Effects of salt and ash deposits on corrosion behaviour of Ni-25Cr in Ar-60CO2-20H2O gas at 650 oC.

A Ni-25Cr (wt.%) alloy coated with the salts and mixtures of these salts with an industrial coal ash was exposed to Ar-60CO2-20 H2O at 650 °C for 300 h. The alloy without any deposit developed a uniform internal oxidation zone (IOZ) and an external metallic nickel layer surmounted by a thin NiO scale. Salt deposits changed the scale constituents and morphology. Predominantly, a duplex scale of NiO over a Cr2O3 layer grew with an underlying IOZ containing large Cr-rich oxide precipitates. Under a deposit of ash plus salts, the basic reaction morphology was the same as in the gas-only case, but the oxide layer was non-uniform. Local voids were formed within the alloy beneath chloride plus ash deposits. Under ash with sulphates, the alloy formed both partially protected areas and non-protective multi-layered scales. Spherical ash particles were enveloped by NiO in both salt-ash deposits. [ABSTRACT FROM AUTHOR]