July 1949
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
of the catalyst a change in color occurred after intermittent use and the activity decreased. After several hundred grams of furfural had been passed over a given sample of catalyst the surface became partially coated with a n organic film not readily wet by water. Leaching with water caused a slight acidic reaction in the aqueous media. No deleterious effects on the activity of the catalyst were noted up to conversion ratios of 68 to 1. However continued use in commercial units could result in complete coating of the active surface. Presumably this could be prevented by an occasional washing of the catalyst with 2-methylfuran or other solvent to remove the polymer or decomposition product responsible for the film. ACKNO w LEDGMENT
The authors are grateful to The Quaker Oats Company for a grant which made this investigation possible. Special thanks are due F* Peters, Jr*,and A*p*Dunlop for their in the problem,
1335
LITERATURE CITED
Adkins, “Reactions of Hydrogen with Organic Compounds over Copper-Chromium Oxide and Nickel Catalysts,” p. 12, Madison, Wis., University of Wisconsin Press, 1937. (2) Adkins and Connor, J . Am. Chem. Soc., 53,1091 (1931). (3) Brown, Gilman, and Van Peursem, Iowa State Coll. J . Sei., 6, (1)
133 (1932). (4)
Burnette, Johns, Holdren, and Hixon, IND. ENQ.CHES., 40, 502
(1948). ( 5 ) Calingaert and Edgar, Ibid., 26, 878 (1934). (6) Dunlop and Trimble, Ibid., 32, 1000 (1940). (7) Graves, U. S.Patent, 2,077,409 (April 20, 1937). (8) Kauffman and Adams, J . Am.Chem. SOC.,45,3029 (1923). (9) Komatsu and Masojiro, Bull. Chem. SOC.J a p a n , 5, 241 (1930). Padoa and Pont, Atti +.accad. Lincei (V), 15,ii, 610 (1906). Paul, Bull. soc. chim., 1946, 208. Ricard and Guinot, U. 5.Patent 1,739,919 (Dec. 17, 1929). (13) Rittmeister, Ibid., 2,201,347 (May 21, 1940). (I4) U. S. Dept. Commerce, Office of Technical Services, PB 671.
RECEIVED August 25, 1947. Abstraoted from t h e Ph.13. thesis of H. D. . Brown presented to the faculty of Iowa S t a t e College.
Kinetic and Structural Factors Involved in Oxidation of Metals EARL A. GULBRANSEN Westinghouse Research Laboratories, East Pittsburgh, Pa. O n e of the interesting questions in the protection of metals against high temperature oxidation and corrosion reactions is whether the protection observed can be correlated with fundamental physical processes, such as ion and electron formation, and with the several structural factors of the oxide film, oxide interface, and metal. In general, the protection is associated with an oxide film of a few Angstroms to several thousand Angstroms in thickness. A study of the rate of formation and structural characteristics of such a thin film requires the use of sensitive microbalances and instruments, such as the electron diffraction camera and the electron microscope. Even with these instruments the structural factors cannot be completely determined. This paper discusses the methods used and some results o f a study on several metals. The role of the secondary structure of the oxide film is presented.
T
H E theory and nature of the surface oxidation of metals have been a subject of increasing interest to both chemists and physicists. The usefulness of studies on the oxidation reaction t o the understanding of corrosion and high temperature behavior of materials as well as to the understanding of batteries, rectifiers, and other electrochemical processes is well known. During recent years numerous attempts have been made to correlate the experimentally observed reaction rates with fundamental physical processes such as the formation and diffusion of the metal ions and electrons ( 1 7 ) . These efforts have led to the application of many of the developments in solid state physics t o the various physical processes involved in these reactions. However, the theory of the surface oxidation reaction is still confused. The author has felt for some time that only by a systematic attack upon the structural as well as the kinetic factors can the theory be placed on firmer ground. It is the purpose of this communication t o discuss the methods and results of the author’s kinetic studies together with an analysis of the structural factors studied by the electron microscope and electron diffraction techniques.
TYPE REACTIONS
Five basic reactions are fundamental to the study of the oxidation reaction.
1. M ( g ) eM 2. z M ( s )
+ g - degassing of the metal
+ g 0 ( 9 )cl;M,O,(s)
3. MzO,(s) 4 yHt ( 9 ) with hydrogen
4. MzOy(s)
+ yC (metal)
+ -
rization
5.
