Erosion corrosion properties of HVOF coatings for municipal solid waste incinerator protection

Abstract

Municipal solid waste incinerators (MSWI) usually have erosion–corrosion (EC) problems. A possible solution for the damage caused by EC processes in the heat exchanger zone has been found in thermal spray coatings with Ni based and Fe based high velocity oxygen fuel (HVOF) coatings showing promising results. High velocity oxygen fuel coatings generally have a low porosity and good adherence. These properties help to prevent damage to exchanger tubes. In the present paper Ni and Fe alloys were compared under different laboratory tests and in service inside a MSWI heat exchanger. Coatings were also characterised by standardised tribological and microhardness tests. A eutectic mixture of KCl–ZnCl2 found on the boiler ashes was used as a corrosion agent in the laboratory tests. Scanning electron microscopy (SEM) images of sprayed coating structures before and after the tests were compared. Energy dispersive spectroscopy (EDS) analysis and X-ray diffraction (XRD) were used to characterise the coatings and corrosion products. Studies on bulk materials with similar composition help to understand some steps in the corrosion processes that take place during incineration. It is concluded that the low level of protection provided by Fe based alloy HVOF coating in these conditions may be attributed to the role of non-protective chromium oxides, and the good behaviour of Ni based alloy HVOF coating may be attributed to the formation of more protective Ni, Mo and Nb oxides.