- Email: [email protected]
Oxidation of an austenitic stainless steel with or without alloyed aluminum in O2 + 10% H2O environment at 800 °C
Accepted Manuscript Title: Oxidation of an austenitic stainless steel with or without alloyed aluminium in O2 + 10% H2 O environment at 800 ◦ C Authors: Zhongdi Yu, Minghui Chen, Changbin Shen, Shenglong Zhu, Fuhui Wang PII: DOI: Reference:
S0010-938X(16)30763-6 http://dx.doi.org/doi:10.1016/j.corsci.2017.03.015 CS 7035
To appear in: Received date: Revised date: Accepted date:
15-9-2016 14-3-2017 15-3-2017
Please cite this article as: Zhongdi Yu, Minghui Chen, Changbin Shen, Shenglong Zhu, Fuhui Wang, Oxidation of an austenitic stainless steel with or without alloyed aluminium in O2+10% H2O environment at 800◦ C, Corrosion Sciencehttp://dx.doi.org/10.1016/j.corsci.2017.03.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Oxidation of an austenitic stainless steel with or without alloyed aluminium in O2 + 10% H2O environment at 800 °C Zhongdi Yu1,2, Minghui Chen*1, Changbin Shen2, Shenglong Zhu3, Fuhui Wang1,3
1
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education),
School of Material Science and Engineering, Northeastern University, Shenyang 110819, China 2
College of Materials Science and Engineering, Dalian Jiaotong University, Dalian
116021, China 3
Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese
Academy of Sciences, Shenyang 110016, China *E-mail: [email protected]; [email protected]
Tel: +86-24-83691562; fax: +86-24-23893624.
1
Research highlights 1. Minor Al addition increases substantially oxidation resistance of stainless steel 2. Scale on the Al-free steel becomes porous and spalls off after 20 h oxidation 3. Scale on the Al-containing steel is dense and protective for 100 h oxidation 4. A thin Al-rich inner layer forms at the interface of scale/Al-containing steel 5. The Al-rich layer inhibits formation of holes and ensures the compactness of scale
Abstract
Oxidation behavior in O2 + 10% H2O at 800 °C of an austenitic stainless steel with or without aluminum is investigated. The Al-free steel enters into breakaway oxidation stage after only 15 h exposure, while the one with 2.7 wt% aluminum shows high oxidation resistance. This minor amount of aluminum is lower than the critical content to form an intact alumina scale, but it promotes the formation of a thin Al-rich inner layer which is favor in prohibiting holes formation due to volatilization of CrO2(OH)2 and Kirkendall effect. A compact scale develops and protects the Al-containing steel very well.
Keywords: A. austenitic stainless steel; A. High temperature water vapor; C. oxidation
2
1. Introduction Water vapor is generally found in hot oxidant environment of many industries, e.g. power generation, petrochemical, and aerospace sectors [1-4]. The presence of water vapor main influences the process of high temperature oxidation of metals and alloys by modifying structure characteristic of the growing oxide scale and the oxidation kinetics [5-9]. Generally, oxide scales formed in wet high temperature are porous and multiphase. Meanwhile, some volatile metal hydroxides may accelerate consumption of the protective oxides [10, 11]. So, oxidation and protection of metals or alloys served at high temperatures with water vapor becomes increasingly important and is now attracting more and more attention. In the presence of water vapor, ion-based ferritic or austenitic stainless steels can serve safely below 600 °C by forming compact and protective oxide scale, thus showing wide applications in this environment [12, 13]. However, higher efficiency always requires higher operation temperature in many industrial fields, bringing in many challenges to the performance of materials, especially to oxidation [14]. The Cr-rich oxide scale which performs very well at temperature lower than 600 °C, will degenerate quickly and no longer protect the underlying stainless steel when the temperature rises to above 700 °C in the presence of water vapor. Generally, Cr2O3 and (Cr,Fe)2O3 may translate to some volatile oxy-hydroxides like CrO2(OH)2 in wet high temperature, which expedites the consumption of Cr [11-17]. In addition, water vapor accelerates diffusion of oxygen along the Cr2O3 grain boundary by the 3
penetration of hydrogen, further promoting the depletion of Cr [18, 19]. So these Cr-rich oxides can’t exist steadily at high temperature with water vapor. It is well known that alumina is more stable than chromia in wet high temperature atmosphere, so it is meaningful to promote formation of an alumina scale on stainless steels in terms of enhancing oxidation resistance [20, 21]. An appropriate method is to add moderate amount of aluminum to stainless steels during smelting process. Generally, the critical content of aluminum is 4-5 wt % for the growth of an external alumina scale on stainless steel depending on its grain size and composition [22]. However, such high amount of Al greatly affects the creep strength of the austenitic matrix, because of the very strong body-centered cubic (bcc)–stabilizing effects of Al on Fe [23]. In order to combine high creep strength with high oxidation resistance, a new series of alumina-forming austenitic (AFA) stainless steel is developed by Oak Ridge National Laboratory [24-26], which develops an external alumina scale on oxidation in dry air. Through adding proper amount of austenitic-stabilizing elements (such as Ni) and an optimized preparing process, they successfully decreased the content of Al in an austenitic to 3.5 wt % that is still able to form a continuous alumina scale while keeps a moderate creep strength. Nevertheless, more than 3 wt % Al in AFA alloy increases the risk of NiAl phase precipitation, which consumes austenite stabilizer element, Ni [27]. Recently, Yamamoto et al. [24] found that an AFA alloy (based on Fe–20Ni–14Cr–2.5Al) shows high resistance to oxidation at 800 °C for as long as 2000 h in air with 10% water vapor, meanwhile possesses a long creep-rupture lifetime (exceeding 2000 h) at 750 °C under 100 MPa. This alloy 4
develops an external alumina scale on oxidation in dry air and shows very low oxidation rate at 800 oC. However, it is worthy to note that such a low content of aluminum is beneficial to high creep strength but does not ensure the formation of an external alumina scale in wet air. The interesting thing is that even in the absence of the protective aluminum scale, the oxide scale could still prevent the substrate from breakaway oxidation. This phenomenon is different from previous researches which ascribes the high oxidation resistance of these Al-containing alloys to the formation of an external alumina scale [27]. So the protection mechanism of this low Al-addition AFA alloy at high temperature with the existence of water vapor is different from the normal consideration. It is meaningful to further study the oxidation behavior, especially at the earlier stage, of this AFA alloy in this harsh environment. To elucidate the oxidation-resisting mechanism of this low Al alloyed in AFA alloy at high temperature in the presence of water vapor, a sequence of researches are involved in this paper. The alumina-forming austenitic (AFA) stainless steel researched in this paper is based on Fe–20Ni–14.5Cr–2.7Al, which is similar with a classical series of AFA alloy (Fe–20Ni–14Cr–2.5Al).The initial oxidation behavior of the AFA alloy is compared with a contrast stainless steel free of aluminum. Meanwhile, the effect of low Al alloyed on the protection mechanism of AFA is discussed.
2. Materials and experimental procedure 2.1 Materials The nominal compositions of the AFA stainless steel and the contrast stainless steel 5
(CSS) are listed in Table 1. Firstly, commercially-pure elements were mixed. Then, alloy ingots were prepared by arc melting. Finally, they were cast into an Φ30 × 70 mm copper mould. After that, these alloys were homogenized at 1200 °C for 2 h and cooled in air. Samples were cut into 15 × 12 × 2 mm by electrical discharge machining, with surface grounded to a final 1000# SiC papers and edges rounded. Before oxidation test all the samples were degreased by distilled water, acetone and alcohol within an ultrasonic cleaner.
2.2 Oxidation experiment Intermittent oxidation was conducted for the AFA and the CSS alloys in O2 + 10% water vapor at 800 °C for 100 h. Their weight change was discontinuous recorded every 20 h exposure. Initial oxidation at the early 20 h was investigated in detail by Steam-TGA, where their weight change was recorded in situ. Though the oxidation time in this work is short, the former (100 h oxidation) is named as long-term oxidation and the latter (20 h) is short-term oxidation for comparison hereafter. Three parallel specimens were tested for each group. During oxidation, samples were all set in an alumina tube which was heated by a resistively tube furnace. In this alumina tube the flowing water vapor is controlled at a rate of 100 cc/min. In order to avoid electrochemical corrosion between different metals, samples were hung by alumina wire rather than by alloy wire. When the sample is weighed during long-term oxidation test, some oxides may peel off the substrate during cooling process. So the kinetics of this sample only reveals its oxidation tendency, but not for calculation.
2.3 Characterization 6
Phase constituent was characterized by X-ray diffraction and/or grazing incidence XRD (X’Pert PRO, PANalytical Co., Almelo, Holland, Cu Ka radiation at 40 kV). The GI-XRD patterns were recorded in 2θ range 10–90°, and a step-scanning mode was employed with a step size of 0.5°. Morphology, microstructure and composition were examined by scanning electron microscopy (SEM, Inspect F 50; FEI Co., Hillsboro, OR) with an energy dispersive X-ray spectroscopy (EDX, X-Max; Oxford Instruments Co., Oxford, UK.) and transmission electron microscopy (TEM, JEM-2100F; JEOL Co., Tokyo, Japan) in conjunction with energy-dispersive X-ray spectroscopy.
