Model oxide characterization
The surface of the TT Cr consists in a 150-nm-thick continuous Cr2O3 film (as calculated from the sputtering speed), covered by an outer Cr(OH)3 layer. From the ToF-SIMS depth profiles, it was possible to calculate a CrO2−/CrO− ratio of 1.2 in the Cr2O3 film. The external Cr(OH)3 thin layer was characterized by both an intense OH− signal and a higher CrO2−/CrO− ratio (≈1.7). In addition to the CrO− or CrO2− ions, CrO3− and CrO4− ions presented the same intensity variations until the interface with bulk Cr.
The analyses performed on thermally oxidized Ni presented very similar results: the NiO continuous 130-nm-thick thin film (as determined from the sputtering speed) was nicely profiled by NiO−, NiO2−, or NiO3− ions. In the oxide film, the NiO2−/NiO− ratio equals unity. There was no NiO4− signal.
The thermal treatment was applied on Ni-30Cr to form NiCr2O4.[11, 12] According to the ToF-SIMS depth profile (Fig. 1), the oxidized surface of TT Ni-30Cr can be described by up to three oxide layers (total oxide thickness >500 nm). The outermost layer is composed of a mixture of NiO and Cr2O3. An intermediate layer can be described by the NiCrO− signal and by a NiO2−/NiO− ratio that is equivalent to 4.0 × 10−1, significantly different from the value in pure NiO (~1.0). The third Cr2O3-rich layer is characterized by a CrO2−/CrO− ratio of 1.9, also different from the value in pure Cr2O3. On the basis of other published data,[11, 12] we can consider that we have prepared a layer of pure NiCr2O4 at the interface with the Cr-rich oxide layer. In this chromite layer, the NiCrO−/CrO− ratio is equivalent to 8.0 × 10−3, NiCrO2−/CrO2− is constant and is equivalent to 6.0 × 10−3. The CrO2− ion has its maximum intensity in this layer.
Figure 1. ToF-SIMS depth profile (negative mode) of Ni-30Cr thermally treated in air for 4 h at 900 °C (Cs+ 2 keV, current 80 nA).
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Additional reference data on mixed oxides have been obtained from the analysis of the NiFe2O4 powder: the NiFeOx− and NiFe2Ox− signals were observed in the ToF-SIMS spectrum (negative mode), confirming that these mixed ions are markers of mixed oxides (or spinels), in agreement with Aubriet et al. The ionic NiFeO−/NiO− ratio is equivalent to 0.053 in the powder (Table 1). The analysis of bulk Ni(OH)2 powder gave us a new series of ions to be distinguished both from the ionic response of the NiO thin film (which may be hydroxylated in its outermost layer) and from the external NiO-rich layer of the TT Ni-30Cr, which may also be hydroxylated. Characteristic ions are NiOH−, NiOH2−, NiO2H− (the more intense signal), and NiO2H2−, the intensity ratio of NiO2H−/NiO− allows us to discriminate between hydroxylated NiO, Ni(OH)2, and hydroxylated NiCr2O4 (Table 1).
