Effect of Zn and Mg alloying on microstructure and anticorrosion mechanisms of Al-Si based coatings for high strength steel

(1) Galvanic protection of steel by coating dissolution. (2) Formation of corrosion products (barrier protection). (3) Dissolution of corrosion products by pH increase for AlSiZn (prevented by Mg alloying). [Display omitted] •AlSiZn(Mg) coatings sacrificially protect high strength steel in cyclic corrosion test.•Sacrificail effect is attributed to new Al(Zn) and Mg2Si phases.•AlSiZn patinas formed in scratch dissolve with cycling loosing barrier effect.•AlSiZnMg patinas formed in scratch doesn’t dissolve with corrosion advancement.•Long lasting barrier effect is attributed to higher pH stability of Mg-rich patinas. The effect of Zn and Mg alloying on the microstructure and anticorrosion mechanisms of AlSi coatings for steel was studied combining electrochemical tests, material balance after accelerated corrosion tests, visual observation, phase identification on corroded and uncorroded surfaces by X-ray diffraction, Raman spectroscopy and electron microscopy. Zn alloying promoted an AlZn solid solution therefore improving sacrificial protection of steel at first cycles of the test. With cycling, the AlSiZn coatings were rapidly consumed by selective dissolution of AlZn coupled with cathodic Si. Additional Mg alloying increased the coating life, binding Si in more anodic Mg2Si and stabilizing corrosion products.