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The oxidation of nickel in atmospheres containing sulphur dioxide
Corrosion Science, 1972, Vol. 12, pp. 829 to 841. Pergamon Press. Printed in Great Britain
THE OXIDATION OF NICKEL IN ATMOSPHI~RES CONTAINING SULPHUR DIOXIDE* M. R. WOOWrON Central Electricity Generative Board, Nuclear Research Laboratories, Berkeley, Gloucestershire, England and N. BIRKS Department of Metallurgy, University of Shemeld, Sheffield, England Abslxaet--The reaction occurring between nickel and argon atmospheres containing 10% sulphur dioxide has been studied over the temperature range 475-900 °. The reaction kinetics are complex and do not conform to any single simple rate law over the whole course of the reaction period. However, in most cases the reaction rate settles down to a more or less constant value, which is thought to be controlled by a reaction step at the scale-gas interface and which varies with temperature in a complex manner, showing maxima at 600° and 750°. Microprobe analysis indicates that substantial concentrations of sulphur may be dissolved in NiO. The development of the relatively complex microstructures further indicates that the dissolved sulphur migrates quite rapidly through the NiO and also enhances cation diffusion rates. R~sum&---On a ~tudi~, entre 475 et 900°, la r~action de Ni et d'une atmosphere d'argon renfermant 10%SOs. La cin~tique r~actionnelle complexe n'y obi:it pas ~ une seule loi cin~tique sim~e valable pour tout le cours de la r~ction. Cependant, le plus souvent, la r~ction prend une allure sensiblement constante qui serait r~gie par une r~ction/t rinterface ~:aille/gaz et varierait de mani~re complexe avecla teml~rature, montrant des maximums ~. 600 et ~. 750°. Des analyses par microsondage r~v~lent que de notables concentrations de soufre peuvent f:tre dissoutes dans NiO. Le d~veloppement des microstructures relativement complexes indiquent en outre que S dissous migre tr~s rapidement darts NiO et augmente ainsi la vitesse de diffusion des cations. Zusammenfassung--Die R.eaktion, die zwischen Nickel und Argonatmosph~.ren, die 10% Schwefeldioxyd enthalten, verl~.uft, wurde im Temperaturbereich 475-900 ° untersucht. Die R.eaktionskinetik ist kompliziert und stimmt iiber den ganzen Verlauf der R.eaktionsperiode nicht mit einem einzigen einfachen Geschwindigkeitsgesetz i~berein. In den meisten F~.llen jedoch stellt sich die R.eaktionsgeschwindigkeit auf einen mehr oder weniger konstanten Weft ein, yon dem angenommen wird, dass er sich nach einer R.eaktionsstufe an der Grenzfl~che zwischen der Metalltiberzugsschicht und Gas richtet und sich mit der Temperatur in komplexer Weise /indert, wobei sich Maxima bei 600 und 750° zeigen. Mikroprobenanalysen zeigen, dass sich betr~ichtliche Konzentrationen yon Schwefel in NiO aufl6sen k6nnen. Die Entwicklung der relativ komplexen Mikrostrukturen deutet ferner darauf bin, dass der gel6ste Schwefel ziemlich rasch dutch das NiO wandert und auch die Kationendiffusionsgeschwindigkeiten steigert. INTRODUCTION ALTHOUGH the sensitivity o f nickel to o x i d a ti o n in sulphurous at m o sp h er es at elevated temperatures is appreciated, I-1° there has been little systematic or reproducible work on the reactions involved. ~-7 M u c h o f this w o r k has been concerned with the conditions under which turbines operate in generating plant a n d experience o f co r r o si o n by fuel ash when m o s t o f the sulphur is present as sulphates. T h e general result is that the presence o f sulphur dioxide in the a t m o s p h e r e causes the reaction rates to be increased. In a t m o s p h e r e s c o n t a i n i n g no free oxygen, A l c o c k e t al. g and F o n t a i n e 7 f o u n d very rapid but irregular attack by sulphur dioxide. P r e - o x i d at i o n in pure oxygen was found s to reduce the sensitivity to sulphur dioxide a n d where layers thicker than 2000 A were f o r m e d no effect o f SO2 was subsequently observed. *Manuscript received 15 September 1971 ; in revised form 5 June 1972. 829
830
M . R . W o o T r o s and N. BrP,gS
There is general agreement 3's'7,s that the reaction rate for any given gas composition goes through a maximum and then decreases as the temperature is increased. The position of the maximum rate has been found to vary over the range 600-800 °3, 5.7.s and has not been explained satisfactorily. Alcock e t al. 8 suggest that the maximum is due to changes in the stabilities of the reaction products NiO and NisS=. This is not confirmed by the relevant thermodynamics although it is valid for the cobalt-oxygen-sulphur system to which the explanation was originally applied. Arkharov 5 proposes that an increase in sulphide solubility in NiO suppresses the formation of sulphides, which is not borne out by metaUography. Fontaine 7 suggests that the effect is due to the presence of traces of oxygen in the argon diluent although, after acceleration at lower temperatures, such a retardation is not expected at higher temperatures. There is general agreement that the sulphide formed is Ni.~S= with one exception 4 in favour of NiS. According to thermodynamics NiaS 2 is not stable even under one atmosphere of SO2. The occurrence of Ni=S2 in the scales formed has been explained 5 by diffusion of sulphur down grain boundaries in the oxide. This then reacts with the metal to form sulphide with subsequent oxidation producing NiO and the sulphide dispersion. 5 Alternatively, it has l~een suggested 8 that there is a dissociation of a thin, undetectable, layer of sulphate. However, a sulphate layer has never been detected and, at these temperatures, nickel sulphate is not stable under even one atmosphere of SO=. The present study was undertaken to clarify the reactions occurring in various sulphur dioxide-argon atmospheres, to specify the temperature at which the maximum rate of attack occurs and to propose a mechanism to account for the results. EXPERIMENTAL PROCEDURE Discs of nickel, containing less than 0.01wt% of carbon and only a trace of iron (as determined by X-ray spectroscopy) were prepared with the dimensions 12 mm diameter and 3 mm thick. The specimens were machined from 13 mm diameter cold swaged bar, ground wet on 600 grade silicon carbide paper and degreased in acetone. The atmospheres were provided by mixing purified argon and sulphur dioxide as described previously, u The oxygen partial pressure was estimated to be about l 0 - 5 to 10-e atm. The apparatus was identical to that used by Flatley and Birks, x2 the atmosphere being admitted to the furnace through an internal silica tube containing a thermocouple. Kinetic data were obtained using an automatic recording balance, giving an accuracy of 4- 0.2 mg, which was protected from attack by the sulphurous exit gases by using a counter stream of nitrogen. 13 The reaction was started by lifting the furnace to surround the reaction tube through which the chosen atmosphere was already flowing. Since the hot zone was 5 cm long over -{- 1° no difficulty was experienced in positioning the hot zone correctly. The furnace had previously been set 100° above the required temperature and was adjusted to the correct setting immediately after positioning. By this means the time for thermal equilibration was reduced to 10 min at all temperatures investigated. Tests were terminated by lowering the furnace and, after I0 min, purging with argon. The scales were studied by optical metallography, X-ray diffraction, microprobe
The oxidation of nickel
831
analysis and using the scanning electron microscope. The colours of the reaction products were such that etching was unnecessary. RESULTS The kinetics of the reactions and the metallography of the scales are complex and vary over the temperature range studied. To reduce confusion the kinetics and metallography will be presented separately in sections according to temperature range and then the whole system will be discussed in later sections. Initially the effect of temperature on the reaction in argon containing 10-1 atm SO2 was studied and then the effect of varying the SO~ content was investigated at various temperatures. Kinetics. The general form of the weight-gain vs. time curves, shown in Figs. 1--4, is complex and does not conform to any single rate law. Comparison of the kinetics under different conditions of atmosphere and temperature is therefore difficult except on a qualitative basis. In most cases shown in Figs. 1-3 the reaction settles down to a fairly constant rate of attack, usually after an initial period during which the reaction rate increases. At lower temperatures the reaction rate eventually slows down to very low values (Fig. 4). Table 1 shows values of the constant rates achieved under various conditions, in general these correspond to the reaction rates established during the later stages of the reactions represented in Figs. 1-3. 25
Sc~ ir
20
,~
15
E IG <~
0
30
60 Time,
Fro. I.
90
;20
1,50
rain
Effect of temperature on scaling kinetics of nickel in argon-10% dioxide 4 7 5 - 5 9 0 ° .
sulphur
M. R. WOOTrON and N. Bra~s
832
25r
Scohng Rlnetlc$ of ntckel in ~-r-IO%SO2
~y
E
/
E
I
30
FIG. 2.
