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ELECTRICAL PROPERTIES OF ALUMINA SCALES DEVELOPED ON β NiAl AT 1100°C
D. Nicolas-Chaubet, A. Huntz, Florence Millot
To cite this version:
D. Nicolas-Chaubet, A. Huntz, Florence Millot. ELECTRICAL PROPERTIES OF ALUMINA
SCALES DEVELOPED ON β NiAl AT 1100°C. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-
1015-C1-1020. �10.1051/jphyscol:19901158�. �jpa-00230254�
CI.1015 COLLOQUE DE PHYSIQUE
Colloque CI, supplement au n°l. Tome 51, Janvier 1990
ELECTRICAL PROPERTIES OF ALUMINA SCALES DEVELOPED ON p NiAl AT 1100"C
D. NICOLAS-CHAUBET, A.M. HUNTZ and F. MILLOT
I.S.M.A.. Un±versit6 Parls-XI, F-91405 Orsay, France
Résumé - La variation de la conductivité électrique dans l'épaisseur d'une couche oxydée a été étudiée par une méthode électrochimique dansle cas de l'alumine formée par oxydation à 1100"C d'un alliage NiAl & .
Les contributions respectives de la diffusion des e s p è c e s , dans le volume de l'alumine dopée par le nickel et le long des joints de grains, sur la conductivité de la couche sont discutées .
11 est ainsi montré que la conduction électronique est une propriété du volume de l'alumine, alors que la conduction ionique est intergranulaire . Par a i l l e u r s , la croissance de la couche d'alumine est assurée principalement par la diffusion d'espèces chargées le long d e s joints de grains .
Abstract - The variation of electrical conductivity in the thickness of an oxide scale was determined by an electrochemical method applied to alumina scale formed by oxidation at 1100°C of a p" NiAl alloy . Respective contributions of the charged species diffusing in the volume of nickel doped alumina and of that diffusing along grain boundaries are discussed .
So, it is shown that electronic conduction is a volume conduction, whereas ionic conductivity is intergranular . Moreover, the growth of alumina scale is mainly ensured by charged species diffusion along grain boundaries of the scale .
1- INTRODUCTION
Thermally grown alumina is one of the most protective coatings for refractory alloys . S o , it is important to have information about the transport properties which are a determining factor for the protective character of such scales .
Problems settled by the study of transport properties of alumina scales formed by oxidation are complex : alumina is a very stoichiometric oxide ana its transport properties are mainly extrinsic, i.e. are strongly dependent on the elements and/or impurities coming from the alloy . b e s i d e s , chemical potential gradients and electrical gradient existing in the scale can lead to variations of the transport characteristics with the thickness of the oxide scale .
In this study, transport properties of alumina scales formed on a f> NiAl alloy, at 1100"C in pure oxygen, are investigated . Such an alloy offers the advantage to develop only an <x alumina scale . The method used to study transport consists in an electrochemical method based on the analysis of intensity-potential curves obtained with thermally grown alumina .
The choice of this method was supported by a preliminary theoretical study about the physical. meaning of such c h a r a c t e r i s t I C curves / l / . Main conclusions of tiiis theoretical approacn will be briefly reminded . This will offer the interest to show the possibilities and the advantages of this method for deter-mim ng transport properties of oxide scales .
2- INTEREST uF V - 1 CORVES KOK STuDtiNG TRANSPORT
The experiment consisted in imposing a stabilized current 1 to a preliminary grown oxide scale, using platinum e l e c t r o d e , and in measuring the resulting potential d i f f e r e n c e , V, between the inner and outer surfaces of the scale
I big. 1 ) .
In the theoretical approach, a formalism was developed, based on assumptions which foresee neither the diffusing species nature nor the paths by which diffusion occurs :
- only charged species contribute to transport,
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901158
COLLOQUE DE PHYSIQUE
-
the electrodes are assumed to be reversible both for ions electrons,-
local equilibrium is realized everywh'ere inside the scale,-
the matter flux is considered to occun only in one direction,-
the temperature is uniform.
Fig.1
-
Scheme of the electrical circuit used for plotting the characteri curves V-I.
So V , I , dV/dI could be expressed versus local transport parameters ir the scale, such as the total electrical conductivity U , the ionic t r a m number ti (ti = U, / U )
...
Then, it appearsa that the variation of the slope of the V-I curves obta for a stationnary state, is closely linked to the variations of conduction propkrties with the oxygen chemical potential in the scale
.
As summarized in figure 2 and table 1, the inverse of the slope of a curve may largely vary
.
As V and I varies from -a to +a, the inverse of slope is successively proportional to the conductivity for the maximum v of ti, then to the average conductivity for the highest ti values, the the average conductivity for medium values of ti, then to the ave conductivity for small values of ti, and finally to the conductivity for minimum value of ti.Table 1
-
Characteristics properties of particular points of the V-I curvFig,.2
-
a ) Relation between the evolution of the slope of the V-I curves and the transport properties .b) Scheme of the conductivity variations related to the characteristic curve of Fig.2 a).
However, it is possible to associate to a given V-I curve, different variations of conductivity vith P,, 1
.
