Pore structure in nickel created by ammonia at 873 K

Nickel spheres (of 0.76-mm diameter and purity 99%) were exposed to ammonia at 873 K for 140 h. The spheres were removed and examined using FEGSEM (Field Emission Gun Scanning Electron Microscopy) and compared with a fresh sample. Formation of a porous layer, several microns thick, was observed at the surface with a pore size distribution concentrated in the range 50–200 nm. Additionally, cracking occurred along the grain boundaries, in some cases reaching the centre of the spheres. Particles of alumina were also observed embedded in the surface of the spheres. Similar experiments with high purity nickel wire (99.999%) showed the same type of pore formation indicating that the observations for the spheres were not likely to be caused solely by impurities in the nickel.