Study On Chemical Conversion Coatings And Electroless Depositing Ni-P Alloy On Chemical Conversion Coatings Of Magnesium Alloys

The technologys of chemical conversion coating and electroless deposit nickel phosphorus alloy are the research hotspots of magnesium surface treatment at present. In this dissertation, based on the study of chemical conversion coating process, a smooth composite coating had been prepared on AZ91D magnesium alloy by chemical conversion coating and electroless depositing Ni-P alloy. The composite coating had very compact interior structure and high anti-corrosion capability, and it could provide more perfect protection for magnesium alloy from corroding.Two new conversion coating system for the corrosion protection of die-casting AZ91D magnesium alloy were studied in this dissertation. One was composed of 17.5g/LNa₂WO₄,4g/LNa₂SiO₃,0.5g/L KMnO₄,1g/L Na₂MoO₄;the other was composed o f 5g/L Na₃PO₄,15g/L NH₄H₂PO₄,1g/L KMnO₄,0.5g/L (NH₄)₆Mo₇O₂₄ and a little additive. The thickness,surface look,structure and the phase constitution of conversion film were studied by scan electron microscope(SEM) and X-ray diffraction(XRD).The results showed that the compositions of the conversion coating made by the first system were MgO,MgSiO₃,Mg₂SiO₄ and some other things. While the coating made by the second system were made up of Mg,Al₁₂Mg₁₇ and some non-crystal structure.The elements in the solution could be found in the coating as their oxides or compounds.The researchs of dynamic potential polarization showed that the corrosion current potential of the coating got by the two system separately increase 0.45V and 0.75V.The corrosion resistance of the films got by the two system was all obviously improved and the self-corrosion current density decreased.The low concentration Sn²⁺ sensitization and Pd²⁺ activation process were used to pretreat the chemical conversion coatings, then a crystal structure Ni-P coating could be prepared on it in alkaline electroless plating solution. The orthogonal test optimization experiments were applied to obtain a good alkaline electroless plating Ni-P alloy process,as follows: 30g/LNiSO₄·6H₂O,28g/LNa_H2PO2·H₂O,10g/LNH₄F, 20g/LNa₂C₄H₆O₆·2H₂O,pH is 8.5-9.0,temperature is 85℃.The thickness,surface look,structure and the phase constitution of conversion film were studied by scan electron microscope(SEM) and X-ray diffraction(XRD). The results showed that the conversion coating was non-crystal structuer, and the composions were made up of Ni, Ni₅P₂, Ni₃P₂, MgO and eta.The study of electrochemistry test showed the system reflected the corrosion information of the nickelplate coating.The self-corrosion current potential of the composite plate increased 0.93V, and the deactivated electric potential sector of polarization curve increased, and the self-corrosion current density increased a order of magnitude. The sealed hole function of electroless deposit nickel phosphorus alloy, further enhanced the corrosion resistance of the coating.