Effect of Mg2+ on the Corrosion Behavior of Zinc

Abstract

Zinc-coated steels have been widely used in various industrial fields such as automobiles and construction since they have excellent corrosion protection in atmospheric corrosion environments. To improve corrosion resistance of the coating, zinc-alloy coatings containing Mg have been develped and they have demonstrated superior corrosion resistance in terms of duration to appearance of red rusts on steel compared with zinc coatings. However, the effect of Mg on the improvement of corrosion resistance of the zinc-alloy coatings is still unclear. In this study, the effect of Mg2+on the corrosion behavior of zinc in Cl-containing aqueous environmnts is investigated by electrochemical measurments and surface analysis. A pure zinc plate (purity: 99.5%) was used as a sample. The zinc plate was grounded with SiC papers up down to JIS #2000 grit and then rinsed in EtOH for several minutes. Test solutions used in this study were aqueous solutions of 1 M MgCl2and 2 M NaCl. Corrosion potential and polarization resistance for zinc were monitored during the immersion in the test solutions. The reciprocal of polarization resistance was used as a measure to evaluate corrosion rate. After the immersion, the surface of zinc was investigated by SEM-EDX, XRD and FE-AES. From the results of the reciprocal of polarization resistances measured in the test solutions, it was confirmed that corrosion rate of zinc in MgCl2 is lower than that in NaCl during the immersion. Furthermore, it was also found that the corrosion rate in MgCl2 decreases obviously ca. 4 h after immersion in the solution. At this time, corrosion potential for zinc in MgCl2 shifted in the noble direction. These results indicate that corrosion rate of zinc is decreased due to the inhibition of anodic dissolution of zinc. From the results of surface observation and anaysis of zinc after the immersion, it was found that finer corrosion products of zinc were formed in the case of MgCl2 compared with that of NaCl. This result suggests that the differenrce in the morphology of zinc corrosion products can be related to the difference in the corrosion resistances in both solutions.