Diffusion coefficients and heats of solution for hydrogen in Pd and Pd-Ag alloys at low hydrogen concentration were determined from 260 to 640°. The change in activation energy for hydrogen diffusion with increasing mole fraction of silver (XAg) from Pd to Pdo.BAg0.5 can be represented empirically by E = 3800XV + 1500XAfr2 + 5260 cal. The preexponential factor, Z)0 showed only a slight linear decrease D0 = -2.65 X 10-sXAg + 2.9 X 10-3. The molar heat of desorption, AHm, increased with rising Ag concentration from 4000 cal (Pd) to a maximum between 8020 (Pdo.eAgo.d and 7090 cal (Pd0.BAgo.6). The entropy of desorption, ASh" remained quite constant at values between 23.3 and 24.4 cal/deg mol.
REDUCTION OF C1 ON C 1161Studies of the mechanism of chlorine evolution on graphite (9) in molten lithium chloride suggests, as the rate determining step, the combination of chlorine atoms on a surface with appreciable coverage. In fact, when scanning the rotating carbon electrode from the region of chlorine evolution to more cathodic potentials, one indeed observes, up to overvoltages of 250 mV, a lower Tafel slope which changes to 2RT/F at higher overvoltages probably indicating the change from an intermediate to low coverage. On the anodic going sweep, this low coverage is maintained until reaching anodic potentials. Thus, we can conclude that the chlorine reduction on vitreous carbon in AlCls-KC1-NaC1 melts occurs according to path 1 with step la being most probably rate determining at higher polarizations with a low chlorine coverage on the electrode.
Using a rotating‐vitreous, carbon disk electrode, the kinetic parameters for chlorine reduction in an
AlCl3‐normalKCl‐normalNaCl
(57.5‐12.5‐30 mole per cent [m/o]) melt were determined. It was found that the reduction of chlorine occurs according to the paths Cl2+e−⇌Cl−+Clnormalads Clnormalads+e−⇌Cl− with the first step being most probably rate determining. The apparent exchange currents were
false(1±0.15false)10−4A/cm2 normalat 130°C
and
false(2.1±0.3false)10−4A/cm2 normalat 150.6°C
.
The galvanic interaction between electronically conductive polyaniline films doped with camphor sulfonic acid (PAni-CSA), and aluminum alloy (AA) 2024-T3 has been investigated for the purpose of assessing PAni-CSA as a corrosion protection coating on AAs. Potentiodynamic polarization and zero resistance ammeter measurements of galvanic couples between PAni-CSA and AA 2024-T3 in 3.5% NaCl show that the AA dominates the mixed potential response and polarizes the PAni-CSA to approximately the potential of the freely corroding alloy (Ϫ0.54 V vs. Ag|AgCl). This observation indicates that conductive PAni coatings on Al are rapidly reduced to the nonconductive form on immersion in an aqueous electrolyte. The results also suggest that any corrosion protection of Al by PAni coatings is due to the barrier properties of the reduced polymer rather than anodic stabilization of passivity as observed with ferrous alloys. Mechanisms by which a PAni-CSA coating might provide corrosion protection of exposed 2024-T3 at defects in the coating are discussed.
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