Chemically depleting the noble impurities from AZ91-T4 magnesium alloy: A new and efficient pretreatment method to improve the corrosion resistance of phosphate conversion coatings

摘要

Alx(Mn,Fe)y phase, as a common derivative of Fe-containing impurities, significantly deteriorates the protectiveness of phosphate conversion coatings (PCC) on magnesium alloys due to the micro-galvanic effect. This work presents a novel pretreatment method, aiming at chemically depleting the noble Alx(Mn,Fe)y phase from Mg alloy surface before PCC treatment. The pretreatment bath contains both oxidant and complexing agent, with the pH value and concentration of different constituents being determined by thermodynamic calculation. The corrosion resistance of PCC being preceded by exploiting the novel pretreatment method was improved, which was attributed to the mitigation of micro-galvanic corrosion.