Manjrekar, S. K.

Salgaonkar, D. P.

Soman, S. V.

English

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radIan Joumal of ChemIstryVol. 15A, February 1977,pp. 88-91Characteristics of Anodic Film on Aluminium in Borate BathS. V. SOMAN, D. P. SALGAONKAR & S. K. MANJREKARDepartment of Chemistry, Institute of Science, Bombay 400032Received 19 January 1976; accepted 14 June 1976The growth of anodle films over aluminium in an electrolyte contalntng sodium carbonate-sodium borate in dioxane-water mixtures of varying compositions has been studied. Theactivation energy and other relevant parameters have been calculated by a new method employ-ine a simple anodizing assembly wherein the use of constant current device has been done awaywith.THE type of anodic oxide film that can beproduced on aluminium depends upon severalfactors, one of the important factors beingthe nature of the electrolyte". The electrolytesin which the anodic oxide film is completely insolublegive rise to a barrier type of oxide film whereaselectrolytes which have a slight dissolving actionproduce a porous type of film. The barrier andporous type of films are quite different in theiroverall physical appearance and other characteristicproperties; however, these two types of films co-exist. A porous film is actually a porous extensionof the thin compact barrier layer. But the overallcomplexity of the film increases as the formationvoltage is increased.In an electrolytic bath that has dissolving actionon the anodic films the balance between the forma-tion and the dissolution of the oxide film is complext.The process is then strongly influenced by factorslike the composition of solvent, concentration ofelectrolyte, anodizing temperature, impressed volt-age current density and the period of anodization.The present work deals with the anodic filmsobtained on an aluminium foil from borate-carbo-nate electrolytic bath (sodium borate + sodiumcarbonate) with dioxane-water mixtures as solvent.This mixture was used with a view to studyingthe effect of (a) lowering of water content in theanodizing electrolyte and (b) replacing water bypoor ionizing solvent like dioxane.Materials and MethodsAn aluminium ribbon 40 mm wide and 30 !L inthickness and of 99·95% purity was used as ananode. The film was held firmly between tworectangular pieces of teflon from the centre of eacha square piece of 25 X 25 mm was removed; thus6·25 ern" area of the aluminium film was exposedfor anodization. The cathodes were made of pairof graphite rods which could be fixed on either sideof the anode at a desired distance. The directcurrent was obtained from de power supply unit.This consisted of a stabilized stepdown transformerproviding output voltages of 10,20,30,40 and 50 V.With silicon rectifiers and smoothening condensersthe unit gave a nearly ripple free direct current atvarying voltages.88Immediately before the experiment the alumi-nium film was degreased with toluene. The surfacewas then chemically cleaned by dipping the filmin 5% solution of NaOH at 80° for 15 sec. Thefilm was then washed with water and dipped in anacid mixture containing 70% phosphoric, 25%sulphuric and 5% nitric acids. Finally it wasflushed with plenty of distilled water to removeany adhering chemicals.Other chemicals used were of BDH Analar qualityand were used as such; dioxane, he wever, wasdistilled befr re use.Current measurements - At the instant of applica-tion of the potential across the electrodes, a surgeof current passes through the system, which fallsand acquires a steady low value after about 20 to200 see depending upon the applied voltage. Theextent of the current flowing through the systemany time during the run being of interest wasmeasured by introducing a high wattage nichromewire exactly of one ohm resistance in series withthe system (Fig. 1).For any current passing through the system thepotential that developed across this resistance wasmeasured continuously with Carl-Zeiss strip chartmillivolt recorder. From the time-millivolt graphsthe current in the anodizing circuit and hence theresistance at any time could be determined. Theresults are given in Table 1.Measurement of the field - A fairly accurateestimate of the variation in the field in the oxideD c. POWER SUPPLY TIME POTENTIAl.0 RECORDER0 0o jI 0LSJ.