Metal Forming

Effect of Oxide Scale Formation on the Behaviour of Cu in Steel during High Temperature Oxidation in O2-N2 and H2O-N2 atmospheres

Seong-Woo KIM1) and Hae-Geon LEE2)

1) Automotive Steel Products Research Group, POSCO Technical Research Laboratories, POSCO, Jeonnam 545-090, Korea 2) Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, San 31 Hyoja-dong Pohang 790-784 Korea.

Cu enrichment at the steel-scale interface and its migration from there was investigated during the heating of steel cast at 1200°C under various oxidizing conditions. The behaviour of Cu enrichment was found to be largely dependent on the morphology of oxide scale formed during oxidation. At the early stage of oxidation, Cu-rich phase formed and accumulated at the steel-scale interface in both O2-N2 and H2O-N2 atmosphere. However, as the oxidation proceeded, the enrichment was vastly different for each oxidizing atmosphere. In the caseof O2-N2 oxidation, an oxide layer was formed initially at the steel surface, but a gap was developed soon after at the steel-scale interface and grew in size, which practically separated the scale from the steel substrate. The scale layer formed under this condition was porous. The Cu-rich phase initially formed at the interface seemed to migrate to the scale layer, leaving no Cu-rich phase at the interface. In the case of H2O-N2 oxidation, however, the scale layer formed was dense and tightly attached to the steel surface, and the Cu rich-phase continued to accumulate at the interface. Regarding the behaviour of the Cu-rich phase formed at the interface, it is proposed withexperimental evidences that, when a gap forms at the steel-scale interface, it is the vaporization of Cu in the Cu-rich phase through the gap that brings Cu to the scale.

Keywords: steel, oxidation, copper, copper enrichment, scale

DOI: 10.2374/SRI08SP119; submitted on 10 June 2008, accepted on 12 August 2008

Introduction

From the viewpoint of resource recycling, an increased use of steel scrap in steelmaking has a great advantage. However, the use of steel scrap induces unwanted elements such as Cu, Ni and Sn that are difficult to remove, if not impossible, due to their thermodynamically stable nature in steel during the steelmaking and refining stage particularly. Especially, Cu tends to accumulate at the steel-scale interface by preferential oxidation of Fe during reheating for hot working. Once the Cu enrichment exceeds the solubility limit, the Cu-rich phase may precipitate and penetrate into the grain boundary of steel inducing surface cracking during hot working. On the other hand, if the Cu-enriched phase at the oxidation front moves to the oxide layer (scale) instead of penetrating into the grain boundaries of the steel matrix, the Cu-induced hot shortness can be avoided. In order to overcome this surface problem, a clear understanding of the enrichment and migration behaviour of Cu during oxidation of steel is essential. A number of investigations have been carried out to examine the effect of temperature, alloying elements and atmospheric conditions on the Cu behaviour during the surface oxidation of steel cast [1-5].

It has been reported [6] that the enrichment behaviour of Cu during oxidation was strongly dependent on the oxidizing condition of the gas phase: the Cu behaviour during oxidation under dry air was vastly different from that under LNG combustion gas. The difference in the Cu behaviour was attributed to the difference in the role of water vapour and oxygen. However, the mechanism of Cu enrichment and migration in particular has not been clearly elucidated.

The present study attempted to identify the mechanism of Cu enrichment at and migration from the steel-scale interface during oxidation with oxygen and with water vapour. Furthermore, it investigated the difference of Cu migration from the steel-scale interface under different oxidizing conditions, i.e., oxygen and water vapour.

Experimental

Steel containing 0.3% Cu was prepared with a high-frequency induction furnace (30 kW, 40 kHz) in Ar atmosphere. The compositions of the steel are shown in Table 1. The steel chemistry was analysed using a C/S spectrometry for carbon and sulphur, an N/O spectrometry for nitrogen and an ICP-AES for other elements. An ingot cast after homogenizing the steel melt was cut into pieces in rectangular shape to achieve the dimension of 15 10 2mm. The surface of each specimen was polished and cleaned ultrasonically in ethanol. The oxidation was carried out using a thermo-gravimetric analyser (TGA). The specimen was suspended by Pt wire, and the chamber was flushed and maintained inert atmosphere by flowing Ar gas purified by passing Mg chips at 450°C. The furnace was then heated to 1200°C. After stabilizing the temperature, the gas mixture of N2+O2 or N2+H2O was introduced, and the specimen was allowed to be oxidized for a predetermined length of time. The gas flow rate was

Table 1. Chemical composition of the Cu containing steel (in wt.%).