3. Result 3.1 Oxidation kinetics and phase constituents Figure 1 shows intermittent oxidation kinetics of the CSS and the AFA alloys in O2 + 10% H2O atmosphere at 800 °C. For the CSS alloy, it lost weight dramatically after 40 h exposure, which makes its weight change invalid. So its oxidation was stopped thereafter. For the AFA alloy, its kinetics roughly follows parabolic law during the whole 100 h oxidation (Fig. 1a). It is of great importance to study comparatively the initial oxidation behavior of these two alloys since oxide scale of the CSS alloy has spalled off after 20 h oxidation. So the precise Steam-TGA was applied. It is found that the oxidation kinetics of the CSS alloy shows three characteristic periods, including: (i) fast oxidation period at the first 5 h; (ii) platform period from 5 h to 15 h with an almost zero mass gain and (iii) the second fast oxidation period from 15 h to 20 h. It indicates that breakaway oxidation takes place at the point of 15 h oxidation 7
and this is not revealed by the long-term oxidation kinetics (Fig. 1a). In fact, the difference in mass gain during the initial 20 h oxidation between the long-term (Fig. 1a) and the short-term (Fig. 1b) oxidation test of the CSS alloy hints that breakaway oxidation should happen at this stage as well. For the continuous oxidation test (short-term oxidation), all the samples are weighed in time at high temperature within the TGA set, while samples have to be taken out of the furnace and cooled down to room temperature for weight measurement for the discontinuous oxidation. At such a case, scale spallation of the CSS alloy oxidized for 20 h should occur during its cooling process and this contributes to its lower mass gain in the discontinuous oxidation than it obtained by the Steam-TGA. For the AFA alloy, no spallation occurs during the initial 20 h oxidation and the growth rate of oxides on it is orders of magnitudes lower than that on the CSS alloy. Figure 2 shows GI-XRD and XRD patterns of the CSS and the AFA alloys after oxidation. For the CSS alloy, its oxide scale is very thin after the first 20 h oxidation. XRD peaks of the oxides are very hard to detect as shown in Fig. 2b. GI-XRD reveals that spinel-type oxide (Mn(Fe,Cr)2O4) and (Cr,Fe)2O3 are the two major phases (Fig. 2a). After 40 h oxidation, XRD patterns show that Fe2O3 and Ni(Fe,Cr)2O4 are the two major phases then. For the AFA alloy, its oxide scale is very thin even after the whole 100 h oxidation. Thus only GI-XRD patterns are present here. As shown in Fig. 2c, the oxide scale is mainly consisted of (Cr,Fe)2O3 with a small amount of MnO2 within the initial 20 h oxidation. When its exposure time is extending to 100 h, a few Al2O3 can be detected except for the above two oxides. 8
3.2 Surface morphology Figure 3 shows morphology of the CSS alloy after oxidation for different times in O2 + 10% H2O at 800 °C. Initially, fast-growing (Fe,Mn)-rich particulate oxides are detected on surface, which are then gradually transformed to oxide clusters at a diameter of 15 μm (Fig. 3a and 3b). These cluster oxides are enriched with Cr in addition to Fe and Mn. According to the XRD patterns, they are Mn(Fe,Cr)2O4. With increasing oxidation time, those oxide clusters further grow to oxide nodules, upon which blade-like (Cr,Fe)-rich oxide becomes notable (Fig. 3c and d). They are (Cr,Fe)2O3 according to the XRD pattern. In addition, cracks begin to nucleate at the bottom of the nodules where holes have been formed. The holes develop quickly, leading to the final loose structure of the oxide scale (Fig. 3e). These loose oxides are lean in Cr and composed mainly of Fe and O as compared to (Cr,Fe)-rich blades. From this point of view, volatile Cr hydroxide has been formed at this stage, and its volatilization should account for the formation of holes. It is no doubt that the cracks and holes contribute mainly to the final spallation of the oxide scale. After 40 h oxidation, in most areas the outer scale spalls off and (Ni, Fe and Cr)-rich oxides grow on the spalled area as shown in Fig. 3e, which are Ni(Fe,Cr)2O4. For the AFA alloy, no macroscopic spallation of oxide occurs after 100 h oxidation as shown in Fig. 4. What’s more, oxide scale on the AFA alloy is so thin that original grinding traces can still be seen after 100 h oxidation. Initially, some protruding oxides are found (see Fig. 4a and b). These oxides are rich in Mn and Fe. Thereafter, (Fe,Cr)-rich oxide begins to grow and spread quickly on the alloy surface(see Fig. 4c 9
and d). Anyway, these oxide nodules still keep small size after 100 h oxidation (Fig. 4e).
3.3 Cross-sectional microstructure Figure 5 shows cross-sectional microstructure of the oxide scale formed on the CSS alloy within the earlier 20 h oxidation. Oxide scale is very thin for the first 5 h oxidation and can’t be differentiated precisely from the alloy surface just by SEM (Fig. 5a). After 10 h oxidation, the oxide scale has grown to a thickness of 4~10 μm and becomes uneven. As shown in Fig. 5b, a large amount of holes are observed within the oxide scale, which divide the scale into two sublayers, i.e. the inner and the outer one. At some places, some holes even connect with each other to form cracks parallel to the alloy surface. With further oxidation, holes and cracks merge together to form a big gap between these two sublayers. As shown in Fig. 5c and d, the development of holes and cracks leads to chipping of the scale. Element analysis shows that Cr is rich at the inner sublayer, lean at the central and then rich again at the outer sublayer after 10 h oxidation. The distribution of elements in the oxide scale after 20 h oxidation is similar with the result of 10 h. TEM was applied to investigate in detail the oxide scale formed on the CSS alloy after the first 5 h exposure. Bright field TEM image and EDS analysis are presented in Fig. 6. The oxide scale is only 70 nm thick, below which a big hole is observed. EDS mapping of this scale show that chromium is concentrated at the bottom while Fe and Mn is rich at the outer part. To be noted, Mn is especially rich at places, below which the big hole dwells. 10
For the AFA alloy, the bright field TEM images of the oxides formed after 5 and 10 h oxidation are shown in Figs. 7 and 8, respectively. According to the EDS maps, its scale is consisted of two layers after 5 h oxidation (Fig. 7). The outer layer consists mainly of Fe and Cr with a few Al. and the inner scale shows especially high content of Al. A detail EDS line scan across the oxide scale shows nearly 20 at% Al concentrated at the inner layer where other alloy elements greatly decrease with oxidation time (selected area “C” in Figs. 7 and 8). When oxidation extending from 5 h to 10 h, the sum content of the other alloy elements (except Al) at the alloy/oxide interface further decreases from 45 at% to less than 10 at% (as seen in Table 2).
4. Discussion In this paper, oxidation behavior of the AFA and the CSS alloys in O2+10% H2O at 800 C is investigated. For the AFA alloy, no macroscopic spallation occurs and mass gain is below 0.2 mg/cm2 after 100 h oxidation. While scales formed on the CSS alloy is not able to protect the substrate from fast oxidation in this harsh environment. Widespread spallation takes place after only 40 h oxidation. In order to explore out the oxidation mechanism, a short-term oxidation (20 h) is carried out by using Steam-TGA. In the following parts, breakaway oxidation mechanism of the CSS alloy and the protection mechanism of the AFA alloy will be discussed. Especially, the effect of Al addition on oxidation is studied. Finally, a model is built to explain the oxidation behavior of these two alloys comparatively.
4.1 Breakaway oxidation of the CSS alloy Though many studies have reported the breakaway oxidation of the conventional 11
CSS alloy [4-10], the underlying mechanisms are still unknown. During oxidation, Mn(Cr,Fe)2O4 and (Cr,Fe)2O3 oxides are initially formed on the CSS alloy. In general, these oxides can protect well the steel substrate from oxidation in dry oxygen atmosphere at 800 C. However, in the presence of water vapor these chromium oxides in (Cr,Fe)2O3 will react with water to form volatile chromium hydroxide (CrO2(OH)2), whose volatilization finally leads to the porous structure of the scale. Consequently, the initially formed blade-like (Cr,Fe)2O3 oxides gradually transform to Fe2O3 due to the Cr depletion. As shown in Figs. 3 and 5, the amount of Cr at the outer sublayer becomes less with increasing oxidation time from 10 h to 15 h while more and more holes develop there. To be noted, this stage where Cr depletes and holes develop quickly corresponds very well with the platform period in oxidation kinetics (Fig. 1b). The mass loss by volatilization of chromium hydroxide compensates with the mass gain by oxidation. However, the TEM image reveals a different formation mechanism of holes (as show in Fig. 6). After the first 5 h oxidation, the initially formed thin oxide scale can further be divided into two layers: the inner and the outer one. The inner scale is mainly composed of (Cr,Fe)2O3 and the outer one consists Mn(Cr,Fe)2O4. Due to the high affinity of Cr with O, chromium oxide forms firstly on the austenitic stainless steel. It possesses high oxidation resistance as chromium and oxygen ions diffuse difficulty in it [28]. However, Fe and Mn ions have a large solubility and mobility in chromia. Thus oxides of Fe and Mn will develop then upon the initially formed chromium oxides. As compared to the corundum-type oxide (Cr,Fe)2O3, defects and 12
vacancies are more in the spinel-type oxide Mn(Cr,Fe)2O4 which thus grows at a high speed [29]. Especially, the diffusion rate of manganese in chromia is higher than Fe and other alloy elements [30]. Because of the fast consumption of Mn, Kirkendall holes are formed beneath the oxide scale. This is the reason why the oxide scale exactly upon the holes is especially rich in Mn (Figs. 5 and 6). No matter what mechanism is dominating, these holes contribute mainly to the final spallation of the oxide scale. Besides, these two mechanisms may co-work. On the one hand, gas phase transfers rapidly in initial formed Kirkendall holes, where vacancies are easy gathered within them. On the other hand, the volatilization of CrO2(OH)2 leads high oxygen pressure within this porous layer. Therefore, many spinel-type oxides (Mn(Cr,Fe)2O4) are formed at high oxygen pressure. With more vacancy in spinel’s lattice, gas phase reaction occurs more quickly within it, which leaves a plenty of cation vacancies in this layer. With oxidation continuing, the buried holes connect with each other to form parallel cracks and even big gaps, separating the oxide scale into several sublayers and leading to the final spallation of scale. Breakaway oxidation occurs after the spallation of scale at the point of 15 h exposure. This oxidation stage is correlated with the formation of Fe2O3 and Ni(Fe,Cr)2O4 oxides as shown in Figs. 2 and 3.
