Indian Journal of Chemical TechnologyVol. 1, September 1994, pp. 287-292

Dissolution and passivation characteristics of titanium and its alloy (VT-9) inhydrochloric acid

V B Singh & SMA HosseiniDepartment of Chemistry, Banaras Hindu University, Varanasi 221 005, India

The potentiostatic technique has been used to examine the anodic behaviour preceeding cathodicpolarization ofTi and its alloy (VT-9) in the concentration range I-11M ofdeareated hydrochloric acidsolution at room temperature. Effect of temperature on the electrochemical behaviour ofTi and VT-9 hasbeen studied for a few selected compositions of the acid solution. An active/passive transition and a largepassivity range of potential are observed for all concentrations and for different temperatures. Cathodiccharging of the metal and alloy resulted in a shift of the corrosion potential (Ecorr) towards more noblepotentials, a decrease in the critical current density for passivity (ic,) and a slight increase in the passivationcurrent (ip). Hydrogen evolution is the most probable cathodic reaction in this medium. Titanium and VT-9showed similar corrosion behaviour, however, the passive current density is found to be somewhat higher incase of the alloy. The film formed on the surface of the metal and alloy, after polarization under differentexperimental conditions, remained fairly stable in prolonged exposure of the specimens. SEM examinationshows formation of uniform passive surface on both the specimens. The electrochemical impedancemeasurements, at different potentials, resulted in non-ideal semicircles and analysis yielded a linearrelationship between reciprocal capacitance and formation potential of the passive film like other valvemetals.

Titanium and Ti-alloys are finding newer uses inpetroleum installation 1. During refining of crude oilseveral compounds result in corrosion of the metalssince the crude oil contains sulphides and chloridesthat turn into hydrogen chloride vapour whichcondense during the processes and thus providing acorrosive environment. Though substantial work? - 5

has been reported on Ti and some of its alloys indifferent aqueous solutions, the corrosion problem inreducing acids has received a little attention.Moreover, the studies are limited to only certainconcentration ranges of acid and no systematicinvestigations have been reported+".

The present paper deals with a systematicelectrochemical investigation of Ti and itscommercially important high strength alloy (VT-9)in hydrochloric acid over a wide concentration rangeat different temperatures. Corrosion parameters andapparent activation energies have been determinedfor these in the experimental solution.

Experimental ProcedureTitanium and VT-9 (S.47 AI, 2.9 Mo, 1.9 Zr, 0.2

wt% Si from Mishra Dhatu Nigam, Hyderabad,India) were cut into pieces in sheet form and were usedas working electrode of2 ern- exposed area. Differentconcentrations of acid solutions were prepared bydiluting the stock concentrated hydrochloric acid

(A R grade) solution. The experimental acid solutionwas deareated by purging with purified nitrogen gasfor 4 h. The experimental set up, electrodepreparation and working procedures adopted werethe same as reported earlier"!". The polarizationwas performed by starting at a negative potentials(shown in respective figures) and moving to positivepotentials, in different concentrations of the acidunless otherwise stated. Selected compositions of theacid were taken to study the effect of temperaturebetween 20-S0°C. The impedance measurementswere done at different potentials (- 300, - 100,+ 100, + 300, + SOO, + 700, +900 mY) in SM HCIsolution within frequency range 5 Hz to 100 kHzusing Electrochemical Impedance system (EG & GPAR Model 378-3).

Results and DiscussionFigs 1 and 2 depict the cathodic and anodic

polarization curves of titanium and VT-9 at 30T,respectively. It is seen that the current densityincreases as the potential increases towards negativedirection for both the specimens. The cathodiccurrent densities generally increase with increasingacid composition at a given cathodic potential. Thesecathodic curves are similar for Ti and VT-9.However, the current densities are lower for Ti thanVT-9 at any potential. The cathodic polarization

288 INDIAN J. CHEM. TECHNOL., SEPTEMBER 1994

101__ --------------------------------------,

'"'E••"t...c.....

c c: :!i ~u u

...•.. 1 It Hel

-0- 3" •....•...5" •

~ 'M •-0- ••••...•.11M •

Pete"'I.! ••.• ora seE-100 -AGO o

Fig. l-Cathodic and anodic polarization of Ti In differentconcentrations of HCl at 30 ± 1°C

105~---.------------------------------------~

..•c;•...c: ,02,u

...•.. , •• Met-0- 3 M ..-- 5 •• •...•.. ,.. ·-0- ••• •-- 11M ·

• • • • • • • • •

-100 -AGO

Fig. 2-Cathodic and anodic polarization of VT-9 in differentconcentrations of HCI at 30± 1°C

..•.. , ••• HC!