I
I
60 90 Time, rain
I
I
120
150
Effect o f temperature o n scaling kinetics of nickel in a r g o n - ] 0 % dioxide 600-700 ° . 2C
ScohnCJ klnehc$ of nickel m ~r-IO%SO~
sulphur
./
810 °C~79o.c
/'~
,s
,~
ji o,c .i
•
/
120
150
" ,I// 0
30
60
90
Time, rain
FIG. 3.
Effect o f temperature o n scaling kinetics o f nickel in a r g o n - l O % dioxide 750-900 ° .
sulphur
The oxidation of nickel
833
12jScalingl kinetics of nickel in various It-S02 atmospheres I° F
-
f
PsozxlO
a~'+
61 ,111
~ 3"0
t
./"
2
0 F r o . 4.
TABLE 1.
I
3o
60
I
l
90 120 Time, rain
I
150
Effect o f s u l p h u r d i o x i d e p a r t i a l p r e s s u r e ( g i v e n in a t m o s p h e r e s ) o n s c a l i n g kinetics of nickel at 500 °.
VALUES OF CONSTANT RATES kL g c m -2 s -1 ON SCALING NICKEL IN ARGON-SULPHUR DIOXIDE ATMOSPHERES BETWEEN 4 7 5 - - 9 0 0 °
S u l p h u r d i o x i d e p a r t i a l p r e s s u r e in a t m o s p h e r e s ToC 475 500 525 550 570 580 590 600 610 620 645 700 750 790 800 810 850 870 900
5 × 10- =
9 . 0 × I 0 -~
1"70 X 10 - 6
1"18 X 1 0 - 6
10 -1 4.51 1.11 1"58 2"52 3"96 4"54 5.31 4"67 3"75 3"17 2"25 1"59 1"42 1"51 1"95 1.91 1.36 1.19
× x X X X X x X X X X X X X X x x x
1 0 -7 10- e 10- e 10 -6 10 -6 10 -6 10- 6 10- 6 10 -6 10 -6 10- 6 10 -6 10- 6 10 -6 10- 6 10- 6 10 -6 10- 6
2 × 10 -1
2 . 4 × 10 - e
5 x 10 - I
7-5 × 10 -x
3 . 3 0 × 10 - e
4 - 1 4 × 10 - e
1"16 X 10 -6
2"58 X 1 0 - 6
7"9 X 1 0 - 6
1"67 x 10 -6
4 . 1 4 x 10 -6
1"39 X 10 -6
834
M.R. Woorror~ and N. Bw.r.s
475-590 °. The constant rate increased with temperature and was succeeded by a decreasing rate period which did not conform to any obvious rate law. Above 570 ° this effect was only observed after prolonged oxidation. The results are shown in Fig. 1. An initial period of accelerating rate was only observed at 550° and above. 600-700 °. The constant rate decreased as the temperature was increased. At times much longer than those shown in Fig. 2 the rate of attack began to decrease. The initial acceleration was observed up to 620 ° above which the rate decreased gradually from the start of the reaction. 750-900 °. The kinetics followed a similar pattern but the rate increased from 750-810 ° and then decreased as the temperature was increased further. This was found to be quite reproducible and the results are given in Fig. 3. Above 810 ° a slight initial acceleration was observed. At 900 °, in this atmosphere, the kinetics were somewhat confused by the reactions occurring during the heating period. After about 45 min, however, the scaling rate settled down to a low value as shown in Fig. 3. Effect of SO~ partial pressure on the reaction kinetics. This effect was studied a, 500, 620, 790 and 900 °. In most cases, although no simple relationship was obvioust the rate of the constant rate period was increased and the decreasing rate period began earlier as the SO~ partial pressure was increased. Exceptions to this were with an SO2 partial pressure of 7.5 × 10-x atm at 620 and 790 ° where it was difficult to define a constant rate period in the curve. At 900 ° the general form of kinetics only develops for SO2 partial pressures greater than 2 × 10-1 atm. The above effects at 500° are shown in Fig. 4. All of the results of consistent rate measurements are shown in Table 1 and in Fig. 5 they are plotted against temperature, the iso-SO2 lines shown are drawn in optimistically assuming that the curves will follow the same general trend. Metallography. As reported in the literature the scale consisted of a layer of goldencoloured NisS2 adjacent to the metal covered with an outer layer of oxide. The oxide layer was NiO and contained some sulphide phase in varying modes of dispersion according to the temperature. 475--525 °. In all cases three layers were formed, a sulphide layer next to the layer surmounted by an oxide layer completely free from sulphide and finally a layer of oxide containing large irregular masses of sulphide which will be referred to as "flames". Microprobe analysis showed the flames to consist of Ni and S only with an indiction of about 5wt% of sulphur dissolved in the oxide. A typical structure is shown in Fig. 6(c). The development of the scale is shown in Fig. 6(a--c), observation of the layers after oxidizing for various times shows that the uniform oxide and sulphide layers are present from the very beginning and grow together as the time of oxidation is increased. When the flame structure develops growth of the underlying sulphide and oxide layers ceases, voids begin to form at the metal-sulphide interface and scalemetal separation begins at the rim. On increasing the sulphur dioxide partial pressure the thicknesses of the sulphide and oxide layers are both reduced and the flames in the duplex structure become a little finer. At 525 °, besides the development of flames, a fine dispersion of sulphide is also formed as shown in Fig. 7. 550--610°. In this temperature range flames do not develop but are replaced
The_oxidation of nickel
835
20
Reoction rote constant of nickel In
various Ar-SO z atmospheres
x -
\
i ,.= I0
-
;00
500
600
~:,o
\
\
700
800
900
Temperafure, =C
FIG. 5. Effect of temperature and sulphur dioxide partial pressure (given in atrno-
spheres) on the constant rate.
completely by the finer dispersion of sulphide initially seen at 525°. As the temperature is increased voids are formed within the duplex layer and scale-metal separation moves in rapidly from the specimen rim. Tests interrupted after short periods show that whereas the voids within the duplex layer develop when scale-metal separation has extended to the centre of the specimen, they do not form at the corner regions where the separation occurs much earlier. The sulphide layer also contains a fine dispersion of nickel, which is thought to be precipitated from the sulphide on cooling from the relatively disordered Nks+~,)S2 to the ordered NiaSt below about 550 °. 620-850 °. Over this range a sulphide layer adjacent to the metal was surmounted by an oxide layer in which sulphide particles are seen uniformly distributed through the oxide. The metal-sulphide interface above 6 2 0 ° is smooth with indications of some grain boundary penetration into the metal indicating that the sulphide has been liquid. A fine dispersion of nickel is also found in this sulphide layer, as at all temperatures above 550 °. On increasing the sulphur dioxide partial pressure at temperatures above 637 °, the niekel-NisS= eutectic temperature, large nickel dendrites also form as shown in Fig. 8. Large voids develop at the sulphide-oxide interface as also shown in Fig. 8. These are evident at 620 ° using normal metallography and just detectable at 610 ° using the scanning electron microscope. A series of interrupted tests at 790 ° shows that both these voids at the oxide-sulphide interface and the uniform sulphide dispersion within the oxide were present
836
M.R. WoorroN and N. BmKS
from the beginning. Also the duplex and sulphide layers are of about the same thickness and grow together. Between 790 and 850° the void density of the duplex-sulphide interface decreases as temperament is increased. On ilacreasing the sulphur dioxide partial pressure the void density and the thicknesses of both layers increase. 900 °. These scales are complicated by reactions occurring on heating through the temperature range. In 10-1 atm SO~, although a very thin sulphide layer is formed, the oxide layer contains virtually no dispersed sulphide. In places the sulphide layer has penetrated deeply down grain boundaries in the metal but whereas voids were formed at lower temperatures, at 900 ° these have apparently filled with oxide as shown in Fig. 9. On increasing the sulphur dioxide partial pressure to 5 × 10 -1 atm the structure is similar to those found in the temperature range 790-850 °. Marker studies. Platinum markers, applied as a platinum resinate solution to the metal surface, were not successful since on heating to decompose the resinate a thin oxide layer impervious to sulphur was formed on the surface. Instead, sputtered gold markers were used and although some gold dissolved in the nickel or Ni3Sz during reaction, isolated markers were observed at the sulphide-oxide interface indicating that this probably was the original metal surface and that the oxide layer grows by outwards migration of cations. This was confirmed by using the scanning electron microscope to examine the oxide surface which had been in contact with the sulphide. Figure 10 shows that this surface has, together with some adhering sulphide particles, parallel score lines which probably correspond to grooves remaining on the metal surface after preparation. Sulphur penetration. Samples were heated in purified argon, containing about 10-s atm oxygen, at 700 ° to produce oxide thicknesses of between 400-2000 A. On admitting sulphur dioxide, weight-gain kinetics and subsequent microstructures were characteristic of non-pre-oxidized samples at 900°C. With preformed oxide layers greater than about 2000 A admitting SO~ did not lead to further reactions confirming the results of Alcock et al. s DISCUSSION Since NiO and NiaS2 are the only phases to form the relevant thermodynamic data are :14 SOz = 02 + ½S~; AG~I =
86,620 -- 17-31T cal.
