Effect of 1200 °C pre-oxidation on oxidation kinetics at 1050 °C

The following two-stage oxidation experiments with in-situ temperature change were carried out:

 15 min. at 1200 °C followed by oxidation at 1050 °C up to the final exposure time

 1 hour at 1200 °C followed by oxidation at 1050 °C up to the final exposure time

 10 hours at 1200 °C followed by oxidation at 1050 °C up to the final exposure time

 50 hours at 1200 °C followed by oxidation at 1050 °C up to the final exposure time

The oxidation tests were carried out for thick and thin specimens for totally 100 h and the thermogravimetric data are presented in figure 10.1. The results are compared with single-stage oxidation data. The short term pre-oxidation (15 min.) shows no big difference in the oxidation rate compared to the single-stage oxidation results. However, as expected, the data shows that with increasing pre-oxidation time at higher temperature the total weight change of both thick and thin specimens is increasing.

It is important to note that until 10 h pre-oxidation time, no difference in the oxidation rate observed between thick and thin specimens occurred whereas a significant change was observed for the specimen pre-oxidized for 50 h at higher temperature, the thin specimen exhibiting a

lower oxidation rate than the thick specimen. This can be explained by the Zr depletion effects in specimens of different thicknesses in section 5.4.

The pre-oxidation times of say 10 hours or more might have a pronounced effect on the 1050 °C oxidation rate, however, this seems to be of no practical use because of the high weight gains, with corresponding extensive bulk Al depletion, occurring at 1200 °C already after relatively short pre-oxidation times of a few hours.

Figure 10.1: Weight change data of thick and thin specimens of low Zr doped FeCrAlY alloy (alloy 1 in table 8.1, batch JJG) during 100 h two-stage oxidation with in-situ temperature change in air (pre-oxidation for 15 min, 1h, 10 h and 50 h at 1200 °C followed by 1050 °C oxidation) compared with single-stage oxidation results

An important observation from figure 10.1 is that the overall weight gain after 100 h for the specimens oxidized for 15 minutes is smaller than that of the specimen without pre-oxidation. The weight gain after the 15 minutes oxidation at 1050 °C is obviously much smaller than the weight gain after the pre-oxidation of 15 minutes at 1200 °C. Consequently, the actually prevailing oxidation rate at 1050 °C between 15 minutes and 100 hours must be smaller for the pre-oxidized specimen than that of the specimen which was only oxidized at 1050 °C. A similar difference in oxidation rate at 1050 °C due to oxidation is also apparent after 1 hour

The measured apparent k_p values of thick and thin specimens after 15 min., and 1 h pre-oxidation at 1200 °C and subsequent oxidation at 1050 °C until a final exposure time of 100 h are shown as function of exposure time and weight change in figure 10.2 and 10.3. The k_p values are calculated from the weight change data in figure 10.1 and derived by differentiation of Δm²(t) plots. The graphs show the kp values including the pre-oxidation time. The kp values show that the instantaneous oxidation rate at 1050 °C immediately after the temperature change from 1200 to 1050 °C is lower than that of the single stage oxidized specimen especially if data for the same oxide scale thickness are compared (figure 10.2 b and 10.3 b).

The metallographic cross sections (SEM images) of thick specimens (figure 10.4) exhibit an increase in oxide scale thickness with increasing pre-exposure time (1 h, 10 h and 50 h) compared to the single-stage oxidized specimen which is in agreement with the observed weight change. In addition, the amount of Zr/Y rich precipitates in the alumina scale is also increasing.

In case of thin specimens (figure 10.5) a similar trend was observed, however, after 50 h pre-oxidation a small precipitate free zone starts to form in the inner oxide scale. This accounts for the slower oxidation rate observed for the corresponding specimen compared to the thick specimen, as already discussed in section 5.4.

The difference in overall scale thickness after totally 100 h oxidation is only minor in case of the 15 minutes pre-oxidized specimens and therefore not easily detected in the (SEM) cross sections.

