Study On Electroless Ni-P-Zn Alloy Plating On Surfaceof Low-Carbon Steel Tube

The Ni-P-Zn alloy layer with bright appearance and excellent corrosion resistance on surface of low-carbon steel tube(20^#) was prepared using electroless plating(auto-catalytic reaction) in this research.The optimal technical conditions of electroless Ni-P-Zn plating were determined by orthogonal test method to be:nickel sulfate 30g·L^-1;sodium hypophosphate 16g·L^-1;sodium citrate 86g·L^-1;ammonium chloride 50g·L^-1;zinic sulfate 8 g·L^-1;pH 9 and temperature 95℃.The effects of the optimal technical conditions of electroless plating on the Ni-P-Zn deposition rate were investigated and the results showed that higher deposition rate was obtained at these conditions.The researches on thermodynamics and dynamics under the optimal conditions were carried out and their mathematical models were established respectively.By experimental verification,these models were basically consistant with the experimental results.The brightness of the Ni-P-Zn layer was increased by adding brightener in the plating solution.The brightener was made of self-prepared OP-10 esters sulfate and other chemicals.Under the optimal conditions with adding agents the phosphor content in the film nearly remained unchanged but the zinic content in the film increased first and then decreased with increasing zinic sulfate concentration in bath.When the zinic sulfate concentration in bath was 8g·L^-1 the zinic content in film was about 12% with which the film had best corrosion resistance.The separate addition of La,Ce and Yb elements in the plating solution with adding brightener increased the metal deposition rate and improved the surface quality and corrosion resistance of the coatings.The corrosion resistance time of the ternary alloys with adding rare earths was higher than those of Ni-P film prepared by electroless plating and Ni-P +Ni composite film prepared by elctroless plating and electroplating.Compared with the three kind of rare earths elements,La exhibited the best effects.The quality and corrosion resistance of the prepared alloy by rare earth modified electroless plating were further increased by the heat treatment of it at 200℃.The heat-treated Ni-P-Zn layer was passived and its corrosion resistance time in salt spray test could reach more than 2100h.