(1)
2Ni + 02 = 2NiO; A6-~2= -- 116,900 + 47.10T cal.
(2)
3Ni + $2 = NiaS~; AG-~3= -- 79,240 + 39"01T cal.
(3)
The oxygen partial pressure of the atmosphere is probably controlled at 10- s a t m by the purified argon. Thus, under the experimental conditions, NiO is always expected to form since, according to equation (2), the dissociation pressure for NiO is less than 10-5 atm below about 1400°. The sulphur potential of the atmosphere determined by the ambient SO2 and Oa partial pressures, is given according to equation (1) by:
. !C,’
. ‘.
. ,,,.
, ,..
-Ni
P,
b (cl
FIG. 6.
x
.
.
.-Ni
660. Scale structures on nickel exposed to argon-IO% sulphur 500” for (a) 10 min, (b) 45 min and (c) 90 min.
dioxide at
‘-Ni
9 FIG.
7.
x330.
Scale
formed
on
nickel exposed to argon-10% sulphur dioxide at 525” for 3 h. FIG. 8. x33. Scale formed on nickel exposed to argon-50% sulphur dioxide at 790” for 3 h. Etched in Merica’s reagent. FIG. 9. x460. Scale formed on nickel exposed to argon-100,i sulphur dioxide at 900” for 3 h. FIG. IO. x330. Scanning electron micrograph of scale formed on nickel in argon10% sulphur dioxide at 620” for 3 h. Oxide surface normally in contact with sulphide layer is shown. Small sulphide particles are adhering.
The oxidation of nickel log Ps, = 2 log Pso~ + 7"57 -- 379__00 ~o~ T
837 (4)
Furthermore the dissociation partial pressure of Ni3S2, P~, can be written according to equation (3), as log P~, = 8.53
17310 T
3 log etNi.
(5)
Thus NisS 2 can form under conditions where equation (6) is positive log Ps__~= 2 log Pso, __ 0"96 -- 2059_____0+ 3 log aNi. Psi /'oz T
(6)
Under the most favourable atmospheres used (1 atm SOs, 10-5 atm Oz) equation (6) does not become positive until well above the temperatures for which the data hold and consequently Ni3S~ is not expected to form from these atmospheres. The fact that sulphide has been found under almost all conditions investigated indicates that, at the scale-metal interface, the ratio PsoJPo, is not as would be expected from the bulk gas composition. Since NiO can form quite readily from these atmospheres it is logical to assume that the oxygen partial pressure is reduced at the scale-metal interface. Depending on the nickel activity at the scale-metal interface the dissociation oxygen partial pressure of NiO, P~,, from equation (2) is: log P~, = 10.3 -- 25,700/T -- 2 log ctNi.
(7)
Substituting this in equation (6) we have p~ 30810 log rS,~ = 2 log Pso, -- 21"56 + T + 7 log aNi.
(8)
According to this equation NiaS2 can form at the scale-metal interface when the righthand side is positive. For an SOs partial pressure of 10-x arm it is observed that no NiaS2 is formed at 900 °. Thus, using equations (7) and (8), it can be shown that under these conditions the oxygen partial pressure is maintained at rather more than about 6.5 times that in equilibrium with nickel and NiO. If the equilibrium oxygen partial pressure P ~ , as defined by equation (7), has been maintained then sulphide-oxide duplex structures would have been expected to occur, under 10-1 atm SO2, up to about 1040°, according to equation (8). Having defined the conditions under which NiaS 2 is formed it is now pertinent to consider the metallographic and kinetic features of the reaction. The several outstanding features are: (1) The formation of a sulphide layer next to the metal then, depending on temperature, an oxide layer surmounted by a duplex oxide-sulphide layer.
838
M. 1L Woorror~ and N. Brags
(2) The establishment of constant kinetics at nearly all temperatures. (3) The reduction of the reaction rate, after more or less long times, to almost zero. (4) The presence of the two maxima in the constant rates found at approximately 590 and 800°. The position of gold markers at the sulphide-oxide interface indicates that the oxide grows by outward migration of cations through the oxide layer. The sulphide layer formed between the metal and the oxide layer must therefore grow by inward migration of sulphur through the oxide layer reacting with cations and electrons at the sulphide-oxide interface. This is confirmed by the observation that, according to microprobe analysis, about 5wt % of sulphur is in solution in the nickel oxide and also by the smooth sulphide-oxide interface which would not be expected if diffusion down cracks were responsible. Furthermore the dissolution of sulphur in NiO appears to have increased the diffusivity of cations quite remarkably. For instance an oxide layer 1.8 × 105 A thick at 500° in 10 -1 atm SO2 transports about 1.2 × 10-9 eq. cm -2 see -1. At 500° a parabolic rate constant of kp = 2.37 × 10-13 gm 2 cm -4 sec -t is obtained by extrapolating the results 1~ of nickel oxidizing in pure oxygen, thus a transport rate of 1.2 × 10-' eq. em -~ see -1 corresponds to an oxide thickness of about 8 A! A similar observation has recently been made in the ease of copper oxides. 18 In view of the large difference in atomic radii of O 2- and S2- (1.32 A and 1.84 A respectively), it is surprising that high concentrations of sulphur can dissob, e in the oxide. Furthermore inclusion of S2- on anion sites in the NiO lattice is not expected to affect cation diffusivities. The conclusion must be that sulphur does not dissolve in NiO as S~- ions. The actual mechanism operating has not been established but it is likely to involve either neutral or positively charged sulphur species on interstitial sites, or electron co-ordination beteeen sulphur on interstitial sites and oxygen on anion sites; 15 both mechanisms leading to the formation of cation vacancies. This aspect is presently being investigated further. The formation of the sulphide fixes the sulphur partial pressure at the sulphideoxide interface at about, but slightly above, the equilibrium value for co-existence of nickel and nickel sulphide. Since sulphur diffuses in through the oxide and the sulphide layer grows continuously together with the oxide then the sulphur activity at the oxidegas interface must increase regularly with time to maintain the sulphur gradient within the oxide layer. Since initially oxide and sulphide do not form as a duplex layer, it can be inferred that nucleation of sulphide on oxide is not easy. However as the sulphur activity at the gas-scale interface increases, and since the nickel activity there is likely to be high, the activity product a~i • t~ becomes so high that eventually nucleation of NiaS2 can occur on, or just within, the oxide-gas interface. This explains the development of the microstructure as shown in Fig. 6. Once this condition has been reached the reaction can occur completely at the scale-gas surface. This can have two effects, the growth of the inner sulphide layer will cease, as observed and, secondly, the excess sulphur gradient within the oxide layer will be removed as the sulphur diffuses either to the inner sulphide layer or to the outer duplex structure. It is not clear why the flame structure seen up to 525° should give way to a fine duplex structure characteristic above 550°. It may be due to a difference in nucleation behaviour as the sulphide formed changes from highly stoiehiometric NisS2 to the
The oxidationof nickel
839
somewhat disordered Nia+xS~ at higher temperatures. In fact, the appearance of the large flame sulphides is consistent with subcutaneous nucleation and growth leading to large masses of sulphide bursting the oxide apart whereas the fine .duplex is consistent with nucleation of both phases on the scale surface simultaneously. Although the preceding discussion shows that a mechanism can be proposed to account for the main metallographic features of the scales an explanation of the various features of the kinetics is less obvious. Neither the kinetics nor the scale structures support diffusion through the scale as the rate-controlling step. Similarly the reaction rate was found to be insensitive to gas flow rate which rules out rate control by diffusion of sulphur dioxide through a boundary layer in the gas at the specimen surface. According to the thermodynamics discussed above, the scale appears to be much closer to equilibrium with the metal than with the gas and since the main kinetic feature is to show a constant rate it is appropriate to postulate that a reaction at the scale-gas phase boundary is rate controlling. If this is so then it is clear, from the deviations from constant rate shown by most of the runs for some period, that the control of rate by a phase boundary reaction is also modified by other factors. Assuming that a phase boundary reaction at the scale-gas interface is responsible for rate control, the steps involved may be represented as SO2(,) = SO2(ad) = Sad q- 20ad = O z- + SfNiO)
(9)
where S(Nio) indicates sulphur in solution in NiO. The rate of all these steps would be expected to increase with sulphur dioxide partial pressure at constant temperatures, as is in fact observed. However, the rate of the final step where adsorbed oxygen is incorporated into the lattice is also expected to depend upon the nickel activity at the scale-gas interface according to the reaction Oaa = NiO + N i m -
+2~.
