Studies on Initial Stage of High Temperature Oxidation of Fe - 9 to 12%Cr Alloys in Water Vapour Environment

Akbar Kaderi1, Mohd Hanafi Ani1*, Sukreen Hana Herman2,

and Raihan Othman3

1Department of Manufacturing and Materials Engineering , Kulliyyah of Engineering, International Islamic University Malaysia (IIUM), Jalan Gombak, 53100, Kuala Lumpur, Malaysia.

2Centre for Electronic Engineering, Faculty of Engineering, Universiti Teknologi MARA, Shah Alam, 40450, Selangor, Malaysia.

3 Department of Sciences in Engineering, Kulliyyah of Engineering, International Islamic University Malaysia (IIUM), Jalan Gombak, 53100, Kuala Lumpur,Malaysia.

*mhanafi@iium.edu.my

Keywords: Fe - 9 to 12%Cr alloys, water vapor condition, initial stage oxidation, external oxide scale, internal oxidation, outer oxidation, internal oxidation zone, oxide scale, cyclic oxidation

Abstract Fe - 9 to 12%Cr alloys are a material for the thick sections boiler components and steam lines of a power plant. The role Fe - 9 to 12%Cr alloys is becoming more prominent in the development of a new generation of Ultra-Supercritical (USC) Power Plant due to the target operating temperature is reaching 620 °C (893 K), in 100% steam condition as well as pressure in excess of 300 bar (30 × 106 Pa). In such condition, the integrity of Fe - 9 to 12%Cr alloys relies on the oxide scale formed during the time of exposure. However due to the high temperature and water vapor condition, it is a well known fact that, the formation of oxide scale is accelerated thus depleting the structural integrity of the Fe - 9 to 12%Cr alloys over the time. Studies show that not only the formation of protective oxide scale was suppressed but the formation of non-protective oxide scale was accelerated instead. Decades of studies done by various groups around the globe has yet to have consensual on the exact mechanism of this phenomenon. Initial stage oxidation of these alloys plays great roles in hope to understand the formation of oxide scale in water vapor condition at high temperature. This paper reviews previous research works to understand the initial stage oxidation of Fe - 9 to 12%Cr alloys at high temperature in water vapor condition.

Introduction

The international research and development projects on advanced steam cycle power plants in Japan, USA and Europe beginning in late 1970s has triggered the development of various heat resistant steels [1]. Due to its low coefficient of thermal expansion (CTE), Fe - 9 to 12%Cr alloys has been the material of choice for the superheater and reheater of a steam power plant. Fe - 9 to 12%Cr alloys are also more cost competitive compared to other group of heat resistant steels. Thus the frequent start and stop cycles in order to follow load demands could benefit from the low CTE of this group of these alloys improving the component of a power plant from thermal stress [2]. Steels of Fe - 9 to 12%Cr alloys have been developed to operate in an ultra - supercritical pressure steam temperature of 903 K in 100% steam condition as well as pressure in excess of 30 × 106 Pa [3].

Due to the extreme environment of high steam pressure and high temperature of a power plant, the ability of the Fe - 9 to 12%Cr alloys to form the slow growing protective Cr2O3 scale was impeded. The formation of Fe3O4, Fe2O3, and Fe, Cr spinel which are inferior in terms of protectiveness was promoted instead. The rapid formation of these oxides increased the oxidation rate of these steels thus eventually reduce the service life of a superheater and the reheater themselves. Albeit the well known problems and active research engagement on this problems done for decades, the convincing reason for the problem to occur has yet to be concluded.

Advanced Materials Research Vols. 557-559 (2012) pp 100-107Online available since 2012/Jul/26 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.557-559.100

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The presence of water vapour in the environment caused the Fe - 9 to 12%Cr alloys have the tendency to promote the formation of void in the oxide scale. While different case will occur if the water vapour is absent from the environment. Studies on the initial stage oxidation give insight into the possibility of various formations of oxide scales, whether it will be a gas-tight type or void filled type of oxide scale. Fe - 9 to 12%Cr alloys both type of oxides have the possibility to occur if it is exposed in different environment. There is a need to look into the studies done by various researchers around the world on the initial stage oxidation of Fe - 9 to 12%Cr alloys. However an attempt to consolidate the findings by these various researchers has never been attempted before. This paper aims at understanding the initial stage oxidation in water vapour environment at high temperature of Fe - 9 to 12%Cr alloys done previously by other researchers.

Initial stage oxidation

In this review, the initial stage oxidation time up to 626.4 ks will be taken into consideration. Figure 1 show previous works arranged according to the time of oxidation. The works were on various types of commercial and model Fe - 9 to 12%Cr alloys. The temperature are varied but in a close range to each other; which is between 823 K to 1323 K. Works on the effect of water vapour on initial stage oxidation involving these steels will be discussed in the following sections.

