Indian .Journal of ChemistryVol. 16A, September 1978, pp. 741-744

High Field Ion Transport in Anodic Oxide Films on SuperpurityAluminium in an Eutectic Mixture of Molten Alkali Nitrates

R. K. NIGAM & R. KAPOORChemistry Department, Maharshi Dayanand University, Rohtak 124001

Received 24 February 1978; accepted 1 April 1978

Anodic film formation characteristics of superpurity aluminium in molten alkali nitratesare quite different from those in aqueous electrolytes. The breakdown voltage, 74·5 V, in thepresent electrolyte has been found independent of current density and temperature. The steadystate data have been represented, within experimental error, in the light of ,Dignam's approach,by only three empirical constants, viz. the pre-exponential factor, the activation energy and theMorse function distance parameter. Inclusion of quadaratic term in the steady state equationis Justified. Parameters !l*' C and W* are temperature-dependent quantities and not indepen-dent as assumed by Dignam [Can. J. Chetru, 42 (1964), 1155]. This accounts for field and ano-malous temperature dependence of Tafel slope.

THE phenomenon of ~ig~ation of charg~dspecies across an activation energy barnerplays a major role in the kinetic behaviour

of a number of systems", Dignam- e,xplain~d thehigh-field ion transpor~ p~lenomena In sO~lds .byrepresenting the net activation energy f~r migrationof the ionic species as a power senes In the fieldstrength E. The quadratic expansion representedthe dat~ extremely well and the inclusion of acubic term lead only to a negligible improvement.In a similar equation derived on empirical basis,Young+ justified the quadratic t~rm .inclusion on ~hebasis of an increase in the activation energy withincreasing condenser pressure. As .the conden~erpressure is proportional to E2, this explana~IOnpredicts a quadratic term of t~e. corre~t sign.The controversial Tafel slope variation with fieldstrength and temperature has be~n accounted forquantitatively. However, the major drawbacks ofthese studies are a limited temperature range with amaximum of 70° (ref. 4) and t~e us~ of. not verypure metal. Br~ce ei .at. 5, in a~ investigation on thetesting of anodic OXide coatings, found that thebreakdown voltages were affected by alumini~mpurity and alloying const.ituents. The present In-vestigation aims at studying the Tafel slope beha-viour over a wider range of temperature andexamining critically ionic conduction d~ta in thelight of various current theories of IOIUCgrowt~.Superpurity aluminium has been empl?y~d f?f thisstudy. The film formation characteristics In themolt'en salt electrolyte at elevated temperatures havebeen compared with those" in aqueous electrolytesat room temperature.

Materials and MethodsSuperpurity aluminium specimens of 1 em? area

were cut and their surfaces prepared", The moltensalt electrolyte, furnace, temperature ~ontr?l, ano-dizing cell and the procedure for .anodlc o?,l~e filmformation were the same as described earher .

742

Results and DiscussionSteady state kinetic data were recorded up to a

temperature of 648·0 K at constant current densities,viz. 2, 3, 5, 7'5, 10, 12·5 and 15 mAJcm2.

A preliminary X-ray report of anodic oxide filmsformed in molten salt electrolyte shows them to becompact and hard, with a non-porous partiallycrystalline structure as compared to the porouscharacter of the films formed in aqueous electro-Iytes", The film growth rate differs significantlyfrom that in the latter electrolytes. While, undergalvanostatic conditions, voltage of formation variedin a parabolic fashion for the aqueous electrolytes",a linear plot was observed for molten salt films.This implies a constant field strength independent offilm thickness. Current efficiency for these filmswas found to be 100%. The density of the oxidefilms was taken as 2·7 g em:" (accuracy ± 0·1 gcm-3)lO.

