The oxidation of nimonic 80A in oxygen at reduced pressure

Corrosion Science, 1974, Vol. 14, pp. 553 to 562. PergamonPress. Printed in Great Britain

THE OXIDATION OF NIMONIC 80A IN OXYGEN AT REDUCED PRESSURE* R. HALES and A. C. HILL Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, GIoucestershire, England Abstract--The oxidation of Nimonic 80A in the temperature range 1048-1143K in oxygen at pressures in the range 2 x 10-7 to 1'5 x l0 -4 tort has been studied by weight gain measurements. Initially the reaction proceeds according to a linear rate law and subsequently follows a parabolic law. These observations are discussed in terms of the formation of two titanium oxides on the metal surface. Metallographic examination of the oxidized metal showed grain boundary attack in the metal substrate. The reaction product found in the metal grain-boundaries was a mixed oxide of aluminium and titanium. R~um6---On a 6tudi6 l'oxydation du Nimonic 80A darts la gamme de temp6ratures de 1048 tt 1143K darts l'oxyg6ne b. des pressions du domaine de 2 x l0 -7 b. 1,5 x l0 -4 torr par des mesures de gain de poids. La r6action 6volue initialement suivant une loi de vitesse lin6aire et ulterieurement suit une loi parabolique. On a discut6 ces observations en termes de formation de deux oxydes de titane sur la surface m6tallique. Un examen m~tallographique du m6tal oxyd6 montre une attaque des joints de grain darts le substrat m6tallique. Le produit r6actiormel qu'on a trouv6 darts les joints de grain du m6tal 6tait un oxyde mixte d'aluminium et de titane. Zusammenfassung--Die Oxydierung von Nimonic 80A in dem Temperaturbereich von 1048 bis 1143K in Sauerstoff bei Driicken in einem Bereich yon 2 x t0 -7 his 1,5 × 10-4 Torr ist dutch Messungen yon Gewichtszunahme untersucht worden. Die Reaktion erfolgt anftinglich auf Grund eines linearen Geschwindigkeitsgesetzes und folgt danach einem Parabelgesetz. Die Beobachtungen werden in Form der Bildung von zwei Titanoxyden auf der Metallfl/iche besprochen. Metallographische Untersuchung des oxydierten Metalls zeigte Korngrenzenangriff in dem Metallsubstrat. Das in den Metallkorngrenzen gefundene Reaktionsprodukt war eine Oxydmischung yon AIuminium und Titan. 1. I N T R O D U C T I O N THE RATE and m e c h a n i s m o f reactions between metals and gases at low pressures has been o f theoretical interest for m a n y years. With the current d e v e l o p m e n t o f h el i u m - co o l ed nuclear reactors the p r o b l e m has now gained industrial importance. In the reactor m a n y structural and boiler tube materials are exposed to helium at high temperatures. H e l i u m is inert, o f course; but the core o f the reactor sustains a low oxygen potential due to impurities in the h e l i u m ? It is these impurities which react with the structural materials o f the reactor. O n e a p p r o a c h to the study o f reactions which occur in the high t e m p e r a t u r e reactor ( H T R ) and are controlled by solid state processes rather than by gaseous diffusion is to ignore the helium a n d simply expose the material in question to the reactive constituents at their a p p r o p r i a t e chemical potential. This a p p r o a c h allows high v a c u u m techniques to be e m p l o y e d rather than high pressure technology, 2 thereby permitting *Manuscript received 15 February 1974. 553

