540
ANDREW DRAVNIEKS
ACTION OF HOT IONIZED GASES UPON ZIRCONIUM AND COPPER ANDREW DRAVNIEKS’
Department of Cheviistry, Illinois Institute of Technology, Chzcayo, Illinois Reeetved M a y 1 , 1960 INTRODUCTION
In many combustion processes the combustion chamber walls are in contact with a gas phase containing free atoms, radicals, and ions, a+s evidenced by spectra and Conductivities of flames (8, 21, 26). These particles are intermediate by-products in the combustion and are short-lived. After the completion of the combustion these unstable particles disappear rapidly. An ordinary analysis of the combustion products describes the gas phase merely in terms of the concentration of carbon oxides, water, etc. The introduction of a synthetic flue-gas mixture into a space kept at the approximate temperature of the combustion chamber will fail to produce the same gas phase that exists during the combustion reaction, since only negligible amounts of ions, radicals, and atoms mould be produced; some change in composition due to the shift of thermodynamic equilibria between normal species with temperature would be the only result. Thus it is possible to distinguish two types of gas phase of the same apparent composition: one that exists during reaction and is rich in unstable fragments, and another formed from the first after the completion of the reaction. An adsorption of reacting gas on the surfaces is a necessary intermediate step in gas-metal interaction (10,22). The ions and radicals exhibit profound influence on adsorption. Consequently, the reaction rates of metals with the two types of gas phase may be different. The present study was undertaken to explore the direction and magnitude of these rate differences. The ionization of a gas or gas mixture by electric discharge generates an artificial ion-rich medium and mas used to produce the “reacting” gaseous media. An experimental survey showed that the rate effects caused by ionization were most pronounced with zirconium and copper. The present paper reports a series of measurements on these metals. LITERATURE
Zirconium is known to react with almost every gas. The literature evidence may be summarized briefly as follows (3,4, 5 , 6, 12, 14, 1.5): Oxygen is taken up by the metal to form a solid solution up to 38 atom per cent oxygen. With the solution of oxygen the metal becomes brittle, with little change in the lattice constant. If the oxidation is prolonged or rapid, an oxide film is also produced. Hydrogen is adsorbed below 400°C. and desorbed a t higher temperatures. Xitrogen forms a solid solution in the metal as well as a nitride film, but the reaction is generally much slower than with oxygen and does not occur in the Present address: Standard Oil Company (Indiana), Chicago, Illinois.
ACTION OF IONIZED GASES ON METALS
541
presence of oxygen in the gas. Oxides of carbon give off oxygen, which reacts with zirconium, and a t higher temperatures carbide formation is possible. The oxidation of copper has been studied extensively by a great many investigators. At 750°C. cuprous oxide is formed a t low oxygen pressures, and the rate obeys the parabolic law (1, 23, 24, 30) at large thicknesses of the oxide layer. The oxidation of copper in monatomic (so-called “active”) oxygen has been studied by the author (6). EXPERIMENTAL
The progress of the reaction between metal and gas was followed by measuring the change of electrical conductivity of metal strips, since this is the only method which permits following the reaction with a rapidly flowing low-pressure ionized gas in a semicontinuous fashion on a single specimen. The flow method must be employed to keep the condition in the gas phase constant. The technique is represented in figure 1. A ceramic (Lava A) body A with appropriate holes carries four connection screws (B) and two double-bore silicaglass tubes (C). A sample of metal 40 cm. in length and 0.2-0.3 cm. in width is cut in the form shown in D (not drawn to scale in the figure). During the experiment the resistance of the center part of the specimen is measured. The four strip-like leads of the specimen pass through the four channels of the silicaglass tubes and are clamped by screws B. Flexible Formvar-enamelled copper wires connect the screws to tungsten lead-in wires sealed through the glass side arms in the stopper E, which has a standard-taper joint. An iron rod (F) and an external magnet enable one to introduce the assembly into the hot zone in the furnace (€1) at a predetermined moment. The furnace temperature is kept within f 2 ” C . by a chromel-alumel thermocouple (I) in combination with a Micromax S temperature