The corrosion behaviour of Ti-Cu (2%) in phosphoric acid

0 Elsevier, Paris Corrosion of Ti-Cu (2%) in phosphoric

acid

Ann. Chim. Sci. Mat, 2000,25,

pp. 447-455

THE CORROSION BEHAVIOUR OF Ti-Cu (2%) IN PHOSPHORIC ACID. Mohy-eddine

KHADIRI’,

Abdelaziz

BENYAkHa,

Abdelkader

OUTZOURHITb,

a Laboratoire d’Electrochimie et de Chimie Analytique, Dkpartement BP 2390, Mamkech, Maroc. b Laboratoire de Physique du Solide et des Couches Minces, Dkpartement BP 2390, Mamkech, Maroc.

Elhassani

de Chimie, de Physique,

Facult6 Facultk

Lahcen AMEZIANEb des Sciences

Semlalia

des Sciences Semlalia

Abstract - The electrochemicalbehaviour of Ti-Cu(2%) was investigated in phosphoric acid at various concentrationsand temperatures.The alloy showed an active-passivebehaviour in this medium over a wide concentrationrange. An increasein the acid concentration resulted in an increasein the peak current density and a decreasein the passivation current density. The apparent activation energy calculatedfrom Arrhenius plots is in closeagreementwith thosereportedin the literature, and showsthat the Ti-Cu alloy can resistscorrosionin a wide temperaturerange. The voltammetry study showed that the dissolution passivation of this alloy is governed by the Calandra- Mtiller mechanism.In addition, the kinetics of the formation of the passivefilm in 5 M phosphoricacid obeysthe Okorie -Novak model. Rksum6 - Etude de la corrosion de I’aUiage Ti-Cu(2%) dans I’acide phosphorique. Le comportementClectrochimiquede I’alliage Ti-Cu(2%) a ett6 &udie dans l’acide phosphoriqueA differentesconcentrationset temperatures.Cet alliagepresenteun comportementactif - passifdans un large domainede concentration.L’augmentation de la concentration de I’acide, entralne une augmentationde la densitede courant de pit de passivationet une diminution de la densitedu courant de passivation.L’energie d’activation apparentecalculeea partir de la loi d’Arrhenius est en accord aveccellesreportcesdansla litt&-ature et montre quel’alliage Ti-Cu resisteQla corrosion dam un large intervalle de temperature.L’etude voltamm&ique, montre que le mecanismede la dissolution/ passivation est gouvernepar le mecanismede Calandra-Miiller.En outre, la cinetique de formation du film passifdana l’acide phosphoriquede concentration5M, obeit au modelede Okorie-Novak. 1. INTRODUCTION Becauseof their high mechanicalcharacteristicsand their excellent corrosion resistance, titanium and its alloys find a large field of application in aeronautic and spaceindustries,heat exchangers,chemical and electrochemicalindustry, plants and others...[l-61. They exhibit a high corrosionresistancein all acidic media.This is due to the formation of an oxide layer, mainly TiOz [7,8], but other oxides may be involved [9,10]. Reprints: M. Khadiri, Departementde Chimie, F.S.S. M, BP 2390, Marrakech, Maroc. Titanium and its alloys have been the subject of many investigations in acidic media.

M.-E Khadiri et al.

448

Titanium and its alloys have been the subject of many investigations in acidic media. Nevertheless,the few studiesperformedin orthophosphoricacid [11,12] have revealeda changein the solution color at potentials less than -700 mV/SCE. The color then disappearedupon polarization in the anodic direction. This changeof color hasbeen attributed to the formation of trivalent titanium or to the reduction of complexesor speciesof titanium to lower oxidation statesas follows [ 1I] : TiO(H#O&

+ le‘------>

Ti3+ + 2HlP04- + O*-

It was also found that the peak current density (iJ increasedwith increasing H9P04 concentrationuntil 11M. However, at 13M acid concentrationthe peak current density decreased. This phenomenonhasbeenattributed to possiblechangesin the acid structure[ 121. The study of the corrosionbehavior of sputter-depositedbinary alloys Mo-Ti, Ti-Cr and WTi in HCl [13-161hasrevealedthat their strongcorrosionresistanceresultsfrom the formationof an oxyhydroxide of the two metallic cationsasdeterminedby X.P.S. measurements. Electrochemicalstudy of binary Ti-Cu(2%) alloy hasbeenperformedin a 2SM H2SO4and showedthat the shapeof the polarization curves was closer to that of the pure metal until the passivation peak was reached.Differences occurred in the passive domain where a new hump appearedat -10 mV/SCE, and was essentiallyattributed to dissolutionof surfacecopper [17]. To our knowledge no detailed study of the corrosion behavior of this alloy has been performed in H,P04. The aim of the present work is, therefore, to examine electrochemicaland corrosion behavior of Ti-Cu(Z%) in H3P04 media of different acid concentrations and at different temperatures.In addition, the kinetics of the formation of the protective film is investigatedusing the polarization curvesand chronoamperometry. 2. EXI’ERlMENTAL

