1056
Anodic Polarization Behavior of Zn in Aqueous MgCl2 Solutions

Tuesday, 15 May 2018: 09:00
Room 304 (Washington State Convention Center)
Y. Tsurumaki, E. Tada (Tokyo Institute of Technology), A. Ooi (Tokyo Institute of Techonology), and A. Nishikata (Tokyo Institute of Technology)
Zn-coated steels have been widely used in various engineering fields such as automobiles and construction since they have excellent corrosion resistance in mild atmospheric corrosion environments. In order to improve the corrosion resistance of the coated steels, Zn-alloy coatings containing Mg have been newly developed and they have shown superior corrosion resistance compared with Zn coatings. However, Mg should dissolve into aqueous solutions preferentially in the early stage of immersion because of one of the active metallic elements. Therefore, it is considered that better corrosion resistance of Zn-alloy coatings containing Mg is related to the presence of Mg2+ ions in corrosion media. However, the effect of Mg2+ on the improvement of corrosion resistance of Zn and Zn-alloy coatings is still unclear. In this study, the effect of Mg2+ on the anodic polarization behavior of Zn is investigated by polarization measurements.

The sample used in this study was Zn plate (99.7%, Goodfellow) and it was ground down to JIS#2000 grit with SiC abrasive papers. Then the sample was degreased in EtOH ultrasonically and then dried in air. The test solutions was aqueous solutions of MgCl2 of various concentrations ranging from 0.05 M to 1 M. Some of the test solutions were deaerated by Ar bubbling at least for 20 h. Anodic polarization curves for Zn were measured at a sweep rate of 0.5 mV/s.

Anodic polarization curves for Zn was measured both in aerated and deaerated MgCl2 solutions. For the measurements in deaerated solutions, cathodic polarization treatment for reduction of air-formed films on Zn was done in NaCl solutions without Mg2+ ions. It was found from the results of anodic polarization curves for Zn in aerated MgCl2 solutions that anodic dissolution was enhanced with increasing concentration of MgCl2 and that Tafel slopes of the curves were relatively small in the order of tens of mV/dec independent of MgCl2 concentration. On the other hand, anodic polarization behaviors of Zn in deaerated MgCl2 solution were almost the same with Tafel slopes of 40 mV/dec independent of MgCl2 concentration. These results suggests that surface film formed on Zn in aerated solutions is related to anodic polarization behavior in MgCl2 solutions and that Mg2+ ions improve anti-corrosion performance of the film.