Only a comparative study of the different intensit'y-potential curvek obtained for different oxygen partial pressures imposed to the scaleallows. to propose schemes for the variations of the ionic conductivity, Qi,
the electronic conductivity C r p and the total conductivity U , in the oxygen potential range existing from tLe inner to the outer surfaces of the alumina film.
This method offers the advantage to describe local transport properties, contrarily Co previous studies which only allowed to determine average transport properties of the scale /2/
.
Though it is semi-quantitative, it gives access to their variations inside the scale.
3
-
EXPERlMENTAL STUDY OF ELECTRlCAL PROPERTIES OF ALUMISA SCALES GROWN ONP
NiAl AT 1100°C
Experimental procedure : samples of @ NiAl (20s2x9mm3 are preliminary oxidized at 1100°C in pure 0
,
,/3/, for four days before the electrical measurements.
Characteristic curves are recorded in an original apparatus for electrical measurements on oxide scales, developed in our laboratory /2/.Different oxygen pressures can be imposed to the outer alumina surface with various gas mixtures (Ar/02 or CO/C02/Arl
.
Electrochemical measurements : from the evolution of the slope of the \-I curves recorded for oxygen pressure ranging from lom5 to 10' Pa, a diagram of the conductivity variation inside the scale is proposed (Fig.3)
.
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Fig.3
-
Evolution of the conductivity vs. the oxygen chemical potenl inside the alumina scale developed on P NiAl at 1100'C.It can be seen that for high oxygen chemical potentials
,
alumina shot dominant electronic conductivity.
On the other hand, for low oxygen chemj potentials, ionlc conductivity predominates.
These properties are interpreted in relation with the evolution of nic solubility in the thickness of the alumina scale, /1,3/, the gradient of oxygen chemical potential and the diffusion paths.
Electronic conductivit~ : an attempt was made to compare the electrc conductivity determined in our alumina scales, to those measured in synthe alumlnas, in conditions as close as possible of our experimental tests
.
hon synthetic alumina for temperature as low as 1100'C are not numerous I so the literature results obtained at the lowest temperatures, ( 1 1300-140O'CI, uere extrapolated at 1100°C
.
It was then remarked that our values are lower than those deduced from the literature data, whose ordex magnitude is 1 0 - ~ - 1 0 - ~ ' ~ 12'
.cm-' / 5 , 6 i.
An increase of the electrc conductivity by grain boundaries, as it has been suggested by Kitazawa Coble 4 or Norby and Kofstad /7/ is not obvious in our case, in spite the high density of grain boundaries in thermally grown oxides.
As electronic conductivity dominates for the highest oxygen chemj potential, it suggests that alumina is doped by an acceptar element
.
lcould be due to nickel incorporated in the scale, detected by SIMS and / l /
.
With such an assumption, a model can be proposed on the basis of Krc results on monocrystalline synthetic alumina doped with nickel /5/ : According to Kroger, nickel may dissolve in alumina with twopossible st:
Ni "ions for low oxygen chemlcal potentials, and ~ i ~ions for higher oxy ' chemical potentials
.
Thus, at 150O0C, nickel dissolved in alumina is mai constituted of ~ i j * in the oxygen pressure range from 10-' to 105 Pa /5/.
By writing the various defect equilibrium equations, it is possible express the electronic conductivity, Ue
.
It is proportional to [Ni:,]'13~(Kroger and Vink notations)
.
Extending Kroger results, it is assumed that, for our alumina scales, 110O0C, all nickel dissolwed is in xi3' state for high oxygen pressure
.
U is proportional to s ' ' ~ D ~ ~ ~ , S being the nickel solubility in alumina
.
It was shown elsewhere /l./ that, for high oxygen pressure, nickel solubil in the volume of alumina ,varies as P ; : 1 2
.
So, this model predicts variation of Ue with the "6Aygen pressure.
When P o Z decreases, on one h Ni3' will not still be the predominant defect and, on the other hand, it shown /l/ that nickel solubility strongly decreases for lower oxygen press All these reasons lead to a decrease of Ue uhen the oxygen pressure goes o a given threshold.
Thus, this model seems to esplain quite satisfactnrilv the variations of
determined in our alumina scales
.
Ionic conductivity : again, it was tried to compare the order of magnitude of our U, values with those given in the literature
.
This cpmparison was made with bcceptor doped synthetic aluminas, as suggested by our diagram of conductivity variations / 5 , 6 /.
The order of magnitude given in the literature for 6, is g 3 to 7.10''' R" .cm-' (by extrapolation).
It is lower than the ionic conductivity values estimated in our case.
An attempt was made to.see if the model used for the electronic conductivity could also explain the ionic conductivity variation
.
But, in this case, the model of conduction by the vol.ume of nickel doped alumina predicts unnegligible variations of the .ionic conductivity :-
for high Po,,
as Ni:, is the major defect, the ionic conductivity, which is proportional to [~i;,], decreases rapidly.
-
for low P O 2 , the major defect is ~ i and the ionic conductivity (ensured ; ~ by oxygen vacancies according to Kroger work / 5 / ) must vary like the nickel solubility, i.e. must decrease /1/.