Figure 1: A summary of work by various groups with respect to time of oxidation

Fe - 9 to 12%Cr alloys Oxide Scale Morphology

The formation of oxide scales is very much different for the Fe - 9 to 12%Cr alloys in water vapor environment than that of in dry environment. The multiple oxide scale has developed even at the initial stage oxidation of these groups of alloys. Various accounts on the complex structure of

Advanced Materials Research Vols. 557-559 101

these oxide scales have been studied for many years. The formation of multiple oxide scale is influenced by the increasing oxygen partial pressure, PO2 as the scale grows further from the Fe-Cr alloy. Figure 2 shows the Ellingham Diagram (constructed from thermodynamic data compiled by Kubaschewski [15] and Barin [16]) and the schematics of multiple oxide scale formed on Fe-Cr based alloys during the high temperature oxidation with the presence of water vapor. It was also shown in the Ellingham Diagram the temperature of interest studied by researchers, and the formation of multiple oxide scale at this particular temperature is related to schematics of the multiple oxide scale.

Figure 2: (a) The Ellingham Diagram for Fe2O3, Fe3O4, FeO, and Cr2O3 along with the temperature of interest by many studies; (b) The schematic of multiple oxide scale of Fe – 9 to 12%Cr alloys

oxidized in water vapor environment at high temperature.

Study on P91 (Fe – 9Cr) alloys by Ehlers et al. [9] reported that outer scale layer is consisting of Fe2O3 and Fe3O4. The inner layer is consisting of Fe3O4 with (Fe, Cr)3O4. Close to the alloy side there is a small existence of FeO appeared together with Fe3O4 and (Fe, Cr)3O4. The same identical layers arrangement also reported by Nakai et al. [14] for Fe – 10Cr - 0.08C oxidized in steam. Peraldi et al. [8] on their experiment on Fe – 10Cr alloys at temperature of 923 K and 1073 K in air + 10% water vapor shows as well the same oxide scale.

The oxide scale development

Ani et al.[4] confirmed that the oxide scale formation influenced by humid environment. They reported that for the transition from internal to external oxidation to occur, the Cr content of Fe-Cr alloys has to be increased. The transition occurred at 8 mass% Cr and 12 mass% Cr for dry

and humid environment respectively. The oxygen permeability, ��(�)�� as expressed in equation

(1) by Wagner[16].

�� =�

�( )

��

����� (1)

102 Advanced Materials and Processes II

where � is the thickness of IOZ, � is the oxidation time,��(�)

is the mole fraction of oxygen at the

metal surface, ��� is the Cr concentration and ��(�)�� is oxygen permeability. The parabolic

constant, ��, is directly proportionate to ��(�)�� . They discovered that the oxygen permeability in α

– Fe increased by a factor of 1.4 in humid compared to that in dry environment. However, it was observed that it is independent of water vapor pressure, PH2O. They proposed the ratio of oxygen permeability in humid and dry environment is about 1.2.

The formation and disappearance of internal oxidation zone (IOZ) on Fe-9Cr-0.26Si has been explained extensively by Ueda et al.[5]. The work has demonstrated the disappearance of IOZ within 100 ks of oxidation in steam (Ar-15%H2O). The reason for disappearance of IOZ is because of the amorphous SiO2 layer formed at the alloy/IOZ interface. SiO2 layer prevents the permeation of oxygen further into the alloy, thus increased the oxygen potential at the IOZ/SiO2 interface. It promotes the oxidation of the matrix in the IOZ; hence the IOZ disappears over time.

Based on these works, a summary on the formation of oxide scale in different environment can be represented as in Figure 3. It has been stated previously that the formation of continuous external scale (in Figure 3 (a)) will be observed in dry environment, if the Cr content is greater than 8 mass%. In humid environment however, the formation of internal oxide (in Figure 3 (b)) is observed instead, due to the increased Cr content needed to form external oxide. Figure 3 (c) validates the disappearance of internal oxidation is possible, if the oxygen potential at the IOZ/SiO2 increased over time.

Figure 3: Schematics representation of mechanism on (a) the formation of external oxide layer in dry environment on Fe-12Cr alloy [4]; (b) the formation of internal oxide zone in wet environment

on Fe-12Cr alloy [4]; and (c) the disappearance of internal oxide zone [5].

Advanced Materials Research Vols. 557-559 103

Void formation in oxide scale

The formation of gap contributes to the poor adherence of Fe2O3 to the rest of oxide scale. For ferritic 9-10%Cr steels oxidation in Ar-H2O environments, Żurek et al.[7] has discovered the formation of large gap during the first 18 ks of oxidation at 923 K. This has contributed to the formation of Fe2O3 layer along the most outer side of the scale. They have found a strong correlation of the time of gap healing with an increased in temperature. At 923 K the time of gap healing took a few hours while at 823 K the time of gap healing took more than several hundred hours.

