Different corrosion behaviour of autoclaves made of nickel base alloy 718 in ammonobasic and ammonoacidic environments

• Published in: The Journal of supercritical fluids Vol. 95; pp. 158 - 166
• Main Authors: Hertweck, Benjamin, Steigerwald, Thomas G., Alt, Nicolas S.A., Schluecker, Eberhard
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-11-2014
• Subjects: Ammonothermal crystal growth; Auger electron spectroscopy; Autoclaves; Bonding; Corrosion; Dissolution; Nickel base alloy; Nitrides; Optical data processing; Stress corrosion cracking; Supercritical ammonia; Supercritical ammonia; Nickel base alloy; Corrosion; Ammonothermal crystal growth
• ISSN: 0896-8446; 1872-8162
• DOI: 10.1016/j.supflu.2014.08.006

•Metallographic investigation of corrosion behaviour of ammonothermal autoclaves.•Ammonobasic conditions lead to formation of corrosion layer.•Ammonoacidic environment cause vast general corrosion and stress corrosion cracking.
The ammonothermal process is a promising technique for cost-effective growth of nitride based bulk-semiconductors. Degeneration of growth reactors made of a NiFeCr alloy is a limiting factor of the growth process. Corrosion is initiated by supercritical ammonia and acidic as well as basic mineralizer up to 600°C and 300MPa. We investigated the corrosion processes with microscopic (optical and scanning electron microscopy) and spectroscopic (energy dispersive X-ray and auger electron spectroscopy) methods. It showed that ammonobasic environments lead to the formation of a nitrited layer. Composition and thickness changes depending on the number of loads. The corrosion layer does not reach a steady state and therefore degenerates as consequence of cyclic ammonothermal loads. For ammonoacidic conditions on the other hand significant general corrosion occurred which is connected to the dissolution of the metal phase in the supercritical. Moreover, a high tendency to stress corrosion cracking was observed.