Effect of Annealing on Corrosion Resistance of Zn-Mg Coatings Deposited by Vacuum Plating

The Zn-Mg alloy coatings were synthesized by vacuum plating on galvanized steel sheet.The effect of the annealing temperature/time on the corrosion resistance was investigated with X-ray diffraction and in acetic acid-salt spray (AASS), Na₂SO₄ full immersion corrosion and electrochemical corrosion tests.The results show that the optimized annealing conditions significantly enhanced the corrosion resistance of the Zn-Mg alloy coatings.To be specific, annealing at 410℃ for 30 s increased the corrosion resistance by 40 times in full immersion corrosion test, possibly because the corrosion resistance of the eutectic MgZn₂ and Mg₂Zn₁₁ phases, formed at the interface of Zn and Mg layers in annealing, were five times higher than that of Zn-coating.Besides, the corroded products in AASA test comprise the compact Zn₅ (OH) ₈Cl₂·H₂O, Zn₄CO₃ (OH) ₆·H₂O, Zn₅ (CO₃) ₂ (OH) ₆ and a little ZnO, being similar to those of Zn-coating.