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Thermochimica Acta 581 (2014) 87–91

Contents lists available at ScienceDirect

Thermochimica Acta

j ourna l h om epage: www.elsev ier .com/ locate / tca

itanizing on the surface of iron metal foam

u-In Leea,c, Jung-Yeul Yunb, Tae-Soo Limb, Byoung-Kee Kimc, Young-Min Kongc,ei-Pil Wangd, Dong-Won Leea,∗

Titanium Department, Korea Institute of Materials Science (KIMS), Changwon, Kyungnam 641-010, Republic of KoreaPowder Technology Research Group, Korea Institute of Materials Science (KIMS), Changwon, Kyungnam 641-010, Republic of KoreaSchool of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, Republic of KoreaDepartment of Metallurgical Engineering, Pukyong National University, Busan 608-739, Republic of Korea

r t i c l e i n f o

rticle history:eceived 31 October 2013eceived in revised form 11 February 2014ccepted 14 February 2014vailable online 24 February 2014

a b s t r a c t

Titanium coating on the surface of iron foam was performed by “titanizing process” in a high temperatureand vacuum conditions to enhance the oxidation resistance of ferrous porous metal. For titanizing, thesponge titanium and titanium hydride were prepared as source materials. They were evaporated at1223 K and 1.3 × 10−6 kPa for 10 h and the formed titanium gases were coated and alloyed on the surfaceof porous iron. It was revealed that the titanium hydride helped more effective coating behavior because

eywords:itaniumerrous porous metalron foamponge titanium

the released hydrogen before starting titanizing made the surface oxide to be reduced thus cleanedinto oxide-free state and the oxidation resistance in elevated temperature at air atmosphere was alsorelatively enhanced in titanized sample by titanium hydride.

© 2014 Elsevier B.V. All rights reserved.

hermo-gravimetric analysis

. Introduction

Recently as a result of development in chemical and envi-onmental industries, the application of high-performance porouslters is being actively implemented for the purification ofxhaust gases. The materials for producing environmental purifi-ation filters are mainly classified into metal and ceramic. In theeramic material, there is problem of brittleness of the mate-ial itself although it has the advantages of excellent heat- andorrosion-resistance. On the other hand, metallic material is excel-ent in ductility enabling production of complicated shapes oforous parts. However, heat- and corrosion-resistance are inferiorompared with ceramic material, so that much interest on post-reatment studies for its improvement is magnified [1–3].

In the present study, possibility for improvement in heat- andorrosion-resistance of porous metal was therefore investigated byeveloping a post-treatment process of coating titanium on the sur-ace of metallic porous bodies. While there exist some cases for the

ethods of coating titanium on or alloying in ferrous metallic sur-

ace, a thermal diffusion process using titanium, namely titanizingrocess, in this study has been applied [4].

∗ Corresponding author. Tel.: +82 55 280 3524.E-mail address: ldw1623@kims.re.kr (D.-W. Lee).

ttp://dx.doi.org/10.1016/j.tca.2014.02.008040-6031/© 2014 Elsevier B.V. All rights reserved.

This involves vaporizing titanium under high vacuum atmo-sphere at the range of temperature of 1023–1273 K to diffuse andpermeate titanium in the surface of porous iron metal, which hasbeen attempted for the improvement in corrosion-resistance ofthe inside surface of a large-scale ferrous system reactor mainlyin chemical plants [4]. Meanwhile, the vaporization of titaniumwith a high melting point at this temperature range is possible onlythrough the use of sponge titanium with a high purity and large sur-face area [4,5]. It was considered in this study that the sponge shapeof titanium hydride (TiH2) might be better performance for titaniz-ing than pure titanium because the releases of titanium gases inhigh temperatures were considered easier than use of pure tita-nium [6–8]. In this study, the possibility of surface modificationin porous metal was investigated by conducting titanizing treat-ment with the use of sponge titanium and sponge titanium hydride.The structure, phase, and oxidation resistance were comparativelyanalyzed in treated samples.

2. Experiment

The high-purity (99.8%) of sponge titanium made by Ukrainewas used for titanizing and its chemical compositions are shown in

Table 1. For titanizing treatment, 50 g of sponge titanium of about1–5 mm in size was prepared shown in Fig. 1 and it was placedin a rectangular molybdenum container above which plate-typeferrous porous metal with 1 mm of thickness and 85% of porosity

88 S.-I. Lee et al. / Thermochimica Acta 581 (2014) 87–91

Fig. 1. SEM microstructure of sponge titanium.

Table 1Chemical compositions of sponge titanium.

