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Home , Corrosion, Crystal bar process, Hafnium, Lanthanide contraction, Zirconium

© 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 13B, : Materials (#06508G)

Corrosion of 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 , , and IVA of the with of hafnium, other properties make hafnium use- are left behind, along with some of the impurity and . Hafnium is always associated ful in chemical equipment. It is relatively easy to . Electron beam melting is also effective in with zirconium in such as and form and join, sufficiently strong, ductile, and purifying hafnium. In this process, hafnium is , usually in the range of 1 to 5%. resistant to withstand the abuse of industrial slowly double-melted under high vacuum. This The chemical similarity between hafnium and applications. Its coefficient of 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 at ambient , and its vapor pressure of hafnium, which is approxi- the inert . This similarity in chemistry of 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 , manganese, and aluminum, along forms intermetallic compounds with most and Graham state that “there seems to be no with , are moved. Only the higher- metallic elements, except the alkali metals and report of the , or its alloys with nontoxic melting-point metals, such as and some alkaline (Ref 1). Hafnium is constituents, causing physiological reaction” , are not removed. To get the highest obtained as a by-product of the extraction pro- (Ref 3). Like zirconium, dissolved hafnium 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 () 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 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 of van Arkel-de Boer process, also known as the higher transition (a!b) for haf- 1.72 V versus the normal electrode iodine bar or 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 (1.63 V) or zirconium loaded into a cylindrical Ni-Cr-Fe vessel. The these elements is the . Both zirconium and (1.53 V). This high chemical reactivity allows vessel lid contains insulated electrical lines from hafnium exhibit anisotropy; their mechanical a thin, tenacious protective 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, and by treatment in steam autoclaves (2200 to 2700 F). The tetraiodide thermally due to the 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 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 . 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 - 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 (48% 72 commercial applications and alloying. Higher- concentration) with 25 to 59% (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 . 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) per g at 0.42 · 106 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), 1.62 · 1012 nium 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, , J 6.25 · 1019 Hafnium and Hafnium Alloys strength, tensile elongation, and bend ductility Thermal absorption 104 depend on the orientation of mill products. , barns Anisotropy is a characteristic of zirconium, It is not the purpose of this article to describe titanium, and hafnium but not of or Alpha 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 , of reactive metals for corrosion , K ( F) 4875 (8312) nickel-base alloys, and more common materials · 6 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 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. At 373.2 K (212.4 F) 22.4 Steam at 400 C (750 F),” is specifically Specific heat, J/kg K, at 298.2 K 117.0 Hafnium can be sensitive to minor impurities directed at zirconium and hafnium and their (77.4 F) in corrosive environments. The presence of Vapor pressure, Pa alloys for use by the nuclear (Ref 8). 5 fluoride in low-pH solutions is particularly At 2040 K (3215 F) 10 Different test methods are required for the At 2280 K (3645 F) 104 damaging to hafnium and other reactive metals. 