Corrosion performance of Zn-Al-Mg coatings in open and confined zones in conditions simulating automotive applications

摘要

Panels coated by hot dipping with zinc (HDG), Zn-5Al (Galfan) and Zn-1.5Al- 1.5Mg coatings at different thicknesses were phosphated and painted on an industrial line. Crevice panels with non-painted bare parts modelling conditions in hem flanges, reference panels with open surfaces and formed non-painted panels were exposed to a cyclic accelerated automotive test. Zn-Al-Mg coatings with the thickness of 10 μmprovided similar or even better protection than HDG and Galfan at 20 μmin both confined and open configurations. In comparison to 10-μm HDG, the Zn-Al-Mg coating delayed red rust appearance in crevices by a factor of 2 and the maximal depth of corrosion in the steel substrate was by 42% lower. Confined areas were more corroded than open surfaces. For HDG, the time to red rust appearance dropped by 50-75%, corrosion attack in steel was from 3.5 to 7 times deeper and mass gain was about 2.3 times higher in crevices than on open surfaces. Corrosion of Zn-Al-Mg may be more affected by local environmental conditions created by the crevice configuration than for HDG. Red rust appearance on formed panels of 20-μm Galfan, 7-, 10- and 14-μm Zn- Al-Mg was delayed to 10-μm HDG by a factor of 2.8, 3.5, 3.8 and >4.5, respectively. No adverse effect of forming was noticed. The results indicate that 2- to 3-fold reduction of the coating thickness for Zn-Al-Mg alloy coatings in comparison to traditional HDG may be possible without compromising the corrosion performance.