Microstructure and initial corrosion behavior of double-layer Zn-Al-Mg coatings produced by PVD

摘要

In this study, double-layer Zn-Al-Mg coatings were manufactured by the PVD process and heat treatment. The microstructure and initial corrosion behavior were investigated during the 20-day immersion experiment. And the influence of heat treatment at different temperatures was discussed. The double-layer Zn-Al-Mg coating showed the superior initial corrosion resistance compared to the Zn55Al coating. And the best corrosion resistance can be achieved after 300 degrees C heat treatment (ZAM3). According to EIS and fitting results, the resistance value of ZAM3 corrosion products layer decreased first from 1.5 x 10(8) to 3.8 x 10(6) Omega.cm(2) and then increased to 7.8 x 10(9) Omega.cm(2) during 20-day immersion. And this resistance value of Zn55Al coating decreased gradually from 3.3 x 10(3) to 5.6 x 10(2) Omega.cm(2). The double-layer Zn-Al-Mg coating can provide two types of protection in different stages, the barrier protection of surface passive film from the beginning to the 8th day and the formation of double layer corrosion products from the 8th day to the 20th day. After 20-day immersion, the corrosion products of Zn-5(OH)(8)Cl-2 center dot H2O and Zn6Al2(OH)(16)CO3 center dot 4H(2)O are easier to form on the surface of ZAM. And on the outermost surface of ZAM3 corrosion products, it tends to be covered by zinc carbonate hydroxide. This is different from the fact that the outermost surface of ZA corrosion products is often covered by zinc hydroxide.