5104 ·-0- ,... ·-- 11 M ·

Potlntl.l,mV v. seE400 800 1200 1600 2000 2t,()()-800 -10()() o

Fig. 3-Cathodic and anodic polarization of Ti in differentconcentrations of HCI at 40 ± 1°C

Fig. 4--Cathodic and anodic polarization of VT-9 in differentconcentrations of HCI at 40 ± I"C

..•.. , ••• HC!

-6--5M.

-0-- 9" It

~11M •

Potential, mY•• SeE

SINGH & HOSSEINI: ELECfROCHEMICAL INVESTIGATION OF Ti AND ITS ALLOY (VT-9) 289

curves for Ti and VT -9 in different concentrations ofhydrochloric acid at 40°C (Figs 3 and 4) are also quitealike and are similar to those observed at 30°C. Thecurrent densities at any potential, increased withincrease in acid concentration for both samples andare higher at 40°C. The similarity of the cathodiccurves for Ti and VT-9 at 30 and 40°C indicates asimilar reaction on both the specimens at differenttemperatures. At higher negative potentials (>-700 mY) the gas evolution was vigorous and thecurrent was found to increase continuously eventhough the' potential was held constant. The gasevolution and rapid increase of current werepronounced in concentrated (? 5M) acid solution.Cathodic reaction in acidic solution is usuallyhydrogen reduction and its subsequent evolution.Cathodic Tafel slopes, derived from the curves, werefound to be between 90-135 mV/dec I for titaniumand VT-9. These values are very close to the values forhydrogen evolution reaction in acidic solutions!".

Active-passive behaviour was observed forTitanium and VT-9 in different concentrations ofhydrochloric acid at 30 and 40°C (Figs 1-4). Themetal and alloy underwent active dissolution andbecame passive in the negative potential region. Theonset of passivity moves slightly towards positivepotentials as the concentration of acid increases. The

-0-- 2O·C......0- 1O·C___~'c___ sO'c

-toO -400 0 'I)() 100 1200 IlOO

Pot.""." "",.,. sc [

Fig. 5---Cathodic and anodic polarization of Ti in 9M HCI atdifferent temperatures

critical current density for passivity (icr) and thepassive current density (ip) increased with increasingconcentration of the acid for the metal and the alloy.The maximum value of critical current density andpassive current density (ip) was obtained at II Mconcentration of the acid. The increase in ier is greaterthan the increase in ip for a given concentration ofHC!. When the passive current density values for themetal and alloy are compared it is usually higher [orthe alloy, in most of the concentrations of the acid.The anodic curves for the metal and alloy at 40°C(Figs 3 and 4) and 30°C (Figs I and 2) are similar hutthe critical current density and the passive currentdensity are higher at 40°C.

Figs 5 and 6 represent the cathodic and anodicpolarization curves at different temperatures in 9Mhydrochloric acid for Titanium and VT-9,respectively. The current densities (ier and ip) arcfound to be considerably higher for Ti and its alloy at50°C in comparison to 20°C. In general, thecharacteristics of the anodic curves in the activeregion are almost similar at different concentrationsand temperatures, for the Ti and VT-9, suggesting theoccurrence of some anodic reaction in each case. Astable and broad passive potential range is observedfor Ti and VT-9 in most of the acid concentrationsand at different temperatures too. The passivity

'05~ -~-_El20C---6--- 'JO·C....•... 4O·C

.......-so·c

"'e•..e

u u103-- 103 ....

