LIFE EVALUATION OF ZrO_2/Ni BASE ALLOY PLASMA-JET COATING SYSTEMS IN HIGHEFFICIENCY WTE BOILER SUPERHEATERS

摘要

In order to improve the durability and cost performance of superheater in high efficiency WTE boiler, new Ni base alloy/YSZ dual coating systems using plasma-jet spray technology has been developed, and then the life of these coatings sprayed on 500 deg C/9.8MPa high-efficiency WTE boiler superheater was evaluated by the long term field corrosion test. Three kinds of coating layer of alloy 625/YSZ, NiCrSiB alloy/YSZ and Cr_3C_2 centre dot NiCr sprayed by 100kW plasma-jet system both on site and on shop were installed in tertialy and secondary superheater having high metal temperature of 450 - 500 deg C, and exposed for more than 1.3 years. As the results of the field corrosion test, alloy 625/YSZ and NiCrSiB alloy/YSZ coatings with inorganic sealant showed excellent durability more than 1.5 years compaired with conventional NiCrSiB alloy HVOF system in tertialy superheater. Based on the detail investigation of cut-off tube samples, A durability of each coatings reduce with increase in corrosivity of combustion gas. YSZ and inorganic sealant were clarified to work as a penetration barrier for corrosive components in deposits to coating/base metal interface. Especially, in soot blower affected position, Cr_3C_2 centre dot NiCr layer sprayed in secondary superheater peeled off by thermal cyclic factor after four months exposure, while large corrosion prevention effect was observed in Ni base alloy/YSZ layer regardless of partial peeling-off of YSZ layer. An application of Ni base alloy/YSZ coatings for actual boiler is expected in the future.