Study On Electrodeposited Zn-Ni Alloy Of High Corrosion Resistance Form A Alkaline Zincate Bath

Zn-Ni alloy coating is considered an excellent protective coating for steel materials because of its excellent corrosion resistance,and also low hydrogen embrittlement and high thermal stability.The alkaline zincate electrodeposition Zn-Ni alloy system is called an environmentally friendly process.It has good homogeneous plating ability.The nickel content of the coating is stable over a wide range of conditions.The corrosion to plating equipment is small,the cost is low.It is easy to operate.Therefore,the study on alkaline zincate electrodeposition Zn-Ni alloy has great value and far-reaching significance.Aiming at the problems that galvanized coatings on steel parts have insufficient protection time in marine climates,cadmium plating and cyanide plating baths have a great impact on environmental pollution in aviation companies,this paper has developed an environmentally friendly Zn-Ni alloy coating with high corrosion resistance and low hydrogen embrittlement to replace zinc,cadmium and cyanide plating systems.The Zn-Ni alloy coating meets the requirements of aviation enterprises that the white rust time of the coating after trivalent chromium passivation treatment is not less than 2000 h in the salt spray experiment,and the tank fluid maintenance is simple and convenient.The paper selects the appropriate complexing agent,rationally designs and optimizes the composition of the electrolyte,analyzes the optimal ratio of brightener through Hull tank experiment,and uses orthogonal experiments to obtaint the optimal concentration of the main salt and complexing agent and the optimal process parameters.In the experiment,the composition and microstructure of the coating were analyzed using EDS and SEM,the brightness of the coating was analyzed using a MN268 gloss meter,and the corrosion resistance of the coating was analyzed using a Tafer curve.The single-factor variable method was used to study the effects of main process parameters(plating solution composition,brightener content,current density,and plating temperature)on current efficiency,coating brightness,nickel content of the coating,microstructure of the coating,and corrosion resistance of the coating.In addition,the effects of duty cycles and frequency on the coating composition and the microstructure of the coating were analyzed using a single-pulse power supply,and compared with the coating obtained by the DC power supply.The Cathodic Polarization Curve(LSV)and Cyclic Voltammetry Curve(CV)were used to explore the co-deposition mechanism of Zn-Ni alloy.The Ultraviolet spectrophotometer and complexometric inverse titration method were used to measure the content of Zn²⁺?Ni²⁺and other components in the plating solution,and a corresponding simple and convenient method for adding main salts was given.The research results show that under the optimized process conditions,a bright and good adhesion Zn-Ni alloy coating can be obtained.Its current efficiency reaches71.01%,the nickel content of the coating is maintained at about 19%(wt),and the coating has a flat and fine micro structure.In the salt spray experiment,the red rust time of the coating without passivation was 785 h,and the coating with passivation with trivalent chromium did not show white rust after 4000 h.The research work has reached the following conclusions:(1)The best technological scheme determined by experiments is:DC power supply,zinc oxide 6.5 g/L,sodium hydroxide 120 g/L,nickel sulfate hexahydrate 11 g/L,complexing agent ME 25 g/L,triethanolamine 3m L/L DPE(II)3 g/L,coumarin 0.46 g/L,vanillin 0.26 g/L,synthetic additive B 8 g/L,sodium dodecyl sulfate 0.28 g/L,current density 1.6 A/dm²,temperature 40?.(2)The content of Zn²⁺and Ni²⁺in the plating solution has the greatest influence on the coating composition and the microstructure of the coating;too much brightener in the plating solution will cause excessive stress and generate more micro-cracks;in the current density range studied,current density has a greater impact on current efficiency,and larger current density has almost no effect on the brightness of the coating;temperature has the greatest effect on the brightness of the coating,the coating obtained at 20°C is the brightest,but the corrosion resistance is slightly poor.(3)Streaks,blackening,and uneven colors appear on the coating obtained under a single pulse power supply.(4)The type of co-deposition of the obtained plating solution system is abnormal co-deposition,and the calculated apparent diffusion coefficient(D²⁺²⁺_Zn+Ni)of Zn²⁺+Ni²⁺in the Zn-Ni alloy plating solution is 2.2714×10^-7cm²/s;addition of brightener and decrease of temperature can slow down Zn-Ni alloy co-deposition and increase cathode polarization;the electroplating solution was electrolyzed for 12 h after electrolysis at a low current(0.2 A/dm²),and its cathodic polarization was enhanced,and the co-deposition type changed from normal co-deposition to abnormal co-deposition.(5)Ultraviolet spectrophotometry and complexometric inverse titration method were used to accurately determine the content of Zn²⁺and Ni²⁺in the plating solution.The relative errors of the standard additions were less than 5%,and the average recovery rate is 107%,which met the production requirements of the factory and gave convenient addition method.