Crrrprrn 4

EX pE Rr M E NrA L rEcll§l3 %li lrxt,ir-fl?ifl

N G A N o Dr c ox I D E

4.01 fxrnoDucrroNA vunv considerable inoroase in the accuracy of experiments on thekinetics of formation has boen obtained in the last few years, duelargely to the use of suitable instrumentation. The improvecl methodsnow available have not been applied to many metaÉ. since almostany experiment on anodic oxide films involves first forming the film,a:,.d since the properties of the film depend on the formation lroceclure,tho use of adequate instrumentation is required eyen when tie kineticsof the formation process are not the prime object of stucly.

4.02 cnenecrnnrzArroN on F,lt rv Foauarrox pnocnssFilms are usually made either at constant current, constant voltage,

or by a soquence of constant current and constant voltage. trilms formedaü constant current aro the best characterized for scieniific purposes. rtis a complete description to statc that a film rvas formecl at a^cerlain ioniccurrent donsity to a given ovcrpotential in a cortain solution at a certaintemporaturc on & given crlstal face of rnctal of given purity, give, stateof colcl worl<, and givcn rnoclo of sur.facc prcparation.

- ri ilrc same

curront density is not uscd throtrghorrt, it is sometimes cliÍficult todctermino for horv Iong the final current clonsity must bo passecl toobtain the samo result as woulcl bc obtaincd úy formatioil at thiscurront density only. rt is not a cornplcto clcscripiion moraly to state,as has often been donc, that a filnr rvas formá at a givãn voltageeithor for an rrnstated tirnc, or for a "long tinlo", o, .r.r-til ,,very lÃvcurrents wero reached" (a stato knorvn in the olclcr literaturo as ,-,fullyformcd"). such descriptions rvoulcl, of courso, bo aclequato for manj,purposes, but not if exact values of a paramcter-such as tho À/vratio-are to be reported.

During formation at colstant voltage (or, better, ovorpotential) thecurront falls continuously until leakage current preãominates orrecrystallization starts (section g.0r). rn theory, a steady state isnover reached, since although the current eventuaily changes only very

38

cdüirqtt

l

OC

pd*etrtrdttÊ(euraftGdnçit€!:raFdqoftryaúbyrtimofhThGmod

l-'and rwith r

the Ir.o. üCAP:EpoinüordirmetcrlcuEt[curtrElIforcr

INSTRIIIIIENTÀTIONr'OBSTEÁDY-STATT STUDIES 39

slowly, the time required to pass the charge necessary to obtain thesteady state is very long. Ilowever, in practice, fllms formed to a givenvoltage at a given constant current may be almost ind.istingúshablein thickness and o;$er ProPerties from fllms formed to the same

current at a constânt voltage. This is the case, for example, withtantalum in dilute solutions.

4.03 h.TstnuMENTÀTroN roR Srneorr-srern Stuorns

For qualitative work, or for work at low current densities, manualcontrol of the eurrent or voltage using a variable series resistance or apotential-divider resistance system of some soú may be used, thougheleotronically regulated constant current or constant voltage suppliesare readily available. At low currents the current may be switched on

and off manually and timed with a stopwatch. Manual regulation ofthe current or voltage is not practicable at very high current densities(ancl hence high rates of change ofvoltage at constant current, or highrates of change of current at bonstant voltage). Electronic formationcontrol devices are then essential. Two principal methods have been

used in constant-current erperiments. Thus, the current may be

terminated when a predetermined periocl of time has elapsed, or whena predeterrninecl ovetpotential has been reached. The first methoddoes not require specially made equiprnent ancl has been used moreoften, but the second has tho aclvantage that the more importantvariable is controlled. The cluration of current flow may be oontrolledby using & commercial electronic timer preferably rvith a separate

time-rncasrtring clevice (c.g. an clectronic cottnter rvhich cottnts pulscs

of known frequency) to cletelmine thc actttal duration of current florv.