4.2 Protection mechanism of the AFA alloy Based on the designing principle of alumina-forming austenitic (AFA) stainless steel, 2.7 wt% Al is lower than the critical level to form continuous external alumina scale. Generally, to achieve this alumina layer in such harsh environment more than 3 13
wt% Al should be alloyed [31]. However, in our research such a low addition of Al affects significantly the oxidation behavior of stainless steel. In the following parts the protection mechanism is discussed. Initially, oxygen pressure is high enough for all the alloy elements to be oxidized, but the content of each element and the growth rate of each oxide are different. Considering the percentage and their affinity to oxygen of the alloy elements comprehensively, the external oxide scale is major consisted of (Cr,Fe)2O3 with a small amount of MnO2 and Al2O3. Initially formed oxides reduce the oxygen pressure greatly at the alloy surface, promoting the selective oxidation of Al beneath them and the consequent formation of a thin Al-rich inner layer. This is agreed well with the oxidation kinetics (Fig. 1), and with the distribution of alloy elements in scale after different oxidation times. Within the inner Al-rich layer (marked as “C” in EDS line scan in Figs.7 and 8) , the total content of other alloy elements (except Al) is about 45 at% after 5 h oxidation, while it decreases to less than 10 at% after 10 h oxidation. In addition, a few of aluminum oxides diffuse into the initially formed (Cr,Fe)2O3. So Al rich oxides are detected by XRD within the outer (Cr,Fe)2O3 scale. Due to the blocking effect of the inner thin Al-rich layer, further oxidation of Mn and Fe is dramatically slowed down, thus no spinel-type oxides are detected in scale formed on the AFA alloy even after 100 h oxidation by GI-XRD (Fig. 2). Thermodynamically, the formation of Mn(Fe,Cr)2O4 is related to the solid state reactions [32]:
MnO+Fe2O3 MnFe2O4
(1) 14
MnO+Cr2O3 MnCr2O4
(2)
Zurek et al. [33] has reported that the effect of Mn addition on the growth rate of corundum-type oxide is various with oxygen pressure. The above-mentioned reactions occur only at high oxygen pressure, where Mn ions have high solubility in the corundum-type oxide. Once the spinel-type oxide forms, the ion diffusion or growth rate increases. However, at low pressure of oxygen, the limit solubility of Mn doesn’t support these reactions to take place and the oxides still maintain their corundum-type. This explanation may also suitable for explaining oxidation process of the AFA alloy. Because the compact Al-rich inner layer significantly decreases oxygen pressure and reduces solubility of Mn in oxides, the solid reactions are prohibited. In the absence of the Cr-rich oxide and the fast-growing Mn(Fe,Cr)2O4 oxide, holes formation due to volatilization of chromium hydroxide or Kirkendall effect is then prohibited. The dense oxide scale which is stable at this O2+10% H2O environment protects well of the steel substrate from oxidation. In conclusion, the protection mechanism of this low-Al content AFA alloy relies on the formation of a dense Al-rich layer between the oxide scale and the steel substrate. Due to that the content of alloyed Al is not sufficient for the transformation of internal growth alumina to external, only an Al-rich inner layer left. However, it slows down the diffusion of Cr and Mn from the substrate to the scale surface. The formation of spinel-type Mn(Fe,Cr)2O4 and volatile chromium hydroxide, as well as the widespread holes, are prohibited.
4.3 Oxidation modeling 15
4.3.1 For the CSS alloy Oxidation process of the CSS alloy is schematically illustrated in Fig 9. Three periods are identified, i.e. fast oxidation period, platform period, and breakaway period. They are marked as ‘‘I’’, ‘‘II’’ and ‘‘III’’ in the illustration, respectively. During period “I”, original surface is covered by a thin (Cr,Fe)2O3 inner layer, above which a spinel-type [Mn(Cr,Fe)2O4] intermediate layer and some (Fe,Mn)-rich top oxides develop. In the presence of water vapor, chromium hydroxide CrO2(OH) forms as a consequence of inter-reaction between chromium oxide and water vapor, and its volatilization gradually leads to a porous microstructure of the oxide scale during period ‘‘II’’. Simultaneously, fast growth of the Mn-rich spinel oxides results in Kirkendall holes in steel immediately beneath the inner (Cr,Fe)2O3 layer. They connect with each other to form a gap at the original alloy surface. Once the gap has grown large enough, peeling of the oxide scale takes place. Thereafter, breakaway oxidation starts and oxidation enters into the period ‘‘III’’ accompanied with macroscopic spallation. At spallation area, Ni(Fe,Cr)2O4 oxides form. In addition, Cr depletion results in the transformation of the blade like (Cr,Fe)2O3 to porous Fe2O3. 4.3.2 For the AFA alloy For the AFA alloy, its oxidation process can also be divided into three periods, i.e. fast nucleation period, selective oxidation period, and stable oxidation period. They are marked as ‘‘I’’, ‘‘II’’ and ‘‘III’’, respectively. During period ‘‘I’’, oxide formed on the surface is mainly (Cr,Fe)2O3, with a small amount of MnO2 and Al2O3.This oxide scale reduces the oxygen pressure greatly at the alloy surface, promoting the selective 16
oxidation of Al beneath them, which belongs to the selective oxidation period (period ‘‘II’’). The third-element effect, first proposed by Wagner also accounts for this oxidation [34]. Generally, the third element “Cr” acts as an ‘‘oxygen getter’’ in Fe-Cr-Al alloy. These chromium oxides limit oxygen diffused inward efficiently that quite a few Al is oxidized initially, which also reduce the critical level of Al for the transition from internal to external oxidation. In this paper, although these chromium oxides gather the oxygen efficiently, insufficient Al can support the transition of alumina. Meanwhile, the presence of water vapor also increases the critical level of Al than in dry oxygen [35]. So an Al-rich inner layer which is not exclusively composed of Al2O3 left. Anyway, it is just because of such an Al-rich inner layer, the diffusion of Cr and Mn ions from alloy to the outer surface is mostly reduced. The formation and subsequent volatilization of hydroxide CrO2(OH)2, as well as the formation of Kirkendall holes, are prohibited. At last, the dense and adherent oxide scale provides high oxidation resistance for the underlying AFA alloy in this wet environment and oxidation enters into the third period: stable oxidation. From this point of view, Al addition in a minor content, lower than the critical one to support forming an external alumina scale, still increases largely the oxidation resistance of steel in wet environment. This positive effect relies on the formation of Al-rich inner layer, which prohibits formation of holes and ensure a dense oxide scale.
5. Conclusion From above study, oxidation behavior of austenitic stainless steel with or without 17
aluminum in O2 + 10% H2O at 800 °C is investigated. The following conclusions can be drawn: (1) In the atmosphere of O2 + 10% H2O at 800 C, the Al-free steel (CSS) enters into breakaway oxidation after only 15 h exposure, while the one containing 2.7 at% aluminum (AFA) shows high oxidation resistance during the whole 100 h oxidation. (2) Initially, oxide scale formed on the CSS alloy is consisted of an outer Mn(Fe,Cr)2O4 layer and an inner (Cr,Fe)2O3 one. Thereafter, Fe2O3 and Ni(Fe,Cr)2O4 develops once breakaway oxidation takes place. For the AFA alloy, its oxide scale is consisted of the outer (Cr,Fe)2O3 layer and inner Al-rich oxide layer. (3) Holes formed due to Kirkendall effect and volatilization of chromium hydroxide (CrO2(OH)2) results in the porous structure and final spallation of oxide scale on the CSS alloy. (4) The compact Al-rich oxide inner layer on the AFA alloy significantly decreases the oxygen pressure and slows down the fast diffusion of Cr and Mn from the substrate to the scale surface. So the formation of spinel-type Mn(Fe,Cr)2O4 and volatile chromium hydroxide, as well as the widespread holes, are prohibited.
Acknowledgements This project is financially supported by National Natural Science Foundation of China (No.51671053), the Youth Innovation Promotion Association CAS under Grant No. 2016178, and by the Fundamental Research Funds for the Central Universities under Grant No. N160205001. 18
References [1] S.R.J. Saunders, M. Monteiro, F. Rizzo, The oxidation behavior of metals and alloys at high temperatures in atmospheres containing water vapor: a review, Prog. Mater. Sci. 53 (2008) 775-837. [2] I.G. Wright, R.B. Dooley, A review of the oxidation behavior of structural alloys in steam, Int. Mater. Rev. 55 (2010) 129–167. [3] K.I. Choudhry, S. Mahboubi, G.A. Botton, J.R. Kish, I.M. Svishchev, Corrosion of engineering materials in a supercritical water cooled reactor: Characterization of oxide scales on Alloy 800H and stainless steel 316, Corros. Sci. 100 (2015) 222-230. [4] T. Fujii, K. Tohgo, A. Kenmochi, Y. Shimamura, Experimental and numerical investigation of stress corrosion cracking of sensitized type 304 stainless steel under high-temperature and high-purity water, Corros. Sci. 97 (2015) 139-149. [5] M. Nezakat, H. Akhiani, S. Penttilä, S.M. Sabet, J. Szpunar, Effect of thermo-mechanical processing on oxidation of austenitic stainless steel 316L in supercritical water, Corros. Sci. 94 (2015) 197-206. [6] M. Calmunger, R. Eriksson, G. Chai, S. Johansson, J. J. Moverare, Surface phase transformation in austenitic stainless steel induced by cyclic oxidation in humidified air, Corros. Sci. 100 (2015) 524-534. [7] J. Ehlers, D.J. Young, E.J. Smaardijk, A.K. Tyagi, H.J. Penkalla, L. Singheiser, J. Quadakkers, Enhanced oxidation of 9% Cr in water vapor containing environment, Corros. Sci. 48 (2006) 3428-3454. 19
[8] N.K. Othman, J.Q. Zhang, D.J. Young, Temperature and water vapour effects on the cyclic oxidation behavior of Fe–Cr alloys, Corros. Sci. 52 (2010) 2827-2836. [9] H. Asteman, J.E. Svensson, L.G. Johansson, Evidence for chromium evaporation influencing the oxidation of 304L: the effect of temperature and flow rate, Oxid. Met. 57 (2002) 193–216. [10] J.T. Yuan, W. Wang, S.L. Zhu, F.H. Wang, Comparison between the oxidation of iron in oxygen and in steam at 650–750 °C, Corros. Sci. 75 (2013) 309-317. [11] E.J. Opila, D.L. Myers, N.S. Jacobson, I.M.B. Nielsen, D.F. Johnson, J.K. Olminsky, M.D. Allendorf, Theoretical and experimental investigation of the thermochemistry of CrO2(OH)2(g), J. Phys. Chem. A. 111 (2007) 1971-1980. [12] J.T. Yuan, W. Wang, H.H. Zhang, L.J. Zhu, S.L. Zhu, F.H. Wang, Investigation into the failure mechanism of chromia scale thermally grown on an austenitic stainless steel in pure steam, Corros. Sci. 109 (2016) 36-42. [13] H. Astemana, J.E. Svenssonb, L.G. Johanssona, Oxidation of 310 steel in H2O/O2 mixtures at 600 °C: the effect of water-vapour-enhanced chromium evaporation, Corros. Sci. 44 (2002) 2635-2646. [14] G.R. Holcomb, High pressure steam oxidation of alloys for advanced ultra-supercritical conditions, Oxid. Met. 82 (2014) 271-295. [15] C.T. Fujii, R.A. Meussner, The mechanism of the high-temperature oxidation of iron-chromium alloys in water vapor, J. Electrochem. Soc. 111 (1964) 1215-1221. [16] D.J. Young, B.A. Pint, Chromium volatilization rates from Cr2O3 scales into flowing gases containing water vapor, Oxid. Met. 66 (2006) 137-153. 20
[17] T. Jonsson, B. Pujilaksono, H. Heidari, F. Liu, J.E. Svensson, M. Halvarsson, L.G. Johansson, Oxidation of Fe–10Cr in O2 and in O2 + H2O environment at 600°C: A microstructural investigation, Corros. Sci. 75 (2013) 326-336. [18] J.N. Shen, L.J. Zhou, T.F. Li. High Temperature oxidation of Fe-Cr alloys in wet oxygen, Oxid. Met. 48 (1997) 347-356. [19] H. Asteman, J.E. Svensson, L.G. Johansson, M. Norell, Indication of chromium oxide hydroxide evaporation during oxidation of 304L at 873 K in the presence of 10% water vapor, Oxid. Met. 52 (1999) 95-111. [20] H. Götlind, F. Liu, J.E. Svensson, M. Halvarsson, L.G. Johansson, The effect of water vapor on the initial stages of oxidation of the FeCrAl Alloy Kanthal AF at 900 °C, Oxid. Met. 67 (2007) 252-266. [21] H. Asteman, M. Spiegel, A comparison of the oxidation behaviors of Al2O3 formers and Cr2O3 formers at 700 °C - Oxide solid solutions acting as a template for nucleation, Corros. Sci. 50 (2008) 1734-1743. [22] G. Berthome´, E. N’Dah, Y. Wouters, A. Galerie, Temperature dependence of metastable alumina formation during thermal oxidation of FeCrAl foils, Mater. Corros. 56 (2005) 389-392. [23] R. Rahimi, B.C.D. Cooman, H. Biermann, J. Mola, Microstructure and mechanical properties of Al-alloyed Fe–Cr–Ni–Mn–C stainless steels, Mater. Sci. Eng. A 618 (2014) 46-55.
