3 mass%NaCl solution under high oxygen pressures (0-2 MPa) and high temperatures (298-423 K). Reversal of polarity for coupling current between iron and zinc occurred under an oxygen pressure of 2 MPa and above 393 K. The period until the polarity reversal occurred, became shorter with increasing the temperature and with increasing the oxygen pressure. Before the polarity reversal occurred, galvanic corrosion potential of the iron-zinc couple was close to the corrosion potential of zinc anode. After the polarity reversal, the potential was close to the corrosion potential of iron anode. Polarization curves of iron before and after the polarity reversal were similar to each other. On the other hand, zinc corrosion potential shifted to nobler direction after the polarity reversal. X-ray diffraction analysis showed that zinc surface changed into ZnO. It was concluded that iron-zinc galvanic couple which acted as iron cathode and zinc anode was changed to the couple which acted as iron anode and zinc cathode due to the change of zinc to ZnO.