Table 1. ToF-SIMS ionic ratios obtained on model oxides
|Species||Material||Analysis conditions||Mean ratio (standard deviation)|
|Cr2O3||TT pure Cr||Depth profiling (0.5 keV, 30 nA)||CrO2−/CrO− = 1.0 (2 × 10−2)|
|Depth profiling (2 keV, 80 nA)||CrO2−/CrO− = 1.2 (7 × 10−3)|
|NiO||TT pure Ni||Depth profiling (0.5 keV, 30 nA)||NiO2−/NiO− = 6.4 × 10−1 (2 × 10−2)|
|Depth profiling (2 keV, 80 nA)||NiO2−/NiO− = 1.2 (3 × 10−2)|
| ||NiO2H−/NiO− = 0.13|
|NiCr2O4||TT Ni-30Cr||Depth profiling (2 keV, 80 nA)||CrO2−/CrO− = 2.6 (2 × 10−1)|
|NiO2−/NiO− = 4.0 × 10−1 (1 × 10−1)|
|NiCrO−/NiO = 1.2 × 10−2 (4 × 10−3)|
|NiCrO−/CrO− = 8.0 × 10−3 (1 × 10−1)|
|Ni(OH)2||Powder||Surface spectra (no sputtering)||NiO2−/NiO− = 6.8 × 10−1 (standard deviation not defined)|
|NiO2H−/NiO− = 3.2 (standard deviation not defined)|
|NiFe2O4||Powder||Surface spectra (no sputtering)||NiO2−/NiO− = 2.2 (standard deviation not defined)|
|NiFeO−/NiO− = 1.1 × 10−1 (standard deviation not defined)|
|NiFeO−/FeO− = 6.0 × 10−2 (standard deviation not defined)|
Application to industrial SG tubes
Figure 2 presents the ToF-SIMS depth profile of a TT SG tube, sputtered at 0.5 keV (30 nA) as a function of the sample thickness. The profile presents the ions of interest as determined from the previous reference analysis: CrO−, CrO2−, NiO−, FeO−, as well as a set of the more intense mixed ions, NiCrO−, NiFeO−, and CrFeO−. It is possible to distinguish three different parts in the oxide layer. The outer part would be composed of a mixed Ni and Fe-rich oxide layer, the intermediate layer would be chromium oxide–rich, the inner layer, mainly characterized by an intense signal of NiCrO-, would correspond to a spinel-rich NiCr2O4 portion. It would be located at the interface with the alloy.
Figure 2. ToF-SIMS depth profile (negative mode) of an SG tube (alloy 690) thermally treated in air for 4 h at 500 °C (Cs+ 0.5 keV, current 30 nA).
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We have brought special attention to the intermediate Cr2O3-rich layer chemical composition, which would be the main protection against Ni diffusion in the case of the two other treatments: the SG tube directly immersed in PW and the TT SG tube immersed in PW (TT + PW) for 9 weeks in both cases. The depth profiles obtained with the PW and TT + PW samples (data not shown) present a similar three-layered structure. Figure 3 presents the CrO2−/CrO− and NiCrO−/NiO− ratios as a function of thickness for TT, PW, and TT + PW samples. For TT, the CrO2−/CrO− ratio is approximately 0.9, significantly lower than in the case of pure Cr2O3 (Table 1). This difference is tentatively interpreted by the loss of the continuous character of the Cr2O3 layer in favor of an island-like distribution, which was recently described after immersion in PW.[3, 12] This hypothesis still has to be confirmed, taking into account the effect of the energy of the sputtering ions on the ionic ratios.
Another hypothesis is that it corresponds to a mixed chromium oxide compound, but not to pure NiCr2O4, as, in this case, the CrO2−/CrO− ratio of 1.5 is by far larger than 0.9 (Table 1). For the PW sample, the same ionic ratio is higher, in satisfactory agreement with pure Cr2O3. Besides these results, the TT oxide is thinner than the PW: 27 nm versus 35 nm for the total oxide thickness, respectively. At this point, we can conclude that the two processes do not lead to the formation of the same intermediate oxide layer.
For TT + PW, the CrO2−/CrO− ratio of the intermediate layer increases from the TT sample (0.9) to the TT + PW sample (1.2; Fig. 3) as a function of sputtering time, the total oxide thickness being in the same range as for the TT sample (25 nm). For the value of 1.2, as in the case of PW, we tend to suggest that it corresponds to a continuous Cr2O3 layer, formed in PW. We propose that the TT oxide layer is not stable in composition in PW conditions, but is stable in thickness, thus protecting the metallic bulk. The chemical modifications, occurring at constant total oxide thickness, tend to result in the same composition as the PW layer.
Regarding the inner layer, taking into account a NiCrO−/NiO− ratio reaching a value of approximately 9 × 10−2 for all treatments (Fig. 3), we propose that it is systematically composed of a chromite spinel. The NiCrO−/NiO− ratio increases gradually in the inner layer from 5 × 10−2 to 9 × 10−2: we attribute these changes either to a change in the spinel stoichiometry or to different mixtures between the spinel and the pure oxides (NiO or Cr2O3). The three-layer structure has also been proposed by Zhiming et al. after immersion in PW but not with the same oxide order (the chromite being located in the intermediate position). The ToF-SIMS approach seems to bring a new and important analytical insight for the establishment of the chemical composition of the oxide, the properties of which are of major interest.