21
[24] Y. Yamamoto, M.P. Brady, Z.P. Lu, P.J. Maziasz, C.T. Liu, B.A. Pint, K.L. More, H.M. Meyer, E.A. Payzant, Creep-resistant, Al2O3-forming austenitic stainless steels, Science. 316 (2007) 433–436. [25] Y. Yamamoto, G. Muralidharan, M.P. Brady, Development of L12-ordered Ni3(Al,Ti)-strengthened alumina-forming austenitic stainless steel alloys, Scripta Mater. 69 (2013) 816-819. [26] M.P. Brady, Y. Yamamoto, M.L. Santella, B.A. Pint, Effects of minor alloy additions and oxidation temperature on protective alumina scale formation in creep-resistant austenitic stainless steels, Scripta Mater. 57 (2007) 1117–1120. [27] X.Q. Xu, X.F. Zhang, X.Y. Sun, Z.P. Lu, Effects of silicon additions on the oxide scale formation of an alumina-forming austenitic alloy, Corros. Sci. 65 (2012) 317-321. [28] D.J. Young, High temperature oxidation and corrosion of metals, first ed. Elsevier, London, 2008. [29] B. Pujilaksono, T. Jonsson, H. Heidari, M. Halvarsson, J.E. Svensson, L.G. Johansson, Oxidation of binary FeCr alloys (Fe–2.25Cr, Fe–10Cr, Fe–18Cr and Fe–25Cr) in O2 and in O2 + H2O environment at 600 °C, Oxid. Met. 75 (2011) 183–207. [30] R.E. Lobnig, H.P. Schmidt, K. Hennesen, H.J. Grabke, Diffusion of cations in chromia layers grown on iron-base alloys, Oxid. Met. 37 (1992) 81-93.
22
[31] X.Q. Xu, X.F. Zhang, G.L. Chen, Z.Q. Lu, Improvement of high-temperature oxidation resistance and strength in alumina-forming austenitic stainless steels, Mater. Lett. 65 (2011) 3285–3288. [32] I. Saeki, H. Konno, R. Furuichi, T. Nakamura, K. Mabuchi, M. Itoh, The effect of the oxidation atmosphere on the initial oxidation of type 430 stainless steel at 1273K, Corros. Sci. 40 (1998) 191-200. [33] J.Zurek, D.J. Young, E. Essuman, M.H änsel, H.J. Penkalla, L. Niewolak, W.J. Quadakkers. Growth and adherence of chromia based surface scales on Ni-base alloys in high- and low-pO2 gases. Mater. Sci. Eng. A 477 (2008) 259-270. [34] C. Wagner, Passivity and inhibition during the oxidation of metals at elevated temperatures. Corros. Sci. 5 (1965) 751-764. [35] M.P. Brady, Y. Yamamoto, M.L. Santella, L.R. Walker, Composition, microstructure, and water vapor effects on internal/external oxidation of alumina-forming austenitic stainless steels. Oxid. Met. 72 (2009) 311–333.
Figure Captions: 23
Figure 1. Intermittent oxidation kinetics of the CSS and the AFA alloys in O2 + 10% H2O at 800 °C: (a) long-term, (b) short-term Figure 2. (a,c) GI-XRD and (b) XRD patterns of the CSS and the AFA alloys after oxidation in O2 + 10% H2O at 800 °C for different times (1-substrate, 2-(Cr,Fe)2O3, 3-Mn(Fe,Cr)2O4, 4-Fe2O3, 5-Ni(Fe,Cr)2O4, 6-Al2O3, 7-MnO2) Figure 3. SEM surface morphologies of the CSS alloy after oxidation in O2 + 10% H2O at 800 °C for: (a) 5 h, (b) 10 h, (c) 15 h, (d) 20 h, (e) 40 h Figure 4. SEM surface morphologies of the AFA alloy after oxidation in O2 + 10% H2O at 800 °C for : (a) 5 h, (b) 10 h, (C) 15 h, (d) 20 h, (e) 100 h Figure 5. Cross-sectional microstructures of the CSS alloy after oxidation in O2 + 10% H2O at 800 °C for: (a) 5 h, (b) 10 h, (c) 15 h, (d) 20 h and EDS line scan along arrows direction in (b) and (d) Figure 6. TEM image and EDS mapping of oxide scale formed on the CSS alloy after 5 h oxidation in O2 + 10% H2O at 800 °C Figure 7. TEM image and EDS mapping of oxide scale formed on the AFA alloy after 5 h oxidation in O2 + 10% H2O at 800 °C Figure 8. TEM image and EDS mapping of oxide scale formed on the AFA alloy after 10 h oxidation in O2 + 10% H2O at 800 °C Figure 9. Schematic diagram illustrating oxidation of the CSS alloy in O2 + 10% H2O at 800 °C Figure 10. Schematic diagram illustrating oxidation of the AFA alloy in in O2 + 10% H2O at 800 °C 24
Figure 1. Intermittent oxidation kinetics of the CSS and the AFA alloys in O2 + 10% H2O at 800 °C: (a) long-term, (b) short-term
25
Figure 2. GI-XRD (a,c) and XRD (b) patterns of the CSS and the AFA alloys after oxidation in O2 + 10% H2O at 800 °C for different times (1-substrate, 2-(Cr,Fe)2O3, 3-Mn(Fe,Cr)2O4, 4-Fe2O3, 5-Ni(Fe,Cr)2O4, 6-Al2O3, 7-MnO2)
26
Figure 3. SEM surface morphologies of the CSS alloy after oxidation in O2 + 10% H2O at 800 °C for: (a) 5 h, (b) 10 h, (c) 15 h, (d) 20 h, (e) 40 h
27
Figure 4. SEM surface morphologies of the AFA alloy after oxidation in O2 + 10% H2O at 800 °C for : (a) 5 h, (b) 10 h, (C) 15 h, (d) 20 h, (e) 100 h
28
Figure 5. Cross-sectional microstructures of the CSS alloy after oxidation in O2 + 10% H2O at 800 °C for: (a) 5 h, (b) 10 h, (c) 15 h, (d) 20 h and EDS line scan along arrows direction in (b) and (d)
29
Figure 6. TEM image and EDS mapping of oxide scale formed on the CSS alloy after 5 h oxidation in O2 + 10% H2O at 800 °C
30
Figure 7. TEM image and EDS mapping of oxide scale formed on the AFA alloy after 5 h oxidation in O2 + 10% H2O at 800 °C
31
Figure 8. TEM image and EDS mapping of oxide scale formed on the AFA alloy after 10 h oxidation in O2 + 10% H2O at 800 °C
32
(I)
(Mn,Fe)-rich oxide Mn(Fe,Cr)2O4
Alloy surface Holes
(Cr,Fe)2O3 Kirkendall holes
Substrate
(Cr,Fe)2O3
(II) Holes Alloy surface
(Cr,Fe)2O3
Substrate
(III)
Mn(Fe,Cr)2O4
Ni(Fe,Cr)2O4
Holes Fe2O3 Mn(Fe,Cr)2O4
Alloy surface Gap
Substrate
(Cr,Fe)2O3 Ni(Fe,Cr)2O4
Figure 9. Schematic diagram illustrating oxidation of the CSS alloy in O2 + 10% H2O at 800 °C
33
(I)
Al2O3
Alloy surface
(Mn,Fe)-rich oxide (Cr,Fe)2O3
Substrate
(II)
Fe-rich oxide
Alloy surface
(Cr,Fe)2O3 Al-rich oxide
Substrate
(III) Fe-rich oxide
Alloy surface
(Cr,Fe)2O3
Substrate
Al-rich oxide
Figure 10. Schematic diagram illustrating oxidation of the AFA alloy in O2 + 10% H2O at 800 °C
34
Table 1 Compositions of the aluminum-containing austenitic (AFA) stainless steel and the contrast stainless steel (CSS) (wt%)
Fe
Ni
Cr
Al
AFA Bal
20
14.5
2.7
CSS
20.5
Bal
14.9
Mn
Mo
Nb
C
B
1.5
2.6
1
0.075 0.01
1.54
2.67
1.03
0.075
S
N
0.004
0.15 0.0031
0.01 0.004
0.15 0.0031
Table 2 Major compositions in selected area “C” in Fig. 7 and Fig. 8 (at %)
Fe
Ni
Cr
Fig. 7
11.23
2.21
28.09
Fig. 8
2.3
0.53
4.6
Al
Mn
O
19.66
1.32
36.62
0.13
66.13
25.74
35
P
S0010-938X(16)30763-6 http://dx.doi.org/doi:10.1016/j.corsci.2017.03.015 CS 7035
To appear in: Received date: Revised date: Accepted date:
15-9-2016 14-3-2017 15-3-2017
Please cite this article as: Zhongdi Yu, Minghui Chen, Changbin Shen, Shenglong Zhu, Fuhui Wang, Oxidation of an austenitic stainless steel with or without alloyed aluminium in O2+10% H2O environment at 800◦ C, Corrosion Sciencehttp://dx.doi.org/10.1016/j.corsci.2017.03.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Oxidation of an austenitic stainless steel with or without alloyed aluminium in O2 + 10% H2O environment at 800 °C Zhongdi Yu1,2, Minghui Chen*1, Changbin Shen2, Shenglong Zhu3, Fuhui Wang1,3
1
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education),
School of Material Science and Engineering, Northeastern University, Shenyang 110819, China 2
College of Materials Science and Engineering, Dalian Jiaotong University, Dalian
116021, China 3
Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese
Academy of Sciences, Shenyang 110016, China *E-mail: [email protected]; [email protected]
Tel: +86-24-83691562; fax: +86-24-23893624.