The effect of short term pre-oxidation at 1200 °C on 1050 °C oxidation might be related to a more rapid incorporation of RE’s into the alumina scale, lower amounts of transient oxide and/or a larger grain size of the oxides formed in the 1200 °C oxidation stage.

Figure 10.2: Measured apparent k_p values of thick specimens of low Zr doped FeCrAlY alloy (alloy 1 in table 8.1, batch JJG) during 100 h two-stage oxidation with in-situ temperature change in air (pre-oxidation for 15 min and 1h at 1200 °C followed by 1050 °C oxidation) as function of a) exposure time b) weight change

a)

b)

0.00001 0.0001 0.001 0.01 0.1

0 20 40 60 80 100 120

Exposure time [h]

log (Kp(t) / [mg2 ·cm-4 ·h-1 ])

0 min.

15 min.

1 h

0.00001 0.0001 0.001 0.01 0.1

0 0.1 0.2 0.3 0.4 0.5

Weight change / [mg*cm^-2] log (Kp(t) / [mg2 ·cm-4 ·h-1 ])

0 min.

15 min.

1 h

Thick specimens

Pre-oxidation times at 1200 °C

Pre-oxidation times at 1200 °C

Figure 10.3: Measured apparent k_p values of thin specimens of low Zr doped FeCrAlY alloy (alloy 1 in table 8.1, batch JJG) during 100 h two-stage oxidation with in-situ temperature change in air (pre-oxidation for 15 min and 1h at 1200 °C followed by 1050 °C oxidation) as function of a) exposure time b) weight change

a)

b)

0.00001 0.0001 0.001 0.01 0.1

0 20 40 60 80 100 120

Exposure time [h]

log (Kp(t) / [mg2 ·cm-4 ·h-1 ])

0 min.

15 min.

1 h

0.00001 0.0001 0.001 0.01 0.1

0 0.1 0.2 0.3 0.4 0.5

Weight change / [mg*cm^-2] log (Kp(t) / [mg2 ·cm-4 ·h-1 ])

0 min.

15 min.

1 h

Thin specimens

Pre-oxidation times at 1200 °C

Pre-oxidation times at 1200 °C

Figure 10.4: Metallographic cross-sections (SEM images) of alumina scales on “thick specimens” of low Zr doped FeCrAlY alloy (alloy 1 in table 8.1, batch JJG) after two-stage oxidation in air for totally 100 h; Pre-oxidation for a) 0 min, b) 15 min, c) 1h, d) 10 h and e) 50 h at 1200 °C followed by 1050 °C until end of the experiment

The macro images of the corresponding thick specimens (figure 10.6) show no significant effect of the high temperature pre-oxidation on the oxide scale appearance up to a pre-oxidation time of 10 h. On the other hand, a pre-oxidation of 50 h results in a substantial darkening of the oxide

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scale. The macro images of thin specimens showed a similar trend, however, as expected from the results in section 5.4, the specimen pre-oxidized at 1200 °C for 50 h was found to be less dark than the thick specimen after the same oxidation treatment.

Figure 10.5: Metallographic cross-sections (SEM images) of alumina oxides on “thin specimens”

of low Zr doped FeCrAlY alloy (alloy 1 in table 8.1, batch JJG) after two-stage oxidation in air for totally 100 h; Pre-oxidation for a) 0 min, b) 15 min, c) 1h, d) 10 h and e) 50 h at 1200 °C followed by 1050 °C until end of the experiment

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Zr / Y depleted zone

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d) e)

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Figure 10.6: Macro images of alumina scales on “thick specimens” of Zr doped FeCrAlY alloy (alloy 1 in table 8.1, batch JJG) after 100 h two-stage oxidation in air (1200 and 1050 °C). Pre-oxidation times at 1200 °C; a) 0 min., b) 15 min., c) 1h, d) 10 h and e) 50 h pre-Pre-oxidation at 1200

°C followed by 1050 °C until end of the experiment