Evaluation of the Corrosion Resistance of Heat Resistant Cast Alloys in Ammonia Containing Atmospheres: The Role of Oxide Scales

Ammonia is desired to become a large-scale energy vector for future H₂-based, decarbonized industry applications. The introduction of ammonia as a cracking feedstock introduces a new corrosion hurdle, nitridation under high temperatures on the SMR-like tubular reactors. To maximize the reliability and the process stability of those cracking units, the materials used for those tubular furnaces have to be chosen with respect to their mechanical properties as well as their resistance against nitridation by ammonia under elevated temperatures. This study demonstrates, that controlled preoxidation of cast high-temperature resistant alloy grades can effectively reduce nitridation of those materials when exposed to NH₃ containing gas phases at 900°C. Combination of isothermal experiments with metallographic investigations of the specimens enabled comparison of the different alloy classes and identify the occurring corrosion mechanisms. Further, the spallation tendency of the scale was identified to reduce the nitridation resistance of the surface.