controller-recorder. The thermocouple is surrounded by a grounded chrome1 spiral to avoid disturbance in the control circuit by stray currents arising during the ionization of the gas. The gas is pumped continuously through the silica-glass tube by a Megavac vacuum pump, and the flow rate is read on the Amoil flowmeter (J) and regulated by a Hoke highvacuum needle-valve (K). To ionize the gas, a Tesla circuit shown schematically as L is connected to two outside metal sheet electrodes (M). The circuit for recording the changes in the resistance of specimens is shown in figure 2. A storage battery (A) supplies a current of approximately 0.15 amp. through the specimen B. The potential drop across the specimen is recorded by a Brown Electronik multipoint recording potentiometer (C). To account for small drifts in the current strength, the potential drop across a series resistance (D) is also recorded. Thus, a record of the change of resistance of the center part of the specimen in the course of the reaction is obtained. When a reaction in the ionized gas is observed, the ionization is interrupted for a brief moment to connect the potentiometer circuit and make a reading. From records the decrease of the conductivity is calculated in terms of the fraction of the initial conductivity. In experiments with zirconium, the resistance of the specimen increases even
542
ANDREW DRAVNIEKS
in the best attainable vacuum if the walls of the vessel are kept hot (13);the high gettering act,ivity of this metal is well known. To obtain an initial value for each experiment, the initial procedure was arbitrarily standardized. The specimen was introduced into the reaction zone, and after 10 min. of heating in
c
D
A
D
Pump
-
Manomet e r
Gas Inlet IO inches
FIG.1. Experimental system
vacuum the gas flow was started and readings begun. The specimen came to the temperature of the furnace within 4 min. after introduction. Electrolytic copper sheets of 0.005 cm. thickness were used. Zirconium sheet, 0.0125 cm. thick, was obtained from the Foote Mineral Company. A typical analysis of the zirconium shows: 2.5-3 per cent hafnium; 0.04 per cent iron; 0.03 per cent oxygen; 0.01 per cent nitrogen; balance, zirconium.
ACTION OF IOKIZED GASES ON METALS
543
Commercially pure oxygen, nitrogen, ethane, carbon monoxide, helium, and medical-grade carbon dioxide were used. These gases were investigated a t 0.6 f 0.02 mm. pressure. Water vapor was generated from carefully de-aerated water and studied a t 0.3 mm. pressure, since higher pressures could not be reached owing to the experimeptal limitations inherent in the dimensions of the system. At the temperatures used the vessel walls are not absolutely impermeable to the atmospheric gases; therefore special purification methods were considered purposeless (13).
I6 II
I
F I G .2. Schematic representation of recording circuit RESULTS
The results of the experiments with zirconium are shown in figures 3 , 4 , and 5. In figures 3 and 4 the ordinates are squares of the loss of the conductivity; this plot is used to reduce the curvature and has a theoretical basis in the parabolic law of film growth and diffusion of gas into a metal (18).Each curve represents the average of duplicate runs, except where no difference in rates was observed between the nonionized and the ionized gases. The conductivity may change 'as the reaction progresses because of (1) the formation of a solution of oxygen or other gas or element in the zirconium, the conductivity of solid solution being very considerably smaller than that of pure zirconium, and (8) the conversion of the metal phase to practically nonconducting oxide scale. In the reactions of zirconium with nonionized carbon dioxide, carbon monoxide, ethane, and technical helium, which is essentially a mixture of traces of oxygen and other gases with helium, only a slight discoloration of metal was observed. In other reactions a thin black oxide layer was formed during the periods of time indicated in the figures. From cross-section it was observed that the oxide film never exceeded a few per cent of the total thickness of the strip. No significant differences in the thicknesses of these films were caused by ionization of oxygen, air, and nitrogen containing 0.5 per cent oxygen.
544
::m
ANDREW DRAVNIEKS
Thus the curves describe chiefly the conductivity change caused by solution of oxygen or some other element, and rates of solution are expressed on a relative scale by these curves. The minor corrections due to the fraction converted to 06
0.4
0 2
0.2
50
0
100
50
0
100
50
0
100
Ethane
0.2 0.4
O 001
0.I
0
o
2
m
0.2
0
100
50
100 '0
50
'0
50
100
FIG.3.
