Mass-balance interpretation of erosion–oxidation degradation in NiCr-based thermal spray coatings under fly ash impingement at 600 °C

High-temperature degradation of protective thermal-spray coatings under fly ash impingement is governed by coupled oxidation, deposition, and erosion-assisted material removal. Consequently, transient mass change does not necessarily represent post-cleaning residual degradation because oxide growth, fly ash deposition, and particle embedment can partially compensate for material removal. This study introduces a paired-specimen mass-balance methodology to distinguish oxidation-related mass gain from erosion-assisted residual loss in Co-, WC-, and Mo-modified NiCr-based coatings exposed to fly ash at 600 °C. For each coating, one specimen was exposed to hot air and the paired specimen to hot air with fly ash under matched thermal histories. Time-resolved and post-cleaning mass measurements were combined with scanning electron microscopy, X-ray diffraction, and Vickers hardness to relate degradation behaviour to coating structure and properties. All coatings showed a positive pre-cleaning mass change after 600 s, while interval-resolved responses alternated between mass gain and mass loss. However, post-cleaning paired mass-balance analysis revealed substantial differences in residual response. NiCr and NiCrCo exhibited substantial apparent erosion-assisted residual loss, whereas WC-containing coatings exhibited markedly lower residual loss. NiCrWCCo showed the lowest apparent paired residual-loss response, with no additional erosion-assisted residual loss resolved relative to the oxidation reference under the present conditions; this response was consistent with its compact lamellar structure, improved inter-splat cohesion, and carbide reinforcement. The results support three mass-balance regimes: oxidation/deposition-dominant, balanced (quasi-stable), and erosion-assisted residual-loss. The paired approach provides a complementary framework for interpreting high-temperature erosion–oxidation when conventional net mass-change measurements cannot separate simultaneous mass-gain and mass-loss processes.