Oxidation Behavior of an Austenitic Stainless Steel ...

Download PDF
56 downloads 0 Views 982KB Size Report
Comment
EDF Energy and its chemical composition is given in Table 1. Although this material is representative of a typical AGR boiler tube, it was provided in a solution ...
Oxidation Behavior of an Austenitic Stainless Steel used in the UK Advanced GasCooled Reactors B. Chen1*, J. Lindsay1, R.A. Ainsworth2, F. Scenini1 School of Materials, The University of Manchester, Manchester M13 9PL, UK 2 School of Mechanical, Aerospace & Civil Engineering, The University of Manchester, Manchester M13 9PL, UK *[email protected] 1

ABSTRACT The UK’s advanced gas-cooled (AGR) nuclear reactors have operated over the last 30 years, with the austenitic stainless steel sections primarily operating at temperatures ranging from 470 °C up to 650 °C. The coolant gases used in this type of nuclear system contain a mixture of carbon dioxide, carbon monoxide, hydrogen, methane and water vapor. A number of cracks have been reported in superheater boiler components made from 4 mm thick austenitic stainless steel tubes. The mechanism underlying the initiation of cracks is believed to be creep-fatigue which may be exacerbated by carburization of the metal surface, associated with the presence of a duplex oxide layer. In this paper, complementary microstructural characterization techniques have been used to investigate the oxidation behavior of Type UNS S31609 stainless steel in the simulated AGR environments. A primary focus was given to the effects of surface finish and the water vapor content on oxidation. The experimental results show that surface deformation promotes the formation of a thin oxide layer, whereas a deformation-free surface leads to formation of thick duplex oxide layers. Furthermore, the presence of water vapor in the mixed gas environment accelerated the growth of the oxides. Key words: High temperature oxidation, Surface finish, Electron microscopy, Stainless steel

INTRODUCTION Type 316H austenitic stainless steel [UNS S31609] is widely used in the UK’s advanced gascooled reactors (AGRs) for the superheater of the boilers because of its good oxidation and creep resistance. UNS S31609 stainless steel sections primarily operated at temperatures ranging from 470 ºC up to 650 ºC,1 and the coolant gas is a mixture of carbon dioxide (CO2), carbon monoxide (CO), hydrogen (H2), methane (CH4) and water vapor (H2O), where the CO2 is the dominated gas.2 The presence of carbonaceous gases leads to the formation of a carburized layer on the surface of UNS S31609 stainless steels which is believed to be an

