No abstract

Some aspects of the mode of formation of anodic oxidation films in the potential region below oxygen evolution were examined for a number of metals under as nearly constant experimental conditions as possible. The metals selected were Al, Ti, Hf, V, Nb, Ta, and Cr. Results for Zr were reported in earlier publications. Electrolytic parameters and formation fields were evaluated from the unitary formation rates. Local currents were estimated using the method described previously.

Tantalum carbide was prepared in the composition range TaC0.73 to TaC1.26 and consolidated by hot pressing. Compositions at ends of the range were two phase. A modified cantilever‐beam test method was used to determine the ductile‐brittle transition temperature ( Tnormaldb ) at a given strain rate and for this geometry as a function of carbon content. The Tnormaldb range from TaC0.98 was 1825°–1925°C, and the median nominal bend stress in this range was 23,000 psi (1617 kg/cm2). At TaC0.90 the median nominal bend stress was at a minimum of 14,000 psi (984 kg/cm2) for the Tnormaldb range 1750°–1850°C. Tnormaldb increases approximately 150°C with increasing x across the single‐phase cubic region of TaCx .

The a‐c electrical conductivity of normalCaO‐ZrO2 and normalCaO‐HfO2 was measured from 800° to 2000 °C. The two systems are similar. The conductivity maximum was found near 12 mole % normalCaO in both zirconia and hafnia.

The total electric potential difference between a metal and a solution across an oxide film being built up by a constant anodic current is decomposed into three contributions: differences at the interphases metal‐oxide and oxide‐solution, both subject to activation overvoltage, and the difference across the oxide layer. For one or the other of the interphases the case of linearity of overvoltage with the logarithm of the current (in the range of large currents) and that of proportionality of overvoltage with current (in the range of small currents) are examined in some detail, and previously obtained experimental data for three types of titanium, for hafnium, and for tantalum are analyzed in terms of the proposed theoretical treatment. Numerical values are obtained for the various parameters appearing in the equations: thickness of the interphase, exchange current, electric potential difference, and electric field at zero current. Possibilities for refinement and extension of the treatment are mentioned.