TechnolTechnology Of Zinc-Nickel Alloy Electroplating From Cyanide-Free Alkaline Solutions

Zinc-nickel alloy coating, which has been widely utilized in electroplating area, was regardedas one of the most promising coatings because of its high corrosion resistance, lowhydrogenous brittleness and excellent facile jointing when plated on steel and iron substrate.The traditional technology of zinc-nickel alloy plating process was alkaline cyanide system.However, the alkaline cyanide technology could pollute the environment and had a seriouslybad effect on the public safety because of the virulent cyanide. Therefore, investigation on thecyanide-free plating process was of great vital. Based on experiment study in laboratory,zinc-nickel plating in alkaline cyanide-free system and the passivation process were discussedin the present paper. The results from practical application with optimum paprametersrevealed that the stability of the product was obviousely improved.The corrosion resistance properties of the zinc-nickel alloy coating were tested by NSS,Electrochemical impedance spectroscopy and Weight-loss measurements. The morphology ofthe zinc-nickel alloy coating after corrosion was characterized by SEM. The results indicatedthat the appearance and properties of the zinc-nickel alloy coating were greatly dependent onthe Ni content. The best performance of corrosion resistance was found at 13.0% of nickel inthe zinc-nickel alloy coating which performed the best protection to the substrate.From the results of experimental in Hull cell and square cell, it was found that the Ni contentin zinc-nickel alloy coating was mainly influenced by the concentration ratio of [Ni²⁺]/[Zn²⁺]and complexing agent concentration, and little influenced by current density and temperature.The optimized parameters found in experiment and employed in practical production weregiven as below:Zn: 8.0±2.0 g/L[Ni²⁺]/[Zn²⁺]:0.1±0.02NaOH: 120±20 g/LComplexing agent 50～60 mL/LBrightener 1.5 mL/LASD 2 A/dm²Temperature 20-30℃(room temperature)Agitation: non-agitation Filter twice/hFrom the study on trivalent chromium passivation on the alkaline cyanide-free zinc-nikellayer, it was found that the appearance and corrosion resistance of the passivation film wereinfluenced by the passivation time, dripping time and drying temperature, and the optimizedparameters were given as followed:Passivation agent－A: 100 mL/LPassivation agent－B: 50 mL/LpH: 3.5±0.3Passivation time:45 sDripping time:30 sDrying temperature:95℃The above working parameters were applicated in production, the nickel concentration inzinc-nickel coating fluctuated in the range of 12.5±1.5%, with the trivalent chromiumpassivation process, the stability of the coating was guaranteed while the corrosion resistancewas improved. Therefore, the application of alkaline cyanide-free zinc-nickel plating andchromium passivation process could gain technical and economic benefit, also was beneficialto the protection to the environment.