Study On The Formation Mechanism Of The Conversion Coating And The Alternating Current (AC) Modification On The Anti-corrrosion Property Of Magnesium Alloy

The formation mechanism of the stannate conversion coating (CC) on AZ91D alloy was investigated firstly. The effect of the temperature and the concentration of natrium stannate on the growth of the CC was discussed. Then the application of the alternating current (AC) passivation was confirmed in the magnesium and mangeium alloy system. On this basis, the formation process of the stannate conversion coating on AZ91D alloy could be controlled by using AC-passivation. AC-passivation removed-re repaired the weak sites of the conversion coating (CC) to improve the quality of the CC and enhance the corrosion resistance of the magnesium alloy. The results showed that:Electrochemical noise (EN) method monitored the rapidly electrochemical reactions occurring at the interface of electrode and CC solution during the stannate conversion coating formation process on AZ91D magnesium alloy. The formation process of the stannate conversion coating on AZ91D alloy was divided into two stages: an incubation stage (0⁵min) and a periodical growth stage (5⁶0min). The incubation stage involved nucleation-nuclei dissolution-Re nucleation process. Moreover, the periodical growth stage consisted of nucleation, growth and dissolution process.The reaction rate, the weight gain and the anti-corrosion of CC reached its peak value under the condition of the temperature 60℃. The fastest reaction rate and the maximum weight were gained under the concentration of natrium stannate 33.3g/L.The AC passivation could be used to the magnesium and magnesium alloy system. The chemical composition of the surface film on pure magnesium and AZ91D alloy was changed after AC passivation. An MgO film was formed on pure magnesium surface. The dual-layer was formed on AZ91D alloy surface: an inner layer rich in MgO with a little Al₂O₃, and an outer layer of Mg(OH)₂, MgO with a little Al(OH)₃. The uniformity and the stability of the surface film were enhanced just for this change of the chemical composition. Meanwhile, AC-passivation increased the compactness and decreased the donor density of the surface film, which led to the block of the pitting corrosion progress and the great improvement of the corrosion resistance of pure magnesium and AZ91D alloy.The formation process of the stannate conversion coating on AZ91D alloy was controlled by using AC-passivation. The weak sites of CC would be removed-re repaired and then a continuous and compact dual-layer CC was formed on AZ91D alloy surface after AC-passivation: an inner layer was consisted of magnesium-aluminum-stannum metal-based part crystallization of the amorphous matrix, and the outer layer was primarily magnesium-stannum hydroxides and oxides. AC-passivation increased the density and reduced the porosity of the CC, which led to the great improvement of the corrosion resistance of the conversion coating.