Corrosion and Protection Characteristics of Phosphate Coatings on Aluminum Alloy 6061 Using Yttrium Oxide as Additive

The corrosion and protection characteristics of phosphate coatings formed on aluminum alloy (AA)6061 (UNS A96061) in a phosphating solution containing mainly zinc oxide (ZnO), phosphoric acid (H₃PO₄), and sodium fluoride (NaF), using yttrium oxide (Y₂O₃) as additive, were investigated through scanning electron microscopy (SEM), polarization curves, electrochemical impedance spectroscopy (EIS) diagrams, the salt water immersion test, and the adhesion test. The results show that the corrosion and protection of phosphate coatings have been improved when Y₂O₃ is added to the phosphating solution, making the free corrosion potential shift to the positive direction and corrosion current to decrease. The phosphate coatings formed in the phosphating solution with 10 mg/L and 20 mg/L Y₂O₃ have finer crystal structures, and they exhibit better corrosion resistance and adhesion than those with 0 or 40 mg/L Y₂O₃.