Synthesis of Zn–Mg coatings using unbalanced magnetron sputtering and theirs corrosion resistance

• The Zn–Mg coatings with various Mg contents were synthesized using an unbalanced magnetron sputtering process.
• Columnar and featureless microstructures were observed depending upon the Mg content in the coatings.
• The corrosion resistance of the Zn–Mg coatings increased with increasing the Mg content in the coatings.
• The Zn5(OH)8Cl2·H2O phase (simonkolleite) was responsible for the enhanced corrosion resistance of the Zn–Mg coatings.

Over the past several decades, the Zn–Mg coatings have been studied extensively to enhance the corrosion resistance of steel. According to previous studies, Zn–Mg coated steel exhibits excellent corrosion resistance compared to that of pure Zn coated steel in a corrosive environment. In this work, the Zn–Mg coatings with various Mg contents were synthesized using an unbalanced magnetron sputtering process. The microstructure, chemical composition, crystal phase and corrosion behavior of the synthesized Zn–Mg coatings were investigated. The pure Zn coating showed a porous microstructure while the Zn–Mg coatings with low Mg content (5.1–12.2 wt.%) exhibited a columnar microstructure. With increasing Mg content up to 15.8 wt.%, a featureless microstructure was developed. The corrosion resistance of the Zn–Mg coatings increased with increasing Mg content in the coatings, which could be attributed to the transition from porous microstructure to dense and compact microstructure. From the corrosion product after the salt spray test, the formation of zinc oxide and Zn5(OH)8Cl2·H2O phase (simonkolleite) was confirmed and it was noted that the increasing simonkolleite was detected along with increasing Mg content in the Zn–Mg coating, which improved the corrosion resistance of the coatings by forming a dense and stable protection layer on the surface of the Zn–Mg coatings