After the steam oxidation test, the surface oxides were observed under SEM and the results are shown in Figure 7
. Compared to the specimens tested in air, the as-received and Cr-IDHTed specimens showed somewhat different surface oxide morphology in steam environment. For the as-received specimen, some of surface oxide layer was spalled off and Fe-rich oxides were detected by point EDS analysis. On the other hand, only fine chromia were formed on the Cr-IDHTed specimen. Figure 8
shows the XRD results of the specimens after steam oxidation test at 650 °C for 500 h. While all peaks of the as-received specimen matched to those of hematite (Fe2
), XRD peaks of the Cr-IDHTed specimen matched to those of the Cr2
and the surface layer (Cr23
). The absence of the peaks of substrate for the as-received specimen indicates the hematite layer would be thick, which was confirmed in a cross-sectional SEM image in Figure 9
. Meanwhile, the XRD peaks of the Cr23
surface layer were observed for the Cr-IDHTed specimen, which suggest that the oxide layer would be thin (Figure 10
) even in steam environment.
The cross-section of the specimens tested in steam was analyzed using SEM and the results are shown in Figure 9
. Thick oxide layers were formed on the as-received specimen, which was consisted of three zones such as a thick (~15 μm) outer Fe oxide layer, (Fe, Cr, Mn) oxide layer and the isolated Fe-rich or Cr-rich oxides (Figure 9
b). In the outer Fe oxide layer, voids and cracks were found. Meanwhile, cross-sectional STEM analyses were conducted on the oxide layer of the Cr-IDHTed specimens because the oxide layer was too thin to be observed by SEM. Figure 10
shows cross-sectional STEM/EDS mapping and EDS line scanning results for Cr-IDHTed specimens oxidized in steam. A thin and continuous chromia layer (about 400 nm thick) was formed. Some voids and Cr depletion below them were observed under the chromia layer. The formation of the void and Cr depletion will be discussed in the following section.
Based on the oxide analysis results, it could be said that thick Fe-rich oxides (~25 μm) on the as-received specimen and the thin chromia layer on the Cr-IDHTed specimen resulted in huge difference in the weight change shown in Figure 2
for the as-received specimen, 0.38 mg/cm2
for the Cr-IDHTed specimen). The rapid oxidation behavior and formation of thick Fe-rich oxides on 9Cr FM steels in steam environment was reported previously [21
], suggesting the presence of water vapor changed the oxidation behavior of 9Cr steels. For the as-received ARROS, the failure of forming the protective chromia in steam environment could be explained considering two mechanisms. One is the chromia volatilization and the other is accelerated oxygen diffusion by water vapor. Because chromia volatilization can be negligible at temperature below 800 °C in pure steam [22
], the accelerated oxygen diffusion in oxide layer in steam would be the main reason for the enhanced oxidation rate. At the early stage of oxidation, Cr-rich oxides were formed on the surface of the as-received specimen. However, Cr would be depleted rapidly due to rapid oxygen diffusion and relatively slow Cr diffusion. Therefore, Fe-rich oxides with fast kinetics were produced by breaking the Cr-rich oxide layer [11
]. According to the literature, the outer Fe-oxide layer was hematite, which was consistent with the observation of this study. Underneath the hematite, (Fe, Cr, Mn) oxide layer was present, which would be (Fe, Cr, Mn)3
and large fluctuation of Cr and Fe contents was present near the oxide/metal interface as shown in EDS line scanning results (Figure 9
b). The Fe-rich and Cr-rich particles could be FeO and Cr2
, respectively, as reported previously [21
]. Therefore, it could be said that although ODS-FM steel showed relatively lower oxidation rate than commercial FM steel, a thick Fe-rich oxide layer was found in steam environment due to low Cr content. On the contrary, the Cr-IDHTed specimen showed a thin and continuous chromia layer because of the presence of the surface layer highly enriched with Cr (~60 wt.%). Similar to the air oxidation test, the Cr and Fe contents in the surface layer were maintained after the steam oxidation test.