Progress in Materials Science, Vol. 24. pp. 51-142. Pergamon Press Ltd. 1979. Printed in Great Britain. THE PROPERTIES OF METALLIC COBALT W. Betteridge formerly Chief Scientist International Nickel Ltd., London, England (Submitted 7 February 1979) CONTENTS 1. INTRODUCTION 52 2. ATOMIC AND NUCLEAR PROPERTIES 52 2.1 Atomic properties 52 2.2 Nuclear properties 54 3. CRYSTAl_ STRUCTURE AND ALLOTROPIC TRANSFORMATION 56 3.1 Allotropes 56 3.2 Allotropic transformation 58 3.3 Deformation textures, recovery and recrystallization 60 4. PHYSICALPROPERTIES 60 4.1 Optical properties 60 4.2 Thermal and thermodynamic properties 64 4.3 Electrical properties 73 4.4 Magnetic properties 77 5. MECHANICAl_ PROPERTIES 91 5.1 Density 91 5.2 Elastic properties 91 5.3 Hardness 95 5.4 Tensile properties 96 5.5 Compressive properties 97 5.6 Formability 98 5.7 Creep properties 101 5.8 Frictional properties 101 6. SPECIAL FORMS OF COBAI.T 104 6.1 Single crystals 104 6.2 Whiskers 106 6.3 Small particles 107 6.4 Thin films 110 7. ELECTROCHEMICAL PROPERTIES 112 7.1 Reversible potentials 112 7.2 Irreversible potentials 113 7.3 Cathodic processes 113 7.4 Anodic processes 114 7.5 Potentials in fused salts 115 8. CORROSION RESISTANCE AND GAS SOLUBILITY 116 8.1 Oxidation and hot corrosion 116 8.2 Wet corrosion 120 8.3 Gas solubility in cobalt 122 9. LIQUID COBALT 124 9.1 Density 124 9.2 Emissivity 124 9.3 Viscosity 125 9.4 Surface tension 125 9.5 Miscellaneous properties 126 ACKNOWLEDGEMENTS 127 REFERENCES 128 ~.P ~,t S 24 2 A 51 52 PROGRESS IN MATERIALS SCIENCE i. INTRODUCTION The present article deals with the physical, mechanical and chemical properties of unalloyed cobalt. The subject is not new, and mny compilations have been devoted to it. The most comprehensive of these I-3 date back to 1960-1961, with one excep- tion 4 dating back to 1965. While the following sections draw to some extent on these earlier reference works as well as on general collections of numerical data S-?, they also embody, in a condensed form, the wealth of information that has become available up to 1975. In some cases, corresponding data on iron and nickel are quoted with a view to comparing and contrasting cobalt with these two associated elements. The reader's attention is drawn to the fact that there are occasionally wide variations among the values reported for a given property of cobalt. Many such discrepancies can be accounted for by the varying purity of the metal used for the determination. An uncertain metallurgical structure, resulting mainly from the sluggishness of the cubic-to-hexagonal transformation, may also be responsible for some of the differences, while, particularly with a hexagonal crystal structure, the orientation of the crystals may actually have a marked effect on directional properties. 2. ATOMIC AND NUCLEAR PROPERTIES 2.1. Atomic properties Cobalt is one of the transition metals, appearing between iron and nickel in the first long period of the periodic table. Its symbol and general atomic properties are as follows: ? Symbol Co Atomic number 27 Atomic mass 58.9332 based on carbon 12. The electronic structure of iron, cobalt and nickel atoms are compared in Table i. The energy levels of cobalt have been derived from characteristic X-ray frequencies and from photoelectric data; they are as follows : K L I LII LII I M I MII,III MIV,V 7708.9 925.6 793.6 778.6 100.7 59.5 2.9 eV. ±0.3 ±0.4 ±0.3 ±0.3 ±0.4 ±0.3 ±0.3 The temperatures at which electronic transitions occur in atoms have been found by Ham and Samans 8 to be expressed by the equation given below : where Tn is the absolute temperature of a transition, A is a constant for a given element and n is an integer greater than n o . They report the following constants for iron, cobalt, and nickel : n o A, K Fe 3 12,500 Co 3 13,870 Ni 3 8,910 PROPERTIES OF METALLIC COBALT 53 TABLE i. Electronic structure of iron, cobalt and nickel atoms * Iron Cobalt Nickel Ground state -3d64s 2 --3d74s 2 --3d84s 2 Ground state of the atom 5D 4 4F3/2 3F 4 First ionization potential, eV 7.87 7.864 a 7.633 b Ion ground state 6D9/2 3F 4 2D5/2 Second ionization potential, eV 16.18 17.05 18.15 C C C Resonance potentials, eV, c 2.39 2.91 3.18 3.20 3.50 3.64 Resonance lines, A d 5166.29(7D5) 4233.99(6FII/2)3884.58(SD4) 3859.91(5D4) 3526.85(4F9/2) 3670.43(3P2) ~Amer. Inst. Phys. Handbook 7 aNormal state of ion 3d 8. bNormal state of ion 3d 9. CResonance potential is the