Corrosion of Hafnium and Hafnium Alloys

HAFNIUM is element number 72. It resides in group IVA of the periodic table with titanium and zirconium. Hafnium is always associated with zirconium in minerals such as zircon and baddeleyite, usually in the range of 1 to 5%. The chemical similarity between hafnium and zirconium is more pronounced than between any other two elements in the periodic table, except the inert gases. This similarity in chemistry of hafnium and zirconium makes separation extremely difficult. Alongwith zirconium, hafnium forms intermetallic compounds with most metallic elements, except the alkali metals and some alkaline earths (Ref 1). Hafnium is obtained as a by-product of the extraction process to produce hafnium-free nuclear-grade zirconium. Commercial hafnium typically contains 0.2 to 4.5% Zr. The van Arkel-de Boer (iodine) process is used to obtain hafnium purities of greater than 99.99%. Annual world consumption of hafnium was approximately 60 tons in 2003 (Ref 2). Hafnium was first identified by x-ray analysis in 1923 by Coster and von Hevesy in Copenhagen, Denmark. Von Hevesey and Jantzen were the first to separate hafnium from zirconium by repeated recrystallization of fluoride salts. The name hafnium comes from the Latin name for Copenhagen, which is Hafnia. Van Arkel and de Boer were the first to produce metallic hafnium. Their process of passing hafnium tetraiodide vapor over a heated filament is the basis for the refining process used today to produce higherpurity hafnium metal. With a standard reduction potential of 1.72 V versus the normal hydrogen electrode at 24 C (75 F), hafnium is more reactive than either titanium ( 1.63 V) or zirconium ( 1.53 V). This high chemical reactivity allows a thin, tenacious protective oxide layer to form when exposed to most chemical environments. Like other reactive metals, hafnium resists attack by many chemicals as long as this thin oxide layer is not penetrated by reactants. Oxide layers can also be developed by anodizing and by treatment in steam autoclaves and in air at elevated temperatures. The most commonly formed oxide is hafnium dioxide (HfO2). Because of its high melting point of 2222 C (4032 F), hafnium may be considered refractory. In addition to the inherent corrosion resistance of hafnium, other properties make hafnium useful in chemical equipment. It is relatively easy to form and join, sufficiently strong, ductile, and wear resistant to withstand the abuse of industrial applications. Its coefficient of thermal expansion is approximately 60% lower than that of 304 stainless steel at ambient temperature, and its thermal conductivity is approximately 40% higher at ambient temperature (Ref 3). Hafnium appears to be nontoxic. Haygarth and Graham state that “there seems to be no report of the metal, or its alloys with nontoxic constituents, causing physiological reaction” (Ref 3). Like zirconium, dissolved hafnium ions are colorless—an important attribute for some applications.