So a non negligible variation is expected for the low Po,.
This does not correspond to the constant value of ai experimentally found for P,, < 10'~ Pa (see Fig.2).
So this model does not seem to be adapted for the description of ionic conduction.Then, more information could be obtained by comparing the experiaental results on conductivity in our alumina scales and thoserelated to the thermogravimetric tests, performed during an oxidation kinetics study /1/
.
If it is assumed that only charged species move during oxidation, the oxidation constant Kc can be related to the conductivity in the scale by : Kc =
-
ti (1-ti) dp12 F~
V A t Z O 3 is the molar volume of alumina, F is the Faraday constant,
p is the oxygen chemical potentia1,ll) and ( 2 ) refer to the inner and the outer interfaces of the scale,respectively,
cr is the total electrical conductivity
.
In our case. where the variation of U is not important, kc can be calculated with some reasonable approximations / I /
.
So, it was found Kc cz 2. 1 0 - l 4 cm2s".
The comparison of this calculated constant, Kc, and the experimental oxidation constant obtained by thermogravimetry, K c c x p , can allow to share the respective contribution of charged or uncharged species to transport during oxidation.
The experimental value, K c e X p -X 3.10-l4 cm2 .s-'
,
satisfactorily agrees the calculated value.
So, it can be deduced that the growth of our alumlna scales at 1100.C onP
NiAl mainly occurs by diffusion of charged species.
Besides, the order. of magnitude of our Kc values was compared with the diffusion coefficients given in the literature in the alloy and in alumina
.
Diffusion coefficients in 6 NiAl are much greater (D,
"
DA 2lo-"
cm2 s-' /8/ than our Kc values
.
It clearly indicates that the scale grouth is not controlled by the diffusion in the underlying alloy.
Moreover, Kc value appears to be too large to agree the lattice selfdiffusion coefficients in alumina : the values found in the literature for oxygen and aluminum diffusion in alumina extend between lo-" and cm2 .s-' /9,10/.
But, theorder of magnitude of the few results concerning the selfdiffusion coefficients in grain boundaries of alumina appears to be in good agreement with our Kc value : D:,,
-=
5.10-'' cm2.s-' /9/.
Studies going through /11/also seem to indicate that Dtb and ::D are of the same order of magnitude than the experimental Kc value
.
So, it can be concluded that the alumina scale developed on
P
SiAl mainly grows by grain boundary diffusion of charged species,
and that the ionicCl-1020 COLLOQUE DE PHYSIQUE
conductivity of such a scale is mainly of intergranular type
.
This ccexplain the high a, values obtained
.
It must be noted that these conclusions are in contradiction with the I
suggested in the literature by Kroger, accounting for an import contribution of neutral species in the grain boundary diffusion in alum /12,13/
.
4
-
CONCLUSIONVariations of electrical conductivity in alumina scales formed by oxidat of P NiAl at 1100°C was studied, using an electrochemical method based on analysis of the intensity-potential curves measured in alumina
.
Evolution o'f electronic conductivity with the oxygen chemical potent existing in the scale was related to a volume conductivity in a nickel dc alumina
.
On the contrary, the ionic conductivity variation is due to an intergran~
conductivity and does not show any dependence with the oxygen pressure c low oxygen chemical potential range
.
The comparison with thermogravimetric data, previously obtained, allows show that the growth of alumina on P NiAl alloy, at 1100°C and in F oxygen, is mainly ensured by the diffusion of charged species along gr boundaries
.
REFERENCES
/ l / Nicolas Chaubet D., Thesis, University Paris XI, Orsay,
812,
(1989).be published in Mbtaux, Corrosion, Industrie
.
/2/ Ben Abderrazik 'G., Millot F., Moulin G., Huntz A.M., J. Amer. Soc., (1985), 302 and 307
.
/3/ Nicolas Chaubet D., Haut C., Picard C., Millot F., Huntz X . Proceedings of the "Int. Symp. on High Temperature Corrosion", Les Embi may 1989, to be published in Mat. Sci. and Eng.
/4/ Kitazawa K., Coble R.L., J . Amer. Ceram. Soc.,
57,
6 , (19741, 245 250./5/ Chen J.S., Kroger F.A., J. Mat. Sci., 20, (1985), 3191.
/6/ Koripella C.R., Kroger F.A., J. Phps. Chem. Soc., G , 6 , (1986), 566
.
/7/ Norby T., Kofstad P., 5th Int. Conf. High Temperature and Energy, Ro Italy, may 1987, High Temperature High Pressures, to be published
.
/8/ Shankar S., Seigle L.L., Met. Trans. A , S , (1978), 1467.
/9/ Smialek J.L., Gibala H., High Temp. Corros., hACE, €j,.(1983), 274.
/10/ Kroger F.A., High Temp. Corros., NACE, 6, (1983), 89.
/11/ Prot D., Monty C., Proceedings of IlB, Paris, sept.1989
.
Les Editide Physique
.
/12/ Hou L.D., Tiku S.K., kang H.A., Kroger F.A., J. Mat. Sci.,
14,
(197 1877./13/ Kroger F.A., Adv. Ceram., 10, (1984), 100.
