ISSN 2070�2051, Protection of Metals and Physical Chemistry of Surfaces, 2010, Vol. 46, No. 5, pp. 587–592. © Pleiades Publishing, Ltd., 2010.Original Russian Text © L.A. Elshina, V.Ya. Kudyakov, V.B. Malkov, S.V. Plaksin, 2010, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2010, Vol. 46, No. 5,pp. 515–520.

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INTRODUCTION

Titanium is one of the most�used structural materi�als. Therefore, numerous investigations aimed atimproving the corrosion characteristics of titaniumand its alloys in diverse corrosive environments haverecently been undertaken [1, 2]. The possibility ofchanging only the surface state rather than affectingthe volume properties of the material (e.g., by alloy�ing), which may also lead to the improvement ofimportant parameters, such as corrosion resistance,hardness, and wear resistance, is very promising [3–5].One of the most up�to�date high�energy methods ofthe surface treatment of metallic materials is the treat�ment with high�temperature pulsed plasma (HTPP)[6, 7]. The HTPP treatment of metallic materialsresults in structural changes of the surface layer.Depending on the material and its original structure,HTPP treatment fuses and smoothens the surface;decreases the sizes of metal grains; and, as a conse�quence, results in the formation of fine�crystalline,amorphous, or amorphous–crystalline structures [9].

Previously, we found [8] that this treatment of alu�minum leads to a decrease in its corrosion rate in mol�ten salts at temperatures no higher than the meltingpoint by an order of magnitude. The object of thisstudy is the thorough consideration of the effect pro�duced by this high�temperature pulsed plasma treat�ment of VT�1 titanium on its corrosion rate in a mol�ten mixture of chlorides and nitrates of alkaline metalscontaining up to 30 wt % sodium nitrate in a tempera�ture range of 790–900 K.

EXPERIMENTAL

Titanium specimens were treated on an MKT�4coaxial plasma accelerator in a hydrogen atmosphere,as well as on a Desna�250 cylindrical accelerator in ahelium environment at the Department of MagneticSystems in Troitsk Institute for Innovation and FusionResearch (Moscow region). Specimens were flag�shaped with an immersed surface area of nearly 2 cm2

and were cut from a VT1 titanium foil with a thicknessof 170 µm containing no more than 1 wt % admixturealuminum, iron, and vanadium, as well as 0.12% oxy�gen and 0.07% carbon.

The specimens were treated with HTPP in the fol�lowing variants:

(i) one shot of hydrogen plasma at an acceleratingvoltage of 12.5 kV at each side of the specimen on anMKT�4 unit (H–Ti) and

(ii) a similar shot of helium plasma at an accelerat�ing voltage of 12.5 kV on a Desna�250 unit (He–Ti).

The conditions were selected in such a way that nosites of visible remelting, which are defects, appearedon the plasma�treated surface. In our previous studiesof the effects produced by HTPP, corrosion was shownto begin at the sites of visible remelting and the meancorrosion rate of specimens with a pronouncedlyremelted surface could be even higher than that of theoriginal metal [8].

Upon the HTPP treatment in a hydrogen or heliumatmosphere, titanium specimens had polished exteri�ors with mirror lusters. Metallographic studies of thecross sections of a titanium specimen upon the treat�ment revealed the presence of a 13–20�µm�thick lightmodified layer with a columnar structure, which is notobserved in the metal base and is not etched in acidsolutions.

Effect of Plasma Treatment on Corrosion–Electrochemical Interaction Between Titanium and Chloride–Nitrate Melt

L. A. Elshina, V. Ya. Kudyakov, V. B. Malkov, and S. V. PlaksinInstitute of High�Temperature Electrochemistry, Ural Branch of Russian Academy of Sciences,

ul. S. Kovalevskoi/Akademicheskaya 22/20, Ekaterinburg, 620990 Russiae�mails: yolshina@ihte.uran.ru; yolshina06@rambler.ru

Received January 23, 2009

Abstract—The corrosion–electrochemical behavior of titanium in a chloride melt containing 1–30 wt %sodium nitrate at a temperature of 790–900 K in an argon atmosphere is studied. Depending on the sodiumnitrate content, either oxide layers of various structures can appear on the titanium surface or titanium diox�ide nanopowder can form in the bulk of the melt. Treating VT�1 titanium with hydrogen or helium high�tem�perature pulsed plasma substantially changes the morphology and protective properties of the oxide films pro�duced on titanium.