'Ihe potcntial with rcspect to a leference elcctrocle may then be

rnonitoretl in several ways:I. Thc r:.o. betweerr the cathocle and tlte attocle may be measurecl

anrl a correction applied for the potential of the reference electrodowith lespect to thc ca,tltode. It is easily shorvn that the oltmic p.o. inihc solution may bc allo'wecl for by adding to the anocle-to-cathodep.n. tho cathocle-to-refcrenco P.D., il,s nteasttrecl rvith a Haber-Luggincapillary (section 4,05 c) lcading from tlte referencc electrode to a

point close to the anode. A low-impedanco measuring device (e.g' an

ordinary multi-range voltmeter or, a fortiori, an accurate voltagemeter, 'lvhich usually will take more current) may only be usecl if thecurrent flow is measured on the cell side of the device, so that thecnrrent taken by the clevice is not includecl in the measured current.Holever, if a constant-current supply is used, any low-irnpedance

40 ExpxnrMrNTAL TEcuNreursvoltage-measuring device must be connected. to a point in the cirouitbefore the control stage in th" cro"rri deüce-otherwise the currentsupplied to the specimen w,r not be constant. x',or exampre, with acurrent supply which regulates by controlring the p.n. ;;;;." a resis_tance through which thJoutput ãurrent fl.ows, the voltmeter must beconnected on the supply side of the control r:eüistance. A correctionfor the p.D. âcross thãrásistanoe is then required. A potential üviderfoliowed by a potentiometer is more accurate but sloier to operate.2' The potential ofthe anode with respect to the reference erectrodemay be measured direcüly-using a high.impedance d"ri;;, if necessaryagain with

_1 Luggin capilaryl ,o aioid including ãn-r" p.». in thesolution.- "High impedance" -úeans

here that the current drain fromthe anode must be negrigible with respect to the applied cu*ent.The range of cu*ent aensútes which is normaily rikery to be requireclis lO-e to l0-1Acm-2. Lower cument densiti_"es _ul, tu-r"ed withmeasuring equipment of given impedance by increasiig iiu .p""i*",arca. r

3' rf discontinuous vortage readings are taken, say at equar timeintervals, and the current is terminãted at u gi*rr" u*ul tnu n.rut(terminating) potentiar musr be obtainecr ty "*t?rfÀi*iiã' This canbecome very tedious if many films have to be formed., and is avoidedif continuous recording can be made with a -r_ecorder

precedecr by asuitable amprifier or attenuator. However, a wide range of paper speeclsis likely to be requirecl and the accuracy may be insufficient. The time rto form a given thickness is, of coursJ, irrru"*uty p";;;;;;".1 to thecun'ont clensity; normaily several clecacles or "úrà.rt'ã;"rfu rv,r berequi.ed so that the time

-m&y vary from secona. to

-roury

'ours.Accuracy nray be improvcd by úacking_off par"t of the ,.o.

-

no,r"-ngr""&cc,r&cy rnay bo orrtained with cligitar vortLeters *t i.nl." rrow q*iterapid in response.The method of controning tho potentiar rather than the time oftcrmination, and of recorclinglhe tiÃes to reacrr a ser.ios of intermediatev«rltages, as well as the time to reach the finar vortage, i* rr".y ãàrvenientand may bc nracle very accur&te. The r.equired numericar results areobtained directly without the neecl for plotting graphs o,

"u"ai,grecorder charts' The final thicknoss is more acouratery conürored sincethe overfield varies far less with current efficiency than cloes the totalcharge to form the firm. rn one clevice 1r'igure +.oif-ro"-"f,""ro"-rrrgthese functions (designecl and built by R. C. Cuthbert and tr. C. Curzonfor the use of the present author) cámparison .,rottog".- n* dieveropedacross a chain of resistors. Each suJcessive

"o*p'J.or, àttugu i,

INSTB,UMENTATION T'OB STUDIES OT, TBANSIENTS 4Iselected by stepping switches (urriselector switches). The potentiar ofthe cell is compared wif,h the eomparison voltage. When-equality isreached, a trigger circuit operates stepping swilches cont.oiring ihereference voltage, and diverts electronicallyà chain of pulses from onecounter to another. The first counter then indicatãs the time totraverse the voltage interval betv'een the last two comparison voltages,whilst the second counter colleots the pulses for the r,ot ti*e interval.

Stort srgnoltY

To onode I Control I

I reroy [---r-Constontcurrentsource

Itril To cothode*r I l--ul +To reference electrode Voltoge

comporotorReíerencevoltoge

Timeindicotors

Fig, 4.0r. Circuit ot "r""tl?r?tj,S:1,,"a""?ffi."rrolring rhe formarion of

The voltage intervals may be varied from I to 20 v. The first com-parison voltage may

-be any integer to 200 v or any even integer to

400 v. The termination voltage mây then be chosen as the first com-pariso, voltage plus any desirecl n,mtrcr of thc chose, voltage intervalsto 400 V.

