Abstract:

The properties and microstructure of the zinc phosphate coatings on the AZ61D magnesium alloy prepared in phosphating baths with different pH values were studied. Weight loss test, anodic polarization measurement and electrochemical impedance spectroscopy were used to access the corrosion resistance of phosphate coatings. The structure and surface morphologies of phosphate coatings were observed by a metallographical microscope and analytical electron microscope. Results indicated that the pH values of phosphating baths had obvious effect on the anticorrosive performance and surface morphologies of phosphate coatings on the magnesium alloy. Phosphate coatings obtained in the baths with higher pH values were porous  with poor anticorrosive performance, while the coatings fabricated at lower pH values were coarse  with poor anticorrosive performance.The coatings prepared at a moderate pH value (pH value was 2.5) were uniform and with good anticorrosive performance.

Key words: magnesium alloy, phosphating, pH value, corrosion resistance