JOURNAL DE PHYSIQUE
Colloque Cl, suppl6ment au n 0 2 , Tome 47, fgvrier 1986
ELECTRIC PROPERTIES OF A1,0, SCALES DEVELOPED BY OXIDATION, IN RELATION WITH THE DOPANT NATURE AND DISTRIBUTION IN THE SCALE
G. BEN ABDERRAZIK, G. MOULIN, F. MILLOT* and A.M. HUNTZ Laboratoire de MBtallurgie Structurale, UA 1107, Universite Paris X I , F-91405 Orsay Cedex, France
" ~ a b o r a t o i r e de Themodynamique et Physicochimie des MatBriaux, UA 446, Universite Paris XI, F-91405 Orsay Cedex, France
EXTENTED A B S T R A E ( f o r more i n f o r m a t i o n s , see references /l/ t o / 3 / ) .
The understanding of oxidatlon mechanisms of alloys requires the knowledge of the oxide transport properties. In most cases, up to now, these properties were determined from conductlvity and diffusion measurements on "pure" and doped single crystalilne or poiycrlstaiiine oxides whose behaviour can be different from thermally grown oxide behaviour.
Consequently, a laboratory apparatus / l / was developed i n order to determine the transport properties of thermally grown oxides. i t consists:
- either In measuring the residual potential difference Vo between the outer and inner oxide interfaces, which gives the ionic transport number ti,
- or In plotting the electrical characteristic curves V = f ( i ) of the thermally grown oxide scale, which leads to the average conductivity U, then the ionlc average conductivity ui and the electronic average conductivity U,,
- or in accelerating the oxide growth while oxidizing under an applied electric fields. Now,the effective charge Z' of the moving species In the oxide scale is obtained.
The formalism used Is developed in ref. /l/. This procedure was applled to the determlnation of the transport properties of alumina scales developed on synthetic FeCrp3A15.
Such alumina scale growth depends on the incorporated impurities which are detected and anaiyzed by SCAN+EDAX, electron microprobe and XPS. Most of the resuits are detailed i n ref. /2/ and / 3 / .
It appears that:
- The alumina scale Is characterized by a residual potential Vo (without an applied electrlc field, i.e. for i = 0). The Vo value is dlrectiy related to the oxygen chemlcai potentlai gradient in the oxide and to the ionic transport number:
Fig. 1, relative to characteristic curves V = f ( i ) indicates that Vo(i=O) decreases wlth the outer P o p . Moreover, for low outer equillbrlum Pop, the diode effect of the alumina scale, as reported by others works (4) for high applied voltages, is no longer observed.
- Whatever the outer Pop, the alumina scale behaves as a mixed conductor, wlth tl varylng from 0.45 to 0. 24 when the outer Pop decreases from 1 to - 10-zsatm.
This result indicates that thermally grown alumina can differ from synthetic alumina /5,6/.
- All the transport parameters: Vo, a, oi, ae, ti, depend on the oxidation temperature and tlme. Particularly, increasing the oxidation time induces impurity incorporation in the scale, as shown for example by Fig. 2 and 3, and a sllght Increase of the transport parameters ( i . e . U, U@, UI and ti ).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986126
JOURNAL DE PHYSIQUE
F i g . 1 : C h a r a c t e r i s t i c V = f ( i ) c u r v e s o f a l u m i n a d e v e l o p e d on a F e C r A l a l l o y a t 1080°C
-1- Oxidation time:llOh at 105OoC
Alloy
4
3d5 300 295 290 285 280 275 270 2 6 5 2 6 0 2 5 5
-
BINDING ENERGY ( e V )-
BINDING ENERGY ( e V )Fig.2 and 3: XPS analyses f o r chemical state o f chromium (2) and carbon (3) i n alumina scales developed on a not very pure FeCrAl a l l o y by o x i d a t i o n f o r various times under l a t m 02.
- Applying an electric field modifies the aiumina growth rate and indicates that movlng spocies is an ionic one. Assuming that aiumina growth preferentially occurs by oxygen diffusion, the effective charge of this moving species is found equal to -2. Thus, it appears that aiumina growth mainly occurs by %c oxygen diffusion: neutral or molecular oxygen diffusion is ??giigible. Nevertheless the nature of the defect responslbie for this diffusion, Vi)' . o r O i cannot be pointed out. More experlments are needed.
- The mlgratlon activation energy of the main atomlc point defect ( V 6 or 0i') Is equal to 366KJ. m o r f (3.8eV).
- A breakdown is observed In the curves ai and me versus temperature (for -
9 5 0 0 , due to either the influence of oxide grain boundaries or precipitation phenomena.
REFERENCES
/ l / Ben Abderrazlk, G. , Moulin, G. , Millot, F. and Huntz,A. M., J. Amer. Ceram.
Soc. 68 (1985) (June) I
/ 2 / Ben Abderrazik,G.. Moulln,G., Miilot F. and Hunt2.A. M., J. Arner. Ceram.
Soc. 68 (1985) (June) Ii
/3/ Ben Abderrazlk, G. , Moulln, G. , Mlllot, F. and Huntz, A. M.. , to be presentend at the "7Bme Congrds Europben de Corrosion' - Nice (1985).
/ 4 / Dlls, R. R. and Follansbee, P. S. Report n* 74.005. Pratt and Whltney Aircraft.
Division of United Alrcraft Corporation, Middletown, Connecticut (28. 5. 1974).
/ 5 / Br0ok.R. J., Yee, J. and Krbger,F. A., J. Amer. Ceram. Soc. B, 9 (1971) 444.
/ 6 / El-Alat. M. M. and Kri)ger, F. A. , J. Amer. Cerarn. Soc. 65, 6 ( 1982) 280.