(lO)
Thus a lower nickel vacancy concentration and therefore higher nickel activity would tend to increase the rate of oxygen ionization and, if this is the rate-limiting step, therefore increase the rate of the whole reaction. During the period of accelerated rate the scale grows in two distinct layers of sulphide next to the metal surmounted by oxide. It has already been shown that the growth of this structure involves dissolution of sulphur in the oxide layer with transport of sulphur through the oxide to react with nickel forming sulphide at the sulphide-oxide interface. Other nickel ions migrate out through the oxide layer to form nickel oxide at the scale-gas interface. The nickel activity there depends on the conductivity of the oxide layer for cations which is thought to increase as the sulphur content of the oxide increases. The accelerating period may therefore correspond to a period during which a stable sulphur gradient is established through the oxide layer, and during which the conductivity for cations and thus the nickel activity at the scalegas interface increases. This would lead to a continual increase in the rate of the surface reaction as discussed above. Although the mechanism appears to be feasible it is difficult to explain the variation in occurrence of the initial period of accelerated rate as a function of temperature.
840
M.R. Wooa-roNand N. B1RKS
The importance of the formation of sulphide at the metal surface in maintaining scale-metal contact may be seen from the molar volumes involved: VNi = 6"6 ml and VNi,S, = 41"2 ml. According to the overall reaction 7]2 Ni + SO'. = 2 NiO + ½Ni3S'. for each tool of SO,. reacted 23.1 ml of nickel are removed from the metal surface and replaced by 20-6 ml of NisS2. Although these are room temperature volumes it is clear that this mechanism allows relatively stress-free scales to form maintaining good scale-metal contact, perhaps accounting for the long periods over which high reaction rates can be maintained at temperatures where the creep rates of the oxide should not be high. When eventually both sulphide and oxide nucleate together at the scale-gas interface, the sulphide no longer forms between the scale and metal and therefore voids develop and scale-metal separation begins at the specimen rim as observed. The uniform dispersion of voids will progressively reduce the effective conductivity of the whole scale for cations and cause a progressive fall in nickel activity and reaction rate at the scale-gas interface. The same thing happens at the specimen rim where total scale-metal separation occurs and the reaction can only be maintained by feeding from areas of the specimen which remain in contact with the scale. The nickel activity and therefore surface reaction rate will correspondingly be much lower over these areas. The progressive advance of scale-metal separation across the whole specimen eventually leads to the rapid reduction in rate seen in the results at 500 and 525° in Fig. 1, and under other conditions at longer testing times. Temperatures greater than 637 °. Since the sulphide is molten, there can be no tearing or shear forces exerted, and the low temperature form of scale-metal parting commencing at the specimen rim is not observed. The oxide grows and behaves like a rigid envelope, with voids forming by vacancy condensation at the sulphide-oxide interface, presumably because the increase in surface energy involved there is less than would be the case at the sulphide-metal interface. The rapid and random increase in void concentration and size leads to a uniform reduction in nickel activity over the outer scale surface. This causes the interface reaction to fall, producing a correspondingiy lower reaction rate. The rate of growth of the voids in terms of their radius will be rapid at first, but will slow down as the void volume increases. This will lead to a progressive decrease in the rate of separation between oxide and sulphide, until a point is reached when the kinetics appear to be constant. The final removal of contact between sulphides and oxides gives rise to the slow rates eventually seen on exposure. This position is reached more quickly at higher SO'. partial pressures simply because the higher reaction rates mean that the voids will form more rapidly. Maxima in kinetics. The increase in the constant rate with temperature up to about 600 ° appears to be due simply to the normal effect of temperature on reaction rates. Above 600° the sulphide is very plastic and above 637 ° where the sulphide is liquid only surface tension forces can operate at the oxide-sulphide interface. These forces are not sufficient to deform the rigid oxide layer and consequently quite large voids form quickly at the sulphide-oxide interface restricting the outward cation migration.
10
0
0
9
v
u
Ii
I
1
a
mm
FIG. 5. The structure of nickel oxide scales formed at 1100”.
I -2mm
l-02
3 -02mm
FIG. 6. The distribution
Tmm
Tmm
) 2mm’
Zmm
4 of voids and internal oxide in cylindrical
a
specimens.
3*2mm
b
a
1-2mm C
d
FIG. 7. The structure of oxidized nickel rods and bars.
The oxidation of nickel
841
This is responsible for the first maximum at about 595 °. Above about 750 °, the rate of void formation apparently decreases and the reaction rate begins to increase again. This suggests that it would be informative to examine the expansion coefficients o f nickel sulphide and nickel oxide over this temperature rartge. Although less voids are being formed above 800 ° the rate now begins to fall off since, as can be seen from equation (8), the thermodynamic driving force for the formation o f sulphide is failing. It is, in fact, debatable whether the microstructure observed above 800 ° is really representative or whether it formed during the heating-up period. If the SO2 partial pressure is increased at 900 ° the driving force is increased and sulphide is found to form at this temperature giving rise to structures similar to those found a r o u n d 800 ° , although the rate is still somewhat lower than those observed in the same atmosphere at the lower temperatures. CONCLUSIONS The reaction o f pure nickel and sulphur dioxide confirms the results obtained with pure copper in that dissolution o f sulphur in the oxides appears to lead to an increase in cation diffusivity through the oxide. In the case o f nickel oxide up to 5 % sulphur is found to dissolve in the oxide and to have a high diffusivity t h r o u g h the oxide. There are difficulties in nucleating nickel sulphide on nickel oxide which leads to the formation o f a layer of sulphide at the metal surface initially, followed by the development o f a duplex layer o f sulphide and oxide growing at the scale-ga~ interface. The reaction rate is suspected o f being controlled by a phase b o u n d a r y reaction, which is sensitive to nickel activity, at the scale-gas interface. The maxima observed at 600 and 750 ° are t h o u g h t to be due to the interplay between these factors brought about by loss o f contact between scale and metal. In this case the relative values o f expansion coefficients of the various phases involved appear to be important. Acknowledgements--The authors wish to thank Professors A. G. Quarrell and G. W. Greenwood
for the provision of laboratory facilities and for encouraging the project. M.R.W. also gratefully acknowledges the award of research scholarship from the Science Research Council. REFERENCES I. W. H. J. VERNON)J. Inst. Metals 48, 121 (1932). 2. P. HANCOCK, First International Congress on Metallurgical Corrosion, p. 193. Butterworths, London (1961). 3. Y. V. RUMYANTSEVand D. M. CHIZHIKOV,ISV. Acad. Nauk. S S S R Otdel Tekh. Nauk. 147 0955). 4. V. V. IPAT'EVand D. V. ZHELLUKIN,Metallov. Obrab. Metall. 12, 42 (1958). 5. V. I. ARKHAROV,V. N. KON~V,A. F. NESTESOV,B. P. A~RJANOVSKnand I. M. P. GLAZKOV^, Issled Sharopr. Splay. Inst. Met. ,4. A. Baikova 10, 239 (1963). 6. G. PANNETIERand L. DAVIGNON,Bull. Soc. chim. Fr., 2304 (1964). 7. P. I. FONTAINE,Br. CorrosionJ. 4, 154 (1969). 8. C. B. ALCOCK,M. G. HOCrONGand S. ZADOR,Corros. Sci. 9, 11 (1969). 9. A. U. SEYBOLTand A. BELTRAN,Hot corrosion problems associated with gas turbines. A S T M Spec. Tech. Pub. 421, 21 (1967). 10. P. HANCOCk,Corrosion of Alloys in Atmospheres conMsting of Fuel Combustion Products. HMSO, London (1968). 11. N. BmKs and T. FL^TLEY,J. Sci. Instrum. (Physics E) 2, 436 (1969). 12. T. FL^TLEYand N. BmKS, J.LS.L 209, 523 (1971). 13. N. BmKs and N. TATTAM,J. Sci. Instrum. (Physics E) 2, 628 (1969). 14. O. KUnASCttEWS~,E. L. L. EVANSand C. B. ALCOCK,Metallurgical Thermochemistry. Pergamon, Oxford (1967). 15. N. BIsz~sand N. TATrAM, Corros. Sci. 10, 857 (1970).