Luggincopillor

4.04 INsrnurltENTÂTroN lroR Stuorns on TnewsrrNr:sThe most easily interpreted type of transient experiment invorves

monitoring the current after a sudclen change of voltage. The formationbefore the change is nrost logically clone at constant current. wo con-sider first the problem of making the change-over and then that ofmonito'ing the current. An obvious ru.ethod is to use either a fastelectr-omagnetic relay, or (at higher currents) erectronic switching, totransfer the anode-cathode system from a constant-current generatorto a constant-voltage supply. rf a reray is usecl, the cell will bã on opencircuit for the periort in which the reray armature is moving. rni.period cannot bo made much less than 0.5 msec with presently-availablerelays of sufficient current rating, and this time ls Iong

"enough to

cause an appreciable self-discharge (section B.r2) which rnay alter the

42 EXrIR,TMENTAL TrcrNrerrosfi-lm sufficientry to i=trod-u= appreciabre errors. The period on openc*curt may be eriminated if i[; t.unsiiion is produceã by using therelay simply to cramp a constant-voltage generator acro§s the constant-current generator' Thu lrtt"" *tx;;-i' as_sociated. capacitances, butthe vbltage transition^wi,t, ,o".tt

"tuJ.,"p"ora,,y r" *rã"-iãiiry rapid.Additional capacitance-pay be addecr uáJo." the output of the constantvoltage suppty. Ir wi,] uJuu,y ;;;;;;;.rry to add a resisriye toadacross the constant-voltage g""""rü;rr"" tti"--ãip.Jurtry "rr,

To constontvoltoge supply trolytic

cell

ohms

lltlFTo recorder

tr'ig. 4.02. Circuiü for transient studies (vermilyea, 11g57).

regulato'when giving a positive current. The transition may be madeautomatica'y usingã voitage*ensingãuoi"" (as described for terminat_mg constant-current formation) to i*iicr. the relay. Manuar conüror

rf the preriminary formation is ma{g at constant potential, thochange lv,r be made when tt u "ur.u.ri'ialrs to a p.edete'"minecr varue.One sirnple methocl lve"mitycn, l;;ilof eff""ti,g_the change (X,igure4.02) is by shorting out a series i,"si.to" nr"o.. rvhich a batbü or other.

Xli]i,!ill'"'"tains a n.o. (with a suitable ";;;'ffi#il;rotect the

Thc cu.rent mav.be rnonitorccr by recorcring thc r,.o. âcross a seriesresist'r'through úrich tto ..,.Ài'fl"r*. This resistor must be smallenoug' not to rinrit unclury tl,o .or.t".,ler.-charging current ancr thusmake the voltagc change of th" ."tt toorioru. Thc recording instrumentmusü respo,cr to srnalr vortages. r";;;;"" sufficient ,.";;;y it maybe rteccssu,ry to suppress thc-zero of tt," rl""o"cler,. Tempor.ary ,,d1sable_rnent" of tho amplifier is avoicreà ty 'r.""ping

the sensing resistorshorted by a .erav rvhich op"n. o r.JJtion of a milrisecond oafter

thechange-over. 'Ihis'aho allows tf," .onãurr,

I:]"rit of "t

n"gu'-fos.ua ir, u," t*r.i",tl:'r::#ff,;ü1fi:r#;rT:current, consequently the ti*u ,.rtu l. irrrr".""ly proportional to the

CEITL DESIGN 43

current,. chart recorders (with photographic recording) are availablewith resolution times as small as I msec. pen-recorders have ratherslower resolution, usually more than l0 msec which limits the experi-ment to current densities below about 100 pcAcm-2. cathode-rayoscilloscope techniques are required for the faster transients at higúcurrent densities.

rn the second tgle of experiment, the potentiar is measured and thechange is from one constant current t,o a second constant cument. Thetransition may be accomplished by switching the range of a singlecurrent supply (in which case transients due to the supply may intãr-fere at high currents). otherwise two supplies may be

"üá, trr" secondworking initially into a dummy load.