FIG.4. FIG.3. Oxidation of zirconium a t 986°C. Abscissas: time of oxidation in minutes; ordinates: (conductivity loss)*, (fraction)*. A , ionized gas; 0, nonionized gas. FIG.4. Action of several gases upon zirconium. Abscissas: reaction time in minutes; ordinates: (conductivity lous)', (fraction)*, A , ionized gas; 0 , nonionized gas.
30
0.6 3
a
a
0
3
0.5
t t
z
0
Y
0 0.4
t
2t 20
z
0
0
a
40
w w
0.3
2
vi
4 00
s 0.2 t
t
0.1 0
0
0
20
40
60
T I M E , MINUTES
BO
100
*'
TIM:
60
MINUTES
80,
FIG. 5 (Left). Action of several nonionized gases upon zirconium a t 986°C. Pressure of water vapor, 0.3mm.; pressure of other gases, 0.6 mm. FIG. 6 (Above). Oxidation of copper in several gases at 750°C. and 0.6 mm. pressure. A, ionized gas; 0, nonionized gas.
the oxide film may be disregarded, as they would change the slopes of both curves to be compared and the relative positions of the curves would not be significantly influenced.
ACTIOS OF IOSIZED GASES OX METALS
545
The oxidation of copper is shown in figure 6. Here the resistance changes are caused only by the formation of a cuprous oxide film, which is practically nonconducting when compared with the metal. The thickness of the copper consumed may therefore be calculated from the conductivity changes, and the parabolic plot shows the approximate adherence to the parabolic film-growth law (23): y2 = kt
Here y is the thickness of metal phase converted to oxide phase, k is the rate constant, and t is the time counted from the start of the oxidation. The is denoted as penetration. Some additional experiments are described in the discussion. DIfiCUSSION
The ionization of a gas or gas mixture may cause two effects as far as the reaction rate with a metal is concerned. First, the rate of reaction may be increased either by a preferential adsorption of the reactive atoms or ions on the surface of the metal or oxide film, or by an increase in the amount of the reacting component: e.g., oxygen produced by the dissociation of carbon dioxide in the electric discharge. Second, the rate of reaction may be decreased by a preferential adsorption of the nonreactive components of the gas phase. The ions are strongly adsorbed because of the large interaction forces between charged ions and the electric fields of solid crystalline surfaces. Even rare gas ions may be active and, before their charge is lost,.form compounds in the surface phase (9). In the case where the adsorbed ions are nonreactive with the metal, they may displace reactive components frsm the adsorbed layer in the surface. A similar mechanism is important in vacuum technique, where, in evacuation of the vacuum systems, ionizing the gas in the system by a glow discharge considerably accelerates the clean-up of adsorbed gases from the walls. The curves presented in figures 3 to 6 illustrate some of these cases, as ifell as cases where the reaction rate is not affected by the ionization. In pure oxygen the ionization has no influence on the rate of oxidation of zirconium. The oxidation of this metal is known to be relatively insensitive to pressure changes of oxygen, and proceeds rapidly even at extremely low concentrations of oxygen in the gas phase (13), as indicated by the use of zirconium as an efficient getter. Thus the increased supply of oxygen ions and atoms fails to bring about an accelerated oxidation. The ionization seems not to be the rate-limiting reaction in the oxidation. The supply of oxygen atoms and ions is already plentiful in normal oxygen. Similar is the case with zirconium in ionized air. Contrary to the case of zirconium, the oxidation of copper is sensitive to changes in oxygen concentration (31). Ionization of the gas produces a slight increase of the oxidation rate in pure oxygen (figure 6). It has been found that oxygen atoms under certain conditions can cause an accelerated oxidation of
546
ANDREW DRAVNIEKS
copper (6) and a similar mechanism is probably effective also in ionized oxygen. I n low-pressure air, the concentration of oxygen atoms is kept down by the presence of nitrogen and the formation of nitrogen oxides; hence there is no change in the oxidation rate on ionizing the air. I n nitrogen with 0.5 per cent oxygen only oxygen diffuses into the zirconium and forms a very thin oxide film on the surface. It has been shown (11) that if the oxygen content in nitrogen exceeds 0.001 per cent, only oxygen reacts with zirconium. I n the ionized mixture the nitrogen ions obviously produce a clean-up effect on the surface, displacing oxygen, and the oxidation is retarded (figure 3). The thin f ilm of the reaction product was identified as oxide. Thus the ionization did not