7 commercial chemical processing industry. Electrical resistivity, V m, at 3.51 · 10 The data in Table 4 indicate the effect of fluoride Reference 9 is specifically designed for testing of 298.2 K (77.4 F) in nitric acid solutions. contamination Temperature coefficient of 3.82 · 1011 zirconium in commercial aqueous environments; can come from unexpected sources, such as electrical resistivity, V m/K, hafnium would require the same guidelines. at 298.2 K (77.4 F) boiling chips and the breakdown of some 5 ASTM G 31, “Practice for Laboratory Immer- Latent heat of fusion, J/kg 1.35 · 10 fluorinated (Ref 13). Beyond this, sion Corrosion Testing of Metals,” may also be 3þ Latent heat of vaporization, J/mol oxidizers such as ferric (Fe ) in hydro- At 298.2 K (77.4 F) 7.02 · 105 adapted for corrosion testing of hafnium. The 5 chloric acid environments may pose a problem At 1000 K (1340 F) 6.99 · 10 subject of corrosion testing of reactive metals is At 2000 K (3140 F) 6.96 · 105 Hardness also covered by Yau (Ref 10). Surface condition Brinell, MN/m2 1700 is more important with hafnium than with many Mohs 5.5 other metals. In applications, Table 3 Chemical composition of hafnium Vickers, MN/m2 1760 for instance, a well-pickled surface is recom- grades and crystal bar Source: Ref 6 mended. Pickling removes impurities, specifi- Composition cally embedded iron, from the surface and Grade R1, Grade R3, Crystal bar, Element wt% wt% ppm Table 2 Typical mechanical properties for fully annealed products Aluminum 0.010 0.050 70 Temperature Ultimate tensile strength Yield strength (0.02% offset) 0.010 0.050 20 Elongation in ... 0.010 25 C FTest direction MPa ksi MPa ksi 5 cm (2 in.), % Iron 0.050 0.0750 250 0.0020 ... 5 Rod Nickel 0.0050 ... 50 RT Longitudinal 483 70 241 35 25 Niobium 0.010 ...... 315 600 Longitudinal 310 45 124 18 40 0.010 0.050 50 Tantalum 0.020 ...... Plate 0.0050 ...... RT Longitudinal 469 68 193 28 25 Titanium 0.010 0.050 50 RT Transverse 448 65 310 45 25 Tungsten 0.0150 0.0150 10 315 600 Longitudinal 367 53 124 18 45 Uranium 0.0010 ... 5 315 600 Transverse 234 34 165 24 48 0.0050 ...... Zirconium (a) (a) (a) Strip Hafnium bal bal bal RT Longitudinal 448 65 172 25 30 Carbon 0.015 0.025 0.0030 RT Transverse 448 65 376 55 30 Hydrogen 0.0025 0.0050 ... 315 600 Longitudinal 376 55 97 14 45 Nitrogen 0.010 0.0150 0.0010 315 600 Transverse 241 35 165 24 50 Oxygen 0.040 0.130 0.0100 RT, room temperature. Source: Ref 6 (a) Zirconium content varies. Source: Ref 6, 7 © 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 13B, Corrosion: Materials (#06508G)

Corrosion of Hafnium and Hafnium Alloys / 3 for the corrosion resistance of hafnium. Because more abrasive conditions, a heat treatment at specifically mentioned (Ref 15). Nielson states of its high reactivity, hafnium will generally be 566 C (1051 F) for 1 to 4 h in air may be that the tendency of hafnium to form inorganic the in most galvanic couples. This helpful in reducing erosion. complexes with anions decreases in the follow- 7 7 37 27 27 will accelerate the corrosion of the com- ing order: OH 4F 4PO4 4CO3 4SO4 4 7 7 7 ponent and allow hydrogen buildup in the NO3 Cl 4ClO4 (Ref 4). Data presented in cathode (hafnium) component of the cell. For Corrosion Resistance of Hafnium the tables that should be considered only as a this reason, care must be exercised in the use guide for potential applications. As with all of dissimilar wire or bolting systems when In aqueous solutions, hafnium is soluble in materials, corrosion performance should be placing hafnium coupons in multicoupon test hydrofluoric acid (HF) and concentrated sulfuric determined in situ in the actual process stream, racks. Any suspension system must be well acid (H2SO4). Hafnium is resistant to dilute because even small process impurities may have insulated to get meaningful