Fe Si Cl C N O Mn Mg H Purity

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Fig. 2. Photographs of ferrous porous metal coated by sponge: (a) titanium and (b)titanium hydride at 1223 K for 10 h.

wt.% 0.06 0.01 0.06 0.02 0.02 0.06 0.01 0.06 0.005 99.7%

atio, was positioned, followed by being charged in a vacuum fur-ace. While the temperature of normal titanizing process is knowno be about 1273 K [5], a lower temperature of 1223 K was set to

inimize a damage by deformation of the ferrous porous plate,nd the pressure and time for titanizing treatment were selected to.3 × 10−6 kPa and 10 h, respectively. In addition, for the titanizingrocess using titanium hydride, sponge-type titanium hydride wasroduced separately by means of hydrogenation process of spongeitanium at 923 K for 2 h [7]. Particle size and process method foritanizing treatment with titanium hydride were same with thosen above routes when using pure titanium.

Titanium content of the surface layer and phases were exam-ned by EDS (Energy Dispersive X-ray Spectroscopy, Zeiss SUPRA5 VP-25-78) and XRD (X-ray diffraction, RIKAKU-R2000), respec-ively. And the microstructures of the specimen were also observedsing SEM (Scanning Electron Microscope, Zeiss SUPRA 55 VP-5-78). Oxidation behaviors at a high-temperature region in airtmosphere with titanized ferrous porous metals were studied forvaluation of oxidation resistance using TGA (Thermo-gravimetricnalysis, SETARAM SYSTEM Evolution).

. Results and discussion

As the vapor pressure of pure titanium is significantly low.5 × 10−9 kPa at 1223 K, the evaporation of titanium solid seemso be impossible at the normal condition of vacuum heat treat-

ent. However, it has been revealed that the titanium sponge withigh purity and in particular large surface area could be evapo-ated kinetically even at 1.3 × 10−6 kPa and thus made titanizingrocess possible [4,5]. The porous iron plate, which vapor pressure

s 5.0 × 10−8 kPa, could be also vaporized by same mechanism buthe effect of iron release was not considered in this study becauseaporized amount was negligibly small.

Fig. 2 shows the appearance of two specimens after the titaniz-ng treatment under the same conditions, and the surface coated

ith titanium hydride appears to be slightly darker although theifference is not great. The microstructures before and after titaniz-

ng are shown in Fig. 3. While the porous body before coatinghows clean surface condition as indicated in (a) of Fig. 3, the

Fig. 3. SEM microstructures of ferrous porous metal: (a) non-coated and (b and c)coated by sponge titanium.

S.-I. Lee et al. / Thermochimica Acta 581 (2014) 87–91 89

btain

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by it.Fig. 7 illustrates the structural concepts and comparisons on the

titanizing behaviors by sponge titanium and titanium hydride. In

Fig. 4. Cross-sectional SEM microstructures and line profiles analyses o

icrostructures changed by alloying effects in the case of theurface with the titanizing treatment are shown in (b) of Fig. 3.articularly, typical forms of dual phase structure were observedn titanized surface where particular particles are formed andispersed in a single-phase matrix. Meanwhile, such dual phasetructure appeared to be similar irrespective of the type of sourceaterials of Ti and TiH2. X-ray diffraction patterns analyzed for

wo specimens were also similar, that is, �-Fe main peaks wereetected from inner substrate and small amounts of minor TiFend �-Ti from coated layer. The precise phase evaluation of coatedayer was required by tool such as low-angle X-ray analysis and its being processed.

SEM microstructures in cross-section and analysis of line pro-les were observed to check the coated thickness (Fig. 4). We knewere that the coated layer treated in TiH2 was 1.2 �m, relativelyhicker than 0.8 �m by pure titanium. Also, the results of EDS anal-sis for the coated surface layer shown in Fig. 5 indicated that areater amount of titanium reacted with the surface of ferrousorous body when titanium hydride was applied as the sourceaterial.It can be guessed by above results that vaporization of titanium

as becomes relatively easier in titanizing by titanium hydride.o investigate such phenomena, firstly, thermodynamic behavioror the dehydrogenation of titanium hydride (TiH2 → Ti + H2) haseen examined in Fig. 6. Considering the free energy change inig. 6, it is noted that hydrogen is decomposed under 101.3 kPabove 1048 K. However since the titanizing treatment conductedn the present study proceeds in a vacuum condition, it should beonsidered that pressure of the reactor was maintained approxi-ately at 1.3 × 10−3 kPa. Namely, the free energy changes under

uch pressure should be studied and here we knew that theehydrogenation under vacuum could be initiated at near 630 K,epresenting a markedly low temperature as shown in Fig. 6. Thats to say, when titanium hydride is employed as the source material,

ed in samples coated by sponge: (a) titanium and (b) titanium hydride.

dehydrogenation can be formed firstly at a relatively low temper-ature upon heating for titanizing, leading to the re-formation intopure sponge titanium and the final titanizing at 1223 K is followed

Fig. 5. EDS profiles analyzed in samples coated by sponge: (a) titanium and (b)titanium hydride.

