OBSERVATION ON VOID FORMED IN OXIDE SCALE OF Fe-Cr-Ni ALLOY AT 1073K IN DRY AND HUMID ENVIRONMENTS

Void formation in oxide scale during high temperature oxidation is a common phenomenon. Over a long period of time voids will affect the mechanical property of scales by influencing the cracking and spalling. Voids formed in dry environment are different than that of formed in humid environment. With the presence of water vapor in humid environment the formation of void will increase, thus greater number of void compared to that in dry environment. Fe-Cr-Ni alloy samples were exposed isothermally at 1073 K in air ( = 0.21atm = 2.1 10 Pa) and humid (air + steam) environments. XRD analysis done to all samples confirms that Fe2O3, Fe3O4, NiCr2O4, FeCr2O4, Cr2O3 and NiO phases exist in the scale. EDX analysis done shows varying compositions of Fe,Cr,Ni and O in outer and inner oxide scale, oxide scale/metal interface and metal. Field emission scanning electron microscope (FE-SEM) was used to investigate voids formed in the cross sections of the oxidized samples. Volume fraction of voids in the oxide scale was calculated in accordance to the cross sectional area fraction of voids in the scale. It shows that Fe-Cr-Ni alloy samples exposed in humid environment has as high as 71% more voids than that exposed in dry environment. It is concluded that the humid environment increased the number of void formed in the oxide scale, thus facilitates the exfoliation of protective scale during the high temperature oxidation. ABSTRAK: Pembentukan gelembung udara di dalam lapisan oksida ketika proses pengoksidaan di suhu tinggi merupakan satu fenomena biasa. Pada satu jangka masa yang panjang gelembung-gelembung ini akan memberi kesan kepada sifat mekanikal oksida dengan mempengaruhi pembentukan keretakan dan pengelupasan oksida. Gelembung udara yang terbentuk di dalam persekitaran kering berbeza daripada yang terbentuk di dalam persekitaran lembap. Dengan adanya wap air, pembentukan gelembung akan bertambah berbanding yang terbentuk di dalam persekitaran kering. Sampel aloi Fe-Cr-Ni telah dioksidakan secara isoterma pada suhu 1073 K di dalam udara ( = 0.21atm = 2.1 10 Pa) dan lembap (udara + wap air). Analisis Pembelauan Sinar – X (XRD) kepada semua sampel menunjukkan oksida yang terbentuk ialah Fe2O3, Fe3O4, NiCr2O4, FeCr2O4, Cr2O3 dan NiO. Analisis Penyebaran Tenaga Sinar – X (EDX) menunjukkan komposisi Fe, Cr, Ni dan O yang berubah ubah di lapisan oksida luar dan dalam, oksida/ antara muka logam dan logam. Mikroskop Imbasan Elektron-Pancaran Medan (FE-SEM) digunakan untuk meneliti gelembung di dalam oksida pada keratan rentas sampel. Pecahan isi padu gelembung yang terbentuk pada oksida dikira dengan merujuk kepada pecahan luas keratan rentas pada oksida tersebut. Sampel aloi Fe-Cr-Ni yang dioksidakan di dalam persekitaran lembap IIUM Engineering Journal, Vol. 12, No. 5, 2011: Special Issue -1 on Science and Ethics in Engineering Kaderi et al. 70 mempunyai kandungan gelembung udara 71% lebih banyak berbanding dengan yang dioksidakan di dalam persekitaran kering. Kesimpulannya persekitaran lembap meningkatkan bilangan gelembung yang terbentuk di dalam lapisan oksida, sekaligus memudahkan pengelupasan oksida semasa pengoksidaan suhu tinggi.


