Corrosion resistance of composite NI-P coatings in various aggressive media

Abstract

The paper studies corrosion resistance in highly aggressive media of composite nickel-phosphorus coatings after isothermal annealing at different temperatures accompanied by crystallization. The phase composition of chemically deposited Ni-P coating containing about 1% dispersed SiC was analyzed. Gravimetric method was used to determine the mass loss of the samples as a result of daily exposure to various acids and in a solution of nitric acid with a concentration from 5 to 65%, which is extremely aggressive for Ni-P coatings. At each heat treatment, the steel witness samples were used to determine the microhardness by the Vickers method at a load of 100 g. The dependence of the parameters of the corrosion process on the presence of a dispersed additive and the phase composition of the coating has been established. At low holding times the dispersed phase exhibits a barrier effect reducing crystalline phosphide Ni3P formation during annealing and corrosion resistance; meanwhile, prolonged holding at lower temperatures produces about 70% Ni3P, stable high hardness values and improved corrosion resistance values. Lowering the coating heat treatment temperature in an oxidizing environment reduces the intensity of phosphorus burn-off from the surface and decreases all coating properties. The concentration of nitric acid in the solution at the level of 5-15% is critical and contributes to the dissolution of all coatings, regardless of their composition.The conducted research and revealed regularities made it possible to determine the contribution of the phase composition and presence of the dispersed additive to the formation of the main service characteristics of the nickel-phosphorus coatings - microhardness and resistance to aggressive media, as well as to determine the technological modes of heat treatment that allow the formation of optimum properties of products used in the oil and gas industry.