90 S.-I. Lee et al. / Thermochimica

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other hand, in the case of coated specimen, a tendency of extremely

ig. 6. Free energy changes for the reaction, TiH2 = Ti + H2 at various temperaturesnd pressures.

eneral, in the case of pure sponge titanium, the oxide layer on theurface is not reduced even under a high vacuum, interfering withffective vacuum vaporization of titanium. Meanwhile, hydrogentoms present in titanium hydride will experience dehydrogena-ion process (Eq. (1)) under vacuum condition in the region of about

73–1023 K. Namely, combined hydrogen atoms existing in therystal lattice in TiH2 will be present in an atomic state rather than

hydrogen molecule until meeting with hydride surface oxides

Fig. 7. Schematic concepts on the evaporation behaviors of titanium from sp

Acta 581 (2014) 87–91

before releasing into H2 gas. The reaction formula along with thefree energy change according to the above mentioned behavior wasthe investigated at 800 K and 1.3 × 10−3 kPa (Eq. (2)).

TiH2 = Ti + H2 �G◦800 K, 1.3×10−3 kPa

= −45.6 kJ/mol (1)

4H (atoms) + TiO2 = Ti + 2H2O (g)�G

◦800K, 101.3 kPa = −315.7 kJ/mol (2)

It should be noticeable that in Eq. (2), the surface oxide layerof TiO2 can be reduced by hydrogen atoms with a sufficiently highdriving force of −316 kJ/mol for the free energy change. Namely,upon raising temperatures in vacuum for titanizing, the oxidesexisting in particle surface can be effectively reduced by hydrogenatoms passing through the surface to finally produce clean sur-face condition with oxides-free state, by which the titanium can bemore effectively vaporized in the subsequent process of titanizing[9]. Thus, an increase of titanium component in surface layers bythe use of titanium hydride as shown in Fig. 5 might be attributed bythe purification with surface reduction and the resultant activationeffect of high-temperature vaporization.

Fig. 8 shows the results for the profiles of weight changesobtained from two coated specimens by heating in air atmosphereup to 1073 K at a rate of raising temperature of 10 K min−1 using thethermo-gravimetric analysis. In the case of a porous metal with-out titanizing, it began to be oxidized from about 873 K, showinga drastic weight increase up to about 42% around 1073 K. On the

small weight increase was shown although temperature was raisedup to 1073 K. An increase rate within about 1% was shown par-ticularly in the case of application of titanium hydride, indicating

onge: (a) titanium and (b) titanium hydride during titanizing process.

S.-I. Lee et al. / Thermochimica

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ig. 8. Oxidation behaviors at elevated temperatures of ferrous porous metal withon-coated and coated by sponge titanium and titanium hydride.

relatively excellent oxidation resistance. It can be explained ashe improvement of corrosion resistance due to an increase of theoated amounts of titanium. In the present study, the possibilityor improvement in of corrosion resistance has been affirmed bylloying the surface of porous metal with titanium through con-ucting titanizing process as a vacuum vaporization process usingponge titanium and sponge titanium hydride. Since the titaniz-ng treatment technique is also applicable to general steel platesther than porous metal, panels for construction requiring cor-osion resistance can be applicable as well and further studies isherefore in progress.

. Conclusions

The surface of porous metal was alloyed by titanium thermaliffusion in the titanizing process under vacuum condition below

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Acta 581 (2014) 87–91 91

1.3 × 10−6 kPa for 10 h at a temperature of 1223 K using titaniumand titanium hydride. While the titanium contents in the surfacealloyed layer were shown to be 56 at.% and 73 at.%, respectivelywhen titanium and titanium hydride were employed as sourcematerials, the observation of cross-sectional SEM microstructuresand line profiles let us know that the coated thickness treatedby titanium hydride was also thicker (1.2 �m) than 0.8 �m bypure titanium. The reason why alloying is realized with a rela-tively greater amount of titanium in use of titanium hydride as thesource material, was considered to be attributable to the surfacepurification effects due to self-reduction that occurred upon thedehydrogenation of hydrogen atoms during a rise of temperatureunder a vacuum. When the porous iron metal without coating treat-ment was heated to 1100 K in the atmosphere, oxidation occurredfrom about 873 K exhibiting a drastic increase rate of about 42% at1073 K. On the other hand, in the case of two coated specimens, theextremely small increasing tendencies were observed even at thetemperature raised up to 1100 K, indicating a relatively more excel-lent oxidation resistance in the specimen prepared by titaniumhydride.

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