corrosion rates. The hydrochloric acid (HCl) and . a profound on corrosion in a particular effect of impurities generated by corrosion of Hafnium is unaffected by nitric acid (HNO3) application. The corrosion resistance of hafnium other metals contained on the test rack may affect in all concentrations. Aqua regia dissolves haf- in various media is given in Table 5. the corrosion of hafnium. With few precautions, nium. The addition of small amounts of soluble Table 6 gives the corrosion behavior of haf- any standard test method and coupon config- fluoride salts in other acidic solutions can result nium in some mixed acid solutions. Because uration can be used or adapted for use with haf- in an appreciable increase in corrosion rates. hafnium is not resistant to aqua regia, caution nium. Researchers report no corrosion for hafnium in should be exercised when applying hafnium in Aeration is not generally needed to passivate hypochlorite (HClO4) solutions and hydriodic mixed involving hydrochloric acid and hafnium in aqueous environments. Hafnium acid (HI) at room temperature (Ref 14). Pourbaix nitric acid. forms a protective oxide film in the presence of gives some indication of possibly higher corro- even small amounts of water. The oxide film sion rates for hafnium in substances that can Corrosion in Specific Media developed on hafnium also forms an effective form soluble complexes or insoluble salts; barrier to erosion under normal conditions. For fluorinated, oxalic, and salicylic complexes are Water and Steam. Hafnium is superior to commercial zirconium (UNS R60702) and zir- caloys (UNS R60804 and R60802), which are nuclear grades of zirconium, in corrosion resis- Table 4 Corrosion of hafnium in nitric acid solutions tance in water at elevated temperatures to 450 C Temperature Corrosion rate (840 F). Table 7 gives weight gains in standard Duration of testing, corrosion tests in water at 360 C (680 F) and Concentration of nitric acid C FCondition days mm/yr mils/yr 2690 psi (18.5 MPa). 10%(a) 35 95 Unknown 14 0.008 0.3 Hydrochloric Acid. In hydrochloric acid, 30% BP(b) Nonwelded 8 0 0 þ hafnium has slightly higher corrosion rates than 30% 1% NaCl BP(b) 240 Nonwelded 8 0 0 50% 115 240 Nonwelded 8 0 0 zirconium. In 10 and 37% acid at 35 C (95 F), 50%þ1% NaCl 115 240 Nonwelded 8 0 0 for instance, hafnium will experience corrosion 70% 116 240 Nonwelded 8 0 0 rates of 0.007 and 0.033 mm/yr (0.3 and 70%þ1% NaCl 116 240 Nonwelded 8 0 0 70% 116 240 Nonwelded 8 0 0 1.3 mils/yr), respectively (Ref 11). Surface 70% 116 240 Welded 8 0 0 finish is an important factor to consider for cor- 30%þ50 ppm fluoride(c) 80 176 Nonwelded 1 1.8 73 rosion resistance in hydrochloric acid environ- 30%þ100 ppm fluoride(c) 80 176 Nonwelded 1 3.2 126 ments. Surface impurities such as iron may have 50%þ50 ppm fluoride(c) 80 176 Nonwelded 1 2.8 113 70%þ50 ppm fluoride(c) 80 176 Nonwelded 1 5.1 202 a substantial impact on pitting resistance. Fuming HNO3 35 95 Nonwelded 14 0.01 0.43 Nitric Acid. Hafnium is resistant to all con- (a) Ref 11. (b) BP, boiling point. (c) Added as hydrofluoric acid. Source: Ref 12 centrations of nitric acid, including fuming nitric

Table 6 Corrosion rates of hafnium in mixed Table 5 Average corrosion rates of hafnium in various boiling solutions acid solutions at 35 C (95 F) Corrosion rate Corrosion rate Exposure time, Exposure time, Medium days mm/yr mils/yr Medium days mm/yr mils/yr 10 0 0 þ 5 5 HCl H2SO4 (1 : 1) 14 0.0 0.5 Saturated (NaCl) at pH 1 21 0.0025 0.1 HCl þ HNO (1 : 1) 14 3.3 130 Saturated (NaCl) at pH 1 with crevice device attached 21 50.0025(a) 50.1(a) 3 H SO þ HNO (1 : 1) 14 4.7 185 3% sodium chloride (NaCl) (alternate immersion) 7 0 0 2 4 3 70% chloride (CaCl2)1000Source: Ref 6 40% hydrobromic acid (HBr) 10 0.025(b) 1.0(b) 48% hydrobromic Acid (HBr) 10 0.025 1.0 40% cupric nitrate (Cu (NO3)2)1000 40% sodium bisulfate (NaHSO4) 10 1.1 44 