554

R. HALESand A. C. FIP~L

easy variation o f some o f the m o r e i m p o r t a n t variables o f the gas c o m p o s i t i o n a n d allowing m o r e accurate d e t e r m i n a t i o n o f the gas c o m p o s i t i o n in question. A recent p a p e r by W i l d 3 r e p o r t e d a study o f the o x i d a t i o n o f N i m o n i c 80A by A u g e r spectroscopy. A l t h o u g h this technique usefully shows the phases f o r m i n g at the gas/oxide interface it is not possible to measure reaction rates. In the present investigation the oxidation kinetics o f N i m o n i c 80A have been m e a s u r e d over a range o f temperatures a n d oxygen partial pressures. Results are c o m p a r e d with d a t a f r o m experiments using high pressure helium. 2,4 2. E X P E R I M E N T A L Samples for o x i d a t i o n were p r e p a r e d f r o m N i m o n i c 80A Strip supplied by H e n r y Wiggin & Co. T h e c o m p o s i t i o n o f the alloy was 0.08%C, 0 - 0 3 % M n , 0-33%Fe, 0.005%S, 19.45%Cr, 1.32%AI, 2.31%Ti and the balance nickel. T h e material was reduced f r o m 0-5 to 0.1 m m thick by a b r a s i o n on silicon carbide p a p e r a n d finally electro-polished in a solution o f 55 % HaPO4, 22 %H2SO4 and 23 % H20. The specimens were weighed a n d d i m e n s i o n e d before being placed on the microbalance. The reaction vessel a n d balance have been described previously. 5 T h e " h o t b a r e " p r o c e d u r e was a d o p t e d ; the sample was heated slowly to t e m p e r a t u r e in the best v a c u u m attainable, < 10 -7 torr, a n d the oxygen a d m i t t e d to the desired pressure when the system was at the selected temperature. Exposures were carried out over a range o f pressure f r o m 2 x 10 -7 t o r r to 1.5 × 10 -4 t o r r at t e m p e r a t u r e s between 1048 and 1143 K. The test conditions are listed in Table 1. A f t e r exposures o f ~ 4 × l0 t rain oxidized specimens were sectioned and examined m e t a l l o g r a p h i c a l l y and by E P M A . T h e second piece o f each specimen was retained for e x a m i n a t i o n in the scanning electron microscope. 3. R E S U L T S 3.1 Kinetics T h e change o f weight o f the specimens was r e c o r d e d as a function o f time a n d TABLE 1.

EXPERIMENTAL REACTION CONDITIONS

Specimen number

Temperature (°K)

1 2 3 4 5 6 7 8 9 10 ll 12 13 14 15

1093 1093 1093 1093 1093 1093 1093 1093 1048 1093 1143 1118 1093 1143 1093

Pressure of oxygen (torr) 2 2 2 7 6 8 5 1"5 7 2 7

× 10-5 × 10-6 × 10-7 × 10-6 × 10-6 × 10-7 :x: 10-7 × 10-' x 10-6 × 10-7 × 10-6 7 x 10-6 7 × 10-a 7 x 10 - a 7 × 10-6

The oxidation of Nimonic 80A in oxygen at reduced pressure

555

these results are summarized in the double logarithmic plots shown in Figs. 1 and 2. It can be seen in Fig. 1 that the rate of reaction increases with temperature as usual; the rate of reaction is also a function of the oxygen pressure as can be seen'in Fig. 2. After the linear portion, the reaction kinetics slow down and obey a parabolic law and after about 104 rain the weight increase of all the specimens oxidized at the same temperature is approximately the same, irrespective of the oxygen pressure. 3.2 Structure Cross-sections of the oxidized specimens were prepared metallographically; the specimens were nickel-plated before being mounted to preserve details of structure of the edge of the specimens. The microstructures observed are shown in Fig. 3 and comprise the metal specimen, which exhibits grain boundary attack, with an overlying oxide scale. (The protective layer of plated nickel can be seen between this surface oxide and the mounting plastic.) The two phases formed by the reaction with oxygen were identified by EPMA as: (i) intergranular aluminium and titanium oxide formed by internal oxidation. (ii) surface oxide formed by the reaction of titanium which diffuses outwards from the matrix. This structure is similar to that reported by Pearce and Sparry4 for material exposed to impure helium in the Dragon reactor. The surface of the oxidized material was examined in the scanning electron microscope and the structures recorded are shown in Fig. 4. In general the oxide is 7 x 10-6~0rr •

oJ

E

it

E

o

.E u

.~

io" '4•

I

10z

4 9

1090K 1048 K

I

io4

i 0 't

Time,

Specimen Specimen

rain

FtG. 1. A plot of weight gain vs. time for Nimonic 80A oxidized in oxygen at 7 x 10- e t o r r .

556

R. HAL~ and A. C. HILL

i0 °

Z i 0 -I o

o 10-2 .c

._~

_.,,,• i" y

i0-~ 10 2

///

./" .I + +,,

/+"

,, ~ +

,

'

,<~,.] ,

103 Time,

~,,<~° f ~,,<~" I ~,

104 rain

FIG. 2. A plot of weight gain vs. time for Nimonic 80A oxidized at 1090K. continuous with a few needles growing from the surface. However, a few of the specimens spelled badly on cooling; spalling was not a continuous process during the oxidation since no discontinuities were observed in weight-change vs. time plots. The result of spalling is clearly shown in Fig. 4. The outer oxide spalls, breaking along lines which seem to correspond to metal grain boundaries. The underlying material clearly shows grain boundaries which appear decorated by different phases. 4. DISCUSSION 4.1

