Corrosion behavior of Fe–14Cr–2W and Fe–9Cr–2W ODS steels in stagnant liquid Pb with different oxygen concentration at 550 and 650°C

Journal of Nuclear Materials xxx (2013) xxx–xxx

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Corrosion behavior of Fe–14Cr–2W and Fe–9Cr–2W ODS steelsin stagnant liquid Pb with different oxygen concentration at 550 and650�C

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⇑ Corresponding author. Tel.: +380 32 292 81 74; fax: +380 32 264 94 27.E-mail address: [email protected] (O. Yeliseyeva).

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Olga Yeliseyeva a,⇑, Valentyn Tsisar a, Zhangjian Zhou b

a Physical-Mechanical Institute of National Academy of Sciences of Ukraine, 5 Naukova St., 79601 Lviv, Ukraineb School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China

a r t i c l e i n f o

Article history:Available online xxxx

a b s t r a c t

Corrosion behavior of ferritic (Fe–14Cr–2W + Y2O3) and ferritic–martensitic (Fe–9Cr–2W + Y2O3) oxidedispersion strengthened (ODS) steels in the static isothermal liquid Pb was investigated at 550 and650 �C depending on the oxygen concentration CO in the melt for duration up to 1000 h. It was deter-mined that the interaction mode of steels changes from the dissolution in the pure Pb (CO 6 10�14 wt%O)to the formation of protective oxide layers on the surface of steels in the oxygen-added Pb (CO � 10�6 -wt%O) and to formation of multiphase non-protective scales in the Pb saturated by oxygen (CO � 10�3 -wt%O). In general, the observed corrosion behavior of ODS steels coincides with that of traditionalchromium steels. However, specific structure of ODS steels causes some variations in corrosion process.In the pure Pb (CO 6 10�14 wt%O) the fine-grained structure promotes inter-granular corrosion attack andpenetration of lead into steel matrix along grain boundaries. Increase in Cr content in the steel promotescorrosion attack. In the oxygen-added Pb (CO � 10�6 wt%O) the fine-grained structure, vice versa, ensuresformation of oxide layers with higher Cr content due to fast diffusion of Cr into growing oxide along grainboundaries. The protective properties of oxide layers are improved with temperature rise (550 ? 650 �C)and chromium content in steel. In the oxygen-saturated Pb (CO � 10�3 wt%O) the ODS steels undergosevere oxidation accompanied by the formation of non-protective multiphase scale which consist of mix-ture of different oxide phases: plumboferrite, magnetite, Fe–Cr spinel and free Pb. The oxidation kineticsintensifies drastically with temperature and decelerates with increasing chromium content in the steel.Based on the experimental data the scheme of interaction of components in the "steel - liquid Pb’’ systemdepending on temperature and oxygen content is proposed.

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1. Introduction

Oxide dispersion strengthened (ODS) steels are considered ascandidate structural materials for both fission and fusion reactorconcepts, while lead based melts (Pb, Pb–Bi, Pb–Li) are the mainfunctional cooling/breeding media [1–4]. An application of ODSsteels allows the working temperature limit to be increased up toabout 700 �C. However, the corrosion rate of steels in liquid metalsincreases with temperature as well. Moreover, the specific phase-structural features of ODS steels (fine-grained structure with highlength of boundaries, presence of the dispersion oxides, residualporosity etc.) can affect substantially their corrosion response incomparison with traditional steels. It is well known also that oxy-gen impurity in the lead melts can alter the interaction mode be-tween solid metal and liquid metal from dissolution in the puremelt to severe oxidation in the melt saturated by oxygen. In the lead

