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Effect of oxygen pressure and experimental method on the high temperature oxidation of pure Fe
Corrosion Science, 1970, Vol. 10, pp. 1 to 8. Pergamon P r c ~ Printed in Great Britain
EFFECT OF O X Y G E N PRESSURE A N D EXPERIMENTAL METHOD ON THE HIGH TEMPERATURE OXIDATION OF PURE Fe* D . CAPLAN, M . J. GRAHAM a n d M . COHEN Division of Applied Chemistry, National Research Council, Ottawa, Canada Abstract--Thermogravimetric measurements were carried out on annealed and cold-worked Fe at 500°C in O3 at 10 and 760torr. Runs were started by bringing the specimens rapidly to temperature in 02 or by admitting O2 to hot H2-reduced specimens maintained free of oxide in ultrahigh vacuum. Less oxidation occurred at 10torr than 760 owing to greater separation between oxide and metal. Still greater separation developed in the hot-bare type of experiment and oxidation was correspondingly slower. Cold-worked Fe showed separation and slower oxidation at 10torr but only after the oxide grew thick enough. The explanation proposed is that plastic deformation of the oxide is greater at 760torr causing collapse of the voids that form by condensation of cation vacancies at the Fe~O(-Fe interface. Oxidation is faster because there is less separation to hinder transfer of metal into the oxide. Oxidation of cold-worked Fe is the same initially at both pressures because extra vacancy sinks are present to suppress nucleation of voids; oxidation at 10torr becomes relatively slow subsequently when the thickening oxide has developed sufficient hot strength to resist the squashing effect of 10torr but not of 760torr. R6sumg---Des mesures thermogravim6triques ont 6t6 effectu6es h 500°C dans 03, sous 10 et 760torr, sur du fer recuit et sur du fer 6croui. Ces essais ont d6but6 en amenant rapidement les ~hantillons ~t temp6rature, en pr6sence de 02 ou en introduisant 02 a u p r ~ d'~chantillons r6duits par I-~ chaud et maintenus ~ l'abri de l'oxydation dans un vide pouss6. I1 se produit une oxydation plus faible ~t 10tort qu'/~ 760torr, ~t cause d'une plus grande s6paration entre l'oxyde et le m6tal. Une s6paration plus grande encore se produit lors des essais avec r6duction h chand, et l'oxydation est plus lente encore. Le fer 6croui montre une s6paration et une oxydation plus lentes ~. 10torr, reals seulement apr~s une certaine croissance de l'oxyde. L'explication propos6e est que la d6formation plastique de l'oxyde est plus importante/t 760tort, et cause une ouverture des vides f o r m s par condensation de lacunes eationiques ~t l'interface FesO4-Fe. L'oxydation est plus rapide parce qu'il y a moins de s6paration pour emp~cher un transfert de m6tal dans roxyde. L'oxydation initiale du fer 6croui est identique aux deux pressions, car des pr6cipitations en dehors des lacunes emp6chent la n u c l ~ t i o n des vides; l'oxydation ~t 10torr se ralentit quand l'6palssissement de l'oxyde d6veloppe suffasarnment de tension/t chaud pour r6sister/t l'6crasement sous 10torr mais non sous 760tort. Zusaramenfassung--Die Oxidation yon kaltverformtem und yon gegliihtem Eisen wurde thermogravimetrisch bei 500°C in O3 bei 10 und 760 Torr untersucht. Der Versuchsbeginn erfolgte teils dureh schnelles Aufheizen der Proben in O2 oder durch Zulassen yon Sauerstoff zu Proben, die zuvor mit Wasserstoff yon Oxidsehichten befreit und in Ultrahoch-Vakuum gehalten wurden. Der Umfang der Oxidation war bei 10 Torr geringer als bei 760 Torr, was auf unterschiedliche Abl6sung der Oxidschicht vom Metall beruhte. Die wasserstoff-reduzierten Proben zeigten starke Abl/Ssung der Oxid. schicht. Ihre Oxidationsgeschwindigkeit war entsprechend gering. Kaltverformtes Eisen zeigte bei 10 Torr Abl/Ssung der Oxidschicht und geringe Oxidationsgesehwindigkeit, wenn die Oxidschicht dick genug geworden war. Diese Befunde werden damit erkl~t, dab bei 760 Tort eine starke plastisehe Deformation der Oxidsehicht eintritt, wodurch die H o h l r i u m e zusammengedriickt werden, welche sich durch Kondensation von Kationenleerstellen an der Phasen grenze FesO(-Fe ausbilden. Die Oxidation ist dann schnell, weii die Hohlr~iume den Obertritt yon Metall in die Oxidschicht behindern. Die Geschwindigkeit der Oxidation des kaltverformten Eisens ist bei beiden Driicken zu Versuehsbegirm die gleiehe, well die Kaltverformung zusitzliche Leerstellensenken sehafft, welche die Ausbildung von Hohlr/iumen unterdriicken. Bei I0 Torr wird die Oxidationsgeschwindigkeit sp~ter jedoeh verh~iltnism~il3ig gering, weil dann die Dicke der Oxidschicht ausreicht, u m dem Gasdruck yon 10 Tort geniigenden Verformungswiderstand entgegenzusetzen. *Manuscript received 10 December 1968.
1
2
D. CAPt.AN,M. J. GRAHAMand M. Conm~
INTRODUCTION A RECiter publication x and two subsequent notes 2,a have described the effect of cold work on the oxidation of Fe. Below 600°C oxidation was found to increase, and oxidemetal contact improve, the greater the degree of cold-work. It was proposed that cold-worked Fe provided extra sinks for the cation vacancies diffusing to the Fe surface through the growing Fe304. Annealed Fe contained too few sinks so that the vacancies condensed into voids* at the F%O4-Fe interface and slowed oxidation by hindering transfer of Fe into the oxide. The main purpose of the present work was to observe the effect of a lower O2 pressure. It was anticipated that at reduced pressure, plastic deformation of the oxide might be less so that interfacial detachment between the oxide and retreating metal would be enhanced and oxidation thus slowed. A second purpose was to determine how the reaction kinetics would be affected if oxidation were initiated by admitting O2 to hot oxide-free Fe (hot-bare) rather than by bringing the specimen to temperature in 02.