The breakdown voltage, which was found to bedependent on current density and electrolyte concen-tration in aqueous electrolytess, was 74·5 V, indepen-dent of current density and temperature in themolten salt electrolyte. However, as also reportedearlier for aqueous electrolytes", this breakdownvoltage is higher than that for EC-F 1050 Hindalcoaluminium (AI = 99'5%, Si-l-Fe-l-Cu = 0·45%,Ti+V = 0·008%). The low breakdown voltageof 74·5 V in molten salt electrolyte studied can beexplained on the basis of oxide properties of theproducts of cathodic reduction of solvent and of thecathodic materials undergoing dissolution. It hasbeen shown experimentally+ that the first reductionprocess of nitrate melts is

NO;+2e --+ NOi+O-2At constant current density, the anodic film growthwas accompanied by an increase in the concentrationof Al-ions, which had to pass through Al203

film and an equivalent number of oxygen ionswhich were adsorbed on its outer surface. This is

NIGAM & KAPOOR: HIGH FIELD ION TRANSPORT IN ANODIC oxroa FILMS

constituting an oxygen electrode at which Al-ionscombined during the film formation. Simultaneousproduction of Al203 and oxygen gas would resultif the concentration of the adsorbed ions at thiselectrode increases sufficiently to establish thepotential required for oxygen evolution .. This indi-cates that the electrons are released not by thecombination of Al-ions with oxygen ions to produceAlz03 but by the simple phenomenon of oxygen gasevolution by means of the reaction 202--4e = O2 (g).The electrons cross the oxide/electrolyte interfaceand pass through Al20a film and complete thecircuit. This penetration of barrier electrons mightbe regarded as a first step towards dielectric break-down. In the molten salt electrolyte at elevatedtemperatures, such a barrier of electrons couldset-up conveniently which, in turn, might stop theformation of new oxide layers, thereby resulting infilm breakdown at such a low voltage (74'5 V).

Fig. 1 presents the plot of field strength, E vsIn i (i = current density) at various temperatures.The non-linearity of this plot implies incompatibilityof the Gunterschulze and BetzI2 theory of ionicconduction. The fact that the values of theconstants A and B, calculated by taking twodifferent current density pairs at the same tem-perature were not found to be constant confirmsthis observation. Again, the E vs lIT plot (Fig. 2)shows non-parallel and non-linear curves. The plotsin both Figs. 1 and 2 suggest firstly, the field andtemperature dependence of Tafel slope and secondly,the non-applicability of Cabrera and MottI3theory.

Dignamt! explained the anomalous temperaturedependence and also the field dependence of Tafelslope for steady state anodic oxidation of Ta, Aland Nb by considering a simple model according towhich the field independent component of potentialenergy function for displacement of mobile chargedspecies was assumed to resemble a Morse function.Assuming the overall rate to be controlled by

1·9

o .501·0·K •.••,515·7 ,a.530·4 ,.••,545~ •• ,559·8 ••• ,574·5 •ow.589·2 i• ,603·9 I• ,618'6 ,• ·633'3 Io ,~48'O •

1·5

0·7

5'5 5'0In I ioA cm-21

Fig. 1 - Field variation with In i (ionic current) at differenttemperatures

'·9

0·7

oo • 15.0mAc,;,2.•• 12.5 •A ,10.0.••• 7·5x • 5.0• , 3.0c .2.0

18 1·9

Fig. 2 - Plots of field strength (E) versus reciprocalabsolute temperature at various current densities

high field transfer of one kind of ionic species,the ionic conduction equation is given by

i =io exp [-{~-~*E (1 - £L~f)} /KT]

where ~*.~ and C are not independent constantsbut related through the form of potential energyfunction making two of them independent. ~ isthe. zero-field activation energy; ~*, the zero-fieldactivation dipole; and C. a dimensionless constant.For the sake of simplicity, Eq. (1) may be writtenasi = 0( exp (~'E-YE2)where0( = io exp (-~/KT)~' =~*/KTY =~*2/C~KTThe logarithmic form of Eq. (2) isIn i =In O(+~'E-Y£2 ... (6)Eq. (6) was solved for different values of currentdensity, i, and mean field strength E at varioustemperatures of study by the least squares regressionand the values of 0(, ~' and Y were found. FromEq. (3),In 0( =In io-Y/KT ... (7)If io and ~ are temperature-independent quantities,the plot of In 0( vs lIT should be linear. Fig. 3shows such a linear plot indicating io and ~ to betemperature-independent. io and ~ were determinedr.espectively from the intercept and slope of thishne by the least squares method as io = 109.63A/cm2

