Comparative high temperature oxidation studies of HVOF IN 625 coating on T22 boiler steel at 900 °C and 700 °C

Abstract

Abstract IN 625 coating was deposited on T22 boiler tube steel by high velocity oxy fuel (HVOF) process to increase high-temperature corrosion resistance. High-temperature oxidation behavior of steel with and without coating has been examined at 700 °C and 900 °C under cyclic conditions. Each cycle consisted 1 h of heating in the silicon carbide tube furnace followed by 20 min of cooling in air. The cyclic oxidation conditions have been chosen as these conditions resembles actual industrial environment conditions where occurrence of shutdown is very frequent. Cyclic studies give the more severe conditions for testing and may be similar to the actual industrial environment of frequent breakdown/shutdown occurrence. Thermo gravimetric technique was implemented to study the kinetics of corrosion by observing weight change after each cycle. The bare steel experienced higher weight gain, which may be credited to the development of un-protective Fe2O3 dominated oxide scales. Evaluation of corrosive products was done using X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDAX) and X-ray mapping techniques. It was observed that the uncoated steel suffered corrosion in the form of spalling and peeling up of its oxide scale. On the converse, the IN 625 coating provided significant resistance to the corrosion. The IN 625 coating was found to be more protective at 700 °C than at 900 °C. The better air oxidation resistance in the coatings may be attributed to the phases discovered in the oxide scale of the coated specimens which mainly consisted of oxides and spinals of nickel and chromium. The information gathering from the investigation will be useful to explore the possibility of the use of the HVOF spray coatings on the boiler tubes working in the range of 700–900 °C.