Table 7 Corrosion of hafnium in 10% sulfamic acid (NH2SO3H) 10 0 0 high-temperature water 25% aluminum (Al2(SO4)) 10 0 0 2 70% chloride (ZnCl2)1000Weight gain, mg/dm 88% zinc chloride (ZnCl2) 10 0 0.1 20% hydrochloric acid (HCl) 8 0.005 0.2 Material 28 days 56 days 60% (H3PO4) 8 0.22 8.5 Zircaloy(a) 20–22 25–28 4 4 85% phosphoric acid (H3PO4)1200 7900 Hafnium 3–6 5–7(b) Hydrogen sulfide (H2S) in water 12 0 0 Sour (vapor) 18 0 0 (a) Nuclear grade of zirconium containing iron, chromium, nickel, and tin as important alloy additions. (b) Estimated range; test not run on routine (a) No pitting or crevice attack. (b) Pitting. Source: Ref 12 basis. Source: Ref 12 © 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 13B, Corrosion: Materials (#06508G)

4 / Corrosion of Nonferrous Alloys and Specialty Products acid. Table 4 gives corrosion information for Table 8 Corrosion of hafnium in sulfuric acid solutions both welded and nonwelded materials. As with Temperature Corrosion rate most solutions, the presence of fluoride ions in Acid Duration of nitric acid will greatly increase the corrosion of concentration, % C FCondition testing, days mm/yr mils/yr hafnium. 60 140 284 Nonwelded 8 0 0 Sulfuric acid is an environment where haf- 60 140 284 Welded 8 0 0 65 110 230 Nonwelded 1 0.03 1 nium could see more use. Zirconium is a suitable 65 154 310 Nonwelded 9 0.03 1 material of construction for acid concentrations 65 151 303 Nonwelded 8 0 0 up to 70 to 75% at boiling temperatures. Above 65 151 303 Welded 8 0 0 80%, materials of construction that can be used 70 165 329 Nonwelded 8 0.05 2 70 165 329 Welded 8 0.05 2 include some stainless steels and carbon steels. 70 120 248 Nonwelded 8 0 0 In the range of 70 to 80% acid concentration, 70 120 248 Welded 8 0 0 hafnium is more corrosion resistant than zirco- 70 180 356 Nonwelded 8 0.1 4 nium. Table 8 gives corrosion rates for other 70 110 230 Nonwelded 1 0.03 1 75 185 365 Nonwelded 4 0.2 8 temperatures and concentrations of sulfuric acid. 75 185 365 Nonwelded 4 0.5 20 Table 9 gives data showing the effect of ferric 75 185 365 Welded 8 0.13 5 ions on the corrosion of hafnium in various sul- 75 182 360 Nonwelded 8 0.13 5 furic acid concentrations. 75 150 302 Nonwelded 12 0.03 1 75 100 212 Nonwelded 12 0 0 Alkalis. Hafnium appears to have a very high 80 202 396 Nonwelded 7 1.8 70 resistance to corrosion by strong alkali solutions. 80 204 400 Nonwelded 4 3 118 solutions do not attack 80 200 392 Nonwelded 7 2.2 86 hafnium until the concentration reaches 70 wt%, 80 150 302 Nonwelded 4 0.97 38 80 80 176 Nonwelded 2 1.7 66 and then only at near-boiling temperatures. 80 80 176 Welded 2 1.1 42 Sodium hydroxide is more aggressive toward 80 65 149 Nonwelded 2 1.6 64 hafnium. An upper limit appears to approxi- 80 65 149 Welded 2 2.4 95 mately 38% concentration at boiling tempera- 80 50 122 Nonwelded 11 0.03 1 80 50 122 Welded 11 0.1 4 tures. The presence of a weld does not appear to 80 50 122 Nonwelded 8 0.1 4 alter the corrosion resistance to a significant 80 50 122 Welded 8 0.4 16 degree. Table 10 gives the corrosion resistance of 80 21 70 Nonwelded 8 0.13 5 hafnium in various alkali solutions. 80 21 70 Welded 8 0.05 2 85 210 410 Nonwelded 1 45 4200 Organics. The only reported corrosion infor- 85 210 410 Welded 1 45 4200 mation for hafnium in an organic solution is 85 21 70 Nonwelded 1 45 4200 testing performed in . No significant 85 21 70 Welded 1 45 4200 corrosion was reported at room temperature; the Source: Ref 12 duration of testing was not given (Ref 14). Molten Metals. Hafnium is superior to zir- conium and zircaloy in molten alkali metals. In , zirconium is rated good to 300 C þ (570 F) and limited to 600 C (1110 F). In Table 9 Corrosion of hafnium in sulfuric acid containing Fe3 sodium at 500 C (930 F), hafnium is as resis- Temperature Corrosion rate tant as , the corrosion rate being Acid concentration Duration of þ 1.2 mg/cm2 yr, or 0.001 mm/yr (0.04 mils/yr) plus Fe3 C FCondition testing, days mm/yr mils/yr (Ref 11). 