THE OXIDATION OF NICKEL IN ATMOSPHI~RES CONTAINING SULPHUR DIOXIDE* M. R. WOOWrON Central Electricity Generative Board, Nuclear Research Laboratories, Berkeley, Gloucestershire, England and N. BIRKS Department of Metallurgy, University of Shemeld, Sheffield, England Abslxaet--The reaction occurring between nickel and argon atmospheres containing 10% sulphur dioxide has been studied over the temperature range 475-900 °. The reaction kinetics are complex and do not conform to any single simple rate law over the whole course of the reaction period. However, in most cases the reaction rate settles down to a more or less constant value, which is thought to be controlled by a reaction step at the scale-gas interface and which varies with temperature in a complex manner, showing maxima at 600° and 750°. Microprobe analysis indicates that substantial concentrations of sulphur may be dissolved in NiO. The development of the relatively complex microstructures further indicates that the dissolved sulphur migrates quite rapidly through the NiO and also enhances cation diffusion rates. R~sum&---On a ~tudi~, entre 475 et 900°, la r~action de Ni et d'une atmosphere d'argon renfermant 10%SOs. La cin~tique r~actionnelle complexe n'y obi:it pas ~ une seule loi cin~tique sim~e valable pour tout le cours de la r~ction. Cependant, le plus souvent, la r~ction prend une allure sensiblement constante qui serait r~gie par une r~ction/t rinterface ~:aille/gaz et varierait de mani~re complexe avecla teml~rature, montrant des maximums ~. 600 et ~. 750°. Des analyses par microsondage r~v~lent que de notables concentrations de soufre peuvent f:tre dissoutes dans NiO. Le d~veloppement des microstructures relativement complexes indiquent en outre que S dissous migre tr~s rapidement darts NiO et augmente ainsi la vitesse de diffusion des cations. Zusammenfassung--Die R.eaktion, die zwischen Nickel und Argonatmosph~.ren, die 10% Schwefeldioxyd enthalten, verl~.uft, wurde im Temperaturbereich 475-900 ° untersucht. Die R.eaktionskinetik ist kompliziert und stimmt iiber den ganzen Verlauf der R.eaktionsperiode nicht mit einem einzigen einfachen Geschwindigkeitsgesetz i~berein. In den meisten F~.llen jedoch stellt sich die R.eaktionsgeschwindigkeit auf einen mehr oder weniger konstanten Weft ein, yon dem angenommen wird, dass er sich nach einer R.eaktionsstufe an der Grenzfl~che zwischen der Metalltiberzugsschicht und Gas richtet und sich mit der Temperatur in komplexer Weise /indert, wobei sich Maxima bei 600 und 750° zeigen. Mikroprobenanalysen zeigen, dass sich betr~ichtliche Konzentrationen yon Schwefel in NiO aufl6sen k6nnen. Die Entwicklung der relativ komplexen Mikrostrukturen deutet ferner darauf bin, dass der gel6ste Schwefel ziemlich rasch dutch das NiO wandert und auch die Kationendiffusionsgeschwindigkeiten steigert. INTRODUCTION ALTHOUGH the sensitivity o f nickel to o x i d a ti o n in sulphurous at m o sp h er es at elevated temperatures is appreciated, I-1° there has been little systematic or reproducible work on the reactions involved. ~-7 M u c h o f this w o r k has been concerned with the conditions under which turbines operate in generating plant a n d experience o f co r r o si o n by fuel ash when m o s t o f the sulphur is present as sulphates. T h e general result is that the presence o f sulphur dioxide in the a t m o s p h e r e causes the reaction rates to be increased. In a t m o s p h e r e s c o n t a i n i n g no free oxygen, A l c o c k e t al. g and F o n t a i n e 7 f o u n d very rapid but irregular attack by sulphur dioxide. P r e - o x i d at i o n in pure oxygen was found s to reduce the sensitivity to sulphur dioxide a n d where layers thicker than 2000 A were f o r m e d no effect o f SO2 was subsequently observed. *Manuscript received 15 September 1971 ; in revised form 5 June 1972. 829
830
M . R . W o o T r o s and N. BrP,gS
There is general agreement 3's'7,s that the reaction rate for any given gas composition goes through a maximum and then decreases as the temperature is increased. The position of the maximum rate has been found to vary over the range 600-800 °3, 5.7.s and has not been explained satisfactorily. Alcock e t al. 8 suggest that the maximum is due to changes in the stabilities of the reaction products NiO and NisS=. This is not confirmed by the relevant thermodynamics although it is valid for the cobalt-oxygen-sulphur system to which the explanation was originally applied. Arkharov 5 proposes that an increase in sulphide solubility in NiO suppresses the formation of sulphides, which is not borne out by metaUography. Fontaine 7 suggests that the effect is due to the presence of traces of oxygen in the argon diluent although, after acceleration at lower temperatures, such a retardation is not expected at higher temperatures. There is general agreement that the sulphide formed is Ni.~S= with one exception 4 in favour of NiS. According to thermodynamics NiaS 2 is not stable even under one atmosphere of SO2. The occurrence of Ni=S2 in the scales formed has been explained 5 by diffusion of sulphur down grain boundaries in the oxide. This then reacts with the metal to form sulphide with subsequent oxidation producing NiO and the sulphide dispersion. 5 Alternatively, it has l~een suggested 8 that there is a dissociation of a thin, undetectable, layer of sulphate. However, a sulphate layer has never been detected and, at these temperatures, nickel sulphate is not stable under even one atmosphere of SO=. The present study was undertaken to clarify the reactions occurring in various sulphur dioxide-argon atmospheres, to specify the temperature at which the maximum rate of attack occurs and to propose a mechanism to account for the results. EXPERIMENTAL PROCEDURE Discs of nickel, containing less than 0.01wt% of carbon and only a trace of iron (as determined by X-ray spectroscopy) were prepared with the dimensions 12 mm diameter and 3 mm thick. The specimens were machined from 13 mm diameter cold swaged bar, ground wet on 600 grade silicon carbide paper and degreased in acetone. The atmospheres were provided by mixing purified argon and sulphur dioxide as described previously, u The oxygen partial pressure was estimated to be about l 0 - 5 to 10-e atm. The apparatus was identical to that used by Flatley and Birks, x2 the atmosphere being admitted to the furnace through an internal silica tube containing a thermocouple. Kinetic data were obtained using an automatic recording balance, giving an accuracy of 4- 0.2 mg, which was protected from attack by the sulphurous exit gases by using a counter stream of nitrogen. 13 The reaction was started by lifting the furnace to surround the reaction tube through which the chosen atmosphere was already flowing. Since the hot zone was 5 cm long over -{- 1° no difficulty was experienced in positioning the hot zone correctly. The furnace had previously been set 100° above the required temperature and was adjusted to the correct setting immediately after positioning. By this means the time for thermal equilibration was reduced to 10 min at all temperatures investigated. Tests were terminated by lowering the furnace and, after I0 min, purging with argon. The scales were studied by optical metallography, X-ray diffraction, microprobe
The oxidation of nickel
831
analysis and using the scanning electron microscope. The colours of the reaction products were such that etching was unnecessary. RESULTS The kinetics of the reactions and the metallography of the scales are complex and vary over the temperature range studied. To reduce confusion the kinetics and metallography will be presented separately in sections according to temperature range and then the whole system will be discussed in later sections. Initially the effect of temperature on the reaction in argon containing 10-1 atm SO2 was studied and then the effect of varying the SO~ content was investigated at various temperatures. Kinetics. The general form of the weight-gain vs. time curves, shown in Figs. 1--4, is complex and does not conform to any single rate law. Comparison of the kinetics under different conditions of atmosphere and temperature is therefore difficult except on a qualitative basis. In most cases shown in Figs. 1-3 the reaction settles down to a fairly constant rate of attack, usually after an initial period during which the reaction rate increases. At lower temperatures the reaction rate eventually slows down to very low values (Fig. 4). Table 1 shows values of the constant rates achieved under various conditions, in general these correspond to the reaction rates established during the later stages of the reactions represented in Figs. 1-3. 25
Sc~ ir
20
,~
15
E IG <~
0
30
60 Time,
Fro. I.
90
;20
1,50
rain
Effect of temperature on scaling kinetics of nickel in argon-10% dioxide 4 7 5 - 5 9 0 ° .
sulphur
M. R. WOOTrON and N. Bra~s
832
25r
Scohng Rlnetlc$ of ntckel in ~-r-IO%SO2
~y
E
/
E
I
30
FIG. 2.