INTRODUCTION
Formation of voids in oxide scale especially in the proximity of metal/oxide interface or outer oxide scale/inner oxide scale interface is common in metals during high temperature oxidation. The formation will affect the oxidation mechanism and mechanical properties of the scale and also the metal itself. The tendency of voids to concentrate at outer oxide scale/inner oxide scale interface will result in spalling of the outer oxide scale [1].
Voids within iron oxide scales and the iron-rich oxides formed on dilute Fe-Cr and Fe-Al alloys developed far more faster in the presence of water vapor. Rahmel et al. [2,3] have reported that presence of H 2 O(g) caused the scale to develop porosity. Ehlers et. al. [4] observed that 9 mass% Cr ferritic steels form rapidly growing scales of porous magnetite plus spinel during oxidation at 923 K in gases containing water vapor.
Contemporary literatures [2][3][4][5][6] on effect of water vapor on Fe-Cr or Fe-Al alloys mostly describe formation of voids in qualitative manner but few are attempting to study it quantitatively. Understanding on the formation of voids quantitatively is very important because voids formation affect development of scales microsructure. Maruyama et al. [7] proposed that void formation in a growing oxide scale during high temperature oxidation of a metal closely related with the divergence of ionic fluxes. Further improvement of the treatment by Ueda et. al. [8], by including all ionic fluxes into the calculation of the chemical potential distribution and quantitatively explained the position and the volume of voids formed in the magnetite scale. Until recently, Maruyama et al. [9] has successfully applied the quantitative estimation on the void formed in a single phase magnetite scale grown on iron substrate at 823 K in oxdizing environment containing oxygen partial pressure, ܲ మ = 4.2 ൈ 10 ିଵଷ Pa. However, most qualitative and quantitative studies on void formation cited earlier done in a strictly controlled oxidizing environment especially by lowering the ܲ మ , so that only specific oxide scale grows on the metal substrate at specific temperature. Therefore in this study, quantitative observation was focusing on void formed in a multiple oxide scale grown on Fe-Cr-Ni alloy at 1073 K in normal air (ܲ మ = 0.21atm = 2.1 ൈ 10 ହ Pa) and humid (air + steam) environment.

Sample Preparation of Fe-Cr-Ni Alloys
Fe-Cr-Ni alloys were prepared from high-purity Fe (99.99%), Cr (99.99%), Ni (99.99%) plates by arc melting in Ar gas. The alloys were further annealed for 86.4 ks at 1373 K in vacuum. After annealing the alloys were sliced into coupons of 1mm thick and the surfaces of the coupons were ground with emery paper up to #2000. Then the samples were polished with 3µm alumina paste to a mirror-like finish. The samples were cleaned with acetone via ultrasonic agitation. The chemical composition of the alloys were analyzed through Energy Dispersive X-Ray (EDX). The chemical composition of Fe-Cr-Ni alloys are shown in Table 1.  Figure 1 shows the experimental setup. The sample was placed at the isothermal zone of the furnace. An R-type thermocouple was placed at the isothermal zone beside the sample to monitor the temperature. The distance between the sample and the thermocouple is 5 mm. The isothermal zone is 30 mm in range. For the oxidation in dry environment, the sample was heated to 1073 K and held for 86.4 ks and 172.8 ks respectively in air. For the oxidation in humid environment, a boiler is placed directly below the furnace. The water vapor was channeled into the furnace once the heating process started. The continous supply of water to the boiler will ensure that the water vapor will be produced continously throughout the oxidation process. The oxidation process in humid environment was also hold for 86.4 ks and 172.8 ks respectivey. After oxidation, the samples are cooled to room temperature in the furnace.

Phase Identification
Oxide phases formed were analyzed by X-ray diffraction (XRD) using the divergence slit of 1 degree. The diffraction angle is from 20° to 80° and the scanning speed was 3.0000 °/min.

Chemical Composition of Each o Scale, Inner Oxide Scale, Inner Oxide Scale/ Alloy
The concentration of main elements of each samples after being oxidized were quantitatively determined by EDX. The analysis was done on 4 spots; Spot 1: Outer oxide scale, Spot 2: Inner oxide scale, Spot 3: Metal/Oxide interface and Spot 4: Metal. Figure 2 shows an example of an area of a sample's 4 spots being analyzed to determine the main elements concentration.