C Si Mn Cu S O

0.10 0.112 1.52 0.31 0.0071 0.010

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maintained at 300cc/min, and the flow of N2 and O2 was controlled by the mass flow controller while H2O vapour was generated by passing distilled water in the isothermal bath. The mass gain of the specimen was measured during oxidation, and the specimen was taken out and quenched in liquid nitrogen. The experimental conditions are listed in Table 2.

The oxidized specimens were observed using a scanning electron microscope (SEM) and an electron microprobe analysis (EPMA). The composition and element distribution were analyzed by the wave-length dispersive spectroscopy (WDS) and the energy dispersive X-ray spectra (EDS).

Results and Discussion

Behaviour of Cu during oxidation. Figure 1 shows BSE (Back Scattered Electron) images and EPMA analysis results of the specimens after being exposed to the

N2-O2 (20%) atmosphere at 1200°C for different lengths of time. It is clearly seen that the Cu-enriched phase appears at the metal-scale interface at the early stage of oxidation and then soon disappears as the oxidation proceeds. The oxidation product, namely the scale, grows in thickness with time, which consists of three layers: FeO, Fe3O4, and Fe2O3 in sequence from the metal side. The morphology and amount of scale depend on the exposure time. It is noted that in the early stage of oxidation (up to about 5 minutes), the FeO layer grows rapidly and the gap at the steel-scale interface begins to form. The gap continues to expand as the oxidation proceeds. The Fe3O4 and Fe2O3layers become thicker apparently at the expense of the FeO layer. It is also noted that the FeO layer becomes porous while the Fe3O4 and Fe2O3 layers show a condensed structure as seen in Figure 2. The Cu mapping in Figure 2 shows that particles of the Cu-enriched phase are mostly sitting at the grain boundaries of the oxides.

Figure 3 shows BSE images and EPMA analysis results of the specimen after oxidation in the H2O(20%)-N2atmosphere. It is clearly visible that the Cu-enriched phase forms at the early stage of oxidation and continues to exist at the metal-scale interface. Much of the Cu-enriched phase still remains at the metal-scale interface even after the oxidation for one hour. This behaviour of the Cu-enriched phase is distinctly different from that of O2-N2oxidation.

Figure 4 shows BSE images of the microstructure at the metal-scale interface after oxidation for an hour in the O2(20%)-N2 atmosphere (Figure 4a) and H2O(20%)-N2atmosphere (Figure 4b). The bright part in Figure 4b

Table 2. Atmosphere conditions and exposure times.

Exposure time / min Atmosphere condition 0.5 1 3 5 20 60

2% O2 - - - - -10% O2 - - -O2-N2

20% O2

5% H2O - - - - -12% H2O - - -H2O-N2

20% H2O -

Figure 1. EPMA results of specimens after oxidation for different exposure times in 20%O2-N2 at 1200°C.

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represents the Cu-enriched phase which contains about 94% Cu. As shown in Figure 4, the Cu enriched phase forms with oxidation in the H2O-N2 atmosphere but not with oxidation in the O2-N2 atmosphere.

The scale formation in the case of the H2O-N2atmosphere is also different from that in the case of the O2-N2 atmosphere. With the comparison of Figures 1 and 3, the differences can be summarized as follows: (1) The FeO layer forms and grows initially but soon

afterwards decreases and eventually disappears in the case of oxidation under the O2-N2 atmosphere, whereas the FeO layer continues to grow in the case of oxidation under the H2O-N2 atmosphere.

(2) The Fe3O4 layer grows with time apparently at the expense of FeO under the O2-N2 atmosphere, whereas under the H2O-N2 atmosphere it forms a very thin layer even after oxidation for an hour.

(3) The Fe2O3 layer forms and grows with time in the case of O2-N2 oxidation, but it does not form in the case of H2O-N2 oxidation. This is because the oxygen partial pressure of the given gas mixture of H2O (20%)-N2 is too low to form Fe2O3.

