高温高圧 O2-CO2 下における NaCl 水溶液中の Fe-Zn ガルバニ腐食

Abstract

Electrochemical characteristics of a Fe-Zn galvanic couple were studied under O2-CO2 mixtures of 2 MPa in a 3 mass% NaCl solution at 423 K for 14.4 ks. In 100% O2 and in 10% CO2+90% O2, both the galvanic corrosion current and corrosion potential of the couple remarkably changed after a certain period. Then, the polarities of the galvanic corrosion currents of the couple were reversed, indicating that Fe was not protected any more and rather corroded at an accelerated rate. On the other hand, the polarities of the current of the couple in 20% CO2+80% O2 and in 100% CO2 were not changed during the immersion time, indicating that Fe was still protected, although the galvanic currents were very low in 100% CO2. The galvanic corrosion potentials were always close to the corresponding cathodic potentials, indicating that the corrosion is governed by cathodic control. In 10% CO2+90% O2 and in 100% O2, the polarization characteristics of Zn specimen before and after the immersion test drastically changed, indicating that the galvanic couple of Zn anode and Fe cathode before the test changed to the couple of Fe anode and Zn cathode after the test. But, in 20% CO2+80% O2 and in 100% CO2, the galvanic couple of Zn anode and Fe cathode was hold during the immersion time. As CO2 ratio decreased, the corrosion products on the surface of Zn changed in the sequence: ZnCO3→Zn5(CO3)2(OH)6→Zn5Cl2(OH)8→ZnO. Above results show that Zn specimen in a Fe-Zn couple acts as a cathode due to the formation of ZnO on Zn. Therefore, whether Zn acts as a sacrificial anode or not, depends on the possibility of the formation of ZnO on Zn.