EXPERIMENTAL
Specimen preparation Specimens 1 × 5 x 0.02cm were cut from zone-refined ultrapure Battelle Fe (99.997~o) cold-rolled sheet containing 4ppmC, 1Mn, 10Si, 1Cu, 3Ni, 5C0, 3Cr, and 7Mo. Details of procedure for specimen preparation have been described, x,~,4 The sequence is to electropolish, anneal in vacuum or Ar, and electropolish again. In some runs, prior oxide was then removed by reduction in H2. 4 Cold-worked specimens 2 were prepared by abrading the annealed and electropolished sheet with 600-grit SiC followed by 6~ diamond and cleaning ultrasonically in organic solvents.
Procedure The prepared specimen was suspended from an automatic microbalance sealed in a bakeable ultrahigh vacuum apparatus. 4 In one procedure (hot-bare) the specimen was reduced in H2 to remove the prior oxide, the H2 pumped out, temperature adjusted to 500°C, and 02 admitted at 10torr or 760torr to the hot oxide-free specimen. 4 In a second procedure (furnace-raised) an annealed or cold-worked specimen already in I0 or 760torr 02 at room temperature was brought to 500 ° by quickly raising the hot furnace around the quartz hangdown tube. Runs were ended by lowering the furnace. Correction factors to establish the hot weight at zero time were checked by facsimile runs and by weighing before and after at room temperature. In a variation (cold-insertion) of the second procedure a different apparatus was used in which 02 flowed up through a vertical furnace tube open at the top. Experiments at latm only were possible. An annealed or cold-worked specimen was lowered abruptly into the hot zone and quickly connected to an automatic balance. 1 Specimens reached temperature somewhat faster than in the furnace-raised procedure -- 30s vs. 3rain. Table I gives a summary of the oxidation runs. *Such voids or cavities have previously~.3 been termed pores. Henceforward, the term "pore" will be reserved to describe perforations through the oxide, as pores in skin.
The high temperature oxidation of pure Fe
3
RESULTS F i g u r e 1 shows the o x i d a t i o n curves o f a n n e a l e d specimens at the two p r e s s u r ~ for the different o x i d a t i o n procedures. I t is evident t h a t after the first h a l f h o u r , o x i d a t i o n is slower at 10torr t h a n a t 760torr for b o t h the h o t - b a r e (curves l ' a n d 2) a n d furnace-raised p r o c e d u r e s (curves 3 a n d 4) a n d the 10torr curves show a p r o n o u n c e d wave. A t 760torr, b o t h furnace-raised a n d cold-insertion o x i d a t i o n are faster t h a n h o t - b a r e (curves 4 a n d 5 vs. 2). F i g u r e 2 shows t h a t the o x i d a t i o n o f c o l d - w o r k e d F e u n d e r the furnace-raised p r o c e d u r e is as fast at 10torr as at 760 for the first 40rain - - 0.7mg/cm = or 5~t o x i d e - then slows c o n s i d e r a b l y (curves 7 a n d 8). C o l d - i n s e r t i o n (curve 6) is slightly slower
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FIO. l. Oxidation of annealed Fe at 500oC in O=. Curve 1 and 2, l0 and'760torr, hot-bare (H.B.); 3 and 4, 10 and 760tort, furnace-raised (F.R.); 5, 760tort, coldinsertion (C.L).
TABLE I. OXIDATIONRUNSOF ANNEALEDAND COLD-WORKEDULTRAPUREFe AT 500°C
Run 1 2 3 4 5 6 7 8
Spec. prep. Ann. Ann. Arm. Ann. Ann. C.W. C.W. C.W.
Oxidn. proc. H.B.* H.B. F.R.T F.R. C.I.~: C.I. F.R. F.R.
Parabolic rate constant§ and oxide thickness a t 20h
O= press, (torr)
Run at O. 1 h time (h) (mg=cm~h -x)
I0 760 10 760 760 760 10 760
42 42 42 42 24 42 42 42
0.010 0"012 0.039 0.032 0.028 0.72 0.82 0.91
(iz) (mgScm~h -I) (~) 0.31 0"31
0"36 0-36 0.52 1.9 2.3 2.1
*Hot bare. TFurnace raised. :[:Cold insertion. §Apparent Kp values obtained by computer calculation of 2wdw/dt.
0.010 0"025 0.009 0"025 0.030 0.16 0.081 0.21
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D. CAPLAN,M. J. GRAHAMand M. COHEN
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Fro. 2. Oxidation of cold-worked Fe (dashed curves) at 500°C in 02. Curve 6, 760torr, cold-insertion (C.I.); 7 and 8, 10 and 760torr, furnace-raised (F.K.). Curve 4 (annealed Fe) repeated from Fig. 1 for reference.
than furnace-raised at 760torr (curve 8); curve 6 does not start off quite as quickly as curves 7 and 8. Curve 4 is repeated from Fig. 1 to illustrate that cold-worked Fe oxidizes faster than annealed Fe under any of these conditions. Figures 3-6 show the structure of the oxide layers,* consisting in every case of FeaO¢ and Fe2Os. The proportion of FeaO4 is higher, the better the contact between oxide and metal. Hot-bare oxidation at 10torr (curve I) yields a double layer of Fe2Os (Fig. 3a, b), consistent with the wave in the curve, in such poor contact with the metal that it erupts plastically into blisters over most of the specimen during cooling (Fig. 3e). At 760torr (curve 2) a single layer forms containing considerable Fe.aO4 (Fig. 3c, d); oxide-metal contact is somewhat better so that the differential contraction during cooling causes only minor blistering (Fig. 3t"). In furnace-raised oxidation at 10torr (curve 3) the oxide again is a non-adherent double layer of Fe2Os (Fig. 4a, b) that blisters almost entirely during cooling (Fig. 4e). At 760torr (curve 4) the oxide is an FesO4-FezOs single layer, showing cavities at the Fe.sO4-Fe interface (Fig. 4e, d), but still having sufficient contact that no blistering occurs during cooling (Fig. 4f). Cold-insertion oxidation (curve 5) yields the same oxide structure (Fig. 5a) as 760torr furnace-raised (curve 4; Fig. 4c, d). Cold-worked Fe oxidized by raising the furnace shows good oxide-metal contact at 760torr (curve 8; Fig. 6c) but at 10torr (curve 7) has an inner band of mixed voids and columnar oxide (Fig. 6a, b) which leads to slight blistering during cooling. Coldworked Fe oxidized by cold-insertion shows intermediate contact (Fig. 5b) consistent with its intermediate weight-gain (curves 6 vs. 7 and 8). DISCUSSION
As previously described, x when annealed Fe is oxidized below 600°C, condensation of cation vacancies causes voids to develop at the Fe~O4-Fe interface. The significant *Authentic sections through oxide layers are difficult to prepare. The cavities in the oxide seen in Figs. 3-6 have been created or exaggerated to various degrees by the metallographic preparation. If repolished on lead laps, only the larger cavities are seen.
