STUDIES ON THE HOT CORROSION BEHAVIOUR OF HVOF COATINGS ON SOME Ni-AND Fe-BASED SUPERALLOYS

Abstract

Hot corrosion degradation of metals and alloys has been identified as a serious problem for many high temperature aggressive environment applications such as boilers, internal combustion engines, gas turbines, fluidized bed combustion and industrial waste incinerators. Hot corrosion has become a topic of continuous investigations and great concern as it consumes the material at an unpredictably rapid rate. Consequently, the load-carrying ability of the components reduces quickly, leading eventually to their catastrophic failure. The inability to either totally prevent the hot corrosion or at least detect it at an early stage has resulted in several accidents, leading to loss of life and/or destruction of engines/infrastructures. Super-heater and re-heater components of boilers used in steam-generating systems are subjected to fireside corrosion due to condensation/accumulation of low melting-point salts on tube surface such as alkali-iron trisulphates compounds in the coal fired boilers or compounds of vanadium, sodium and sulphur, mainly as Na2SO4—V205 complex and sodium—vanadates mixtures in the case of oil fired boilers. Some combinations of these compounds have low melting points 550°C. These compounds easily liquefy at the operating temperatures of boilers and cause accelerated corrosion (hot corrosion). The hot corrosion problem of the boilers has become more alarming as the increasing demand for electricity is forcing the power plants to increase steam temperature and pressure of the boilers. Increased steam parameters in turn lead to accelerated corrosion of boiler components. Currently, the superalloys are used to increase the service life of the boilers especially in the super-heater zones of the new generation ultra-supercritical boilers. However, the presence of combustion gases constitutes an extreme environment and the hot corrosion is inevitable when the superalloys are used at high-temperatures for longer periods of time. A number of countermeasures are presently in use or under investigation to combat the hot corrosion such as, the use of inhibitors, control of the process parameters, development of suitable industrial alloys, and deposition of protective coatings. However, the use of protective coatings is most practical, reliable, and economically viable method to control or prevent the hot corrosion problems of the super-heater and re-heater tubes of the boilers.