To Study the Corrosion Behavior of D-Gun Sprayed NiCr Coated 347H Stainless Steel under Simulated and Actual Biofuel Fired Boiler Environment

Abstract

High temperature corrosion has lead to failure of different components in field of aerospace, thermal power plants, gas turbines etc. In power generation industries, ferritic and austenitic steels have been used in the high temperature areas because of its ability to withstand at high temperature. However, the current steels are limited to 620ºC. But due to the requirement of increase in efficiency of boiler, the temperature in the boilers is constantly increasing. Moreover, coal is generally used as fuel in the boilers which produces greenhouse gases. Hence, to reduce the problem of green house gases emission, the fuel has been replaced by bio-fuel such as wood, husk etc. Although use of bio-fuel gives lot of advantages such as reduction in green house gases, reduction in fuel cost but burning of such waste can leads to the formation of salt species which can deteriorate the life of the components used in construction of boilers. This can causes unwanted shutdown of the boiler as well as can lead to accidents due to failure of components. To reduce such unwanted shutdown and fatal accidents has lead to purpose of work that is to enhance the corrosion characteristics of 347H SS. It is known that ferritic and martensitic steels that are used in construction of boiler components do not have the required spallation and oxidation properties for boiler tube applications as service temperature of boiler increases. Spallation inside the tube can lead to blockage, overheating, creep rupture and turbine erosion. To obviate these problems austenitic stainless steel grade 347H has been chosen as base metal to carry out the experimentation. 75Ni25Cr coating powder was deposited on 347H austenitic SS sheet by the process of Detonation Gun. Oxidation and hot corrosion studies were done on bare and coated specimens at 800ºC under cyclic condition for 50 cycles. The tests were carried out in laboratory using tubular furnace. Oxidation study was carried out in air whereas hot corrosion study was performed under the simulated bio-fuel fired boiler environment. Mixture of 40%K2SO4+10KCl+40%Na2SO4+10%NaCl salts were used to simulate the boiler environment. To compare the laboratory test, the samples were also hung in the actual husk fired boiler environment. The boiler was available in Nectar II Company situated at Dera Bassi, Punjab and the temperature inside the boiler was 750C±50C. The samples were hung for 150h duration. Oxidation kinetics was recorded using weight gain technique for laboratory samples. For specimens hung in actual boiler thickness loss measurements were carried out which can help in finding the life of the samples in milli-inches per year. The oxide morphology was characterized using SEM and EDS analysis The bare steel specimen experienced higher weight gain, which v may be due to the formation of protective Cr2O3 and unprotective Fe2O3 oxide layers in case of samples that were tested in laboratory. Whereas coated samples showed lower weight gain because of the formation of protective Cr2O3 and NiCr2O4 oxide layer. In actual husk fired boiler both the bare and coated samples got intensively corroded. However Ni25Cr coated 347H SS sample was less corroded as compared to bare 347H SS sample.