© 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 13B, Corrosion: Materials (#06508G) Corrosion of Hafnium and Hafnium Alloys D.R. Holmes, ATI Wah Chang, Allegheny Technologies HAFNIUM is element number 72. It resides in In addition to the inherent corrosion resistance impurities such as oxygen, carbon, and nitrogen group IVA of the periodic table with titanium of hafnium, other properties make hafnium use- are left behind, along with some of the impurity and zirconium. Hafnium is always associated ful in chemical equipment. It is relatively easy to metals. Electron beam melting is also effective in with zirconium in minerals such as zircon and form and join, sufficiently strong, ductile, and purifying hafnium. In this process, hafnium is baddeleyite, usually in the range of 1 to 5%. wear resistant to withstand the abuse of industrial slowly double-melted under high vacuum. This The chemical similarity between hafnium and applications. Its coefficient of thermal expansion process removes impurities having partial pres- zirconium is more pronounced than between any is approximately 60% lower than that of 304 sures at the surface of the melt greater than the other two elements in the periodic table, except stainless steel at ambient temperature, and its vapor pressure of hafnium, which is approxi- the inert gases. This similarity in chemistry of thermal conductivity is approximately 40% mately 0.1 Pa (0.75 mm Hg) at 2500 K hafnium and zirconium makes separation ex- higher at ambient temperature (Ref 3). (4040 F). The more volatile metallic impurities, tremely difficult. Along with zirconium, hafnium Hafnium appears to be nontoxic. Haygarth such as iron, manganese, and aluminum, along forms intermetallic compounds with most and Graham state that “there seems to be no with chlorine, are moved. Only the higher- metallic elements, except the alkali metals and report of the metal, or its alloys with nontoxic melting-point metals, such as tantalum and some alkaline earths (Ref 1). Hafnium is constituents, causing physiological reaction” tungsten, are not removed. To get the highest obtained as a by-product of the extraction pro- (Ref 3). Like zirconium, dissolved hafnium ions purity possible, the van Arkel-de Boer process cess to produce hafnium-free nuclear-grade zir- are colorless—an important attribute for some and electron beam melting are used in sequence. conium. Commercial hafnium typically contains applications. The United States Government allows the 0.2 to 4.5% Zr. The van Arkel-de Boer (iodine) export of high-hafnium (0.2% or greater) process is used to obtain hafnium purities of commercial-grade (nonnuclear) zirconium greater than 99.99%. Annual world consumption Production (UNS R60702) to most countries. This level of of hafnium was approximately 60 tons in 2003 hafnium content prevents commercial-grade (Ref 2). Because hafnium is always associated with zirconium from being modified for use in nuclear Hafnium was first identified by x-ray analysis zirconium, two main processes have been used to production facilities. This requirement limits the in 1923 by Coster and von Hevesy in Copenha- separate hafnium from zirconium. A European amount of hafnium metal available for other gen, Denmark. Von Hevesey and Jantzen were producer uses an extractive distillation process, uses. the first to separate hafnium from zirconium by while in the United States a liquid-liquid repeated recrystallization of fluoride salts. The extraction is the preferred method. Two primary name hafnium comes from the Latin name for operational methods of metal production are also Physical and Mechanical Properties Copenhagen, which is Hafnia. Van Arkel and de possible. The Kroll process is the primary of Hafnium Boer were the first to produce metallic hafnium. reduction process used in the United States, Their process of passing hafnium tetraiodide while in Europe an electrowinning process is Hafnium is often considered for the same vapor over a heated filament is the basis for the preferred. Ullman gives a detailed description for applications as zirconium, because of their very refining process used today to produce higher- each of these processes (Ref 4). similar physical and chemical characteristics. In purity hafnium metal. Highest-purity metal is obtained by using the fact, Clark (Ref 1) states that other than the With a standard reduction potential of van Arkel-de Boer process, also known as the higher transition temperatures (a!b) for haf- À1.72 V versus the normal hydrogen electrode iodine bar or crystal bar process. In this process, nium, the only other chemical and physical at 24 C (75 F), hafnium is more reactive hafnium sponge produced by the Kroll process is property showing a major difference between than either titanium (À1.63 V) or zirconium loaded into a cylindrical Ni-Cr-Fe vessel. The these elements is the density. Both zirconium and (À1.53 V). This high chemical reactivity allows vessel lid contains insulated electrical lines from hafnium exhibit anisotropy; their mechanical a thin, tenacious protective oxide layer