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a
lop
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b
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d
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f
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F IG . 3. Oxide structures formed on annealed Fe at 10 and 760torr by hot-bare oxidation. (a) and (b) Run I, IOtorr; sections at x 1000; poorly-adherent double layer of Fe-OS, contact with metal lost during cooling; some Fe,O, at metal surface. (c) and (d) Run 2, 760torr; sections at x 1000; single layer of Fe,Ol-Fq,OS, separation by voids at Fe,O,-Fe. (e) Run I, IOtorr; surface photo at x 3, major blustering occurred during cooling. (f) Run 2, 760torr; surface photo at x 3, slight blistering during cooling.
a
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b
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nl
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7 6 0 torr
d
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-
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f
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7 6 0 torr
FIG. 4. Oxide structures formed on annealed Fe at 10 and 760torr by furnace-raised oxidation. (a) and (b) Run 3, 10tort; sections at ~-. 1000; poorly-adherent double layer of F~O3, contact with metal lost during cooling; some F%O.s at metal surface. (c) and (d) Run 4, 760tort; sections at > 1000; single layer of Fe30~-Fe~Oa, separation by voids at Fe30.i-Fe. (e) Run 3, 10torr; surface photo at ". 3, major blistering occurred during cooling; note pattern of metal grains still evident on oxide surface indicating that firstformed oxide was epitaxial. (f) Run 4, 760tort; surface photo at ;.: 3, no blisters.
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b
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FIG. 5. Sections through oxide formed on annealed and cold-worked Fe by coldinsertion oxidation. (a) Run 5, annealed; single layer of Fe304-Fe~.O~, separation by voids at Fe30,-Fe. (b) Run 6, cold-worked; voids at Fe30~-Fe but less continuous than in (a): voids, in Fe~O~ partly artifacts. × 1000.
a
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I0 torr
b
I0 torr
C
7 6 0 torr
FIG. 6. Sections through oxide formed on cold-worked Fe at 10 and 760torr by furnace-raised oxidation. (a) and (b) Run 7, 10torr; relatively solid outer layer of F~O3 and Fe30~ (voids mainly artifacts); inner layer of voids and Fe30~. (c) Run 8, 760torr; good contact between FeaO4 and metal; voids mainly artifacts. 7", 750.
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FIG. 7. Transmission electronphotomicrograph through outer layer of run 1; see Fig. 3a. Bright spots are interpreted to be perforations rendering layer permeable to 02. • I 0,000.
The high temperature oxidation of pure Fe
5,
observations in the-present work are that oxidation is slower at 10 than 760torr, slower if O, is adn~itted to hot oxide-free Fe than if the cold specimen is introduced to hot oxygen, and tliat the slower oxidation is accompanied by greater separation between oxide and metal. These results can be satisfactorily explained by considering how 02 pressure, cold work, and the procedure for initiating oxidation affect the separation between oxide and metal. The degree to 'which oxidation is inhibited by voids depends on their continuity and location in the oxide and how effectively they are eliminated by plastic deformation. Development of voids is promoted by a large vacancy flux and high oxide hot strength, and lessened by an adequate supply of vacancy sinks and high gas pressure.* The balance between these factors determines the oxidation rate. It follows that oxidation of annealed Fe at 10torr, either hot-bare or furnace-raised (curves 1, 3 and 7), is slower because 760torr is more effective than 10torr in compressing the oxide on to the retreating metal surface. (Even at 760torr, contact still is poorer and oxidation slower than with cold-worked Fe at 10 or 760torr where cavity nucleation is suppressed by extra vacancy sinks.) Relatively flat segments of the oxidation curves --curves 1 and 3 before and after the break, and the later stage of curve 7--then signify an oxide separated from the metal over a considerable area of the specimen, as the photomicrographs show. Interruption of metal transfer into the oxide causes Fe304 to be.progressively converted to Fezes. FesO4 is less dense but its oxidation to FeBOa involves a volume increase of 2.2 ~o owing to the uptake of oxygen. As the Fe~Os-Fe, O4 interface migrates inwards, the compressive stress generated by the increase in volume causes the separated oxide to bow outwards at temperature as seen in Fig. 3a. Tension stresses consequently are induced in the Fezes as it thickens, causing the oxide layer to become permeable to oxygen, and leading to the breaks seen in curves 1 and3. An inner second layer of oxide forms under which vacancies once more accumulate as cavities, and metal transfer again is stifled. The apparently plastic behaviour observed during cooling at the end of runs I and 3, when the double layer of oxide erupts gently into blisters (owing to differential contraction between oxide and metal), indicates that failure o f the first layer at temperature would not have been by brittle fracture. Some type of perforation or preferred path for diffusion must develop during the stifled period preceding the break in the oxidation curve, t Examination of the outer layer by transmission electron microscopy (Fig. 7) suggests perforations as large as 150(O, although this may not be representative of the structure as it was 20h earlier at the time of failure. The effect of pressure on the oxidation of cold-worked Fe seems straightforward. Cavity formation at the Fe304-Fe interface does not occur initially at either pressure on account of the extra sinks available to annihilate vacancies. Hence curves 7 and 8 *Void formation may be affected also by specimen shape, grain size, purity and orientation, but these were not investigated. tThe plasticity of the oxide is surprising since 500°C is a low temperature relative to the melting point of Fete( and FeaOaboth of which are above 1500°C. Conceivably, stress caused by conversion of Fe,O4 to FesOs could create intercrystalline micro-cracks able to simulate plastic behaviour under differential contraction, as well as act as easy diffusion paths at temperature. However, the blistering observed on specimen 7 during cooling, even though minor, seems to be evidence for some plasticity since little thickening of the Fete3 layer has occurred (Fig. 6a, b).