Volume Fraction of Voids
Oxidized samples were mounted individually in resin. Prior to mounting, 2 mm of each of sample was crosssamples after being cross-sectioned. The exposed cross ground up to #2000. Then the samples were polished with 0.05 µm alumina to a mirror like finish.

omposition of Each of Main Elements after Oxidation: Inner Oxide Scale/Fe-Cr-Ni Alloy Interface a
The concentration of main elements of each samples after being oxidized were by EDX. The analysis was done on 4 spots; Spot 1: Outer oxide scale, Spot 2: Inner oxide scale, Spot 3: Metal/Oxide interface and Spot 4: Metal. Figure 2 shows an example of an area of a sample's 4 spots being analyzed to determine the main SEM micrograph shows the spots for EDX analysis.

f Voids in the Oxide Scale
Oxidized samples were mounted individually in resin. Prior to mounting, 2 mm of sectioned by diamond saw. Figure 3 is a schematic of the sectioned. The exposed cross -section part of the samples was ground up to #2000. Then the samples were polished with 0.05 µm alumina to a mirror Schematics of samples after being cross-sectioned.
Surface morphology was observed by field emission scanning electron microscopy (FE SEM). Grids of uniformed size were traced on each images and area fraction of voids in the scale was measured. It was assumed that the volume fraction of void in the scale equal sectional area fraction of voids in the scale [10]. The same assumption was [11], in order to calculate voids formed in NiO grown of Ni

Outer Oxide and Fe-Cr-Ni
The concentration of main elements of each samples after being oxidized were by EDX. The analysis was done on 4 spots; Spot 1: Outer oxide scale, Spot 2: Inner oxide scale, Spot 3: Metal/Oxide interface and Spot 4: Metal. Figure 2 shows an example of an area of a sample's 4 spots being analyzed to determine the main SEM micrograph shows the spots for EDX analysis.
Oxidized samples were mounted individually in resin. Prior to mounting, 2 mm of diamond saw. Figure 3 is a schematic of the section part of the samples was ground up to #2000. Then the samples were polished with 0.05 µm alumina to a mirror-Surface morphology was observed by field emission scanning electron microscopy (FE-SEM). Grids of uniformed size were traced on each images and area fraction of voids in umed that the volume fraction of void in the scale equal . The same assumption was , in order to calculate voids formed in NiO grown of Ni  The EDX Profile was examined as shown in Fig. 5a       Nevertheless, the increases in volume fraction of voids for samples oxidized in humid environment shown in Fig. 7 and 8 are phenomenal than that of samples oxidized in dry environment. The increase is ranging from 59% to 86%.

Volume Fraction of Voids in the Oxide Scale
It is interesting to note that the volume fraction of voids in outer oxide scale in this study are in a range of 0.06 to 0.29. Studies on void formation in magnetite scale by Maruyama [9] shows very small volume fraction of voids in a range of 0.05 to 0.06. Meanwhile, NiO scale on Ni by Akiba [11] shows volume fraction of voids less than 0.01. Both studies were conducted in a controlled ܲ మ environment. Therefore, the fact that the oxidation for the current study was done in normal air with higher ܲ మ has contributed to the formation of greater magnitude of void in the outer scale.

CONCLUSION
The void formation in oxide scales of Fe-Cr-Ni alloy samples exposed isothermally at 1073 K in air (ܲ మ =0.21atm = 2.1 ൈ 10 ହ Pa) and humid (air + steam) were investigated. It has been shown that the protective Cr 2 O 3 phases and NiO phases occurred in samples oxidized in dry environment while non-protective Fe 3 O 4 phase occurred in all samples oxidized in dry and humid environment. The volume fraction of voids formed in inner scale increased by 84% for 86.4 ks and 86% for 172.8 ks when oxidized in humid environment. On the other hand, volume fraction of voids formed in outer scales increased by 76% for 86.4 ks and 59% for 172.8 ks when oxidized in humid environment. The oxygen partial pressure, ܲ మ greatly influenced the volume fraction of voids formed in the oxide scale of Fe-Cr-Ni alloy. Hence the oxidation in air contributes greater formation of void than in controlled environment.