Formation and oxidation resistance of Al/Ni coatings on low carbon steel by flame spray

In this work the effect of heat treatment on the structure and the durability of double layer Ni/Al coatings was investigated. An initial Ni-Cr layer was primarily deposited by thermal spray, followed by a successive Al metallic layer in a second stage. The specimens were then heat treated in inert atmosphere. Nickel aluminide compounds were formed as a result of the diffusion between the two layers upon annealing. Indeed, scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy examinations of the coated samples revealed that the as sprayed coatings contain two distinct Al and Ni-Cr layers while the heat treated coatings were gradually transformed to Al3Ni depending on the heat treatment parameters. The as sprayed and annealed specimens were tested by subjecting them in an air environment at 700 °C and 800 °C, for the evaluation of their durability and resistance. It was found that the annealed samples have superior high temperature resistance compared with the as sprayed coatings and bare steel, which is attributed to the presence of Ni-Al compounds, to the low coating porosity and to the formation of dense oxides on the coating surface.