(4) A gap forms between the metal and the scale in the case of oxidation with the O2-N2 gas mixture, and it grows in width with time. On the other hand, the scale layer under the H2O-N2 atmosphere shows a tight adhesion to the metal, leaving virtually no gap in between. This happens irrespective of the H2O content and exposure time. The difference in the scale formation at the metal-scale interface is more clearly visible in Figure 5.

As it appears that the behaviour of Cu during oxidation of the steel is closely related to the morphology and structure of the scale layer, closer consideration of the

Figure 2. EPMA results of the scale formed after oxidation for 3 minutes in O2-N2,(a) the lower part of the FeO layer, (b) around the FeO-Fe3O4 interface .

Figure 3. EPMA results of specimens after oxidation for different exposure times in 20%O2-N2 at 1200°C.

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scale formation under different oxidation conditions is necessary.

Mechanism of scale formation. Figure 6 shows the results of mass gain during oxidation in various O2-N2 and H2O-N2 atmospheres at 1200°C. The figure shows a

distinctive difference in oxidation behaviour between oxygen and water vapour. In Figure 6a, the mass gain in the O2-N2 atmosphere increases rapidly with time at the initial stage, followed by a gradual retardation in mass gain. The initial oxidation rate increases with an increase in the oxygen content in the gas mixture. As the oxidation proceeds, the oxidation rate sharply decreases to a large extent: this phenomenon is clearly visible in the case of oxidation with 10% and 20% O2. This change in the rate suggests the shift of the rate-determining step of oxidation as the oxide layer grows. On the contrary, as shown in Figure 6b, the mass gain in the H2O-N2 atmosphere increases nearly in a linear manner with time up to an hour of exposure time. The oxidation rate increases with an increase in the H2O content in the gas mixture.

In the present system of oxidation, there will be three important steps which may influence the oxidation rate: (1) the gas phase mass transport, (2) the transport through the product layer and (3) the chemical reaction. If the oxidation is controlled by either step (1) or (3), the rate will follow a linear relationship with time. Furthermore, the rate should also be dependent on the concentration of the oxidant in the gas phase. On the other hand, if the oxidation is limited by step (2), the rate should show a parabolic relationship with time. In this case, the effect of the concentration of the oxidant on the rate will be dependent on whether the rate-determining step is the transport of the oxidant through the oxide layer or that of Fe. If the former is the case, the rate will be proportional to the concentration of the oxidant. If the latter is true, on the other hand, the rate will be independent of the concentration of the oxidant.

Abuluwefa et al. [7] reported that the initial oxidation rate of low carbon steel at 1000-1250°C in O2-N2atmosphere obeyed the linear rate relationship and claimed that the gas phase diffusion of oxygen was the rate-determining step. Some other researchers reported that oxidation kinetics of steel followed the parabolic rate relationship at 700-1250°C in air or oxygen [8]. As they examined the oxidation for long exposure times of several hours, the oxidation behaviour for the first few minutes during which the oxidation exhibits a linear relationship with time may have been ignored. Considering the above analysis and the experimental results shown in Figure 6, it is clear that the oxidation with the O2-N2 gas mixtures which shows a parabolic relationship except for the first few minutes is mainly governed by the mass transfer in the product layer. In all other cases in the present study, the major factor that controls the rate is the gas phase transport.

The Fe-O phase diagram in Figure 7 indicates the formation of three kinds of oxides, namely, wustite (FeO), magnetite (Fe3O4) and hematite (Fe2O3) when iron is exposed in the oxidation atmosphere at temperatures higher than 570oC. It is generally agreed that the growth rate of FeO is high due to its large defect concentrations, and its oxide is sufficiently plastic at high temperature to maintain a good contact with the receding metal surface [10]. Figure 8 is the schematic expression of the oxidation mechanism of iron with oxygen which has commonly been accepted [11, 12]. In the figure,

Figure 4. Micrograph of a cross section of the steel-scale inter-face of specimens oxidized for an hour. (a) In 20%O2-N2.

(b) In 20%H2O-N2.

Figure 5. BSE images of a cross section of specimens oxidizedfor an hour. (a) In 20%O2-N2, (b) in 20%H2O-N2.

