Corrosion of Zinc and Zn-Mg Alloys with Varying Microstructures and Magnesium Contents

This work addresses the effect of the Mg-content on the corrosion performance of Zn-Mg alloys. Since only low Mg-contents are used in modern Zn-Mg-X coatings, the magnesium in the investigated Zn-Mg alloys was varied in the low content range from 0 wt% to 3 wt%. In addition pure, intermetallic MgZn 2 was investigated for comparison. Furthermore, for the case of the alloy with 2 wt% magnesium, the effect of different microstructure at same composition was investigated, which was adjusted by applying different cooling rates during solidification from the melt. The alloys can be ranked in order of the fineness of their microstructure. With increasing magnesium content the lamellar eutectic becomes finer and more evenly distributed. The slowly cooled specimen exhibits coarser microstructure than the quenched alloy. The corrosion of the investigated alloys can be described in three stages, i.e. activation, active corrosion and formation of a protective layer. While the development of the corrosion resistance in the first two stages is very similar for all investigated alloys, in the last stage distinct differences were found for the different samples. It was observed that an increase of the fineness of the microstructure leads to an improved corrosion performance.