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Anodic behaviour of titanium in acidic chloride containing media (HCl NaCl). Influence of the constituents of the medium—III. Analysis of the electrochemical impedance. General dissolution — passivation mechanism
ANODIC BEHAVIOUR OF TITANIUM IN ACIDIC CHLORIDE CONTMEDM (HCl - NaCl). INFLUENCE OF THE CONSTITUENTS OF THE MEDKJM-III. ANALYSIS OF THE ELECTROCHEMICAL XMPEDANCE. GENERAL DISSOLUTION - PASSIVATION MECHANISM J. P. FRAYRET l_.&oratoirc d’Ener&!ique, lkole Nationale zz~_~E
Mkanique, 1 rue de la Noi, 44072 Nantes
and A. &,,up
de
~~rche
CAPRANI
no 4 du C.N.R.S. “F’bysique dcs Liquidca et Elcctrocbimie”, associk I l’Universit.6 Pierre et Marie curie, 4 place Jussku, 75230 Paris Cedex 05, France
(Received 18 May 1981)
.
~-AMl3Magtkdcotrockguodtmpedarrcs d~~ploctadinsLorpfrequarcy~(lO-‘Hz1~ m) cna#cr to anapkte and support tbe dlssolution-passivatlon scheme of titanium elaborated from stauiy state mts. ~PILP1YdlOttlieR,.Z~(~~tranrfsrrasistmaby~tiaury cWalt~-theplwalceof two active disduti~ paths in terratent titaniukn, one of wbicb depends on the ptmaca of the hydride the tntnwdmt oxide TiOp in the establisbmcnt of the (Tsi,). MOit ElMBrms the ngthechCmicalnat.umofalltlwintemwdiatcaandfinal StPbldWW.ACOmPkfC
INTRODUCITON In a preceding paper[l], the analysis of the variations of tire current wd of the diilution valence with the potaktial has allowed us to prv for titanium in concentrated hydrochloric solutions a disklutionpnesivation model charmeked essentially by three lxaction pethe. Thisamdelwhichincludc~~ratedeterminingstcps current potential relationisre6alledbCIOWsoasthe ship associated:
In this scheme, Ti is the metallic substrate assumed to be uncovereel by the oxide, Ti(IIIh is an adsorbed tervaknt intenncdkte which partially covers the surface (coverage 0,); TimI is the adsorbed tetravalent intermediate comxponding to the first pamivation path (coverage 0,); Ti(a, is the a&o&cd tetravalent interme&te correspond to the secoud passivation path (coverage 0,); Ti(III),, Ti(IV),, and Ti(IV),, are the tcrmd tetravalent species which leave tbeinterface. AS@Yistherateofthe&’ ’ ‘dstcp of the global reaction Ti -WTi(III).,,,,, B,eb~Y and
Ti(III),
Ti . . . . _ . _ ‘a - ‘@‘:
Ti(IIIh Ti(IVb,
I=
%Ti(Iv),,
F(3D,+4B1eb~Y+4B,eb~~ D. 1+ -+ As.@”
B,eblY hv (
+&)+w+-&+g-) 391
392
J. P. FRAYRETAND A. CAPRANI
B,ebiY are the rates of the two electrochemical steps which lead to the formation of Ti(Iu, and Ti(lVhs2from Ti(IIIh.D,, ZJ,,, and Dp, are the chemical reaction rates which lead lo the formation of Ti(III),, Ti(IVb, and Ti(IV)_ respectively. Our purpose here is by studyingthe electrochemical impedance, which depends on the whole steps of a mechanism, to verify the vali&ty of the model on the one hand, particularly the existence of the three reaction paths,and to obtain informations on the rapid steps unreachable with the stationary measurements on the other. To perform this study, we shall use essentially the analysis of the product R, _Z (change transfer resistance x stationary current). Resides, having noticed the -presence of hydride on the electrode surface in the activity range, from the corrosion potential till the current peak[2], without being able to explain its role in the dissolution kinetics, we shall try consequently to do it here with the analysis of the electrochemical impedance.
_wumLmuu________-___,
I
AND
EXPERIMENTAL
The experimental device used for the electrochemical impedance measurement (Fig. 1) is constituted by a potentiostat having a negative intern resistance[3], a generator and a function correlator Schlumherger type 1172, an bterface Sch&mberger 1180 which enables plotting the impedance diagram in the frequency range 1O- 3 Hz to 10“ Hz on a XY Sefram recorder. The high frequency origin of the diagram is shifted of the electrolyte resistance which is negligeable in our study since it is lower than 2n. The impedance is measured from the response of the current AZ to a sine wave pertuibation (of angular frequency w) of the potential of low magnitude A V. The ratio [A vg’o)]/[AZ (jw)] provides the coinplex impedance Z(jcu) = R -jG which includes beyond the Faradaic impedance, the double layer capacitance and the electrolyte resistance. On the diagrams, the real part of the impedance is reported on the x-axis, the imaginary part in ordinate. The low frequency limit of the impedance diagram is identical to the polarization resistance R,, defined by the inverse of the slope of the stationary polarization curve (I, V): AI/AK It can be positive or negative. The electrochemical cell is the same that the one used for the plotting of the stationary curves (I, V) described elsewhere[4].
EXPERIMENTAL
d&am dution
RESULTS
diagrams
Within the whole frequency range two types of impedance diagrams have been observed: the one is characteristic of the positive slope of the (Z,V) curvy ie the active dissolution range, the other one is characteristic of the negative slope of the (I, V) curve, ie the passivation range. In Figures 2-6 we give some significant examples for various concentrations in HCl. We must note that for potential values. more anodic than - 250 mV us xe it becumes Impossible to plot the diagram in the whole freq+ncy range because of a very high irreproducibility of the ~remenis for the low frequencies. This difficulty increases when the concentration of the medium decreases and the sole high frequency loop of the diagram can be obtained. As a first approximation, all the diagrams plotted in the whole frequency range, are constitnted of two capacitive loops. In the active dissolution d&main the two loops are hated in the first dial of the imaginary plane. For the pas&&ion, the low f&quency loop passes through the imaginary axis and finishes in the second dial. We must note that in all cases, the low frequency loop differ significantly of a sdmi-circlc We have always verified that its extrapolation on the x-axis is near the polarization resistance R, arlcuhttcd from the slope of the (I, V) curve. Resides we can assume that the shape of the diagrams in the near passivity range will be the same as in paasivation since the values of the polarization resistance deduced from the slope
R / Fig. 2. ]mpedance a-dcaerated
‘7Jm
Fig. 1. Ektrochemiad impedance measurement device. r(cu) = 4 v* out/ Scheme of principle. Admittance AV*in*l/r.D.
Impedance
METHOD
/\
I
,--
for 8 M.I-’ HCl at %“C, --t R, c&. = 750 (OM)
Bhr
OS see, n= 1000rcv/min. --o_ V = -600mV b-aerated solution R, talc. = 820 (Ohms).
Anodic
bohaviour
of titanium in acidk chloride containing media @ICI-NaCl)
sao-
Fig. 3. Impcdanct~gramforSM.I-‘H~d~t~,rt21”C,V R, de.