65%þ1000 ppm 151 305 Nonwelded 2–4–2 0.05–0.01–0.05 2–0.4–2 Hafnium begins to react slowly with 65%þ1000 ppm 151 305 Welded 2–4–2 0.04–0.01–0 1.6–0.4–0.1 Gases. þ air or oxygen to form hafnium oxide at 70% 1000 ppm 165 330 Nonwelded 2–4–2 0.06–0.2–0 2.4–7.9–0.1 70%þ1000 ppm 165 330 Welded 2–4–2 0.07–0.020.02 2.8–0.8–0.8 approximately 400 C (750 F), with nitrogen 75%þ1000 ppm 185 365 Nonwelded 2–4–2 1.5–0.65–0.68 59–26–27 to form nitrides at approximately 900 C 75%þ1000 ppm 185 365 Welded 2–4–2 1.2–0.6–0.6(a) 47–24–24(a) (1650 F), and rapidly with hydrogen at 85%þ200 ppm 21 70 Nonwelded 1 45 4200 þ 4 4 approximately 700 C (1290 F) to form 85% 200 ppm 21 70 Welded 1 5 200 hydrides. Hydrogen can be readily removed from (a) Localized attack. Source: Ref 16 hafnium by heating in a vacuum at 900 C (1650 F). The and nitrides formed at these elevated temperatures tend to remain at the surfaces; however, hydrogen diffuses rapidly and forms hydrides throughout the metal. Ox- Table 10 Corrosion of hafnium in alkaline solutions ides, nitrides, and hydrides all to impaired Temperature Corrosion rate Duration of ductility. Hafnium (containing 2.5 wt% Zr) has a high reaction rate with nitrogen but less than that Solution concentration Condition C Ftesting, days mm/yr mils/yr for titanium and zirconium (Ref 11). Hafnium Sodium hydroxide 38% Nonwelded 115 240 3 0.15 6 reacts with carbon at 4500 C (930 F) (Ref 5). Sodium hydroxide 38% Welded 115 240 3 0.2 8 Sodium hydroxide 50% Nonwelded 140 284 4 0.67 27 At elevated temperatures, hafnium will react Sodium hydroxide 50% Welded 140 284 4 0.63 25 with , , and silicon, while Sodium hydroxide 50% Nonwelded 35 95 14 0.005 0.2 react directly to form tetrahalides. As a com- 50% Nonwelded BP(a) 4 0 0 parison, hafnium, at 740 C (1360 F), reacts Potassium hydroxide 50% Welded BP(a) 4 0 0 Potassium hydroxide 70% Nonwelded 170 338 2 0.15 6 with air at approximately the same rate as zir- Potassium hydroxide 70% Welded 170 338 2 0.15 6 conium but at one-half the rate of zirconium at (a) BP, boiling point. Source: Ref 12 900 C (1650 F) (Ref 11). © 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 13B, Corrosion: Materials (#06508G)

Corrosion of Hafnium and Hafnium Alloys / 5

Forms of Corrosion The pitting resistance of these alloys was improved grain-boundary ductility and increased evaluated in boiling 20% HCl containing oxidation resistance (Ref 18). 3 þ . Hafnium is a very 200 ppm Fe (as FeCl3) and in chlorine-gas- Hafnium is also used in several niobium- and noble (cathodic) metal, probably similar in saturated water. Test results are given in tantalum-base alloys to increase strength at high ranking to silver and zirconium on the galvanic Tables 11 and 12. Although all alloys showed temperature. The niobium-base alloys are C-103 series for seawater. With few exceptions, when some pitting in these oxidizing chloride solu- (Nb-10Hf-1Ti) and C-129Y (Nb-10Hf-0.07Y). hafnium is galvanically coupled with another tions, Hf-59.5Zr appeared to have the highest These alloys are used in turbojet and rocket metal, the other metal will become the anode and pitting resistance. engine parts. Alloys such as WC-3015 (Nb- suffer an increase in corrosion as a result of the Hafnium-Tantalum Alloys. All hafnium- 30Hf-1.5Zr-15W) and WC-3009 (Nb-30Hf- couple. Although the corrosion rate of hafnium tantalum alloys tested exhibit no significant 1.5Zr-9W) are used in applications requiring will remain low, its mechanical properties can corrosion in boiling j70% nitric acid with or high-temperature strength and oxidation resis- suffer from hydrogen pickup as a result of the without 1% sodium chloride (Table 13). The tance. Hafnium provides dispersed particle couple with the less . tensile strength of hafnium-tantalum alloys strengthening and -solution