TECHNIOUES

The voltammetry experiments i = f(E) were performed with the conventional threeelectrode configuration using a computer controlled EG&G Princeton Applied Research273A Potentiostat / Galvanostat. The working electrode (WE) was a 1 cm2 Ti-Cu(2%) alloy plate embeddedin an inert resin. Prior to eachscan, the WE was mechanically polishedwith 220 and 400 grade abrasive paper. It was subsequentlycleanedin an ultrasoundbath and rinsed with acetoneand distillated water. The referenceelectrodewas a saturatedcalomelelectrode(SCE), and the counter electrodewasa platinum sheet. The solutions were preparedfrom an 85% analysis-gradeHzP04. 12, 10 and 5M H3P04 solutionswere obtainedby successivedilution. The working electrodewas dipped in the solution for about 30 secondsin order to stabilize its open-circuit potential. The sweepingrate (vb) varied from 1 to 20 mV/s for each concentration.The solutionswere energetically stirred, aerated,and maintainedat a constanttemperatureusing a thermostatedbath. The temperaturewas varied from room-temperatureto 50°C. The chronoapmerometrystudy of the Ti-Cu alloy was performed with potential steps ranging from the open-circuit potential to 1200 mV/SCl$ thus spanningthe active and passive region of-the alloy. The variation of the current densitywith time wasrecordedfor eachtemperature and concentration. 3. RESULTS AND DISCUSSION The polarization curves of the Ti-Cu alloy in phosphoricacidic media at 15, 12, 10 and 5M are showninfigure I. An active passivebehavior is observedfor all the concentrations.

Corrosion

of Ti-Cu (2%) in phosphoric

acid

449

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1ZM IOM SM

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0,04-

fz $ 0,02 .- o,oo-,.

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, -500

, 0

,

I 500

.

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E (mV/SCE) Figurel. Polarization curvesofthe Ti-Cu(2%) alloy for different concentrationsof HaPOd. It can be seen that when the acid concentration increasesthe corrosion potential shifts towards cathodic values. This is accompaniedby an increasein the peak current density and a decreasein the passivationcurrent. This result suggeststhat phosphateanionsmay contribute to the passivationprocess.However, taking into accountthe concentrationsor the pH used,it is unlikely that these anions are involved in the passivation processof the Ti-Cu alloy, since at these concentrationsno significantdissociationof H3P04is excepted.

0,35

0

A 0 0

25%. 3o”c.

40°C. 50%.

10

E(mVL3.C.E) Figure 2. Polarization curvesof the Ti-C!u(2%)alloy at different temperaturesin 1OM H3P04, Vb = 2 mvts.

450

M.-E Khadiri et al.

The polarization curves at different temperaturesare reported in figure 2 for the selected concentration 10 M. In this case,the peak current density increaseswith increasingtemperature, and the corrosion potential shifts towards the cathodic region. The polarization curves at different temperaturesare similar, however, new humps appearedin the passive region. Their current densitiesincreasewith increasingtemperatures,and their respectivepotentialsmove cathodically. This behavior may be attributedto the dissolutionof the metal throughthe oxide layer. In addition the passivationcurrent density increaseswith increasingtemperature,andthe activation domainof the alloy becomeswider asthe temperatureis increased. The broad similarity between the polarization curves, in this case, suggeststhat the dissolutionprocessof the Ti-Cu alloy is the sameover the explored temperaturerange,and that only its rate changeswith the temperatureor the concentration. The apparentactivation energiescalculatedfrom Arrhenius plots in the active and passive regionsare shown in figure 3 for the selectedconcentration.The activation energy in the active region is of the sameorder asthosereported in the literature for other alloys in HzP04 or in other solutions[11,12].

-7

3,1x1o-3

I

at thepeakpotential

0

at 1100 mVK’2.E

3,2x10”

3,3x1o-3

3,4x1o-3

l~(JC’) Figure

3. Arrhenius plots of the current density for Ti-Cu(2%) in 1OM phosphoricacid.

To fkther clarify the dissolution/ passivationmechanismof Ti-Cu(2%), the effect of the sweepingrate was investigatedvigure 4). When the sweepingrate increases,the peak current density(i,,) increasesand its potential (IS,,) moves in the anodic direction. In addition, the passivationcurrent density increaseswhile the corrosionpotential remainspractically constant.