6
D. C ~ L ~ , M. J. GRAHAMand M. Com~ 0.6
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I I 13'1 20 30 40 TIME, hours Fzo. 8. A p p a r e n t parabolic rate constants calculated as 2wdw/dtf r o m slope o f oxidat i o n curves, and plotted against time. C o ] d - w o r k e d specimens shown by dashed cmwcs. 0
~ 10
3
at '10torr and 760torr coincide for the first 40min. Then, when the oxide is 5iz thick, its hot strength, combined with the considerable vacancy flux, is sufficient to resist the squashing effect of 10torr so that voids begin to accumulate. (The decrease in vacancy sinks by annealing and by consumption of the cold-worked surface metal also would contribute to void formation. ) Oxide hot strength, however, is inadequate to resist 760torr pressure, plastic deformation collapses the voids, and no separation develops. The oxidation curve at 10torr thus drops below the 760torr curve as the separation at the oxide-metal interface increases. The degree of separation over the succeeding 40h depends on the balance between a decreasing vacancy flux and increasing resistance to oxide collapse due to increasing thickness. The outer solid layer continues to thicken, despite the cavities in the inner layer, finally reaching 18~ compared to 24~t for the 760torr run. The mechanical squashing effect of pressure discussed above must be distinguished from the classical effect of pressure in which the defect concentration at the surface of the oxide, and hence the oxidation rate, is changed by a change in oxygen pressure. With some metals under some conditions of oxidation there is a risk of confusing the two effects such that an increase in oxidation rate with pressure may be ascribed incorrectly to a change in defect concentration rather than simply to an improvement in oxide-metal contact.
The high temperatureoxidationof pure Fe
7
As discussed previouslyx rate constants calculated for the growth of separated oxide layers have little significance. Of greater interest is their v.ariation with time. Figure 8 shows the apparent parabolic rate constants, Kp, calculated as 2wdw/dt from slopes of the oxidation curves. (The right-hand columns in Table 1 show Kp values at 0-1 and 20h and the corresponding oxide thicknesses.) The low values in runs 1-5 are the result of the poor oxide-metal contact of oxides formed on annealed Fe. For cold-worked Fe, Kp is very high initially and decreases considerably over 10h or more before becoming relatively constant. This cannot be the result merely of an increase of cavities with time, particularly since in run 8 no interface separation is evident (Fig. 6e). It follows that some additional factor is acting. It is believed that with cold-worked metal the diffusion of cations through the oxide is enhanced relative to annealed metal by reason of either easy diffusion paths or a steeper vacancy concentration gradient. Oxide formed on cold-worked metal may contain a large number of leakage paths, or a lower concentration of cation vacancies at its inner surface and hence a steeper gradient through the oxide.5 Annealing that occurs during oxidation causes the effect to decrease with time. To put these results in perspective it should be stated that the prime interest in examining the effects of pressure, experimental procedure and cold work is not for their importance, per se, but to obtain information on the mechanism of oxidation. Inhibiting the oxidation of Fe by choosing conditions that produce a separated oxide layer has no practical application since a poorly-adherent oxide could not be protective for extended periods. On the other hand the possibility should be noted that problems of corrosive wear, such as fretting and some bearing problems, may be aggravated by the faster oxidation rate resulting from the elevated temperature and continuous cold working generated by the rubbing metal surfaces. CONCLUSIONS
1. The oxidation rate of Fe at 500°C in O~ is decreased by voids at the FeaO4-Fe interface and increased by cold work. Void formation in turn is enhanced by a large vacancy flux and high oxide hot strength and suppressed by cold work and high gas pressure. The oxidation behaviour observed represents the net effect of the interaction of these factors. 2. The oxidation of annealed Fe is slow at 10torr because insufficient plastic deformation of the oxide occurs for adequate contact between oxide and metal to be maintained. Voids formed by condensation of cation vacancies separate the oxide from the retreating metal and oxidation is stifled. Thus the 10torr oxidation curves become fiat, rise abruptly when the separated oxide layer fails, and flatten again when oxide-metal contact is lost once more. The resulting oxide is predominantly a nonadherent double layer of F%Os. Oxidation is faster at 760torr because the cavities at the F%O~-Fe interface are squashed closed by the higher pressure. The oxidation curves are continuous and the oxide is a single Fe~Oa-FeaO4 layer showing only moderate separation at the oxide-metal interface. 3. Oxidation is still slower and the pressure-effect enhanced if the experimental procedure is modified so that O3 is admitted to hot oxide-free Fe rather than bringing the specimen to temperature in 03. The higher initial vacancy flux of hot-bare oxida-
8
D. CAPLAN,M. J. GRAHAMand M. Com~
tion increases void formation and creates even greater separation at the oxide-metal interface. 4. Cold-worked Fe oxidizes at 10 and 760torr at the same rapid rate initially; no voids nucleate because the cold work supplies extra sinks for cation vacancies; as a result oxide-metal contact is maintained at both Oz pressures. W h e n the oxide thickens, however, it develops sufficient hot strength to resist the squashing effect o f 10torrO2 (but not 760torr) and voids accumulate at the interface which slows the oxidation rate by hindering metal transfer. Acknowledgements--The authors wish to thank Mr. G. I. Sproule and P. E. Beaubien for their part of the experimental work and Dr. R. J. Hussey for helpful discussion.
REFERENCES 1. D. CAPLANand M. COHEN, Corros. Sci. 6, 321 (1966). 2. D. CAPLAN,Corros. Sei. 6, 509 (1966). 3. D. CAPLANand M. COHEN, Corros. Sci. 7, 725 (1967). 4. M. J. GRAHAMand M. COHEN,J. eleetrochem. Soc. to be published. 5. D. CAPLAN,G. L SPROULEand R. J. HUSSEy,Corros. Sci. 10, 9 (1970).