I
I
60 90 Time, rain
I
I
120
150
Effect o f temperature o n scaling kinetics of nickel in a r g o n - ] 0 % dioxide 600-700 ° . 2C
ScohnCJ klnehc$ of nickel m ~r-IO%SO~
sulphur
./
810 °C~79o.c
/'~
,s
,~
ji o,c .i
•
/
120
150
" ,I// 0
30
60
90
Time, rain
FIG. 3.
Effect o f temperature o n scaling kinetics o f nickel in a r g o n - l O % dioxide 750-900 ° .
sulphur
The oxidation of nickel
833
12jScalingl kinetics of nickel in various It-S02 atmospheres I° F
-
f
PsozxlO
a~'+
61 ,111
~ 3"0
t
./"
2
0 F r o . 4.
TABLE 1.
I
3o
60
I
l
90 120 Time, rain
I
150
Effect o f s u l p h u r d i o x i d e p a r t i a l p r e s s u r e ( g i v e n in a t m o s p h e r e s ) o n s c a l i n g kinetics of nickel at 500 °.
VALUES OF CONSTANT RATES kL g c m -2 s -1 ON SCALING NICKEL IN ARGON-SULPHUR DIOXIDE ATMOSPHERES BETWEEN 4 7 5 - - 9 0 0 °
S u l p h u r d i o x i d e p a r t i a l p r e s s u r e in a t m o s p h e r e s ToC 475 500 525 550 570 580 590 600 610 620 645 700 750 790 800 810 850 870 900
5 × 10- =
9 . 0 × I 0 -~
1"70 X 10 - 6
1"18 X 1 0 - 6
10 -1 4.51 1.11 1"58 2"52 3"96 4"54 5.31 4"67 3"75 3"17 2"25 1"59 1"42 1"51 1"95 1.91 1.36 1.19
× x X X X X x X X X X X X X X x x x
1 0 -7 10- e 10- e 10 -6 10 -6 10 -6 10- 6 10- 6 10 -6 10 -6 10- 6 10 -6 10- 6 10 -6 10- 6 10- 6 10 -6 10- 6
2 × 10 -1
2 . 4 × 10 - e
5 x 10 - I
7-5 × 10 -x
3 . 3 0 × 10 - e
4 - 1 4 × 10 - e
1"16 X 10 -6
2"58 X 1 0 - 6
7"9 X 1 0 - 6
1"67 x 10 -6
4 . 1 4 x 10 -6
1"39 X 10 -6
834
M.R. Woorror~ and N. Bw.r.s
475-590 °. The constant rate increased with temperature and was succeeded by a decreasing rate period which did not conform to any obvious rate law. Above 570 ° this effect was only observed after prolonged oxidation. The results are shown in Fig. 1. An initial period of accelerating rate was only observed at 550° and above. 600-700 °. The constant rate decreased as the temperature was increased. At times much longer than those shown in Fig. 2 the rate of attack began to decrease. The initial acceleration was observed up to 620 ° above which the rate decreased gradually from the start of the reaction. 750-900 °. The kinetics followed a similar pattern but the rate increased from 750-810 ° and then decreased as the temperature was increased further. This was found to be quite reproducible and the results are given in Fig. 3. Above 810 ° a slight initial acceleration was observed. At 900 °, in this atmosphere, the kinetics were somewhat confused by the reactions occurring during the heating period. After about 45 min, however, the scaling rate settled down to a low value as shown in Fig. 3. Effect of SO~ partial pressure on the reaction kinetics. This effect was studied a, 500, 620, 790 and 900 °. In most cases, although no simple relationship was obvioust the rate of the constant rate period was increased and the decreasing rate period began earlier as the SO~ partial pressure was increased. Exceptions to this were with an SO2 partial pressure of 7.5 × 10-x atm at 620 and 790 ° where it was difficult to define a constant rate period in the curve. At 900 ° the general form of kinetics only develops for SO2 partial pressures greater than 2 × 10-1 atm. The above effects at 500° are shown in Fig. 4. All of the results of consistent rate measurements are shown in Table 1 and in Fig. 5 they are plotted against temperature, the iso-SO2 lines shown are drawn in optimistically assuming that the curves will follow the same general trend. Metallography. As reported in the literature the scale consisted of a layer of goldencoloured NisS2 adjacent to the metal covered with an outer layer of oxide. The oxide layer was NiO and contained some sulphide phase in varying modes of dispersion according to the temperature. 475--525 °. In all cases three layers were formed, a sulphide layer next to the layer surmounted by an oxide layer completely free from sulphide and finally a layer of oxide containing large irregular masses of sulphide which will be referred to as "flames". Microprobe analysis showed the flames to consist of Ni and S only with an indiction of about 5wt% of sulphur dissolved in the oxide. A typical structure is shown in Fig. 6(c). The development of the scale is shown in Fig. 6(a--c), observation of the layers after oxidizing for various times shows that the uniform oxide and sulphide layers are present from the very beginning and grow together as the time of oxidation is increased. When the flame structure develops growth of the underlying sulphide and oxide layers ceases, voids begin to form at the metal-sulphide interface and scalemetal separation begins at the rim. On increasing the sulphur dioxide partial pressure the thicknesses of the sulphide and oxide layers are both reduced and the flames in the duplex structure become a little finer. At 525 °, besides the development of flames, a fine dispersion of sulphide is also formed as shown in Fig. 7. 550--610°. In this temperature range flames do not develop but are replaced
The_oxidation of nickel
835
20
Reoction rote constant of nickel In
various Ar-SO z atmospheres
x -
\
i ,.= I0
-
;00
500
600
~:,o
\
\
700
800
900
Temperafure, =C
FIG. 5. Effect of temperature and sulphur dioxide partial pressure (given in atrno-
spheres) on the constant rate.
completely by the finer dispersion of sulphide initially seen at 525°. As the temperature is increased voids are formed within the duplex layer and scale-metal separation moves in rapidly from the specimen rim. Tests interrupted after short periods show that whereas the voids within the duplex layer develop when scale-metal separation has extended to the centre of the specimen, they do not form at the corner regions where the separation occurs much earlier. The sulphide layer also contains a fine dispersion of nickel, which is thought to be precipitated from the sulphide on cooling from the relatively disordered Nks+~,)S2 to the ordered NiaSt below about 550 °. 620-850 °. Over this range a sulphide layer adjacent to the metal was surmounted by an oxide layer in which sulphide particles are seen uniformly distributed through the oxide. The metal-sulphide interface above 6 2 0 ° is smooth with indications of some grain boundary penetration into the metal indicating that the sulphide has been liquid. A fine dispersion of nickel is also found in this sulphide layer, as at all temperatures above 550 °. On increasing the sulphur dioxide partial pressure at temperatures above 637 °, the niekel-NisS= eutectic temperature, large nickel dendrites also form as shown in Fig. 8. Large voids develop at the sulphide-oxide interface as also shown in Fig. 8. These are evident at 620 ° using normal metallography and just detectable at 610 ° using the scanning electron microscope. A series of interrupted tests at 790 ° shows that both these voids at the oxide-sulphide interface and the uniform sulphide dispersion within the oxide were present
836
M.R. WoorroN and N. BmKS
from the beginning. Also the duplex and sulphide layers are of about the same thickness and grow together. Between 790 and 850° the void density of the duplex-sulphide interface decreases as temperament is increased. On ilacreasing the sulphur dioxide partial pressure the void density and the thicknesses of both layers increase. 900 °. These scales are complicated by reactions occurring on heating through the temperature range. In 10-1 atm SO~, although a very thin sulphide layer is formed, the oxide layer contains virtually no dispersed sulphide. In places the sulphide layer has penetrated deeply down grain boundaries in the metal but whereas voids were formed at lower temperatures, at 900 ° these have apparently filled with oxide as shown in Fig. 9. On increasing the sulphur dioxide partial pressure to 5 × 10 -1 atm the structure is similar to those found in the temperature range 790-850 °. Marker studies. Platinum markers, applied as a platinum resinate solution to the metal surface, were not successful since on heating to decompose the resinate a thin oxide layer impervious to sulphur was formed on the surface. Instead, sputtered gold markers were used and although some gold dissolved in the nickel or Ni3Sz during reaction, isolated markers were observed at the sulphide-oxide interface indicating that this probably was the original metal surface and that the oxide layer grows by outwards migration of cations. This was confirmed by using the scanning electron microscope to examine the oxide surface which had been in contact with the sulphide. Figure 10 shows that this surface has, together with some adhering sulphide particles, parallel score lines which probably correspond to grooves remaining on the metal surface after preparation. Sulphur penetration. Samples were heated in purified argon, containing about 10-s atm oxygen, at 700 ° to produce oxide thicknesses of between 400-2000 A. On admitting sulphur dioxide, weight-gain kinetics and subsequent microstructures were characteristic of non-pre-oxidized samples at 900°C. With preformed oxide layers greater than about 2000 A admitting SO~ did not lead to further reactions confirming the results of Alcock et al. s DISCUSSION Since NiO and NiaS2 are the only phases to form the relevant thermodynamic data are :14 SOz = 02 + ½S~; AG~I =
86,620 -- 17-31T cal.
