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

The ammonothermal process is a promising technique for cost-effective growth of nitride based bulk-semiconductors. Degeneration of growth reactors made of a Ni-Fe-Cr 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 degrees C and 300 MPa. 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. (C) 2014 Elsevier B.V. All rights reserved.