EFFECT OF O X Y G E N PRESSURE A N D EXPERIMENTAL METHOD ON THE HIGH TEMPERATURE OXIDATION OF PURE Fe* D . CAPLAN, M . J. GRAHAM a n d M . COHEN Division of Applied Chemistry, National Research Council, Ottawa, Canada Abstract--Thermogravimetric measurements were carried out on annealed and cold-worked Fe at 500°C in O3 at 10 and 760torr. Runs were started by bringing the specimens rapidly to temperature in 02 or by admitting O2 to hot H2-reduced specimens maintained free of oxide in ultrahigh vacuum. Less oxidation occurred at 10torr than 760 owing to greater separation between oxide and metal. Still greater separation developed in the hot-bare type of experiment and oxidation was correspondingly slower. Cold-worked Fe showed separation and slower oxidation at 10torr but only after the oxide grew thick enough. The explanation proposed is that plastic deformation of the oxide is greater at 760torr causing collapse of the voids that form by condensation of cation vacancies at the Fe~O(-Fe interface. Oxidation is faster because there is less separation to hinder transfer of metal into the oxide. Oxidation of cold-worked Fe is the same initially at both pressures because extra vacancy sinks are present to suppress nucleation of voids; oxidation at 10torr becomes relatively slow subsequently when the thickening oxide has developed sufficient hot strength to resist the squashing effect of 10torr but not of 760torr. R6sumg---Des mesures thermogravim6triques ont 6t6 effectu6es h 500°C dans 03, sous 10 et 760torr, sur du fer recuit et sur du fer 6croui. Ces essais ont d6but6 en amenant rapidement les ~hantillons ~t temp6rature, en pr6sence de 02 ou en introduisant 02 a u p r ~ d'~chantillons r6duits par I-~ chaud et maintenus ~ l'abri de l'oxydation dans un vide pouss6. I1 se produit une oxydation plus faible ~t 10tort qu'/~ 760torr, ~t cause d'une plus grande s6paration entre l'oxyde et le m6tal. Une s6paration plus grande encore se produit lors des essais avec r6duction h chand, et l'oxydation est plus lente encore. Le fer 6croui montre une s6paration et une oxydation plus lentes ~. 10torr, reals seulement apr~s une certaine croissance de l'oxyde. L'explication propos6e est que la d6formation plastique de l'oxyde est plus importante/t 760tort, et cause une ouverture des vides f o r m s par condensation de lacunes eationiques ~t l'interface FesO4-Fe. L'oxydation est plus rapide parce qu'il y a moins de s6paration pour emp~cher un transfert de m6tal dans roxyde. L'oxydation initiale du fer 6croui est identique aux deux pressions, car des pr6cipitations en dehors des lacunes emp6chent la n u c l ~ t i o n des vides; l'oxydation ~t 10torr se ralentit quand l'6palssissement de l'oxyde d6veloppe suffasarnment de tension/t chaud pour r6sister/t l'6crasement sous 10torr mais non sous 760tort. Zusaramenfassung--Die Oxidation yon kaltverformtem und yon gegliihtem Eisen wurde thermogravimetrisch bei 500°C in O3 bei 10 und 760 Torr untersucht. Der Versuchsbeginn erfolgte teils dureh schnelles Aufheizen der Proben in O2 oder durch Zulassen yon Sauerstoff zu Proben, die zuvor mit Wasserstoff yon Oxidsehichten befreit und in Ultrahoch-Vakuum gehalten wurden. Der Umfang der Oxidation war bei 10 Torr geringer als bei 760 Torr, was auf unterschiedliche Abl6sung der Oxidschicht vom Metall beruhte. Die wasserstoff-reduzierten Proben zeigten starke Abl/Ssung der Oxid. schicht. Ihre Oxidationsgeschwindigkeit war entsprechend gering. Kaltverformtes Eisen zeigte bei 10 Torr Abl/Ssung der Oxidschicht und geringe Oxidationsgesehwindigkeit, wenn die Oxidschicht dick genug geworden war. Diese Befunde werden damit erkl~t, dab bei 760 Tort eine starke plastisehe Deformation der Oxidsehicht eintritt, wodurch die H o h l r i u m e zusammengedriickt werden, welche sich durch Kondensation von Kationenleerstellen an der Phasen grenze FesO(-Fe ausbilden. Die Oxidation ist dann schnell, weii die Hohlr~iume den Obertritt yon Metall in die Oxidschicht behindern. Die Geschwindigkeit der Oxidation des kaltverformten Eisens ist bei beiden Driicken zu Versuehsbegirm die gleiehe, well die Kaltverformung zusitzliche Leerstellensenken sehafft, welche die Ausbildung von Hohlr/iumen unterdriicken. Bei I0 Torr wird die Oxidationsgeschwindigkeit sp~ter jedoeh verh~iltnism~il3ig gering, weil dann die Dicke der Oxidschicht ausreicht, u m dem Gasdruck yon 10 Tort geniigenden Verformungswiderstand entgegenzusetzen. *Manuscript received 10 December 1968.
1
2
D. CAPt.AN,M. J. GRAHAMand M. Conm~
INTRODUCTION A RECiter publication x and two subsequent notes 2,a have described the effect of cold work on the oxidation of Fe. Below 600°C oxidation was found to increase, and oxidemetal contact improve, the greater the degree of cold-work. It was proposed that cold-worked Fe provided extra sinks for the cation vacancies diffusing to the Fe surface through the growing Fe304. Annealed Fe contained too few sinks so that the vacancies condensed into voids* at the F%O4-Fe interface and slowed oxidation by hindering transfer of Fe into the oxide. The main purpose of the present work was to observe the effect of a lower O2 pressure. It was anticipated that at reduced pressure, plastic deformation of the oxide might be less so that interfacial detachment between the oxide and retreating metal would be enhanced and oxidation thus slowed. A second purpose was to determine how the reaction kinetics would be affected if oxidation were initiated by admitting O2 to hot oxide-free Fe (hot-bare) rather than by bringing the specimen to temperature in 02.
EXPERIMENTAL
Specimen preparation Specimens 1 × 5 x 0.02cm were cut from zone-refined ultrapure Battelle Fe (99.997~o) cold-rolled sheet containing 4ppmC, 1Mn, 10Si, 1Cu, 3Ni, 5C0, 3Cr, and 7Mo. Details of procedure for specimen preparation have been described, x,~,4 The sequence is to electropolish, anneal in vacuum or Ar, and electropolish again. In some runs, prior oxide was then removed by reduction in H2. 4 Cold-worked specimens 2 were prepared by abrading the annealed and electropolished sheet with 600-grit SiC followed by 6~ diamond and cleaning ultrasonically in organic solvents.
