Elevated temperature erosion of several thermal-sprayed coatings under the simulated erosion conditions of in-bed tubes in a fluidized bed combustor

A series of elevated temperature erosion tests were carried out on cooled AISI 1018 low-carbon steel and four thermal spray-coated mild steel specimens using a nozzle-type elevated temperature erosion tester. The thermal-sprayed coatings included a high velocity oxy-fuel (HVOF) 75Cr(3)C(2)-25NiCr cermet coating, HVOF Cr3C2, low velocity flame-sprayed Cr2O3 ceramic coatings and an are-sprayed FeCrSiB coating. Test conditions attempted to simulate the erosion conditions at the in-bed tubes of fluidized bed combustors, The specimens were water-cooled on the backside. Material wastage was determined from thickness loss measurements of the specimens. Erosion test results were compared with those from testing isothermal specimens. The morphologies of specimens were examined by scanning electron microscopy ( SEM). It was found that the cooled specimens had greater material wastage than the isothermal specimens. The effect of cooling the specimen on the erosion wastage for 1018 steel was greater than that for the thermal-sprayed coatings. The lower material wastage of coated specimens compared to 1018 steel was related to the coatings' composition and morphology. The HVOF Cr3C2 coating specimen exhibited the lowest erosion wastage due to its favorable composition and morphology. It had a finer structure and smaller splat size than the other three coatings. The erosion mechanism of the cooled specimens was similar to that of the uncooled specimens, despite the different material wastage.