Microprocesses in nickel accompanying metal dusting

Abstract

Polycrystalline nickel was reacted at 680 °C with a carbon-supersaturated CO/H2/H2O gas. Graphite deposited on the metal surface as (a) very thin (20-200 nm) layers on a smooth metal surface, (b) thick (1-2 μm), relatively uniform layers which developed serrated incursions into the metal, and (c) nodules which stood proud of, and protruded into, the metal surface. The thin, benign graphite deposits formed on {0 0 2} nickel surfaces but thicker graphite layers grew on {1 1 1} surfaces. Thickening of that layer, and graphite nodule growth, were both associated with the inward growth of graphite basal planes parallel to nickel {1 1 1} and {1 1 3} planes located within the metal interior. Microanalysis showed that the graphite contained nickel, which was identified by dark-field imaging and selected-area diffraction as nanoparticulate metal. These nanoparticles are thought to have resulted from metal disintegration caused by the volume expansion accompanying graphite nucleation and growth.