1
Research highlights 1. Minor Al addition increases substantially oxidation resistance of stainless steel 2. Scale on the Al-free steel becomes porous and spalls off after 20 h oxidation 3. Scale on the Al-containing steel is dense and protective for 100 h oxidation 4. A thin Al-rich inner layer forms at the interface of scale/Al-containing steel 5. The Al-rich layer inhibits formation of holes and ensures the compactness of scale
Abstract
Oxidation behavior in O2 + 10% H2O at 800 °C of an austenitic stainless steel with or without aluminum is investigated. The Al-free steel enters into breakaway oxidation stage after only 15 h exposure, while the one with 2.7 wt% aluminum shows high oxidation resistance. This minor amount of aluminum is lower than the critical content to form an intact alumina scale, but it promotes the formation of a thin Al-rich inner layer which is favor in prohibiting holes formation due to volatilization of CrO2(OH)2 and Kirkendall effect. A compact scale develops and protects the Al-containing steel very well.
Keywords: A. austenitic stainless steel; A. High temperature water vapor; C. oxidation
2
1. Introduction Water vapor is generally found in hot oxidant environment of many industries, e.g. power generation, petrochemical, and aerospace sectors [1-4]. The presence of water vapor main influences the process of high temperature oxidation of metals and alloys by modifying structure characteristic of the growing oxide scale and the oxidation kinetics [5-9]. Generally, oxide scales formed in wet high temperature are porous and multiphase. Meanwhile, some volatile metal hydroxides may accelerate consumption of the protective oxides [10, 11]. So, oxidation and protection of metals or alloys served at high temperatures with water vapor becomes increasingly important and is now attracting more and more attention. In the presence of water vapor, ion-based ferritic or austenitic stainless steels can serve safely below 600 °C by forming compact and protective oxide scale, thus showing wide applications in this environment [12, 13]. However, higher efficiency always requires higher operation temperature in many industrial fields, bringing in many challenges to the performance of materials, especially to oxidation [14]. The Cr-rich oxide scale which performs very well at temperature lower than 600 °C, will degenerate quickly and no longer protect the underlying stainless steel when the temperature rises to above 700 °C in the presence of water vapor. Generally, Cr2O3 and (Cr,Fe)2O3 may translate to some volatile oxy-hydroxides like CrO2(OH)2 in wet high temperature, which expedites the consumption of Cr [11-17]. In addition, water vapor accelerates diffusion of oxygen along the Cr2O3 grain boundary by the 3
penetration of hydrogen, further promoting the depletion of Cr [18, 19]. So these Cr-rich oxides can’t exist steadily at high temperature with water vapor. It is well known that alumina is more stable than chromia in wet high temperature atmosphere, so it is meaningful to promote formation of an alumina scale on stainless steels in terms of enhancing oxidation resistance [20, 21]. An appropriate method is to add moderate amount of aluminum to stainless steels during smelting process. Generally, the critical content of aluminum is 4-5 wt % for the growth of an external alumina scale on stainless steel depending on its grain size and composition [22]. However, such high amount of Al greatly affects the creep strength of the austenitic matrix, because of the very strong body-centered cubic (bcc)–stabilizing effects of Al on Fe [23]. In order to combine high creep strength with high oxidation resistance, a new series of alumina-forming austenitic (AFA) stainless steel is developed by Oak Ridge National Laboratory [24-26], which develops an external alumina scale on oxidation in dry air. Through adding proper amount of austenitic-stabilizing elements (such as Ni) and an optimized preparing process, they successfully decreased the content of Al in an austenitic to 3.5 wt % that is still able to form a continuous alumina scale while keeps a moderate creep strength. Nevertheless, more than 3 wt % Al in AFA alloy increases the risk of NiAl phase precipitation, which consumes austenite stabilizer element, Ni [27]. Recently, Yamamoto et al. [24] found that an AFA alloy (based on Fe–20Ni–14Cr–2.5Al) shows high resistance to oxidation at 800 °C for as long as 2000 h in air with 10% water vapor, meanwhile possesses a long creep-rupture lifetime (exceeding 2000 h) at 750 °C under 100 MPa. This alloy 4
develops an external alumina scale on oxidation in dry air and shows very low oxidation rate at 800 oC. However, it is worthy to note that such a low content of aluminum is beneficial to high creep strength but does not ensure the formation of an external alumina scale in wet air. The interesting thing is that even in the absence of the protective aluminum scale, the oxide scale could still prevent the substrate from breakaway oxidation. This phenomenon is different from previous researches which ascribes the high oxidation resistance of these Al-containing alloys to the formation of an external alumina scale [27]. So the protection mechanism of this low Al-addition AFA alloy at high temperature with the existence of water vapor is different from the normal consideration. It is meaningful to further study the oxidation behavior, especially at the earlier stage, of this AFA alloy in this harsh environment. To elucidate the oxidation-resisting mechanism of this low Al alloyed in AFA alloy at high temperature in the presence of water vapor, a sequence of researches are involved in this paper. The alumina-forming austenitic (AFA) stainless steel researched in this paper is based on Fe–20Ni–14.5Cr–2.7Al, which is similar with a classical series of AFA alloy (Fe–20Ni–14Cr–2.5Al).The initial oxidation behavior of the AFA alloy is compared with a contrast stainless steel free of aluminum. Meanwhile, the effect of low Al alloyed on the protection mechanism of AFA is discussed.
2. Materials and experimental procedure 2.1 Materials The nominal compositions of the AFA stainless steel and the contrast stainless steel 5
(CSS) are listed in Table 1. Firstly, commercially-pure elements were mixed. Then, alloy ingots were prepared by arc melting. Finally, they were cast into an Φ30 × 70 mm copper mould. After that, these alloys were homogenized at 1200 °C for 2 h and cooled in air. Samples were cut into 15 × 12 × 2 mm by electrical discharge machining, with surface grounded to a final 1000# SiC papers and edges rounded. Before oxidation test all the samples were degreased by distilled water, acetone and alcohol within an ultrasonic cleaner.
2.2 Oxidation experiment Intermittent oxidation was conducted for the AFA and the CSS alloys in O2 + 10% water vapor at 800 °C for 100 h. Their weight change was discontinuous recorded every 20 h exposure. Initial oxidation at the early 20 h was investigated in detail by Steam-TGA, where their weight change was recorded in situ. Though the oxidation time in this work is short, the former (100 h oxidation) is named as long-term oxidation and the latter (20 h) is short-term oxidation for comparison hereafter. Three parallel specimens were tested for each group. During oxidation, samples were all set in an alumina tube which was heated by a resistively tube furnace. In this alumina tube the flowing water vapor is controlled at a rate of 100 cc/min. In order to avoid electrochemical corrosion between different metals, samples were hung by alumina wire rather than by alloy wire. When the sample is weighed during long-term oxidation test, some oxides may peel off the substrate during cooling process. So the kinetics of this sample only reveals its oxidation tendency, but not for calculation.
2.3 Characterization 6
Phase constituent was characterized by X-ray diffraction and/or grazing incidence XRD (X’Pert PRO, PANalytical Co., Almelo, Holland, Cu Ka radiation at 40 kV). The GI-XRD patterns were recorded in 2θ range 10–90°, and a step-scanning mode was employed with a step size of 0.5°. Morphology, microstructure and composition were examined by scanning electron microscopy (SEM, Inspect F 50; FEI Co., Hillsboro, OR) with an energy dispersive X-ray spectroscopy (EDX, X-Max; Oxford Instruments Co., Oxford, UK.) and transmission electron microscopy (TEM, JEM-2100F; JEOL Co., Tokyo, Japan) in conjunction with energy-dispersive X-ray spectroscopy.