Corrosion

of Ti-Cu (2%) in phosphoric

0925 . 0,20-

0.15 es-

-

0 0 A v t X

acid

451

1 mv/s zmv/s 4mVls 6mVls 12mVis ZOmV/s

i

-5im

0 E (mV/S.C.E)

Figure 4. Evolution of the polarization curvesof the Ti-Cu(2%) alloy with the sweepingrate at 12M phosphoricacid. The variations of iP and Ep with the squareroot of sweepingrate are linear with a good correlation. Furthermore,the polarization curves of the Ti-Cu alloy presentbefore the passivation peak many linear segments,where the current density varies linearly with the potential, i.e. : E = R*I, where R is the combinedresistanceof the formed film and the electrolyte. This resistance increasesandreachesits maximum value at the peak. This behavior hasbeenobservedfor iron in phosphatemedium [ 181,where the pure activation is maskedby the passivationprocess,sincethe dissolutionoccurssimultaneouslywith the passivation.However, in other cases,the current density showedan exponentialdependence,due to the activation, before varying linearly with the potential u91. These observations suggestthat the titanium dissolution/paasivationis governed by the Calandra - Mtiller [20] model which states that the passivation of the metal is based on the formation of an insolublefilm on the metal surfaceasfollows : M+nX-

=======> MX,, + ne’

whereX- is an anion. The film nucleatesat certain points and then growslaterally until only a small Inaction of the surface remains uncovered. The surface where the dissolution occurs decreases accordingly until the reaction kinetics becomeslimited by the solution resistancein the pores.In addition, this model also predictsthat the peak current density (ir,) and its correspondingpotential (EJ dependon the squareroot of the sweepingrate . As showninfigure 5, the plots of i,, and Epvs. v”’ are linearwhich further confirms that the dissolution/ passivationof the alloy obeysthe Calandra- Mtiller model. Since it is unlikely that the phosphateanionsparticipate in this process,this behavior can only resultfrom the decompositionof water into H30Cand OH- on the surfaceof the alloy. Indeedusingthe X.P.S analysisX-Y. Li et al. [ 151showedthat there is a hydroxide layer at the metal - solutioninterface, in the caseof sputteredTi-Cr alloy. Underneaththis, an oxide layer is present.In addition, all the X.P.S study performedon titanium and its alloy subjectedto treatments in aeratedor deaeratedmedia revealedthe presenceof OH‘ ions at the surface,and 0’. deeperinto the film [13-161

452

M.-E

Khadiri

et al.

0,20

0.16

rG%I2 E 0 3 0.08 ..P 0.04

Figure 5. Evolution of the peak potential and of the corresponding current density with the square root of the sweeping rate vb in 12M phosphoric acid. Since the titanium dissolves in the active region to Ti3+ state, it can be stated that the dissolution / passivation of titanium copper alloy occurs as follows : Ti + 30H‘

====>

Ti(OH)j

+ 3e-

This is in accordance with the observation that when the acid concentration increases the water concentration and consequently the OR concentration decreases, which considerably increases the peak current density. Subsequently, the dissolution valency after the passivation peak changes slowly to 4. Indeed, using X.P.S analysis Kim et al. [ 141 showed that the apparent charge of Ti ions on the surface of sputtered titanium films is lower than 4. Therefore, the formed hydroxide gradually changes at the hydroxide I metal interface to the oxide as follows : Ti”3)(r.d3,Tt .c+41

(x13)0(‘-‘(2x/3)0H(-)(3-x)

The formed film is porous, as predicted by the Calandra-Mtiller model, and allow metal dissolution through the pore of the oxide layer. This is evidenced by a non-zero current density in the passive region. Furthermore, this dissolution competes with a repassivation, since the current density remains constant in this region. The chronoamperometric curves are shown infigure 6 for various applied voltage steps. The overall shape of the curves remains the same for the acid concentration and temperatures explored vigure7). A fast drop in the current density when the circuit is closed, and then the current tends to stabilize at a value which depends on the applied potential.

Corrosion

of Ti-Cu (2%) in phosphoric

5-

‘I-

oz.-

acid

453

7

o o x p

-100 (mV/S.C.E) 0 (mVk3.C.E) 200 (mVIS.C.E) 1000 (rnWS.C.E)

go xv

0;o

0;2

0;4

0;6

Oh

t 6)

Figure 6. Chronoamperometric curvesfor various appliedvoltagesin SM phosphoricacid at room

temperature.