melt with optimal content of oxygen (�10�5 to 10�7 wt%O) the dis-solution of steel’s components is mitigated due to formation of pro-tective oxide layer on the steel’s surface while PbO oxide does notprecipitate in the cold leg of liquid-metal loop [4–6]. An applicationof so-called passivation technology has been widely studied for theconventional austenitic and ferritic–martensitic steels at moderatetemperatures (6550 �C) [4–6]. At the same time the data regardingcompatibility of ODS steels with lead melts are still scarce [7–14].The limited results testify that chromium ODS steels oxidize in leadmelts similar to the conventional chromium steels, i.e. with forma-tion of double-oxide layer Fe3O4/Fe(Fe1�x,Crx)2O4, outer part ofwhich can be destroyed by flowing melt. It was determined alsothat with decrease in oxygen content in the melt from 10�4 to10�6 and then to 10�8 wt%, the concentration of chromium in theinner spinel layer increases up to 15, 30, and 41 wt%Cr respectively[8]. Development of a thick and pronounced inner oxidation zone(IOZ) consisting of Cr-rich oxide particles finely dispersed in theCr-depleted steel matrix, was found, to be a specific feature ofoxidation of ODS steels possessing by fine-grained structure in lead

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melts under fluctuating conditions of oxygen concentration in therange between 10�5 to 10�9 wt %O [14]. Summarizing scarce liter-ature data it can be indicated that corrosion tests are performedmainly at temperatures from 500 to 650 �C and oxygen concentra-tion ranged from 10�4 to 10�6 wt%. The conditions representing ex-treme modes of interaction such as the severe oxidation andintensive dissolution are practically missed as well as the effect ofspecific phase-structural state of ODS steels and Cr content on theprevalence of oxidation or dissolution processes is not elucidatedyet. Therefore, in this work the corrosion behavior of chromium fer-ritic and ferritic–martensitic ODS steels was investigated at 550 and650 �C in stagnant liquid Pb with different concentration of oxygenproviding dissolution, passivation and severe oxidation of steels.

2. Experimental procedure

Samples (£10 � 3 mm) of ODS ferritic–martensitic Fe–9Cr–1.5W and ferritic Fe–14Cr–1.5W steels strengthened by Y2O3 oxideparticles were supplied in as-HIPed state by University of Scienceand Technology Beijing (China). Then, the samples were polishedand cleaned in acetone followed by the vacuum heat treatmentat 1000 �C for 1 h for homogenization of structure and removal ofresidual stresses.

Corrosion tests of ODS steels were carried out in stagnant iso-thermal Pb with different concentration of oxygen impurity at550 and 650 �C for up to 1000 h. Fig. 1 shows the schematicrepresentation of interaction modes of steels facing liquid Pb withdifferent concentration of oxygen plotted using experimental datapresented in [6]. Based on the scheme, three intervals ofoxygen concentration were chosen for corrosion tests: pure Pb(CO� 10�7 wt%) providing domination of dissolution of solid met-als; oxygen-added Pb (CO � 10�6 wt%) providing passivation ofsteels and oxygen-saturated Pb (CO � 10�3 wt%) providing inten-sive oxidation of steels.

In order to obtain pure Pb, the samples were fixed in the Nb am-poules. Then, ampoules were filled by liquid Pb in glow box underthe argon atmosphere in order to mitigate contamination of meltwith oxygen impurity. Argon atmosphere in the glow box waspurified with respect to oxygen by liquid Li getter heated to�350 �C. After the solidification of Pb, the Nb ampoules wereplaced into protective stainless steels capsules, which were sealedby welding in the same glow box. Then, the capsules were exposedin furnace at 550 and 650 �C. It was assumed that during test, oxy-gen dissolved in the liquid Pb should be absorbed by the inner sur-

Fig. 1. Schematic representation of corrosion rate and interaction modes of steelsfacing liquid Pb with different concentration of oxygen depending on temperature.

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face of Nb capsule. According to thermodynamic evaluation theequilibrium concentration of oxygen in the melt should be reducedto about 10�14 wt%. At the same time the fine-dispersed yttriumoxides, strengthening Fe–Cr matrix, should be stable in the Pb meltsince free energy of formation of Y2O3 is more negative in compar-ison to liquid Pb containing �10�14 wt%O.