strengthening in Crevice Corrosion. Hafnium appears to be increases with increasing tantalum concentra- niobium-base alloys (Ref 4). Tantalum-base resistant to crevice corrosion in saturated sodium tion. Average tensile strengths of hafnium, alloys also use hafnium as an important alloying chloride solutions. Table 5 indicates a limited Hf-1Ta, Hf-3Ta, and Hf-5Ta are 480, 520, 560, element. Alloys T-111 (Ta-8W-2Hf-0.003C), T- tendency for pitting attack in other saturated and 690 MPa (70, 75, 81, and 100 ksi), respec- 222 (Ta-9.6W-2.4Hf-0.012C), and Astar 811-C chloride solutions and as a result of exposure to tively (Ref 17). (Ta-8W-1Re-1Hf-0.25C) are used to make hon- other halide salts. eycomb-type heat shields for use in re-entry . Hafnium appears to be vehicles and for fasteners for high-temperature resistant to pitting in a wide variety of solution Applications conditions. Hafnium is also an important addi- compositions. Pitting appears to be limited to tion to certain titanium, tungsten, and molybde- hydrobromic acid (Table 5) and to high con- The limited availability of hafnium leaves num alloys where, along with carbon, it forms centrations of sulfuric acid containing additions very little material for uses other than nuclear second-phase dispersions. of ferric ion (Table 9). applications and as an alloying element. Nuclear. The second major use of hafnium is Alloy Component. The primary use of haf- in control rods for nuclear reactors. Hafnium is Corrosion of Hafnium Alloys nium is as an alloying element in nickel-base used in pressurized light water reactors used superalloys. These include cast nickel-base primarily to power naval vessels (Ref 5). The Hafnium-Zirconium Alloys. The hafnium- alloys such as MM 002, IN-713 Hf (MM 004), B- combination of excellent hot water corrosion zirconium system is one of the few metallic 1900 Hf (MM 007), Rene´ 80 Hf, Rene´ 125 Hf resistance, good ductility, and good machin- systems in which thermochemical properties are (MM 005), MAR-M 200 Hf (MM 009), MAR-M ability, along with a large neutron absorption almost ideal. That is, hafnium and zirconium can 246 Hf (MM 006), MAR-M 247 (MM 0011), and cross section, makes hafnium ideal for this form isomorphous alloys for all ratios of the CM 247LC. Each of these alloys contains from application. Hafnium has been proposed as components. Moreover, hafnium and zirconium 0.05 to 2% Hf. Single-crystal alloys CMSX-3, separator sheets to allow closer spacing of spent exhibit similar excellent corrosion-resistant CMSX-4, and CMSX-6 use 0.05 to 0.1% Hf in rods in interim holding ponds, properties, although they differ greatly in neu- their formulation. In these alloys, hafnium serves because of it superior neutron absorption cap- tron absorption. Therefore, hafnium-zirconium as a strong former and provides abilities (Ref 5). Hafnium absorbs both thermal alloys offer a broad range of neutron absorption for special applications in which corrosion resistance and neutron absorption are both important. Table 12 Corrosion of hafnium-zirconium alloys in boiling water saturated with chlorine gas Hafnium alloys with 2.9, 17.3, 42.4, 59.5, and Average corrosion rate 81.4% Zr were evaluated for their corrosion First test (2 days) Second test (4 days) resistance in various media. All of the alloys Pitting exhibited low corrosion rates (50.0025 mm/yr, Alloy mm/yr mils/yr mm/yr mils/yr ranking(a) or 0.1 mil/yr) in the following boiling solutions: Hf-2.9Zr 0.015 0.6 0.005 0.2 3–4 5 5 30% HNO3 with or without 19% NaCl, 50% Hf-17.3Zr 0.0025 0.1 0.005 0.2 3–4 HNO with or without 1% NaCl, and 70% HNO Hf-47.4Zr 50.0025 50.1 50.0025 50.1 2 3 3 Hf-59.5Zr 0 0 50.0025 50.1 1 with or without 1% NaCl. Transverse-cut U- Hf-81.4Zr 50.0025 50.1 50.0025 50.1 5 bend specimens of these alloys were tested in (a) 1, best; 5, worst 90% HNO3 at room temperature for 60 days. No cracking was observed.

