2017 Volume 58 Issue 10 Pages 1386-1391
Electron backscatter diffraction (EBSD) measurements were performed for welded 308 stainless steel corroded under proton irradiation at 473 K. After chemical cleansing and subsequent mechanical polishing to remove oxide layers on the specimen surface for EBSD measurements, selective dissolution of delta ferrite (δ) phase in the dendritic microstructure was detected only for the in-beam corroded specimen. The susceptibility to δ phase dissolution is dependent on the deviation angle of its orientation from the (101) plane on the specimen surface when δ phases are in a grain of austenitic gamma (γ) phase as well as on the γ-γ phase grain boundaries except for the coincidence grain boundaries. Selective dissolution of δ phase would be explained by not only radiolytic products such as radical oxygen acting as electric charge carriers but also the radiation-induced point defects and dislocation structures playing an important role in sustaining anodic reactions at δ phases.
The use of austenitic stainless steels as the primary construction materials of nuclear reactors or nuclear waste containers requires the joining of components by welding. Although the formation of passive oxide film on the surface of austenitic stainless steels ensures excellent resistance to general corrosion, the welding process can increase the susceptibility to localized corrosive attack on the passive film through the introduction of metallurgical changes and residual stresses in the materials1,2). In particular, a dendritic microstructure, namely a cast structure with delta ferrite (δ) phases in the austenitic gamma (γ) phase matrix designed to prevent hot-cracking during weld metal solidification, is responsible for the localized deterioration of passive film due to the depletion of chromium3) or the mismatch of lattice structures4) at the boundaries of the δ-γ phases. Furthermore, many studies1,5–8) indicate that the selective dissolution of δ phases in the electrochemical reactions can lead to the preferential formation of corrosion pits and subsequent initiation of stress corrosion cracking (SCC) along the δ-γ phase grain boundaries. Besides these metallurgical factors, it is well accepted that neutron irradiation can significantly modify SCC behavior through radiolysis of the coolant water and microstructural/microchemical changes in the materials produced by displacement damage. Some radiolytic products such as hydrogen peroxide and other radicals can act as strong oxidants in a water solution, leading to enhanced cathodic reactions in the electrochemical reaction9,10). On the other hand, the development of dislocation structures under irradiation could provide additional internal areas to be oxidized through a higher oxygen diffusion rate along dislocations11), and thereby contribute to the increase in anodic reactions. From the viewpoint of microchemical changes, the radiation-induced segregation of minor elements at the grain boundaries would affect the SCC behavior12).
In the previous study13), stress corrosion tests loaded at 0 and 300 MPa were conducted under 17 MeV proton irradiation for welded 308 stainless steel in aerated water conditions at 473 K. In situ measurement of electrochemical corrosion potential (ECP) and subsequent SEM analysis were also performed for all specimens. The process of SCC was significantly accelerated under irradiation, and the in-beam effect on SCC was discussed in terms of both water radiolysis and radiation-induced microstructures. The progress of internal oxidation at the δ-γ phase grain boundaries was suggested even for the in-beam specimen loaded at 0 MPa, because a slight decrease of ECP was measured during the test. However, in the previous study, the microstructural locations susceptible to internal oxidation in a dendritic structure could not be estimated by a crystallographic approach. In the present study, we conducted EBSD measurements for the specimens of welded 308 stainless steel corroded at 0 MPa in aerated water at 473 K under 17 MeV proton irradiation in the previous study. The objective was to investigate the crystallographic features of the δ and γ phases and their relationship at the boundaries susceptible to localized corrosive attacks for internal oxidation.
The specimen examined by EBSD measurement was welded 308 stainless steel corroded at 0 MPa and 473 K for 2.16 × 105 s in aerated water conditions (DO: 0.15–0.22 ppm, DH: <10 ppb) under proton irradiation in the previous study13). The chemical composition of welded 308 stainless steel and the specimen preparation process for SCC tests are shown in Table 1 and Fig. 1, respectively. The total amount of dose level was estimated at 0.028 dpa for the in-beam specimen under a constant displacement damage rate of 1.3 × 10−7 dpa/s. Details of the welding conditions, specimen preparation, in-beam stress corrosion apparatus, and irradiation conditions are described elsewhere13,14).
| Ni | Cr | Mn | V | C | Si | P | S | Fe | |
|---|---|---|---|---|---|---|---|---|---|
| 308 | 10.55 | 19.05 | 1.73 | 0.064 | 0.02 | 0.23 | 0.026 | 0.004 | Balance |
Specimen dimensions for EBSD measurement. (a) Sampling of specimen from welded joint, (b) Specimen dimensions for SCC test, (c) Optical microscopic photo of metallic structure on fusion zone at specimen gauge
The in-beam corroded specimen was chemically cleansed by hydrochloric acid solution (35%, 200 ml) with pure water (200 ml) and hexamethylenetetramine (1.4 g) for 600 s at room temperature, and then mechanically polished by a choroidal silica suspension (powder diameter of 0.06 μm) for about 1800 s to a mirror finish for EBSD measurements. The depth of surface layer removed during chemical cleansing and subsequent mechanical polishing was estimated by monitoring the diameter of a Vickers hardness testing mark introduced on the specimen prior to these polishing procedures, and adjusted to 4.0 μm in depth from the original specimen surface. Microscopic measurements of dendritic structures in the weld metal were performed at 40 areas using a field-emission scanning electron microscope (FE-SEM, JSM7001-F) equipped with an EBSD detector (EDAX, DVC-4). For each area, the crystallographic orientation was examined at 0.20-μm steps over a surface dimension of 30 × 20 μm2. As for the description of the inverse pole figure (IPF) maps and plots, a standard direction was assigned as the normal direction (ND) of the specimen surface. As a control experiment, microscopic measurements were also conducted for both off-beam corroded and uncorroded specimens after the same cleansing and polishing procedures. All the data derived from EBSD measurements was analyzed using the OIM Version 7.3 software package (TSL Solutions).