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- At the Fe/FeO interface, Fe2+ ions form at the expense of the metallic Fe:

Fe = Fe2+ + 2e- (1)

- In the FeO layer, Fe2+ ions and electrons migrate outward through the iron vacancies and electron holes, respectively.

- At the FeO/Fe3O4 interface, FeO forms at the expense of Fe3O4:

Fe2+ + 2e- + Fe3O4 = 4FeO (2)

- Surplus Fe2+ ions and electrons left after the reaction (2) move through the Fe3O4 layer. Some Fe2+ ions may be further oxidized to Fe3+.

- At the Fe3O4/Fe2O3 interface, Fe3O4 forms at the expense of Fe2O3:

Fen+ + ne- + 4Fe2O3 = 3Fe3O4 (3)

where n is 2 for Fe2+ and 3 for Fe3+. At this interface, Fe2O3 may also form owing to O2- ions which have migrated into the Fe2O3 layer from the gas phase:

2Fe3O4 + O2- = 3Fe2O3 + 2e- (4)

(a) (b)

Figure 6. Mass gain of specimens during oxidation at 1200°C in various (a) O2-N2 and (b) H2O-N2 atmospheres.

Figure 7. Fe-O phase diagram [9].

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Figure 8. Mechanism of oxidation of iron with oxygen before gap formation at the Fe/FeO interface.

Figure 10. Mechanism of oxidation of iron with oxygen after gapformation at the Fe/FeO interface.

Figure 9. Change in thickness of each oxide layer with time in O2(20%)-N2 atmosphere at 1200°C.

Figure 11. Positions of the Pt marker after oxidation for onehour. (a) In 20%O2-N2. (b) In 20%H2O-N2.

- In the Fe2O3 layer O2- ions which have formed at the Fe2O3/gas phase boundary migrate inward, and electrons move outward to the Fe2O3/gas phase boundary. Migration of iron ions in the Fe2O3 is known difficult to occur [11].

- At the Fe2O3/gas phase boundary, oxygen ionizes with the help of electrons which have migrated through the Fe2O3 layer:

1/2O2 + 2e- = O2- (5)

As mentioned earlier, the growth rate of FeO is high; hence, the scale layer should be predominantly FeO. As can be seen in Figure 1 and Figure 9, which shows the change of the thickness of each oxide layer with time, however, this is the case only at the initial stage of oxidation. Initially the FeO layer sharply increases in its thickness with the thin Fe3O4 layer and the Fe2O3 layer is negligibly thin. Soon afterwards (3-5 mins. in the present study) the FeO layer begins to shrink. On the other hand, the Fe3O4 layer becomes thicker and the Fe2O3 layer also becomes appreciable. It is noted that a gap forms at the metal/FeO interface and it practically separates the two phases. It should also be noted that widening of the gap and shrinking of the FeO layer occur simultaneously. This

implies that they are interrelated to each other. It is generally considered that the FeO layer is sufficiently plastic at high temperature to maintain a good contact with the receding metal surface. However, the outward movement of iron ions from the metal through the scale to the reaction site can cause loss of the scale adhesion particularly when the reaction rate is rapid, and hence induce a gap at the metal/scale interface [11].

Once the gap forms, the oxidation mechanism should be altered due to the supply of iron ions becoming limited. Considering the above and the observation in the present study, it is suggested that the oxidation sequence given in Figure 10 be operative. Once the gap has been developed, it throttles the ionization reaction of iron by Eq. (1) at the metal/FeO interface by reducing the effective area of the reaction. However, the demand of iron ions at the Fe3O4/Fe2O3 interface, Eq. (3), continues to persist, and hence the reaction at the FeO/Fe3O4 interface reverses in its direction to cope with the demand:

4FeO = Fe3O4 + Fe2+ + 2e- (6)

At the same time, FeO facing the gap will dissociate into iron ions and oxygen as an effort to meet the local equilibrium:

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Figure 12. SEM image of (a) the inner surface of scale formed after oxidation for 3 min in 20%O2-N2 (A: area adhering to the metal surface, B: area receded from the metal surface). (b) The enlarged view showing Cu-rich particles attached preferentially to the area adhering to the metal surface.

Figure 13. Vaporization of Cu from the Cu-rich phase. The amountof Cu to be accumulated due to selective oxidation of Fe in thedifferent atmospheres s also included for comparison.