M,O,(s)
- oxidation
+ qH20(g) - reduction z M ( s ) + yCO(g) - decarbu-
zM(s)
M,O,(g)
- volatilization
I n the above equations M refers to the metal and s and g refer to the solid and gaseous states, respectively. The rate of formation of the oxide film, Reaction 2, is the main reaction for study. The presence or absence of gas in the metal F a y not only influence the reactiqn rate b u t interfere with the measurements. This gas may be removed by heating to elevated temperatures in a high vacuum according to Reaction 1. Studies on metals or alloys free from oxide films may be made if the oxide films can be reduced with hydrogen (Reaction 3), or if the oxide can be removed by the decarburization reaction, 4. The decarburization reaction makes possible the elimination of oxide films on metals or alloys which are impossible to reduce with pure, dry hydrogen. Reaction 5 provides information on the volatility of the oxide film, an important factor in the oxidation of molybdenum and tungsten. METHODS OF STUDY
LITERATURE.Three methods have proved useful in the study of the kinetics of thin oxide film formation. The polarimetric method has been adapted by Lustman and Mehl (16) for the measurement of the oxidation of copper in a continuous manner a t elevated temperatures. A differential pressure method has been developed by Campbell and Thomas ( 1 ) and has been used for the study of a series of metals and alloys a t elevated temperatures in
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1386
a continuous manner. The present author chose to use a weight gain method in which a sensitive quartz microbalance (7, 8) is placed within the vacuum system. This method also allows a continuous study of the oxidation reaction over a wide pressure and temperature range.
n_
pfzzkzlL+1 TUNGSTEN WIRE
ISTUBING
L,5,rJ
COUNTERBALANCE
Figure 1.
Microbalance System
VAccurd MICROBALAXCE METHOD.The vacuum microbalance method, in brief, is to suspend a thin sheet of the metal from the beam of a very sensitive quartz microbalance operating in an allglass vacuum system. The weight change of the specimen is followed continuously by a niicrometei microscope as various operations are performed upon it. These operations include evacuation to 10-6 mm. of mercury, degassing, reduction of the initial film by hydrogen or by the carbon in the metal, oxidation of the metal, and finally the stability of the oxide film to high vacuums. These reactions can be studied by the use of suitable reaction vessels from liquid nitrogen temperatures to 1200” C. The reaction vessel for the highest temperatures is constructed from zircon. Pressures of better than 10-6 mm. of mercury c a n be achieved at the higheat temperatures. The balance together with the furnace tube is shown in Figure 1. A sensitivity of approximately 0.3 X lou6 gram can be readily achieved with very small temperature and pressure corfficients and a stable zeio point. The samples are prepared from thin sheets of the metal or alloys, approximately 0.13 mm. thick for metals having a density of 7 to 9 and 0.26 mm. thick for the lighter metals. The samples weigh 0.6840 gram and have surface areas of 10 to 20 sq. cm. The metals are in general abraded under kerosene to minimize the formation of films. The specimens are then cleaned with several solvents before use. Special experiments are made with the “as received” specimens and with speciniens having other abrasion or polishing treatments. Various annealing and other pretreatments can be given to the metal speciniens before placing them on the balance.
Vol. 41, No. 7
At higher temperatures the oxidation rate for both aluminum and magnesium is a constant as the thickness of the film increases The oxidation curve for aluminum a t 500’ C. is shown in Figure 3. The linear rate law (constant rate of reaction) is followed even nith the thinnest films. A comparison of the aluminum curve in Figure 2 n i t h the curves in Figure 3 indicates that the reaction mechanism has changed. If one applies the oxide to metal volume rule developed by Pilling and Bedworth (19) all of the metals shown in Figure 2 should form protective oxides with the exception of magnesium. The oxidation of magnesium was studied and it was found to be protective at temperatures below 400’ 6. (9). TEMPERATURE. I n the study of the temperature effect on the oxidation rate of metals, one of the interesting questions is the nature and thickness of the room temperature film. This was studied for a number of metals. Figure 4 shows the study on a specimen of pure iron which has had its original oxide film removed bv reduction with hydrogen. After the hydrogen is removed, oxygen is added and the results shown in Figurz 4 are obtained. A weight gain equivalent to an oxide film of 19 A. is calculated assuming a surface roughness ratio of 1. The film is stable to a vacuum oi 1 0 - 6 mm. of mercury and no additional film formation is noticrd on adding further oxygen. The effect of increasing the temperature on the oxidation oi pure ironois shown in Figure 5 . The oxide film forms to a thickness of 1560 A. after a 2-hour oxidation a t 450” C . The relationship between the thickness and the weight gain depends upon the pal ticular crystal structure present, its density, ant1 thc ouygc’n content. One microgra? per square centimeter of oxygen take-up corresponds to 69.5 il. of iron oxide (FeaO4), the normal oxide found by electron diffraction studies for this temperature. ,4 brief analysis of the curves in Figure 5 would show that the oxidation rate is an exponential function of the temperature. Thr details of this relationship 15111 be discussed later.
