CORROSION MECHANISM OF ZIRCONIUM AND ITS ALLOYS--DIFFUSION OF OXYGEN IN ZIRCONIUM DIOXIDE

The diffusion rate of O in anion-deficient zirconia, ZrO/sub 1.994/, was determined by the interface migration of stoichiometric oxide and is represented by the equation D = 0.055 exp (--33,400 surface proces 3100/RT). A comparison was made with other processes that occur in the metal and the oxide. Excellent agreement was noted between activation energies of O diffusion in ZrO/sub 1.944/ and those for parabolic or cubic oxidation in both air and water. It appears that O diffusion in the oxide is rate-controlling during oxidation of the metal. The corrosion and oxidation behavior of Zr and some alloys are discussed in terms of the oxide defect structure and the electric conductivity behavior in the oxide. A speculative mechanism for corrosion transition to linear rates was suggested on the basis of preferential oxidation of a grain boundary metallic phase. The nature of the phase and of its formation and elimination are discussed. (auth

Internal oxidation of Nb-Zr alloys over the range 1555-1768°C at low oxygen pressures

Three Nb alloys, containing 1 w/oZr, 2.5 w/oZr, and 10W-2.5Zr, were internally oxidized in the range of 1555 to 1768°C at oxygen pressures ranging from 5 x 10-5 to 1 x 10-3 torr. Linear kinetics were measured suggesting that oxygen arrival at the surface and not oxygen diffusion in the substrate was rate controlling. Both tetragonal and monoclinic ZrO2 formed, the tetragonal form being favored by high nucleation rates (lower temperatures), lower alloy content, and location in the reaction zone (small particles near the surface). Monoclinic ZrO2 formed at higher temperatures and deeper within the reaction zone where larger precipitates formed. The high solubility product of ZrO2 in Nb-Zr alloys gives rise to non-Wagnerian behavior so that the solute is not precipitated out at the reaction front, additional precipitation occurring after the reaction front has passed. This causes a variation in the precipitate volume fraction with distance in the zone. Experimental observations are discussed in terms of various models for non-classical internal oxidation

THE EFFECT OF PRIMARY ALPHA ON THE BETA DECOMPOSITION OF ZIRCONIUM-URANIUM- OXYGEN ALLOYS

The effect of primary alpha present during the solutiontreating cycle on the incubation period for the decomposition of beta and on the resultant structure has been determined for Zr-base alloys containing from 7 to 15 wt.% U and from 0.094 to 0.29 wt.% 0. Prior-existing alpha decreased the time at which precipitation of Widmanstgtten alpha started and increased the rate of precipitation. The microstructure of transformed alloys was correlated with tentative isothermal-transformation diagrams. (auth

SPECIFIC ZIRCONIUM ALLOY DESIGN PROGRAM. First Quarterly Progress Report, February-June 1962

A program was initiated for the design of steam-service Zr base cladding alloy. Specifications were derived for an acceptable alloy for such service based on considerations of neutron economy, cost, H/sub 2/ embrittlement, corrosion resistance, fabricability, and strength. The selection of the field of alloy compositions having promise for meeting these specifications was made. Thirty-one alloys whose compositions were chosen from the alloy field were arcmelted and are being fabricated to sheet. Specimens cut from the sheet will be tested for response to composition. The response parameters will measure corrosion resistance at 300, 400, and 500 deg C, mechanical properties before and after corrosion exposure, and H/sub 2/ take-up during corrosion. The experiment is designed to permit analysis of the results employing computer programing to determine the optimum composition within the selected alloy field. Fundamental studies in support of the alloy design were also initiated. Film stripping techniques were developed to permit partitioning studies of alloying element between the corrosion film and the alloy substrate. Hydrogen overvoltage measurements were made for the intermetallic phases Zr/sub 2/Cu, Zr/sub 2/Ni, for Zr-90% Nb and for crystal bar Zr. The diffusion of O/sub 2/ in bulk nonstoichiometric ZrO/sub 2/ was measured: D = 0.055 exp (33,400 plus or minus 3,100/RT) for ZrO/sub 1.955/ at 700 to 1000 deg C. The activation energy is in agreement with that for parabolic corrosion of Zr alloys indicating that bulk O/ sub 2/ diffusion in the corrosion films and not short circuiting diffusion paths are rate controlling. Oxygen diffusion in alloy oxide-doped ZrO/sub 2/ is now being studied. (auth

Developing a Nigerianspecific food and nutrient coding database

SupplementA Nigerian-specific food and nutrient coding database was developed by adding Nigerian-specific foods, recipes, measure descriptions, and gram weights to information from the US Department of Agriculture (USDA) databases used for the 1994–1996, 1998 Continuing Survey of Food Intakes by Individuals. The Nigerian database was used to analyze intakes from the 2001 Nigerian Food Consumption and Nutrition Survey. The Food Intake Analysis System (FIAS) (developed by The University of Texas School of Public Health and the USDA Agricultural Research Services Food Surveys Research Group) and food and nutrient data from the USDA databases were used to aid creation of foods and recipes specific to Nigeria. USDA data had only 214 of the basic foods reported consumed by Nigerians. Researchers developed food descriptions and nutrient profiles for another 117 foods specific to Nigerian intake. An additional 14 foods were created from ingredients in the USDA database. Complete profiles of 14 nutrients were included for every new food from food composition books and tables. A total of 422 recipes were created from the USDA and Nigerian foods using the nutrient retention factor recipe methodology and the FIAS software. Recipes were kitchen tested to obtain a weight before and after cooking, and the weight of a typical serving. It was necessary to add measure descriptions and gram weights to 359 USDA and 164 Nigerian foods. Development of this unique population-specific database of foods and recipes for this African country constitutes an important milestone for future diet and nutrition studies