•i LJ\.I .~-II=-=- - - 1=-=STIRRER_~ ~~ ~~ERMOSTATTHFig. 1 - Circuit diagram for measuring current flowsSOMAN et al.: CHARACTERISTICS OF ANODIC FILM ON ALUMINIUMfilm could also be made by measuring the changesin the resistance of the oxide film. For a substancelike Al20a it can be assumed that the thickness ofthe film will be proportional to resistance, if theapplied voltages are not high. Knowing the specificresistance of Al203 the thickness of the oxide filmcould be calculated. The value of the field canthen be calculated by dividing the voltage by thethickness of the film. However, the actual valueof the field being of no practical interest, a figureproportional to this actual value was obtained bydividing the voltage by the resistance of the film.It is this value which has been used as an index ora measure of the field in this investigation.Results and DiscussionUsing mildly alkaline carbonate-borate mixturesas the electrolyte and water-dioxane mixtures assolvent, anodization of aluminium foils was carriedout under different experimental conditions. Therelationship between the current and the field hasbeen studied. The results have been grouped underfollowing categories. Only the representative resultshave been presented in each case.(i) Effect of increase of voltage on field vs lnl plots-In this group it has been observed that the fieldvs lnl plots were linear without any exception.The data are given in Table 2. It was furtherobserved that with the increase in impressed voltagethe slopes of these plots increased. The slopevalues for impressed voltages of 10, 20, 30 and 40 V(de) are 4'14,4'44, 5·0 and 5·52 respectively.Nigam and Arora" found that (1) the Tafel slopefor a given electrolyte is dependent en the fieldstrength and thickness of the oxide film, and (ii) thevalues of Tafel slope are different for different elec-trolytes. They attributed the difference in thevalues of Tafel slopes to thickness controlling para-meters of oxide and to the density of the material.According to Dewald' a modified form of hisgeneral equation (Eq. XIII) allows for a rise in theTafel slopes with increasing current density whenvalues of a are near about 1. Since Dewaldstheory involves inclusion of two extra parametersas compared to the Cabrera and Motts theory! hesuggests that further experimental verificationwould be highly desirable.An explanation of increase in Tafel slope withincreased applied voltage seems difficult at thisstage.(ii) Effect of change in temperature - This effectwas studied by carrying out the anodization atthree different temperatures, viz. 30°, 60°, and100°. Since this caused a profound and a visiblechange in the appearance of the film, microphoto-graphs of the surface were taken with a reflectancemicroscope. The variations in the field versuscurrent relationship observed in these experimentsare included in Table 3.TABLE 1 - VARIATION OF RESISTANCE (VALUES IN OHM)OF THE ANODIC FILM WITH TIME TABLE 3 - EFFECT OF TEMPERATURE ON[Bath composition = 20% dioxane + 80% water; electrolyte FIELD VS lnl PLOTS= 15% Na2C03 + 5% Na2B.o,J [Applied voltage (V) = 10 V de; bath composition, 20%Time 30° 60° 100°dioxane + 80% water; electrolyte, 15% Na2C03 + 5%Na2B.O,J(see) ------10 V 20 V 10 V 20 V 10 V 20 V Temperature(de) (de) (de) (de) (de) (de)5 