3. Result 3.1 Oxidation kinetics and phase constituents Figure 1 shows intermittent oxidation kinetics of the CSS and the AFA alloys in O2 + 10% H2O atmosphere at 800 °C. For the CSS alloy, it lost weight dramatically after 40 h exposure, which makes its weight change invalid. So its oxidation was stopped thereafter. For the AFA alloy, its kinetics roughly follows parabolic law during the whole 100 h oxidation (Fig. 1a). It is of great importance to study comparatively the initial oxidation behavior of these two alloys since oxide scale of the CSS alloy has spalled off after 20 h oxidation. So the precise Steam-TGA was applied. It is found that the oxidation kinetics of the CSS alloy shows three characteristic periods, including: (i) fast oxidation period at the first 5 h; (ii) platform period from 5 h to 15 h with an almost zero mass gain and (iii) the second fast oxidation period from 15 h to 20 h. It indicates that breakaway oxidation takes place at the point of 15 h oxidation 7
and this is not revealed by the long-term oxidation kinetics (Fig. 1a). In fact, the difference in mass gain during the initial 20 h oxidation between the long-term (Fig. 1a) and the short-term (Fig. 1b) oxidation test of the CSS alloy hints that breakaway oxidation should happen at this stage as well. For the continuous oxidation test (short-term oxidation), all the samples are weighed in time at high temperature within the TGA set, while samples have to be taken out of the furnace and cooled down to room temperature for weight measurement for the discontinuous oxidation. At such a case, scale spallation of the CSS alloy oxidized for 20 h should occur during its cooling process and this contributes to its lower mass gain in the discontinuous oxidation than it obtained by the Steam-TGA. For the AFA alloy, no spallation occurs during the initial 20 h oxidation and the growth rate of oxides on it is orders of magnitudes lower than that on the CSS alloy. Figure 2 shows GI-XRD and XRD patterns of the CSS and the AFA alloys after oxidation. For the CSS alloy, its oxide scale is very thin after the first 20 h oxidation. XRD peaks of the oxides are very hard to detect as shown in Fig. 2b. GI-XRD reveals that spinel-type oxide (Mn(Fe,Cr)2O4) and (Cr,Fe)2O3 are the two major phases (Fig. 2a). After 40 h oxidation, XRD patterns show that Fe2O3 and Ni(Fe,Cr)2O4 are the two major phases then. For the AFA alloy, its oxide scale is very thin even after the whole 100 h oxidation. Thus only GI-XRD patterns are present here. As shown in Fig. 2c, the oxide scale is mainly consisted of (Cr,Fe)2O3 with a small amount of MnO2 within the initial 20 h oxidation. When its exposure time is extending to 100 h, a few Al2O3 can be detected except for the above two oxides. 8
3.2 Surface morphology Figure 3 shows morphology of the CSS alloy after oxidation for different times in O2 + 10% H2O at 800 °C. Initially, fast-growing (Fe,Mn)-rich particulate oxides are detected on surface, which are then gradually transformed to oxide clusters at a diameter of 15 μm (Fig. 3a and 3b). These cluster oxides are enriched with Cr in addition to Fe and Mn. According to the XRD patterns, they are Mn(Fe,Cr)2O4. With increasing oxidation time, those oxide clusters further grow to oxide nodules, upon which blade-like (Cr,Fe)-rich oxide becomes notable (Fig. 3c and d). They are (Cr,Fe)2O3 according to the XRD pattern. In addition, cracks begin to nucleate at the bottom of the nodules where holes have been formed. The holes develop quickly, leading to the final loose structure of the oxide scale (Fig. 3e). These loose oxides are lean in Cr and composed mainly of Fe and O as compared to (Cr,Fe)-rich blades. From this point of view, volatile Cr hydroxide has been formed at this stage, and its volatilization should account for the formation of holes. It is no doubt that the cracks and holes contribute mainly to the final spallation of the oxide scale. After 40 h oxidation, in most areas the outer scale spalls off and (Ni, Fe and Cr)-rich oxides grow on the spalled area as shown in Fig. 3e, which are Ni(Fe,Cr)2O4. For the AFA alloy, no macroscopic spallation of oxide occurs after 100 h oxidation as shown in Fig. 4. What’s more, oxide scale on the AFA alloy is so thin that original grinding traces can still be seen after 100 h oxidation. Initially, some protruding oxides are found (see Fig. 4a and b). These oxides are rich in Mn and Fe. Thereafter, (Fe,Cr)-rich oxide begins to grow and spread quickly on the alloy surface(see Fig. 4c 9
and d). Anyway, these oxide nodules still keep small size after 100 h oxidation (Fig. 4e).
3.3 Cross-sectional microstructure Figure 5 shows cross-sectional microstructure of the oxide scale formed on the CSS alloy within the earlier 20 h oxidation. Oxide scale is very thin for the first 5 h oxidation and can’t be differentiated precisely from the alloy surface just by SEM (Fig. 5a). After 10 h oxidation, the oxide scale has grown to a thickness of 4~10 μm and becomes uneven. As shown in Fig. 5b, a large amount of holes are observed within the oxide scale, which divide the scale into two sublayers, i.e. the inner and the outer one. At some places, some holes even connect with each other to form cracks parallel to the alloy surface. With further oxidation, holes and cracks merge together to form a big gap between these two sublayers. As shown in Fig. 5c and d, the development of holes and cracks leads to chipping of the scale. Element analysis shows that Cr is rich at the inner sublayer, lean at the central and then rich again at the outer sublayer after 10 h oxidation. The distribution of elements in the oxide scale after 20 h oxidation is similar with the result of 10 h. TEM was applied to investigate in detail the oxide scale formed on the CSS alloy after the first 5 h exposure. Bright field TEM image and EDS analysis are presented in Fig. 6. The oxide scale is only 70 nm thick, below which a big hole is observed. EDS mapping of this scale show that chromium is concentrated at the bottom while Fe and Mn is rich at the outer part. To be noted, Mn is especially rich at places, below which the big hole dwells. 10
For the AFA alloy, the bright field TEM images of the oxides formed after 5 and 10 h oxidation are shown in Figs. 7 and 8, respectively. According to the EDS maps, its scale is consisted of two layers after 5 h oxidation (Fig. 7). The outer layer consists mainly of Fe and Cr with a few Al. and the inner scale shows especially high content of Al. A detail EDS line scan across the oxide scale shows nearly 20 at% Al concentrated at the inner layer where other alloy elements greatly decrease with oxidation time (selected area “C” in Figs. 7 and 8). When oxidation extending from 5 h to 10 h, the sum content of the other alloy elements (except Al) at the alloy/oxide interface further decreases from 45 at% to less than 10 at% (as seen in Table 2).
4. Discussion In this paper, oxidation behavior of the AFA and the CSS alloys in O2+10% H2O at 800 C is investigated. For the AFA alloy, no macroscopic spallation occurs and mass gain is below 0.2 mg/cm2 after 100 h oxidation. While scales formed on the CSS alloy is not able to protect the substrate from fast oxidation in this harsh environment. Widespread spallation takes place after only 40 h oxidation. In order to explore out the oxidation mechanism, a short-term oxidation (20 h) is carried out by using Steam-TGA. In the following parts, breakaway oxidation mechanism of the CSS alloy and the protection mechanism of the AFA alloy will be discussed. Especially, the effect of Al addition on oxidation is studied. Finally, a model is built to explain the oxidation behavior of these two alloys comparatively.
4.1 Breakaway oxidation of the CSS alloy Though many studies have reported the breakaway oxidation of the conventional 11
CSS alloy [4-10], the underlying mechanisms are still unknown. During oxidation, Mn(Cr,Fe)2O4 and (Cr,Fe)2O3 oxides are initially formed on the CSS alloy. In general, these oxides can protect well the steel substrate from oxidation in dry oxygen atmosphere at 800 C. However, in the presence of water vapor these chromium oxides in (Cr,Fe)2O3 will react with water to form volatile chromium hydroxide (CrO2(OH)2), whose volatilization finally leads to the porous structure of the scale. Consequently, the initially formed blade-like (Cr,Fe)2O3 oxides gradually transform to Fe2O3 due to the Cr depletion. As shown in Figs. 3 and 5, the amount of Cr at the outer sublayer becomes less with increasing oxidation time from 10 h to 15 h while more and more holes develop there. To be noted, this stage where Cr depletes and holes develop quickly corresponds very well with the platform period in oxidation kinetics (Fig. 1b). The mass loss by volatilization of chromium hydroxide compensates with the mass gain by oxidation. However, the TEM image reveals a different formation mechanism of holes (as show in Fig. 6). After the first 5 h oxidation, the initially formed thin oxide scale can further be divided into two layers: the inner and the outer one. The inner scale is mainly composed of (Cr,Fe)2O3 and the outer one consists Mn(Cr,Fe)2O4. Due to the high affinity of Cr with O, chromium oxide forms firstly on the austenitic stainless steel. It possesses high oxidation resistance as chromium and oxygen ions diffuse difficulty in it [28]. However, Fe and Mn ions have a large solubility and mobility in chromia. Thus oxides of Fe and Mn will develop then upon the initially formed chromium oxides. As compared to the corundum-type oxide (Cr,Fe)2O3, defects and 12
vacancies are more in the spinel-type oxide Mn(Cr,Fe)2O4 which thus grows at a high speed [29]. Especially, the diffusion rate of manganese in chromia is higher than Fe and other alloy elements [30]. Because of the fast consumption of Mn, Kirkendall holes are formed beneath the oxide scale. This is the reason why the oxide scale exactly upon the holes is especially rich in Mn (Figs. 5 and 6). No matter what mechanism is dominating, these holes contribute mainly to the final spallation of the oxide scale. Besides, these two mechanisms may co-work. On the one hand, gas phase transfers rapidly in initial formed Kirkendall holes, where vacancies are easy gathered within them. On the other hand, the volatilization of CrO2(OH)2 leads high oxygen pressure within this porous layer. Therefore, many spinel-type oxides (Mn(Cr,Fe)2O4) are formed at high oxygen pressure. With more vacancy in spinel’s lattice, gas phase reaction occurs more quickly within it, which leaves a plenty of cation vacancies in this layer. With oxidation continuing, the buried holes connect with each other to form parallel cracks and even big gaps, separating the oxide scale into several sublayers and leading to the final spallation of scale. Breakaway oxidation occurs after the spallation of scale at the point of 15 h exposure. This oxidation stage is correlated with the formation of Fe2O3 and Ni(Fe,Cr)2O4 oxides as shown in Figs. 2 and 3.
