Scale development on impure nickel at high temperatures

The growth of scales on impure Ni at 1373 and 1473 K was examined by microscopy and by oxygen-tracer-imaging techniques. The outward transport of cations was the basic rate-controlling step, and on the simple initial columnar scales such cation movement was the only kinetic process. However, oxygen movement was essential for the continued growth of more complex scales. Three types of sub columnar oxide were observed: (1) relatively coarse equiaxed crystals, (2) fume-like crystals, and (3) oxide that filled the pores between the crystals of the first two types. The oxygen for the first two types was provided by dissociation of the base of the columnar layer and also from voids that developed in the columnar layer. Oxygen for the third type came directly from the atmosphere following scale cracking. Such cracks healed after forming. The origin of the major features of scales was accounted for.