and ~ = 1'7345 eV.The values of ~* and C h .ve been calculated with

the help of Eqs. (4) and (5) for different tempera-tures. Both ~* and C are observed to be tempe-rature-dependent. However, this dependency on

... (1)

... (2)

... (3)

... (4)

... (5)

743

iNDIAN j. CHEM., VOL 16A, SEPTEMBER 1978

z·o

lS 13·0

"o

o17.0 0 0

Fig. 3 - Plot of In ex versus lIT

temperature could not be observed by Dignam-sdue to the non-availability of steady state data ofanodic oxidation for a wide range of temperature.The reciprocal Tafel slope, ~, may be derived fromEq. (1) as

B = (0 In i) =p.*/KT [1 _ 2p.*E] ... (8}. oE T CcpIt is clear from Eq. (8) that Tafel slope isdependent both on temperature and field. Using aMorse function for mobile charged species, theequation for ionic conduction can be readily writtenas

i =io exp [ - {cp-W*E (1 -In ~~E - ~~~) }/KT]... (9}

where w* is a Morse function parameter having thesame dimensions as those of (J.*. w* is given by

... (10)

744

On substituting the value of w* in Eq. (9), thevalue of i agreed, within experimental error, with theexperimental value. This justifies the validity ofthe quadratic term in Eq. (1) or (2), and henceof the Morse function. It is interesting to notethat Ib}15 arrived at a similar equation in a simpleand general manner from transition-state theorytaking into account the energy stored in thedielectric. He has also discussed the significance ofY, which, according to him, represents the differencebetween the derivative of the dielectric constantwith respect to concentration for the normal andthe activated particles. Hence, the results arereadily explained if the effective field strengthassisting the ion transfer process includes anappreciable contribution from the electrostatic polari-zation of the oxide dielectric medium.

AcknowledgementOne of the authors (R.K.) acknowledges his thanks

to the Maharshi Dayanand University, Rohtak, forthe award of a junior research fellowship.

References1. YOUNG, L., Anodic oxide filn.s (Academic Press, ~~e\V

York). 1961.2. DIGNAM, M. J., ]. ptiys. Chem, Solids, 29 (1968), 249.3. YOUNG, L., J. electrochcm, Soc., 110 (1963), 589.4. ADAMS, G. D. & KAO, T., J. electrochem, Soc., 107

(1960), 640.5. BRACE, A. \V. & POCOCK, K., Trans. Inst. Metal Finish-

ing, 35 (1958), 277.6. KIGAM, R. K. & KAPOOR, R., Indian ]. cu«. 16A

(1978), 283.7. KIGAM, R. K. & ARORA, I. K., Indian]. Chem., 9 (1971),

578.8. ::-:IGAM R. K. & CHAUDHARY, R. 5., Indian ]. Chem.,

8 (1970), 343.9. NIGAM, R. K. & KAPOOR, R., Indian ]. Chem., 15A

(1977), 92.10. CAMPANELLA, L. & CO~TE, R., Plating, 56 (1969), 813 .11. SWOFFORD, H. S. & LAITINEN, H. A., J. electrochem,

Soc., llO (1963). 814.12. GUNTHERSCHULZE, A. & DEITZ, n., Z.Phys.,92 (1934), 367.13. CAlmERA, N. & MOTT, X. F., Rep. Prog. Phys., 12 (1948-

49), 163.14. DIGNAM, M. J., Can. ]. Chen:., 42 (1964), 1155 .15. IBL, x., Electrochim . Acta, 12 (1967), 1043.