High Temperature Corrosion Behavior of Cold Spray Ni-20Cr Coating on a Boiler Steel in Molten Salt Environment at 900 oC

Cold spray is an emerging technology which produces high density coatings with low oxide contents and high thermal conductivity, which makes them ideal for applications on boiler tubes in thermal power plants for high temperature corrosion resistance. In the current investigation, Ni-20Cr alloy powder was deposited on SA 516 boiler steel by cold spray process. Oxidation kinetics was established for the uncoated and cold spray Ni-20Cr coated boiler steel in an aggressive environment of Na₂SO₄ -60%V₂O₅ at 900^oC for 50 cycles by the weight change  technique. The coated as well as bare steel followed the parabolic rate law of oxidation. The parabolic rate constant of Ni-20Cr coated steel was lower than that of the bare boiler steel.  X-ray diffraction, FE-SEM/EDAX and X-Ray mapping techniques were used to analyse the oxidation products of the coated and uncoated boiler steel. The uncoated steel suffered corrosion in the form of intense spalling and peeling of its oxide scale, which may be due to formation of unprotective Fe₂O₃ oxide. The Ni-20Cr coating was successful in reducing the weight gain of the steel by of 82.7%.  The formation of oxides of nickel and chromium may be contributing to the development of hot corrosion resistance in the coated boiler steel.