The formation of void, gap and pore are common phenomena in Fe3O4 scale of Fe 9-10%Cr steels. It has been reported that higher quantity of void formed in the Fe3O4 scale grown on Fe substrate exposed in higher PO2 atmosphere (PO2 = 1.6 × 10-11 Pa) equivalent to H2/H2O ratio of 7 × 10-5 [18, 21]. Figure 4 is a compilation of quantitative studies on volume fraction of voids with respect to oxygen partial pressure, PO2. Studies by Kaderi et al. [19] on void formation during the oxidation of Fe-10Cr-10Ni at 1073 K in air + steam environment shows that the quantity of void increased up to 86% for inner scale and 76% for outer scale. The formations of voids in the outer scale tend to congregate at the interface of outer scale / inner scale. Similar observation has been confirmed by latest studies by Ueda et al. [20] on Fe-5Cr alloy at 773 K in a more controlled environment (Ar-H2-H2O gas mixture in which the PO2 was stabilized at 2.5 × 10-22 Pa).

Figure 4: A comparison of volume fraction of voids vs. oxygen partial pressure, PO2 by previous studies, all samples oxidized up to 86.4 ks.

Cyclic oxidation of Fe – 9 to 12%Cr alloys

Cyclic oxidation of Fe – 9 to 12%Cr alloys at high temperature has been studied by many researchers. Cyclic oxidation greatly affects the oxide scale adherence to the alloys thus initial stage of its oxidation is worth of a discussion. The cyclic oxidation of this group of alloys has been studied by Othman et al. [12] and Peraldi et al. [8].

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Both works covered 100 cycles of oxidation with 3 600 s of soaking time in the furnace for each cycle. Figure 5 (a) and (b) summarize the oxide morphology of Fe-10Cr (adapted from Peraldi et al.) and Fe-12Cr (adapted from Othman et al.) respectively.

The oxidized sample of Fe-10Cr (Figure 5 (a)) shows cavity formed initiated by the crack propagation. Cavities are also formed in the spinel oxide layer area. However the overall oxide scale morphology is nearly the same as the schematics shown in Figure 2 (b).

Oxide scale of Fe-12Cr (Figure 5 (b)) does not follow the schematics shown in Figure 2 (b). FeO is of outer scale while the inner scale is of FeO + Spinel and FeCr2O4. There are formations of voids in the vicinity of FeO + Spinel layer. Cracks are also shown to propagate from the outer layer through the inner layer.

The differences in oxygen partial pressure, PO2 might explain the differences of oxide morphology formed on each alloy. If air was considered, PO2 = 2.1 × 104 Pa, therefore Fe-10Cr was exposed at a higher PO2 than that of Fe-12Cr sample which was exposed at PO2 = 1.1 × 10-11 Pa. But one might need to consider the different composition of alloy (Peraldi’s 10Cr and Othman’s 12Cr) used for their work.

It is well known that during the cyclic oxidation, the Cr depletion will occur if the Cr content is low, thus Fe- rich oxides formation will be formed instead [12]. Moreover, the mechanical defect of the oxide scales, caused by the cyclic oxidation will caused the cracks, thus exposing the alloys surface. This mechanism will cause the formation of protective scales very difficult [9]. Finally, this will not contribute to the protective scale in the long – term.

Figure 5: Oxide Scale of (a) Fe-10Cr [8], and (b) Fe-12Cr [12] subjected to cyclic oxidation in humid condition.

Advanced Materials Research Vols. 557-559 105

Conclusion

The initial stage of high temperature oxidation up to 626.4 ks on Fe – 9 to 12%Cr alloy from 823 K to 1323 K in humid environment had been discussed. The formation of oxide scales in humid environment follows a multiple scale pattern regardless of its temperature. The critical Cr concentration for the transition from internal oxidation to external oxidation was influenced by the presence of water vapour. The disappearance of IOZ in the oxidation of Fe-9Cr-0.26Si is due to the increase oxygen at the IOZ/SiO2 interface thus promoting the oxidation of the matrix. The duration of gap healing beneath the Fe2O3 dependant is dependent on the temperature. Cyclic oxidations initiate and promote crack propagation of non-protective oxide scale on Fe – 9 to 12Cr alloys at high temperature in humid environment. It has been shown from this review various factors affect the oxidation in water vapor. More studies on the following field must be done to contribute to the understanding of the water vapor’s effect on the mechanism of oxidation of Fe – 9 to 12Cr alloys. Suggestions on these studies are as follows:

a) Solid – state reaction in the oxide or interdiffusion of the alloy constituents in the oxide with the presence of H atom, particularly their effect at interface vicinity, i.e. diffusivity in metal/oxide interface at IOZ.

b) The formation of void with the presence of H. c) The effect of water vapor on surface chemistry/ energy. (Modelling might help to

understand the mechanism/ matter).

Acknowledgement

This work was supported by the Malaysian Ministry of Higher Education (MOHE) through its Fundamental Research Grant Scheme (FRGS 0510 - 127). The authors gratefully acknowledged the support.

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