Influence of surface pretreatment on the chromium-free conversion coating of magnesium alloy

The conversion treatment process of die-cast magnesium alloy in this study mainly contains steps of acid cleaning, immersion in fluoride solution and chemical conversion treatment. The effects of these steps on surface morphology and composition variation have been studied using SEM and EDX. The results indicated that the conversion coating of sample with acetic acid cleaning was quite coarse and the conversion coating of sample with sulfuric acid cleaning was not formed completely. The conversion coating of sample with phosphate acid cleaning contained O, Mn, P, Ca as well as Mg. The weight loss of all samples was evaluated with a photoelectric analytical balance. The results demonstrated that the sulfuric acid cleaning had the weakest effect on the weight loss of sample among all acid cleaning processes. Potentiodynamic polarization curves were measured to determine the corrosion current density of the samples with conversion coating. The results showed that better conversion coating could be formed by the procedure of phosphoric acid cleaning, immersion in NaHF2 solution and then with conversion treatment, which had lower corrosion current density and better corrosion resistance.