Table 13 Corrosion of hafnium-tantalum alloys at 120 C (248 F) Table 11 Corrosion of hafnium-zirconium þ Corrosion rate alloys in boiling 20% HClþ200 ppm Fe3 Alloy Corrodant concentration mm/yr mils/yr Average corrosion rate Hf-1Ta 70% nitric acid 0 0 First test Second test 70% nitric acidþ1% NaCl 0 0 (2 days) (4 days) Pitting 20% HClþ200 ppm Fe3þ 0.08(a) 3.1(a) ranking Alloy mm/yr mils/yr mm/yr mils/yr (a) Hf-3Ta 70% nitric acid 0 0 70% nitric acidþ1% NaCl 0 0 Hf-2.9Zr 0.19 7.4 0.058 2.3 2 20% HClþ200 ppm Fe3þ 0.09(a) 3.5(a) Hf-17.3Zr 0.18 7.0 0.053 2.1 5 Hf-5Ta 70% nitric acid 0 0 Hf-47.4Zr 0.22 8.7 0.053 2.1 4 70% nitric acidþ1% NaCl 0 0 Hf-59.5Zr 0.09 3.5 0.036 1.4 1 20% HClþ200 ppm Fe3þ 0.04(a) 1.6(a) Hf-81.4Zr 0.13 5.1 0.058 2.3 3 Note: Samples both as-received and vacuum annealed at 816 C (1500 F) for 45 min were tested. No appreciable difference in corrosion rate was 3þ 3þ Note: Fe added as FeCl3. (a) 1, best; 5, worst detected. Fe added as FeCl3. Corrosion rate is the average of the results from 3 to 48 h test periods. (a) Some corrosion at edges. Source: Ref 12 © 2005 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook, Volume 13B, Corrosion: Materials (#06508G)

6 / Corrosion of Nonferrous Alloys and Specialty Products and epithermal , and its absorption cross 6. “Hafnium Brochure,” Wah Chang, an Alle- Applications, Outlook, Vol 10 (No. 2), Wah section decreases only slowly after long periods gheny Technologies Company, Albany, OR, Chang, an Allegheny Technologies Com- of neutron irradiation. The combination of high 1987 pany, 1989, p 3–5 corrosion resistance in nitric acid and high neu- 7. “Standard Specification for Hafnium and 18. Properties and Selection: , Steels, and tron absorption cross section allows hafnium to Hafnium Alloy Strip, Sheet, and Plate,” B High-Performance Alloys, Vol 1, ASM be used in spent fuel reprocessing plants that 776, Annual Book of ASTM Standards Handbook, ASM International, 1990, p 982 utilize nitric acid. The unique properties of haf- 02. 04, ASTM, 1996 nium help avoid criticality in this application 8. “Corrosion Testing of Products of Zirco- (Ref 17). nium, Hafnium, and Their Alloys in Water at Chemical Processing Industry. While haf- 680 F (633 K) or in Steam at 750 F SELECTED REFERENCES nium has good corrosion resistance, zirconium (673 K)”, G-2 (G2M), Annual Book of has similar corrosion properties, costs less, is ASTM Standards 03.02, ASTM Interna- W.W. Allison, “Zirconium, Zircaloy and approximately half as dense, and is more avail- tional, 1999 Hafnium Safe Practice Guide for Shipping, able. 9. T.