Procedure The prepared specimen was suspended from an automatic microbalance sealed in a bakeable ultrahigh vacuum apparatus. 4 In one procedure (hot-bare) the specimen was reduced in H2 to remove the prior oxide, the H2 pumped out, temperature adjusted to 500°C, and 02 admitted at 10torr or 760torr to the hot oxide-free specimen. 4 In a second procedure (furnace-raised) an annealed or cold-worked specimen already in I0 or 760torr 02 at room temperature was brought to 500 ° by quickly raising the hot furnace around the quartz hangdown tube. Runs were ended by lowering the furnace. Correction factors to establish the hot weight at zero time were checked by facsimile runs and by weighing before and after at room temperature. In a variation (cold-insertion) of the second procedure a different apparatus was used in which 02 flowed up through a vertical furnace tube open at the top. Experiments at latm only were possible. An annealed or cold-worked specimen was lowered abruptly into the hot zone and quickly connected to an automatic balance. 1 Specimens reached temperature somewhat faster than in the furnace-raised procedure -- 30s vs. 3rain. Table I gives a summary of the oxidation runs. *Such voids or cavities have previously~.3 been termed pores. Henceforward, the term "pore" will be reserved to describe perforations through the oxide, as pores in skin.
The high temperature oxidation of pure Fe
3
RESULTS F i g u r e 1 shows the o x i d a t i o n curves o f a n n e a l e d specimens at the two p r e s s u r ~ for the different o x i d a t i o n procedures. I t is evident t h a t after the first h a l f h o u r , o x i d a t i o n is slower at 10torr t h a n a t 760torr for b o t h the h o t - b a r e (curves l ' a n d 2) a n d furnace-raised p r o c e d u r e s (curves 3 a n d 4) a n d the 10torr curves show a p r o n o u n c e d wave. A t 760torr, b o t h furnace-raised a n d cold-insertion o x i d a t i o n are faster t h a n h o t - b a r e (curves 4 a n d 5 vs. 2). F i g u r e 2 shows t h a t the o x i d a t i o n o f c o l d - w o r k e d F e u n d e r the furnace-raised p r o c e d u r e is as fast at 10torr as at 760 for the first 40rain - - 0.7mg/cm = or 5~t o x i d e - then slows c o n s i d e r a b l y (curves 7 a n d 8). C o l d - i n s e r t i o n (curve 6) is slightly slower
,oF /
~".Z
o
Io
..s.
I 20
TIME, hour'=
,
! ~o
I 40
FIO. l. Oxidation of annealed Fe at 500oC in O=. Curve 1 and 2, l0 and'760torr, hot-bare (H.B.); 3 and 4, 10 and 760tort, furnace-raised (F.R.); 5, 760tort, coldinsertion (C.L).
TABLE I. OXIDATIONRUNSOF ANNEALEDAND COLD-WORKEDULTRAPUREFe AT 500°C
Run 1 2 3 4 5 6 7 8
Spec. prep. Ann. Ann. Arm. Ann. Ann. C.W. C.W. C.W.
Oxidn. proc. H.B.* H.B. F.R.T F.R. C.I.~: C.I. F.R. F.R.
Parabolic rate constant§ and oxide thickness a t 20h
O= press, (torr)
Run at O. 1 h time (h) (mg=cm~h -x)
I0 760 10 760 760 760 10 760
42 42 42 42 24 42 42 42
0.010 0"012 0.039 0.032 0.028 0.72 0.82 0.91
(iz) (mgScm~h -I) (~) 0.31 0"31
0"36 0-36 0.52 1.9 2.3 2.1
*Hot bare. TFurnace raised. :[:Cold insertion. §Apparent Kp values obtained by computer calculation of 2wdw/dt.
0.010 0"025 0.009 0"025 0.030 0.16 0.081 0.21
2.7 I~ 4"3 ][.~ 2"4 | 5"14 ! 5.0 14.8 ',~ 12.4~ ! 16.1 "T
D. CAPLAN,M. J. GRAHAMand M. COHEN
4
i
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i
~ 6 0 ~°~f~- ' ' ~ ' ' ~ " 8 F . ~ , . ~ . ~ "~'" ....,.~ 6
11/~ Z"
2
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i
/
F.R. 760 t°_.~.~--~ 4
2'o
TIME,hours
4'0
Fro. 2. Oxidation of cold-worked Fe (dashed curves) at 500°C in 02. Curve 6, 760torr, cold-insertion (C.I.); 7 and 8, 10 and 760torr, furnace-raised (F.K.). Curve 4 (annealed Fe) repeated from Fig. 1 for reference.
than furnace-raised at 760torr (curve 8); curve 6 does not start off quite as quickly as curves 7 and 8. Curve 4 is repeated from Fig. 1 to illustrate that cold-worked Fe oxidizes faster than annealed Fe under any of these conditions. Figures 3-6 show the structure of the oxide layers,* consisting in every case of FeaO¢ and Fe2Os. The proportion of FeaO4 is higher, the better the contact between oxide and metal. Hot-bare oxidation at 10torr (curve I) yields a double layer of Fe2Os (Fig. 3a, b), consistent with the wave in the curve, in such poor contact with the metal that it erupts plastically into blisters over most of the specimen during cooling (Fig. 3e). At 760torr (curve 2) a single layer forms containing considerable Fe.aO4 (Fig. 3c, d); oxide-metal contact is somewhat better so that the differential contraction during cooling causes only minor blistering (Fig. 3t"). In furnace-raised oxidation at 10torr (curve 3) the oxide again is a non-adherent double layer of Fe2Os (Fig. 4a, b) that blisters almost entirely during cooling (Fig. 4e). At 760torr (curve 4) the oxide is an FesO4-FezOs single layer, showing cavities at the Fe.sO4-Fe interface (Fig. 4e, d), but still having sufficient contact that no blistering occurs during cooling (Fig. 4f). Cold-insertion oxidation (curve 5) yields the same oxide structure (Fig. 5a) as 760torr furnace-raised (curve 4; Fig. 4c, d). Cold-worked Fe oxidized by raising the furnace shows good oxide-metal contact at 760torr (curve 8; Fig. 6c) but at 10torr (curve 7) has an inner band of mixed voids and columnar oxide (Fig. 6a, b) which leads to slight blistering during cooling. Coldworked Fe oxidized by cold-insertion shows intermediate contact (Fig. 5b) consistent with its intermediate weight-gain (curves 6 vs. 7 and 8). DISCUSSION
As previously described, x when annealed Fe is oxidized below 600°C, condensation of cation vacancies causes voids to develop at the Fe~O4-Fe interface. The significant *Authentic sections through oxide layers are difficult to prepare. The cavities in the oxide seen in Figs. 3-6 have been created or exaggerated to various degrees by the metallographic preparation. If repolished on lead laps, only the larger cavities are seen.
