Reserch On Nickel Plating Process And Its Coating Performance On The Surface Of AZ31 Magnesium Alloys

Magnesium alloy,as the green material of 21 century,have a broad development prospect because of its lightweight.However,the corrosion resistance of magnesium alloy is poor.Electroless plating and plating can significantly change the surface properties such as wear resistance and corrosion resistance.In this paper,in order to acquire optimum pretreatment process,4 different pretreatment processes were contrastive studied using AZ31 magnesium alloy as the substrate;Beside,the analysis of the morphology,structure,element and performance of the electroless nickel plating and nickel electroplating was studied by SEM,XRD,XPS.Also the effects of process parameters on electroless nickel plating and nickel electroplating were studied.The results show that:?1?A large number of MgF₂ and a small amount of MgO and Mg?OH?2 was generated in the substrate using the pretreatment process of HNO₃+CrO₃ pickling and HF activation;After the pretreatment process of HNO₃+CrO₃ pickling and NH4HF₂+H3PO₄ activation,the substrate generated MgF₂ and the precipitation of Mg3?PO₄?2 and AlPO₄;Using the pretreatment of HNO₃+H3PO₄ pickling and HF activation,MgF₂,MgO and Mg?OH?2 was generated in the substrate;Also,a large number of Mg3?PO₄?2,AlPO₄ and a small amount of MgF₂ was generated in the substrate using the pretreatment of HNO₃+H3PO₄ pickling and NH4HF₂+H3PO₄ activation.?2?The electroless nickel coatings which the pretreatment is HNO₃+H3PO₄ pickling and NH4HF₂+H3PO₄ activation has the best surface properties.?3?The electroless nickel coating using the pretreatment of HNO₃+CrO₃ pickling and HF activation,also HNO₃+H3PO₄ pickling and HF activation belonging to low phosphorous coatings;However,the coatings using the pretreatment of HNO₃+CrO₃ pickling and NH4HF₂+H3PO₄ activation,also HNO₃+H3PO₄ pickling and NH4HF₂+ H3PO₄ activation belonging to high phosphorous coatings.?4?The best pH value of electroless nickel plating is 6.5,the optimum temperature is 90-95 ?,and the optimum concentration of NaH2PO2 is 24 g/L,the concentration of thiourea is no more than 2.4 mg/L,and the best plating time is 60 min;The optimal current density of nickel electroplating is 0.02 A/m2.?5?XRD showed that the electroless Ni-P layer was amorphous.The grain orientation of nickel electroplating coatings is Ni?111?,Ni?200?,Ni?220?,and the preferred orientation is Ni?111?,the nickel electroplating coatings resents typical crystal state.?6?XPS analysis showed that the existence state of nickel on the surface of electroless Ni-P plating layer is mainly pure nickel?27%?,NiO?33%?and Ni?OH?2?40%?,also existing state of phosphorus is pure P?20%?,PO₄2-?17%?and PO₄3-?63%?.However,the existence of nickel in the intermediate layers of the coating is almost 100% pure nickel,also existing state of phosphorus is pure P?74%?and PO₄2-?26%?;The surface of the nickel electroplating coatings is composed of C,O,N and Ni,the presence of nickel is Ni,NiO and Ni?OH?2.