exacerbating factor for the initiation of creep dominated creep-fatigue cracks.3 It is generally recognized that the carburization is associated with the presence of poorly protective oxides such as Fe-rich spinels duplex oxide layers, which are porous and permeable to carbon. 4-6 The carburized stainless steel has the potential to adversely impact the local material creep ductility, which reduce the resistance of the material to crack initiation at high temperature. 1 Thus it is important to understand the oxidation behavior of UNS S31609 stainless steel in a CO2 based high temperature environment. Many factors could affect the oxidation behavior of UNS S31609 stainless steels, but two of concerns in this paper are (i) surface finish and (ii) H2O content in the CO2 based environment. The effect of surface finish on oxidation behavior of Cr-containing steels has been investigated by Ostwald and Grabke.4 Five surface finishes were considered for both the 9% Cr-1% Mo and 20% Cr-32% Ni steels: (a) electropolished, (b) 1 μm diamond polished, (c) 600-grit SiC ground, (d) 25 μm fine grain sandblasted, and (e) 150 μm coarse grain sandblasted. Weight gain measurements were undertaken to evaluate the oxidation rate of these two steels after oxidation at 600 ºC for up to 100 h in three gaseous environments: H2-2.5% H2O, N2-20% H20.49% H2O, and N2-20.5% O24. A general increase in weight gain was obtained for the intensified surface deformation, with the slowest oxidation for the electropolished surface finish (a) and the fastest oxidation for the sandblasted surface finish (e). 4 However, it is unclear whether this effect of surface finish also applies for the oxidation behavior of UNS S31609 stainless steel in the CO2 based environment considering that the effect of surface finish on the types of the oxides is believed to be dependent on the oxygen partial pressure of the environment.4 The effect of water content on gaseous oxidation has been widely investigated in binary Fe-Cr and ternary Fe-Cr-Ni alloys in Ar-20% CO2 based gas mixtures at 650 ºC for time durations of up to 240 h.7 Three different H2O contents were considered: 0%, 5% and 20% H2O. It has been concluded that additions of H2O had no significant effect on the extent of Fe-rich spinel oxide formation on Fe-Cr-Ni alloys.7 In addition, there was no H2O effect on the formation of Cr2O3 oxides formed on both the Fe-Cr and Fe-Cr-Ni alloys.7 However, this observation appears to be inconsistent with the fact that additions of H2O are known to accelerate the growth of Cr2O3 oxides.5, 8-11 For example, Putatunda11 studied the oxidation behavior of Type 304 austenitic stainless steel [UNS S30400] in both dry and wet (10% H2O) CO2 environments at temperatures between 200 ºC and 400 ºC for time durations of up to 32 h. It was found that a higher oxidation rate occurred in the CO2 environment containing 10% H2O compared with that in the dry CO2 environment. In the present study, oxidation tests of UNS S31609 stainless steels were undertaken at 550 ºC in CO2 based environments containing 1% CO and different contents of H 2O (0%, 300 vppm, and 3%). The oxidation environment containing 1% CO and 300 vppm H 2O represents a typical AGR coolant gas. In fact, the content of CO in the AGR coolant gas was reported to be in a range from 0.6% to 1.2%, whereas the content of H 2O is reported to range between 174 vppm and 395 vppm.2 Two different surface finishes were considered: 600-grit SiC ground and colloidal silica OPS polished. These two surface finishes were not aimed at representing an engineering condition but were chosen for mechanistic understanding purposes. The effects of both the H2O content and surface finish on the oxidation behavior of UNS S31609 stainless steels were investigated by using complementary microstructural characterization techniques.

MATERIAL AND OXIDATION TEST A 4 mm wall thickness superheater tube made of UNS S31609 stainless steel was provided by EDF Energy and its chemical composition is given in Table 1. Although this material is representative of a typical AGR boiler tube, it was provided in a solution annealed condition (1100 °C for 3 mins followed by water quenching) and was not service aged. The grain size of this material was measured to be 34 ± 4 μm using the linear intercept method. The superheater tube was sliced by using a Struers Accutom-5† precision cutting machine. Each 7 mm thick slice was then segmented as shown in Figure 1 (a) to obtain six pieces of specimens for oxidation tests. The top surface of each specimen, Figure 1 (b), was then prepared to either (A) 600-grit SiC ground or (B) colloidal silica OPS polished surface finish. The OPS surface finish was believed to produce a deformation-free surface.12-14 Two rectangular shaped side surfaces were 600-grit SiC ground surface finished, while the curved surfaces were left in the as-received state. The evaluation of the oxidation rate will be described later. Table 1 Chemical Composition of the UNS S31609 Stainless Steel Superheater Tube (wt.%)

C

Si

0.05 0.53

Mn

P

1.55

0.03

S

Cr

0.005 16.89

Mo 2.04

Ni

Co

11.25 0.09

Figure 1: Dimensions of UNS S31609 stainless steel specimens removed from the superheater tube: (a) six specimens were segmented from each ring of the superheater tube; and (b) oxidation test specimen with a height of 7 mm and a width of 4 mm, where the top surface was prepared to two different surface finishes (OPS polished and 600-grit SiC ground).

