Chlorination of Cobalt in an Argon-1 Pct Oxygen-1 Pct Chlorine Mixture at 1000 K
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MICHAEL J. McNALLAN is Associate Professor of Metallurgy in the Department of Materials Engineering at the University of Illinois at Chicago, P.O. Box 4348, Chicago, IL 60680. C.T. KANG, formerly Graduate Student in the Materials Department at the University of Wisconsin-Milwaukee, is now Graduate Student in the Department of Metallurgy and Materials Engineering at the University of Pittsburgh, Pittsburgh, PA 15261. WINSTON W. LIANG, formerly Assistant Professor in the Materials Department at the University of WisconsinMilwaukee, is now with the Gas Research Institute, 8600 Bryn Mawr Road, Chicago, IL 60631. Manuscript submitted August 5, 1982. METALLURGICALTRANSACTIONS A
chemical potential of oxygen decreases to 8.5 • 10-18 atm at the scale-metal interface. 5'8 When 0.01 atm of C12 is also present, another important corrosion product can be CoC12(g). The vapor pressure of CoC12(g) over solid Co304 is given by Eq. [1]:
Co304(s ) At- 3C12(g) = 3CoC12(g) + 202(g)
[1]
for which 3
Kl00O = Pc~176
P312
2
= 9.74 • 10 .7
[2]
When the partial pressures of oxygen and chlorine are 0.01 atm, the vapor pressure of CoC12(g) at equilibrium at 1000 K is 2.14 x 10 .3 atm. When the partial pressure of oxygen is reduced with the partial pressure of chlorine held at a constant level, then the vapor pressure of CoC12(g) increases until it reaches a pressure of 6.87 x 10 .3 atm at a partial pressure of oxygen of 1.73 x 10-3 atm. At lower partial pressures of oxygen solid COC12 would be the thermodynamically stable condensed phase in the presence of 0.01 atm of chlorine at 1000 K and the vapor pressures of CoC12(g) would no longer increase with decreasing partial pressures of oxygen. The rate of chlorination of flat plates of CoO in flowing argon-oxygen-chlorine mixtures has been studied previously. 9 These specimens also exhibit a linear decrease in mass with time, with the rate of the reaction being controlled by mass transfer of COC12 in the gas phase according to Eq. [3]: 1~
kl
= 0.664R~
D2/3
-~
[V,~ 1`2 ~,TJ Pco%
[3]
Where M is the atomic weight of CoO, R is the gas constant (82.05 cm 3 atm mole -1 ~ T is the temperature in Kelvin, D is the diffusion coefficient of CoCl2(g) in the gas mixture, v is the kinematic viscosity of the gas mixture, L is a characteristic dimension of the system (in this case 7r/4 times the diameter of the disc), V, is the superficial velocity of the gas, Pcoc12is the vapor pressure of the volatile chloride at the surface of the specimen, and k~ is the rate constant in g cm -2 s -l. The rate of chlorination of CoO at 1000 K was modeled adequately when Pcoc~2 was taken from the equilibrium of Eq. [1], D was taken as 0.66 cm 2 s -~, and u was taken as 1.1 cm 2 s-l. 9 The apparatus used for these experiments has been described elsewhere.7'9 It consists of a Kanthal wound furnace with associated controller, a gas train for the preparation of controlled gas mixtures, and a Cahn microbalance for recording changes in the mass of the specimen. Gas flow through the furnace is from top to bottom to av
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