A study of the dissolution of Zr-O alloys in hydrofluoric acid

The purpose of this investigation was to ascertain and explain, if possible, the effect of oxygen in the metal on the kinetics and electrochemistry of the reaction of zirconium in hydrofluoric acid solutions. This entailed the determination of the effect of oxygen on the dissolution rates, activation energies, dissolution potentials, and difference effect for zirconium in hydrofluoric acid and hydrofluoric-hydrochloric acid mixtures. The alpha-phase solid solutions of oxygen and zirconium were synthesized by the powder-metallurgy technique. Both the alloy analysis and dissolution rates were obtained by the gas evolution method according to the reaction: ZrOx + 4HF → ZrF4 + (2 - x)H2 + xH2O The dissolution rates of the alloys were determined in a hydrofluoric acid range of 0.10 to 0.50 N and a temperature range of 10 to 50⁰ C. It was found that: (1) the alloy-HF reaction rate decreases with Increasing oxygen content, (2) the reaction is first order with respect to the un-ionized HF concentration over the range studied and the reaction order is independent of oxygen content, and (3) the activation energy for the reaction increases as the oxygen content increases (i.e., from 4.1 ± 0.1 kcal/mole for 1.52% b.w. oxygen to 5.2 ± 0.2 kcal/mole for 6.97% oxygen). Additions of hydrochloric acid caused the rates of dissolution of both zirconium and the alloys in hydrofluoric acid to increase greatly whereas the order of the reaction and the activation energies were not altered. The dissolution potentials of the alloys were determined in hydrofluoric acid solutions at 25⁰ C. The initial potentials of the alloys were very near the initial potential of zirconium sheet metal. The potentials rapidly became more noble with time reaching essentially constant values in 50 to 60 minutes. In general, the dissolution potential was more noble the higher the oxygen content. Additions of hydrochloric acid caused the potentials of both zirconium and the alloys to shift to more noble values whereas similar additions of potassium chloride caused shifts to less noble potentials. Difference effect measurements on the alloys showed that: (1) anodic currents have a very strong influence on the dissolution reaction, (2) the difference effect is directly proportional to the current density and independent of the hydrofluoric acid concentration but the proportionality constant increases slightly with oxygen content, (3) hydrochloric acid additions result in an increase in the proportionality constant which relates the difference effect to the current density, (4) there is no noticeable temperature dependency on the proportionality constant in the range 10 to 50⁰ C, and (5) anodic currents cause a slight increase in the activation energy of the dissolution reaction. The results of this investigation strongly suggested that the slow step in the dissolution process is the diffusion of un-ionized HF through effective diffusion layers to the metal surface. It was shown that the experimental results could be correlated with the diffusion rate theory supplemented with Langmuir\u27s adsorption theory. A reaction mechanism for the dissolution process was proposed. It was postulated that oxygen in the metal does not affect the hypothesized mechanism; the free or unbound zirconium of the alloy matrix reacted as if there were no oxygen present --Abstract, pages 1-3