to form which a hafnium wire filament, in a hairpin properties depend on the direction in which they when exposed to most chemical environ- shape, is suspended. Iodine is added to the vessel, are measured. Table 1 gives the physical prop- ments. Like other reactive metals, hafnium which is then evacuated and heated to vaporize erties of hafnium, while Table 2 gives the resists attack by many chemicals as long as the iodine. Volatile hafnium tetraiodide forms mechanical properties. this thin oxide layer is not penetrated by reac- and diffuses to the central hafnium filament, Chemical Properties. The ionic radii of tants. Oxide layers can also be developed by which is resistance heated to 1200 to 1500 C zirconium and hafnium are almost identical, anodizing and by treatment in steam autoclaves (2200 to 2700 F). The tetraiodide thermally due to the lanthanide contraction. This results and in air at elevated temperatures. The most dissociates at the hot filament, depositing haf- in very similar chemical properties between commonly formed oxide is hafnium dioxide nium and releasing iodine to react again with the the two metals. As a result, a review of the (HfO2). Because of its high melting point of sponge feed. This cycle continues until the effi- chemical behavior of zirconium can be a use- 2222 C (4032 F), hafnium may be considered ciency diminishes (Ref 4, 5). Under conditions ful indicator of the chemical behavior of refractory. optimized for hafnium transfer, interstitial hafnium (Ref 4). © 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 13B, Corrosion: Materials (#06508G) 2 / Corrosion of Nonferrous Alloys and Specialty Products Table 1 Physical properties of hafnium Specifications and Standards. Hafnium removes other surface imperfections where pit- comes in two grades: grade R1 for nuclear ting may be initiated. Pickling hafnium requires Property Value applications (low zirconium) and grade R3 for a solution of 3 to 7% hydrofluoric acid (48% Atomic number 72 commercial applications and alloying. Higher- concentration) with 25 to 59% nitric acid (70% Atomic weight 178.49 Density, kg/m3 (lb/in.3) 13.31 · 103 (0.481) quality material can be produced by the crystal concentration) and the balance of water. Heat Modulus of elasticity, MPa (ksi) bar process. Grade R3 hafnium is available in treatment for weldments may also be beneficial At 294 K (70 F) 13.7 · 104 (2.00 · 104) specific grades from 0.5 to 4.5% Zr content (Ref for corrosion resistance in specific applications. 4 4 At 533 K (500 F) 10.6 · 10 (1.54 · 10 ) 6). No Unified Numbering System number has Heat treatments may be either a solution anneal At 644 K (700 F) 9.5 · 104 (1.38 · 104) Magnetic susceptibility per g at 0.42 · 10À6 been assigned to hafnium. ASTM B 776, requiring high temperatures or a stress anneal at 298.2 K (77.4 F) “Standard Specification for Hafnium and Haf- lower temperatures. The choice of heat treatment Hall effect at 25 C (77 F), À1.62 · 10À12 nium Alloy Strip, Sheet, and Plate” (Ref 7), depends on the specific environment and appli- Vm/AT includes chemistry requirements. Table 3 gives cation. Spectral emissivity, 0.40 the chemical composition of three grades of For corrosion tests involving stress-corrosion 1750–2300, e0.65u Electron emission, A/m2 hafnium. or environmental cracking, such as tensile, U- At 1900 K (2960 F) 4.80 · 10 bend, or C-ring specimens, consideration should 2 At 2000 K (3140 F) 2.62 · 10 be given to the anisotropy in hafnium. Specimen At 2100 K (3320 F) 1.23 · 103 At 2200 K (3500 F) 4.85 · 103 Aqueous Corrosion Testing of properties such as thermal expansion, yield Work function, J 6.25 · 10À19 Hafnium and Hafnium Alloys strength, tensile elongation, and bend ductility Thermal neutron absorption 104 depend on the orientation of mill products. cross section, barns Anisotropy is a characteristic of zirconium, Crystal structure It is not the purpose of this article to describe titanium, and hafnium but not of niobium or Alpha phase Hexagonal close-packed the recommended procedure for corrosion tantalum. Table 2 gives examples of changes in Beta phase Body-centered cubic testing. Reactive-metals corrosion testing, spe- Alpha-to-beta transformation 2033 (3200) mechanical properties between materials tested cifically zirconium and hafnium, is, however, temperature, K ( F) in the longitudinal and transverse directions. Melting point, K (F) 2500 (4041) sufficiently different from stainless steels, Welding of reactive metals for corrosion Boiling point, K ( F) 4875 (8312) nickel-base alloys, and more common materials · À6 samples also requires some care. Although Coefficient of linear thermal 5.9 10 of construction to warrant a few comments. expansion, (m/m)/K, commonly welded, hafnium and other reactive Testing procedure ASTM G 2, “Corrosion at 273–1273 K (32–1830 F) metals are very sensitive to contamination by Thermal conductivity, W/m Á K Testing of Products of Zirconium, Hafnium and oxygen, nitrogen, hydrogen, and carbon during At 298 K (77 F) 23 Their Alloys in Water at 360 C (680 F) or the welding process.