TIME (MIN)
Figure 2. Oxidation of Molybdenum, Iron, Tungsten, Aluminum, and Magnesium at 400” C.
RESULTS O F RATE STUDIES
TIME. Figure 2 shows a comparison of the oxidation curves for molybdenum, iron, tungsten, aluminum, and magnesium at 400’ C. The pressure of the oxidizing atmosphere is 7.6 cm. of oxygen except for magnesium, where the pressure is 20.0 cm. of oxygen. The weight gain in micrograms per square centimeter is plotted against the time in minutes. The time behavior of the several metals and alloys is variable. The shapes of the rate curves for a polycrystalline metal are a function of the temperature, pressure, the presence of gas in the metal, surface area, and surface preparation. Aluminum and magnesium oxidize slowly a t 400 ’ C. and the reaction rate diminisheswith time,indicating that the film has protective qualities.
PRESSURE. The influence of pressure on the time curves for the oxidation of iron and molybdenum is shown in Figures 6 and 7 The effect is not large for iron but more important for molybdenum. Wilkins (25) has discussed the pressure effect and has derived an expression for the pressure below which the oxidation ir affected by pressure. This limiting pressure is based on the concept that adsorption precedes diffusion and that the oxygen adsorbed on the oxide surface is capable of lateral diffusion. The limiting pressure will be a function of the rate of reaction, this irt turn being affected by the temperature, time, etc. SURFACE AREA. One of the most interesting studies the authoi made on the effect of surface area is t h a t for the Oxidation of tung-
INDUSTRIAL
July 1949
1387
A N D ENGINEERING CHEMISTRY
TIME (MINJ
Oxidation of Aluminum at 500' C. and 7.6 Cm. of Oxygen Pressure
Figure 3.
Effect of abrasion
rb.6
Figure 6.
0 X
Oxidation of Pure Iron
Effect of pressure at 450' C. and 7.6 t o 0.03 om. of oxygen pressure
N.
€ 9
s
In 5 2
a
i
U
f t'
2 0
20
40
60 TIME
80
100
120
140
160
I
[MIN.)-
Figure 4. Study of Room Temperature Oxide Film Formation and Stability to Vacuum of Iron
Figure 7.
Oxidation of Molybdenum
Effect of pressure at 400' C.
I
210
4b
60
EkI
IO0
I20
I40
Id0
TIME (MIN.)
Figure 5. Oxidation of Iron a t Various Temperatures and 0.1 Atmosphere of Oxygen Pressure
sten. Figure 8 shows three oxidation curves. The first is the normal oxidation, while the second and third curves show the effect of reduction of the previously formed oxide. It is evident that the reaction rate is greatly affected by the surface area. The reduction of the oxides of tungsten develops a large number of small crystals of tungsten which greatly increases the surface area. Figure 8. STRUCTURAL FACTORS
It is essent,ial to develop a physical and chemical picture of the oxidation reaction if a theory is eventually to be erected to explain the reaction which will allow predictions of other metal and alloy systems. I n the past a continuous film has been postulated and the reaction attributed to diffusion of metal ions or oxygen through the continuous and uniform film. A more detailed picture has been
Oxidation of Tungsten Polished a t 500' C. and 7.6 Cm. of Oxygen Pressure Effect of reduction
developed of the surface oxidation reaction as a result of the work of Lacombe and Beaujard (14) using polarized light micrographs and by the author and his co-workers (11-13, 18) using the electron microscope and electron diffraction methods.
1388
I N D U S T R I A L A N D E N GI I N E E R I N G C H E M I S T R Y TABLE I.
PHYSICilL
Vol. 41, No. 7
PROCESSES INVOLVED Ix GAS METAL REACTIONS
1. T r a n s p o r t of gas t o t h e surface
2. 3. 4.
5.
6. 7. 8.
Figure 9.