= -SSOmVossce,SI-lMlOrev/min. -
2350 (Ohms).
Fig. 4. Impedance diagmm for 2 M.1 - ’ HCl, dtsasrt&, at 21°C V I R, cdc = 7200 (c%lnlI).
hi
5. 1mpedamc
diagram for 8 h4. I- l BCL daeratod et 21% V R* m&z = - 1050 (#Muus).
- 550 mV us $ce, n = loo0 rw/min.
-35omvusace,n~1ooorev/min,
393
J. P. F~yaE’r
394
AND
A.
CAPRAN
n
61
$
1111
30
.*.r II*
>wsr
II
l#l
#/IL.‘ Fig-b Imped== at2t*C,v=--3Wm
HCI, dcaerar=t, usstz,n-tuODr+nia,R,,cek for 5M.l-’
T= -143o(Ohms)
of tbe (I, V) curve remain negative (for instance in HCl 8 M-I-‘, R,~2OOOOOhms for V= -100mV us see). Finally, the importance of the deoxygenation is reveakd by Fig. 2; for 8 M . I- 1 HCl, deaerated, (curve a~theimpubmEdiagramisstabkintimeandreproducibk whatever the rotation speed of -he ‘disc eketrade LXIn contrast for 8 M-. I - r HCl aerated there is a shift of the diagram towards the positive values of R, which depcndx on the rotation speed of the electrode (curve b corresponds to fI = laoOrw/min). These results are consistent with those deduced from stationary mmsurunents[2].
-1 L
41
-*IO V/d.,
-8s
I
0.C.E.
Fig 7. Variationof the R, _I product withthe potcntiulV, for various activitim in HCI. (standard deviation of not more A 13.5; x 8.4; 2%). l ana = 49.8; 0 22.4 cl 4.4;1 2.1
Product R,. I Theanalysisoftheprcductofthechargatrans&r resistance R, E (aF’/aZ), by the stationary current allows one to cjetect all the reaction paths of a mhanism[5J. Conse+ently we have investigated its variation with various parameters. In Figures 7-9 are represented the variations of the product R, . I with the potential for various activities, in HCI (cr.&, in H+ ions (au+) the activity in Cl- being maintained conatant and in Cl- ions (aa-) the pH being maintained constant, rcspoctively[l, 2,6]Let us recall that the charge tram& iesistance is cxperimentahy determined by extrapolating in low frequency, on the x-axis the high Zkequency capacitive loop of the diagram. Indeed this loop is as a 6rst approximation attributed toa R,, Cdcircuit, where C,, is the double layer capacitance. Figure 7 shows that the variation of R,. I with the potential presents two plateaus. The fir@,one, the lesx anodii, has prackally the same width for all the activities studied whereas ;the width of the second one decrcazzs when the activity in HCI decreases. After the second plateau the product R,. I strongly decreases and tends towards a thirdplateaubadly defined. This is amdally due to the inawwaq on a@ measluenlent of the passivity current which is very leW[4]. For this reason we have no~.~represented the values of the product 4. Z in passivity for the various activities in HQ iower than 49.8 as well as for the vurioua activities in H+ (Fig. 8) and Cl- (Fig. 9). The variations ofR,.Z,giveninFig.gaPd9havet~~shrpess those of Fig 7.
Fig. 8. Variation of the R,I product with the potential V. for various activitiesin H+ ions. (standard deviation of not more than 2%), 0 nn+ - 1.35:0 2.1; + 4.4
Fig. 9. Variation of the R,f product with the potential V, for variousactivitiesinCl-ions.( standud &v&don of not more than 2 7.). a oa- = 3; o 12.5
INTERPRETATION The conservation of the shape of the im&r dfqguu~tbs4st=ofyte~ namalus that the dksolWon-pussivation meckmsm ranrlasunckk&.Thereaetionnaodel.recahed klkWsLwbichincl~~adsorbsdin~ “tee should give rise to &, Ti(lv&. Tim,).
Anodic bchaviour of titanium in acidic chloride wnuining mental diagram has to exhibit three loops, or two only Nevertheless, taking into account the intheirpraanoc. sp~oftheftequencyrange,whichislimitedinhigh f-w by tha existence of the loop associated to R,.Cdandto10-3Hzinlowfisquency,~~the nece&ty to have time wnstauts dSrring by about two orders of magnitude in order to ObSrve sejk3rate loope, it isnot possible to ohewe more than two loops, In fact measurements revu& OIlc loop only (secolld loop of the diagram) which markedly d&r from a semi-circle. This loop coukl be simulaW by several time cm4tant4, am4i4tmt with the r&on model. Though thehigh f?equesxy loop is not always a semi circle neither perfeot nor complete, we shall assume, a priori, as it is wy admitted that it is associated to the time constant R, . Cd. This loop can then provide the charge transfer resistance and allow a quantitative analyrls of the R,.Z product. To this end it is necessary to know the theoretical expression of R, . Z in the framework of the reaction model reca&d above. TO establish this expression, all themoaoelectronic stepa have to be OonsiduM. We are then led to use the following reaction model:
395
media (HCl-NaCI)
(11) (12) (13) The current is given by: z = F(v,+u,+Vs+V5+u,) and at the stationary
(14)
state:
T= F (3Kb + 4~;;, + 4~;;,)&
(IS)
The perturbation of the potential of low amplitude A V applied for the meas-nt of the eIectrochemiCal impedance provides a response AI of the current I( v, 6,C) whichcan be writtenas follows:
(16)
Ti(III),
Keeping the calculation hypothesis, Mned previously[lJ, the rates of the elementary steps per geometrical area unit are given by: VI =K,(l-e,-e81-e83--e4-eJ
t)a= K,B, uj = KS9,
(1) (2) (3)
vq = K&
(4)
v5 = K&B,
(5)
v6
=
DpLe4
(6)
v,
=
K&,8,
(7)
V8 = D,,@s
If we assume that the concentrations C remain unchanged when the potential is peArbed, ie mkd transfer does not limit the reaction rates, we can write:
(8)
The equations of the mass balance between the formation and the consumption of the species provide the set of ditTerential eqt~tigqs (9)-(13) with #&,&. $?,, fid and j& the cover& c&t&ants of the species consider&
1
AZ(b-1 ---=($)*+(gg
z - AV(jw)
When AelAV = 0, ie when the covezages are inde pendent of the potential which is true for the high freq+es, the farad& impedance Z, is identical with the charge transfer resistance R,: 1 az =F[alK,(1-881-e2-_~-e~-6e,) R,= ( dV > 0 +a,K,e,
+a,KJ& + b, K:, 8, + b,K;;z 0,]
Taking into account equations ~=F’&[(al+a,+a~)Kb+(a, f
de2 = KtOl -K& dt
(10)
R,T=
(17)
(9)-(13) fa2+aS+b,)K&
+(a,+az+a,+b,)K;,] Setting a1 +a,+a, &-
(17)
(18)
- u 3Kc +4K;;, + ;1K;,
uKb+(u+b,)K~,+(tc+b,)K;;,
(19)
J. P. FRAYRET ANDA. CAPRANI
396
or
R,T=
3Da+4E,ehV+4E,&~V uD.+(~+b,)El~~V+(ufbs)B,eb*v
(20) tht potenli0I inereaeD,, B,eblvand B2eblv predm and the product R,Z trccomar~~tot~~tyalue3/u,4/u+b,and 4/u +. b2. These three w&s can correspond in a first amwoxiination to the three ex@erirnental plateaus f&7; uwa = 49.8). SO, the thret ieaction oath model enables to account for fhe tvpe of the e~pt&&ntal variation of R,I with the potential. Moreover the quantitative analysis of the first pIateau provides the mean value of the transfer 9 ets of the two rapid steps (Ti(0) 4 TiQ and Ti(1) + Ti(II) of the reaction model_ This value equal to RTf2F (u-a,) whioh falls between 0 and 1, is cbnsis&-nt with the physical meaning of a transfer ooe%klent. Despite these poilits of agreement we must however note that the experhnental second plateau of R,Z is shifted towards the cathodic potentials with respect to that w&h can be predicted by (20) taking into account the values of the kinetics parameters determined elsewhere[ 11. Jn contrast, the narrowing of this plateau whereas theactivity in HCl or in H+ decreases isconsistmt with the reaction model since the second passivation has a reaction order equal to + 1 with respect to OH-. After the second plateau, the experimental product &Z ma&e&y deawraes and becomes very low which is ia went with (20). The lhnit value 4/(u + bz) is i@eed of one order of magnitude higher. Fiy, taking into aocount the hypothesis on which the iCBCtion model is based, one can expect a non &i&on of the height of the plateaus with the cohcerttration of the constituents of the medium. Ia f+ RJ ax&consequently certain transfer coef&zients vary with the activity in HCl and in H+ (Figs 7 and 8). I&t us note Iioweyer that expeiimental variations of tmmfer cme#iciust with the eonccntration of the medium have baen reported in the literature[8]. These various experimental obsemtions cannot be -plained easily. We shall nonetheless try to interpret #cm by considering the inI&nce of the solid layers we have identifted at the surface of the electrode[2,9]_ So, tpe Brst plateau of R, Z can be related to the presw of the hydri& that we have indeed detected by X-rays, diffraction, with a .