The growth and corrosion mechanism of Zn-based coating on AZ31 magnesium alloys by novel hot-dip process

A Zn-based coating with excellent corrosion resistance was prepared on the surface of AZ31 magnesium alloy by novel hot-dip process. The microstructure, growth process and corrosion mechanism of coatings were investigated. The results show that good metallurgical bonding and integrated multi-layer phase structure were observed in coatings with sufficient immersion time. This multi-layer phase structure is divided roughly into an alloy layer consisting of Zn + Mg2Zn11 + MgZn2 phases and a diffusion layer consisting of MgZn + Mg7Zn3 phase, which is mainly attributed to the diffusion reaction of Zn and Mg atoms and the subsequent solidification. Furthermore, the corrosion resistance of coated AZ31 is dramatically improved, the low frequency value of the coated AZ31 up to 3541 Ω cm2, more than 90 times higher than bare AZ31 which is 38.1 Ω cm2, the icorr of 4 s coating is 1.948 × 10−5 A cm−2, reduced to 6.6% of bare AZ31 which is 2.895 × 10−4 A cm−2. The outstanding corrosion performance is mainly due to an effective barrier to Mg matrix by the dense multi-layer structure and the compact corrosion products.