829When ~ ~ 1, the Tafel slope approaches kT/fiq and the parameters of the metal-oxide interface are the rate-determining ones. While, if ~ ~ 1, the rate of oxidation is determined almost entirely by the bulk properties of the film, and the space charge contributions are all important. Therefore, the half width, given in Table II, should be intermediate ones between a and ft. However, when the rate-determining mode approaches the Cabrera and Mott picture at the high temperature, a condition of U--a//~ ~ 0, pointed out by Dewald, should hold, where U is the activation energy for diffusion through the film, and r the energy for entrance into the film.Assuming ~b ----1.02 e.v. and U = 1.47 e.v., the terminal values of half width which are consistent with U --a/fl~ ~ 0 are nearly equal to 1.6 and 1.2A, so that kT/flq was expressed, in Fig. 4, by a dotted line nearly passing the plot of 70~ and another dotted line was assigned to kT/aq. Thus, with the data obtained from 12 ~ to 70~ the energy parameters, U, r a, and /~ of Dewald's model have the values 1.47 e.v., 1.02 e.v., 1.6A, and 1.2A, respectively.The data at 85~ seem to be rather anomalous, since the resulting value of X is about 0.9A, which is too small to visualize. The reason for deviation at 85~ is not entirely clear, but probably attributed to disturbances by side reactions and erroneous measurement.In general, the ionic conduction in the titanium oxide film is much greater than those of Ta and A1 at the same temperature, so that the large film thickness would result. Therefore, further experiments at the much lower temperatures are desirable to investigate the kinetics of the film growth at high fields. ABSTRACTThe kinetics of the oxidation of ingot iron in 19 mm O,~ has been studied over the temperature range of 260~176 Both Fe~O4 and a-Fe.~O.~ were observed to be present at all temperatures and times. Initially Fe~O4 grows to reach a value which becomes constant for some time and then starts to increase again. When the Fe~O4 layer recommences growth there is also an increase in the over-all oxidation rate. The rate constant for the early part of the oxidation is about half that of the latter part. In all cases the total oxidation rate is essentially parabolic.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.248.155.225
The annealing reaction in vacuum of thin films of magnetite and hematite on iron has been studied at 463 ~ The gravimetric curves obtained for growth of magnetite and decay of hematite have been compared with the curves of growth of both during oxidation at 5.75 cm pressure of oxygen. The flow of iron through the magnetite during annealing is one fourth its value during the oxidation.A preceding paper (1) has described a technique of measuring curves of gain in weight, as a function of time for the growth of magnetite and hematite on iron heated in oxygen at temperatures between 268 ~ and 463~The sensitivity was such that studies could be made on thin films ranging in thickness from 100 to 20,000A. The resulting data were displayed as three curves at any one given temperature, one each for total oxide, hematite and magnetite. It was obvious from the results that the reactions of growth were complex, with~the major product changing with time from hematite to magnetite and back again.In the oxidation there is a flow of both oxygen and metal to the site of reaction. If, after an oxide layer has been established, the flow of oxygen is shut off by pumping the gas from the reaction vessel, then the iron may continue to flow to the reaction zone. A possible result is the following reactionThis may be a complex reaction in itself but for the present purposes can be considered elementary. There is strong evidence (2, 3, 4) in the literature for the occurrence of the reaction and some measurement of its rate, but the technique used here of measuring the splitting of the whole film into its component layers offers a unique opportunity to study the annea]ing action directly and to measure its rate and compare it with the over-all rate. Experimental ProcedureThe method and materials for experiments of this type have been fully described in (1). It is sufficient to say here that weighed specimens of pure iron are heated in oxygen after reduction in hydrogen; after definite times of oxidation they are "quenched" rapidly to "freeze" the ratio of the oxides and reweighed. The gain in weight is the weight of the whole film; subsequent analysis for hematite by a method of electrolytic reduction enables the weights of the component layers of the duplex films to be determined.In the particular set of experiments described here a group of oxidations at 463~ and 5.75 cm pressure of oxygen were each stopped at the same time, 100 rain, by rapidly pumping out the oxygen; they were then kept at that same temperature in the vacuum for different spedified times of annealing before being rapidly quenched to room temperature by immersion of the quartz reaction cell in cold water. Subsequent analysis showed the change caused by the annealing in the ratio of the thicknesses of the layers while similar oxidations witht 463~ -OXYGEN PRESSURE -5.75 crn/Hg. o so 4soo ~ o o~ / . ~ oAo~ ~' o ,~o ,~o ~Go MINUTES Fig. 1. Weight of oxygen vs. time for oxidation at 463~ and 5.75 cm/Hg pressure of oxygen. Upper curve, total oxygen; middle curve, oxygen as ...