Metallography

The microstructures presented in Fig. 3 are indistinguishable from those observed by Pearce 6 in sections of Nimonic 80A exposed to impure helium in a prototype HTR. The structures consists of grain boundary attack underlying a compact surface oxide. A discrepancy arises in comparing the relative amounts of grain boundary and surface oxide. The ratio of depth of grain boundary attack to thickness of surface oxide is about 2 : 1 in the micrographs shown in Fig. 3 whereas the ratio for specimens exposed in the helium loops is ~ 6 : 1.6 An important difference between the studies performed in vacuum and those carried out in helium is the level of chromium in the surface oxide, Pearce and Sparry 4 reported an oxide enriched in chromium on the surface of Nimonic 80A exposed to impure helium in the Dragon reactor for 3000 h. The EPMA examination carried out by Wild, 3 in agreement with the present work, on the other hand found no evidence of chromium enrichment in the surface oxide on specimens exposed to oxygen at reduced pressure. It was first thought that this discrepancy discredited the use of vacuum

I u.uDmm

FIG. 3.

I

The microstructure of Specimen 11 showing internal oxidation at grain boundaries below a compact surface oxide.

Fro. 4.

Scanning electron micrographs of the surface of oxidized Nimonic 80 A.

The oxidation of Nimonic 80A in oxygen at reduced pressure

557

techniques for studying reactions appropriate to H T R programmes; however, a recent observation by Pearce (private communication) shows that a chromium-free surface oxide is also formed on Nimonic 80A oxidized in impure helium for 300 h. Thus it appears that the incorporation of chromium into the surface oxide only occurs after prolonged exposure and is not a result of the method used to produce the oxidizing potential. The structure of the oxidized surfaces shown in Fig. 4 exhibit some interesting features. At low temperatures the metal grain boundaries are clearly in relief, as shown on Specimen 5. As the temperature is raised the grain boundaries become less distinct. When the surface oxide spalls, a rough metal surface is exposed in which the metal grain boundaries are clearly visible. Needles, of the type described by Wild, a are not visible on many of the specimens, although they can be seen clearly on Specimen 7. The density and dimensions of these needles are less than those reported by Wild. a 4.2 Kinetics The weight gain curves all show the same general trend as shown in Figs. 1 and 2. The initial rates correspond to a linear reaction followed by subsequent parabolic kinetics. The isothermal tests at 1095K have allowed the linear rate constant to be plotted as a function of the oxygen partial pressure as shown in Fig. 5. The results are consistent with a Po2 + relationship. The parabolic rate constant on the other hand is independent of oxygen pressure and has an average value of (1.27 4- 0.85) × 10 -5 mg2/cm4/min. The variation of the linear and parabolic rate constants, kt and kp respectively, with temperature at 7 x l0 -8 torr oxygen are summarized in Figs. 6 and 7. Activation energies QI and Qp can be ascribed to the two rates such that

ic~ +8

E E o

o t)

Slope • I/2

B

I

10-6

Oxygen pressure,

FIG. 5.

i 10-4

t

I0-5

fort

A plot of the linear rate constant vs. oxygen pressure.

I

10-:3

558

R. HALES and A. C. HILL

'~[+11 °J~

"

\+13

&

Q- 44.6-* 6kals/mole

8.7

FIG. 6.

9.0

9.5 I/TX I04, OK'l

A plot of the linear rate constant vs.

lIT.

T,°C 10.4870,. 845 820 800 775 ,ll

E

E

Q-71.3 + I0 kais/.mole

IQ" 8.7

' 9.0

9i5

I/T x I0,4 o K -I

FlO. 7.

A plot o f the paraboEc rate constant vs. ]/T.