--
--- -
----~---
a
lop
IO tort
u
IO
b
tort
C
760 tom
d
760 tort
IO torr
f
760
torr
F IG . 3. Oxide structures formed on annealed Fe at 10 and 760torr by hot-bare oxidation. (a) and (b) Run I, IOtorr; sections at x 1000; poorly-adherent double layer of Fe-OS, contact with metal lost during cooling; some Fe,O, at metal surface. (c) and (d) Run 2, 760torr; sections at x 1000; single layer of Fe,Ol-Fq,OS, separation by voids at Fe,O,-Fe. (e) Run I, IOtorr; surface photo at x 3, major blustering occurred during cooling. (f) Run 2, 760torr; surface photo at x 3, slight blistering during cooling.
a
I0~
i
I
I0 torr
b
IOtorr
nl
,
C
7 6 0 torr
d
~ 7 6 0 torr
-
C
I0 torr
f
.
~
~
7 6 0 torr
FIG. 4. Oxide structures formed on annealed Fe at 10 and 760torr by furnace-raised oxidation. (a) and (b) Run 3, 10tort; sections at ~-. 1000; poorly-adherent double layer of F~O3, contact with metal lost during cooling; some F%O.s at metal surface. (c) and (d) Run 4, 760tort; sections at > 1000; single layer of Fe30~-Fe~Oa, separation by voids at Fe30.i-Fe. (e) Run 3, 10torr; surface photo at ". 3, major blistering occurred during cooling; note pattern of metal grains still evident on oxide surface indicating that firstformed oxide was epitaxial. (f) Run 4, 760tort; surface photo at ;.: 3, no blisters.
I0~1
a
b
I
~
ann.
c.w.
FIG. 5. Sections through oxide formed on annealed and cold-worked Fe by coldinsertion oxidation. (a) Run 5, annealed; single layer of Fe304-Fe~.O~, separation by voids at Fe30,-Fe. (b) Run 6, cold-worked; voids at Fe30~-Fe but less continuous than in (a): voids, in Fe~O~ partly artifacts. × 1000.
a
IOp~
J
I0 torr
b
I0 torr
C
7 6 0 torr
FIG. 6. Sections through oxide formed on cold-worked Fe at 10 and 760torr by furnace-raised oxidation. (a) and (b) Run 7, 10torr; relatively solid outer layer of F~O3 and Fe30~ (voids mainly artifacts); inner layer of voids and Fe30~. (c) Run 8, 760torr; good contact between FeaO4 and metal; voids mainly artifacts. 7", 750.
IP
m
¢
Ib -
0
|
•
I/a,I
'9
i
FIG. 7. Transmission electronphotomicrograph through outer layer of run 1; see Fig. 3a. Bright spots are interpreted to be perforations rendering layer permeable to 02. • I 0,000.
The high temperature oxidation of pure Fe
5,
observations in the-present work are that oxidation is slower at 10 than 760torr, slower if O, is adn~itted to hot oxide-free Fe than if the cold specimen is introduced to hot oxygen, and tliat the slower oxidation is accompanied by greater separation between oxide and metal. These results can be satisfactorily explained by considering how 02 pressure, cold work, and the procedure for initiating oxidation affect the separation between oxide and metal. The degree to 'which oxidation is inhibited by voids depends on their continuity and location in the oxide and how effectively they are eliminated by plastic deformation. Development of voids is promoted by a large vacancy flux and high oxide hot strength, and lessened by an adequate supply of vacancy sinks and high gas pressure.* The balance between these factors determines the oxidation rate. It follows that oxidation of annealed Fe at 10torr, either hot-bare or furnace-raised (curves 1, 3 and 7), is slower because 760torr is more effective than 10torr in compressing the oxide on to the retreating metal surface. (Even at 760torr, contact still is poorer and oxidation slower than with cold-worked Fe at 10 or 760torr where cavity nucleation is suppressed by extra vacancy sinks.) Relatively flat segments of the oxidation curves --curves 1 and 3 before and after the break, and the later stage of curve 7--then signify an oxide separated from the metal over a considerable area of the specimen, as the photomicrographs show. Interruption of metal transfer into the oxide causes Fe304 to be.progressively converted to Fezes. FesO4 is less dense but its oxidation to FeBOa involves a volume increase of 2.2 ~o owing to the uptake of oxygen. As the Fe~Os-Fe, O4 interface migrates inwards, the compressive stress generated by the increase in volume causes the separated oxide to bow outwards at temperature as seen in Fig. 3a. Tension stresses consequently are induced in the Fezes as it thickens, causing the oxide layer to become permeable to oxygen, and leading to the breaks seen in curves 1 and3. An inner second layer of oxide forms under which vacancies once more accumulate as cavities, and metal transfer again is stifled. The apparently plastic behaviour observed during cooling at the end of runs I and 3, when the double layer of oxide erupts gently into blisters (owing to differential contraction between oxide and metal), indicates that failure o f the first layer at temperature would not have been by brittle fracture. Some type of perforation or preferred path for diffusion must develop during the stifled period preceding the break in the oxidation curve, t Examination of the outer layer by transmission electron microscopy (Fig. 7) suggests perforations as large as 150(O, although this may not be representative of the structure as it was 20h earlier at the time of failure. The effect of pressure on the oxidation of cold-worked Fe seems straightforward. Cavity formation at the Fe304-Fe interface does not occur initially at either pressure on account of the extra sinks available to annihilate vacancies. Hence curves 7 and 8 *Void formation may be affected also by specimen shape, grain size, purity and orientation, but these were not investigated. tThe plasticity of the oxide is surprising since 500°C is a low temperature relative to the melting point of Fete( and FeaOaboth of which are above 1500°C. Conceivably, stress caused by conversion of Fe,O4 to FesOs could create intercrystalline micro-cracks able to simulate plastic behaviour under differential contraction, as well as act as easy diffusion paths at temperature. However, the blistering observed on specimen 7 during cooling, even though minor, seems to be evidence for some plasticity since little thickening of the Fete3 layer has occurred (Fig. 6a, b).