(1)
2Ni + 02 = 2NiO; A6-~2= -- 116,900 + 47.10T cal.
(2)
3Ni + $2 = NiaS~; AG-~3= -- 79,240 + 39"01T cal.
(3)
The oxygen partial pressure of the atmosphere is probably controlled at 10- s a t m by the purified argon. Thus, under the experimental conditions, NiO is always expected to form since, according to equation (2), the dissociation pressure for NiO is less than 10-5 atm below about 1400°. The sulphur potential of the atmosphere determined by the ambient SO2 and Oa partial pressures, is given according to equation (1) by:
. !C,’
. ‘.
. ,,,.
, ,..
-Ni
P,
b (cl
FIG. 6.
x
.
.
.-Ni
660. Scale structures on nickel exposed to argon-IO% sulphur 500” for (a) 10 min, (b) 45 min and (c) 90 min.
dioxide at
‘-Ni
9 FIG.
7.
x330.
Scale
formed
on
nickel exposed to argon-10% sulphur dioxide at 525” for 3 h. FIG. 8. x33. Scale formed on nickel exposed to argon-50% sulphur dioxide at 790” for 3 h. Etched in Merica’s reagent. FIG. 9. x460. Scale formed on nickel exposed to argon-100,i sulphur dioxide at 900” for 3 h. FIG. IO. x330. Scanning electron micrograph of scale formed on nickel in argon10% sulphur dioxide at 620” for 3 h. Oxide surface normally in contact with sulphide layer is shown. Small sulphide particles are adhering.
The oxidation of nickel log Ps, = 2 log Pso~ + 7"57 -- 379__00 ~o~ T
837 (4)
Furthermore the dissociation partial pressure of Ni3S2, P~, can be written according to equation (3), as log P~, = 8.53
17310 T
3 log etNi.
(5)
Thus NisS 2 can form under conditions where equation (6) is positive log Ps__~= 2 log Pso, __ 0"96 -- 2059_____0+ 3 log aNi. Psi /'oz T
(6)
Under the most favourable atmospheres used (1 atm SOs, 10-5 atm Oz) equation (6) does not become positive until well above the temperatures for which the data hold and consequently Ni3S~ is not expected to form from these atmospheres. The fact that sulphide has been found under almost all conditions investigated indicates that, at the scale-metal interface, the ratio PsoJPo, is not as would be expected from the bulk gas composition. Since NiO can form quite readily from these atmospheres it is logical to assume that the oxygen partial pressure is reduced at the scale-metal interface. Depending on the nickel activity at the scale-metal interface the dissociation oxygen partial pressure of NiO, P~,, from equation (2) is: log P~, = 10.3 -- 25,700/T -- 2 log ctNi.
(7)
Substituting this in equation (6) we have p~ 30810 log rS,~ = 2 log Pso, -- 21"56 + T + 7 log aNi.
(8)
According to this equation NiaS2 can form at the scale-metal interface when the righthand side is positive. For an SOs partial pressure of 10-x arm it is observed that no NiaS2 is formed at 900 °. Thus, using equations (7) and (8), it can be shown that under these conditions the oxygen partial pressure is maintained at rather more than about 6.5 times that in equilibrium with nickel and NiO. If the equilibrium oxygen partial pressure P ~ , as defined by equation (7), has been maintained then sulphide-oxide duplex structures would have been expected to occur, under 10-1 atm SO2, up to about 1040°, according to equation (8). Having defined the conditions under which NiaS 2 is formed it is now pertinent to consider the metallographic and kinetic features of the reaction. The several outstanding features are: (1) The formation of a sulphide layer next to the metal then, depending on temperature, an oxide layer surmounted by a duplex oxide-sulphide layer.
838
M. 1L Woorror~ and N. Brags
(2) The establishment of constant kinetics at nearly all temperatures. (3) The reduction of the reaction rate, after more or less long times, to almost zero. (4) The presence of the two maxima in the constant rates found at approximately 590 and 800°. The position of gold markers at the sulphide-oxide interface indicates that the oxide grows by outward migration of cations through the oxide layer. The sulphide layer formed between the metal and the oxide layer must therefore grow by inward migration of sulphur through the oxide layer reacting with cations and electrons at the sulphide-oxide interface. This is confirmed by the observation that, according to microprobe analysis, about 5wt % of sulphur is in solution in the nickel oxide and also by the smooth sulphide-oxide interface which would not be expected if diffusion down cracks were responsible. Furthermore the dissolution of sulphur in NiO appears to have increased the diffusivity of cations quite remarkably. For instance an oxide layer 1.8 × 105 A thick at 500° in 10 -1 atm SO2 transports about 1.2 × 10-9 eq. cm -2 see -1. At 500° a parabolic rate constant of kp = 2.37 × 10-13 gm 2 cm -4 sec -t is obtained by extrapolating the results 1~ of nickel oxidizing in pure oxygen, thus a transport rate of 1.2 × 10-' eq. em -~ see -1 corresponds to an oxide thickness of about 8 A! A similar observation has recently been made in the ease of copper oxides. 18 In view of the large difference in atomic radii of O 2- and S2- (1.32 A and 1.84 A respectively), it is surprising that high concentrations of sulphur can dissob, e in the oxide. Furthermore inclusion of S2- on anion sites in the NiO lattice is not expected to affect cation diffusivities. The conclusion must be that sulphur does not dissolve in NiO as S~- ions. The actual mechanism operating has not been established but it is likely to involve either neutral or positively charged sulphur species on interstitial sites, or electron co-ordination beteeen sulphur on interstitial sites and oxygen on anion sites; 15 both mechanisms leading to the formation of cation vacancies. This aspect is presently being investigated further. The formation of the sulphide fixes the sulphur partial pressure at the sulphideoxide interface at about, but slightly above, the equilibrium value for co-existence of nickel and nickel sulphide. Since sulphur diffuses in through the oxide and the sulphide layer grows continuously together with the oxide then the sulphur activity at the oxidegas interface must increase regularly with time to maintain the sulphur gradient within the oxide layer. Since initially oxide and sulphide do not form as a duplex layer, it can be inferred that nucleation of sulphide on oxide is not easy. However as the sulphur activity at the gas-scale interface increases, and since the nickel activity there is likely to be high, the activity product a~i • t~ becomes so high that eventually nucleation of NiaS2 can occur on, or just within, the oxide-gas interface. This explains the development of the microstructure as shown in Fig. 6. Once this condition has been reached the reaction can occur completely at the scale-gas surface. This can have two effects, the growth of the inner sulphide layer will cease, as observed and, secondly, the excess sulphur gradient within the oxide layer will be removed as the sulphur diffuses either to the inner sulphide layer or to the outer duplex structure. It is not clear why the flame structure seen up to 525° should give way to a fine duplex structure characteristic above 550°. It may be due to a difference in nucleation behaviour as the sulphide formed changes from highly stoiehiometric NisS2 to the
The oxidationof nickel
839
somewhat disordered Nia+xS~ at higher temperatures. In fact, the appearance of the large flame sulphides is consistent with subcutaneous nucleation and growth leading to large masses of sulphide bursting the oxide apart whereas the fine .duplex is consistent with nucleation of both phases on the scale surface simultaneously. Although the preceding discussion shows that a mechanism can be proposed to account for the main metallographic features of the scales an explanation of the various features of the kinetics is less obvious. Neither the kinetics nor the scale structures support diffusion through the scale as the rate-controlling step. Similarly the reaction rate was found to be insensitive to gas flow rate which rules out rate control by diffusion of sulphur dioxide through a boundary layer in the gas at the specimen surface. According to the thermodynamics discussed above, the scale appears to be much closer to equilibrium with the metal than with the gas and since the main kinetic feature is to show a constant rate it is appropriate to postulate that a reaction at the scale-gas phase boundary is rate controlling. If this is so then it is clear, from the deviations from constant rate shown by most of the runs for some period, that the control of rate by a phase boundary reaction is also modified by other factors. Assuming that a phase boundary reaction at the scale-gas interface is responsible for rate control, the steps involved may be represented as SO2(,) = SO2(ad) = Sad q- 20ad = O z- + SfNiO)
(9)
where S(Nio) indicates sulphur in solution in NiO. The rate of all these steps would be expected to increase with sulphur dioxide partial pressure at constant temperatures, as is in fact observed. However, the rate of the final step where adsorbed oxygen is incorporated into the lattice is also expected to depend upon the nickel activity at the scale-gas interface according to the reaction Oaa = NiO + N i m -
+2~.