The oxidation tests were carried out in a horizontal double-walled reaction tube made out of quartz. The furnace had a three zone controller which provided a hot zone of 150 mm long where the temperature was controlled by thermocouples within ± 1 ºC. The gases were mixed using calibrated digital flow meters and injected into the outer wall of the reaction tube where the mixed gas was pre-heated. The mixed gases then flowed into the inner wall of the reaction tube where the specimens were placed and oxidized. All oxidation tests were undertaken at a temperature of 550 °C for 500 h. Three oxidation gas environments were selected: (i) 1% CO / CO2, (ii) 1% CO / CO2 + 300 vppm H2O, and (iii) 1% CO / CO2 + 3% H2O. The total pressure of the mixed gas in the reaction tube was at 1 atmospheric pressure (atm). The oxygen partial

†

Trade name

pressure, PO , for the 1% CO / CO2 environment (i) was determined following the wellestablished thermodynamic equations:15 2

CO 2  CO  12 O 2 ; K 

PCO PO1 / 2

(1)

2

PCO

2

where K is the temperature dependent equilibrium constant for the CO2 associated reaction. The PO can be calculated by knowing the equilibrium PCO PCO ratio. For the gas environment 2

2

(i) 1% CO / CO2, it has been assumed that the equilibrium partial pressure ratio, PCO PCO , is equal to the volumetric fraction of the injected mixed gases, 1% CO / 99% CO2. The temperature dependent equilibrium constant K was calculated by: 2

K  exp(

ΔG 0 ) RT

(2)

using the standard energies for the CO2 reaction,15 temperature T of 550 ºC and gas constant R of 8.314 JK-1mol-1, the oxygen partial pressure PO is calculated to be 1.62 × 10-23 atm. For the gas environments (ii) and (iii), which contain H2O contents of 300 vppm and 3% respectively, it was not possible to calculate the oxygen partial pressure since the equilibrium 16 PH PH O ratio was unknown and the gases are in dynamic equilibrium . However, it is 2

2

2

reasonable to assume that the addition of water into the 1% CO / CO2 oxidation environment leads to a higher oxygen partial pressure because of the larger value of Gibbs free energy for the H2O reaction compared with that for the CO2 reaction.15 The wet gases were generated by passing the relevant gases through a water bubbler. For the oxidation environment (iii) 1% CO / CO2 + 3% H2O, the pre-mixed gases with a composition of 1% CO / CO2 were injected into the water bubbler to pick up 3% H2O. The water bubbler filled by the distilled water was kept at the room temperature. Since the total pressure of the gas environment was at 1 atm., the proportion of the H 2O uptake by the gas is theoretically ~3% based on the partial pressure calculation. 17 The total gas flow rates were fixed at ~0.037 L/min and this results in a refresh rate of 2.2 L/h for the oxidation testing rig. As described above, oxidation tests were carried out at 550 °C for 500 h. Table 2 shows the oxidation testing environments for all the specimens. The top surface of each specimen was prepared to either colloidal silica OPS or 600-grit SiC ground surface finish. These specimens were then exposed to three gas environments at 550 °C for 500 h, specimens 2 to 4 and 6 to 8, Table 2. Additional specimens were also prepared to provide the reference condition, i.e. not subjected to high temperature oxidation, specimens 1 and 5 in Table 2. POST OXIDATION CHARACTERIZATION TECHNIQUES After oxidation tests in the three selected CO2 based environments, specimens were removed from the reaction tube. These oxidized specimens, as well as additional reference specimens 1 and 5, Table 2, were visually examined with a primary focus of understanding the effect of surface finish on oxides. The specimens were then examined by using both optical

microscopy and field emission gun scanning electron microscopy (FEG-SEM) FEI Quanta 650† and Philips XL30† instruments. To evaluate the oxidation rate, the thickness of oxides was measured on SEM images of the cross-sectioned specimens 2 and 6. These two specimens represent the OPS surface finish and 600-grit SiC ground surface finish, respectively. The cross-sections were made along the perpendicular direction to the original top surface of the oxidized specimen, see Figure 1 (b) for the geometry of the specimen. Table 2 Summary of Specimens Exposed to Oxidation Testing Environments at 550 ºC for 500 h

