Corrosive Degeneration of Autoclaves for the Ammonothermal Synthesis: Experimental Approach and First Results

• Published in: Chemical engineering & technology Vol. 37; no. 11; pp. 1903 - 1906
• Main Authors: Hertweck, Benjamin, Steigerwald, Thomas G., Alt, Nicolas S. A., Schluecker, Eberhard
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01-11-2014; WILEY‐VCH Verlag
• Subjects: Ammonia; Ammonothermal crystal growth; Autoclaves; Corrosion; Degeneration; High-pressure equipment; High-temperature corrosion; Nickel base alloys; Nickel-base alloy; Nitrides; Optical microscopy; Superalloys
• ISSN: 0930-7516; 1521-4125
• DOI: 10.1002/ceat.201300719

The ammonothermal growth of different bulk group III nitride materials enables cost‐efficient production of high‐quality single crystals. Despite its advantages this process is limited due to the corrosive ammonothermal environment. This environment consists of supercritical ammonia with acidic or basic additives at high temperatures and pressures in high‐pressure cells made of a nickel‐base alloy. As a consequence of corrosion, large amounts of metallic impurities are detectable in the grown crystals. The mechanisms of corrosion are not well understood. A method is presented to investigate corroded autoclaves, caused by the interaction with supercritical fluid, together with first results regarding the degeneration of autoclave material.
Alloy 718, a material of high corrosion resistance, degenerates in supercritical ammonia with acidic mineralizers. EDX spectroscopy in combination with optical microscopy and micro indentation is a suitable tool for analyzing the corrosion behavior and allows a detailed investigation of the corrosion phenomena for Alloy 718 under ammonothermal conditions.