Preparation Process Optimization of Ni-W-Mo-P Quaternary Alloy Coating

The Ni-W-Mo-P quaternary alloy coating was prepared by chemical plating on N80 steel. The effects of activation time （3, 5, 8 min） of steel plate before plating and sodium molybdate mass concentration （0.2, 0.4, 0.5, 0.7 g·L⁻¹） in plating solution on the morphology and hardness of the coating were studied through single-factor experiments. The effects of hexahydrate nickel sulfate mass concentration （25, 30, 35 g·L⁻¹）, tungstate sodium mass concentration （15, 20, 25 g·L⁻¹）, sodium hypophosphite mass concentration （30, 35, 40 g·L⁻¹） and plating solution temperature （70, 80, 90 ℃） on the phosphorus content were studied through orthogonal experiments. The optimal process parameter combination was determined, and the morphology and properties of the coating prepared under the optimal process were studied. The results show that as the activation time was shortened, the cellular structure of the coating became more compact, the coating became thick, and the surface became smooth. As the sodium molybdate mass concentration increased, the size of the cellular structure first decreased and then increased, the arrangement first became compact and then became loose, and the hardness of the coating increased. The factors affecting phosphorus content of the coating, in descending order of their effect, were sodium hypophosphite mass concentration, hexahydrate nickel sulfate mass concentration, tungstate sodium mass concentration and plating solution temperature. The phosphorus content increased with the increase of the mass concentration of sodium hypophosphite and hexahydrate nickel sulfate, and increased first and then decreased with the increase of the mass concentration of tungstate sodium or the decrease of the plating solution temperature. The optimal process was activation time of 3 min, bath temperature of 90 ℃, and plating solution composition of 35 g·L⁻¹ hexahydrate nickel sulfate + 20 g·L⁻¹ tungstate sodium + 40 g·L⁻¹ sodium hypophosphite + 0.5 g·L⁻¹ molybdate sodium + 30 g·L⁻¹ citric acid sodium + 20 g·L⁻¹ ammonium chloride. The coating prepared under the optimal process was composed of small-sized and closely arranged cellular structures, with a flat surface and a dense structure. No pit-like defects and gaps were observed at the interface between the coating and the substrate, and the coating had high hardness （547.8 HV） and strong corrosion resistance （corrosion rate of 0.377 mm·a⁻¹）.