4.2 Protection mechanism of the AFA alloy Based on the designing principle of alumina-forming austenitic (AFA) stainless steel, 2.7 wt% Al is lower than the critical level to form continuous external alumina scale. Generally, to achieve this alumina layer in such harsh environment more than 3 13
wt% Al should be alloyed [31]. However, in our research such a low addition of Al affects significantly the oxidation behavior of stainless steel. In the following parts the protection mechanism is discussed. Initially, oxygen pressure is high enough for all the alloy elements to be oxidized, but the content of each element and the growth rate of each oxide are different. Considering the percentage and their affinity to oxygen of the alloy elements comprehensively, the external oxide scale is major consisted of (Cr,Fe)2O3 with a small amount of MnO2 and Al2O3. Initially formed oxides reduce the oxygen pressure greatly at the alloy surface, promoting the selective oxidation of Al beneath them and the consequent formation of a thin Al-rich inner layer. This is agreed well with the oxidation kinetics (Fig. 1), and with the distribution of alloy elements in scale after different oxidation times. Within the inner Al-rich layer (marked as “C” in EDS line scan in Figs.7 and 8) , the total content of other alloy elements (except Al) is about 45 at% after 5 h oxidation, while it decreases to less than 10 at% after 10 h oxidation. In addition, a few of aluminum oxides diffuse into the initially formed (Cr,Fe)2O3. So Al rich oxides are detected by XRD within the outer (Cr,Fe)2O3 scale. Due to the blocking effect of the inner thin Al-rich layer, further oxidation of Mn and Fe is dramatically slowed down, thus no spinel-type oxides are detected in scale formed on the AFA alloy even after 100 h oxidation by GI-XRD (Fig. 2). Thermodynamically, the formation of Mn(Fe,Cr)2O4 is related to the solid state reactions [32]:
MnO+Fe2O3 MnFe2O4
(1) 14
MnO+Cr2O3 MnCr2O4
(2)
Zurek et al. [33] has reported that the effect of Mn addition on the growth rate of corundum-type oxide is various with oxygen pressure. The above-mentioned reactions occur only at high oxygen pressure, where Mn ions have high solubility in the corundum-type oxide. Once the spinel-type oxide forms, the ion diffusion or growth rate increases. However, at low pressure of oxygen, the limit solubility of Mn doesn’t support these reactions to take place and the oxides still maintain their corundum-type. This explanation may also suitable for explaining oxidation process of the AFA alloy. Because the compact Al-rich inner layer significantly decreases oxygen pressure and reduces solubility of Mn in oxides, the solid reactions are prohibited. In the absence of the Cr-rich oxide and the fast-growing Mn(Fe,Cr)2O4 oxide, holes formation due to volatilization of chromium hydroxide or Kirkendall effect is then prohibited. The dense oxide scale which is stable at this O2+10% H2O environment protects well of the steel substrate from oxidation. In conclusion, the protection mechanism of this low-Al content AFA alloy relies on the formation of a dense Al-rich layer between the oxide scale and the steel substrate. Due to that the content of alloyed Al is not sufficient for the transformation of internal growth alumina to external, only an Al-rich inner layer left. However, it slows down the diffusion of Cr and Mn from the substrate to the scale surface. The formation of spinel-type Mn(Fe,Cr)2O4 and volatile chromium hydroxide, as well as the widespread holes, are prohibited.
4.3 Oxidation modeling 15
4.3.1 For the CSS alloy Oxidation process of the CSS alloy is schematically illustrated in Fig 9. Three periods are identified, i.e. fast oxidation period, platform period, and breakaway period. They are marked as ‘‘I’’, ‘‘II’’ and ‘‘III’’ in the illustration, respectively. During period “I”, original surface is covered by a thin (Cr,Fe)2O3 inner layer, above which a spinel-type [Mn(Cr,Fe)2O4] intermediate layer and some (Fe,Mn)-rich top oxides develop. In the presence of water vapor, chromium hydroxide CrO2(OH) forms as a consequence of inter-reaction between chromium oxide and water vapor, and its volatilization gradually leads to a porous microstructure of the oxide scale during period ‘‘II’’. Simultaneously, fast growth of the Mn-rich spinel oxides results in Kirkendall holes in steel immediately beneath the inner (Cr,Fe)2O3 layer. They connect with each other to form a gap at the original alloy surface. Once the gap has grown large enough, peeling of the oxide scale takes place. Thereafter, breakaway oxidation starts and oxidation enters into the period ‘‘III’’ accompanied with macroscopic spallation. At spallation area, Ni(Fe,Cr)2O4 oxides form. In addition, Cr depletion results in the transformation of the blade like (Cr,Fe)2O3 to porous Fe2O3. 4.3.2 For the AFA alloy For the AFA alloy, its oxidation process can also be divided into three periods, i.e. fast nucleation period, selective oxidation period, and stable oxidation period. They are marked as ‘‘I’’, ‘‘II’’ and ‘‘III’’, respectively. During period ‘‘I’’, oxide formed on the surface is mainly (Cr,Fe)2O3, with a small amount of MnO2 and Al2O3.This oxide scale reduces the oxygen pressure greatly at the alloy surface, promoting the selective 16
oxidation of Al beneath them, which belongs to the selective oxidation period (period ‘‘II’’). The third-element effect, first proposed by Wagner also accounts for this oxidation [34]. Generally, the third element “Cr” acts as an ‘‘oxygen getter’’ in Fe-Cr-Al alloy. These chromium oxides limit oxygen diffused inward efficiently that quite a few Al is oxidized initially, which also reduce the critical level of Al for the transition from internal to external oxidation. In this paper, although these chromium oxides gather the oxygen efficiently, insufficient Al can support the transition of alumina. Meanwhile, the presence of water vapor also increases the critical level of Al than in dry oxygen [35]. So an Al-rich inner layer which is not exclusively composed of Al2O3 left. Anyway, it is just because of such an Al-rich inner layer, the diffusion of Cr and Mn ions from alloy to the outer surface is mostly reduced. The formation and subsequent volatilization of hydroxide CrO2(OH)2, as well as the formation of Kirkendall holes, are prohibited. At last, the dense and adherent oxide scale provides high oxidation resistance for the underlying AFA alloy in this wet environment and oxidation enters into the third period: stable oxidation. From this point of view, Al addition in a minor content, lower than the critical one to support forming an external alumina scale, still increases largely the oxidation resistance of steel in wet environment. This positive effect relies on the formation of Al-rich inner layer, which prohibits formation of holes and ensure a dense oxide scale.
5. Conclusion From above study, oxidation behavior of austenitic stainless steel with or without 17
aluminum in O2 + 10% H2O at 800 °C is investigated. The following conclusions can be drawn: (1) In the atmosphere of O2 + 10% H2O at 800 C, the Al-free steel (CSS) enters into breakaway oxidation after only 15 h exposure, while the one containing 2.7 at% aluminum (AFA) shows high oxidation resistance during the whole 100 h oxidation. (2) Initially, oxide scale formed on the CSS alloy is consisted of an outer Mn(Fe,Cr)2O4 layer and an inner (Cr,Fe)2O3 one. Thereafter, Fe2O3 and Ni(Fe,Cr)2O4 develops once breakaway oxidation takes place. For the AFA alloy, its oxide scale is consisted of the outer (Cr,Fe)2O3 layer and inner Al-rich oxide layer. (3) Holes formed due to Kirkendall effect and volatilization of chromium hydroxide (CrO2(OH)2) results in the porous structure and final spallation of oxide scale on the CSS alloy. (4) The compact Al-rich oxide inner layer on the AFA alloy significantly decreases the oxygen pressure and slows down the fast diffusion of Cr and Mn from the substrate to the scale surface. So the formation of spinel-type Mn(Fe,Cr)2O4 and volatile chromium hydroxide, as well as the widespread holes, are prohibited.
Acknowledgements This project is financially supported by National Natural Science Foundation of China (No.51671053), the Youth Innovation Promotion Association CAS under Grant No. 2016178, and by the Fundamental Research Funds for the Central Universities under Grant No. N160205001. 18
References [1] S.R.J. Saunders, M. Monteiro, F. Rizzo, The oxidation behavior of metals and alloys at high temperatures in atmospheres containing water vapor: a review, Prog. Mater. Sci. 53 (2008) 775-837. [2] I.G. Wright, R.B. Dooley, A review of the oxidation behavior of structural alloys in steam, Int. Mater. Rev. 55 (2010) 129–167. [3] K.I. Choudhry, S. Mahboubi, G.A. Botton, J.R. Kish, I.M. Svishchev, Corrosion of engineering materials in a supercritical water cooled reactor: Characterization of oxide scales on Alloy 800H and stainless steel 316, Corros. Sci. 100 (2015) 222-230. [4] T. Fujii, K. Tohgo, A. Kenmochi, Y. Shimamura, Experimental and numerical investigation of stress corrosion cracking of sensitized type 304 stainless steel under high-temperature and high-purity water, Corros. Sci. 97 (2015) 139-149. [5] M. Nezakat, H. Akhiani, S. Penttilä, S.M. Sabet, J. Szpunar, Effect of thermo-mechanical processing on oxidation of austenitic stainless steel 316L in supercritical water, Corros. Sci. 94 (2015) 197-206. [6] M. Calmunger, R. Eriksson, G. Chai, S. Johansson, J. J. Moverare, Surface phase transformation in austenitic stainless steel induced by cyclic oxidation in humidified air, Corros. Sci. 100 (2015) 524-534. [7] J. Ehlers, D.J. Young, E.J. Smaardijk, A.K. Tyagi, H.J. Penkalla, L. Singheiser, J. Quadakkers, Enhanced oxidation of 9% Cr in water vapor containing environment, Corros. Sci. 48 (2006) 3428-3454. 19
[8] N.K. Othman, J.Q. Zhang, D.J. Young, Temperature and water vapour effects on the cyclic oxidation behavior of Fe–Cr alloys, Corros. Sci. 52 (2010) 2827-2836. [9] H. Asteman, J.E. Svensson, L.G. Johansson, Evidence for chromium evaporation influencing the oxidation of 304L: the effect of temperature and flow rate, Oxid. Met. 57 (2002) 193–216. [10] J.T. Yuan, W. Wang, S.L. Zhu, F.H. Wang, Comparison between the oxidation of iron in oxygen and in steam at 650–750 °C, Corros. Sci. 75 (2013) 309-317. [11] E.J. Opila, D.L. Myers, N.S. Jacobson, I.M.B. Nielsen, D.F. Johnson, J.K. Olminsky, M.D. Allendorf, Theoretical and experimental investigation of the thermochemistry of CrO2(OH)2(g), J. Phys. Chem. A. 111 (2007) 1971-1980. [12] J.T. Yuan, W. Wang, H.H. Zhang, L.J. Zhu, S.L. Zhu, F.H. Wang, Investigation into the failure mechanism of chromia scale thermally grown on an austenitic stainless steel in pure steam, Corros. Sci. 109 (2016) 36-42. [13] H. Astemana, J.E. Svenssonb, L.G. Johanssona, Oxidation of 310 steel in H2O/O2 mixtures at 600 °C: the effect of water-vapour-enhanced chromium evaporation, Corros. Sci. 44 (2002) 2635-2646. [14] G.R. Holcomb, High pressure steam oxidation of alloys for advanced ultra-supercritical conditions, Oxid. Met. 82 (2014) 271-295. [15] C.T. Fujii, R.A. Meussner, The mechanism of the high-temperature oxidation of iron-chromium alloys in water vapor, J. Electrochem. Soc. 111 (1964) 1215-1221. [16] D.J. Young, B.A. Pint, Chromium volatilization rates from Cr2O3 scales into flowing gases containing water vapor, Oxid. Met. 66 (2006) 137-153. 20
[17] T. Jonsson, B. Pujilaksono, H. Heidari, F. Liu, J.E. Svensson, M. Halvarsson, L.G. Johansson, Oxidation of Fe–10Cr in O2 and in O2 + H2O environment at 600°C: A microstructural investigation, Corros. Sci. 75 (2013) 326-336. [18] J.N. Shen, L.J. Zhou, T.F. Li. High Temperature oxidation of Fe-Cr alloys in wet oxygen, Oxid. Met. 48 (1997) 347-356. [19] H. Asteman, J.E. Svensson, L.G. Johansson, M. Norell, Indication of chromium oxide hydroxide evaporation during oxidation of 304L at 873 K in the presence of 10% water vapor, Oxid. Met. 52 (1999) 95-111. [20] H. Götlind, F. Liu, J.E. Svensson, M. Halvarsson, L.G. Johansson, The effect of water vapor on the initial stages of oxidation of the FeCrAl Alloy Kanthal AF at 900 °C, Oxid. Met. 67 (2007) 252-266. [21] H. Asteman, M. Spiegel, A comparison of the oxidation behaviors of Al2O3 formers and Cr2O3 formers at 700 °C - Oxide solid solutions acting as a template for nucleation, Corros. Sci. 50 (2008) 1734-1743. [22] G. Berthome´, E. N’Dah, Y. Wouters, A. Galerie, Temperature dependence of metastable alumina formation during thermal oxidation of FeCrAl foils, Mater. Corros. 56 (2005) 389-392. [23] R. Rahimi, B.C.D. Cooman, H. Biermann, J. Mola, Microstructure and mechanical properties of Al-alloyed Fe–Cr–Ni–Mn–C stainless steels, Mater. Sci. Eng. A 618 (2014) 46-55.
