Effect of ageing on KCl corrosion attack of AISI 347H FG steel

One of the major drawbacks of biomass firing as compared to coal firing is the increased corrosion rate on the fireside of superheater tubes where the corrosive environment is characterized by the presence of alkali chlorides such as KCl.To assess whether corrosion in a KCl containing environment is influenced by microstructure evolution at high temperature, laboratory corrosion experiments were performed using as received and aged AISI 347H FG austenitic steels taken from coal-fired reheater tubes. The corrosion exposure was performed using KCl-covered samples in a 15 % (v/v) H2O (g) + 5% (v/v) O2 (g) + N2 (g) (balance) atmosphere at 600oC for 168h. Microstructural characterisation was performed before and after the corrosion experiment using light optical microscopy and scanning electron microscopy with energy dispersive x-ray analysis. The aged samples showed higher corrosion attack than the as received sample. Grain boundary internal attack was also observed from the aged samples.