(lO)
Thus a lower nickel vacancy concentration and therefore higher nickel activity would tend to increase the rate of oxygen ionization and, if this is the rate-limiting step, therefore increase the rate of the whole reaction. During the period of accelerated rate the scale grows in two distinct layers of sulphide next to the metal surmounted by oxide. It has already been shown that the growth of this structure involves dissolution of sulphur in the oxide layer with transport of sulphur through the oxide to react with nickel forming sulphide at the sulphide-oxide interface. Other nickel ions migrate out through the oxide layer to form nickel oxide at the scale-gas interface. The nickel activity there depends on the conductivity of the oxide layer for cations which is thought to increase as the sulphur content of the oxide increases. The accelerating period may therefore correspond to a period during which a stable sulphur gradient is established through the oxide layer, and during which the conductivity for cations and thus the nickel activity at the scalegas interface increases. This would lead to a continual increase in the rate of the surface reaction as discussed above. Although the mechanism appears to be feasible it is difficult to explain the variation in occurrence of the initial period of accelerated rate as a function of temperature.
840
M.R. Wooa-roNand N. B1RKS
The importance of the formation of sulphide at the metal surface in maintaining scale-metal contact may be seen from the molar volumes involved: VNi = 6"6 ml and VNi,S, = 41"2 ml. According to the overall reaction 7]2 Ni + SO'. = 2 NiO + ½Ni3S'. for each tool of SO,. reacted 23.1 ml of nickel are removed from the metal surface and replaced by 20-6 ml of NisS2. Although these are room temperature volumes it is clear that this mechanism allows relatively stress-free scales to form maintaining good scale-metal contact, perhaps accounting for the long periods over which high reaction rates can be maintained at temperatures where the creep rates of the oxide should not be high. When eventually both sulphide and oxide nucleate together at the scale-gas interface, the sulphide no longer forms between the scale and metal and therefore voids develop and scale-metal separation begins at the specimen rim as observed. The uniform dispersion of voids will progressively reduce the effective conductivity of the whole scale for cations and cause a progressive fall in nickel activity and reaction rate at the scale-gas interface. The same thing happens at the specimen rim where total scale-metal separation occurs and the reaction can only be maintained by feeding from areas of the specimen which remain in contact with the scale. The nickel activity and therefore surface reaction rate will correspondingly be much lower over these areas. The progressive advance of scale-metal separation across the whole specimen eventually leads to the rapid reduction in rate seen in the results at 500 and 525° in Fig. 1, and under other conditions at longer testing times. Temperatures greater than 637 °. Since the sulphide is molten, there can be no tearing or shear forces exerted, and the low temperature form of scale-metal parting commencing at the specimen rim is not observed. The oxide grows and behaves like a rigid envelope, with voids forming by vacancy condensation at the sulphide-oxide interface, presumably because the increase in surface energy involved there is less than would be the case at the sulphide-metal interface. The rapid and random increase in void concentration and size leads to a uniform reduction in nickel activity over the outer scale surface. This causes the interface reaction to fall, producing a correspondingiy lower reaction rate. The rate of growth of the voids in terms of their radius will be rapid at first, but will slow down as the void volume increases. This will lead to a progressive decrease in the rate of separation between oxide and sulphide, until a point is reached when the kinetics appear to be constant. The final removal of contact between sulphides and oxides gives rise to the slow rates eventually seen on exposure. This position is reached more quickly at higher SO'. partial pressures simply because the higher reaction rates mean that the voids will form more rapidly. Maxima in kinetics. The increase in the constant rate with temperature up to about 600 ° appears to be due simply to the normal effect of temperature on reaction rates. Above 600° the sulphide is very plastic and above 637 ° where the sulphide is liquid only surface tension forces can operate at the oxide-sulphide interface. These forces are not sufficient to deform the rigid oxide layer and consequently quite large voids form quickly at the sulphide-oxide interface restricting the outward cation migration.
10
0
0
9
v
u
Ii
I
1
a
mm
FIG. 5. The structure of nickel oxide scales formed at 1100”.
I -2mm
l-02
3 -02mm
FIG. 6. The distribution
Tmm
Tmm
) 2mm’
Zmm
4 of voids and internal oxide in cylindrical
a
specimens.
3*2mm
b
a
1-2mm C
d
FIG. 7. The structure of oxidized nickel rods and bars.
The oxidation of nickel
841
This is responsible for the first maximum at about 595 °. Above about 750 °, the rate of void formation apparently decreases and the reaction rate begins to increase again. This suggests that it would be informative to examine the expansion coefficients o f nickel sulphide and nickel oxide over this temperature rartge. Although less voids are being formed above 800 ° the rate now begins to fall off since, as can be seen from equation (8), the thermodynamic driving force for the formation o f sulphide is failing. It is, in fact, debatable whether the microstructure observed above 800 ° is really representative or whether it formed during the heating-up period. If the SO2 partial pressure is increased at 900 ° the driving force is increased and sulphide is found to form at this temperature giving rise to structures similar to those found a r o u n d 800 ° , although the rate is still somewhat lower than those observed in the same atmosphere at the lower temperatures. CONCLUSIONS The reaction o f pure nickel and sulphur dioxide confirms the results obtained with pure copper in that dissolution o f sulphur in the oxides appears to lead to an increase in cation diffusivity through the oxide. In the case o f nickel oxide up to 5 % sulphur is found to dissolve in the oxide and to have a high diffusivity t h r o u g h the oxide. There are difficulties in nucleating nickel sulphide on nickel oxide which leads to the formation o f a layer of sulphide at the metal surface initially, followed by the development o f a duplex layer o f sulphide and oxide growing at the scale-ga~ interface. The reaction rate is suspected o f being controlled by a phase b o u n d a r y reaction, which is sensitive to nickel activity, at the scale-gas interface. The maxima observed at 600 and 750 ° are t h o u g h t to be due to the interplay between these factors brought about by loss o f contact between scale and metal. In this case the relative values o f expansion coefficients of the various phases involved appear to be important. Acknowledgements--The authors wish to thank Professors A. G. Quarrell and G. W. Greenwood
for the provision of laboratory facilities and for encouraging the project. M.R.W. also gratefully acknowledges the award of research scholarship from the Science Research Council. REFERENCES I. W. H. J. VERNON)J. Inst. Metals 48, 121 (1932). 2. P. HANCOCK, First International Congress on Metallurgical Corrosion, p. 193. Butterworths, London (1961). 3. Y. V. RUMYANTSEVand D. M. CHIZHIKOV,ISV. Acad. Nauk. S S S R Otdel Tekh. Nauk. 147 0955). 4. V. V. IPAT'EVand D. V. ZHELLUKIN,Metallov. Obrab. Metall. 12, 42 (1958). 5. V. I. ARKHAROV,V. N. KON~V,A. F. NESTESOV,B. P. A~RJANOVSKnand I. M. P. GLAZKOV^, Issled Sharopr. Splay. Inst. Met. ,4. A. Baikova 10, 239 (1963). 6. G. PANNETIERand L. DAVIGNON,Bull. Soc. chim. Fr., 2304 (1964). 7. P. I. FONTAINE,Br. CorrosionJ. 4, 154 (1969). 8. C. B. ALCOCK,M. G. HOCrONGand S. ZADOR,Corros. Sci. 9, 11 (1969). 9. A. U. SEYBOLTand A. BELTRAN,Hot corrosion problems associated with gas turbines. A S T M Spec. Tech. Pub. 421, 21 (1967). 10. P. HANCOCk,Corrosion of Alloys in Atmospheres conMsting of Fuel Combustion Products. HMSO, London (1968). 11. N. BmKs and T. FL^TLEY,J. Sci. Instrum. (Physics E) 2, 436 (1969). 12. T. FL^TLEYand N. BmKS, J.LS.L 209, 523 (1971). 13. N. BmKs and N. TATTAM,J. Sci. Instrum. (Physics E) 2, 628 (1969). 14. O. KUnASCttEWS~,E. L. L. EVANSand C. B. ALCOCK,Metallurgical Thermochemistry. Pergamon, Oxford (1967). 15. N. BIsz~sand N. TATrAM, Corros. Sci. 10, 857 (1970).