FeO = Fe2+ + 1/2O2 (g) + 2e- (7)

As a result, the FeO layer continues to shrink from both sides by the reactions of Eqs. (6) and (7), whereas the Fe3O4 and Fe2O3 layers grow in thickness. Furthermore, the oxygen generated by Eq. (7) is transported to the metal surface through the gap and reacts with the metal to produce the secondary layer of FeO, which is evidenced in Figures 1 and 5. The porous nature of the FeO layer as shown in Figure 2 indicates that the reaction of Eq. (7) should also occur through invading into grain boundaries of the FeO layer.

For the case of the oxidation with the H2O-N2 atmos-here, the initial oxidation rate is, as seen in Figure 6b, much lower than that in the O2-N2 atmosphere so that the gap can hardly form. Even if a gap forms, it will be healed by hydrogen migrated from the gas/oxide interface [13]:

- At the FeO surface: FeO + H2 = Fe2+ + 2e- + H2O (8)

- At the metal surface: Fe + H2O = FeO + H2 (9)

For these reasons, no gap forms in the case of oxidation with the H2O-N2 atmosphere as shown in Figure 3.

In order to more clearly understand as to whether the oxidation occurs at the metal/oxide interface or not, a set of oxidation experiments were carried out with a Pt wire marker initially placed at the metal surface. The result is given in Figure 11, in which the marker sits at the bottom of the scale layer for the case with the O2-N2 atmosphere (Figure 11a). However, it is located in the middle of the scale for the case with the H2O-N2 atmosphere (Figure 11b). This result confirms that the oxide layer grows outward only for the case of oxidation with O2, whereas the layer grows both outward and inward when oxidized with H2O. The reactions given by Eqs. (8) and (9) are considered responsible for the inward growth.

Mechanisms of Cu migration to the oxide scale. When a steel containing Cu is subjected to an oxidizing environ-ment, due to the difference in thermodynamic stability, Fe is preferentially oxidized and Cu is accumulated in the receding metal surface. When the accumulated Cu exceeds the solubility limit, the Cu-rich phase is precipitated. This is already seen in Figures 1, 3 and 4. Now a question arises as to how the Cu-rich phase precipitated at the metal surface interacts with the scale. Kondo [14] has suggested that the metallic Cu migrates from the metal/scale inter-face up to the Fe3O4/Fe2O3 interface, where it oxidizes to copper oxide. They observed a number of metallic Cu par-ticles at the grain boundaries of FeO and Fe3O4 phases, but not at the Fe2O3 layer. The results of the present study with the O2-N2 atmosphere tend to support their view as seen in Figures 1 and 2, but the results with the H2O-N2 atmos-phere shown in Figure 3 do not agree with their suggestion.

In order to understand the migration behaviour of Cu during oxidation in the O2-N2 atmosphere, the scale surface facing the gap was observed. The scale was carefully removed by separating it through the gap between the steel and scale. Figure 12 shows the SEM images of the scale surface of the gap-scale interface after oxidation for 3 minutes in the O2(20%)-N2 atmosphere. It is seen in Figure 12a that the surface consists of two different types of grains: grains apparently adhered to the metal surface (indicated by A) and grains apparently receded from the metal surface (indicated by B). When the scale surface is enlarged in Figure 12b, it is clearly visible that Cu-rich particles are found at the grains attached to the metal surface but not at the grains receded from the metal surface. The above arguments do not apply to the case of oxidation with the H2O-N2 atmosphere, in which the Cu-rich phase continues to accumulate and stay at the metal/scale interface as seen in Figure 3.

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The above observations lead to the following summary: The Cu-rich phase forms and accumulates at the steel-scale interface at the initial stage of oxidation. If the scale layer formed is dense and tightly attached to the steel surface (as in the case of H2O-N2 oxidation), the Cu-rich phase continues to accumulate and stay at the interface. On the other hand, if the scale layer formed at the interface is porous (as in the case of O2-N2), the Cu-rich phase formed at the interface migrates to the porous scale through pores, and then moves to the upper part of the scale though the grain boundaries. Now a question arises: By what mechanism does the Cu-

rich phase pass through the gap between the steel and scale in the first place? As an attempt to answer this question, the possibility of vapour phase transport of the Cu-rich phase was examined.

