Hafnium-free zirconium has assumed great importance in commercial use in recent years. When economical methods were developed in 1950 to remove hafnium from Zr, a metal was produced which not only exhibited good corrosion resistance and desirable physical properties, but also had a very low absorption cross section for thermal neutrons. Such a metal could obviously be utilized to advantage as a structural material for nuclear reactors. This application is the most important one for Zr at present.
Good corrosion resistance to most mineral acids is one of the outstanding non-nuclear properties of Zr. Hf-free Zr has good corrosion resistance in all mineral acids with exception of hydrofluoric acid, concentrated sulfuric and phosphoric acids, and aqua regia. The resistance of Zr to attack in concentrated nitric acid is excellent, with but negligible attack occurring in fuming nitric acid. The corrosion of Zr in hydrochloric acid is particularly dependent on the purity of the metal. It has been shown that high carbon graphite melted Zr is severely embrittled in HCl because of selective attack of the carbides. In general, Hf-free Zr (containing less than 0.1% Hf) is more corrosion resistant than Zr containing the usual 2.5% Hf. Small percentages of other impurities in Zr such as carbon, nitrogen, and oxygen will also decrease corrosion resistance.
It has been stated that Zr has poor corrosion resistance in HF. It was proposed that a quantitative investigation of the dissolution of Zr in HF be made to provide additional data to that already gathered on the corrosion of Zr in other mineral acids. Since the rate of dissolution of Zr in HF is greater than in any other mineral acid, a study of the rate and mechanism of the reaction and possible passivation effects might be a valuable addition to present knowledge --Introduction, pages 1-2
