3.1. Oxidation Dynamic Curves
The oxide weight gain curves of oxide samples under different oxidation conditions (temperature, time) are shown in
Figure 1. Oxidation rate refers to the mass gain of the samples in unit area and unit time, which can be used to characterize the oxidation intensity of materials [
18].
The weight gain for P91 and MarBN steels increased with increasing oxidation temperature and oxidation time. As can be seen from the oxidation curves in
Figure 1, the oxidation of experimental steels was stable during the first 1000 h. During 1000 h of oxidation, a stable weight gain was observed, which can be explained by a thin protective oxide layer formation on the steel surface without its significant breakaway,
Figure 2 and
Figure 3. It is assumed that in this time interval, the surface of the steel was mainly covered by Cr-rich oxides, which could prevent the penetration of oxygen, and thus no significant subsurface oxidation occurred [
13].
This phenomenon indicates that P91 steel had a better antioxidant effect at both 600 °C and 650 °C. Steels with a ferrite content or ferritic steels often show increasing oxidation resistance with increasing oxidation temperature [
19,
20].
The analysis of different steel oxidation behavior in water vapour is a tool for better understanding of the oxidation mechanism and leads to the following conclusions, to which several authors have come in their works [
7,
14,
21]: the rate-limiting step of the whole corrosion process is the outward iron migration to form the outer layer and iron migrates when the ions increase the oxygen potential towards high oxygen potential that is present on the outer surface. In this process, the ions oxidize from Fe
2+ to Fe
3+. This leads to a lower Fe
2+/Fe
3+ ratio on the outer layer surface versus the oxide-steel interface. The compact oxide film formation on the surface prevents contact between the steel surface and the furnace area environment. Since the oxidation rate is controlled by diffusion, the mass gain is also lower [
21].
After changing the oxidation rate and especially the last 1000 h of the process (in the time interval of 1000–2000 h), the effect of long-term higher oxidation temperature was significantly manifested, and the oxidation mass gain reached the highest value. After 2000 h, a slowdown in the oxidation rate was observed for all tested steels and temperature conditions. The authors in [
22] described that after the breakaway in the oxide layer, there was a certain re-passivation, and the result was an overlap of nodules in the continuous thick oxide scale. This effect was reflected in a slowing down of the oxidation rate.
3.3. Morphology and Composition of Oxide Layers after Oxidation
Figure 4,
Figure 5,
Figure 6 and
Figure 7 and
Table 3 and
Table 4 present the qualitative and quantitative cross-section analysis for the tested P91 and MarBN steels after 2000 h oxidation at 600 °C. This oxidation time is interesting because of the more significant change in the oxidation rate as shown in
Figure 1. The oxide layer morphology on P91 steel is documented in
Figure 4. The average oxide layer thickness was 35.74 µm. The results of the quantitative EDX analysis, shown in
Table 3, indicated a high 34.4 wt.% Cr at the steel/oxide layer interface (location 3 in
Figure 4 and in
Table 3), and also in the inner layer of 14.5 wt.% (location 2 in
Figure 4 and in
Table 3). The Cr-rich layer at the interface was locally discontinuous at locations with lower Cr contents, as illustrated by the EDS map in
Figure 5b. As reported by the authors in [
24], 28 at.% Cr corresponds to the maximum chromium content in spinel oxide, which is equivalent to the FeCr
2O
4 stoichiometry. The inner layer edge was accompanied by the formation of protrusions, which were covered with Fe oxides, as confirmed by
Figure 5c. In the oxide layer cross section, a location is visible that separates the lower part from the upper part of the oxides. The paper [
24] describes that this is an interface that separates the bottom spinel oxide from the outer oxide layer area. As the authors of the paper further state, the above interface corresponds to the original sample surface. That is, oxide layer regions were formed by diffusion of metal cations towards the outer surface, while the layer below the interface was enlarged by oxygen transport inside the material. Thus, it can be concluded that the formed magnetite and hematite layers, after their loss of compactness in a moisture-containing environment, grew by the iron diffusion in the outward direction. The cross-sectional distribution of elements after 2000 h shows that the oxidation process was in oxide film formation stage which means that the alloy elements diffused outwards from the steel and the oxygen inwards,
Figure 5a–e. Mn and Mo were rather located in the inner diffusion zone part. Isolated islands of oxide nodules (indicated by arrows in
Figure 4) were also part of the layer, which were the result of the water vapour based environment acting on the steel surface [
21,
24].