&creasing thickness from the “&rrosion ,potqatial” till the peak of the current. The presence of the hydride is concomittant with the sole d&solution at the valence 3[9]. Taking into account the thickness of the hydride layer[2, lo], it is difficuh to consider the hydride as an intermediate of the dissolution mechanism, but it can be considered as a stefic fmtor gl&g rise to two types of dissolution sit- As a first approximation, we can consider dissolution, sites surrounded by the sole titanium and diiolvtion sites surroundad by the hydride, the vicinitj! of the hydride giving rise only to a modification of Lhe rates-of the elementary steps. This modilication can be schtmatized in the reaction model by an addition&l reaction path, leading in solution to the tervalent titsinium, which disappear at the vicinity of the peak of the current. The &crease of 4. I until the second plateau can when
ire,sdveIy
then be explained by the progressive disappearance of the hydride layer or more precisely of a non stw&iome?ric compound Ti H,, not detee&d by X dlffmction, whose the presence modifies the Tafel’s exponents associated with the dissolution into tervaleat titanium. With such an hypothesis the second plateau is reached for x = 0, ie for the potential from wbieh hydrogencan no more penetrateinto the lattice. The existence of the second plateau of R, wZ whereas the dissolution valence is changing, implies, as we established elsewhere[2,11], that in the absence of the hydride and of the second passivating species the mean values of the Tafel’s exponents of the elementary electrochemical steps leading to ter- and tetravaknt titanium are identical. Thus, the value of b, cptr then be deduced; we have verifM that this determination of b, is in good agreement with the value provided by the stationary measurements[l], despite the variation of the height of the plateaus with the pH. ‘ra explain the decrease of the product R, . Z after the second plateau one can consider various hypotheses, particularly the existence of a reversible electrochemical reaction or the formation of a highly insulating layer. The first of these two hypotheses is difficult to verify; in contrast, we have detected by electron diffraction, the presence of the tetravalent oxide TiOz at the end of the passivation and in the near passivity[2, 11-J Consequently, we shall relate the decrease of R,Z with the progressive coverage by the oxide which becomes predominant in the near passivity. In this last domain, the experimental value of R,.I provides transfer coefficient values markedly superior to 1, which is incompatible with the physical meaning of these quantities; the discrepancy between these experimental values and that calculated 4/u -t-b, from expression (20) can be attributed to the nature and thickness of the oxide layer. The obtention of abnormally low values of R, . Z at the end of passivation and in passivity, as well as the modifications of the height of the plateaus when the activity in HCl or HC varies, can be partially due to a bad determination of the transfer resistance because of the possible dispersion of the double layer capacitance, which can have very high values[2] or/and interferences with other time constants. In order to obtain information on the behaviour of the double layer capacitance, we have studied, in the high frequency range, the partial derivative of the imaginary part G of the impedance with respect to the frequency. Since in the high frequency range G is identical to 2n/Cdj; the derivative would be equal to
I
. -555
-501
-455 U/d
0
1s ICI.
Fig. 10. Variation of alogG/dlog f as a function of the potential.
Anodic
bchaviour
of
titanium
in acidic chloride amiaining
media (HCI-N&l)
397
- 1 if the double layer capacitance does not vary with the frequency. As an example, we give on Fig. 10, the values of @log G/a log f), as a function of the potential, obtained for 8 M _1-l HCl. One can note that this derivative differ from - 1 in the whole potential range investigated. Moreover this discrepancy increases in the potential range corresponding to the high decrease of R, . Z and in the passivity range. Consequently, it appears that’the variation of the double layer capacitance with the frequency can lead to an inaccuracy in the determination of the R,. I
Moreover, the increase of the height of the fimt platum when the pH decma+es cannot he attribytsa~ to the increase of the hydride layer since a similar ‘mcreaee exist for the second plateau for which the ah&em of solid layers has been established. It reault that the variation of R,. I with the pH can only be cspl&ed by the variation with the pH of certain transfer coefficients. Finally, the reaction scheme, the more compkte, which takes into account the results obtained from the analysis of the electrochemical impedance Can be written as follows:
product; the origin of this dispersion of the double layer capacitance can lie in the presence of solid layers at the surface of the electrode. Indeed, these layers provide their own electric contribution, which can be expressed in the equivalent electric circuit of the interface by a RC circuit, in series with the other elements of the impedance, which could interfere in the high frequency range with the time constant R, .C,[12, 131. In these conditions the high frequency loop corresponds either to the two circuits R,C and R,, CA whose the time constants are not very different, or to the sole RC circuit, the time constant R,. Cd contributing only with the other time constants to the second loop. With the first hypothesis one must expect to measure experimentally a “charge transfer resistance” too high since it corresponds to (R + R,) and consequently an experimental “R,. I” too high, which is in contradiction with the experimental results. With the second hypothesis, one must expect to measure experimentally only the leak resistance R of the solid layer and not at all the charge transfer resistance. We have verified this second hypothesis by considering a dielectric layer of TiOl 10A in thickness. Indeed, by writing R . C -z R, . C,, with values of R, and CI consistent with the phenomena they represent (transfer coefficient comprised between 0 and 1, C,, some 1OpF .cm-‘) we have calculated for the leak resistance R, values which are of the magnitude of the diameter of the high frequency loop, ie the experimental “II,“, for the potential range corresponding to the second passivation and the near passivity. Consequently, it becomes impossible to determine experimentally a true R, . I after the second plateau of R, .I, and its comparison with a theoretical value calculated from the reaction mechanism has no sense. If we consider now the hydride layer, we can established, by a similar reasoning, that it cannot affect by its own electric contribution, the experhnentaI R, . Z determination since TiH, has no dielectric properties.