. 4.05 Cnr,r, DssrcNThe design of the cell may have a consiclerabre effect on the acouracy

of the results and the speed of the experiment. The essontial points willbe listed. i

(a) Thermostat. A water thermostat accurate to better than 0.r"cis convenient, for ordinary work, but care must be taken to avoid pathsto earth from the connections to the anode. with any liquiá, allground joints must be above the liquid level. Leads may. be broughtout along long glass tubes to obtain good insulation. ff currents of10-6 A are to be measured to ro/o at 200 v appriecr, the permissible leakageis l0-8 A, i.e. an insulation resistance of 200/10-8 : 2 | lgro o is requireit"Measurements at very low current densities require electrocles oi lorg*area and good insulation.

(b) Geometry of electrodes. Good eJectrode geometry is necessary for.forming films as rvell as for capacity me&surements (section 12.02).The conclitions a,re met easily unless the concluctivity of tho solution islow. rf they are not met, the films will be uneven in thickness over theelectrode and the capacity measurements may be serio,sly in error.

(c) Reference electrocle. A constant cathoclo potential may bo obtaineclby using a large platinized platinum cathocle lvith the soluiion saturateclwith hydrogen. Horvever, sometimes the presence of hydrogen m&ybe deleterious. A very small cathocle will give a steaclier poterrtirrt r.,tpoor geometry. The best system is to monitor the anocre pote'tialdirectly with respect to a reference electrocle. rf the ohmic p.n. in thesolution is not negligible, electrical connection to the olectrolyte ismade via the electrolyte in a tube with the encl clrawn down and leadingto a point close to the anocle: this is knourn as a Haber-Luggin capillary.since only minute currents will be drawn from the referei"ce electrcrrr,,

44 EXPER,IMENTÂL TECENIQUEsthere will be no appreciable ohmic r.o. in the capinary, and no.por artza_tion of the reference erectrocle.

"h;;;;;.tion of a reference erectrodedepends on the sorution and the;;;r-1 hydrogen electrode ..in thesame solution" is theoretica,y the most rerevant ir"" .";;;;;.05). Asource of pure hvdrogen is rãquired. iti. i,

"ot'aim"rrt, i.d ""p.o_

duÕibility and accuracy sufficilnt for the present pur?ose aro eas,yobtained' The caromuÍ "tu"t"oà" lio"""*r*pre, the erpes with an

l'ig' 4'0:l' celr fo. forrning firr,s rvit'"açricl circrrraüion oíerectrolyte.

l

asbestos plug or a gt'ouncl joint to lcrluce clifftrsion of I(cl) is ofte, veryconvenient, but care is neecled for workaluminium, t

""ru."'or the sensitiv,r, .iril ::il: ;i:l.J:'#,ffi:,::,ll:rn concentratecl sulprruric acicl, puórlprso, courcl be used. The con-version to potentials "with respect to'o rrya"ogen electrode in ,re samesolution" may be rnlle b,r .o""urtirg first tã potentiats with .".pu.tto the normal hyclrogen eieotrode,rritt", allowing for the pH. (Forpractical purposes the potenüial of a hycrrogen electroãe at 25.i becomes2'303 RTIF==59mv Lss positiv" rJ" "á;r,

unit increaseln-pH, th"normal hydrogen electrod.e co"ru.po*tlrrg to pH : g.;

TECENIQUES Or. SUR,T,ÀCE.PB,EPÂR,ATION 45

(d') circuration of the erectroryte. At current densities above r macm-z,the solution must be.circurateá past the anod.e if the tem[""rtr.u of theoxide is not to rise (section o.ú1. tt" system of Figure 4.03 gives asmooth flow past the erectrode which á"oiã. ã""rig^ãr thin sheetelectrodes' A glass,properer working in Teflon bearings (supported instandard ground grass sockets) imp"els the sorutio' ioroa a furoid.Tlis sysiel will give adequate coolilrrg ,rp to 50 or even r00 mAcm-2with tantalum. success L tested "ri,y uy .u"o"aiofiú" ti*"* ro,traversal of equar voltage increments áuring ror-uti?r lt constant

;;il;;f,;.wirh tanratum as test meral, rhe; tim;" ,frà"fa be very

stirring is also advisabre at lorv current densities in order to maintaingood thermostating, particularly in an open cell. Even in a closed cell,at the higher temperatures rãfluxinsdifferentiis f""* rh;l;;;il#;1f".mav cau§e surprisinglv great

(e) Electrod,e mounting. The ãathode may be supportecl by a remov_able-ground joint into which fits a cap holding the anode. pararer-sided and hemisphericar ground joints aitow aalist*""|"iirre positionof the electrode. ce,s mãde of prastic, p.ef""aüly tuáorr, or *hich theflat electrodo forms one face, urã ,r."fúio" ro_""u*pu"i;;..

