Electroless Ni–P coatings on low-Cr steels: A cost-efficient solution for solar thermal applications

Abstract

The corrosion behavior of an electroless Ni–P coating on low-Cr X20CrMoV12-1 steel as a cost-efficient substrate-coating combination is investigated in molten NaNO 3 –KNO 3 nitrates at 600 °C. Results indicated that the as-deposited Ni–P coating was transformed to a thermally treated diffusion coating upon exposure to the molten salt without additional heat treatment prior to the exposure. Detailed evolution of the Ni–P coating and the oxidation behavior during the 1000-h immersion test is discussed. Results showed competitive corrosion resistance of the coating system compared to that of the sophisticated Ni alloy 230, with the advantage of an inherent barrier against Cr dissolution form the substrate. The significant corrosion resistance of the electroless plated Ni–P coating is attributed in part to the protective Ni--rich oxide scale and in other part to the in-situ formed Ni 3 P barrier at the oxide-substrate interphase boundary. - Electroless Ni–P coating was developed on a low-cost X20CrMoV12-1 substrate - Interdiffusion occurred during the immersion test at 600 °C - The as-coated substrate required no additional heat treatment prior to the molten salt exposure - An outer Ni–Cr rich oxide scale and an inner Ni 3 P layer formed during the immersion test - The Ni–Cr scale and Ni 3 P layer protected the substrate against molten salt corrosion and Cr-dissolution