1. ~,. ~••'i ..

c. ." "~ c~ ~:> :>u u

r

'I)() 100 1200

••••••••tI••• "" •• seE

Fig. 6--Cathodic and anodic polarization ofVT-9 in 9M HCI atdifferent temperatures

290 INDIAN J. CHEM. TECHNOL., SEPTEMBER 1994

current, however, increases with increase in acidconcentration and temperature.

It is observed from the Figs 2, 4 and 6 that in case ofalloy (VT -9) another current maximum is observedwithin the passive region and stable passivationregion is obtained beyond the appearance of suchsecond current peak. The current corresponding tosecond peak is found to increase with increase in acidconcentration and temperature. Since the secondcurrent peak is observed in the case of alloy only, it islikely that the presence of molybdenum in alloy isplaying some role. Corrosion of the alloy wouldenrich molybdenum or its compound on the surfacewhich is oxidised when the potential is increasedleading to such maxima.

Earlier observations 12 - 15 suggested that a hydridelayer formed on the surface of titanium whenimmersed in an acid solution of sufficientconcentration and that the rate!+ of the hydrideformation increased when the concentration of acidincreased or when a cathodic potential was applied.In the present investigation hydrogen evolved on thesurface of the metal and alloy under unpolarizedcondition (ocp) which continued to evolve duringcathodic polarization. It is thus quite likely that ahydride layer would form on their surface. Thesurface of the specimens after the cathodicpolarization appeared grey-mat (dull silvery)possibly due to hydride formation whichsubsequently went under dissolution during theanodic polarization. Such dissolution in case of Tialloy has been reported I 6, in the negative potentialregion in 5 M hydrochloric acid. Severalworkers+T"!" pointed out that the hydride layer isformed in acidic solutions but its thicknessdiminishes with increase in anodic potential-". Sincethe presence of hydride is inferred, it can be said thatthis compound may influence the dissolution andpassivation behaviour of the metal and the alloy. Ithas been reported-t+' that, above a certain criticalconcentration, Ti(lV) ions in solution passivate anactive surface. Thomas and Nobe+' concluded that ata concentration below the critical concentrationrequired for passivation, Ti(lV) ions inhibit theanodic dissolution of titanium in the active state. It isbelieved 14 that in presence of hydride on the surface,formation ofTi(IV) ions in solution would not takeplace and the active dissolution of the metal/alloy isnot inhibited. This is also indicated by higherpassivity current obtained particularly in theconcentrated acid solutions (> 5 M). Figs (1-4), ofcourse the acidity may also be responsible for suchbehaviour.

The apparent activation energy has been

calculated for different potential regions for the metaland VT-9 in 5 and 9M HCI from an Arrhenius plot.The activation energy for the metal and the alloy in thesame composition of acid, differs slightly in the activeregion, while the difference is prominent in thepassive region. The observed values in the activeregion for Ti and alloy (l5.8-18.9K cal/mol) suggest achemical reaction as reported elsewhere>'. Thevalues are higher in the passive region (10.6-18.1 Kcal/mol) for VT-9 compared to that ofTi (7.1-12.3Kcal/mol). The activation energies obtained from theplot oflog icr vs I/Tare higher than those reported forTi and its alloys in H2S04 (ref. 6,24) and are close tothose reported in H)P04 (ref. 7).

The performance and stability ofthe passivatedmetal and alloy after polarization, were examinedpotentiostatically. The passive specimens were leftexposed in 1M, 3M, 5Mfor 14h and in 9MHCI for 5 hunder the open circuit condition and the ocp wasnoted at different intervals in this duration. The ocpwas found to remain positive (+ 60 to 260 mV for Tiand + 150 to 340 mV for VT-9 at 30°C) with slightvariation of potential during 14 h of exposure invarying concentration of the acid. Similarly at 20, 30and 40°C the ocp remained more noble when thepassivated specimens of the metal and alloy wereexposed to different acid concentration (1,3 and 5MHCI). The ocp values, after polarization andexposure, were found to be more noble in case ofVT-9 compared to Ti in different concentrations ofthe acid or at different temperatures except in 9MHC!. In 9M HCI experiments after 5 h exposure theocp did not remain noble as in the case of lowerconcentrations (1-5 M) and the difference betweenthe ocp of the passivated and unpassivated sampleswas small, i.e., '" 40 mY. This implies that in 9MHCIthe passivated metal and alloy were not stable and thefilm which was formed tended to loose its protectivenature. This is confirmed by potentiostatic studies.