The alumina crucibles were used for corrosion tests in the oxy-gen-added and oxygen-saturated melts. In order to provide theoxygen-saturated conditions, the liquid Pb was in a contact withdry air during the tests. The red-colored PbO oxides covered themirror of the melt indicating that content of oxygen in Pb wasnearly saturated and according to the equation lgCO[Pb] =3.2 � 5000/T [15] averaged 1.33 � 10�3 and 6.07 � 10�3 wt%O at550 and 650 �C respectively.

In the case of the oxygen-added melt, the lead melt was in con-tact with controlled dynamic vacuum atmosphere (PO2 � 2.7 �10�1 Pa). Based on the previous experience [9,10] the oxygen con-centration in the Pb during corrosion test corresponds to about3 � 10�6 and 10�5wt %O at 550 and 650 �C.

After the tests, the surface and prepared cross-sections of thespecimens were examined using the light optical (LOM) and scan-ning electron (SEM) microscopes, in order to determine the mor-phology and dimensional parameters of scales formed. Thinningof samples depending on exposure time in the oxygen-saturatedPb was measured as a half the difference between initial andpost-test thicknesses of steel’s matrix unaffected by corrosion. Ele-mental composition of scales and corrosion zones were deter-mined by means of energy dispersive X-ray (EDX) analysis. Theweight changes were determined using an electro-balance withan accuracy of 0.01 mg.

3. Results

3.1. Dissolution of ODS steels in pure Pb (CO 6 10�14 wt%O)

After the long term contact with the pure Pb, all samples werecovered by adhered solidified lead, indicating wetting of steel’ssurface by melt and absence of oxide film between liquid and solidmetals. Therefore, the exposed samples were cleaned chemically inCH3COOH + H2O2 + C2H5OH mixture (1:1:1) at room temperaturein order to remove Pb and then to determine the weight changesof samples and morphology of surface facing liquid metal medium.Samples of both steels demonstrated weight losses, which in-creased with temperature (Fig. 2). The weight loss of Fe�14Cr steelwas always greater than that of Fe�9Cr.

After the test at 550 �C, the initially shine and smooth surface ofsamples become mat and relief indicating corrosion attack. It was

Fig. 2. Weight loss of Fe–9Cr and Fe–14Cr ODS steels after exposure to pure Pb(610�14 wt%O) at 550 and 650 �C.

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determined that concentration of chromium decreases slightly inthe near-surface layers of samples especially for Fe�14Cr steel.The depth of chromium depletion did not exceed severalmicrometers.

After the exposure at 650 �C, the highly developed relief withdeep cavities can be observed on the both surface and cross-sec-tions of samples (Fig. 3). The depth of the damaged layer averaged�15 and �40 lm for Fe–9Cr and Fe–14Cr steels, respectively(Fig. 3b and d). In spite of the marked recession of material, the ele-mental composition obtained from the surface by EDX analysesand cross-section line-scan of Fe–9Cr indicates that the composi-tion of corroded layers is near similar to the initial compositionof steel. However, the near-surface layers of Fe–14Cr steel weremarkedly depleted in Cr, concentration of which decreased from14 wt% (initial) to 8–9 wt% (as tested). The depth of chromiumdepletion (�40 lm) coincides with highly damaged structure(Fig. 3d). Thus, it is evident from the results obtained that the in-crease in chromium content favors dissolution attack of steel andinter-granular penetration of liquid Pb into ODS steel.