4.06 TncnxreuEs or Sunnacn pnnpenarrow,The behaviour of most metars is very sensitive to the method ofpreparing the surface. Several methods úave been.,."à.-iá*Uy, theseshould be appried to singre-crystar materiar oí high ,ra r.r,o*., purity.(a) Abrasioz. Abrasiú wiitr a series of .""."r..i.,rury Àrri" -uturro_graphic papers with 90" direction change at each stage has

"t "";;;used. rt p,oduces a surface which is ver] rough on the scale of the oxiclefilm' This reads to low cu.rent efficienJies and mechanicary imperfecbfilms' with tantarum, abracrecl .urfuces require a very Iarge curront toform oxide at all-most of the current produces oxygen. Zirconiunris less sensitive.(b) lretallographic potishing. convention l metarographic porishingyif\ nn9 alro, or Mgo on a.rotating croth-covered wheel leads to afinish which appears mirror-like to th'e eye but which, as recenú workhas shorvn (Samuels, t:521, is essentially-o nrr"ty ut.u,a"*ã"

"rt surface,

ffíi;":Tjjyff: ,|;ch surraces clJnot give the turt,*.urt., being

(c) Bufi,ng. The specimen, is helcl a-Sailyt the edge of a rotatingcircular pad made of several rayers of"carico or other croth with awax*jeweller's rouge or wâx+chromium sesqrüoxide ,rpolish,,. It

46 EXPER,IMENTAL TECENIQUESprobably leads to a heavily flowed surface. euite good currentefficiencies can be obtained. Íhe method was often used in the earüerwork.

(d') chemical polishing. The specimen is d.ipped into a bath whichreacts to leave a shiny surface. The surface*risualy "pp;;;. flai athigh magnifications though some "orange-peer,, efectis oit'"r, p.oao""a

and is mo e apparent at low magnificatiús. most of the baths càmmorrryT.^9 ::" liled by Tegarr

1 r ssoi 1e.g. for ranralum u, r, r. u ir rUy r.otr*u98% H2§O4,7OoÁ I[NO3, 4S%]J-E).

Fig' 4'04. Teflon cell for erecüroporishing with corrosive solutions,

^(e) Electrolyti,c Ttorishi,ng. The electrode is made the anocre in a bathwith current or vortage adjustecl to give conditions between etching(active region) and frm growth lpassi-vity;. The actual f.o.u.r"" orulittle understoocl' rhg balhs ,ro.*-orty ,rr"d nr" ristecl by r'"g"rt (1956).careful variation of,-cretails of p.oceáure m&y be requlreci'to fir.r th"best conditions. A 'r'eflon cel alowing

"o"ro*i,r" mixit res (s,ch as the9 : I mixture by volume of ggo/o Hssó4 : ago/o HF which is usefur fortantalum) to be circuratecl is iliustrated in tr'igure 4.04. Erectroryticpolishing would normalry be used where a surface which is flat on themacroscopic soale is required. Otherwise chemical pofirfrirg is moreconvenient. E lectroryti c porishing wil, therefo.", "orrirrriy

iu"pru"".r"dby abrasion down to a/0 paperãr sometimes onry zro,"aid even bypreliminary metallographic pà[shing.