DOI: 10.1134/S2070205110050151

NEW SUBSTANCES, MATERIALS, AND COATINGS

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The technique of electrochemical experiment isdescribed in detail in [8]. The electrolyte was based onthe eclectic mixture of cesium and sodium chloridesand contained 0.1–30 wt % sodium nitrate. Uponevacuating and filling the space of a quartz electro�chemical cell with argon, the studied electrode wasimmersed in a molten chloride–nitrate mixture at aselected temperature. Measurements of the corrosionpotential of titanium in the melt with respect to a sil�ver–chloride reference electrode were begun immedi�ately and, when a constant value of the steady�statepotential was reached, the anodic polarization wasbegun out under potentiostatic conditions.

Based on the interaction of titanium with a chlo�ride–nitrate melt, the specimen surfaces were coveredwith layers of solid corrosion products, which were byno means always compact, did not have a good adhe�sion to the metallic base and sometimes exfoliated,certain conclusions about the corrosion rate of tita�nium can scarcely be drawn based on the gravimetricdata. Upon cooling down the salt melt, titanium ionsare not typically found because they easily bind toform insoluble titanium oxide in an oxygen�contain�ing melt. Therefore, the corrosion rate was calculatedform the anodic polarization curves constructed insemilogarithmic coordinates, as well as estimatedfrom the passivation current densities.

Images of the surfaces of HTPP�treated specimensbefore and after the exposure to a chloride–nitratemelt were obtained in secondary electrons with aGSM�5900 LV scanning electron microscope. Fur�thermore, the characteristic X�ray spectra of the sur�faces of titanium specimens were recorded and thecontent of each element in the surface layers wasdetermined from them. The phase composition of thesurfaces was studied with a Rigaku D/MAX 2000 PCX�ray diffractometer.

ANALYSIS OF MORPHOLOGYOF LAYERS FORMED

SEM images of surfaces of titanium foil specimensin the original state and upon treatment with hydrogenor helium plasma are shown in Fig. 1. Upon treatingwith hydrogen plasma on a coaxial plasma acceleratorat an accelerating voltage of 12.5 kV (H–Ti, Fig. 1b),the titanium surface is similar to the surface of theoriginal specimens (Fig. 1a); i.e., stripes that appearupon foil rolling are quite pronounced. The character�istic X�ray spectrum of the surface H–Ti layer involvesonly two peaks corresponding to 99.09 wt % titaniumand 0.91 wt % aluminum. Experiments carried out in[10] also revealed the possibility of reducing 70% ofthe admixture of titanium oxide (on titanium) tometallic titanium by the hydrogen plasma.

The plasma treatment of titanium in a heliumatmosphere results in a more noticeable change in thesurface of a titanium specimen (Fig. 1a). A weaklypronounced wavy relief with a horizontal front line isobserved on the titanium surface. Upon treatmentwith helium plasma, a layer containing up to 25 at %oxygen is found on the surfaces of titanium specimens,which may be related to the formation of a thin tita�nium oxide layer on the metallic surface. X�ray dif�fraction analysis of He–Ti surfaces showed the pres�ence of an oxide phase of nonstoichiometric TiO0.48composition along with the metallic titanium, which isthe main phase.

Data given in [7] also show that, upon the helium�plasma treatment of titanium, oxide�tint layers appearon the surface. The authors supposed that surface oxi�dation can take place either during helium implanta�tion or at the moment of cooling in air. The X�rayetching profiles indicate the decrease in the oxygencontent with the depth and the maximum atomic tita�nium�to�oxygen ratio of 1 : 2 (TiO2) in the surface ofspecimens.

At the anodic polarization of titanium underpotentiostatic conditions, titanium oxide layers,whose morphology strongly depends on the metal sur�face state; oxidation temperature; and nitrate concen�tration in the salt melt, are formed. Characteristicspectra of the surfaces of specimens upon exposure toa chloride–nitrate melt were recorded. In a melt con�taining up to 5 wt % sodium nitrate, loose defectivetitanium oxide films of a nonstoichiometric composi�tion and with a lowered oxygen content (about 52 and48 at % Ti and O respectively) are formed on the tita�nium surface. An increase in the content of sodiumnitrate up to 10 wt % enables one to produce transpar�ent titanium oxide films of a stoichiometric TiO2 com�position.