cause the nitrogen in this mixture to react with zirconium. Technical helium is essentially a mixture of small amounts of oxygen, nitrogen, and hydrogen with helium. The reaction product film which was formed after prolonged interaction was very thin and violet-black in color, and could not be identified as oxide, nitride, or any definite compound. Whatever the reacting component of the gas may be, the helium ions generated by the discharge again partially displace this reacting component from the surface and decrease the rate of the reaction (figure 4). A solid solution of oxygen in zirconium and a thin (below 1 micron) film of oxide were products of the interaction with both nonionized and ionized water vapor. I n nonionized gas the oxygen may be obtained from either phase (where it results from thermal dissociation of mater vapor) or directly from the water molecules adsorbed on the surface. The oxidation in ordinary water vapor is faster (figure 5) than in nitrogen with 0.5 per cent oxygen and in pure nonionized carbon dioxide, although the partial pressure of oxygen formed by the thermal dissociation of water vapor is much smaller than in the two other gases. Therefore it seems reasonable to assume that the main source of oxygen taken up by zirconium is from the water molecules adsorbed on the surface. In ionized water vapor, hydroxyl radicals, HzOt ions, and hydrogen peroxide have been detected, together with an enhanced production of oxygen and hydrogen by a dissociation in the electric discharge (2, 19, 20, 25, 28). The ions and radicals again interfere with the mechanism of supplying oxygen to the metal and to the oxide film, as evidenced by the retardation of the conductivity loss produced by ionization (figure 4). The interference is most probably via the preferential adsorption of less reactive species, which displace water and eventually oxygen from the adsorbed layer. The increased concentration of oxygen in gas dissociated by the discharge is apparently of no influence. This supports the view that the concentration of the reactive component in the adsorbed layer is more important than the concentration in the gas phase. The oxidation of zirconium in nonionized carbon dioxide is surprisingly slow as compared with water vapor and with carbon monoxide. In ionizing discharge the carbon dioxide molecule is split and oxygen and carbon monoxide produced, together with various ions (7, 16, 17). In ionized carbon dioxide the rate of oxidation is considerably faster, as expected from the presence of larger concentrations of oxygen supplied by splitting. Retarding influence of any kind is not evident. The rate approaches that in low-pressure air. To ascertain whether
ACTION OF IONIZED QASES ON METALS
547
some definite ions could be responsible for the accelerated oxidation, two samples of zirconium were o x i d i d in ionized carbon dioxide 0.5 cm. apart and with a 180-v. D.C. potential dserence applied between them. Both samples increased in weight similarly, so that the rapid oxidation in ionized carbon dioxide is not caused by any definite charged particles. Copper is not oxidized in nonionized carbon dioxide, but is oxidized slowly in the ionized gas, again by oxygen produced in the ionization. In the ionization of carbon monoxide, carbon and oxygen are among the products formed (9, page 231). During experiments with ionized gas a visible film of carbon is produced on the walls of the tube but not on the specimen of zirconium. One would expect that an acceleration would result from ionization rather than the retardation which actually occurred (figure 4).In this case either a clean-up effect on the surface by nonreacting ions is indicated, or the carbon plays a retarding role. Since the surface of the specimen was free from any visible film, the role of carbon is not simply the blocking of the surface macroscopically. Change of resistance of the zirconium in nonionized and ionized ethane was extremely slow, with no visible film produced. Only a very slight discoloration of the metal was observed. No brittleness resulted from the interaction. Hence the carbide formation is negligibly slow at the temperature of the experiment. A specimen heated in ethane for 2 hr. reacted with low-pressure air as rapidly as did fresh zirconium. Hence a carbide or a solid solution of carbon in zirconium (14)either are not produced or have no retarding influence on the diffusion of oxygen into the metal, at the conditions employed.