Results of Full Scale Tests of Ion-Plasma Coatings in a Fluidized Bed Cyclone Stoker

Abstract

The article presents the results of full scale abrasion and corrosion resistance tests carried out on the experimental specimens of grade St20 and 09G2S steels with protective Cr–CrС and Cr–CrN ion-plasma coatings (IPCs) after they have been held for 576 h in a 25 kW fluidized bed cyclone stoker operated on three kinds of pelletized biofuel: flax shives, straw, and sunflower husk. The temperature in the furnace and in the gas flue, in which the experimental specimens made of grade 09G2S and St20 steels were placed, was from 750 to 650°С. A chemical analysis of the ash produced after combusting these three kinds of pelletized fuel has shown that potassium and chlorine were available in the ash composition, the presence of which may give rise to the occurrence of corrosive compounds in the case of moisture condensation during the equipment outage periods. It has been found that the coatings studied did not experience abrasion or corrosion destruction; the coating thickness changed by a factor of 1.2–1.5 on the average. A deviation in the morphology was noted as a consequence of coating surface layer oxidation at the maximal exposure time; however, no material oxidation under the coating was found on the transverse metallographic sections of coated specimens. The bench tests of grade 09G2S steel specimens with protective Cr–CrN IPC carried out after holding them in the installation’s furnace at an abrasive air flow incidence angle of 90° have shown that the abrasive resistance in terms of steady wear rate has increased by no less than a factor of two in comparison with that of noncoated specimens. In testing the corrosion resistance of specimens made of grade St20 steel with Cr–CrC coating after holding them in the installation’s gas flue followed by holding them for 480 h in a water extract from the ash of each of combusted biofuels, it has been noted that the corrosion rate determined using the gravimetric method has been decreased by 1.2–2.3 times in comparison with the same indicator of noncoated specimens. According to the study results obtained, multilayer Cr–CrС and Cr–CrN ion-plasma coatings are promising ones for ensuring protection against corrosion and abrasion wear for the furnace and gas flue materials in fluidized bed boilers that use agricultural waste (flax shives, straw, or sunflower husk) as biofuel.