Figure 2 shows SEM photos of the polished surface of (a) in-beam, (b) off-beam and (c) uncorroded specimens prepared for EBSD measurements. Chains of irregularly shaped corrosion pits were detected only for the in-beam specimen, as shown in Fig. 2 (a). On the other hand, a few widely scattered corrosion pits were detected for both the off-beam and uncorroded specimens (see Figs. 2 (b) and (c)), and their surface features were quite similar. This indicates hardly any progress of internal oxidation for the off-beam specimen. The few scattered corrosion pits on the surface for both the off-beam and uncorroded specimens may be responsible for the corrosive content in the choroidal silica suspension. Therefore, SEM-EBSD measurements were mainly conducted for the in-beam specimen. Figure 3 shows the sets of a) SEM photo, b) inverse pole figure (IPF) map and c) IPF plots in (a) Area A and (b) Area B for the in-beam specimen, respectively. Schematic unit cells for grains of δ and γ phases and the orientation difference angle at the γ-γ phase grain boundary are described on the IPF map. The crystal orientation of each grain is also indicated in the IPF plots. In a comparison between a) SEM photo and b) IPF map in either (a) Area A or (b) Area B, the chain of corrosion pits (see SEM photo) was certainly attributed to the selective dissolution of δ phase (see IPF map) located along the γ-γ phase grain boundaries as well as in a γ phase grain. As shown in the IPF map in Area A, δ-1 phase was selectively dissolved along the γ1-γ2 phase grain boundary characterized by a random grain boundary with an orientation difference angle of 42°. In contrast, undissolved δ-3 phase was detected along the γ3-γ4 phase grain boundary categorized as a coincidence grain boundary of ∑3, while δ-2 phase was selectively dissolved in a γ-3 phase grain in the IPF map in Area B. Figure 4 presents diagrams of the crystal orientation of dissolved/undissolved δ phases at (a) IPF plots and (b) histogram as a function of the deviation angle from the (101) plane for the in-beam specimen. Selective dissolution of the δ phase was strongly dependent on its crystal orientation: more sensitive to dissolution when the deviation angle was over 30°, more resistant to dissolution below 20°. A scatter diagram of the crystal orientation of dissolved/undissolved δ phases located along the γ-γ phase grain boundary for the in-beam specimen is described as a function of the deviation angle from the (101) plane and the orientation difference angle at the γ-γ phase grain boundary, as shown in Fig. 5. The plots of δ phase located at the coincidence grain boundaries (∑3, 5) are indicated by arrows in the diagram. The border zone of dissolved/undissolved δ phases also appears to be in the range between 20° and 30° of the deviation angle even when δ phases were located along the γ-γ phase grain boundaries, although the undissolved δ phases located along the coincidence grain boundaries (∑3, 5) were detected in the border zone.
SEM photos of polished surface for (a) in-beam, (b) off-beam and (c) uncorroded specimens.
Sets of a) SEM photo, b) inverse pole figure (IPF) map and c) IPF plots in (a) Area A and (b) Area B for the in-beam specimen.
Diagrams of crystal orientation of dissolved/undissolved δ phases at (a) IPF plots and (b) histogram as a function of the deviation angle from the (101) plane for the in-beam specimen.
Scatter diagram of crystal orientation of dissolved/undissolved δ phases located along the γ-γ phase grain boundary for the in-beam specimen.
The Kurdjumov-Sachs (KS) orientation relationship between the γ and δ phases, namely (011)δ//(111)γ and $[1 \bar 1 1]$δ//$[10 \bar 1]$γ, is a crystallographic relationship known to be preserved at the martensitic transformation from austenite to ferrite phase15). As for the γ-3 and δ-2 phases designated in Area B (see Fig. 3 (b)), overlaid pole figures of (a) {110} plane at γ-3 phase with {111} plane at δ-2 phase, and (b) {111} plane at γ-3 phase with {110} plane at δ-2 phase are presented in Fig. 6. With respect to one of the 24 variants of the KS orientation relationship, (011)δ//$( \bar 1\bar 11)$γ and $[\bar 11 \bar 1]$δ//$[0 \bar 1\bar 1]$γ, the respective orientation plots are relatively close but not coincident with each other in the overlaid pole figures shown in Fig. 6 (a) and (b). Similarly, some deviation of the orientation plots in the overlaid pole figures was also detected for the other combinations of γ and δ phases; therefore, the KS orientation relationship between γ and δ phases is hardly applicable to the δ phases examined in the present study.
