| Magnesium is one of the lightest metal structure material, it is widely used inaerospace, automotive and electronics industries, etc. But magnesium with high chemicalactivity, easy to corrode. So provide effective protection for magnesium alloy, magnesiumalloy can wider application fields.Chemical conversion coating method is one of theeffective protection methods of magnesium alloys. In numerous methods of the protectionof magnesium alloy, chemical conversion coating method simple operation, good effect, isit one of the most commonly used anti-corrosion methods of magnesium alloy, includingchemical conversion coating methods mainly include the chromate conversion coating andphosphate conversion coating, stannate conversion coating, molybdate conversion coating,rare earth conversion coating, phytic acid conversion coating, Molybdate salt due to itsstrong oxidizing, can form stable passivation membrane in the material surface corrosioninhibit ion penetration, protect the base material. At present, molybdate has been applied inzinc, aluminum alloy and steel substrate, but applied to magnesium alloy is less reported.So the magnesium salt molybdate conversion film applications there is a huge potential.This article primarily in the preparation of molybdate conversion coating of AZ31magnesium alloy, process conditions on the influence of the corrosion resistance of themembrane layer. According to single factor experiment, by adopting the method ofelectrochemical and corrosion weight-loss method, determine the optimum processconditions of molten magnesium salt molybdate conversion for: Na2MoO425g/L，NaF4g/L，pH3.0，reaction time is65℃, reaction temperature12min.SEM showed that thecoating on substrate is good, there are crack in rule. XRD results show that the coating ismainly composed of MoO2, MoO3and MoO (OH)2. Electrochemical tests show that in3.5%NaCl solution, the optimal conditions of molybdate conversion coating sample thanthe corrosion potential of magnesium alloy substrate corrosion potential was520.4mV,open circuit voltage is0.31V, capacitive reactance is about10times better. By measuringthe molybdate conversion coating weight and open circuit potential-time curve combinedwith the corresponding molybdate conversion coating surface morphology, infercoatingformed through four stages: total dissolved phase matrix, and the coatingdepositionstage, the rapid growth stage, coating deposition and matrix dissolution equilibrium phase.Simple sketch coatingis established, and the mechanism of coating forming are concluded.In molybdate conversion coating on the basis of adding silica nanoparticles in solution,make the coating layer of patience can be further improved. According to single factor experiment, by adopting the method of electrochemical and corrosion weight-loss method,determines the optimum technological conditions for magnesium alloy nano silicon dioxidecomposite membrane coating: nano SiO2of2.5g/L, the stirring speed600r/min, thestirring speed for48h. Nanoparticles composite conversion coating cracks decreasedsignificantly. SEM showed that the nano silicon dioxide composite membrane after layercrack decreased significantly. Coatingcomponent in EDS analysis, coatingelements Si,obviously proved that nanoparticles have been successfully in composite layer on thesurface of magnesium alloy membrane. Electrochemical tests show that in3.5%NaClsolution, the optimal conditions of the polarization curve of the composite coatingthan themolybdate conversion coating corrosion potential was246.2mV, open circuit voltage is0.21V, capacitive reactance is also greatly improved, further improve the corrosionresistance. |
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