Dissolution-passivation
mechanism
We have shown above that the~understamling of the anodic dissolution of titanium is complex because of the Rresenca of solid layers at the surface of the metal. The hydride layer whose thickness is incompatibzl with the calculation ,hypotheaes of intermediates ad: sorbed according to a monolayer, has led us to invoke a particular reaction path leading. to the tervalent titanium, characteristic of the presence of the hydride. It is probable that the intermediate and final species of this path are identical to those of the reaction path taking place in the absence of the hydride; the differences between the two paths lieing in the values of the kinetics parameters. In contrast, the thickness of the oxide layer being sufficiently low (some A in the media investigated), TiOs can be considered as an intermediate of the passive dissolution, at least in the potential range corresponding to the second passivation and’to the near passivity. We shall now try to specify the chemical nature of the species implied in the reaction scheme. This scheme includes the lowest number of monoelectronic steps leading in solution to the ter- and tetravalent titanium. This species as well as TiOr and TiH, have been identi&d[2,9]. Taking into account, the reaction orders and the quantitative analysis of the valence change reported elsewhere[l] as well as the analysis of the R,‘. I product, we can propose for the kinetic pammeters associated with the various rate determiting ste@s the following expressions: -4 = k&I+ A; = k&I+
1ICl- I
in the absence of the hydride
I/cl-(
in the presence of the hydride
BI = k,, A3, = k,lOH-
I
J. P. FMYRETAND
398
A. CAPRAN
:
Da =
in the absence of the hydride
b.lH+ 1
0: = k,]H+ D,, = k&I+
in the hydride
1 I P-
presence
of
the
I
t Ia- I % = bJft’ If we start from the slow steps situated at the end of each path of the reaction scheme, and if we take into account the more probable compounds[l4,15], we can propose for the chemical reactions leading in solution to Ti(III) and Ti(IV) the following ones: [Ti(OH)Cl]’ [Ti(OH)Cl]”
+ H+ D’TiCl’+
+ Hz0
+ H+ +Cl-%TiClf’
TiOr + I-I+ f Cl- k
+ Hz0
TiO(OH)Ci
Let us note that TiO(OH)CI has to be transformed rapidly in a non oxygenated 5peEies before leaving the interface beeausa the formation of such an oxyguuted mmpound implies the consumption of OH- ions wbkh, taking into ziccount theiT comtration in the maliurn, would imply a limitatbn of the currentbymasstxansfcr in mmdiction with the experimental results The compound [TiOHC!l]+ represents Ti(IIb. Thus if we consider the minimum number of elementary steps required to reach the tetravalent intermediate of the two passivation paths from Ti(III),, we can write:
Ti(0I-I) Tf+‘($Ti
[Ti(OH)Cl]’ [Ti(OH)Cl]
fit[TiCl(OH)]Z+
* + OH-
+ e-
% [TiO(OH)]+
+ HCl + e-
TO form TiOs, an additional rapid chemical reaction is required which is probably: [TiO(OH)]+
+OH-
%!TiO,+H,O
Considering the expressions of A, and A; it is possible to propose for the elementay step forming (Ti(OH)Cl)+ the following one: TYOH), + H++ Cl- z
[Ti(OH)Cl]
+ + &O
Ti+H,O-
-Ti(OH)+H++e-
Ti(OH)+H,OsTi(OH)r+H+
+e-
As far as the reaction mechanism remains unchanged in the pH range investigated, the eonsideration of these two rapid steps can provide an exphtnation of the variation of R, . I with the pH. The Tdel’s expnents a3 and bl having been founded indqmdcut
(OH), +_cyC.+g z 9
[Ti (OH)CI] * *TiCI
+
2
rapid
Ti (OH) _+$+20 mpi2Ti
+ e-
Finally, it remains to specify the two electrochemical steps leading to Ti(OH),, we shall assume that these reactions are similar to those proposed in sulfuric medium by Kelly[16]:
+H+ _H20>TiClz+
(OH)2 TTy
-e -Ht.3 [Ti O’OH)] +
[Ti Cl (OH)]’ ’
+OH-
+c1-
-HI0
rapid
\b
t
T$Zl:*
Ti Or +H+ +Cl-
I /’ [Ti 0-(OH) Cl]
-Hz0
-l-H+
,’
/
//
//’
/’
STiCl:’
Anodic behaviour
of titanium
in a&iii
of the pH[l], the variation of R,.Iwith the pH is exclusively due to the variation of one or of the two Tafel’s exponents of these reactions whose rates are pH dependent. All tbe irreversible elementary steps of the dissol&on-passivation of titanium are summarized by the preceding scheme. CONCLUSION The study we have presented here underlines the importance of electrochemical impedance mcasuremerits, in the analysis of a dissolutio~ivation me4zhanismof a meti especiaUythe information provided by the investigation of the R, .I product. We have also shown that the sole impedance is insufficient to elaborate a model; stationary measurements and identification of the species present at the surface have to be carried out jointly[l, 2-J.In particular the determination of the partial reaction orders thanks to the use of mixed electrolytes (HCl-NaCl) has been decisive in specifying the chemical nature of the various reaction intermediates. Finally, using various complementary methods, we have elaborated a coherent reaction mechanism which appear to us the simplest to describe precisely the anodic behaviour of titanium in deaerated acidic chloride media. We have in particular specified the role of the various constituents of the medium (H+, OHand Cl- ions) on the different steps of the mechanism. Whereas the H+ or OH- ions are implied in the most steps, the Cl- anions are implied only in few steps either alone or simultaneously with Hf ions, in
chloride containing
media (HCI-NaCl)
399
particular in the determining electrochemical step of the active dissolution and in the determining chemical reaction of depassivation. This explicitation of the respective roles of the pH and of the chloride anions would have to allow a better understanding of the stress corrosion cracking of titanium in chloride containing media acidic or not. REFERENCES 1. J. P. Frayret and A. Caprani, Electrochim. Acta 26,1789 119811. 2. i. P. kayret, Thtse d’Etat, Nantes (1979). 3. C. Gab&Hi, Th&ae d’Etat, Paris (1973).
