Technology And Performance Of Corrosion Resist Coatings On AZ91D Magnesium Alloy

As to the problem of poor corrosion resistance of magnesium and magnesium alloys, inthis research project, we conducted chemical conversion,magnetron sputtered aluminumfilms and a combination of both on the surface of AZ91D magnesium alloy to improve itscorrosion resistance.We selected Zn(H2PO4)2 as the main salt, used orthogonal tests to analyse the influence ofphosphoric acid, zinc dihydrogen phosphate, temperature and conversion time on the AZ91Dmagnesium alloys, SEM, XRD, EDS, Neutral Salt Spray Test, Electrochemical Tests wereobserved and analyzed the microstructure and properties of the chemical conversion coatings.Through analysis of the main factors, we chosed 8ml/L H3PO4, 70g/L Zn(H2PO4)2, 5g/LNaNO3, 3g/L NaF, temperature 45℃, reaction time 10min as the optimal condition. And then,we carried out sealing treatments after conversion coating.Grid methods proved that the adhesion between substrate and coating was good.Phosphate conversion coatings were observed mainly by the "flower-like crystal" composedof black and white crystals. EDS analysis showed that the coatings mainly contain Na, P, Zn,O, F and other elements. XRD analysis showed that the phase of phosphate coatings mainlycontained Zn3(PO4)2·4H2O and Zn. In the Drop corrosion tests, the corrosion resist timeincreased from the substrate of 69s to 180s. In the 24 hours neutral salt spray tests, thecorrosion resistant level after hole sealing increased two orders of magnitude. Electrochemicaltests showed that after passivation, Ecorr had positive shifts in various degrees(The hole sealedsample was 205.8 mV), corrosion current density Jcorr respectively reduced one and twoorders of magnitude.Also, we conducted DC magnetron sputtering deposition of aluminum films by changingbias and target currents. CSM nano indentation testing mechanical properties showed that thehardness of aluminum film was 3447.9MPa, elastic modulus E was 64.089GPa. Neutral saltspray tests and electrochemical tests showed that corrosion resistance become worse afteraluminum film deposition by PVD method. In addition, using Alodine as chemical convertionformula on the aluminum film, we got the compound processing samples. EDS analysisshowed that the coatings mainly contain chromium, fluorine, aluminum, oxygen, magnesium and other elements. NSS tests showed that after 24h, corrosion resistant level Rp rated gradefive. Electrochemical corrosion tests showed that the current density Jcorr decreased two ordersof magnitude; the samples had obvious passivation area, polarization resistance Rp increasedsignificantly. All these results significantly proved that the compound processing samples hadbetter corrosion resistance than the samples single treated with chemical conversion and DCmagnetron sputtered aluminum film.