Cross Sectional Analysis

Cross sectional analysis using SEM was carried out to investigate the oxide thickness and chemistry for samples exposed isothermally to air saturated steam at 923 K for 50 – 1000 hours. Back scattered electron images of samples exposed for each time tested are shown in Figure 4-3. A duplex oxide consisting of an inwardly growing Fe-Cr-Ni spinel and an outwardly growing magnetite oxide grew during isothermal oxidation. Also visible in the cross sections are voids within the magnetite layer for shorter oxidation times (a-c) and at the magnetite/ spinel interface for oxidation times above 500 hours (d-f). The area fraction of voids and diameter between void walls were measured and are shown in Table 4-2.

Table 4-2 Area fraction of voids and the maximum diameter measured between void walls for each oxidation time in air-saturated steam at 923 K.

Oxidation time/ hours Area fraction of voids/ % Diameter of voids/ µm

50 4.4 2.9 sectional examination for all of the oxidation times investigated in this thesis.

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Figure 4-3 Cross sectional BSE micrographs of TP347H FG post oxidation in air saturated steam at 923 K for (a) 50 hours, (b) 100 hours, (c) 300 hours, (d) 500 hours, (e) 750 hours and (f) 1000 hours.

EDS and WDS analyses were also carried out on cross sections to confirm the oxide chemistry.

(a)

(b)

(c) (d)

(e) (f)

Crack extension

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Optical microscopy of cross sections was also carried out since it was not possible to distinguish between haematite and magnetite using SEM analysis, Figure 4-4.

Results from micrographs of unspalled regions show that haematite appeared as a marginally darker grey than magnetite and confirmed the presence of both Fe oxides.

Figure 4-4 Optical cross sectional micrograph of an unspalled region of TP347H FG exposed to air saturated steam at 923 K for 1000 hours.

Haematite formation was also confirmed using WDS analysis. Figure 4-5 shows the concentration profile of a sample of TP347H FG exposed to air saturated steam at 923 K for 100 hours. At the oxide/ gas interface it is clear that there is an increase in O concentration confirming the haematite/ magnetite interface seen similarly in optical cross sectional analysis.

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Figure 4-5 BSE image and concentration profile of the oxides formed on TP347H FG during oxidation in air saturated steam at 923 K for 1000 hours where the O and Cr concentrations have been obtained using WDS analysis (from [160]).

Identification of the Cr rich layer at the base of the Fe-Cr-Ni spinel was found early on in this project but the resolution of the EDS on the SEM was not high enough to determine the chemistry of the layer. Access to a Transmission Electron Microscope (TEM) with EDS capabilities later on in the project allowed the full characterisation and identification of Cr rich oxides. Further investigation was carried out on a sample of TP347H FG that had undergone 1000 hours oxidation in air saturated steam at 923 K. The oxide grown under these conditions was thick enough at approximately 1.5 µm to use a Focused Ion Beam (FIB) microscope to mill out a small section. The TEM foil produced is shown in Figure 4-6. Higher magnification dark field images of the TEM foil indicated there were two layers of oxide present at the base of the Fe-Cr-Ni spinel, Figure 4-7(a).

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Figure 4-6 TEM foil of Cr rich oxide at the base of the Fe-Cr-Ni spinel produced using a FIB microscope.

EDS analysis from this sample indicated the thicker Cr rich oxide layer adjacent to the Fe-Cr-Ni spinel to be that of chromite, FeCr2O4. The oxide layer beneath that was found to be that of Cr2O3, indicated on the concentration profile at approximately 1350 nm. The width of the Cr rich peak was measured to be approximately 115 µm.

Using approximate values, the following equation was used to determine the rate constant kp for the Cr2O3 layer found in this study [161] which was calculated to be 5.25 x 10^-21 m².s^-1:

𝑘_𝑝 = 2.07𝑥10⁻⁶ 𝑥 𝑒𝑥𝑝 (−31020

𝑇 ) Equation 4-1

Using this value of kp the theoretical chromia oxide thickness for 1000 hours could then be calculated based on Equation 4-2 and was found to be approximately 140 µm.

𝜉² = 𝑘_𝑝𝑡 Equation 4-2

The theoretical value is in good agreement with the value measured from the Cr peak in the concentration profile.

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Figure 4-7 (a) Dark field cross sectional TEM image of TP347H FG exposed to air saturated steam for 1000 hours at 923 K and (b) the corresponding linescan which was taken from left to right on the cross sectional image.

The concentration profile shown in Figure 4-7(b) also indicated an enhancement in Fe directly beneath the Cr2O3layer at approximately 1450 nm. The Cr2O3 layer was therefore thought to be behaving protectively, preventing further diffusion of Fe from within the alloy outwards.

4.2.4 Oxidation Kinetics

Isothermal oxidation kinetics for the Fe-Cr-Ni spinel oxide were determined but the thicknesses of magnetite and haematite were difficult to interpret as a result of the extensive spallation. Oxidation kinetics were therefore not determined for those oxides.

A plot of oxide thickness as a function of oxidation time was used to calculate n, the rate constant, using the following equation, Figure 4-8:

𝜉 = (𝑘_𝑛𝑡)^{1 𝑛⁄} Equation 4-3

where ξ is oxide thickness, m, kn is the rate constant, mⁿ.s^-1 and t is time, s.

0 200 400 600 800 1000 1200 1400 1600 1800 0

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The value of n in Equation 4-3 was calculated to be 2.9 for isothermal oxidation in air saturated steam at 923 K showing near-cubic behaviour. The value of kn was found to be 1.3 x 10^-15 m^2.9.s^-1. Data found in the literature for the same alloy stated that the oxidation kinetics were parabolic with kp values of 1.7x10^-17 m².s^-1 [2]. However, this literature value is for total oxide thickness for TP347H FG from plant trials rather than just the Fe-Cr-Ni spinel. The steam environment is also slightly different and so this value is not a true comparison. Shorter term testing has also been said to result in higher rate constants than do long term testing. Data for spinel on Super 304 H, which has a similar composition to the alloy studied in this thesis, was also found to be parabolic with a kp value of 1.9 x 10^-16 m².s^-1 when exposed to deoxygenated steam at 923 K. This discrepancy may again be as a result of the different steam conditions used. Insufficient data on spinel growth in the literature impedes further comparisons to be made. The cause for the near-cubic oxidation kinetics is thought to be as a result of the Cr rich oxides formed at the base of the spinel.

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Figure 4-8 Plot of mean and standard deviation of Fe-Cr-Ni spinel oxide thickness against time at temperature in air saturated steam at 923 K.

4.3 Spallation Behaviour after Isothermal Oxidation