Oxide Film on Iron (Fe 30-230)
a and d , electron micrograph stripped film; b , electron diffraction transmission stripped film; c, electron diffraction reflection film on metal; e, light micrograph film o n metal
-4breakdown of the structural factors in matter has been given by Riley (20). These are the crystal structure or atomic geomtry of the unit cell, the primary structure including the crystal size and shape, and the secondary structure including the particle size and shape. The author has included, in the latter factor for films, the details of the oxide boundaries at the junction of the particles and other knon-ledge of the film which can be observed with the electron microscope. The crystal structure can be studied by electron diffraction and an estimate given for the crystal size. The primary and secondary structures are studied JT-ith the electron microscope. If the particle consists of many crystals, it would be impossible to observe the size and shape of these individual crystals unless some method of selective etching could be developed. ELECTRON MICROSCOPE STUDYOF IROX;. Figure 9 shows a study of the oxide film on iron by the electron diffraction and electron microscope methods. The oxide in this case is formed a t 250 O C. for 30 minutes in 0.1 atmosphere of oxygen. ,4n electron diffraction pattern is taken by the refkction method a t the temperature of formation. The specimen is then cooled and the film removed by the electrochemical stripping technique developed by Evans and his eo-workers (5,6). The electron diffraction reflection pattern is shown in Figure 9, c, and a transmission pattern in 9, b. The crystal structure is found to be Fe304. Figure 9, cl, shows an electron micrograph a t 7000 diameters and Figure 9, a, a t 38,000 diameters. The effect of the underlying metal grain orientation on the average growth of the oxide is noticed. The details of the oxide structure are noticed more clearly a t higher magnification in Figure 9, a. A careful study of the original negatives of a series of these micrographs shows a closely fitted mosaic structure for the oxide crystals. The thickness is not uniform and a second and third layer of crystals form above the first layer. Overlapping of crystals is noted a t their crystal boundaries. This indicates that there is a random orientation for crystal growth and that the direction of crystal growth varies from crystal to crystal. The effect of the underlying metal orientation on the average oxidation is not new. Lacombe and Beaujard (IC),for example, have shovn this effect in a striking manner by a light micrograph study of anodically oxidized films on aluminum. A schematic picture of their results is shown in Figure 10. From a systematic study of stripped oxide films on metals and alloys Gulbransen has tried to visualize the surface oxidation re-
F o r m a t i o n of chemisorbed layer F o r m a t i o n of metal o r oxvsen ions Diffusion of metal or oxykzn ions t h r o u g h normal or strained sites F o r m a t i o n of electrons Chemical reaction Nucleation a n d growth of crystals S t r u c t u r a l transformations
action as one involving several steps. The details are shown i n Figure 11. Let us assume a polycrystalline metal reacting with oxygen gas. The structure of the initial stage of the reaction involving the first few layers cannot be studied with any known technique. I t may be postulated from other evidence that t h e initial stage of the reaction involves as a first step a c$einisorbed layer. After the film grows to a thickness of 50 t o 100 A., one can observe with the electron microscope small crystals of the order of size of the thickness of the film. These crystals giow at the expense of one another with further reaction and ncw crystals are formed on top of the initial layer. The first layer of crystals it, usually random in orientation and a perfect interlocking mosaic structure is formed. The second and succeeding layers appear to form crystals which follow their known crystal habits. The overall picture is probably a combination of the pictures shown in Figures 10 and 11. It is possible to conclude from this evidence that the ratio of oxide to metal volume is probably not the important quantity it once occupied in the theory of oxidation since the oxide crystals can grow only in one direction. The oxide to metal volume ratio determines only the relative thickness of the film
OXIDE METAL
Figure 10.
Relationship between Surface Oxidation Rate and Metal Grain Orientation
I n considering the mechanism of the reaction it is apparent t h a t the uniform flrn theory is only a n approximation. The effects at the oxide crystal boundaries cannot be neglected and may be verjr important. The crystal size of the oxide film is dependent on thc: method of preparation of the oxide film. Films formed in fused oxide baths a t the same conditions of time and temperature shoiv a much finer crystal size than films formed by gas oxidation Thus the nature of the medium adjacent to the reacting surface may have an important influence upon the reaction. The author tried to visualize also the role of the contact zone ot the individual oxide crystals. A study of many electron micrographs indicates, with several exceptions, that the details of t h e 2 . FINE MOSAIC S T R i C T U R E OF CRYSTALS
I. ABSORBED 0 FILL.?
3. GROWTH OF CRYSTALS
cwsraLTc;e 500A.