The surface state of titanium specimens plays a cer�tain role in the formation of an oxide film with a par�ticular morphology and structure. Upon one to fivehours of exposure of the original titanium specimen toa CsCl–NaCl–2.0 wt % NaNO3 melt at a temperature

(b)

(c)

(а) 5 µm 5 µm

5 µm

Fig. 1. SEM images of surfaces of titanium specimens:(a) original; (b) treated with hydrogen plasma; and(c) treated with helium plasma.

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of 810 K (without polarization), a multilayer fine�crystalline titanium oxide deposit covers the wholesurface. The oxide layers formed are thick and loose,through cracked, and, hence, nonprotective (Fig. 2a).Upon the 5�h polarization of H–Ti specimens in thesame melt at 810 K, only islets of the modifying pro�tective film remained and lamellar or threadlike tita�nium dioxide crystals with a thickness of 20 to 25 nmand a lamina length of 500 nm were seen (Fig. 2b). Inthis case, in the bulk of salt electrolyte, a substantialamount of powder�like titanium oxide deposit is accu�mulated. The powder consists of rectangular or hexag�onal crystallites with a side of about 200 nm and athickness of 20 nm, as well as fibers with diameters upto 20 nm and lengths around 200 nm (Fig. 2c).

Comparing the exterior and faceting of titaniumoxide crystals formed upon the interaction betweenthe metal and a chloride–nitrate melt, which appearunder the modified coating layer due to the plasmashot at the titanium specimen surface, with those ofthe titanium dioxide deposit crystals in the salt meltshown in SEM images (Figs. 2b and 2c), we havegrounds to conclude that these kinds of crystals arealmost identical (isomorphic). Then, it is natural toassume that the deposit consists of the exfoliated andcrumbled coating crystals. Their structure and mor�phology noticeably differ from the structure of tita�nium dioxide nanopowder synthesized with conven�tional sol–gel methods [12] or hydrolysis [13]. At thesame time, amorphous titanium dioxide powdersobtained in individual LiNO3, KNO3, or NaNO3nitrate melts, crystallize upon the additional annealingat 973 K and have similar sizes as in our case, which istherefore the same phase [14].

At the anodic polarization of titanium in a CsCl–NaCl–10.0 wt % NaNO3 melt, a thin layer of thread�like or lamellar titanium oxide crystals is formed onthe original titanium surface. On the surface of H–Tispecimens, a thin titanium dioxide layer composed ofclosely packed crystals of a distorted hexagonal shapeand a size up to 1 µm appears, while the anodic oxida�tion of He–Ti specimens results in the formation of avery thin layer of crystallites with a dodecahedral shape(crystallite faces can be seen in the photo) and a size ofno more than 100 nm (Fig. 3). X�ray pattern of theanodically oxidized He–Ti specimen shows the pres�ence of two phases, namely, the dioxide titanium baseand titanium. It is worth noting that the signals in thepattern are not broadened and have no halo, thoughthe sizes of titanium oxide particles are larger than100 nm.

RESULTS AND DISCUSSION

At the currentless exposure of metallic titaniumspecimens to a molten eutectic mixture of cesium andsodium chlorides containing up to 30 wt % sodiumnitrate, loose titanium oxide layers are formed on themetal surface (Fig. 4). Steady�state titanium poten�

tials in salt mixtures containing 1.0 or 2.0 wt % sodiumnitrate are reached in 2.5–3.0 h; their values in themelts are slightly more negative than those in an indi�vidual chloride melt, which indicates the activation ofthe modified surface layer due to the interactionbetween titanium and sodium nitrate and the baring ofa fresh active titanium surface. Both the time neededto stabilize the corrosion potentials of titanium in theabove melts and the more negative values of the poten�tials are determined by the poor adhesion of the oxidecoating to titanium and, hence, a large amount of tita�nium dioxide nanopowder crumbled and deposited inthe salt melt. Because of the extremely high oxygenaffinity of titanium, already a small amount of sodiumnitrate (1 or 2 wt %) is sufficient for the formation of

(b)

(c)

(а) 1 µm 5 µm

1 µm

Fig. 2. SEM images of titanium surface upon anodic oxi�dation in a CsCl–NaCl–1.0 wt % NaNO3 melt: (a) origi�nal titanium; (b) H–Ti; and (c) titanium dioxide powderseparated from aqueous solution obtained upon dissolvingcooled melt in distilled water.

