Corrosion Science 75 (2013) 309–317

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Corrosion Science

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Comparison between the oxidation of iron in oxygen and in steamat 650–750 �C

0010-938X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.corsci.2013.06.014

⇑ Corresponding author. Tel.: +86 24 23904856; fax: +86 24 23893624.E-mail addresses: jtyuan@imr.ac.cn (J. Yuan), wen@imr.ac.cn (W. Wang),

slzhu@imr.ac.cn (S. Zhu), fhwang@imr.ac.cn (F. Wang).

Juntao Yuan, Wen Wang ⇑, Shenglong Zhu, Fuhui WangState Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China

a r t i c l e i n f o

Article history:Received 5 February 2013Accepted 10 June 2013Available online 18 June 2013

Keywords:A. IronB. SEMB. XRDC. High temperature corrosionC. Oxidation

a b s t r a c t

Oxidation of iron was investigated in oxygen and in steam at 650–750 �C by TGA, OM, SEM, and XRD. Inoxygen, parabolic kinetics and multilayer oxide scales composed of typical three iron oxides wereobserved. Noticeably, the scale adhesion was very poor. In steam, linear-parabolic shape kinetics andmultilayer oxide scales with fewer pores were found. As indicated by Pt marking, inward-growing pro-cess in steam was considered to result in the improvement of scale contact. Based on the experimentalresults, scaling mechanism of iron in steam was discussed.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Steam oxidation has been becoming a more concerning prob-lem for alloys with potential for use as parts in steam boilers, espe-cially as steam parameters including temperature and pressureincrease year by year to improve the thermal efficiency. Nowadays,iron-based alloys are still favourable materials in this applicationdue to their relatively lower cost, better weldability, and higherheat conductivity compared to nickel-based alloys. To well under-stand the oxidation mechanism and further provide effective pro-tections, extensive studies have been conducted to investigatethe oxidation of iron-based alloys in wet gases in the past decade.This has been reviewed in [1,2] recently, however, some importantpoints in our perspective will be briefly illustrated here. First, itwas generally observed that the presence of water vapour signifi-cantly influenced the oxidation behaviour, e.g. the formation ofiron oxide nodules and the acceleration of oxidation rate. Second,the loss of Cr resulting from the evaporation in terms of CrO2(OH)2

was preferably used to interpret the breakaway oxidation of stain-less steels in wet oxygen [3,4]. But this mechanism could not ac-count for the breakaway oxidation observed in pure steam [5]because the evaporation of CrO2(OH)2 can be ignored. Third, ithas been argued that injection of protons into the chromia contain-ing scale would alter the cation transport properties [6]. Fourth, itis considered that carrier gas used in most works would influencethe oxidation process. In sum, steam oxidation mechanism is still

open to debate, and much effort is expected to put forward onthe fundamental studies including oxidation of pure metals andoxide properties in pure steam with/without pressure.

As the base element in iron-based alloys, oxidation of iron hasbeen widely studied in air or oxygen as reviewed by Chen and Yuen[7], while only a few papers investigated the effect of water vapour[8–13]. Some of these results have been reviewed in [14]. Theaccelerated oxidation by the addition of water vapour was ob-served at a wide temperature range (400–600 �C and 850–950 �C). However, the addition of water vapour to oxygen appearedto have no influence on the oxidation rate of iron at 750 �C [12]. Sofar, no published papers have dealt with the steam oxidation ofiron at intermediate temperatures 650–750 �C systematicallyalthough this temperature range is of interest for thermal powerplant applications in future. With these considerations in mind,we introduced flowing pure steam at atmospheric pressure intothermo-gravimetric analysis (TGA) system. Based on this appara-tus, isothermal oxidation of iron in pure steam at temperatures be-tween 650 and 750 �C was investigated by comparing with theoxidation of iron in pure oxygen. Most attention was paid to thesteam oxidation of iron so as to obtain basic understandings onthe scaling mechanism in steam from the oxidation kinetics andscale microstructure.

