12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Zn alloy-coated steels such as ZnMg and ZnMgAl have been recognized as a potential automotive materials replacing traditional Zn-coated steels such as galvanized (GI) and galvannealed (GA) steel with enhanced corrosion resistance under harsh corrosive environment. There are two representative Zn alloy coatings, PosMAC®1.5 (ZnMgAl coating) and PosPVD® (ZnMg coating) for automotive application in POSCO steel products. These have been developed to meet the requirements for automotive parts for weldability, surface quality, phosphating and paintability, and corrosion resistance with low alloy contents in Zn-based coating. Because applications of newly developed steel product require careful consideration, POSCO makes efforts to obtain performance data from various corrosive conditions and find solutions for proper applications.
In this study, the anti-corrosion performance of PosMAC®1.5 and PosPVD® were investigated from the viewpoint of automotive body structures. Corrosion resistances of flat surface, bimetallic joint and lap-joint surface were compared with conventional Zn-coated steels by cyclic corrosion tests (CCTs) and proving ground (P/G) test. In the results of these tests, the Zn alloy-coated steels exhibited better corrosion resistance compared to that of GI and GA steels despite the lower coating weights and prove its potential as a high corrosion resistant automotive steels. Finally, the corrosion life of the coatings were quantitatively predicted based on the vehicle corrosion (P/G) test results.