Surface orientation effects on initial carbon deposition and metal dusting of nickel

Polished polycrystalline nickel was first characterised by orientation imaging microscopy to search for grains with low index faces, and then reacted in a gas of 25%CO-25%H(2)-0.5%H(2)O-49.5%Ar at 650 degrees C for different times. Carbon deposition on and metal dusting of these grains were examined by scanning electron microscopy and cross-section observations made by focused ion beam milling. Carbon deposition was found to be the fastest on nickel (111) face, slowest on nickel (001), and somewhere in between on other faces. The carbon morphology varied from a featureless uniform layer on (111) face to aligned needles or leaves on (101) and other faces. This phenomenon can be explained in terms of a graphite-nickel epitaxial relationship. Initial metal dusting was affected by nickel surface orientation in a more complicated way. The (001) face produced initially little or no carbon deposition, and therefore no carbon attack in most area, but dusted locally after 22h reaction. Local dusting attack was also found to correspond well with localised graphite nucleation. Different surface nickel grains were found to change height with reaction time, a difference attributed to varying extents of carbon dissolution.