Study On The Electrochemical Fabrication And The Related Properties Of Ni-P/Cu And Ni-p-TiN/Ni On AZ91D Mg Alloy

Due to their low density, high strength-weight ratio and excellent dimensional stability, magnesium alloys are always used in different applications such as aerospace, automotive, cellular phones, recreational equipments and computer industries, where weight reduction is important. The activity and poor corrosion resistance of magnesium and its alloys restrict their wide applications. In order to improve the corrosion resistance and mechanical performance of AZ91D Mg alloy, Ni-P/Cu coating and Ni-P-TiN/Ni composite film were elet-odeposited on Mg alloy successfully.Suitable pretreatments were adopted before eletrodeposition on AZ91D Mg alloy, such as:HF one step acid picking-activation, which not only removes the loose film on the magnesium alloy surface, but also avoids fierce replacement, which is propitious to plating smooth layers; then once zinc immersion was applied in pyrophosphate-based solution, which can reduce the difference between a phase and (3phase, in other words, the chemical activity and Volta electrode potential diversity distribution on the substrate is equalized and potential difference between magnesium alloy substrate and subsequent deposits is reduced after zinc immersion process; and then pyrophoshphate copper electrolyte was used to electroplate Cu layer, which can ensure Ni-P coating be eletrodeposited on Mg alloy successfully; Semi-bright nickel was electroplated as under-layer before eletrodeposition Ni-P-TiN composite film, which can improve the stability of Mg alloy in Ni-P-TiN electrolyte and the adhesion between Mg alloy and Ni-P-TiN composite film.Through the single factor experiment, effect of technology parameters (current density, electrolyte pH value, ultrosonice) on physical and chemical behaviour of coatings on Mg alloy was evaluated and the optimize parameters were obtained. For eletrodepositing Ni-P coating, the optimize ranges of the technology parameters of the pH value and the current density are1.0-2.0and2-8A/dm2, respectively, and Ni-P coatings with the phosphorus content from8.8to17.0at.%are obtained; for eletrodepositing Ni-P-TiN composite film, the maximum TiN contents in composite film can be obtained at the technology parameters of the pH2.0and the current density6A/dm2.In terms of morphology, structure and alloy composition of these two coatings, SEM, XRD and TEM were used to analyse, so of corrosion resistance and corrosion behaviour during immersion in3.5wt.%NaCl solution, the electrochemistry impedance spectroscopy (EIS) and the potentiondynamic polarization measurements were used to evalutate. The results are:with increasing the phosphorous content of the Ni-P coatings, the structure was found to gradually transform to amorphous and the corrosion resistance to increase synchronously; phosphorous content is the most important factor to affect the corrosion resistance on Ni-P coating and Ni-P-TiN composite film; compared with Ni-P-TiN composite film, Ni-P alloy coating of amorphous property displays the highest corrosion resistance, however the TiN particles in composite coatings can play a significant role for long-term corrosion resistance.Other properties of these two kinds of coatings were studies, such as hydrogen evolution property, microhardness of coatings with or without annealed, the results are:Ni-P-TiN composite film displays better hydrogen evolution property and higher microhardness than Ni-P coating.