energy in electron volts (eV) required to raise an atom from the ground state to the lowest excited state. The letter C indicates that there are states of the same parity as the ground state between it and the first resonance state. dThe resonance line is the spectrum line absorbed or emitted in this or the reverse transition. The characteristic X radiation from cobalt is given in Table 2. The values of the atomic scattering factor are given in Table 3. and the mass absorption coefficients for X-rays and y-rays will be found in Table 4. The more accurate coefficients determined for characteristic X-rays of different target materials are those rele- vant to X-ray crystallographic studies I0 TABLE 2. Characteristic X radiation of cobalt 7 Line A(0.1nm) Line A(0.1nm) Ka I 1.788965 La I 15.972 K~ 2 1.792850 L~ 1 15.666 KS1, 3 1.62079 L[ 18.292 K65 1.60891 L 17.87 K absorption edge 1.60815 LS3,4 14.31 With regard to the emission spectrum of cobalt the sensitive lines are listed in Table 5, with the relative intensities in both arc and spark spectra. Details of further spectral lines are given in Reference 6, in which over 330 lines varying in wavelength from 9746 to 2011 A are listed. The magnetic properties of the cobalt atom are dealt with in Section 4.4. 54 PROORESS |N MATER;ALS SC;ENCE TABLE 3. Atomic scattering factor, a(% in A)* Sin 0/~ f Sin e/% f 0.0 27 7 9.3 0.i 24.1 8 8.3 0.2 19.8 9 7.3 0.3 16.4 0 6.7 0.4 14.0 1 6.0 0.5 12.1 2 5.5 0.6 10.7 *Metals Reference Book 6 aThe atomic scattering factor, f, of an atom of atomic number Z is defined as the ratio of the amplitude of the wave scattered by the atom (taken as the resultant of that scattered by its Z electrons) to that scattered by the electron. It is a function of the number of electrons, Z, the angle of diffraction, 28, and the wavelength, %. TABLE 4. Mass absorption coefficients of cobalt For l-rays* For X-rays Energy, Coefficient, a Wavelength, Energy, Coefficient, a Ref. MeV ~/p , cm2/g A keY ~/p, cm2/g 0.5 0.0828 0.098 127 0.287 1 0.0598 0.130 96 0.46 2 0.0418 0.175 75 0.92 3 0.0356 0.200 62.5 1.26 [9] 4 0.0326 0.260 48 2.60 6 0.0303 0.417 30 9.45 8 0.0298 Radiation i0 0.0300 0.560 Ag Ks 21.14 ± 0.ii 14 0.0310 0.712 Mo Ks 40.40 ± 0.32 18 0.0322 1.54 Cu Ka (350) [ i0] 20 0.0328 1.79 Co Ka 61.56 ± 0.50 1.94 Fe Ka 78.0 ± 0.6 2.29 Cr Ka 126.0 *Handbook of Chemistry and Physics s aThe mass absorption coefficient, ~, is defined by the relation I =Io exp ~t, often rewritten I =Io exp ~ pt, ~here I is the intensity of a beam of rays, of incident intensity Io, afte~ passing through a thickness, t, of a material of density p. 2.2 Nuclear properties The only naturally occurring isotope of cobalt, 59Co, is stable. There are, however, twelve other known isotopes, all of them radioactive. Their mass numbers range from 54 to 64. Their main characteristics are listed in Table 6. 60Co, by virtue of the reasonable half-life, its ready production by neutron bombard- ment of natural cobalt and the general physical and mechanical properties of cobalt metal, is the most widely-used source of radioactivity for such purposes as radio- graphy, radiotherapy, tracer applications, control devices, food sterilization, etc. Details of the production and use of 60Co are given in Chapter 17 of the Cobalt Monograph I and in Chapter 16 of Young's monograph 3. PROPERTIES OF METALLIC COBALT 55 TABLE 5. Sensitive lines of emission spectrum of cobalt 6 Wavelength, A Intensity Arc Spark 3529.813 i000 30 3465.800 2000 25 3453.505 3000 200 3405.120 2000 150 2519.822 40 200 2388.918 i0 35 2378.622 25 50 2363.787 25 50 2307.857 25 50 2286.156 40 300 R = wide self-reversal TABLE 6. Isotopes of cobalt Mass Half Radiation Particle energy, b Gamma ray energy, c number life, emitted a MeV MeV T 54 0.2 sec B+ _ M 55 18.2 hr B+,EC 0.26(2.3+) ,0.53(4.9+), 0.253(2+) ,0.477(28+) ,0.935(156+), 1.03(39.5+) ,1.50(53.3+)1.41 (26+) ,1.84(0.6+) ,2.17(4+) 56 80 days B+,EC 0.44(4+),1.50(96+) 0.85(100+),1.24(55+),1.75(24+), 2.30(12+), 2.60(14+), 3.25 (24+) 57 270 days B+ 0.320 0.014(~ = Ii x I0-8sec),0.123(15+) I K 0.011 ]M 1,0138(1+)[ K0.14 IE 2 58m 9 hr no B+,IT -- 0.025 [ large]M 3 58 72 days B+,EC 0.472(15%) 0 805 [ 2.9 x 10 -4[E 2 59 stable none 60m i0.i min B- 1.56(0.28%) 0.059 [K 35]M 3 60 5.3 years B- 0.31,1.48(0.15%) 1.1728 [1.6 x 10-41, 1.3325 [1.2 x i0 -q] 61 99 min ~- 1.00(45%),1.42(55%) =0.5 62m 1.6 min B- 62 13.9 min B- 2.8 I .0(40+), 1.17(100+) ,i.