2. Experimental

2.1. Sample preparation

An iron bar with purity of 99.9% was studied in the presentwork, for which the impurity contents are given in Table 1.

Fig. 1. (a) Oxidation kinetics in oxygen and in steam; (b) parabolic plot of oxidationkinetics in oxygen and in steam and (c) Arrhenius plot of oxidation rate constants.

310 J. Yuan et al. / Corrosion Science 75 (2013) 309–317

Specimens with dimensions of 15 � 10 � 2 mm3 were mechani-cally ground to 1000 mesh on SiC papers, and then ultrasonicallycleaned in distilled water and ethanol for 15 min subsequently,and stored in absolute ethyl alcohol.

2.2. Oxidation exposures

Flowing steam was generated by pumping ultra-purified waterinto the preheating device at a fixed rate of 1 ml/min. At room tem-perature, the resistivity of the inlet water is about 18.25 MX � cmand the dissolved oxygen is around 8 wppm. Steam flow rate isdependent on temperature and can be roughly calculated fromthe ideal gas equation. The calculated values are 4.26, 4.49, and4.73 L/min for 650, 700, and 750 �C respectively, correspondingto a linear velocity of 7.38, 7.78 and 8.19 cm/s for each temperaturesubsequently.

Isothermal steam oxidation experiments were carried out in aVersaTherm HM thermobalance connected with the steam genera-tor. During all experiments, the heating rate and cooling rate wereset as 80 �C/min and �5 �C/min respectively. When the furnacetemperature exceeded 100 �C, the pre-heated steam was intro-duced immediately. After oxidation, the steam flow was switchoff and the samples were furnace cooled down to room tempera-ture. For comparison, oxidation exposures of iron in oxygen wereconducted in another TGA system where pure oxygen stream witha flow rate of 50 ml/min was introduced and the same heating/cooling rates were used.

2.3. Characteristic methods

After oxidation, the oxidized specimens were characterizedcarefully in terms of scale microstructure and phase composition.Scale microstructures were investigated by Optical Microscopy(OM) and Scanning Electron Microscopy (SEM). Oxide phases wereidentified by X-ray Diffraction (XRD) layer by layer since the scalesformed on iron are very thick. While the outmost surface oxidesformed in steam were analyzed by Grazing Incidence XRD (GIXRD)with an incidence angle of 0.5�.

3. Results and discussion

3.1. Oxidation kinetics

The measured oxidation kinetics results are shown in Fig. 1a. Itis evident that there is a critical time at each temperature. Duringthe stage before the critical time, mass gains in oxygen are greaterthan those in steam. Contrarily, in the stage after the critical time,mass gains in oxygen are less than those in steam. In addition, thiscritical time increases with temperature. This indication is differ-ent from the results at low temperature range (400–600 �C) [8]where weight changes in dry oxygen were always smaller thanthose in wet oxygen. For the oxidation in oxygen, the kinetics fol-lows the parabolic rate law:

DWA

� �2

¼ kp � t ð1Þ

where DW/A is the mass-gain per unit area (g/cm2), t the time (s),and kp the parabolic rate constant (g2/cm4/s). Unlikely, the reactionkinetics in steam at all studied temperatures are seen to be initiallylinear and subsequently parabolic, as observed in the oxidation of

Table 1Impurities in the iron used in this research (wt.%).

C P S Mn Si Cu

0.011 0.005 0.0094 0.035 0.028 0.023

iron [13] and low carbon steels [15,16] in wet environments. Thelinear rate constants were modelled by the rate equation:

DWA¼ kl � t ð2Þ

where kl is the linear rate constant (g/cm2/s). The subsequent para-bolic kinetics were also modelled by Eq. (1) and the correlations

Ni Al Cr Mo Nb

0.018 0.015 <0.01 <0.005 <0.005

J. Yuan et al. / Corrosion Science 75 (2013) 309–317 311

between (DW/A)2 and t are present in Fig. 1b. All the rate constantsare summarized in Table 2, where R2 approaching 1 indicates goodfitting quality. It can be seen that the parabolic rate constants insteam are somewhat greater than those in oxygen. Then, the appar-ent activation energy for oxidation kinetics can be obtained fromthe Arrhenius equation

ki ¼ k0 exp � EA

RT

� �ð3Þ

where ki represents kp and kl, k0 is a constant with the same unit toki, EA is the activation energy of the oxidation reaction (J/mol), R isthe gas constant (8.314 J/mol/K) and T is the absolute temperature(K). The Arrhenius plots in all cases are shown in Fig. 1c.