The further growth of the layer largely depends on the transport process within the nodules.
Because iron transport is significantly reduced due to the presence of cavities, hematite is formed at the interface with the oxidizing environment of the furnace (location 1 in
Figure 4 and
Table 3). Depending on the oxidation conditions, “depassivation” processes can occur inside the oxide layer or partial peeling of the oxide layers [
22,
25].
Figure 6 shows the oxidized surface of MarBN steel after 2000 h of oxidation at 600 °C. The average oxide layer thickness was 62.86 µm. The quantitative EDX analysis results, shown in
Table 4, showed a high Cr content of 13.0 wt.% at the steel/oxide layer interface (location 8 in
Figure 6 and in
Table 4), and also in the inner oxide layer at 14.4 wt.% (location 9 in
Figure 6 and in
Table 4). The Cr content indicated in locations 6 and 7 was part of the steel substrate chemical composition. As documented in
Figure 6, the outer oxide layer was interrupted by nodules (indicated by arrows in the figure), which are formed as the result of the water vapour action in the oxidizing environment on the steel surface [
21,
24].
Compared to the oxide layer on MarBN steels (
Figure 6 and
Figure 7), the distribution of Cr and Mn in the oxide layer on P91 steels is uniform (
Figure 4 and
Figure 5), although local discontinuities were observed. The lower diffusion activity in the martensitic-bainitic steel MarBN led to a less continuous distribution of Cr and Mn in the inner oxide layer. For the reason mentioned above, the antioxidant effect of the surface of MarBN steel was lower than that of P91 steel with a ferritic matrix [
20,
26]. It was reflected in the larger average oxide layer thickness of 62.86 µm compared to the average oxide layer thickness of 35.74 µm on P91 steel.
This assumption was also confirmed by the EDS map of element distribution that diffused in the forming Fe oxide layer during the oxidation process,
Figure 7a–g. The Cr distribution shows a higher Cr content at the substrate/layer interface and in the inner layer,
Figure 7b. However, this region is locally interrupted by locations with lower Cr content. The outer layer surface contains Cr, however, the EDX analysis showed a low 1.5 wt.% Cr content (location 10 in
Table 4). A compact Cr-enriched layer with Cr content of at least 7.0 wt.% is required for a sufficiently effective oxide layer [
7,
17]. The cross-sectional distribution of elements further showed that Co, W and Si were more a part of the inner oxide layer (
Figure 7d–f) in contrast to Mn, whose distribution in the layer is uniform (
Figure 7g).
According to [
22,
27], the chromium content is essential for these steels, but even small amounts of additional alloying elements (e.g., Mn, Mo, Si) can have a significant effect on the oxidation behavior of steel by influencing either the nucleation behavior of the protective chromium layer (and thus the behavior of cracks in healing and by this also the healing behavior of cracks) or the behavior of chromium removal in the subsurface zone of steel during oxidation or also the chromium depletion behavior of the steel subsurface location during oxidation.
Figure 8 shows the P91 steel after 2000 h oxidation at 650 °C. The average oxide layer thickness was 76.52 µm. The quantitative EDX analysis results, shown in
Table 5, showed a high Cr content of 21.3 wt.% at the steel/inner oxide layer interface (location 12 in
Figure 8 and in
Table 5), and also in the inner layer with a Cr content of 11.7 wt.% (location 14 in
Figure 8 and in
Table 5). The 9.6 wt.% Cr shown in area 11, or 13, was part of the steel substrate chemical composition. The outer oxide layer (locations 15 and 16) had Cr contents at the level of tenths of 0.7–0.9 wt.%. As documented in
Figure 8, the outer oxide layer was interrupted by heterogeneities in its structure (indicated by arrows in the figure), which were formed as the result of the water vapour acting in the oxidizing environment on the steel surface [
21,
24].
