Preheating Mg Alloy Before the Phosphate Conversion Treatment: The Enhanced Crystal Precipitation Kinetics Associated with the Elevated Temperature at Reaction Fronts

Abstract

Temperature is a critical parameter that affects both the electrochemical reactivity  and the crystal nucleation / growth during the conversion treatment of Mg alloys, but the increase of operating temperature negates the bath reactivity, thus degrading the protectiveness of coating. However, preheating the Mg substrate will not alter the bath chemistry. In this work, Mg substrates were preheated to various temperature values, then were conversion treated in a bath at 60 °C. Results prove that a preheating temperature higher than the bath temperature could effectively enhance coating protectiveness, which was attributed to the enhanced crystal precipitation kinetics at reaction fronts.

Keywords: Magnesium alloy, Pretreatment, Conversion coating, Corrosion resistance