3.2. Passivation of ODS steels in the oxygen-added Pb (CO � 10�5 to10�6 wt%O)

At 550 �C, the double-oxide layer was formed on the surfaces ofboth steels (Fig. 4a and b). The dimension of the oxide layersformed (total thickness, thickness of outer layer, thickness of inneroxide layer with inner oxidation zone IOZ) are presented in the ta-ble. The double-oxide layer covers surface of Fe–9Cr steel quiteuniformly (Fig. 4a), while Fe–14Cr oxidizes irregularly showingsmall non-oxidized areas even after 1000 h exposure (Fig. 4c).The outer layer is composed of Fe and O (Fig. 4b and d). The innerlayer, in addition to Fe and O, contains Cr, concentration of which ishigher in comparison with the steel bulk. Based on the results of X-ray diffraction analyses presented in our previous works and liter-ature data, the outer oxide is magnetite (Fe3O4) while the inneroxide layer is spinel (Fe,Cr)3O4 [4,9,10]. The outer and inner oxide

Fig. 3. Surface and cross-section morphologies accompanied by the elemental profiles(610�14 wt%O) at 650 �C for 1000 h.


layers separated by porous band which coincides with initial inter-face (X = 0) between solid metal and liquid metal. This assertion isbased on the following observations and experimental facts, i.e.:the measurements of initial and post-test thicknesses of samplesunaffected by oxidation indicate about coincidence of materialthinning with the thickness of inner oxide layer (Table 1); micro-graphs revealing non-oxidized areas of surface neighboring dou-ble-oxide layer (Fig. 4c) and finally the redistribution of Cr whichacts as a natural marker of steel matrix (Fig. 4b and d).

The porous band in the double-oxide layer formed on Fe–14Crsteel is noticeably broader (Fig. 4d). The IOZ is observed beneaththe inner Cr-rich layer and is characterized by repetitive concen-tration peaks of Cr (Fig. 4b and d). Lead was not detected eitherin outer or inner sub-layers of scale.

At 650 �C, the single Cr-rich oxide film was formed on the sur-face of steels (Fig. 5) instead of double-oxide layer observed at550 �C. The thickness of oxide films does not exceed 1–1.5 lmsince all peaks of steel’s components are present in the EDX spectraobtained from the surface. The Fe–9Cr sample showed abundantareas (2) with exfoliated oxide film denuding bare matrix, whichwas depleted in Cr (�6%) (Fig. 5a). In comparison with Fe–9Cr steel,the surface of Fe–14Cr steel was covered by almost continuousoxide film (Fig. 5b) and Fe peaks were suppressed noticeably inthe EDX spectrum (Fig. 5b, area (3) indicating that oxide filmformed on the steel with higher chromium content is denser orslightly thicker.

3.3. Oxidation of ODS steels in oxygen-saturated Pb (CO � 10�3 wt%O)

The liquid Pb saturated by oxygen (CO � 10�3 wt%) is very infor-mative medium since the morphological features, elemental andphase composition of thick scales could be examined in detail. Un-der the oxygen-saturated conditions the rise in temperature from550 to 650 �C results in insignificant increase of oxygen in the li-quid Pb from 1.33 � 10�3 to 6.07 � 10�3 wt%O (Section 2).

of Fe–9Cr (a and b) and Fe–14Cr (c and d) ODS steels after exposure to pure Pb



Fig. 4. Morphology of scales accompanied by the elemental profiles of Fe–9Cr (a and b) and Fe–14Cr (c and d) ODS steels exposed to oxygen-added Pb (�10�6 wt%O) at 550 �Cfor 500 h. a and b – LOM. c and d – SEM.

Table 1Parameters of scales formed on the surface of Fe–9Cr and Fe–14Cr ODS steels after exposure to Pb melt depending on the temperature, oxygen concentration in the melt and time.