20·00 55·56 20·04 79·37 20·1330° 60° 100°53.33 -------15 21·74 57·14 21·30 83·33 21-54 56'34 Inl Field lnl Field Inl Field25 28,41 22·40 95'69 22·05 60'24 (VjR) (VjR) (VjR)35 24·00 97'56 22·25 60'9845 28·99 61·16 24·40 22·45 4·382 0·5000 4·331 0·499 4·597 0·620065 29'41 25·70 23·28 4'299 0·4599 4·263 0·446 4·484 0·554195 26·600 99·01 23-84 4·031 0·3520 4·190 0·410 4·461 0·5420115 61·54 24·24 63'30 4·011 0·3449 4·095 0·376 4·421 0·5187145 28·08 25·55 4·002 0'3420 3·989 0,340 4·389 0·5040165 62·50 29·41 64·10 3·996 0'3400 3·932 0·316 4·357 0·4881185 29·67 62·70 102'56 26·22 64·92 3'991 0'3380 3,762 0'246 4·277 0·4501245 29·76 103·63 27·70 65·57 3·987 0·3370 3-611 0·230 4·231 0·4299265 29·76 62·70 31·65 28·21 3·984 0,3360 3·527 0·210 4·190 0·4130TABLE 2 - EFFECT OF INCREASE OF VOLTAGE[Temp. 60°; bath composition, 20% dioxane + 80% HoD; electrolyte, Na~B.o, + Na2C03110 V de 20 V de 30 V de 40 V de----------lnl Field (VjR) Inl Fieldj(VIR) lnl Field (VIR) lnl Field (VjR)4·331 0·499 3·762 0·270 3·785 0·273 3·539 0·2154·263 0·446 3-689 0·250 3·556 0·220 3·533 0·2144·190 0·410 3·638 0·240 3·497 0·208 3·454 0·1984·095 0·376 3·497 0·209 3·430 0·200 3·454 0·1963·989 0·340 3·491 0·205 3·415 0·196 3·428 0·1943·932 0·316 3·446 0·199 3·402 0·190 3'418 0·1903·762 0·246 3·441 0·195 3·388 0·1883·611 0·230 3·425 0·1893·527 0·21089tNDtAN J. CHEM., VOL. 15A, FEBRUARY 1977The surface microphotographs (X 800) exhibitedthat with increase in the bath temperature theoxide grains become finer and provide a moreunifrorm coverage for the metal surface.(iii) The change in the solvent composition - Theeffect of the changes in the solvent composition onthe structure of the surface oxide film was alsostudied using 0, 10, 20 and 30% dioxane-watermixtures as solvent. The increased concentrationof dioxane in the solvent mixture did not influencethe field versus InI plots. However, the surfacemicrophotographs clearly revealed that the finegrained and the smoothest films were formed in20% dioxane-water mixtures.The process of formation of oxide consists of twostages, viz. (i) the electrolytic formation of oxidesand (ii) the hydration of the surface. Of thesetwo, the formation of anodic oxide does not appearto be highly temperature sensitive, since changesin temperature brought about very small variationsin the slopes of field versus InI plots. The rateof hydration of the oxide, on the other hand, isknown to be temperature dependent and quitefast at higher temperatures'. As the hydrationprocess is simultaneous we get more hydrated oxideon the surface at higher temperatures than atlower ones.Guntherschulze and Betz? have shown that underhigh field the ionic current density i could be writtenasi=A exp BEwhere A and B are constants. Mott and Cabrera-while explaining the mechanism of ionic conductionproposed that the ionic current density could bewritten asi=, exp [-(w-qaE)/kT]where fW=height of energy barrier over which theion must pass in order to enter the oxide layer,i.e. activation energy, q=charge on the mobileions, and a=activation half jump distance, i.e.the distance from the position of minimum tomaximum potential energy, k=Boltzmann's con-stant and T=absolute temperature.Both the expressions suggest an exponentialcorrelation between the ionic current i and thefield 'E'. This has been experimentally establishedby several workers··8.', who obtained the linearplots of field E against lnl. Further, the Tafelslope values, dE/dlnI, were found to be constantfor a given electrolyte but changed with the natureof the electrolyte",Johansen et al.lO have studied the anodic oxidationof several metals including aluminium in borateelectrolytes at low current densities. They reportthat the films formed were 20 to 100 A thick, i.e.were of such thickness that