Electrochemical faceting of polycrystalline (pc) gold electrodes in 1 M HaSO., was investigated by employing wire and bead-shaped electrodes. Electrochemical faceting was produced by applying a repetitive symmetrical square wave potential signal in the 2-4 kHz range and upper and lower potential limits ranging between 1.44 and 1.60 V (vs. RHE) and 0.10 and 1.10 V, respectively. The degree of faceting was followed voltammetrically principally through the 0-electroadsorption/electrodesorption process. Electrochemical information was complemented with scanning electron microscopic observations. The results are discussed in terms of the equilibrium potential of reactions involving different gold species, and the potential of zero charge, hydrophobicity and anion adsorbability of the (llO), (100) and (111) crystallographic faces of gold. The kinetics of electrochemical faceting of gold can be explained through the two-stage mechanism proposed earlier for polycrystalline platinum in acid electrolyte.
The electrochemical faceting of Rh in 1 M HaSO, can be developed by applying repetitive periodic potentials in the range -0.1 to 1.2 V (vs. RHE) at frequencies greater than 0.5 kI-Iz. The degree of electrochemical faceting, as followed voltammetrically at 0.1 V/s in the H-adatom potential range, exhibits two different situations, depending on the lower and upper limits of the periodic potential. In one case, the voltammogram exhibits two sharp reversible conjugated peaks related to the H-adatom reactions, similar in shape to those reported for Rh (ill), although the peak potentials are closer to those found for Rh (110). In the other case, a complex voltammogram is obtained which can be related to a predominance of ( 111) arm(lOO) crystallographic faces. SEM micrographs confirm the development of electrochemical faceting involving steps, terraces and pyramidal structures which are related to the preferred orientations.
The effects of crystallographic texture, grain size and shape, and tensile stress on the orientation of hydride platelets in Zircaloy-2, Zr-2.5Nb, and Excel tubes have been studied using transverse ring and longitudinal tension specimens.
When the α grains are equiaxed, the hydride platelets precipitate either at grain boundaries or on the basal plane. A sufficiently high tensile stress perpendicular to the basal plane will precipitate the platelets perpendicular to the stress.
When the grains are thin and elongated, the platelets precipitate along the grain boundaries. When the basal plane normals are close to a longitudinal axis of the grains, a sufficiently high stress will precipitate the platelets perpendicular to the stress and the platelets will cut across the grain boundaries.
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