High-temperature degradation of Hastelloy C276 in methane and 99% cracked ammonia combustion: surface analysis and mechanical property evolution at 4 bar

Alnaeli, Mustafa, Goktepe, Burak, Morris, Steven ORCID: https://orcid.org/0000-0001-5865-8911 and Valera-Medina, Agustin ORCID: https://orcid.org/0000-0003-1580-7133 2026. High-temperature degradation of Hastelloy C276 in methane and 99% cracked ammonia combustion: surface analysis and mechanical property evolution at 4 bar. Processes 14 (2) , 235. 10.3390/pr14020235

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Abstract

This study examines the high-temperature degradation of Hastelloy C276, a corrosion-resistant nickel-based alloy, during exposure to combustion products generated by methane and 99% cracked ammonia. Using a high-pressure optical combustor (HPOC) at 4 bar and exhaust temperatures of 815–860 °C, standard tensile specimens were exposed for five hours to fully developed post-flame exhaust gases, simulating real industrial turbine or burner conditions. The surfaces and subsurface regions of the samples were analysed using scanning electron microscopy (SEM; Zeiss Sigma HD FEG-SEM, Carl Zeiss, Oberkochen, Germany) and energy-dispersive X-ray spectroscopy (EDX; Oxford Instruments X-MaxN detectors, Oxford Instruments, Abingdon, United Kingdom), while mechanical properties were evaluated by tensile testing, and the gas-phase compositions were tracked in detail for each fuel blend. Results show that exposure to methane causes moderate oxidation and some grain boundary carburisation, with localised carbon enrichment detected by high-resolution EDX mapping. In contrast, 99% cracked ammonia resulted in much more aggressive selective oxidation, as evidenced by extensive surface roughening, significant chromium depletion, and higher oxygen incorporation, correlating with increased NOx in the exhaust gas. Tensile testing reveals that methane exposure causes severe embrittlement (yield strength +41%, elongation −53%) through grain boundary carbide precipitation, while cracked ammonia exposure results in moderate degradation (yield strength +4%, elongation −24%) with fully preserved ultimate tensile strength (870 MPa), despite more aggressive surface oxidation. These counterintuitive findings demonstrate that grain boundary integrity is more critical than surface condition for mechanical reliability. These findings underscore the importance of evaluating material compatibility in low-carbon and hydrogen/ammonia-fuelled combustion systems and establish critical microstructural benchmarks for the anticipated mechanical testing in future work.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Schools > Engineering
Publisher:	MDPI
ISSN:	2227-9717
Funders:	EPSRC
Date of First Compliant Deposit:	10 January 2026
Date of Acceptance:	23 December 2025
Last Modified:	12 Jan 2026 11:45
URI:	https://orca.cardiff.ac.uk/id/eprint/183775

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