the Cabrera and Mottstheory' would be expected to apply.In the present investigation at a constant appliedvoltage the ionic current decreased with increasein resistance of the growing anodic film. Thisresistance (R) has been calculated at any instanceas applied voltage-current I. It will also be seenthat as R increases its numerical value will givethe extent to which the applied potential will haveto be raised in case we want the current to be90TABLE 4 - W-VALUES, Info UNDER DIFFERENT UNPRESSEDVOLTAGES[Temp., 60°; bath composition, 20% dioxane + 80% water]Applied Intercept Slope InIo w (eV)voltage (C)10 volts 2·79 4·14 4·478 0·48320 volts 2·50 4·44 3·762 0·36330 volts 2-45 5·00 3·685 0·35540 volts 2·40 5·52 3·556 0·333TABLE 5 - W-VALUES AND Inlo AT DIFFERENTTEMPERATURES[Applied voltage, 10 V (dc); bath composition, 20% dioxane+ 80% water]Temp. Intercept Slope In10 w (eV)°C (C)30 3·10 4·80 4·782 0·51360 2·79 4·14 4·478 0·483100 2·42 2·8 4·016 0·459maintained at the initial level (i.e. constant currentcondition).If the resistance R of the anodic film is ameasure of the thickness of the film, the ratio VIRwill be an index or a measure of the applied fieldand the plots VI'R versus InI will have the samesignificance as the Tafel's plots. And hence duringanodization at constant voltage it will be reason-able to assume that the same relationship willexist between the VIR and I as is found betweenfield and I under constant current conditions.The validity of these assumptions can be testedby evaluating the w values (i.e. the height of theenergy barrier). For the straight line plots ofVIR versus InI the Mott and Cabrera's expressionfor ionic current can be written asl=ioe-r",-qaE]!ATI w qahence InI = In o-kT + kT' Eqa wor InI =kT' E + Inlo- rt :The slope of this linear plot will correspond toqaJ.kT and the intercept, for E=O, will be equalto [ln1o- wjkT].For any particular impressed voltage the valueof lnlo. can be calculated from the initial currentin the anodizing circuit. It then allows us tocalculate the w, value for di'fferent experimentalruns under different impressed voltages. The wvalues and those of other relevant parameters, fora representative set of experiment, are includedin Tables 4 and 5.These results indicate that w values thus calcu-lated fall within the order of values earlier reportedby Nigam and Arora" from citric, tartaric, phos-phoric and sulphuric acids baths. Further, it willbe seen that these fundamentally important para-meters can be calculated by this new method, em-ploying a simple anodizing assembly wherein theSOMAN el al.: CHARACTERISTICS OF ANODIC FILM ON ALUMINIUMuse of constant current device has been done awaywith. Some other important properties of theanodic films formed in the carbonate-borate elec-trolyte with water-dioxane, mixtures as solventsare being communicated separately.References1. DIGGLE, J. W., DOWNIE, T. C. & GOULDING, C. W.,Chem. Reu., 69 (1969), 374.2. BARKMAN, E. F., Anodized aluminium (ASTM specialtechnical publication No. 388), 1965, 87.3. NIGAM, R. K. & ARORA, J. K., Ind~an J. Cllem., ,(1971), 578.4. DEWALD, J. F., J. electrochem. Soc., 102 (1955), 1.5. CABRERA,N. S. & MOTT, N. F., Rep. Prog. Phys., 12(1948-49), 163.6. BARKMAN,E. F., A1Iodiled aluminium (ASTM specialtechnical publication No. 388), 1965, 85.7. GUNTHERSCHULZE,A. & BETZ, Z. H., Z. phys., 92 (1934).367.8. VERWEY, E. J. W., Physics, 2 (1935), 1059.9. DEWALD, J. F., J. phys. CAem. Solids, 2 (1957), 55.10. JOHANSEN,H. A., ADAMS,G. B. & PvAN, RYSSltLBltRGBlt.J. electrochem. Soc., 104 (1957), 339.91

Characteristics of Anodic Film on Aluminium in Borate Bath

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Characteristics of Anodic Film on Aluminium in Borate Bath

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