Study On Chemical Nickel Plating Process And Corrosion Resistance Of AZ31B Magnesium Alloy Micro-Arc Oxide Ceramic Layer

The ceramic layer generated by the micro-arc oxidation treatment of magnesium alloy improves the corrosion performance of magnesium alloy,but the insulation property of the ceramic layer itself limits the scope of its application in communication and electronics industries.The compositc treatment or magnesium alloy micro-arc oxidation/chemical nickel plating solves the surface electrical conductivity and prevents the direct contact between magnesium and nickel to produce galvanic corrosion causing serious damage to the magnesium alloy substrate.In this paper,the effects of citric acid concentration.pH and additive type on the growth characteristics and film properties(bonding and hardness)of the chemical nickel plating layer were investigated.and the electrochemical corrosion characteristics and corrosion resistance of the composite plating layer were analyzed by polarization curves and neutral salt spray tests,respectively.The results showed that:(1)The main components of the magnesium alloy micro-arc oxide ceramic layer are MgO and Mg2SiO4,the surface roughness Ra value of the oxide ceramic layer is 0.4874μm,the microarc oxide pore size is about 2.26μm,the porosity of the oxide layer is 17.39%.After the activation treatment,the elemental content of Mg was 46.38%,O was 43.15%,Si was 8.9%,and Pd was 1.57%.The surface of Pd sample provided a more uniform distribution of catalytic activity centers,which could stabilize the chemical reaction at the beginning and maintain the good homogeneous plating characteristics of chemical nickel plating.(2)Process conditions of chemical nickel plating:NiCO3·2Ni(OH)2·4H2O 10g/L.NaH2PO2·H2O 20g/L,C6H8O7·H2O 7.5g/L.C18H29NaO3S 25ml/L,C4H6O2 23ml/L,CdSO41 mL/L.,solution temperature at 80±2℃.The surface of the coating prepared under this process condition is dense and uniform.with fine nickel-phosphorus cell-ike structure,and no defects.(3)The growth and deposition process of electroless nickel plating forms a growth morphology with large size nickel particles uniformly covering the micro-arc oxide and small size particles uniformly accumulating on the surfae of the large particles and the dividing line.The hardness of the plated layer is 884.5 HV0.01.the bonding force of the film base is 18.5 N,the corrosion potential is increased by 1.0 V and the corrosion current density is increased by 2 orders of magnitude compared with that of the magnesium alloy,the salt spray resistance time reaches 600 h,and the phosphorus content of the Ni-P alloy plated layer is 16.63 wt%.(4)Additives added to change the corrosion form of composite plating in the salt spray atmosphere,ordinary plating in the salt spray test showed pitting corrosion,corrosion on the surface of the plating along the plating longitudinal depth of the substrate,in a short period of time penetrate the chemical nickel plating layer and micro-arc oxide layer so as to corrode the substrate,while the addition of additives first in the surface of the plating extended corrosion,the appearance of yellow-brown rust-like surface,and constantly corrode the surface plating,until the surface of serious pitting corrosion,pitting corrosion to the substrate after the occurrence of electrochemical corrosion,corrosion intensified,the plating protection failure.