* Thus none of the types of carbon influence considered so far can account for the slow reaction of zirconium with ionized carbon monoxide. To explain the retardation caused by the ionization one is compelled to assume either ( 1 ) a clean-up mechanism by ions on the surface or (2) interference by carbon atoms or ions adsorbed on an atomic scale rather than in the form of a definite phase. Copper is not oxidized in ionized carbon monoxide nor in nonionized or ionized nitrogen containing 0.5 per cent oxygen. This nitrogen has an estimated pressure of lW6 atm. of oxygen in the experimental system. This figure is well above the equilibrium preasure of oxygen (10-1°atm.) in the copper-cuprous oxide system? A possible explanation may be the difficulty in nucleation of the first lattice cells of cuprous oxide on the metal, in the sense of the considerations of Volmer (29): the chemical potential of oxygen in the nuclei of cuprous oxide may be higher than that of the oxygen in the surrounding gas, so that nuclei if formed disappear again. From the absence of changes in the rates with ionization in some cases it may be concluded that the heating effect of the electric discharge is not a factor in the change in reaction rate at the conditions of high temperature employed. In conclusion, positive or negative rate changes in the interaction of ionized The slow increase of resistance in ethane may be partly due to the unavoidable traces of impurities such as oxygen in the gas, and not necessarily to solution of carbon in zirconium or t o carbide formation. a Calculated from free energies; data from Thompson (27).
548
AXDREW DRAVXIEKS
gases with zirconium and copper may be caused by the changes occurring in the layers of gases adsorbed on metals or scales. The changes in the reaction rates observed vary by a factor of 2, a t most. With the present state of knowledge concerning composit'ions of the ionized gases it is not yet possible to make any quantitative correlations between the concentration of the unstable species in the gas phase and the rate of interaction of the ionized gas and metal. SUMMARY
By measuring changes in electrical resistance the relative reaction rates of zirconium at 986°C. with low-pressure nonionized and ionized oxygen, air, water vapor, carhon dioxide, carbon monoxide, nitrogen containing 0.5 per cent oxygen, ethane, and technical helium were studied. Io some cases an acceleration and in others a retardation of the reaction upon ionizing the gas was observed. The retardation may be explained by a clean-up effect on the surface of the metal or reaction film by the nonreacting components of the gas phase. The oxidation in water vapor seems abnormally fast as compared with the oxidation in oxides of carbon or in nitrogen with 0.5 per cent oxygen. The oxidation of copper in both forms of gases was studied at 750°C. The ionization of carbon dioxide causes a slow oxidation of copper, which proceeds accordingly to the parabolic law. Copper does not undergo oxidation in nonionized and ionized carbon monoxide. A method of recording continuously the progress of the oxidation of metal strips is described. The research work reported in this paper was done under contract with the
U. S. Navy, Ofice of Naval Research. The author wishes to acknowledge the assistance of Dr. D. D. Cubicciotti in the preparation of the manuscript. REFEREKCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)
BARRER, R . M . : Diflusion i n and through Solids. University Press, Cambridge (1911 J. BARTON, H. A , , A N D BARTLETT, J. H . , JR.: Phys. Rev. 31, 822 (1928). CUBICCIOTTI, D . D . : J. Am. Chem. SOC.72, 4138 (1950). DE BOER,J. H . , A N D FAST,J. D . : Rec. trav. chim. 66, 456 (1936). DE BOER,J. H . , A N D FAST,J. D . : Rec. trav. chim. 69, 161 (1940). D R A V N I E KAS, :, J . Ani. Chem. SOC.72, 3761 (1950). FISCHER, F . , KUESTER, H . , A N D PETERS, K . : Brennstoffchem. 11, 300 (1930). GAYDON,A. G . : Proc. Roy. SOC.(London) A l a , 111 (1944). GLOCKLER, G., A N D LINU,S. C . : The Electrochemistry of Gases and Other Dieleclrzca, p. 424. John Wiley and Sons, Inc., New York (1939). GULBRANSE E.NA , . : Trans. Eleetrochem. SOC.I, 301 (1943). GULBRANSEN, E. A , : Paper presented at the Symposium on the Solid State, which was held under the auspices of the Division of Physical and Inorganic Chemistry of the American Chemical Society in Pittsburgh, Pennsylvania, June 20-22, 1949. GULBRANSEN, E. A , , A N D ANDREW,K . F . : J. Metals I, No. 8, 515 (1949). GULBRASSEN, E. A , , ASD ANDREW, K . F . : Ind. Eng. Chem. 41, 2762 (1949). GULDNER,W. G . , . ~ S DWOOTEN,L. A , : Trans. Electrochem. SOC.93, 223 (1948). H I C K M AJ. N ,W . , A N D GULBRANSEN, E. A , : Anal. Chem. 20, 158 (1948). HUNT,H . , A N D S C H W BW , . C . : J. Am. Chem. SOC.62, 3152 (1930).