The oxidation of Nimonic 80A in oxygen at reduced pressure

kt----7.6 × 104 e x p -

(45 :t: 3) × l0 g RT

559

mg/cm2/min

(1)

mg2/cm'/min"

(2)

and kp = 1'9 x 10l° exp

(71 4- 10) x 10 s RT

The dependence of the linear rate constant on oxygen pressure strongly suggests that the reaction is controlled either by dissociation of oxygen at the surface or by dissolution and subsequent diffusion of oxygen into the metal. A surface reaction clearly leads to linear kinetics, but the activation energy for oxygen dissociation, 115 kcal/mole, 7 is ve:y much larger than that measured for the reaction which proceeds with linear kinetics. The activation energy for oxygen diffusion into the metal is similarly large compared with that for the initial reaction, 72 kcal/mole, s-l° although the estimated value for grain boundary diffusion (the activation energy for grain boundary diffusion is usually taken to be 0.6 times that for volume diffusion) u is in much closer agreement. However, internal oxidation should result in parabolic weightgain kinetics and thus neither of these propositions fully explains the experimental observations. The change from linear to parabolic kinetics in the present investigation occurs after ~ l0 s rain oxidation. After similar durations and under similar condition's of temperature and pressure, Wild s observed a dramatic change in the surface concentration of certain elements during the oxidation of Nimonic 80A. Initially, Wild 3 observed an increase in the concentration of titanium and oxygen but these elements were always in the ratio 1 : 1, corresponding to TiO. After about 103 rain the oxygen concentration quickly increased until the ratio 2 : 1 was attained; this new ratio which corresponds to futile persisted for the duration of the experiment. In view of the excellent agreement between the time to change from linear to parabolic kinetics and the time to change from TiO to TiO2 on the oxidizing surface, it is believed that the linear kinetics correspond to the formation of TiO and the parabolic kinetics correspond to the formation of TiO~. The microstructure of the oxidized material indicates why this change should take place. In the early stages of the experiment there is an abundant supply of titanium to the metal surface which is oxidized to TiO. The titanium-to-oxygen ratio is maintained constant by the continued supply of titanium from the matrix and by the dissolution of oxygen into the metal where it reacts to form A1203 in the grain boundaries. This may be compared with the early stages of oxidation of pure titanium in which metal-oxygen solid solutions of the order TiO0.35lz were found near the oxide/metal interface. Unfortunately, little or nothing is known about the defect structure of oxides of titanium lower than TiO2, but if the defect structure of TiO approximates to an ordered solid solution of oxygen in titanium a pressure dependence of Po2~ would be anticipated in agreement with the observation of kt oc Po2½. The parabolic kinetics exhibit a temperature dependence which corresponds to an activation energy of 71 4- 10 kcal/mole, which may be compared with the activation energy for diffusion in rutile of 74 kcal/mole, 13 giving further support to the contention that the parabolic kinetics are due to the formation of rutile.

560

R. HALES and A. C. HILL

The observation of parabolic oxidation kinetics preceded by linear kinetics is not unique and two previous observations are pertinent to the present study and support the qualitative model already outlined. During the oxidation of titanium in pure oxygen in the temperature range 870 to 1170K linear kinetics give way to parabolic kinetics. The linear kinetics last only 20 min but are prolonged by reducing the temperature and it is reasonable to suppose that reducing the oxygen pressure will have a similar effect. By analogy in the present work the linear period can be ascribed to the formation of the lower oxide, TiO, and the later parabolic kinetics can be ascribed to the formation of rutile. The process whereby TiO persists in preference to TiOz may be either the oxidation of outward flowing cations or the dissolution of inward moving anions into the metal according to the reactions: TiO~ -k Ti ~ 2TiO

for cation diffusion

TiO2 ~ TiO q- Odi~solved

for anion diffusion.

or

A similar sequence of events has been reported by Hussey and Cohen 14 for the oxidation of iron at reduced oxygen pressure. These workers reported that the transition from linear to parabolic kinetics occurred when ctFe~.Oa nucleated and covered the surface of the original FeaO4 scale. They suggested that in general FeaO4 is oxidized at the oxide/oxygen interface to the higher oxide but that the flux of cations through the scale causes FeaO4 to be reformed by the reaction 4Fe2Os q- Fe ~ 3FeaOa. Any process which reduces the flux of cations across the scale inevitably interferes with the above reaction and stable Fe2Os grows with parabolic kinetics. This behaviour in the case of iron is only observed at reduced oxygen pressures, in the range 10-e10-3 torr. a4 A further point of similarity between the oxidation of iron 15 and Nimonic 80A is that in both systems the linear kinetics vary with Po2t whilst the parabolic kinetics are pressure independent. Although the foregoing models12,14,x5 fit many of the facts there is no physical basis to explain the initial linear kinetics. Linear kinetics are sometimes ascribed to the oxide growing in islands rather than as a continuous film, but this requires special restrictions on the nucleation and growth modes and gives no explanation for nucleation of a higher oxide phase at the onset of parabolic kinetics. It was suggested earlier that the two-stage kinetics observed in the present work result from the formation of TiO followed by the growth of a rutile scale. If a reaction can be controlled by two different processes then the slowest step is rate controlling, for example a reaction may occur by the transport of an ion across a scale and subsequently by reaction of the ion at an interface. The first reaction leads to parabolic kinetics whilst the latter gives a linear rate law. The instantaneous rate for each reaction can be written for linear kinetics dx

dt -- k~ exp --

(Qt/RT)