CRYSTALS
4 , SECOUD LAYER APPEARS
---c 7. iooca i
E CRYSTALS
Figure 11. Oxidation iMechanism Secondary structure as seen by electron microscope (50,000 X )
July 1949
Figure 12.
1389
INDUSTRIAL AND ENGINEERING CHEMISTRY
Secondary Structure Crystal Boundaries
Scale, 0.25 inch = 8.4 b. (scale reduced one third i n printing), Fen01lattice 100 face up, oxide crystals are 200 t o 400 I . i n size
contact zone are not resolvable by the electron microscope. The observed contact zone is for most crystals a sharp discontinuity. Figure 12 shows a simplified schematic picture of the contact zone of three oxide crystals. In this figure, each square denotes a unit cell of the Fea04structure. The boundary zone will probably have openings into which it is no longer possible to fit the unit cell or one of the fractional parts comprising the unit cell. Some form of fitting and adjustment of bond strengths must occur. Impurity atoms or crystals may also concentrate a t the contract zones. Strains in the lattice may be set up unless the atoms are of similar size and structure. It is a t present impossible to calculate the free energy curves for diffusion in the normal oxide lattice and similarly for the strained lattice sites a t the crystal boundaries. It is possible, however, to visualize the shape of the free energy curves for the two processes (Figure 13). The free energy of the activation process for diffusion is plotted against the distance. Two adjacent lattice sites are shown for the two types of diffusion. I n real films various degrees of strain may be set up a t the boundary zone owing to the degree of lack of fitting of the two crystals or the number of impurity atoms present. The maximum of the free energy curves will be similar for the two types of diffusion processes. However, the distance between. the sites and the minimums in the free energy curves will be different. The strained lattice sites, therefore, may offer a lower free energy path for diffusion than normal sites. If this is the correct picture, then the number of strained sites should play an important role in the theory as well as the degree of strain. These factors will vary with the crystal size and shape as well as impurity atoms in the film. ELECTRON DIFFRACTION STUDY OF IRON.Figure 14 showg the existence diagram obtained from electron diffraction photographs a t the temperature of the experiment. Four oxides are observed, namely, y-FezOs,FeaOa, a-FezOa,and FeO above 450 ' C. Thus, as the temperature is changed, a series of transformations occurs. This has been studied in some detail in a previous work (11). Iron is one of the few metals which form a series of reversible transformations with temperature. The effect of these transformations on the secondary structure including particle size and shape has not been determined. There is evidence from electron diffraction that transformations occur as a function of film thickness. Thus, a t 250" C., the spinel Fe304first forms; then transforms to cu-FezOs as the film thickens. These transformations frequently involve secondary structure changes. One can say, then, that as a result of crystal structure transformations not only may the diffusion rate change in the new lattice but, as a result of secondary structural changes, the number of strained sites and their degree of strain may be changed also. Electron diffraction data do not in themselves identify com-
pletely the chemical species present in the film. Agreement in lattice parameters is usually accepted as identification. However, in the case of oxide films on alloys a number of metals may be present in solid solution in the oxide. If the parameters of such metal oxides are close together, it will be difficult to identify the film completely. The presence of traces of materials unless concentrated in the surface would not be readily identified with the electron diffraction method. A complete microchemical analysis of the film is necessary. DISCUSSION. It is possible to conclude that the several structural factors in the oxide film are very important in the theory of the surface oxidation of metals. Evans (4) in a recent paper has recognized the importance of large scale physical effects in thick films and has developed the theoretical equations accordingly. I n the thin film range the structure factors, as well as the chemical composition, probably play the important role and must be considered in detailed modifications of the present theory. The several physical processes with which one is concerned in gasmetal reactions are tabulated in Table I. The limiting reaction is a function of the temperature and pressure and the particular metal involved. The present state of the theory is concerned with processes 3, 4, and 5 of Table I. The correlation of these processes with rate data will be discussed in the next section. A t the present time the structural factors can only help us visualize the process and explain deviations from the theory. OXIDATION THEORY AND CORRELATIONS
TIME.Three rate laws have been postulated to account for the time behavior of the reaction. The arabolic rate law was first derived by Pilling and Bedworth (le.9fon the basis of diffusion of oxygen through the oxide film. It has been derived more recently by Wagner and Grunewald (22)and by Mott ( 1 7 ) . The equation is: W2 = Kt f C Here W is the weight gain, 1 is the time, and K and C are constants. The linear rate law is given by the equation
~
.