Non-protective oxide layers (locations 15 and 16 in
Figure 8) formed on the surface, composed of an outwards grown Fe
2O
3 with a Cr depleted steel subsurface localization. As the conclusions of the research [
17] state, this process leads to the local formation of fast-growing iron-rich oxide nodules, and this surface was completely covered with a non-protective-oxide scale. As further stated by the authors of the work [
17], with the growing oxidation time, fast-growing nodules start to cover large parts of the surface and grow together, finally ending up in the continuous thick oxide scale. This fact was confirmed even after 2000 h oxidation at 650 °C steel P91, when the average thickness of the outer non-protective oxide layer (
Figure 8 and
Figure 9) increased significantly compared to the outer oxide layer after 2000 h oxidation at 600 °C (
Figure 5 and
Figure 6).
There is a visible interface in the oxide layer structure along the cross-section (similar to the oxide layer of P91 steel oxidized for 2000 h at 600 °C). This interface separates higher Cr content regions from the outer oxide parts with lower Cr content. The EDS map shows the distribution of elements that diffused in the forming Fe oxide layer during the oxidation process,
Figure 9a–e. The Cr distribution shows a higher Cr content at the substrate/layer interface, and at the interface between the lower and upper parts of the oxide layer,
Figure 9c. On the outer surface of the oxide layer, the Cr content is significantly lower, as confirmed by EDX analysis (location 16 in
Figure 8 and
Table 5). The fact that chromium is present in higher amounts only inside the growing layer can be explained by lower diffusivity of Cr
3+ versus Fe
2+ in oxides [
28]. For P91, the outward growing layer is composed of chromium-bearing hematite, while the inward growing oxide reached a composition with higher Cr content in some areas. The cross-sectional distribution of elements further showed that Mn and Mo were more part of the inner oxide layer,
Figure 9d,e.
Figure 10 documents a cross-section of MarBN steel after 2000 h of oxidation at 650 °C. The average oxide layer thickness was 97.75 µm. The results of the quantitative EDX analysis, shown in
Table 6, showed a high Cr content of 15.2 wt.% at the steel/inner oxide layer interface (location 19 in
Figure 10 and
Table 6), and also in the inner layer (location 20 in
Figure 10 and
Table 6) and at the inner/outer oxide layer interface (location 21 in
Figure 10 and
Table 6). The Cr content at locations 18 and 19 is part of the steel chemical composition. There is an interface in the structure of the oxide layer after the cross-section with different Cr and Fe contents in the inner oxide layer, versus the outer oxide layer, as shown in
Table 6 according to
Figure 10. This observation is also confirmed by the EDS map for Fe and Cr in
Figure 11a,c. Such an interface was formed in the layers of all tested steels, only with different “sharpness” (
Figure 4,
Figure 6,
Figure 8 and
Figure 10). The EDS map with distribution of elements that diffused in the forming Fe oxide layer during the oxidation process,
Figure 11a–e, shows at the same time a higher concentration of Co and Mn in the inner oxide layer versus the outer one. This may be due to the lower diffusivity of the above elements from steel, compared to Fe [
28].
The growth of the average thickness of the oxide layer was mainly supported by the formation of non-protective behavior “low-Cr” oxides Fe
2O
3. This process occurs when the kinetics of chromium subsurface location depletion exceeds the formation of a layer with a protective antioxidant effect [
17].
3.4. Surface Morphology of the Oxide Film after 3000 h Oxidation at 600 °C
Figure 12,
Figure 13,
Figure 14 and
Figure 15 document the steel surface morphology after continuous oxidation of 3000 h.
Figure 12 shows the oxidized surface of P91 steel after 3000 h of oxidation at 600 °C. As shown in the above figure, the surface morphology is formed by acicular oxides (locations A and B) and nodular oxides (location C).
According to the authors [
8], acicular oxide fills the voids between the spinel oxides and the surface becomes more compact, which results in improvement in the antioxidant capacity of materials. In case, after a certain period of oxidation, the healing of the steel surface occurs as the authors [
22,
26,
27] have also considered, the course of the oxidation process will slow down. During the research of our tested steels, there was a slight slowdown in oxidation after 2000 h.