(b)

(c)

(а) 0.5 µm 1 µm

0.1 µm

Fig. 3. SEM images of titanium surface upon anodic oxi�dation in a CsCl–NaCl–10.0 wt % NaNO3 melt: (a) orig�inal titanium; (b) H–Ti; and (c) He–Ti.

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titanium oxide. However, under these conditions,adhesion of the oxide to the metallic surface is weakand the oxide layer formed is crumbled in the salt elec�trolyte as a nanopowder deposit such that the rate oftitanium dissolution in the melts is high.

Inmelts containing 10 or 30 wt % sodium nitrate,the corrosion potentials of titanium are reached morequickly, i.e., in 60–80 min. In the first minutes, thepotentials are shifted in the positive direction by nearly200 mV and, in the next hour, change only slightly.

The higher the concentration of nitrate ions in themelt, the quicker the stabilization of the corrosionpotentials of titanium. This is determined by the sur�face formation of more corrosion�resistant protectiveoxide layers with the stronger adhesion to the base. Incontrast to zirconium [14], an increase in the oxida�tion temperature leads to a potential shift in the posi�tive direction at any sodium nitrate concentration,which also unambiguously indicates the formation ofoxide layers on the metallic surface. However, it shouldbe noted that in a melt containing 30.0 wt % sodiumnitrate, the corrosion potential of titanium corre�sponds to the oxide�electrode potential (0.632 V onthe original titanium; –0.608 V on H–Ti; and 0.590 Von He–Ti), i.e., to the potential of titanium com�

pletely covered with the own oxide. In this case, com�pact, continuous, nonporous protective titaniumoxide layers are formed on a titanium surface. Treatingtitanium with hydrogen or helium plasma does notresult in a noticeable change in the corrosion poten�tials; their values correspond to those typical of theoriginal titanium.

Upon the stabilization of the corrosion potentials,titanium specimens were anodically polarized underpotentiostatic conditions. Polarization curvesrecorded in salt mixtures at various concentrations ofsodium nitrate substantially differed. Figure 5 showsanodic polarization curves of titanium in a CsCl–NaCl–1.0 wt % NaNO3 melt. At low polarizationdensities, curves on all specimens, including H–Tispecimens, nearly coincide. Corrosion rates decreasewith an increase in the oxidation temperature, whichmay be caused by the formation of more compact lay�ers, which provide a better protection of the metallicbase against further dissolution. However, at the highpolarization, all curves approach the same limitingcurrent value. This fact indicates the fairly high,though constant, dissolution rate of titanium due tothe crumbling of oxide layer caused by its weak adhe�sion to the titanium base, as well as the presence ofdefects, pores, and cracks in the loose oxide layer.Therefore, the layer is not protective. This suppositionis illustrated by the morphology and structure of thecorrosion products formed on the surfaces of titaniumspecimens upon their polarization in the melts con�taining 1 or 2 wt % NaNO3 (Fig. 2).

Polarization curves of all three specimens in chlo�ride melts containing 10 or 30 wt % sodium nitrate areof the passivation kind with no segment of the activemetal dissolution. Anodic polarization curves of tita�nium, which was treated with hydrogen plasma,recorded in a chloride–nitrate melt depending on thecontent of the oxygen�containing additive at 810 K areshown in Fig. 6. The polarization curve of H–Ti spec�imen in a CsCl–NaCl–1.0 wt % NaNO3 melt is typi�cal of the anodic metal dissolution when passivationbarely takes place. At a sodium nitrate content of 10 wt %in the melt, the shape of the polarization curvechanges, and it becomes a typical passivation curvewith a long (up to 400 mV) passivation segment. Thepassivation current is close to 0.1 mA/cm2. The polar�ization curve of the same specimen recorded in aCsCl–NaCl–30.0 wt % NaNO3 melt shows that thepassivation potential range is broadened and the passi�vation currents decrease to 0.02 mA/cm2. A titaniumoxide layer becomes apparently more compact; lessporous; and, hence, more protective.

Anodic polarization curves of titanium, which wastreated with helium plasma, in chloride melts contain�ing 10 or 30 wt % sodium nitrate are typical passivationcurves with no active anodic dissolution segment and abroad passivation range with passivation currentsabout 0.01 mA/cm2. This means that the oxide layers

1 µm

Fig. 4. SEM image of loose titanium oxide layer formedon metal surface at currentless exposure to CsCl–NaCl–10.0 wt % NaNO3 melt.