4. J. P. Frayrct, A. Caprani and R. Pointeau, Electrochim. Actn 26, 1783 (1981). 5. A. Caprani, I. Epelboin and Ph. Morel, J. electrwmaal. Chem. 43, App2 (1973). 6. A. Caprani, J. P. Frayret and R. Pointeau, 4th Int. Co& titanium, Kyoto, Japan, May 19-22 (1980). Prp. 2585H. Kimura, 0. lzumi Ed. 7. D. Schuhmann, Thbse d’Etat, Paris (1964). 8. J. Kuta and E. Yeager, XIX Meeting CI’TCE, Detroit (1968). 9. A. Moreau. J. P. Frayret, F. Del Rey et R. Pointeau, Met. cm-. ind. Fr., 628, 429 (1977). 10. A. Caprani, Thkse d’Etat, Paris (1974). 11. A. Caprani and J. P. Frayret, Electrochim. Acta 24, 235 (1979). 12. C. K. Dyer and J. S. L. Leach, Electrochim.Acta 23,1387
(1978). 13. J. W. Dig&, Oxides and Oxide Film. Marcel Dckkcr, N.Y. (1972). 14. W. I-L Latimer, Oxidation Potentials, 2nd Ed. Prentiec Hall (1952). 15. R. H. I-I.Clark, The Chprnistry ofTitanium and Vmdium Elsevier, Amsterdam(1968). 16. E. J. Kelly, Corn. 5th ICMC, Tokyo abstract no 92 (1972).
Mkanique, 1 rue de la Noi, 44072 Nantes
and A. &,,up
de
~~rche
CAPRANI
no 4 du C.N.R.S. “F’bysique dcs Liquidca et Elcctrocbimie”, associk I l’Universit.6 Pierre et Marie curie, 4 place Jussku, 75230 Paris Cedex 05, France
(Received 18 May 1981)
.
~-AMl3Magtkdcotrockguodtmpedarrcs d~~ploctadinsLorpfrequarcy~(lO-‘Hz1~ m) cna#cr to anapkte and support tbe dlssolution-passivatlon scheme of titanium elaborated from stauiy state mts. ~PILP1YdlOttlieR,.Z~(~~tranrfsrrasistmaby~tiaury cWalt~-theplwalceof two active disduti~ paths in terratent titaniukn, one of wbicb depends on the ptmaca of the hydride the tntnwdmt oxide TiOp in the establisbmcnt of the (Tsi,). MOit ElMBrms the ngthechCmicalnat.umofalltlwintemwdiatcaandfinal StPbldWW.ACOmPkfC
INTRODUCITON In a preceding paper[l], the analysis of the variations of tire current wd of the diilution valence with the potaktial has allowed us to prv for titanium in concentrated hydrochloric solutions a disklutionpnesivation model charmeked essentially by three lxaction pethe. Thisamdelwhichincludc~~ratedeterminingstcps current potential relationisre6alledbCIOWsoasthe ship associated:
In this scheme, Ti is the metallic substrate assumed to be uncovereel by the oxide, Ti(IIIh is an adsorbed tervaknt intenncdkte which partially covers the surface (coverage 0,); TimI is the adsorbed tetravalent intermediate comxponding to the first pamivation path (coverage 0,); Ti(a, is the a&o&cd tetravalent interme&te correspond to the secoud passivation path (coverage 0,); Ti(III),, Ti(IV),, and Ti(IV),, are the tcrmd tetravalent species which leave tbeinterface. AS@Yistherateofthe&’ ’ ‘dstcp of the global reaction Ti -WTi(III).,,,,, B,eb~Y and
Ti(III),
Ti . . . . _ . _ ‘a - ‘@‘:
Ti(IIIh Ti(IVb,
I=
%Ti(Iv),,
F(3D,+4B1eb~Y+4B,eb~~ D. 1+ -+ As.@”
B,eblY hv (
+&)+w+-&+g-) 391
392
J. P. FRAYRETAND A. CAPRANI
B,ebiY are the rates of the two electrochemical steps which lead to the formation of Ti(Iu, and Ti(lVhs2from Ti(IIIh.D,, ZJ,,, and Dp, are the chemical reaction rates which lead lo the formation of Ti(III),, Ti(IVb, and Ti(IV)_ respectively. Our purpose here is by studyingthe electrochemical impedance, which depends on the whole steps of a mechanism, to verify the vali&ty of the model on the one hand, particularly the existence of the three reaction paths,and to obtain informations on the rapid steps unreachable with the stationary measurements on the other. To perform this study, we shall use essentially the analysis of the product R, _Z (change transfer resistance x stationary current). Resides, having noticed the -presence of hydride on the electrode surface in the activity range, from the corrosion potential till the current peak[2], without being able to explain its role in the dissolution kinetics, we shall try consequently to do it here with the analysis of the electrochemical impedance.
_wumLmuu________-___,
I
AND
EXPERIMENTAL
The experimental device used for the electrochemical impedance measurement (Fig. 1) is constituted by a potentiostat having a negative intern resistance[3], a generator and a function correlator Schlumherger type 1172, an bterface Sch&mberger 1180 which enables plotting the impedance diagram in the frequency range 1O- 3 Hz to 10“ Hz on a XY Sefram recorder. The high frequency origin of the diagram is shifted of the electrolyte resistance which is negligeable in our study since it is lower than 2n. The impedance is measured from the response of the current AZ to a sine wave pertuibation (of angular frequency w) of the potential of low magnitude A V. The ratio [A vg’o)]/[AZ (jw)] provides the coinplex impedance Z(jcu) = R -jG which includes beyond the Faradaic impedance, the double layer capacitance and the electrolyte resistance. On the diagrams, the real part of the impedance is reported on the x-axis, the imaginary part in ordinate. The low frequency limit of the impedance diagram is identical to the polarization resistance R,, defined by the inverse of the slope of the stationary polarization curve (I, V): AI/AK It can be positive or negative. The electrochemical cell is the same that the one used for the plotting of the stationary curves (I, V) described elsewhere[4].
EXPERIMENTAL
d&am dution
RESULTS
diagrams
Within the whole frequency range two types of impedance diagrams have been observed: the one is characteristic of the positive slope of the (Z,V) curvy ie the active dissolution range, the other one is characteristic of the negative slope of the (I, V) curve, ie the passivation range. In Figures 2-6 we give some significant examples for various concentrations in HCl. We must note that for potential values. more anodic than - 250 mV us xe it becumes Impossible to plot the diagram in the whole freq+ncy range because of a very high irreproducibility of the ~remenis for the low frequencies. This difficulty increases when the concentration of the medium decreases and the sole high frequency loop of the diagram can be obtained. As a first approximation, all the diagrams plotted in the whole frequency range, are constitnted of two capacitive loops. In the active dissolution d&main the two loops are hated in the first dial of the imaginary plane. For the pas&&ion, the low f&quency loop passes through the imaginary axis and finishes in the second dial. We must note that in all cases, the low frequency loop differ significantly of a sdmi-circlc We have always verified that its extrapolation on the x-axis is near the polarization resistance R, arlcuhttcd from the slope of the (I, V) curve. Resides we can assume that the shape of the diagrams in the near passivity range will be the same as in paasivation since the values of the polarization resistance deduced from the slope
R / Fig. 2. ]mpedance a-dcaerated
‘7Jm
Fig. 1. Ektrochemiad impedance measurement device. r(cu) = 4 v* out/ Scheme of principle. Admittance AV*in*l/r.D.
