A Comparative Study on Role of Zirconium as Minor Addition in HVOF-Spray Ni-20Cr Coatings in Enhancing Life of Boiler Steels

The aim of this study is to investigate the role of a rare earth element Zirconium (Zr) as minor addition in HVOF-spray coatings to formulate a high temperature oxidation resistant composition. Ni-20Cr coatings with and without 1% Zr addition were deposited on 347H boiler steel. Subsequently the high temperature oxidation behaviour of the steel with and without coatings was investigated by laboratory tests at an elevated temperature of 900⁰C under cyclic conditions. The samples were also subjected to a boiler environment at an elevated temperature of 700ºC under cyclic conditions to ascertain their erosion-corrosion behaviour. Weight change data was evaluated to formulate the kinetics of the high temperature oxidation as well as erosion-corrosion for the coatings. The specimens were physically examined also after each cycle to notice any macrographic physical changes, spallation tendency of their oxide scales etc. The oxidation mechanisms were elucidated through the X-ray diffraction (XRD) analysis and microstructural characterization techniques such as SEM/EDAX analysis. Both the coatings provided resistance to the base steel in actual as well as laboratory conditions. It was observed that the oxidation rate was subsequently low in the steel coated with Ni-20Cr +1% Zr coating, which indicates the usefulness of the Zr addition.