For the oxidation of iron in oxygen or air, despite the rapid ini-tial reactions, the longer-term oxidation rate under isothermalconditions is quite steady and usually follows the parabolic ratelaw. It suggests that the scaling process is controlled by the soliddiffusion. At the temperature range of 700–1250 �C, the observedoxidation activation energy is about 150 kJ/mol [7]. In the presentwork, however, the obtained oxidation activation energy in oxygenis a somewhat greater value 205 kJ/mol. This difference is reason-able if we consider the fact that a more complex oxidation happensat the temperature range 570–700 �C as mentioned by Chen andYuen [7].

No information has been published on the oxidation of iron inpure steam, however, some works has dealt with the oxidation ofiron in wet oxygen [8,10,12,17] and wet carbon dioxide [12]. Inmost cases, a steady-state kinetics obeying parabolic regime wasobserved after an initial rapid oxidation, and the rate was acceler-ated due to the presence of water vapour. Concerning the linearkinetics during the first 2–5 h, it is commonly considered thatthe oxidation rate is controlled by the scale-gas interfacial process[18]. In the cases involving water vapour or carbon dioxide[15,16,19,20], the incorporation of adsorbed water vapour (or car-bon dioxide) into the surface oxide was usually thought to be therate-limiting step. This mechanism can be represented by the steps

H2OðgÞ $ H2OðadsÞ ð4Þ

H2OðadsÞ $ OO þ V00Fe þH2 þ 2h ð5Þ

where h, OO and VFe0 denoting an electron hole, an oxygen ion on anormal oxygen site, and an iron vacancy with two negative effectivecharges, respectively. By applying absolute rate theory to the for-ward and reverse processes of reaction (5), Turkdogan et al. [13] ob-tained the expression

kl ¼ k0 1� a0Oa0O

� �pH2O ð6Þ

where a0O and a00O are the oxygen activities at the scale/gas and scale/metal interfaces, respectively. Experimental measurement of iron inwater-hydrogen gas mixtures at 850–1150 �C indicated a apparentactivation energy about 80 kJ/mol [13]. In the present work, rela-tively greater apparent activation energy for linear kinetics in steam106 kJ/mol was obtained. This discrepancy may be reasonable if weconsider the different temperature range and oxidizing environ-ment. However, this value is similar to the observation in Ref.

Table 2Summary of rate constants.

T/oC Linear stage (steam) Parabolic sta

kl (g/cm2/s) R2 kp (g2/cm4/s)

650 8.80 � 10�8 0.99 7.90 � 10�10

700 2.06 � 10�7 0.99 4.19 � 10�9

750 3.38 � 10�7 0.99 1.47 � 10�8

[21], where the apparent activation energy for the external surfacereactions

H2OðadsÞ þ V00O $ OH0O þH0i ð7Þ

2H0i þ 2e0 $ H2 ð8Þ

was 110 kJ/mol. In this respect, the hydroxyl ion formed at theexternal interface would possibly be the major diffusing defectdue to its smaller size and lower charge than oxygen ion [21]. Thiswill be discussed later.