Impedance
METHOD
/\
I
,--
for 8 M.I-’ HCl at %“C, --t R, c&. = 750 (OM)
Bhr
OS see, n= 1000rcv/min. --o_ V = -600mV b-aerated solution R, talc. = 820 (Ohms).
Anodic
bohaviour
of titanium in acidk chloride containing media @ICI-NaCl)
sao-
Fig. 3. Impcdanct~gramforSM.I-‘H~d~t~,rt21”C,V R, de.
= -SSOmVossce,SI-lMlOrev/min. -
2350 (Ohms).
Fig. 4. Impedance diagmm for 2 M.1 - ’ HCl, dtsasrt&, at 21°C V I R, cdc = 7200 (c%lnlI).
hi
5. 1mpedamc
diagram for 8 h4. I- l BCL daeratod et 21% V R* m&z = - 1050 (#Muus).
- 550 mV us $ce, n = loo0 rw/min.
-35omvusace,n~1ooorev/min,
393
J. P. F~yaE’r
394
AND
A.
CAPRAN
n
61
$
1111
30
.*.r II*
>wsr
II
l#l
#/IL.‘ Fig-b Imped== at2t*C,v=--3Wm
HCI, dcaerar=t, usstz,n-tuODr+nia,R,,cek for 5M.l-’
T= -143o(Ohms)
of tbe (I, V) curve remain negative (for instance in HCl 8 M-I-‘, R,~2OOOOOhms for V= -100mV us see). Finally, the importance of the deoxygenation is reveakd by Fig. 2; for 8 M . I- 1 HCl, deaerated, (curve a~theimpubmEdiagramisstabkintimeandreproducibk whatever the rotation speed of -he ‘disc eketrade LXIn contrast for 8 M-. I - r HCl aerated there is a shift of the diagram towards the positive values of R, which depcndx on the rotation speed of the electrode (curve b corresponds to fI = laoOrw/min). These results are consistent with those deduced from stationary mmsurunents[2].
-1 L
41
-*IO V/d.,
-8s
I
0.C.E.
Fig 7. Variationof the R, _I product withthe potcntiulV, for various activitim in HCI. (standard deviation of not more A 13.5; x 8.4; 2%). l ana = 49.8; 0 22.4 cl 4.4;1 2.1
Product R,. I Theanalysisoftheprcductofthechargatrans&r resistance R, E (aF’/aZ), by the stationary current allows one to cjetect all the reaction paths of a mhanism[5J. Conse+ently we have investigated its variation with various parameters. In Figures 7-9 are represented the variations of the product R, . I with the potential for various activities, in HCI (cr.&, in H+ ions (au+) the activity in Cl- being maintained conatant and in Cl- ions (aa-) the pH being maintained constant, rcspoctively[l, 2,6]Let us recall that the charge tram& iesistance is cxperimentahy determined by extrapolating in low frequency, on the x-axis the high Zkequency capacitive loop of the diagram. Indeed this loop is as a 6rst approximation attributed toa R,, Cdcircuit, where C,, is the double layer capacitance. Figure 7 shows that the variation of R,. I with the potential presents two plateaus. The fir@,one, the lesx anodii, has prackally the same width for all the activities studied whereas ;the width of the second one decrcazzs when the activity in HCI decreases. After the second plateau the product R,. I strongly decreases and tends towards a thirdplateaubadly defined. This is amdally due to the inawwaq on a@ measluenlent of the passivity current which is very leW[4]. For this reason we have no~.~represented the values of the product 4. Z in passivity for the various activities in HQ iower than 49.8 as well as for the vurioua activities in H+ (Fig. 8) and Cl- (Fig. 9). The variations ofR,.Z,giveninFig.gaPd9havet~~shrpess those of Fig 7.
Fig. 8. Variation of the R,I product with the potential V. for various activitiesin H+ ions. (standard deviation of not more than 2%), 0 nn+ - 1.35:0 2.1; + 4.4
Fig. 9. Variation of the R,f product with the potential V, for variousactivitiesinCl-ions.( standud &v&don of not more than 2 7.). a oa- = 3; o 12.5
INTERPRETATION The conservation of the shape of the im&r dfqguu~tbs4st=ofyte~ namalus that the dksolWon-pussivation meckmsm ranrlasunckk&.Thereaetionnaodel.recahed klkWsLwbichincl~~adsorbsdin~ “tee should give rise to &, Ti(lv&. Tim,).
Anodic bchaviour of titanium in acidic chloride wnuining mental diagram has to exhibit three loops, or two only Nevertheless, taking into account the intheirpraanoc. sp~oftheftequencyrange,whichislimitedinhigh f-w by tha existence of the loop associated to R,.Cdandto10-3Hzinlowfisquency,~~the nece&ty to have time wnstauts dSrring by about two orders of magnitude in order to ObSrve sejk3rate loope, it isnot possible to ohewe more than two loops, In fact measurements revu& OIlc loop only (secolld loop of the diagram) which markedly d&r from a semi-circle. This loop coukl be simulaW by several time cm4tant4, am4i4tmt with the r&on model. Though thehigh f?equesxy loop is not always a semi circle neither perfeot nor complete, we shall assume, a priori, as it is wy admitted that it is associated to the time constant R, . Cd. This loop can then provide the charge transfer resistance and allow a quantitative analyrls of the R,.Z product. To this end it is necessary to know the theoretical expression of R, . Z in the framework of the reaction model reca&d above. TO establish this expression, all themoaoelectronic stepa have to be OonsiduM. We are then led to use the following reaction model:
395
media (HCl-NaCI)
(11) (12) (13) The current is given by: z = F(v,+u,+Vs+V5+u,) and at the stationary
(14)
state:
T= F (3Kb + 4~;;, + 4~;;,)&
(IS)
The perturbation of the potential of low amplitude A V applied for the meas-nt of the eIectrochemiCal impedance provides a response AI of the current I( v, 6,C) whichcan be writtenas follows:
(16)
Ti(III),
Keeping the calculation hypothesis, Mned previously[lJ, the rates of the elementary steps per geometrical area unit are given by: VI =K,(l-e,-e81-e83--e4-eJ
t)a= K,B, uj = KS9,
(1) (2) (3)
vq = K&
(4)
v5 = K&B,
(5)
v6
=
DpLe4
(6)
v,
=
K&,8,
(7)
V8 = D,,@s
If we assume that the concentrations C remain unchanged when the potential is peArbed, ie mkd transfer does not limit the reaction rates, we can write:
(8)
The equations of the mass balance between the formation and the consumption of the species provide the set of ditTerential eqt~tigqs (9)-(13) with #&,&. $?,, fid and j& the cover& c&t&ants of the species consider&
1
AZ(b-1 ---=($)*+(gg
z - AV(jw)
When AelAV = 0, ie when the covezages are inde pendent of the potential which is true for the high freq+es, the farad& impedance Z, is identical with the charge transfer resistance R,: 1 az =F[alK,(1-881-e2-_~-e~-6e,) R,= ( dV > 0 +a,K,e,
+a,KJ& + b, K:, 8, + b,K;;z 0,]
Taking into account equations ~=F’&[(al+a,+a~)Kb+(a, f
de2 = KtOl -K& dt
(10)
R,T=
(17)
(9)-(13) fa2+aS+b,)K&
+(a,+az+a,+b,)K;,] Setting a1 +a,+a, &-
(17)
(18)
- u 3Kc +4K;;, + ;1K;,
uKb+(u+b,)K~,+(tc+b,)K;;,
(19)
J. P. FRAYRET ANDA. CAPRANI
396
or
R,T=
3Da+4E,ehV+4E,&~V uD.+(~+b,)El~~V+(ufbs)B,eb*v
(20) tht potenli0I inereaeD,, B,eblvand B2eblv predm and the product R,Z trccomar~~tot~~tyalue3/u,4/u+b,and 4/u +. b2. These three w&s can correspond in a first amwoxiination to the three ex@erirnental plateaus f&7; uwa = 49.8). SO, the thret ieaction oath model enables to account for fhe tvpe of the e~pt&&ntal variation of R,I with the potential. Moreover the quantitative analysis of the first pIateau provides the mean value of the transfer 9 ets of the two rapid steps (Ti(0) 4 TiQ and Ti(1) + Ti(II) of the reaction model_ This value equal to RTf2F (u-a,) whioh falls between 0 and 1, is cbnsis&-nt with the physical meaning of a transfer ooe%klent. Despite these poilits of agreement we must however note that the experhnental second plateau of R,Z is shifted towards the cathodic potentials with respect to that w&h can be predicted by (20) taking into account the values of the kinetics parameters determined elsewhere[ 11. Jn contrast, the narrowing of this plateau whereas theactivity in HCl or in H+ decreases isconsistmt with the reaction model since the second passivation has a reaction order equal to + 1 with respect to OH-. After the second plateau, the experimental product &Z ma&e&y deawraes and becomes very low which is ia went with (20). The lhnit value 4/(u + bz) is i@eed of one order of magnitude higher. Fiy, taking into aocount the hypothesis on which the iCBCtion model is based, one can expect a non &i&on of the height of the plateaus with the cohcerttration of the constituents of the medium. Ia f+ RJ ax&consequently certain transfer coef&zients vary with the activity in HCl and in H+ (Figs 7 and 8). I&t us note Iioweyer that expeiimental variations of tmmfer cme#iciust with the eonccntration of the medium have baen reported in the literature[8]. These various experimental obsemtions cannot be -plained easily. We shall nonetheless try to interpret #cm by considering the inI&nce of the solid layers we have identifted at the surface of the electrode[2,9]_ So, tpe Brst plateau of R, Z can be related to the presw of the hydri& that we have indeed detected by X-rays, diffraction, with a .