,
.._I'
AF ENERGY
1
~
DI TANCE X
Figure 13. Free Energy Curves for Diffusion Normal diffusion i n oxide --- - Strained grain boundaries lattice i n oxide at
'I
W=Kt+C This equation states that the reaction rate is constant with time and is independent of the film \
'
limiting thickness.the T reaction h e f a cmay tor involve the energy barrier in the formation of the ions rather than the diffusion through the lattice. The logarithmic rate law has been derived by Tammann and Koster (21). This equationstates
t
= fi(eW/a
-
I)
I50
,ool
VAS
I
0.05
Figure 14.
0.1
05
1.0 TIME-HOURS5
Io
so
IO@
500
Existence Diagram o f Surface Oxides on Iron
1390
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 41, No.
P
TIME (MIN.)
Figure 15
Heie 01 and p are constants. The physical basis for this rate law has not been well established It has been used largely in an empirical manner. I n attempting to make correlations with theory it has appeared for some time extremely unlikely that one rate law could be applied over a vide temperature or pressure range. However, the correlation with one particular mechanism may be studied over a certain temperature and film thickness range. The author adopted this limited point of view in his studies. T o test the application of the parabolic rate law, the weight gain squared is plotted against the time. Figure 15 shows the results of such a plot for iron. An initial deviation is noticed in these plots during which the oxidation reaction rate is faster than that predicted by the law. M o t t (16) has proposed a mechanism to account for this deviation based on the existence of a potential owing t o adsorbed oxygen ions on the surface. The field resulting from the potential decreases as the thickness increaaes until its effect is negligible. At lower tempeiatures the parabolic rate law does not hold. The logarithmic rate law has been shown by Lustman and Mehl (15) to apply for copper and by Campbell and Thomas (1) for copper and its alloys. This law suffers from its lack of a plausible physical mechanism. TEXPERATURE. The temperature dependence of the parabolic rate law constant was first shown to follow an expression of the Arrhenius type by Dunn ( 2 ) . Mott ( 1 7 )has related the parabolic rate law constant to diffusion theory. The author (10) has applied the transition state theory of diffusion developed by Eyring and eo-workers (6). The parabolic rate law constant is given according to this theory by the following equation
PARABOLIC RATE L A W CONSTANT
T X
Id
Figure 16
the rate-determining process for the several metals and alloys stydied. She values of the energy of activation, E*, vary from 22,600 calories per mole for iron t o 45,650 calories per mole of tungsten. The exceptionally high value for tungsten is interesting. In order to account for the rapid rate of oxidation of tungsten, the temperature independent factor e 4 S Y / Rmust have positive values. This is in contrast to the negative values observed for iron, aluminum, and the stainless steels. The values of A S * varv from -28.7 for 18-8 stainless steel t o +11.2 for tungsten. AS* is a measure of the frequency factor or the probability of an ion with a definite energy getting through the lattice. LITERATURE CITED
Campbcll, W. E., and Thomas, U. B., Trans. Electrochem. Soc., 91. m e n r i n t hTo.24 (1947). Dunn,-J. k . , J . Chem. Soc.,1929, 1149-50. Evans, U. R., Ibid., 1927, 1020. Evans, U. R., Trans. Electrochem. Soc., 91, p r e p r i n t , No. 6 (1947). Evana, U. R., and Stockdale J., J . Chem. Soc., 1929, 2651. Glasstone, Laidler, and E>ring, "The Theory of Rate Proteases," Y-ew York, McGraw-Hi11 Book Co., 1941. Gulbransen, E. A, Rev. Sci. Instrumfnts, 15, 201-4 (1941).