(3)

The oxidation o f Nimonic 80A in oxygen at reduced pressure

561

and for parabolic kinetics ko

dXd=-~x t exp-- (Qa/RT)"

4)

Transition from one reaction mode to the other occurs when the two rates are equal, hence x = (k~/2kT) exp -- (Qp -

Q,)/RT.

(5)

From the reported data for the oxidation of Nimonic 80A (Qp - - Qi) is calculated to be 27 4- 11 kcal/mole. A plot of the log of weight increment per unit area at which the rate law changes vs. I / T is shown in Fig. 8 and a value for (Qp - QI) of 35 4 - 3 kcal/mole is measured. This good agreement strongly suggests a set of reactions linked in series. In sumrllary it has been shown that the two-stage kinetics reported in this paper are associated with a change of oxide from a low oxidation state to a higher oxidation state of the cation, in common with the oxidation of titanium x2 and iron. 14,15 It is not possible at this stage to give a satisfactory explanation for the linear kinetics of the initial reaction in these systems. However, the relationship between the activ.ation energies of the two reactions and the onset of parabolic kinetics gives a clear indication that the two processes are linked in series. It seems likely that other oxide systems in which a number of valence levels exist may also exhibit two stage kinetics of this type.

II

~-

points

~'?¢:0'I ~' =~°

0.018.7

r 9"0

t 9-5 ~F X I04, °K'I

FIG. 8.

A plot of weight gain per unit area to change from linear to parabolic kinetics vs.

I/T.

562

R. HALESand A. C. lilLE 5. C O N C L U S I O N S

The oxidation of Nimonic 80A at reduced pressures occurs in two stages. The first reaction obeys a linear rate law and corresponds to the formation of TiO. The subsequent reaction obeys a parabolic rate law and corresponds to the formation o f TiO2. Although this type of behaviour has also been observed for iron and titanium, it has not been possible to account for the change from linear to parabolic kinetics by a physical model. However, in all the systems which exhibit this type of two stage oxidation, the change from linear to parabolic kinetics is associated with a change of oxide from a low oxidation state to a higher oxidation state of the cation. Metallographic examination has shown that oxygen diffuses into the grainboundaries of the metal where it reacts to form mixed oxides of titanium and aluminium, during the formation of the surface oxide. This complicates any analysis of the oxidation kinetics and may contribute to the two-stage reaction reported. Acknowledgement--This Board.

paper is published by permission of the Central Electricity Generating

REFERENCES 1. A. C. HZLL and R. HALES, C.E.G.B. Report RD/B/N2567 (1973). 2. H. A. G. BAav.s, K. BOrE and N. FoRsx,'rn (Dragon Project). Private communication (1972). 3. 1L K. WILD, Corros. Sei. 13, 105 (1973). 4. R. J. Pr.ARCEand IL SPAPa~Y,C.E.G.B. Report RD/B/N1844 (1972). 5. R. HALES, A. C. HZLL and R. K. WILD, Corros. Sei. 13, 325 (1973). 6. R. J. PEARCE,Private communication. 7. G. ~ B r . R O , Spectra of Diatomie Molecules. Von Nostrand, New York (1950). 8. C. B. ALCOCKand P. B. BROW~, Met. Sei. J'. 3, 116 (1969). 9. R BARLOWand P. J. GRUNDY, Met. Sei. 3". 4, 797 (1969). 10. G. J. L L O ~ and J. W. MARVIN, Met. Sei. J. 6, 7 (1972). 11. D. W. JAMESand G. M. LEAK, Phil. Mag. 12, 491 (1965). 12. P. KOrSTAD,High Temperature Oxidation of Metals, p. 169. Wiley, New York (1966). 13. R. HAUL, D. JUST and G. DUMGEN,Proe. Reactivity of Solids, p. 65. Elsevier, Amsterdam (1961). 14. R. J. HussEY and M. ConE1q, Corros. $ci. 11,713 (1971). 15. M. J. G~,HAM and M. COHEN,J. eleetroehem. Soe. 116, 1430 (1969).