&creasing thickness from the “&rrosion ,potqatial” till the peak of the current. The presence of the hydride is concomittant with the sole d&solution at the valence 3[9]. Taking into account the thickness of the hydride layer[2, lo], it is difficuh to consider the hydride as an intermediate of the dissolution mechanism, but it can be considered as a stefic fmtor gl&g rise to two types of dissolution sit- As a first approximation, we can consider dissolution, sites surrounded by the sole titanium and diiolvtion sites surroundad by the hydride, the vicinitj! of the hydride giving rise only to a modification of Lhe rates-of the elementary steps. This modilication can be schtmatized in the reaction model by an addition&l reaction path, leading in solution to the tervalent titsinium, which disappear at the vicinity of the peak of the current. The &crease of 4. I until the second plateau can when
ire,sdveIy
then be explained by the progressive disappearance of the hydride layer or more precisely of a non stw&iome?ric compound Ti H,, not detee&d by X dlffmction, whose the presence modifies the Tafel’s exponents associated with the dissolution into tervaleat titanium. With such an hypothesis the second plateau is reached for x = 0, ie for the potential from wbieh hydrogencan no more penetrateinto the lattice. The existence of the second plateau of R, wZ whereas the dissolution valence is changing, implies, as we established elsewhere[2,11], that in the absence of the hydride and of the second passivating species the mean values of the Tafel’s exponents of the elementary electrochemical steps leading to ter- and tetravaknt titanium are identical. Thus, the value of b, cptr then be deduced; we have verifM that this determination of b, is in good agreement with the value provided by the stationary measurements[l], despite the variation of the height of the plateaus with the pH. ‘ra explain the decrease of the product R, . Z after the second plateau one can consider various hypotheses, particularly the existence of a reversible electrochemical reaction or the formation of a highly insulating layer. The first of these two hypotheses is difficult to verify; in contrast, we have detected by electron diffraction, the presence of the tetravalent oxide TiOz at the end of the passivation and in the near passivity[2, 11-J Consequently, we shall relate the decrease of R,Z with the progressive coverage by the oxide which becomes predominant in the near passivity. In this last domain, the experimental value of R,.I provides transfer coefficient values markedly superior to 1, which is incompatible with the physical meaning of these quantities; the discrepancy between these experimental values and that calculated 4/u -t-b, from expression (20) can be attributed to the nature and thickness of the oxide layer. The obtention of abnormally low values of R, . Z at the end of passivation and in passivity, as well as the modifications of the height of the plateaus when the activity in HCl or HC varies, can be partially due to a bad determination of the transfer resistance because of the possible dispersion of the double layer capacitance, which can have very high values[2] or/and interferences with other time constants. In order to obtain information on the behaviour of the double layer capacitance, we have studied, in the high frequency range, the partial derivative of the imaginary part G of the impedance with respect to the frequency. Since in the high frequency range G is identical to 2n/Cdj; the derivative would be equal to
I
. -555
-501
-455 U/d
0
1s ICI.
Fig. 10. Variation of alogG/dlog f as a function of the potential.
Anodic
bchaviour
of
titanium
in acidic chloride amiaining
media (HCI-N&l)
397
- 1 if the double layer capacitance does not vary with the frequency. As an example, we give on Fig. 10, the values of @log G/a log f), as a function of the potential, obtained for 8 M _1-l HCl. One can note that this derivative differ from - 1 in the whole potential range investigated. Moreover this discrepancy increases in the potential range corresponding to the high decrease of R, . Z and in the passivity range. Consequently, it appears that’the variation of the double layer capacitance with the frequency can lead to an inaccuracy in the determination of the R,. I
Moreover, the increase of the height of the fimt platum when the pH decma+es cannot he attribytsa~ to the increase of the hydride layer since a similar ‘mcreaee exist for the second plateau for which the ah&em of solid layers has been established. It reault that the variation of R,. I with the pH can only be cspl&ed by the variation with the pH of certain transfer coefficients. Finally, the reaction scheme, the more compkte, which takes into account the results obtained from the analysis of the electrochemical impedance Can be written as follows:
product; the origin of this dispersion of the double layer capacitance can lie in the presence of solid layers at the surface of the electrode. Indeed, these layers provide their own electric contribution, which can be expressed in the equivalent electric circuit of the interface by a RC circuit, in series with the other elements of the impedance, which could interfere in the high frequency range with the time constant R, .C,[12, 131. In these conditions the high frequency loop corresponds either to the two circuits R,C and R,, CA whose the time constants are not very different, or to the sole RC circuit, the time constant R,. Cd contributing only with the other time constants to the second loop. With the first hypothesis one must expect to measure experimentally a “charge transfer resistance” too high since it corresponds to (R + R,) and consequently an experimental “R,. I” too high, which is in contradiction with the experimental results. With the second hypothesis, one must expect to measure experimentally only the leak resistance R of the solid layer and not at all the charge transfer resistance. We have verified this second hypothesis by considering a dielectric layer of TiOl 10A in thickness. Indeed, by writing R . C -z R, . C,, with values of R, and CI consistent with the phenomena they represent (transfer coefficient comprised between 0 and 1, C,, some 1OpF .cm-‘) we have calculated for the leak resistance R, values which are of the magnitude of the diameter of the high frequency loop, ie the experimental “II,“, for the potential range corresponding to the second passivation and the near passivity. Consequently, it becomes impossible to determine experimentally a true R, . I after the second plateau of R, .I, and its comparison with a theoretical value calculated from the reaction mechanism has no sense. If we consider now the hydride layer, we can established, by a similar reasoning, that it cannot affect by its own electric contribution, the experhnentaI R, . Z determination since TiH, has no dielectric properties.
