Comparison of the effects of pre-activators on morphology and corrosion resistance of phosphate conversion coating on magnesium alloy

Abstract

• The effects of three pre-activators (colloidal Ti, oxalic acid, phosphoric acid) on the performance of a PCC was compared. • The pre-activators promote nucleation by peeling off the original oxide film and increasing the specific surface area. • Different pre-activators did not alter the formation mechanism, but significantly changed the growth rate and morphology. • Phosphoric acid was the overall best choice for pre-activating the Mg alloy considering on the PCC performance and cost. In this study, Mg–6.0Zn–3.0Sn–0.5Mn (ZTM630) magnesium alloy was pre-activated by colloidal Ti, oxalic acid, and phosphoric acid, and a phosphate conversion coating (PCC) was prepared on the alloy surface. The morphology and corrosion resistance of the prepared PCCs were investigated. Surface morphology studies showed that the phosphate crystals that formed the coating were the smallest for the sample pre-activated by phosphoric acid. The coating on the colloidal Ti and the phosphoric acid samples had the largest and the smallest thickness and surface roughness, respectively. The reason for the discrepancy was analyzed by comparing the surface morphologies of alloy samples immediately after the pre-activation treatment and various phosphating treatments. X-ray diffraction analysis revealed that all three PCCs contained the same compounds. The corrosion resistance time from the copper sulfate drop test and the electrochemical data from the potentiodynamic polarization curves showed that the coating pre-activated by phosphoric acid had the best corrosion resistance. Finally, the 1500 h neutral salt spray corrosion test confirmed that the phosphating treated magnesium alloy, which was pre-activated by phosphoric acid, exhibited excellent corrosion resistance and behavior.