Figure 7. Chronoamperometriccurves in 15M phosphoricacid for different temperatureswith a 1200mV/S.C.E appliedvoltage. The semi-logarithmicand logarithmic plots of the current againsttime are showninfigures 8 and 9. Three parts in thosecurves canbe easilydistinguished: two exponentialparts followed by a regionwhere the logarithm of current densityvaries linearly with the logarithm of time. The shapeof the chronoamperometrycurves is in good agreementwith the Okorie - Novak [2 l] model suggestingthat the passivationof Ti-Cu alloy occursin three steps: First, the formation and growth of several nuclei on the electrode surface. This is mathematicallydescribedby :

M.-E Khadiri et at.

454

-u% W

Q10

412

414

0.16

0,18

I

Figure 8. Semi-logarithmicplots of the current againsttime at 1 V/S.C.E in 5M phosphoricacid.

Figure

9. Logarithmicplot of the current againsttime at 1 V/S.C.E in 5M phosphoricacid.

Subsequently,the kinetics is governedby the diffusion of metallic cationsthrough the formed film, from the metal surface to the solution/film interface. This is describedby a current in the form :

The two processesresult in a porous film, the pore elimination becomesdominant and the law

Corrosion

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acid

455

governingthis processis :

i = atmn(n I 1) Thesefeaturescanbe clearly observedin the chronoamperometrycurvesgiven injigures 8 and 9. 4. CONCLUSION The Ti-Cu(2%) alloy exhibited an active-passivebehaviour for al1investigatedphosphoric acid concentrations and temperatures. The peak current density increased with increasing concentrations and temperatures.The passivation current density increased with increasing temperature.Dissolution/passivationof the alloy is governedby the Calandra-Mtiller model. Under potential control the kinetics of the formation of the passivef%n in SM of phosphoricacid obeys the Okorie - Novak model. ACKNOWLEDGMENT: program.

This work was partially supported by the PARS No 60-Physique

5. REFERENCES El1 El 133 t41 151 161 [71 t81 191 t101 1111 1121 El31 r141 r151 [161 r171 [181 [191 1201 r211

M. Dechamps,P. L&r, J. LessCommonMetals, 56 (2) (1977) 193-207. T. W. Farthing, Titanium. Sci. Technol. Proc S*Int. ConfTitanium, 1 (1984) 39-54. K. B. Lowrie, Metals Progress,100 (3) (1971) 166-167. W. G. Renshaw,P. R. Bish, Corrosion,11 (1955) 57-63. W. H. Colner, M. Feinleib, J. N. Redin, J. Electrochem.Soc.,lOO(11) (1953) 485-489. R. D. MC Intyre, Mat. Eng., 96 (3) (1982) 40-47. J. C. Pesant,P. Vennereau,J. LessCommonMetals, 69 (1) (1980) 63-72. J. C. Pesant,P.Vennereau,J. Etectroanal. Chem. Interfacial Electrochem 106 (1980) 103113. B. M. Biwer andS. L. Bemasec,Surf. Sci., 167(1) (1986) 207 -230. E. Roman,M. Sanchez-Avedillo and J. L. Desegovia,Applied PhysicsA 143 (2-3) (1984) 482 - 492. V. B. Singh, S. M. A. Hosseini,J. Appl. Electrochem.,24 (1994) 250-255. V. B. Singh, S.M. A. Hosseini,Corr. Sci., 34 (10) (1993) 1723-1732. P. Y. Park, E. Akiyama, H. Habazaki, A. Kawashima,K. Asami and K. Hashimoto,Con. Sci., 38 (10) (1996) 1649-1667. J. H. Kim, E. Akiyama, H. Yoshioka, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto,Con-.Sci., 34 (6) (1993) 975-987. X-Y. Li, E. Akiyama, H. Habazaki, A. Kawashima,K. Asami, K. Hashimoto,Corr. Sci., 39 (5) (1997) 935-948. J. Bhattrai, E. Akiyama, A. Kawashima,K. Asami and K. Hashimoto, Corr. Sci. 37 (12) (1995) 2071-2086. A. I.&argue, J. A. Petit, F. Dabosi,J. Less-CommonMetals 56 (1977) 233-241. J. Benzakour, A. Derja, J. Electroanalyticalchemistry,437 (1997) 119-124. A. Derja, A. El Hanssali,J. Chim. Phys., 94 (1997)569-583. A. J. Calandra, W. J. Mtillcr, Transient Techniquesin Electrochemistry. Plenum Press (1977) 295-298. B. A. Okorie, W. B. Novak, Journalof ElectrochemicalSociety 130(2) (1983)290-296.

(Article recu le 18/08/99,

sousforme definitive le 04/05/00)