Observation of local hydrogen on nickel surfaces
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lightly electropolished again to remove any contamination or deformation layer arising from cathodic charging. Subsequently, the samples were immersed (typically for 10 minutes) in the metal ion solutions described below, rinsed with distilled water, and dried. Inspection of the decorated surfaces was carried out by light microscopy, scanning electron microscopy, and in a few selected cases by transmission electron microscopy (TEM) on thinned samples. In all cases, the dilution of the solution as well as the exposure times were adjusted so that only a very minor or negligible deposition of metal crystals occurred in the "reference areas". Finally, it should be noted that good, reproducible results were obtained only if samples were electrolytically polished. Mechanical polishing and even vibratory polishing led to too many surface artifacts. Ag crystal decoration. The best results were obtained with a potassium-silver dicyanide solution. It was prepared by adding drops of a 4 pct KCN water solution to about 10 ml of a 12 pct AgNO3 solution while stirring, until the white precipitate of AgCN (reaction (a)) just dissolves (reaction (b)):
T. SCHOBER, Staff Scientist, and C. DIEKER, Research Assistant, are both with the Institute fl,ir Festk6rperforschung, Kernforschungsanlage Jiilich, 5170 JiJlich, West Germany. Manuscript submitted December 31, 1982. 2440--VOLUME 14A, NOVEMBER 1983
AgNO3 + KCN ~ AgCN + KNO3 AgCN + KCN ~ K[Ag(CN)2]
[a] [b]
This solution is then diluted with water to 1 part in 50. The specific activity of this solution is most probably due to the following reactions: dissociation: reduction:
[Ag(CN2)]- ~ Ag + + 2CNAg + + e- ~ Ag
[c] [d]
The electron in (d) for the reduction of the Ag + ion can be thought to be provided by the reaction H a a ~ H + + e-
[e]
Here Had is adsorbed atomic hydrogen on the surface. Thus, near the surface there is a local equilibrium between the dissolved "proton" H + in the metal and the adsorbed atomic hydrogen. The latter may provide its electron for the reduction in (d). In Figure 1 a boundary is shown between an uncharged and a heavily charged area. Clearly, the silver crystals were predominantly nucleated in the hydrogen charged areas. Such a check is always essential for a reliable interpretation and was performed on all samples. A typical well-annealed and undeformed Ni sample after hydrogen loading is shown in Figure 2. We note a very large concentration of Ag crystals at the grain boundaries signifying a large flux of hydrogen at these positions. The reference area on the same sample did not display any grain boundary decoration although it was also exposed to the silver dicyanide solution. Often, different densities of Ag crystals were observed in adjacent grains or in annealing twins (Figure 3). Some samples were deliberately given a small amount of plastic deformation prior to hydrogen charging. On the surfaces of such samples a decoration of the slip lines was found (Figure 4). Again, this decoration was absent in the reference areas which had experienced
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