Study On Preparation And Deposition Mechanism Of Micro-arc Oxidation/Self-assembly/Nickel Composite Coatings

Magnesium alloys are formed by alloying of magnesium,which are one of the lightest metal structural materials at present.Magnesium alloys own many excellent material properties as structural materials,such as excellent castability,good electrical shielding,good fatigue resistance,good cutting performance and high heat dissipation which make magnesium alloys a material with broad development prospects.The corrosion resistance of magnesium alloys is the main factor restricting its industrial application.Surface coating technology is an effective corrosion prevention method.However,due to the shortcomings of single coating,it is crucial to prepare composite coatings with better protective performance.In this paper,a three-layer composite coatings were successfully prepared on magnesium alloy surface by micro-arc oxidation?MAO?,self-assembly?SAM?and electroless nickel plating?EN?,and the deposition mechanism of electroless nickel plating was studied.The results are as follows:Firstly,the three-layer composite coatings were prepared by Pd²⁺activation and nickel salt activation respectively in this paper.The micro-morphology and structure of the coatings were analyzed by various test methods,such as SEM,XRD and EDS.The corrosion resistance of the coatings was tested by electrochemical polarization curve and AC impedance.The results show that the morphology and corrosion resistance of the coatings prepared by Pd²⁺activation process are better than those prepared by nickel salt activation process.Then,the electroless nickel plating process was optimized by single factor experiment.The micro-morphology and structure of the three-layer composite coatings and magnesium alloy substrate,MAO coating and MAO/SAM coating were analyzed by means of SEM,EDS,XRD,XPS and AFM.The surface of the three-layer composite coating is compact and deposited without defects.The thickness of the composite coatings is about 40?m obtained by scanning the elemental surface of the cross-section morphology of the three-layer composite coatings.The diagonal surface height changes of the three-layer composite coatings in the three-dimensional stereogram of AFM is the smallest fluctuation range of 5-10 nm indicated that the coatings own the best surface smoothness.The contact angle of the three-layer composite coating is 100.01°,which owns the largest contact angle value and the best hydrophobicity among Mg alloy with different coatings above.The corrosion resistance of the coatings was tested by electrochemical test and salt spray corrosion test.The corrosion current density of the three-layer composite coating was 5.047×10^-8 A·cm^-2,which was three orders of magnitude lower than that of the substrate.The three-layer composite coating exhibited excellent corrosion resistance in 120 h salt spray test.The results show that the MAO/SAM/EN three-layer composite coating provides effective corrosion protection for magnesium alloy substrate.At last,six electroless plating samples were prepared on the active surface of MAO/SAM coating at different plating times.The surface morphology,element composition and material structure of the samples were tested by SEM,XRD and EDS.The deposition model of composite coatings was established,and it was inferred that the deposition mechanism of electroless nickel plating was the deposition process of autocatalytic reduction based on the initial ecological reducibility[H].The growth mode of nickel is cellular three-dimensional growth that the growth rate of perpendicular to the interface is less than that parallel to the direction.