Further these specimens were brought to + 1500mV in the passive region and current vs time wasnoted for about one hour. It was observed that thepassive current densities were lower than thoseobtained during the first course of polarization at+ 1500mVin 1,3 and 5Macid solution, while in 9MHCl a higher value of current was recorded over firstpolarization. The colour of the electrolyte (9M HCI)became yellow during the immersion of the passiveelectrodes which indicates dissolution of the film. Thepassive current density for the passivated metal andalloy was almost same in 1M HCI at 20,30 and 40°C.This indicates the formation ofa uniform protectivefilm of same chemical composition on the eitherelectrode at all the temperatures and became even

------------.-- --.---- --------

SINGH & HOSSEINI: ELECTROCHEMICAL INVESTIGATION OF Ti AND ITS ALLOY (VT-9) 291

0.1

-6- Ti

--0-- YT- 9

••u

o

E,mYW;SCE

Fig, 8-Dependance of reciprocal capacitance on the potential offormation in 5M HCl at 30°C

Fig, 7-SEM micrograph of the passivated metal and alloy, afterexposure in 9M HCl (a) Ti and (b) VT-9 at 20T

more protective after prolonged exposure in the acidsolution. The corrosion reaction occurring at thesurface of the passive film of the specimens may beconsidered to be dissolution-formation reactionduring the exposure and polarization in HC\. Thedissolution rate is slower than the film formation rateand, therefore, the ocp remained noble and thepassive current densities were found to be much lowerin 1,3, and 5MHCl than in 9MHCL It is also observedthat the passive current density is slightly lower in caseofTi than its alloy. The dissolution behaviour of theanodic film formed on the metal and the alloy isattributed to the inherent properties of the oxide film'and the nature and concentration of the electrolyte.The slightly higher passive current density for VT-9compared to Ti shows that the alloying elements donot improve the corrosion resistance.

SEM examination of the surface of passivatedmetal and alloy reveals formation of uniformprotective layer in 9MHCl at 20°C (Figs 7'a and b).

However, the film formed on titanium surfaceappeared to be very uniform consisting of smallergrains compared to that of the alloy. The observeddifference in corrosion between the metal and thealloy may be due to the dual phase microstructure ofthe alloy and ~ phase (dark region) is attacked morethan the rt phase (bright region, Fig. 7b).

The complex impedances did not give idealsemicircles for this system. The derivation can beascribed to the possibility of incorporation ofelectrolyte ions into the passive film. A plot ofreciprocal capacitance vs formation potential (Fig.8), is linear similar to the behaviour observed forother valve metals25, yields a linear dependance. Thelinear relationship is consistent with a-constant meanelectrostatic field in the passive film. It can beconsidered that the linear dependance of reciprocalcapacitance on applied voltage may result becausethe series capacitance depends on th'e voltage.

ConclusionsTitanium and VT -9 showed active/passive

behaviour in the whole composition range ofhydrochloric acid at different temperatures. Thecritical current density (icr) and the passive currentdensity increased with increasing acid concentrationand temperature. The electrochemical corrosionbehaviour of the metal and alloy is nearly same inhydrochloric acid. The passivated metal and alloyretained its stability during prolonged exposure inupto 5M HCl solution, while in 9 and 11M HCl thepassivated specimens underwent severe corrosion.Hydride formation seems to occur on the surface ofthe metal and alloy and its presence does notsubstantially disturb the kinetics of dissolution andpassivation of the metal and alloy. In generaL themetal and the alloy showed comparable corrosionresistance in the entire concentration rangeU -l1M)ofhydrochloric acid.

292 INDIAN J. CHEM. TECHNOL., SEPTEMBER 1994

AckoowledgementsWe are grateful to the Head, Department of

Chemistry, Banaras Hindu University, for providingnecessary facilities.

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