-L. Yau, J.A. Andrews, R. Henson, and Storing, Handling and Scrap Disposal,” Other uses for hafnium are in the production of D.R. Holmes, Practice for Conducting WAPD-TM-17, Bettis Atomic Power specialty chemicals, including , tetra- Corrosion Tests on Zirconium and Its Laboratory, Pittsburgh, PA, 1960 hydridoborates, , halides, nitrides, Alloys, Corrosion Tests and Evaluation, R.J.H. Clark, D.C. Bradley, and P. Thornton, hydrides, sulfides, , acetates, and Silver Anniversary Volume, STP 1000, The Chemistry of Titanium, Zirconium and oxides. These are discussed in detail by Nielsen R. Baboian and S.W. Dean, Ed., ASTM, Hafnium, Pergamon Texts in Inorganic (Ref 5). It is also used in plasma arc metal- 1990, p 303–311 Chemistry, Vol 19, Pergamon Press, 1973 cutting tips, high-temperature , tool 10. R. Baboian, Ed., Corrosion Tests and Stan- K.L. Komaraek, Ed., Hafnium: Physico- bits, and sputtering targets. dards, Application and Interpretation, Chemical Properties of Its Compounds and ASTM, 1995, p 507–510 Alloys, International Atomic Energy Agency, 11. D.E. Thomas and E.T. Hayes, Ed., The REFERENCES Vienna, 1981 of Hafnium, United States and Microstructures, Vol 9, 1. R.J.H. Clark, The Chemistry of Titanium, Atomic Energy Commission, 1960, p 282 ASM Handbook, American Society for Metals, Zirconium and Hafnium, Pergamon Press, 12. Corrosion tests conducted at Wah Chang, an 1985, p 497–502 1973, p 419–490 Allegheny Technologies Company, Albany, National Fire Codes, Vol 13, NFPA 482M, 2. J.B. Hedrick, “ Industry Surveys: OR National Fire Protection Association, Boston, Zirconium and Hafnium,” Mineral Com- 13. T.-L. Yau and D. Holmes, “ from MA, 1978 modity Summaries, U.S. Geological Survey, Overlooked Sources,” presented at the Cor- E. Negishi and T. Takahashi, Aldrichim. Acta, Jan 2003 rosion Applications Conference (Coeur d’ Vol 18 (No. 2), 1985, p 31–48 3. J.C. Haygarth and R.A. Graham, Review of Alene, ID), Wah Chang, an Allegheny C.R. Tipton, Jr., Ed., Reactor Handbook, Extraction, Processing, Properties and Technologies Company, 2003 2nd ed., Interscience Publishers, Inc., 1960, Applications of Reactive Metals, B. Mishra, 14. R. Herold, Hafnium, Metall, Vol 26 (No. 7), p 783–789 Ed., TMS, 2000, p 1–71 July 1972 (in German) B. Venugopal and T.D. Luckey, Metal Toxi- 4. R.H. Nielsen, Hafnium and Hafnium Com- 15. M. Pourbaix, Atlas of Electrochemical city in Mammals: Chemical Toxicity of Metals pounds, Ullman’s Encyclopedia of Indus- Equilibria in Aqueous Solutions, Pergamon and Metalloids, Vol 2, Plenum Press, 1978 trial Chemistry, 5th ed., Vol A12, VCH Press, 1966, p 230–233 P.C. Wailes, R.S.P. Coutts, and H. Weigold, Verlagsgesellschaft mbH, 1989, p 559–569 16. J.H. Schemel, Manual on Zirconium and of Titanium, Zir- 5. R.H. Nielsen, Hafnium and Hafnium Com- Hafnium, STP 639, American Society for conium, and Hafnium, Academic Press, Inc., pounds, Encyclopedia of Chemical Tech- Testing and Materials, 1977 1974 nology, 4th ed., Vol 12, Kirk-Othmer, 1994, 17. T.-L. Yau, Hafnium: A Unique Metal “Welding Brochure,” Wah Chang, an Alle- p 863–881 with Uncommon Properties Finds New gheny Technologies Company, Albany OR