21
[24] Y. Yamamoto, M.P. Brady, Z.P. Lu, P.J. Maziasz, C.T. Liu, B.A. Pint, K.L. More, H.M. Meyer, E.A. Payzant, Creep-resistant, Al2O3-forming austenitic stainless steels, Science. 316 (2007) 433–436. [25] Y. Yamamoto, G. Muralidharan, M.P. Brady, Development of L12-ordered Ni3(Al,Ti)-strengthened alumina-forming austenitic stainless steel alloys, Scripta Mater. 69 (2013) 816-819. [26] M.P. Brady, Y. Yamamoto, M.L. Santella, B.A. Pint, Effects of minor alloy additions and oxidation temperature on protective alumina scale formation in creep-resistant austenitic stainless steels, Scripta Mater. 57 (2007) 1117–1120. [27] X.Q. Xu, X.F. Zhang, X.Y. Sun, Z.P. Lu, Effects of silicon additions on the oxide scale formation of an alumina-forming austenitic alloy, Corros. Sci. 65 (2012) 317-321. [28] D.J. Young, High temperature oxidation and corrosion of metals, first ed. Elsevier, London, 2008. [29] B. Pujilaksono, T. Jonsson, H. Heidari, M. Halvarsson, J.E. Svensson, L.G. Johansson, Oxidation of binary FeCr alloys (Fe–2.25Cr, Fe–10Cr, Fe–18Cr and Fe–25Cr) in O2 and in O2 + H2O environment at 600 °C, Oxid. Met. 75 (2011) 183–207. [30] R.E. Lobnig, H.P. Schmidt, K. Hennesen, H.J. Grabke, Diffusion of cations in chromia layers grown on iron-base alloys, Oxid. Met. 37 (1992) 81-93.
22
[31] X.Q. Xu, X.F. Zhang, G.L. Chen, Z.Q. Lu, Improvement of high-temperature oxidation resistance and strength in alumina-forming austenitic stainless steels, Mater. Lett. 65 (2011) 3285–3288. [32] I. Saeki, H. Konno, R. Furuichi, T. Nakamura, K. Mabuchi, M. Itoh, The effect of the oxidation atmosphere on the initial oxidation of type 430 stainless steel at 1273K, Corros. Sci. 40 (1998) 191-200. [33] J.Zurek, D.J. Young, E. Essuman, M.H änsel, H.J. Penkalla, L. Niewolak, W.J. Quadakkers. Growth and adherence of chromia based surface scales on Ni-base alloys in high- and low-pO2 gases. Mater. Sci. Eng. A 477 (2008) 259-270. [34] C. Wagner, Passivity and inhibition during the oxidation of metals at elevated temperatures. Corros. Sci. 5 (1965) 751-764. [35] M.P. Brady, Y. Yamamoto, M.L. Santella, L.R. Walker, Composition, microstructure, and water vapor effects on internal/external oxidation of alumina-forming austenitic stainless steels. Oxid. Met. 72 (2009) 311–333.
Figure Captions: 23
Figure 1. Intermittent oxidation kinetics of the CSS and the AFA alloys in O2 + 10% H2O at 800 °C: (a) long-term, (b) short-term Figure 2. (a,c) GI-XRD and (b) XRD patterns of the CSS and the AFA alloys after oxidation in O2 + 10% H2O at 800 °C for different times (1-substrate, 2-(Cr,Fe)2O3, 3-Mn(Fe,Cr)2O4, 4-Fe2O3, 5-Ni(Fe,Cr)2O4, 6-Al2O3, 7-MnO2) Figure 3. SEM surface morphologies of the CSS alloy after oxidation in O2 + 10% H2O at 800 °C for: (a) 5 h, (b) 10 h, (c) 15 h, (d) 20 h, (e) 40 h Figure 4. SEM surface morphologies of the AFA alloy after oxidation in O2 + 10% H2O at 800 °C for : (a) 5 h, (b) 10 h, (C) 15 h, (d) 20 h, (e) 100 h Figure 5. Cross-sectional microstructures of the CSS alloy after oxidation in O2 + 10% H2O at 800 °C for: (a) 5 h, (b) 10 h, (c) 15 h, (d) 20 h and EDS line scan along arrows direction in (b) and (d) Figure 6. TEM image and EDS mapping of oxide scale formed on the CSS alloy after 5 h oxidation in O2 + 10% H2O at 800 °C Figure 7. TEM image and EDS mapping of oxide scale formed on the AFA alloy after 5 h oxidation in O2 + 10% H2O at 800 °C Figure 8. TEM image and EDS mapping of oxide scale formed on the AFA alloy after 10 h oxidation in O2 + 10% H2O at 800 °C Figure 9. Schematic diagram illustrating oxidation of the CSS alloy in O2 + 10% H2O at 800 °C Figure 10. Schematic diagram illustrating oxidation of the AFA alloy in in O2 + 10% H2O at 800 °C 24
Figure 1. Intermittent oxidation kinetics of the CSS and the AFA alloys in O2 + 10% H2O at 800 °C: (a) long-term, (b) short-term
25
Figure 2. GI-XRD (a,c) and XRD (b) patterns of the CSS and the AFA alloys after oxidation in O2 + 10% H2O at 800 °C for different times (1-substrate, 2-(Cr,Fe)2O3, 3-Mn(Fe,Cr)2O4, 4-Fe2O3, 5-Ni(Fe,Cr)2O4, 6-Al2O3, 7-MnO2)
26
Figure 3. SEM surface morphologies of the CSS alloy after oxidation in O2 + 10% H2O at 800 °C for: (a) 5 h, (b) 10 h, (c) 15 h, (d) 20 h, (e) 40 h
27
Figure 4. SEM surface morphologies of the AFA alloy after oxidation in O2 + 10% H2O at 800 °C for : (a) 5 h, (b) 10 h, (C) 15 h, (d) 20 h, (e) 100 h
28
Figure 5. Cross-sectional microstructures of the CSS alloy after oxidation in O2 + 10% H2O at 800 °C for: (a) 5 h, (b) 10 h, (c) 15 h, (d) 20 h and EDS line scan along arrows direction in (b) and (d)
29
Figure 6. TEM image and EDS mapping of oxide scale formed on the CSS alloy after 5 h oxidation in O2 + 10% H2O at 800 °C
30
Figure 7. TEM image and EDS mapping of oxide scale formed on the AFA alloy after 5 h oxidation in O2 + 10% H2O at 800 °C
31
Figure 8. TEM image and EDS mapping of oxide scale formed on the AFA alloy after 10 h oxidation in O2 + 10% H2O at 800 °C
32
(I)
(Mn,Fe)-rich oxide Mn(Fe,Cr)2O4
Alloy surface Holes
(Cr,Fe)2O3 Kirkendall holes
Substrate
(Cr,Fe)2O3
(II) Holes Alloy surface
(Cr,Fe)2O3
Substrate
(III)
Mn(Fe,Cr)2O4
Ni(Fe,Cr)2O4
Holes Fe2O3 Mn(Fe,Cr)2O4
Alloy surface Gap
Substrate
(Cr,Fe)2O3 Ni(Fe,Cr)2O4
Figure 9. Schematic diagram illustrating oxidation of the CSS alloy in O2 + 10% H2O at 800 °C
33
(I)
Al2O3
Alloy surface
(Mn,Fe)-rich oxide (Cr,Fe)2O3
Substrate
(II)
Fe-rich oxide
Alloy surface
(Cr,Fe)2O3 Al-rich oxide
Substrate
(III) Fe-rich oxide
Alloy surface
(Cr,Fe)2O3
Substrate
Al-rich oxide
Figure 10. Schematic diagram illustrating oxidation of the AFA alloy in O2 + 10% H2O at 800 °C
34
Table 1 Compositions of the aluminum-containing austenitic (AFA) stainless steel and the contrast stainless steel (CSS) (wt%)
Fe
Ni
Cr
Al
AFA Bal
20
14.5
2.7
CSS
20.5
Bal
14.9
Mn
Mo
Nb
C
B
1.5
2.6
1
0.075 0.01
1.54
2.67
1.03
0.075
S
N
0.004
0.15 0.0031
0.01 0.004
0.15 0.0031
Table 2 Major compositions in selected area “C” in Fig. 7 and Fig. 8 (at %)
Fe
Ni
Cr
Fig. 7
11.23
2.21
28.09
Fig. 8
2.3
0.53
4.6
Al
Mn
O
19.66
1.32
36.62
0.13
66.13
25.74
35
P