After the early linear stage, the oxidation kinetics graduallychanged to be parabolic. The parabolic scaling kinetics providesindications that local equilibrium was achieved at the scale/gasinterface and solid diffusion process tended to be the rate-control-ling step. In air or oxygen, for iron and steel oxidation at tempera-tures above 700 �C, the scaling process is controlled by thediffusion of iron through wustite [16]. The reported oxidation acti-vation energies of iron in wet gases [12,17] always deviated from150 kJ/mol, and no clear rule can be concluded. In the presentwork, the observed apparent activation energy for the steady-statekinetics in steam, 229.9 kJ/mol, is very similar to the case of 0–2%Cralloys oxidation in steam at temperatures above 500 �C [1]. Chenand Yuen [16] applied the Wagner’s theory to calculate the para-bolic rate constant and found that the calculated values were150–175 times greater than the measured values. They pointedout that the difference might be attributed to the ignorance ofmagnetite formation and hydrogen release. It could be concludedthat conventional theories cannot provide a satisfactory explana-tion. In our point of view, there are two possibilities accountingfor the parabolic kinetics and its different activation energy: (1)as mentioned above, hydroxide ion would be one important diffus-ing defect and probably as the co-rate-controlling step; (2) the self-diffusion coefficient of iron in magnetite and/or wustite could beaffected by the presence of hydrogen. In this respect, scaling mech-anism in steam could be significantly different from that in oxygen.

3.2. Scale structures formed in oxygen

After oxidation in oxygen, all samples showed dark grayappearances. The scale adherence was poor so that blistering andeven complete spalling at scale/metal interface was observed.Longer period oxidation led to much severer oxide scale spalling.After 50 h oxidation at 700 �C and 750 �C, the whole scales com-pletely separated from the metal substrate. Since no spalling wasindicated from oxidation kinetics even for 50 h exposure, it canbe speculated that spalling happened during the cooling process.

Cross-section morphologies of oxide scales formed in oxygenafter 10 h are shown in Fig. 2. It can be seen that multilayer scaleswith considerable pores formed at studied temperatures as foundelsewhere [22]. With oxidation temperature increasing, the thick-ness of outer layer consisting of hematite and magnetite increasesfrom 18.5 lm at 650 �C to 59.5 lm at 700 �C and then decreasesto 14 lm at 750 �C. After 50 h exposure, no obvious change was ob-served on the scale structure except that the outer layer becamethicker in some extent and severe spalling took place. The poor scaleadhesion may be attributed to the low plasticity of oxides and the

ge (steam) Parabolic stage (oxygen)

R2 kp (g2/cm4/s) R2

0.99 6.64 � 10�10 0.990.99 2.06 � 10�9 0.990.99 9.10 � 10�9 0.98

Fig. 2. Cross-section SEM images of oxide scales formed on iron after 10 h oxidation in oxygen at (a) 650 �C, (b) 700 �C, and (c) 750 �C, where the outmost white layer is Niplating.

312 J. Yuan et al. / Corrosion Science 75 (2013) 309–317

presence of pores at interfaces [23]. If the oxide is plastic sufficientlyto deform, pores would be progressively collapsed. Otherwise, scaleblistering and/or separation would develop and then hinder the so-lid diffusion. In consequence, the interfacial adhesion and subse-quent scaling rate would be reduced in some extent.

From Fe–O phase diagram, a scale consisting layers of FeO,Fe3O4 and Fe2O3 would form at temperatures above 570 �C, andsuch multilayer scale were expected in the present work. In orderto identify the phase compositions in each layer, XRD patterns asshown in Fig. 3a and b were gradually obtained by polishing outeroxides. At 750 �C, the oxides were prone to spall during the slightpolishing process due to the poor adhesion, so that only the outeroxide layer was identified by XRD as shown in Fig. 3c. It is evidentthat scales formed at 650–700 �C in oxygen consist of three ironoxides. In the case of 750 �C, the outer layer contains hematiteand magnetite. In this respect, the outer darker layer would becomposed of an outmost thin hematite layer and an inner magne-tite layer because these two oxides have the similar contrast. Andthe inner lighter layer would be wustite with visible magnetiteprecipitates. It is agreed that these magnetite precipitates formduring cooling process as the composition range of wustite con-tracts with decreasing temperature until 570 �C where the phasebecomes unstable [24–27]. The dark layer at scale/metal interfaceis also considered as magnetite which transformed from wustiteduring cooling process, subsequently a thin gap formed at thisinterface due to the volume change.