Dissolution-passivation
mechanism
We have shown above that the~understamling of the anodic dissolution of titanium is complex because of the Rresenca of solid layers at the surface of the metal. The hydride layer whose thickness is incompatibzl with the calculation ,hypotheaes of intermediates ad: sorbed according to a monolayer, has led us to invoke a particular reaction path leading. to the tervalent titanium, characteristic of the presence of the hydride. It is probable that the intermediate and final species of this path are identical to those of the reaction path taking place in the absence of the hydride; the differences between the two paths lieing in the values of the kinetics parameters. In contrast, the thickness of the oxide layer being sufficiently low (some A in the media investigated), TiOs can be considered as an intermediate of the passive dissolution, at least in the potential range corresponding to the second passivation and’to the near passivity. We shall now try to specify the chemical nature of the species implied in the reaction scheme. This scheme includes the lowest number of monoelectronic steps leading in solution to the ter- and tetravalent titanium. This species as well as TiOr and TiH, have been identi&d[2,9]. Taking into account, the reaction orders and the quantitative analysis of the valence change reported elsewhere[l] as well as the analysis of the R,‘. I product, we can propose for the kinetic pammeters associated with the various rate determiting ste@s the following expressions: -4 = k&I+ A; = k&I+
1ICl- I
in the absence of the hydride
I/cl-(
in the presence of the hydride
BI = k,, A3, = k,lOH-
I
J. P. FMYRETAND
398
A. CAPRAN
:
Da =
in the absence of the hydride
b.lH+ 1
0: = k,]H+ D,, = k&I+
in the hydride
1 I P-
presence
of
the
I
t Ia- I % = bJft’ If we start from the slow steps situated at the end of each path of the reaction scheme, and if we take into account the more probable compounds[l4,15], we can propose for the chemical reactions leading in solution to Ti(III) and Ti(IV) the following ones: [Ti(OH)Cl]’ [Ti(OH)Cl]”
+ H+ D’TiCl’+
+ Hz0
+ H+ +Cl-%TiClf’
TiOr + I-I+ f Cl- k
+ Hz0
TiO(OH)Ci
Let us note that TiO(OH)CI has to be transformed rapidly in a non oxygenated 5peEies before leaving the interface beeausa the formation of such an oxyguuted mmpound implies the consumption of OH- ions wbkh, taking into ziccount theiT comtration in the maliurn, would imply a limitatbn of the currentbymasstxansfcr in mmdiction with the experimental results The compound [TiOHC!l]+ represents Ti(IIb. Thus if we consider the minimum number of elementary steps required to reach the tetravalent intermediate of the two passivation paths from Ti(III),, we can write:
Ti(0I-I) Tf+‘($Ti
[Ti(OH)Cl]’ [Ti(OH)Cl]
fit[TiCl(OH)]Z+
* + OH-
+ e-
% [TiO(OH)]+
+ HCl + e-
TO form TiOs, an additional rapid chemical reaction is required which is probably: [TiO(OH)]+
+OH-
%!TiO,+H,O
Considering the expressions of A, and A; it is possible to propose for the elementay step forming (Ti(OH)Cl)+ the following one: TYOH), + H++ Cl- z
[Ti(OH)Cl]
+ + &O
Ti+H,O-
-Ti(OH)+H++e-
Ti(OH)+H,OsTi(OH)r+H+
+e-
As far as the reaction mechanism remains unchanged in the pH range investigated, the eonsideration of these two rapid steps can provide an exphtnation of the variation of R, . I with the pH. The Tdel’s expnents a3 and bl having been founded indqmdcut
(OH), +_cyC.+g z 9
[Ti (OH)CI] * *TiCI
+
2
rapid
Ti (OH) _+$+20 mpi2Ti
+ e-
Finally, it remains to specify the two electrochemical steps leading to Ti(OH),, we shall assume that these reactions are similar to those proposed in sulfuric medium by Kelly[16]:
+H+ _H20>TiClz+
(OH)2 TTy
-e -Ht.3 [Ti O’OH)] +
[Ti Cl (OH)]’ ’
+OH-
+c1-
-HI0
rapid
\b
t
T$Zl:*
Ti Or +H+ +Cl-
I /’ [Ti 0-(OH) Cl]
-Hz0
-l-H+
,’
/
//
//’
/’
STiCl:’
Anodic behaviour
of titanium
in a&iii
of the pH[l], the variation of R,.Iwith the pH is exclusively due to the variation of one or of the two Tafel’s exponents of these reactions whose rates are pH dependent. All tbe irreversible elementary steps of the dissol&on-passivation of titanium are summarized by the preceding scheme. CONCLUSION The study we have presented here underlines the importance of electrochemical impedance mcasuremerits, in the analysis of a dissolutio~ivation me4zhanismof a meti especiaUythe information provided by the investigation of the R, .I product. We have also shown that the sole impedance is insufficient to elaborate a model; stationary measurements and identification of the species present at the surface have to be carried out jointly[l, 2-J.In particular the determination of the partial reaction orders thanks to the use of mixed electrolytes (HCl-NaCl) has been decisive in specifying the chemical nature of the various reaction intermediates. Finally, using various complementary methods, we have elaborated a coherent reaction mechanism which appear to us the simplest to describe precisely the anodic behaviour of titanium in deaerated acidic chloride media. We have in particular specified the role of the various constituents of the medium (H+, OHand Cl- ions) on the different steps of the mechanism. Whereas the H+ or OH- ions are implied in the most steps, the Cl- anions are implied only in few steps either alone or simultaneously with Hf ions, in
chloride containing
media (HCI-NaCl)
399
particular in the determining electrochemical step of the active dissolution and in the determining chemical reaction of depassivation. This explicitation of the respective roles of the pH and of the chloride anions would have to allow a better understanding of the stress corrosion cracking of titanium in chloride containing media acidic or not. REFERENCES 1. J. P. Frayret and A. Caprani, Electrochim. Acta 26,1789 119811. 2. i. P. kayret, Thtse d’Etat, Nantes (1979). 3. C. Gab&Hi, Th&ae d’Etat, Paris (1973).
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