Site related nucleation and growth of hydrides on uranium surfaces

The characteristics of hydride nucleation and growth on certain surfaces of pure uranium and of U-0.1 wt.% Cr samples were studied (under 1 atm H-2 at temperatures of 50-75 degrees C) using the hot-stage microscope, microprobe analyzer and atomic force microscope techniques. Different preparation procedures of the samples were applied, in order to check the effects of surface oxidation layer variations on the nucleation and growth characteristics. Four families of hydride nuclei, differing in density, size and growth rates, were observed and classified. The smallest and most dense is in the form of submicron blisters formed instantaneously along mechanical polishing scratches. A larger (1-10 mu m) blister-like family is formed beneath the oxide at point defect sites (but not discontinuities of the oxide), growing very slowly probably due to the compression of the coating oxide layer. The third family is characterized by preferential nucleation and rapid growth around carbide inclusions due to the discontinuity in the oxide at the carbide/oxide interface. The fourth family is found only on the samples having a thick oxide layer, and is characterized by a rapid growth rate, but is not located around inclusions. In this case, the nuclei probably originate at some other oxide discontinuities, such as twins or grain boundaries.