Surface nodular oxides are often separated by voids and cracks, which can lead to the oxide region separation [
10]. Detail from location B shows the occurrence of blade-shaped crystals on the scale surface. As reported by the authors in the paper [
24] the formation of blade-shaped hematite crystallites at the scale surface in wet O
2 was also observed for pure Fe.
Table 7 shows the quantitative EDX analysis results of the experimental P91 steel oxide surface after 3000 h of oxidation at 600 °C. The main elements on the tested steel surface are Fe, O, Cr, Mn and Si.
Figure 13 documents the oxide layer morphology on the surface of MarBN steel after 3000 h of oxidation at 600 °C. A relatively uniform oxide was formed on the surface. The main effect of the water vapour presence is a somewhat higher mass gain than in dry O
2 and the appearance of blade-shaped crystals on the scale surface [
24]. D location detail shows the above type of hematite crystallites, similar to those on the P91 steel in
Figure 12.
Results of quantitative analysis in
Table 8 show the occurrence of the elements: Fe, O, Co, Cr, and Mn. Elements noted above are part of the oxide layer.
The XRD analyses of the samples also provide important information on the oxidation phenomena [
29]. Phase analyses of the surface oxide formed on P91 and MarBN steels by XRD after oxidation exposed in humid atmosphere with ~10% water vapour at 600 °C for 3000 h are shown in
Figure 14. It is evident that both samples show the presence of two iron oxides, that is, Fe
2O
3 (trigonal, space group R-3c (167) and Fe
3O
4 (cubic, space group Fd-3 m (227). Identification of some peaks was not possible due to minor phases in the oxide layer. We can assume the presence of Cr and Co oxides, as shown by the results of the quantitative analysis in
Table 7 for steel P91 and
Table 8 for MarBN steel.
3.5. Surface Morphology of the Oxide Film after 3000 h Oxidation at 650 °C
9Cr steels may show increasing oxidation resistance with increasing temperature. Higher diffusion rates of the elements forming the protective oxide layers, such as Cr, Mn, and Co, lead to more extensive incorporation of these elements into the oxide layer, thus improving their protective effect [
22,
27].
Figure 15 shows the oxidized surface of P91 steel for 3000 h of oxidation at 650 °C. The surface morphology of the oxide layer on steel is formed by nodular-like (locations H and G) and spinel-like forms (location F). According to the authors of [
8], surface nodular oxides are often separated by cavities and cracks, which can lead to the separation of the oxide region [
10]. Detail from location F shows the occurrence of iron-chromium-spinel oxides.
Table 9 shows the quantitative EDX analysis results of the experimental P91 steel oxide surface after 3000 h of oxidation at 650 °C. The main elements on the tested steel surface are Fe, O, Cr and Mn.
At this stage, breakaway oxidation has occurred, resulting in a very rough scale with hematite blades and nodules (
Figure 15) [
21]. The presence of a compact layer (film) of spinel oxides can better prevent further oxidation inside the steel [
8].
Figure 16 shows the surface morphology of the oxide layer on MarBN steel after 3000 h of oxidation at 650 °C. The surface morphology is more striking compared to the surface of P91 steel. On the other hand, it is more homogeneous in terms of Cr content. Iron-chromium-spinel oxides form with more than 7 wt.% can be seen in detail from location M (
Table 10, location M). According to the authors of [
17], the change in oxidation resistance between protective and non-protective behavior occurs at a content of about 7% Cr for 9–12% Cr steel.
Phase analysis of the surface oxide formed on P91 and MarBN steels by XRD after oxidation for 3000 h at 650 °C is shown in
Figure 17. Both samples show the presence of two iron oxides, that is, Fe
2O
3 and Fe
3O
4, as in the case of steels P91 and MarBN oxidized for 3000 h at 600 °C in
Figure 14. As documented in
Figure 17, in contrast to the above-mentioned steels, in this case, Cr and Co oxides were also identified. Identification of Cr oxide with Cr content above 7 wt.% corresponds to iron-chromium-spinel oxides. Identification of some peaks was not possible due to minor phases in the oxide layer.