3.3. Scale structures formed in steam

After oxidation in steam, the samples indicated a light redappearance at low temperatures (e.g. 650 and 700 �C) and a darkgrey appearance at 750 �C. Unlike in the case of oxygen, thescale-to-metal adhesion is much better and no obvious spallingwas observed even after 50 h oxidation.

The cross-section SEM images of scales formed on iron after10 h and 50 h oxidation in steam are presented in Figs. 4 and 5.Similar to findings in oxygen, multilayer scales containing a fewpores were formed in steam at all studied temperatures. However,there are characteristic features which evidently differ from themorphologies in oxygen, including the relatively thinner outerlayer, the innermost thick layer containing fine voids and numer-ous several micron-wide ‘‘fissures’’ perpendicular to the metal sur-face (see Fig. 4d), and excellent contact at the scale/metal interface.A general temperature dependence on the thickness percentage ofouter layer can be concluded that the percentage decreases withincreasing temperature. After 50 h oxidation, the percentage de-creases from �11% at 650 �C to �3% at 750 �C. While, no significanttime dependence was found on this value.

At the studied temperature range, the effective oxygen partialpressure in steam calculated from the equilibrium dissociation ofwater [2] is 10�6–10�8 atm, which is sufficient to sustain the for-mation of hematite, magnetite and wustite. However, the real oxy-gen partial pressure may change with oxidation time due to theinfluence of resultant hydrogen. Therefore, the stability of oxideswould be dependent on the oxidation kinetics and steam flow rate.In the present work, GIXRD were applied to identify the outermostsurface oxides, and the results are shown in Fig. 6a. It can be seenthat some amount of hematite formed on the external surface. TheXRD patterns from gas side to metal side are shown in Fig. 6b–d.The results show that the scale is composed of typical three ironoxide layers, as found in oxygen. This indicates that steam mayhave no significant effect on the phase structure of oxides.

3.4. Effect of exposure environment on scaling mechanism of iron

Oxidation of iron in oxygen at the studied temperatures fol-lowed parabolic rate law, and the formed oxide scales were com-posed of an outer Fe2O3 layer, a middle Fe3O4 layer and an inner

Fig. 3. XRD patterns for the oxide scales formed on iron after 10 h oxidation inoxygen at (a) 650 �C, (b) 700 �C, and (c) 750 �C.

J. Yuan et al. / Corrosion Science 75 (2013) 309–317 313

FeO layer with some Fe3O4 precipitates. Also, significant pores andeven a continuous gap were observed at scale/metal interface.This is related to the mechanism depicted in Ref. [28]. In thismechanism, the scaling process is mainly supported by the out-ward diffusion of iron ions. Since cation outward diffusion occursusually via a vacancies gradient, vacancies are injected into themetal substrate causing a vacancy supersaturation and eventuallya condensation as pores [29]. As mentioned in Ref. [28], the rapidrate of reaction of iron above 570 �C causes thick scales to developquickly and, in spite of the relatively high plasticity of the FeOlayer, scale/metal adhesion is lost and porous inner layer of FeOis formed, next to the substrate. In addition, the existence of poresand continuous gap would mitigate solid-state diffusion to someextent.