Pb melt ODS steel Temperature (�C) Time (h) Thickness of scale (lm)

Total Outer layer Inner layer + (IOZ)

Oxygen-added Pb (�10�6 wt%O) Fe–9Cr 550 500 8.5 4.0 2.5 + (2.0)1000 9.2 4.2 2.2 + (2.8)

Fe–14Cr 500 13.5 5.0 3.0 + (5.5)1000 15.2 6.0 4.0 + (5.2)

Oxygen-saturated Pb (�10�3 wt%O) Fe–9Cr 550 230 22 11 11500 40 20 20

1000 50 25 25650 230 850 820 30

500 1200 1025 1751000 1400 1150 250

Fe–14Cr 550 230 17 8.5 8.5500 24 12 12

1000 30 15 15650 230 20.5 12.7 7.7

500 26.2 16.0 101000 35.0 20.0 15


3.3.1. Morphology and composition of scales3.3.1.1. Fe–9Cr ODS steel. At 550 �C, the typical double-oxide layerforms on the surface of Fe–9Cr steel in oxygen-saturated Pb(Fig. 6a and b). Composition of outer and inner sub-layers is similarto that formed in oxygen-added melt at the same temperature, i.e.the outer layer consists of Fe and O (magnetite) while the inner onecontains Fe, O and is enriched in Cr (spinel). As a rule, outer and in-ner sub-layers have nearly equal thickness (Table 1) and growingsymmetrically with respect to the initial interface X = 0 (Fig. 6).With time, the double-oxide layer becomes more porous and Pbappears at the "oxide/matrix" interface (Fig. 6b and d).

At 650 �C, Fe–9Cr steel demonstrates severe oxidation accom-panied by formation of thick multi-layer scale (Table 1). Fig. 7shows the structure of multi-layer scale obtained by means ofbackscattered electrons (a) and accompanied with general EDXspectrum of scale (b) and corresponding maps of elements (c–f).The scale consists of numerous repetitive longitudinal black andgray-colored layers against a light gray background of scale(Fig. 7a). The black layers consist of O and Cr while the gray ones


are composed mainly of O and Fe. Lead is distributed across wholescale excepting the places with high chromium content. Especiallya lot of Pb was detected nearby the ‘‘scale-matrix’’ interface(Fig. 7f). In spite of the fact that thick scale contains Pb the integrityand adhesiveness of scale to the steel matrix was good enough.

3.3.1.2. Fe–14Cr ODS steel. Fig. 8 shows surface structure of Fe–14Crsteel after the test at 550 �C for 1000 h. The oxidized surface ofsample is highly irregular. Against a background of smooth Cr-reach oxide film (1) the great number of protrusions (2) are ob-served. The latter consist of crystals, which according to the EDXanalysis are composed mainly of Fe and O, i.e. are magnetite.Cross-section examinations confirm the non-uniform character ofoxidation of Fe–14Cr steel at 550 �C (Fig. 9). Similar to Fe–9Cr,the areas covered by double-oxide layer were observed on thecross-section of Fe–14Cr steel (Fig. 9). At the same time, there wereareas without double oxide, which revealed higher oxidation resis-tance due to formation of Cr-based oxide film (Fig. 8), which wasabsent on the surface of Fe–9Cr steel tested under the same



Fig. 5. Surface structure and corresponded EDX spectra of Fe–9Cr (a) and Fe–14Cr (b) ODS steels exposed to oxygen-added Pb (�10�6 wt%O) at 650 �C for 1000 h.

Fig. 6. Morphology and corresponded elemental profiles of scales formed on the surface of Fe–9Cr ODS steel after exposure to oxygen-saturated Pb (1.33 � 10�3 wt%O) at550 �C for a – 230 h and b – 500 h. X = 0 – initial ‘‘solid metal–liquid metal’’ interface. a and b – LOM. c and d – SEM.


conditions. Chromium is distributed uniformly in the inner layer,but closer to the matrix the gathering of Cr is detected with time(Fig. 9d).

