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Small additions of yttrium to NiAl result in a decreasing oxygen transport through the Al2O3 scale on AiAl at 1270 K, as has been shown by nuclear reaction analysis (18O(p, α)15N). A higher concentration, e.g. 0.5 wt%, has the opposite effect. Yttrium segregates to the grain boundaries in the alloy, and an enhanced Y concentration has been observed by means of EPMA in the oxide scale above the grain boundaries of the alloy. Transition aluminium oxides are shown to be formed in the initial stage at 1170 K with (glancing angle) x‐ray diffraction. Only α‐Al2O3 is formed after oxidation for several minutes at higher temperatures (1470 K). The rate of transformation of transition Al2O3 to the stable α‐Al2O3 modification depends on the oxidation temperature. The transition is shown to be retarded by the presence of Y. The primary transport processes in the Al2O3 scales are short‐circuit diffusion of oxygen and/or aluminium. At 1170 K, while transition oxides are still present, Al transport predominates. At higher temperatures, when the oxide has the α‐Al2O3 structure, oxygen transport is enhanced. This would also explain the retardation of oxygen transport by the addition of Y, since Y was found to prevent the formation of the most stable form of Al2O3.

Annealed gold wires were loaded at 0 mV vs. SCE. The specimens were then polarized during 1 rain intervals to successively more negative potentials ranging from -200 to -1800 mV in steps of 200 inV. The extra strains measured as a function of the electrode potential had a characteristic distribution depending on the composition, oxygen content, and pH of the test solution. In general there are two peaks. In deaerated alkaline solutions a first peak occurred between -200 and -600 mV. This peak was completely absent in deaerated acid solutions. It is ascribed to dislocation pinning at the metal surface by chemisorbed OH-ions during loading of the specimen. A first peak probably due to adsorbed H~O2 reappears in oxygenated acid solutions. A second peak was observed in all test solutions. This peak occurs at electrode potentials more negative than -1000 mV vs. SCE.The occurrence of increased creep rates as a result of cathodic polarization of metal wire or strip specimens during constant load creep tests has been reported repeatedly in the literature (1-3). This effect has been attributed to the decrease of the surface stress as the electrode potential deviates from the point of zero charge (pzc). The creation of new surface area in the form of slip steps is then expected to require less energy and creep is expected to be faster. The above explanation is basically in accordance with the original interpretation of the effect of cathodic polarization on hardness measurements as given by Rehbinder et al. (4) and more recently by Latanision et al. (5). The influence of surface active agents has been explained using the same arguments (6-8). However a firmly established relationship between the mechanical behavior and the shape and position of the electrocapillary curve is lacking, although certain results (1, 2) appear to support the surface stress hypothesis. It is obvious that any possibility of oxide films being formed on the metal surface should be eliminated during tests where "Rehbinder effects" are investigated. Many published results in the literature, especially those obtained on less noble metals such as Zn or Pb (1), are of highly questionable value because oxide films are likeIy to have played a role (9, 10).For the present study gold was chosen because this noble metals has a sufficiently large polarizable region where it does not dissolve, form oxides, or hydrides. Surface stress data and pzc values are now available for polycrystalline gold in various media, including solutions similar to those used in the present tests (11,12). The oxidation behavior of gold and the electrochemical oxygen reduction reaction on the surface of this metal have been studied extensively (13-17). The oxygen reduction reaction on gold has been found to be unaffected by residual impurities in the solution (17), in contrast to the results obtained on Pt and Rh electrodes. Experimental ApparatusThe gold wires used in the present creep tests were drawn to a diameter of 0.15 mm from 0.25 mm 99.999% pure gold wire supplied by Johnson...

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