Specimen ID 1 2 3 4 5 6 7 8

Surface finish Colloidal silica OPS polished

600-grit SiC ground

Oxidation environment No oxidation 1% CO / CO2 1% CO / CO2 + 300 vppm H2O 1% CO / CO2 + 3% H2O No oxidation 1% CO / CO2 1% CO / CO2 + 300 vppm H2O 1% CO / CO2 + 3% H2O

A focused ion beam (FIB) workstation, FEI Quanta 3D† dual beam FIB / FEG-SEM, was used to reveal the detailed sub-surface features of the oxidized specimens. FIB cross-sections were milled normal to the specimen surface at selected positions, mainly across grain boundaries, to produce high precision trenches. A 30 keV accelerating voltage was used with the ion beam current adjusted to achieve the milling requirements, ranging from 65 nA for rough milling to 7 nA for final polishing. More details about the application of FIB cross-section technique to reveal sub-surface features of a stainless steel specimen can be found elsewhere. 18 Transmission electron microscopy (TEM) thin foil was cut using a FIB instrument from mainly the outer and inner oxide layers of specimen 1. This specimen was initially prepared to an OPS surface finish, and then oxidized in a 1% CO / CO2 environment at 550 ºC for 500 h, Table 2. The area of FIB-TEM thin foil was representative of that containing two grains and one grain boundary. After the specimen was mounted on a copper grid, a 300 keV scanning transmission electron microscope (STEM) (FEI Tecnai F30 G2†) was used to undertake compositional microanalysis and to image the oxide formed on the OPS surface finished specimen. A Proto† portable residual stress X-ray diffractometer was used to measure the surface residual stresses for both the oxidized and non-oxidized specimens reported in Table 2. This machine is equipped with two fiber optic based solid state detectors to enable two sections of the X-ray diffraction cone to be captured simultaneously. This reduces the data collection time significantly. A Mn X-ray tube anode with an aperture size of 2 mm was used for the residual stress measurement using the classic sin2ψ method and the appropriate elastic constants. A characteristic peak at a 2θ angle of ~152° corresponding to the {311} grain families was used to interpret the residual stresses representative of the bulk material behavior. The X-ray diffraction elastic constants for -ν/E and (1+ν)/E were reported to be -1.87 × 10-6 MPa-1 (S1) and 6.98×10-6 MPa-1 (1/2S2).19 These were used to derive the residual stresses from the †

Trade names

measured lattice spacings. Detailed information about the X-ray diffraction technique to determine surface residual stresses can be found in some specialized publications.20-22 The {311} grain families of an austenitic stainless steel have been shown to be the least affected by the presence of inter-granular residual stresses associated with the prior plastic deformation history.23 Both the x and y directions for each specimen were measured at least three times. The y direction is along the shorter side (4 mm) of the specimen and the x direction is the one perpendicular to the y direction, Figure 2. The averaged magnitudes of residual stresses together with their standard deviations are reported in the results section. RESULTS Oxide Formation and Growth Figures 2 (a) and (b) compare the OPS polished surface finish (specimen 2) and 600-grit ground surface finish (specimen 6), revealed by visual examination. The top surface of the oxidized specimens showed different oxide features, Figures 2 (a) and (b). Both of the specimens 2 and 6 were subjected to a 500 h oxidation at 550 ºC in a 1% CO / CO 2 environment, Table 2. The OPS polished surface of specimen 2 formed a dark colored oxide, Figure 2 (a), whereas the 600-grit ground surface of specimen 6 formed a brighter and yellowish colored oxide, Figure 2 (b). Optical micrographs of specimens 2 and 6 subjected to a 1% CO / CO2 oxidation are presented in Figures 3 (c) and (d) for both the OPS polished and 600-grit ground surface finishes. These are compared with specimens 1 and 5, which were not subjected to high temperature oxidation, Figures 3 (a) and (b). For the non-oxidized OPS surface finished specimen 1, the morphology of the grains was observed, Figure 3 (a). The OPS surface finished specimen 2 showed a thick and dark colored oxide layer after oxidation, Figure 3 (c). The presence of the less oxidized locations can also be observed for the OPS surface finished specimen, see the white colored “islands” visible in Figure 3 (c). However, for the 600-grit SiC ground surface finished specimen 6, a thinner oxide was formed sporadically on the stainless steel specimen, Figure 3 (d). In addition, the 600-grit SiC grinding marks can be still seen on specimen 6 when compared with the non-oxidized specimen 5, Figures 3 (b) and (d).