Here Iz is Goltzman's constant, h is Planck's constant, X is the distance in Angstroms between the diffusional states in the oxide lattice, A S * IS the entropy of activation, and E* is the energy of activation. The free energy of activation is given by the equation A F I = E* - TAS* since A T is negligible. The temperature dependent part of the equation is given by e-E*IRTT IT hile the temperature independent part is given by TABLE 11. P A R A B O L I C RATECOXSTANTS AND DIFFUSIOK CONSTAXTS, EXTROPIES, ENERGIES, X2eAS*/R. The quantity Ic/h AND FREE ENERGIES OF ACTIVATION OXIDATION PROCESS is a universal constant. Figure 16 shows a plot of the parabolic rate law con15,350 37,950 22,600 -24.6 1 2 X 10-16 Iron 350 5 . 4 8 X 10-8 stant as a function of l / T 16,550 39,150 22,600 -24.6 400 4 . 0 9 X 10-16 4 . 6 8 X 10-8 for the temperature range of 17,700 40,300 22,600 -24.5 450 1 5 . 2 5 X 10-15 5 . 5 2 X 10-8 250' to 450" C. for the oxi18-8stainlesssteel 550 2 . 5 1 X 10-16 9 . 3 X 10-9 -28.2 29,600 24,200 53,800 GO0 7 . 7 6 X 10-18 1 . 0 2 X 10-8 -28.2 29,600 24,600 54,200 dation of molybdenum. The 8 . 4 3 X 10-0 -28.7 29,600 26,500 56,100 650 1 . 6 1 X 10-16 results may be interpreted as 38,100 45,650 -7,550 11.2 Tungsten. 400 7 . 6 2 X 10-18 2.89 giving two straight lines of -7,950 37,700 11.0 45,650 2.70 430 7 . 6 2 X 10-14 different slope. An analysis -8,330 37,300 65,650 10.8 500 6 . 4 9 X 10-'a 2.82 of the 350" to 450" C. porMolybdenum 350 2 2 1 x 10-15 7 si x 10-3 0 38 36,500 - 240 36,260 tion gives an energy of activa0 71 36,500 - 460 36,040 375 5 . 6 2 X 10-13 6.3 X 400 2 . 1 7 x 10-14 s 2 x 10-3 0 36 36,500 - 240 36,260 tion of 36,500 cal. per mole. 0.67 36,500 470 36,030 425 5 . 2 0 X 10-14 7 . 8 8 X 10-3 Table I1 shows a comparison 450 1 . 4 7 X 10-14 8.63 X 0 48 36,500 - 350 36,150 of the parabolic rate law con350 4 . 7 X 10-16 2 . 4 7 X 10-8 -25.6 22,800 16.000 38,800 Aluminum stants and diffusion constants 17,000 39,900 -25.4 zz.800 400 2 . 3 4 x 10-16 3.08 x 10-3 -26.2 22,800 19,000 41,800 5 . 2 9 X 10-15 2 . 2 X 10-8 450 and the entropies, energies, and free energies of activation of
July 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
(8) Gulbransen, E. A., Trans. Electrochem. Soc., 81, 327-339 (1942). (9) Ibid., 87, 589-699 (1945). (10) Ibid., 83,301-317 (1943). (11) Gulbransen, E. A , , and Hickman, J. W., Am. Inst. Minzng Met. Engrs., Inst. Metals Div., Metals Technol., 13, No. 7, Tech. Pub. 2068 (1946). (12) Gulbransen, E. A., Phelps, R. T., and Hickman, J. W., IND. ENG.CHEM..ANAL.ED., 18, 640 (1946). (13) Hickman, J. W., and Gulbransen, E . A.,Am.Inst. Mining Met. Engrs., Inst. Metals Div., Metals Technol., 13, No. 7, Tech. Pub. 2069 (1946).
(14) Lacombe, P., and Beaujard, L., “Etudes sur les aspects des pel-
licules l’oxydation anodique,” editions du Cornit6 General d’organization des Industries Mechaniques, 11 Av. Hoche, Paris, 1944. (15) Lustman, B., and Mehl, R. F., Trans. Am. Inst. Mhing M e t . Engrs.,Inst. Metals Div., Tech. Pub. 1317, 143, 246 (1941).
1391
Mott, N. F., J . Inst. Metals, 72, 367-80 (1946). Mott, N. F., Trans.Faraduv Soc., 3 6 , 4 7 2 (1940). Phelps, R. T.,Gulbransen, E. A., and Hickman, J. W., IND. EN@. CHEM.,ANAL.ED., 18, 391 (1946). (19) Pilling, N. B., and Bedworth, R. E., J. Inst. Metals, 39, 529
(16) (17) (18)
(1923). (20)
Riley, H. L., J. Oil & Cobur Chemists’ Assoc., 29, No. 308, 25-36
(21) (22)
Tammann, G., and Koster, W., 2. anorg. Chem., 123, 196 (1922). Wagner, C., and Griinewald, K., 2. physik. Chem., 40B, 455
(23)
Wilkins, F. J., Proc. Royal Sac. (London), 128A, 407-17 (1930).