ADSORPTION OF ORQANIC COMPOUNDS ON STEEL
549
(17) JOLIBOIS,P . : Compt. rend. 199, 53 (1934). (18) JOST, W.: Diffusion und chemisehe Reaktion in festen Slogen, p. 16. Photo-Lithoprint, Edwards Brothers, Ann Arbor, Michigan (1943). (19) LAVIN,G . I . , A N D STEWART, T. B.: Proc. x a t l . Acad. sci. U. s. 16, 839 (1929). (20) LISDER,E. G . : Phys. Rev. 98, 679 (1931). A. E . , A N D LAVROV, F. A , : 2.Physik 69, 690 (1930). (21) MALINOVSKI, (22) MOTT,N. F.: Trans. Faraday SOC. 86, 472 (1940). R . E.: J. Inst. Metals 29, 529 (1923). (23) PILLIKG,N. B., A N D BEDWORTH, (24) RHISES, F. K.,JOHNSON, W. A , , A N D ASDERSON,W. A , : Trans. .4m. Inst. Mining hlet. Engrs. 147, 205 (1942). (25) RODEBUSH, W.H., A N D WAHL,ILI. H . : J. Chem. Phys. 1, 696 (1933). (26) STVECKLEN, H.: Handbuch der Physik, Vol. XIV, p . 108.J. Springer, Berlin (1927). (27) THOMPSON, M.DE K A Y :The Total and Free Energies of Formation of the Ozides o j Thirty-Two Metals. The Electrochemical Society, Inc., New York (1942). (28) UREY,H.C., A N D LAWN,G . I.: J. Am. Chem. SOC.61, 3286,3290 (1929). (29) VOLUER,M.: K i n e t i k der Phasenbildung. Photo-Lithoprint, Edwards Brothers, A n n Arbor, Michigan (1943). (30) WAQNER,C., A N D GRUENEW~ALD, K.: z. physik. Chem. BM, 455 (1938). (31) WILKINS,F. J . , ASD RIDEAL,E . K.: Proc. Roy. SOC.(London) 128. 394 (1930).
ADSORPTION OF POLAR ORGANIC COMPOUNDS ON STEEL' E. L. COOK2
AND
X O R M S S HACKERM.4S
Department of Chemistry, University o j Texas, Auslin, Teras Received April 87, 1950
Adsorption from solution of polar organic compounds on metal surfaces has been studied by numerous investigators because of its importance in corrosion prevention and in lubrication. Rhodes and Kuhn (7) studied the corrosioninhibiting character of acridine and some of its derivatives and found that the higher-molecular-weight compounds of the same homologous series were generally better inhibitors against acid attack of metal, presumably because of greater extent of adsorption. Mann, Lauer, and Hultin ( 5 ) demonstrated the inhibition of acid attack on steel with mono-, di-, and tri-alkylamines containing from one to five carbon atoms in the alkyl group. They found that both increasing the number of substituent radicals and increasing the chain length of the radical improved the effectiveness of the inhibitor. Zisman (11) showed that fatty acids, the more basic amines, and any other polar molecules capable of ionizing at an oil-water interface were many times more adsorbable than were alcohols, esters, ketones, or other molecules not capable of ionization at such an interface. Later work by Bigelow, Glass, and Zisman (1) showed these same large differences in adsorptivity at oil-metal interfaces. Presented before the Division of Colloid Chemistry a t the 115th National Meeting of the American Chemical Society, which was held in San Francisco, California, March, 1919. Present address : Magnolia Petroleum Company, Dallas, Texas.
'