At 650 �C, the double scale was also detected on the surface ofFe–14Cr steel after 1000 h exposure (Fig. 10a). The total thicknessof scale is slightly thicker in comparison to that formed at 550 �C(Table 1). However, contrary to the lower temperature (550 �C),some sub-layers of this scale include Pb. Thus, the outer non-uni-form light-grey layer besides Fe and O contains Pb (Fig. 10a and f


area 1). In contrast, the inner taupe-colored oxide with high con-tent of Cr does not contain Pb (Fig. 10 area 2). The underlyinglight-grey oxide consists of all components of steel and Pb(Fig. 10a area 3). At last, continuous Cr-rich oxide strip separatesthe steel matrix from the scale formed (Fig. 10a, c, e). Based onthe quantitative data from EDX analyses (table in Fig. 10) the atom-ic ratio of elements in different areas of scale corresponds well tothe following complex oxides: PbO Fe2O3 (1), FeO Cr2O3 (2), andPbO (Fe,Cr)2O3 (3).



Fig. 7. Backscattered electron image (a), general EDX spectrum of elements (b) and elemental maps (c–f) of scale formed on the surface of Fe–9Cr ODS steel exposed tooxygen-saturated Pb (1.33 � 10�3 wt%O) at 650 �C for 500 h.


3.4. Oxidation kinetics

Kinetics of scale growth in the oxygen-saturated and oxygen-added melts is shown in Fig. 11. In the oxygen-saturated Pb(�10�3 wt%O), lower-chromium steel Fe–9Cr oxidizes more inten-sively (Fig. 11a, curve 1 and 2) in comparison with Fe–14Cr(Fig. 11a, curve 3 and 4). The oxidation of Fe–14Cr steel acceleratesinsignificantly with temperature (compare curve 3 and 4), whereasthe oxidation of Fe–9Cr steel intensifies drastically at 650 �C (com-pare curve 1 and 2).

In oxygen-added Pb (�10�6 wt%O), the thickness of the scalesformed on the surface of ODS steels at 550 �C does not differ sub-stantially, although the scales formed on the Fe–14Cr were evenslightly thicker in comparison with those on Fe–9Cr (Fig. 11a, com-pare curve (5 and 7). However, contrary to the oxygen-saturatedmelt, with increase in test temperature from 550 to 650 �C the oxi-dation rate of both steels decreases markedly (Fig. 11a, comparecurves 5 and 7 with 6 and 8). Reducing the oxidation of steels withrise in temperature could be caused by increase in chromium dif-fusivity and formation of more protective oxide film with higherchromium content on the steel surface as it was shown above (Sec-tion 3.2. Fig. 5).

Fig. 11b shows thinning of samples depending on time of expo-sure in the oxygen-saturated Pb. Based on the measurements, thethinning of Fe–9Cr and Fe–14Cr steels at 550 �C for 1000 h(Fig. 11b curve 1 and 3) reached 25 and 15 lm, respectively. At


650 �C the thinning of Fe–9Cr steel reaches 250 lm for 1000 h(Fig. 11b, curve 2) while thinning of Fe–14Cr steel does not changesubstantially (Fig. 11 curve 4). The thinning of samples, in general,correlates well with the thickness of inner oxide layer growing to-wards matrix (Table 1).

4. Discussion

Based on the obtained results it can be indicated that corrosionbehavior of ODS steels depends on the oxygen concentration in thePb melt. Fig. 12 summarizes the observed corrosion modes whichchange as it was expected from the dissolution in the pure Pb(Fig. 12a and b), to the formation of protective oxide layers inthe oxygen-added melt (Fig. 12c and d) and finally to the severeoxidation in the oxygen-saturated Pb accompanied by formationof thick multiphase scale (Fig. 12d and e). The obtained resultsagree with existed knowledge about interaction modes betweentraditional steels and Pb or Pb–Bi melts [4–6]. However there areseveral features of interaction caused by the specific phase-struc-tural state of ODS steels. The ambiguous influence of temperatureand/or chromium content on the corrosion behavior of steels needsto be elucidated also.

