Research On Corrosion Resistance Of Coatings Prepared By PTA/TWAS/HVOF Technologies In Waste Incinerators

Incineration has become the most commonly used method of waste disposal in China.However,the corrosive substances released during the incineration process will cause corrosion of the heat exchange surface and affect the safe and operation of boilers.Applying coatings can strengthen the heat exchange surface and alleviate the thermal corrosion.In view of the complex furnace environment,the applicability of coatings prepared with different technologies and materials was still unclear.In this paper,plasma transferred arc welding(PTA),twin wire arc spray(TWAS)and high-velocity oxygen-fuel spray(HVOF)technologies were employed to prepare 10 kinds of Ni Cr-based and Fe Cr-based coatings.The effects of coating technologies and materials on the properties and corrosion resistance of coatings were studied.Laboratory and actual furnace experiments were used simultaneously to evaluate the applicability of coatings under current and high-parameter conditions,and reveal the anti-corrosion mechanism of coatings.The main conclusions were as follows:The oxidation degree of the material in the process of PTA and HVOF was significantly lower than that of TWAS technology,and the TWAS coatings contained more oxides.PTA and HVOF coatings were more uniform and dense.The bonding strength of the coatings was above 70 MPa.The coatings also had excellent thermal shock resistance.The micro morphology of TWAS coatings showed a rich texture structure with the porosity among 2-7%,and the bonding strength of the coatings were among 30-60 MPa.In the thermal shock resistance test,the coating would warp and even peel off.On the whole,compared with TWAS coatings,PTA and HVOF coatings had superior performance in terms of microstructure,bonding strength and thermal shock resistance.It indicated that PTA and HVOF coatings may have better corrosion resistance.Coating samples provided a good protective effect on boiler steel at 450-650 ℃.The difference in corrosion resistance of coatings expended as the temperature increase.From the perspective of coating technologies,the coatings prepared by the three technologies all had excellent corrosion resistance at 450 ℃ and the application of low-cost TWAS coatings could be preferred.At 550 ℃ and above,the TWAS coatings were likely to fall off due to corrosion penetration and oxidative corrosion at the bonding surface.PTA and HVOF coatings still had good applicability because of the compact structure.From the perspective of coating materials,As Cr content increased from 7.7%to 21.6%,and Ni/Cr ratio decreased from 10.0to 3.0,the corrosion resistance of coatings increased linearly.The high Cr content and low Ni/Cr ratio promoted the enrichment of Cr in corrosion layer.The dense Cr-rich oxide film inhibited the occurrence of corrosion and enhanced the thermal stability of the film.In addition,the influence of ash adhesion on thermal corrosion of coatings was more obvious at450-550 ℃.Ash adhesion was positively correlated with the corrosion.Coatings had little difference in ash adhesion at 600-650 ℃.The influence of temperature on corrosion was enhanced,and the correlation between ash adhesion and corrosion was weakened.In actual waste incinerator,the NiCr coatings prepared by the three technologies had shown good corrosion resistance in low-temperature superheater where the temperature of working fluid was 350-420 ℃.In high-temperature superheater where the temperature of working fluid was 380-450 ℃,the TWAS coating on the windward side all peeled off from the substrate after 2700 h of service.There was also serious consumption in local areas of HVOF coating.The flue gas penetration along the pores of the coating was an important factor for the corrosion.The porosity of PTA coating was only 0.17%,and the Cr-rich oxide layer formed on the surface was tightly combined,which inhibited the penetration of corrosion substances.In general,PTA coating had the best corrosion resistance and TWAS coating was the worst.The permeation of flue gas along the pores intensified the corrosion of coatings in actual furnace.By optimizing the preparation process to increase the density of the coating and the use of sealing treatment could prevent the flue gas from penetrating into the interior,and further enhance the performance of coatings.In summary,applying coatings had good corrosion protection effect on heat exchange surface in waste incinerator.Compared with TWAS coating,PTA and HVOF coating had higher applicable temperature and stronger corrosion resistance.Increasing Cr content and reducing Ni/Cr ratio could enhance the corrosion resistance of coatings.This paper had positive guiding significance for coating application on heat exchange surfaces in waste incinerators and the development of high-performance corrosion-resistant materials.