6
D. C ~ L ~ , M. J. GRAHAMand M. Com~ 0.6
,
!a
J
II II II
IE o 0.4 o,I E Z
I~\ Z 0 ul I-e r O.2
F,R. 760 torr
-I 0 II1
C,I.
\
o:
\
8
6
~ " - -
F.R. I0 t0rr 1'
0
I I 13'1 20 30 40 TIME, hours Fzo. 8. A p p a r e n t parabolic rate constants calculated as 2wdw/dtf r o m slope o f oxidat i o n curves, and plotted against time. C o ] d - w o r k e d specimens shown by dashed cmwcs. 0
~ 10
3
at '10torr and 760torr coincide for the first 40min. Then, when the oxide is 5iz thick, its hot strength, combined with the considerable vacancy flux, is sufficient to resist the squashing effect of 10torr so that voids begin to accumulate. (The decrease in vacancy sinks by annealing and by consumption of the cold-worked surface metal also would contribute to void formation. ) Oxide hot strength, however, is inadequate to resist 760torr pressure, plastic deformation collapses the voids, and no separation develops. The oxidation curve at 10torr thus drops below the 760torr curve as the separation at the oxide-metal interface increases. The degree of separation over the succeeding 40h depends on the balance between a decreasing vacancy flux and increasing resistance to oxide collapse due to increasing thickness. The outer solid layer continues to thicken, despite the cavities in the inner layer, finally reaching 18~ compared to 24~t for the 760torr run. The mechanical squashing effect of pressure discussed above must be distinguished from the classical effect of pressure in which the defect concentration at the surface of the oxide, and hence the oxidation rate, is changed by a change in oxygen pressure. With some metals under some conditions of oxidation there is a risk of confusing the two effects such that an increase in oxidation rate with pressure may be ascribed incorrectly to a change in defect concentration rather than simply to an improvement in oxide-metal contact.
The high temperatureoxidationof pure Fe
7
As discussed previouslyx rate constants calculated for the growth of separated oxide layers have little significance. Of greater interest is their v.ariation with time. Figure 8 shows the apparent parabolic rate constants, Kp, calculated as 2wdw/dt from slopes of the oxidation curves. (The right-hand columns in Table 1 show Kp values at 0-1 and 20h and the corresponding oxide thicknesses.) The low values in runs 1-5 are the result of the poor oxide-metal contact of oxides formed on annealed Fe. For cold-worked Fe, Kp is very high initially and decreases considerably over 10h or more before becoming relatively constant. This cannot be the result merely of an increase of cavities with time, particularly since in run 8 no interface separation is evident (Fig. 6e). It follows that some additional factor is acting. It is believed that with cold-worked metal the diffusion of cations through the oxide is enhanced relative to annealed metal by reason of either easy diffusion paths or a steeper vacancy concentration gradient. Oxide formed on cold-worked metal may contain a large number of leakage paths, or a lower concentration of cation vacancies at its inner surface and hence a steeper gradient through the oxide.5 Annealing that occurs during oxidation causes the effect to decrease with time. To put these results in perspective it should be stated that the prime interest in examining the effects of pressure, experimental procedure and cold work is not for their importance, per se, but to obtain information on the mechanism of oxidation. Inhibiting the oxidation of Fe by choosing conditions that produce a separated oxide layer has no practical application since a poorly-adherent oxide could not be protective for extended periods. On the other hand the possibility should be noted that problems of corrosive wear, such as fretting and some bearing problems, may be aggravated by the faster oxidation rate resulting from the elevated temperature and continuous cold working generated by the rubbing metal surfaces. CONCLUSIONS
1. The oxidation rate of Fe at 500°C in O~ is decreased by voids at the FeaO4-Fe interface and increased by cold work. Void formation in turn is enhanced by a large vacancy flux and high oxide hot strength and suppressed by cold work and high gas pressure. The oxidation behaviour observed represents the net effect of the interaction of these factors. 2. The oxidation of annealed Fe is slow at 10torr because insufficient plastic deformation of the oxide occurs for adequate contact between oxide and metal to be maintained. Voids formed by condensation of cation vacancies separate the oxide from the retreating metal and oxidation is stifled. Thus the 10torr oxidation curves become fiat, rise abruptly when the separated oxide layer fails, and flatten again when oxide-metal contact is lost once more. The resulting oxide is predominantly a nonadherent double layer of F%Os. Oxidation is faster at 760torr because the cavities at the F%O~-Fe interface are squashed closed by the higher pressure. The oxidation curves are continuous and the oxide is a single Fe~Oa-FeaO4 layer showing only moderate separation at the oxide-metal interface. 3. Oxidation is still slower and the pressure-effect enhanced if the experimental procedure is modified so that O3 is admitted to hot oxide-free Fe rather than bringing the specimen to temperature in 03. The higher initial vacancy flux of hot-bare oxida-
8
D. CAPLAN,M. J. GRAHAMand M. Com~
tion increases void formation and creates even greater separation at the oxide-metal interface. 4. Cold-worked Fe oxidizes at 10 and 760torr at the same rapid rate initially; no voids nucleate because the cold work supplies extra sinks for cation vacancies; as a result oxide-metal contact is maintained at both Oz pressures. W h e n the oxide thickens, however, it develops sufficient hot strength to resist the squashing effect o f 10torrO2 (but not 760torr) and voids accumulate at the interface which slows the oxidation rate by hindering metal transfer. Acknowledgements--The authors wish to thank Mr. G. I. Sproule and P. E. Beaubien for their part of the experimental work and Dr. R. J. Hussey for helpful discussion.
REFERENCES 1. D. CAPLANand M. COHEN, Corros. Sci. 6, 321 (1966). 2. D. CAPLAN,Corros. Sei. 6, 509 (1966). 3. D. CAPLANand M. COHEN, Corros. Sci. 7, 725 (1967). 4. M. J. GRAHAMand M. COHEN,J. eleetrochem. Soc. to be published. 5. D. CAPLAN,G. L SPROULEand R. J. HUSSEy,Corros. Sci. 10, 9 (1970).