In steam, oxidation of iron followed the linear-parabolic rate re-gime, suggesting transition of rate-determining process as oxida-tion duration elongated. At the initial stage, if the diffusionthrough scale is assumed to be fast enough, the rate-limiting stepwould be the scale/gas interfacial reaction. Since the apparent acti-vation energy derived from linear rate constants is similar to thefinding in Ref. [21], reaction (7), therefore, could be considered asthe rate-determining reaction. In this perspective, the hydroxylions formed at the external surface would be the major diffusionspecies. However, Galerie et al. [30] has pointed out that transportof hydroxyl ions is exactly the same as oxide ions, for instance, ahydroxyl ion jumps from a substitutional site to an adjacent va-cancy, with an activation energy probably lower due to the easiercrossing of the potential barrier (with lower charge and size thanoxide ions). In this respect, the assumption that the diffusion is fastenough during the linear stage would be valid only for the oxidecontaining considerable oxygen vacancies and/or grain boundariesof p-type metal vacancy oxides. The scale structures after 1 h oxi-dation in steam are shown in Fig. 7 and their XRD patterns are pre-sented in Fig. 8. It can be seen that three iron oxides exist withinthin scales. The outer layer containing hematite and magnetite isapproximately 1 lm at all studied temperatures while the thick-ness of wustite layer increases with temperature. After etching,columnar grain structures were observed in the wustite layer atall studied temperatures. As oxidation proceeded, most columnargrains tended to disappear after 5 h. The evolution of scale struc-ture formed in steam at 700 �C is illustrated in Fig. 9, where OMimages of etched samples suffering from different period of oxida-tion exposure are shown. During the initial period (before 5 h), theexistence of single layer of columnar grains may enable the rela-tively rapid transport of hydroxyl ions through grain boundaries.The diffusion would be affected by the length of diffusion path;the longer the length, the slower the diffusion. Once the diffusionis slower sufficiently, the diffusion process tends to be the rate-controlling step. With oxidation time elongated, wustite thickenedrapidly and the columnar grains gradually disappeared, so that thetransport of hydroxyl ions would be lowered to a great extent, andthen parabolic kinetics was observed. Also, there is a significanttemperature dependence on the thickening of wustite layer, asindicated in Fig. 7. As temperature increased, the thickening ofwustite layer was enhanced and consequently the linear period be-came shorter.

The original metal surface was marked by locating Pt particlesplasma on the polished sample. After 10 h oxidation in steam at750 �C, the cross-section morphologies are shown in Fig. 10. Itclearly shows that the innermost layer was grown by inward trans-port of oxidants. As mentioned above, hydroxyl ions could movevia oxygen vacancies and/or grain boundaries of p-type metal va-cancy oxides [30]. For iron oxides, magnetite and wustite have p-type semiconducting properties and their main point defects aremetal vacancies. In usual, the diffusion of iron ions through thesetwo oxides is considered to be much faster than oxygen ions. How-ever, the inward transport of hydroxyl ions through p-type chro-mia scale has been observed by Bamba et al. [6]. Although theauthors did not interpret the diffusion mechanism, Galerie et al.[30] proposed that the same paths (grain boundaries) as oxide ionsare probably used. It indicates that grain boundaries may be diffus-ing paths for hydroxyl ion through magnetite and wustite in thepresent work. On the other hand, it is considered that the inwardtransport of water molecules through micro-channels withinscales is possible in some cases [31,32]. As pointed out in Ref.[33], seemingly dense scales may have pores and microscracks(which are often too fine to be seen by SEM) so that transportthrough the scales may take place by both solid-state diffusionand via gaseous species in pores and cracks. However, if moleculesare to be transported through scales, channels larger than 0.3 nm

Fig. 4. Cross-section SEM images of oxide scales formed on iron after 10 h oxidation in steam at (a) 650 �C, (b) 700 �C, (c and d) 750 �C, where the outmost white layer is Niplating.

Fig. 5. Cross-section SEM images of oxide scales formed on iron after 50 h oxidation in steam at (a) 650 �C, (b) 700 �C, and (c) 750 �C.

314 J. Yuan et al. / Corrosion Science 75 (2013) 309–317

Fig. 6. (a) GIXRD patterns for the outmost oxides formed on iron after 10 h oxidation in steam; XRD patterns for oxide scales formed on iron after 10 h oxidation in steam at(b) 650 �C, (c) 700 �C, and (d) 750 �C.

Fig. 7. Cross-section images of oxides formed on iron after 1 h oxidation in at (a) 650 �C, (b) 700 �C, and (c) 750 �C.

J. Yuan et al. / Corrosion Science 75 (2013) 309–317 315

Fig. 8. XRD patterns for the oxides formed on iron after 1 h oxidation in steam.