In order to confirm whether the vapour phase transport of Cu in the present system is feasible, a separate experiment was carried out to measure the rate of vaporization of Cu from a Cu-rich phase similar to what was found in the above oxidation studies. 3 g of Fe-Cu (94 wt%) was charged in an alumina crucible with the inner diameter of 8.0mm and the mass loss with time was measured using TGA at 1200oC. The result is seen in Figure 13. In the figure the amount of Cu to be accumulated due to selective oxidation of Fe in the different atmosphere is also included for comparison. It is remarkable to know that the Cu evaporation rate from the Cu-rich phase is much faster than the rate of Cu accumulation by the oxidation of steel. This implies that all the Cu precipitated due to metal oxidation can vaporize, provided that required conditions are met. The observations and comparisons given above strongly support the view that the vapour phase transport is the dictating mechanism of Cu migration to the scale layer. By adopting this view, the difference in the Cu migration

behaviour between O2-N2 and H2O-N2 oxidations observed in the present study can be systematically explained. (1) Cu migration in the case of O2-N2 oxidation (Refer to

Figure 14a)(a) As the oxidation proceeds, Cu accumulates at the

steel surface due to selective oxidation of Fe. (b) When the Cu saturation limit is reached, the Cu-

rich phase precipitates. (c) The amount of the Cu-rich phase increases as the

oxidation continues. At the same time the gap between the steel and scale forms and grows due to Fe diffusion through the scale.

(d) Cu in the Cu-rich phase vaporizes through the gap, and continues to move to the upper part of the scale layer utilizing pores and grain boundaries of the layer. As the Cu vaporization rate is faster than the rate of accumulation of Cu, the net amount of the Cu-rich phase at the steel surface decreases when the gap formation becomes appreciable. The Cu-rich phase is eventually exhausted.

(e) In this way the surface of steel is protected from Cu penetration through the grain boundaries of steel.

(f) The migration of Cu in the scale layer stops at the Fe2O3-Fe3O4 boundary where Cu oxidizes to Cu2O.

(2) Cu migration in the case of H2O-N2 oxidation (Refer to Figure 14b)(a) As oxidation proceeds, Cu accumulates at the

steel surface due to selective oxidation of Fe. (b) When the Cu saturation limit is reached, the Cu-

rich phase separates. (c) The amount of the Cu-rich phase increases as the

oxidation continues. (d) There is no gap formation, and hence the Cu-rich

phase is not possible to vaporize, but migrates to

Figure 14. Schematics of Cu enrichment behaviour and Cu migration mechanism during oxidation. (a) In O2-N2 atmosphere. (b) In H2O-N2 atmosphere.

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the scale layer through some pores. The overall rate of migration is slow due to insufficient means of migration.

(e) Therefore, the Cu-rich phase continues to accum-ulate at the interface as the oxidation proceeds.

(f) Eventually the Cu-rich phase penetrates into the grain boundaries of the steel.

Conclusions

The current study was conducted to identify the mechanism of Cu enrichment at the steel-scale interface and its migration from there during oxidation. The oxidation was carried out with the 0.3% Cu containing steel at 1200 C in various O2-N2 and H2O-N2 atmospheres. The oxidation kinetics, scale formation and behaviour of Cu enrichment and its migration were also investigated. From the present study, the following results were obtained:

The oxidation kinetics of 0.3% Cu containing steel depended on the oxidizing condition. In O2-N2 atmosphere, it obeyed the linear rate law at the initial stage but followed the parabolic rate law at later times. In H2O-N2atmosphere, the rate showed the linear relationship with time from the beginning.

At the initial stage of oxidation, Cu-rich phase formed and accumulated at the steel-scale interface due to the selective oxidation of Fe in both of O2-N2 and H2O-N2.

As the oxidation in O2-N2 proceeded, the scale layer formed became porous and was detached from the steel surface, and the Cu-rich phase formed at the interface migrated to the scale layer via vapour phase transport.

In the case of oxidation in H2O-N2 atmosphere the scale layer formed was dense and tightly attached to the steel surface, and the Cu-rich phase continued to accumulate at the interface because the vaporization of Cu-rich phase was suppressed by the tight adhesion of scale with steel.

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