In pure Pb (CO < 10�14 wt%O) both ODS steels underwent disso-lution attack resulted in significant weight losses which intensifiedwith temperature and were always greater for Fe–14Cr steel incomparison with Fe–9Cr (Fig. 2). Traditional ferritic/martensitic



Fig. 8. Surface morphology and composition of Fe–14Cr ODS steel after exposure to oxygen-saturated Pb (1.33 � 10�3 wt%O) at 550 �C for 1000 h.

Fig. 9. Morphology and corresponded elemental profiles of scales formed on the surface of Fe–14Cr ODS steel after exposure to oxygen-saturated Pb (1.33 � 10-3 wt%O) at550 �C for a – 230 h and b – 500 h. X = 0 – initial ‘‘solid metal–liquid metal’’ interface. a and b – LOM; c and d – SEM.


steels with similar chromium content demonstrate substantiallysmaller corrosion losses. Most probably high length of grainboundaries in ODS steels promotes grain-boundary penetrationof liquid metal which intensified with temperature rise (Fig. 12a


and b). As a result, the whole grains are simply leached from thenear-surface layers resulting in the formation of deep-pronouncedsurface relief and noticeable weight loss (Fig. 2 and 3). It is not un-likely that increased segregation of Cr in the vicinity of grain



Fig. 10. Backscattered electron image of scale (a) formed on the surface of Fe–14Cr ODS steel after exposure to oxygen-saturated Pb (1.33 � 10�3 wt%O) at 650 �C for 1000 h,accompanied by composition and elemental maps (c–f).

Fig. 11. Kinetics of scale growth (a) and thinning of solid metal (b) for Fe–9Cr and Fe–14Cr ODS steels exposed to oxygen-saturated (�10�3 wt%O) and oxygen-added(�10�6 wt%O) Pb melts at 550 and 650 �C.


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Fig. 12. Schemes of interaction of ODS steels with liquid Pb with different oxygen concentration depending on the temperature: a and b – pure Pb (610�14 wt%O), c and d –oxygen-added Pb (�10�6 wt%O), e and f – oxygen-saturated Pb (�10�3 wt%O). PF – plumboferrite, M – magnetite, SP – spinel.


boundaries, especially in the case of Fe–14Cr steel, favors liquidmetal attack since solubility of Cr in the Pb melt is higher than thatof Fe [16].

In oxygen-added Pb (CO � 10�6 wt%O) at 550 �C the Fe-baseddouble-oxide layer with inner oxidation zone is formed on the sur-face of both ODS steels (Fig. 12c). Obtained results correlate wellwith that observed for traditional and ODS chromium steels [4,7–14]. The growth of the scale is controlled by preferential diffusionof Fe-cations through the magnetite. As a result, the outer magne-tite (Fe3O4) grows towards the oxygen-containing melt while spi-nel ((Fe,Cr)3O4) grows towards matrix. It was noticed thatconcentration of Cr in the spinel formed on ODS steels is higher(�30 to 40 wt%Cr) comparing to traditional steels (620 wt%Cr)with similar content of Cr. This fact indicates higher diffusivity ofCr in ODS steels due to their specific fine-grained structure.

With increase in test temperature to 650 �C the thin Cr-basedoxide layer forms on the surface of ODS steels instead of Fe-baseddouble-oxide layer (Fig. 12d). Formation of chromium oxide can beexplained by change in thermodynamic and kinetic conditions ofoxidation at 650 �C. Thus on the one hand, approaching equilib-rium between Pb[O] solution and magnetite could result in thedestabilization of latter. On the other hand, temperature promotesfast chromium delivery from the bulk of matrix towards ‘‘solid me-tal/liquid metal’’ interface. Under such conditions, the Cr-based


oxide grows solely on the surface of ODS steels. Comparison of oxi-dation of Fe–9Cr and Fe–14Cr steels indicates that protective prop-erties of oxide film improved with increase in Cr content similar tothe gaseous media.