(1945).
(1938).
RECEIVED June 1, 1948. Presented before the Division of Physical and Inorganic Chemistry at the 113th Meeting of the AVERICANCHBMICAL SOCIETY, Chicago, Ill.
Phosphatic Animal-Feed Supplement -
LABORATORY AND PILOT PLANT PRODUCTION G. L. BRIDGER’, J. W. MOORE2, AND H. M. McLEOD, J R . ~ Tennessee Valley Authority, Wilson Dum, Ala.
THE production of In an investigation of the utilization of by-product ferrofor direct feeding of domestic phosphorus by the elecphosphorus from the electric-furnace production of phosanimals should be not more tric-furnace process, all of the phorus, a process was developed through the pilot plant than 0.30% for cattle, 0.35% iron, manganese, and titascale in which a mixture of pulverized ferrophosphorus for sheep, 0.45% for swine, nium in the furnace charge and limestone is roasted in a rotary kiln. The product and 0.60% for poultry. AS are reduced to the elemental contains 19 to 20yo total P&, which is highly soluble in an emergency measure during state and combine with phos0.4% hydrochloric acid, and less than 0.10% fluorine. the war, the association recphorus and silicon t o form an The product is proposed for use as a phosphatic animalommended that defluorialloy known as ferrophosfeed supplement. nated phosphates containing phorus, which is tapped from not more than 1 part of fluothe furnace periodically. rine to 40 parts of phosphorus The ferrophosphorus usually contains 20 to 25% phosphorus. could be used for feeding; this emergency recommendation is still 65 to 75% iron, and the remainder is comprised principally of in effect ( 1 ) . Since ferrophosphorus contains only traces of fluosilicon, manganese, and titanium. I n the furnaces at the rine, mostly in the form of slag inclusions, i t was believed that Tennessee Valley Authority (TVA) Fertilizer Works about 6% ferrophosphorus might be converted to a product suitable as an of the phosphorus in the furnace charge is recovered in the form animal-feed supplement by oxidation in contact with a material of ferrophosphorus, although this percentage may vary, desuch as limestone or dolomite. The possibility of using such a pending principally on the iron content of the charge. product as a phosphatic fertilizer was considered also. Although there are a few uses for ferrophosphorus, such as for Preliminary experiments therefore were made in which pulalloying purposes in the steel industry, the production of ferroveriaed ferrophosphorus and pulverized limestone were reacted phosphorus greatly exceeds the demand, and this situation is at high temperatures. The phosphorus pentoxide contents of being accentuated by the rapidly increasing electric-furnace phosthe resultant products were soluble to a large extent in dilute phorus capacity in the United States. hydrochloric acid and in dilute citric acid, and fluorine contents During the war, there was a severe shortage of animal-feed were suitably low. An investigation was then made to define the grade phosphates; the principal requirements for these are: optimum conditions for the production of such a material and t o their phosphate content should be assimilable by animals and test its suitability for feeding and fertilizer purposes. their fluorine content low. The National Research Council LABORATORY MATERIALS, EQUIPMENT, AND PROCEDURE Committee on Animal riutrition ( L ) recommends that the maximum permissible level of fluorine in the total dry feed for cattle, The compositions of the principal materials used in the laborasheep, and swine be 0.003%, and for poultry, 0.015%. The tory experiments are shown in Table I, and their particle sizes Association of American Food Control Officials recommends are shown in Table 11. The principal specification for the limesomewhat higher permissible fluorine contents, as follows: The stone and dolomite was low fluorine content. I n most of the fluorine content in the ration should be not more than 0.009% experiments the raw materials were thoroughly mixed in the for cattle, 0.010% for sheep, 0.014% for swine, and 0.035% for desired proportions and spread in a layer 0.0625 t o 0.125 inch poultry; and the fluorine content in minerals or mineral mixtures deep in a shallow nickel tray (for the limestone-ferrophosphorus
IN
.
1 2 8
Present address, Iowa State College, Ames, Iowa. Present sddresa, Southern Alkali Corporation, Corpus Christi, Tex. Present address, Carbide and Carbon Chemicals Corporation, Oak Ridge,
Tenn.
tests) or stainless steel tray (for the dolomite-ferrophosphorus tests). The tray was inserted into an electric muffle furnace at the desired temperature, which was registered by a thermocouple
