High-temperature oxidation resistance of Ni–P and Ni–B electroless coatings on mild steel after long-term tests

The oxidation resistance of Ni-P and Ni-B electroless coatings applied on mild steel has been determined by means of isothermal oxidation tests at 600 degrees C for up to 400 h for uncoated and coated samples. The oxidation rate is evaluated by means of gravimetric analysis. SEM, EDS, XRD and Raman Spectroscopy characterisation allows establishing the morphological and chemical features of the coatings and the oxidation products, as well as their relationship with the oxidation resistance at high temperature. Coated samples show higher oxidation resistance than un-coated samples, being better the behavior of Ni-P coatings. The protective behaviour of electroless coatings is associated with factors such as the formation of a Ni-Fe interdiffusion layer, the incorporation of NiO into the oxide layer and the formation of crystalline phases of Ni3P.