–2

–4

–6

–8

2.01.51.00.50–10

1234

lni,

A c

m–

2

E, V

1

2 34

3

Fig. 5. Anodic polarization curves of titanium in CsCl–NaCl–1 wt % NaNO3 melt at temperature, K: (1) 790;(2) 840; (3) 900; and (4) H–Ti at 790 K.

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formed on He–Ti specimens have better protectiveproperties compared to those of H–Ti specimens.

As follows from the table, potential values reachedon a titanium anode upon the anodic polarization aremuch more positive compared to the corrosion poten�tials of titanium, which did not undergo polarization.This fact can indicate the structure changes in themorphology of titanium oxide during the anodicpolarization. This hypothesis is confirmed by the SEMimages of titanium oxide at the currentless exposure(loose defective structures) and upon the anodicpolarization (thin layers of closely packed microcrys�tals). Thus, we may state unequivocally that there is arelation between the morphology of the oxide film andits protective properties. For example, multilayer loosefilms are cracked during the formation of subsequentlayers and cannot prevent the further dissolution oftitanium from under the oxide film. Thin films, whichare more structured under the effect of the anodicpolarization and consist of closely packed titaniumdioxide crystals, provide the protection of the metallic

base against dissolution, and the finer the crystals ofthe oxide layer, the better the protective effect.

When up to 5 wt % sodium nitrate is added to thechloride melt, the titanium surface is activated irre�spectively of the preliminary surface treatment. Polar�ization of titanium anodes in the melts with the smallconcentration of nitrate ions leads to the formation ofa large amount of titanium dioxide nanopowder.Anodic polarization of titanium in CsCl–NaCl–10 or30 wt % NaNO3 results in the formation of passivatingoxide layers on the titanium surface. At the samesodium nitrate content, the passivation currentsdecrease in the following series: original titanium,H–Ti, He–Ti. Passivation currents are very smalland reach 0.01 mA/cm2 on a He–Ti specimen, whichalso indicates the formation of protective oxide layersat the anodic oxidation of titanium in a chloride–nitrate melt in a temperature range of 790–900 K.

CONCLUSIONS

Corrosion–electrochemical behavior of titaniumin the molten eutectic mixture of cesium and sodiumchlorides containing 1–30 wt % sodium nitrate in atemperature range of 790–900 K in an argon atmo�sphere is studied. Titanium treatment with hydrogenor helium high�temperature pulsed plasma does notnoticeably affect the steady�state potentials, but itdoes substantially change the morphology and proper�ties of oxide layers formed on the titanium surface dur�ing its interaction with a chloride–nitrate melt com�pared to those on the original titanium.

At all temperatures studied, the currentless expo�sure of titanium to a chloride–nitrate melt is accom�panied by the surface formation of a thick loose corro�sion product layer, which does not prevent further cor�rosion of titanium. The anodic oxidation in the meltswith a low sodium nitrate content (1 or 2 wt %) doesnot enable one to obtain a protective oxide filmbecause of the weak adhesion of the oxide particlesformed, which are exfoliated and accumulated in theelectrolyte as a nanopowder during the anodic polar�ization.

It is the oxidation of titanium in melts containing10 or 30 wt % sodium nitrate that solely provides thesurface formation of sufficiently compact nonporousprotective titanium dioxide films composed of closelypacked microcrystals. A relation between the mor�phology of the surface oxide film and its protectiveproperties is found.

In CsCl–NaCl–10 or 30 wt % NaNO3 melts, pas�sivation currents are 0.02–0.01 mA/cm2 on titaniumspecimens treated with hydrogen plasma and0.01 mA/cm2 on those treated with helium plasma.

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3

2

1

9008007006005004000E, mV

I, A

cm

–2

300200100

1

2

3

Fig. 6. Anodic polarization curves of titanium at 810 K inCsCl–NaCl–NaNO3 melt containing sodium nitrate,wt %: (1) 1.0; (2) 10.0; and (3) 30.0.

Corrosion–electrochemical characteristics of titanium in achloride–nitrate melt

NaNO3 content, wt % T, K Ecor, V Est, V

icor ×103, A/cm2

1.0 790 –1.367 –1.360 1.43

810 –1.318 –1.184 1.11

900 –1.292 –1.090 0.77

2.0 790 –1.381 –1.327 1.0

810 –1.224 –1.121 0.77

840 –1.126 –1.100 0.58

10.0 790 –0.854 –0.808 0.58

840 –0.800 –0.680 0.21

Note: Est is the stable potential reached on the electrode upon theanodic polarization.

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