Effect of Mg2+ on the microstructure and corrosion resistance of the phosphate conversion coating on hot-dip galvanized sheet steel

The effect of Mg2+ in the phosphate solution on the microstructural evolution and corrosion resistance of the coating on hot-dip galvanized steel has been explored. Surface morphology observations reveal that increasing the solution Mg2+ concentration increases the population density and refines the grain size of the phosphate grains. In the presence of Mg2+, the phosphate coating is composed of mixed Zn phosphate hydrate and (Zn, Mg) phosphate hydrate. Furthermore, elevating Mg2+ in the solution reduces the porosity of the phosphate coating. As a result, the corrosion resistance of the phosphate coating is improved.

Research highlights
► Mg2+ in the phosphate solution reduces pores in the phosphate coating on HDG steel.
► The phosphate coating with fewer pores display a better corrosion resistance.
► Mg2+ improves the corrosion resistance of phosphated HDG steel.
► Mg2+ distributes uniformly in Zn phosphate and (Zn, Mg) phosphate hydrate coating.
► Mg2+ accelerates the nucleation of (Zn, Mg) phosphate by enhanced impingements.