Effect Of Ultrasound Activation Pretreatment On Corrosion Resistance Of Electroless Ni-W-P Coating On AZ91D Magnesium Alloy

Activation pretreatment has been widely applied in improving the corrosion resistance of electroless coating.Ultrasound activation pretreatment is a new activation technology,which means ultrasound imposes in activation process.The electroless techniques were extensive used in improving the corrosion resistance of magnesium alloys.The effects of plating variables in a Ni-W-P electroless process for improving corrosion resistance of Ni-W-P electroless coating on AZ91D alloy were systematically investigated using response surface methodology?RSM?.Two-quadratic models had been obtained in order to correlate the immersion time of electroless samples with the plating variables,and analyze the interaction between two different plating variables on the corrosion resistance of the coating.The results showed that the optimization electroless parameters was determined that NiSO₄.6H₂O 20g/L,Na₂WO₄.2H₂O 15g/L,Na₂H₂PO₂.H₂O 30g/L,the temperature of the bath was 65?,and the pH-value was 9.3.The effect degree of the variables on the corrosion resistance of the coating was confirmed,namely Sodium tungstate>Sodium hypophosphite>the pH-value>Nickel sulfate>the bath temperature.The interactions of Sodium tungstate with Nickel sulfate,Sodium hypophosphite and the pH-value were significant.The effect of ultrasound activation pretreatment on corrosion resistance of the coating was studied with using ultrasound in the process of activation.The corrosion resistance of the coating was analyzed by POL,immersion test and cumulative probability.The reasons that ultrasound activation pretreatment effected the corrosion resistance of the coating were researched from three points of view,namely the microstructure of the coating,the process of activation and the electroless initial deposition.The microstructure and corrosion resistance of the coating were analyzed by SEM,EDS,XRD,TEM,electrode noise,OCP,and mass gain/reduce test and the following conclusions were drawn:With ultrasound activation,the break potential of coating was increased 60mV,and the coating failure time was lengthened.Therefore,ultrasound pretreatment improved the corrosion resistance of the coating.The effects of ultrasound activation pretreatment on morphology,thickness,deposition rate,phase and microstructure of the coating were not significant.The phases of substrate were distinct under ultrasound activation,and the?phase was covered by compact and thin activation produce,which improved the open circuit potential of the?phase,so that the potential difference between?phase and?phase was diminution.The ultrasound activation had slow the electrochemistry deposition of the initial deposition process in?phase surface down,and decreased the content of the impurity?MgF₂?in coating,which improved the corrosion resistance of the initial coating.Hence,the corrosion resistance of the final coating was improved.