In oxygen-saturated Pb (CO � 10�3 wt %O) the oxidation rate ofboth steels accelerates noticeably (Fig. 11 curve 1–4). Acceleratingof oxidation could be caused by nucleation and competitive growthof plumboferrite phase (PF) (Fig. 12e and f). At moderate tempera-ture of exposure (T 6 550�C) the PF is noncompetitive with magne-tite (M) and spinel (SP) and decays with precipitation of free Pbwithin the outer magnetite layer (Fig. 12e). The intensive reactionof formation of PF starts at temperatures P600 [17–19]. Therefore,at 650 �C from the beginning of contact PF demonstrates competi-tive equilibrium with M and SP and grows quickly (Fig. 12f). Withtime PF creates continual interconnected network through thescale and decays closer to matrix where iron activity is high. As aresult, free Pb accumulates within the inner part of scale. In sucha way the multi-layer scale growths. This scale consists from differ-ent oxide phases and does not possess by protective properties.Nevertheless, under those extreme conditions the high content ofCr mitigates effectively the growth of scale, as is obvious whencompare the oxidation rate of Fe–9Cr and Fe–14Cr steels at650 �C (Fig. 11 curve 2 and 4) or structure of scales formed (com-pare Fig. 7 and 10).




Thus, it can be indicated, that specific fine-grained structure incombination with high content of Cr affect positively on the oxida-tion resistance of ODS steels facing oxygen-added Pb melt resultingin oxidation of Cr in the vicinity of ‘‘solid metal/liquid metal’’ inter-face. However, under the contact with pure Pb the afore-men-tioned factors play negative role intensifying inter-granulardissolution of steel matrix. Oxygen-saturated Pb provokes growthof plumboferrite phase resulting in catastrophic oxidation of steels.

5. Conclusions

Corrosion behavior of Fe–9Cr–1.5W(Y2O3) and Fe–14Cr–1.5W(Y2O3) ODS steels depends on the oxygen concentration inthe Pb melt and changes from the dissolution in the pure Pb(610�14 wt%O) to formation of protective oxide layers in the oxy-gen-added melt (�10�6 wt%O) and then to severe oxidation inthe melt saturated by oxygen (�10�3 wt%O) accompanied by fastgrowth of non-protective scales. In general, the corrosion behaviorof ODS steels coincides with that of traditional chromium steels.Nevertheless, the fine-grained structure of ODS steels causes somevariations in corrosion response, namely:

� In the oxygen-added Pb, with increase in test temperature from550 to 650 �C, the oxidation of ODS steels changes from forma-tion of Fe-based double-oxide layer towards development ofsingle-layer Cr-based film due to approaching equilibriumbetween Fe3O4 and Pb[O] (�10�6 wt%O at 650 �C) beneathwhich Fe3O4 is not stable. Fine-grained structure of ODS steelfavors formation of Cr-based film providing fast diffusion of Crinto growing oxide along grain boundaries. The protective prop-erties of Cr oxide film increases with increasing chromium con-tent in ODS steels from 9 to 14 wt%.� In the pure Pb, the lack of oxygen does not allow the protective

oxide film to be formed. In this case the ODS steels with fine-grained structure suffering from preferential grain-boundarycorrosion attack accompanied by selective leaching of chro-mium and simultaneous inter-granular penetration of lead intosteel matrix. Increase in Cr content in the steel promotes inter-granular corrosion attack.� In the oxygen-saturated Pb the ODS steels undergo severe oxi-

dation accompanied by the formation of non-protective porousmultilayer scale which consist of mixture of different phases:


plumboferrite, magnetite, Fe–Cr spinel and free Pb. The oxida-tion kinetics intensifies drastically with temperature and decel-erates with chromium content in the steel.

Acknowledgements

This work was carried out within the framework of IAEA Re-search Contract No. 16707 ‘‘Compatibility of ODS Steels with Cool-ant/Breeding Lead Melts (Pb, Pb–Li) at Elevated Temperatures’’.

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