Overlaid pole figures of (a) {110} plane at γ-3 phase with {111} plane at δ-2 phase, and (b) {111} plane at γ-3 phase with {110} plane at δ-2 phase in Area B for the in-beam specimen.
It is well known that the selective dissolution of δ phases in the dendritic structure of weld metal for stainless steel can lead to the initiation of SCC in corrosive solutions including chloride (Cl−) ions1,5). Since the passive layer has an inner region rich in chromium oxide and an outer region rich in iron oxide, the Cl− ions act as electric charge carriers to provide donors and acceptors at point defects in the outer and inner layers of the passive film, respectively5). Tensile stress contributes to the increase in concentration of donors and acceptors through the introduction of additional point defects in the passive film. When the passive film preferentially ruptures at the γ-δ phase grain boundaries under tensile stress, a galvanic cell is formed on the fresh metal surface. Because of the difference in concentration of solute elements between the two phases, the δ phase functions as an anodic electrode in the galvanic cell, leading to selective dissolution of the δ phase. The decrease of ECP has been demonstrated during the process of selective dissolution1).
In the present experiments, selective dissolution of δ phases was detected for the in-beam specimen, as shown in Fig. 3. In the in-beam corrosion tests conducted in the previous study13), the in situ ECP measurement confirmed a slight negative proportion of ECP during the tests. Although Cl− ions were not present in the aerated water solution at the in-beam corrosion tests, radiolytic products such as radical oxygens would act as electric charge carriers under irradiation. Furthermore, the anodic reactions at δ phases would be sustainable due to not only the enhanced concentration of electric charge carriers at the radiation-induced point defects in the passive film but also the formation of galvanic cells supported by the higher oxygen diffusion rate along the radiation-induced dislocation structures. Thus, the dissolution of δ phases would be promoted for the in-beam specimen, irrespective of stress-free and Cl− ion-free conditions. In comparison with the contribution of radiolysis and the radiation-induced dislocation structures, the radiation-induced segregation may have little influence on the dissolution of δ phases at dose level of 0.028 dpa in the present study.
The susceptibility to δ phase dissolution dependent on its crystal orientation was evidenced for the in-beam specimen, as shown in Fig. 4. The susceptibility was higher when the deviation angle from the (101) plane was over 30°, while it was lower below 20°. Since close-packed {101} planes for the ferritic phase have the intrinsic property of higher corrosion resistance16), the dependence of dissolved δ phase on its crystal direction could be explained by the anisotropic sensitivity of dissolution to the orientation of δ phase. As for the δ phases located on the coincidence γ-γ grain boundaries (∑3, 5), dissolution could not be detected even when their orientation lay in the border zone between 20° and 30° of the deviation angle, as shown in Fig. 5. It is well accepted that the intergranular stress corrosion cracking (IGSCC) at the coincidence grain boundaries is more effectively suppressed due to the lower grain boundary energy than that of random grain boundaries17,18). Although the lower grain boundary energy at the coincidence grain boundaries may play an important role in suppressing the dissolution of δ phases located on the boundaries, the experimental data on δ phases in the present study would be too limited to clarify whether the location on the coincidence grain boundary is more influential than the crystal orientation in suppressing the dissolution of δ phases. However, there is no doubt that the susceptibility to δ phase dissolution is dependent on the deviation angle of its orientation from the (101) plane of the specimen for δ phases located in the γ phases and on the γ-γ phase grain boundaries except for the coincidence grain boundaries. Thus, the crystallographic features of δ, γ phases and their relationship at the boundaries would strongly influence the localized corrosive attacks for internal oxidation, namely the selective dissolution of δ phase in the weld metal.
In the ferritic-austenitic solidification mode (FA mode) for austenitic stainless weld metals, two types of ferritic morphology have been reported; one is known as a vermicular ferrite, and the other is a lathy ferrite19). Since the KS orientation relationship is established at the γ phase grain boundary for the lathy ferrite phase, the lathy ferrite shows a lower degree of sensitization, leading to a higher resistance to corrosion19,20). However, in the present study, because the δ phases examined did not meet the KS orientation relationship between γ and δ phases (see Fig. 6), almost all δ phases would be the vermicular ferrite phases. The proportion of lathy ferrite to vermicular ferrite in weld metal solidified in FA mode is reported to be strongly dependent on the crystallographic orientation relationship between the primary γ and δ phases during solidification as well as on the welding heat flow rate and direction against the preferential growth direction of γ and δ phases19,20). More extensive efforts to accumulate data on EBSD measurements for the dendritic structures including both vermicular and lathy ferrite phases are necessary to improve the understanding of selective dissolution of δ phase in the weld metal under irradiation.
Electron backscatter diffraction (EBSD) measurements were performed for welded 308 stainless steel corroded under proton irradiation at 473 K. From the analytical results on the dendritic structures in weld metal for the in-beam and off-beam specimens, the following conclusions were drawn:
