Surface nitridation induced by ammonia combustion can alter material properties, reducing lifetime and performance of a practical combustor. Although ammonia-induced surface nitriding has been studied at low and moderate temperatures, its behavior at temperatures above 1200 K in flames remains unexplored. This study investigates high-temperature nitriding and the impact of NH radical of nickel rods exposed to NH3/H2 flames above 1500 K. The presence of abundant radicals and complex chemical reactions in NH3/H2 flames, together with flame–wall interactions between reactive gases and metal surfaces, impose great challenges in understanding the underlying nitridation mechanism. In this work, gas-phase characterization was performed using in-situ planar laser-induced fluorescence (PLIF) for NH radical distribution and Rayleigh scattering for temperature, while surface morphology and chemical composition were analyzed by scanning electron microscopy (SEM) and high-resolution X-ray photoelectron spectroscopy (HRXPS). The results show that nickel nitridation can occur within minutes of flame exposure and is strongly dependent on NH concentration at a same flame temperature. In addition, no distinct nitride layer was observed after more than one hour of flame exposure, while pore structures beneath the nickel surface were detected using ion-beam cutting under high vacuum. These observations suggest that high-temperature nitridation under ammonia combustion conditions may proceed through mechanisms that differ from those typically reported at lower temperatures. Novelty and significance statement This study presents the first high-temperature experimental investigation of nickel nitridation under ammonia/hydrogen combustion conditions exceeding 1500 K, moving beyond the moderate-temperature (<1200 K) and non-flame studies that dominate existing literature. By combining operando planar laser-induced fluorescence (PLIF) for NH radicals with ex-situ high-resolution X-ray photoelectron spectroscopy (HRXPS) and scanning electron microscopy (SEM) analysis, the study directly links radical chemistry in the reacting flow to surface phase evolution and microstructural transformation. The findings, characterized by rapid nitride formation, radical-mediated nitrogen incorporation, and lattice instability, highlight that the flame-wall interactions at 1600 K are different from low and moderate temperatures. These insights are essential for predicting material degradation in ammonia-based energy systems and for guiding the design of high-temperature metal components with improved resistance to nitridation and/or oxidation.
}}, author = {{Bao, Yupan and Ji, Liang and Vogeley, Raphael and Ri, Kaii and Xu, Wenbin and Zhang, Zhiyuan and Xu, Yijie and Mei, Bowen and He, Huixin and Brackmann, Christian and Li, Tao and Ju, Yiguang}}, issn = {{1540-7489}}, keywords = {{Ammonia hydrogen flame; Gas-phase and surface diagnostics; NH radicals; Nickel nitridation}}, language = {{eng}}, publisher = {{Elsevier}}, series = {{Proceedings of the Combustion Institute}}, title = {{Surface nitridation of nickel by NH3/H2 rich flames at elevated temperature}}, url = {{http://dx.doi.org/10.1016/j.proci.2026.106020}}, doi = {{10.1016/j.proci.2026.106020}}, volume = {{42}}, year = {{2026}}, }