..(f) Anod'izationfollowed,.bi_sotutioí,of ozid,e. The cycle (i) anorlization,(ii) solution of film (e.g. in Htr' in thl oase of tantarum), if repeatecrseveral times, sometimes gives an effect equivarent to erectroryticpolishing (Napier and westwood, rg50). This is quite a .,."t,i1".n iqrr"for aluminium, but with tantarum fieíd rec_rystalrization (section 9.0r)is rather encouraged, perhaps because inc'isions ,"a urry .ry.talline

DXFINITION OF WOBKING ÂR,3Â 47oxide already present are not dissolved. during the removal of the:f,,f;;rlff:d

then act as nuclei for recrystaUizarion during subsequent

4.07 DprrxrrroN oF WonxrNc AnneThe probrem of how to define the working .r";;;t;" specimen andinsutate the elecrrical read with s;ffi;entty high ,u;;;;;" is by nomeans trivial. The possibilities are as folows, "(a) Thick pre_formed, otide on n-on_worlring area. This has theadvantage that no organic materiars are in contact with the sorution.However, the thick o*id", if it is i-*"..a, contributes to the measuredcapacity, and, since thick oxide *

"{r:" l"rk;;_id", "it _ry ,aaappreciably to the leakage current. The vottãg"-;-;l# must notexceed a varue which gives the maximum ioni"-cu"àirt iirough theTo form the thick oxiaã with a sharp junction it is, in practice,necessary to use a stopping--off,compould, which is then removed. Avery sharp step may be,prúuced ry tui. technique. rf the crean motaris merely immersed. to the reqür^ed á"p; in solution, oxide forms abovethe water-line due to creepagã of sorutiion,, and the ,Á*i" rr*"ting in thethin layer of solution p.oãrã", ,,sparkino,,(b) Use of ,,sto7ryting_off,_,

cotru?toi"ar. "frfytfrene, paraffin, or ceresinwaxes do not form a goocl seal. Teflon *y- U" .ppii"J ,r ln aqueoussuspension

!e's' »un11t^3flo1enamel), áriecr at abáut 1000c to removewater and fused at 800"C. S"r"rnt .ouii,sutation, t"t foiyu...,u

"1r, r" opilãã'H; l,"JiT"i:ffiT:r""r;to fill pinholes' Teflon a"a potltt er"-o.u, of coursá, ormort idearyresistant crremically, but the pro.À- of application is raüher tirne_consuming if large numbers of Jpecimens are requirecl.Picion or Apiezon_t;4pe ,,rvaiur,, fã.r, good. seals but t'e uppertemperature limit is restrictive. The wabaili or *ort"., ,à* in severar ."ri, ,i'Jlxrrsj,i: #ll"Srfl",il.lOne technique is to âlply a thickish à*à" n".t, tfr", to .lop_o* ancl,finaliy, to remove tÀe áxiae o" ti u"ro.r.ing surfaco b/ chemicalpotishing-avoiding exposure or rhe wax r;il;;il;t;ü"iiln,"g bath.Epoxy .esins fo,m. an ex_cerenü sear, but aro ,;#k;ã';y somesolutions. They may be appliecl ;";;; chemically polished surface,cuted, and the specirnen cleanecl ugoirr-i"foru n.ut "rJ".-iil

catalystand curing times are,chosen to gi"u';pii;rm chemical resistance, andthe "run-offtime" of the uncur"ir"ri"ir i".reased as required by addinga filler such as ,,expanded.,, 1i.e. colloiáaij sihca. J

48 EXpEI,TMENTAT, TEcrÍNreuES. Sevoral other compounds have been used, for example Formvar

enamel diluted. with a I : 1 : I mixture of benzene, toluene and ethylalcohol, and baked (Adams et al., 1957).

(c) Mechani,cal seals. Tefl.on, pol;óhene or rubber pressure-sealedgaskets ofvarious t54pes have been used by several authors (e.g. Sternand Makrides, 1960). Sequences of glasses which accommodate thethermal expansion of the metal to that of borosilicate glass are usuallyobtainable. However, it is quite üfficult to seal the reactive metalssuch as tantalum into glass without introducing impurities.

REFERENCES

ADÂMs, G. B., Boncuuns, C..E. and ver B,vssrrssneuE, P. (1957). A.E.C.A.3388. A.E.C. Information Service, Oak Ridge, Tennessee.

Nerroa, D. If. and'Wrsrwoo», J. V. (f 950). Met. Ind,.76, L23.Se.utrnrs, L. E. (1957). J. Inst. Met.85, L77.Sronn, M. and Merr,rons, A. C. (1960). J. electrochern. Soc. 107,782.Tneer,r, W. J. McG. (1956). "Tho Electrolyüic and Chemical Polishing of

Metals." Pergamon, London. !

Ynaumyne, D. A. (1957). J. electrochem. §oc. lO4, 427.