316 J. Yuan et al. / Corrosion Science 75 (2013) 309–317

or so would be needed. Fukomoto et al. [31] examined the frac-tured scales by SEM and found that the external wustite layer con-

Fig. 9. OM etched morphologies of scales formed on iron after o

Fig. 10. Cross section images indica

Fig. 11. Schematic illustration of oxi

tained numerous several micron-wide fissures perpendicular tothe alloy surface. These fissures may provide paths for the trans-port of water molecules toward the scale/metal interface. In ourwork, fractured samples were not investigated. Anyhow, it can becertain from the Pt marking that inward transport of oxidants oc-curred during the oxidation of iron in steam. In this respect, thesubsequent parabolic kinetics in steam should be supported by acomplicated diffusion mechanism involving of transport of cations,anions and even water molecules.

Regarding the improvement of scale adhesion by steam, Tucket al. [10] proposed that the oxide formed in wet oxygen has great-er plasticity (ability to creep) possibly due to the incorporation ofhydrogen in the lattice. They pointed out that the formation of hy-droxyl ions would result in an increase in cation vacancies, andthen increase the volume of the distorted zone neighbouring dislo-cations and thus enhance their mobility. Bamba et al. [6] found theincreased adhesion of chromia by steam, and thought that inwardprogression of the scale/metal interface resulted from the inwardtransport of hydroxyl ions and the mechanical strengthening ofthe substrate induced by the competitive liberation of hydrogen

xidation in steam at 700 �C for (a) 1 h, (b) 5 h, and (c) 10 h.

ting the original metal surface.

dation process of iron in steam.

J. Yuan et al. / Corrosion Science 75 (2013) 309–317 317

would account for the improvement of scale adhesion. In our pointof view, the inward-growing oxides would fill the pores at scale/metal interface and then increase the contact area to a great extentas found in cross-section images. In addition, the fine pores in theinward-grown layer would improve its plasticity to some extent, sothat stresses could be relieved by plastic deformation without anyfailure. The incorporation of hydrogen may contribute some to theincrease of plasticity as proposed by Tuck et al. [10], because inves-tigations [34–36] have showed that hydrogen was accumulated inthe vicinity of the scale/metal interface. In all, the inward-growingprocess leads to the significant improvement of scale adhesion.

Based on the above discussion, the oxidation process in steamcan be illustrated schematically in Fig. 11. It is far different fromthe oxidation process of iron in oxygen or air [28] where iron cat-ions are the major diffusing species. In steam, oxygen generated bywater dissociation would be the source for iron oxidation. Simulta-neously, hydrogen is released into bulk gas so that the effectiveoxygen partial pressure would be much lower than in oxygen orair. Therefore, the growth of hematite would be significantly sup-pressed. In steam, the scaling process is supported by cations andanions. In particular, the inward transport species possibly as hy-droxyl ions significantly alter the scaling mechanism and resultin the improvement of scale adhesion.

4. Conclusions

Oxidation of iron in oxygen and in steam was studied at 650–750 �C in terms of kinetics and scale microstructure. Based onabove results, several conclusions can be made:

(1) Oxidation of iron in oxygen followed parabolic rate kinetics,and multilayer scales consisting of three iron oxides wereobserved. The scaling mechanism was believed to be out-ward transport of cations so that considerable pores andeven a continuous gap formed at the scale/metal interface,and subsequently reduced the scale adhesion dramatically.

(2) Oxidation of iron in steam followed linear-parabolic ratekinetics, and the steady-stage rate constant was somewhatgreater than that in oxygen at a given temperature. The mul-tilayer scales containing fewer pores were also composed ofthree iron oxides, while the outer layer was much thinnerand much better scale adhesion were observed. It is consid-ered that the rate-determining step changed from the initialscale/gas interfacial reaction to the steady-state diffusion ofcations and anions. The inward-growing process would beresponsible for the improvement of scale contact.

Acknowledgements

The authors are grateful for the financial supports from the Na-tional Nature Science Foundation of China under Contracts of51071163 and the National Key Basic Research Program of China(973 Program) under Grant No. 2012CB625100.

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