An electron ~nicrographic study has been made of iror~ s~~rfaces subjected to electropolishing. I t has been found that although electropolishing produces a relatively sn~ooth surface, the iron is covered with a film. The strengthening of this film by oxidation and its disruption by reduction with hydrogen have been followed. T h e film is assumed to be oxide and its thicktiess has been estimated to be approximately 100 Angstrom ~~n i t s .The effect of reduction of the film on surface area and thus on initial oxidation rates of the reduced specimen has been considered.
I t is assumed that the free energy of a thin film of iron oxide is a function of its thickness. The effect on the dissociation pressure is calculated and the condition for stability of thickness is stated. A possible method for measuring the free energy change involved in oxide film formation on metal surfaces is presented which is based on measurements of the e.m.f. of the dry cell metal (oxide(oxygen. INTRODUCTIONAt ordinary temperatures and pressures the system iron+oxygen is unstable and will go spontaneously to one or more of the various iron oxides. For example, the equilibrium dissociation pressure of a Fe203 is about 10-80mm. pressure of oxygen. Hence, when the process of oxidation halts it is natural to assume t h a t kinetic conditions have imposed a very slow speed on the process, slow enough to give the appearance of a halt.The influence of resistance to change can be seen very clearly in the formation of iron oxide films on massive iron a t pressures of around, say, 3 cm. oxygen and a t temperatures around 370°C. In this region the clear-cut result is obtained, by a variety of methods, t h a t the rate of increase of the film thickness is inversely proportional to the thickness. The most reasonable interpretation of this result is t h a t the rate is controlled by the diffusion of the reagents through the film. Subsidiary experiments on transport processes, such as conduction, in oxides have checked this conclusion directly.This simple diffusion mechanism holds over a very wide range of film thicknesses-under the conditions mentioned above, from 800 A up t o thicltnesses of the order of fractions of a millimeter where the film begins t o lift:from the metal and to scale off the surface. The range from 0 to 800 A behaves anomalously, not following the diffusion law. Fig. 1 shows an oxidation for which the diffusion law is valid; the weight increase varies linearly with the square root of the time which follows from the varying of the rate of increase with the thicltness. Fig. 2 can be regarded a s a "blown-up" view of the anomalous region in Fig. 1; the data were obtained from observation of interference colors and the ordinate is plotted directly as Angstrom units since the interference colors can be correlated with film thickness. I t is seen that the anomalous region has been divided into two stages. At any rate, from Fig. 1 it is clear that given a certain thickness iron oxide will limit the rate of its own further growth. I t is tempting t o apply this conclusion to the data on iron oxidation a t room temperature. At room temperature and below, iron will take up oxygen to form a film in the manner shown in Fig. 3. I t is seen that the rate slows 'Manuscript received September 16, 1954. Contvibution from Division of Applied Chemistry, National Research C O Z L ? L C~~, Ottawa, Canadn. Issued as N.R.C. No. 3495. This paper was presented at the Symposirtm on Problents Relating to the Adsorption of Gases by Solids, held at Kingston, Ontario, September 10-11, 1954. Can. J. Chem. Downl...
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