Authors: Yuchen Cai, Jianqiang Zhang*, David J. Young

ABSTRACT

A model Ni-25Cr (wt.%) alloy coated with chloride salts, industrial coal ash, and mixtures of these salts with coal ash (ash/salt weight ratios: 90/10, 50/50, 25/75) was exposed in Ar-60CO2-20H2O at 650oC for 100 h and 300 h. For comparison, the alloy without any deposit was also exposed to the same gas.

The alloy without any deposit developed a uniform internal oxidation zone (IOZ) containing Cr-rich oxide, below an external layer and metallic nickel nodules surmounted by a thin NiO scale. Ni-25Cr covered by 100% ash behaved quite similarly to the one in the gas-only case, despite the disappearance of extruded metallic Ni layer and the thicker IOZ layer (gas-only: 6 µm, ash-only: 9 µm).

After starting to add chloride salt to the deposit, a localised structure formed. Localised thick Cr2O3 scale containing encapsulated Ni metal with underlying subsurface alloy porosity were found. This localised structure occupied 10% of the whole surface in the 10% salt + 90% ash case, while the rest 90% remained the same as in the gas-only case. As the salt amount increased to 50%, the structure with subsurface voids occupied 80% of the whole surface, while a duplex layer formed on the remaining 20% of the sample surface.

Two different structures are also found in the 100% salt case. Predominantly, a multilayered scale of NiO over a thin Cr2O3 layer formed, with an underlying IOZ containing Cr-rich oxide precipitates. Local regions of thick Cr2O3 scale containing encapsulated Ni metal with underlying subsurface alloy porosity were found.

Keywords: High-temperature corrosion, chlorides, coal ash, Ni-based alloy

Read the complete paper here – Effects of Salt and Ash on high temperature corrosion behaviour of NI-25CR Alloy in Wet CO2 Gas at 650°C – Author J Zhang (1)