x y

x y

(a) (b) Figure 2: Visual examination of two surface finished stainless steel specimens which were subjected to 500 h oxidation tests at 550 ºC in a 1% CO / CO2 environment: (a) colloidal silica OPS polished specimen 2 and (b) 600-grit SiC ground specimen 6.

Figure 3: Optical micrographs of the top surfaces of the oxidized stainless steel specimens (1% CO / CO2 at 550 ºC for 500 h): (a) non-oxidized specimen 1 with OPS surface finish; (b) nonoxidized specimen 5 with 600-girt SiC ground surface finish; (c) oxidized specimen 2 with OPS surface finish; (d) oxidized specimen 6 with 600-grit SiC ground surface finish.

SEM observations were carried out for all specimens 2 to 4 and 6 to 8, which were all oxidized at 550 ºC for 500 h. Specimen IDs have been given in Table 2. Figures 4 (a) to (f) compare the surface oxides formed on OPS polished specimens 2 to 4 with those on the 600-grit SiC ground specimens 6 to 8. Specimens 2 (OPS polished) and 6 (600-grit ground) were oxidized in a 1% CO / CO2 environment, Figures 4 (a) and (b). The presence of thick oxides on the original grain interiors, in contrast with the grain boundaries, can be observed in OPS polished specimen 2, Figure 4 (a). This is consistent with the white colored “islands” observed by optical microscopy, Figure 3 (c). However, the 600-grit SiC ground specimen 6 formed a very thin oxide layer, and the grinding marks remained on the specimen surface, Figure 4 (b). The thickness of oxides was measured on SEM images of the cross-sectioned specimens 2 and 6. The thickness of oxides was measured to be 3 μm for the OPS polished specimen 2, and 0.1 μm for the 600-grit SiC ground specimen 6. Specimens 3 and 7 were oxidized in a 1% CO / CO 2 + 300 vppm H2O environment, whereas specimens 4 and 8 were oxidized in a 1% CO / CO2 + 3% H2O environment. The presence of the water vapor in the mixed 1% CO / CO2 gas environment tended to accelerate the growth of the oxides for both the OPS polished and 600-grit ground specimens, when comparing Figures 4 (a) and (b) with Figures 4 (c) to (f). However no significant differences were found between the oxidation environment containing 300 vppm H2O shown in Figures 4 (c) and (d) and that containing 3% H2O shown in Figures 4 (e) and (f). For the OPS polished specimens 3 and 4,

the oxides also formed at grain boundaries, Figures 4 (c) and (e). Conversely, OPS polished specimen 2, Figure 4 (a), which was oxidized in a dry 1% CO / CO2 environment, formed thick oxide preferentially within the grain interiors.

Figure 4: SEM images of the top surfaces of oxidized specimens at 550 ºC for 500 h: (a) OPS polished specimen 2, subjected to a 1% CO / CO2 environment; (b) 600-grit SiC ground specimen 6, subjected to a 1% CO / CO2 environment; (c) OPS polished specimen 3, subjected to a 1% CO / CO2 + 300 vppm H2O environment; (d) 600-grit SiC ground specimen 7, subjected to a 1% CO / CO2 + 300 vppm H2O environment; (e) OPS polished specimen 4, subjected to a 1% CO / CO2 + 3% H2O environment; and (f) 600-grit SiC ground specimen 8, subjected to a 1% CO / CO2 + 3% H2O environment.

Sub-surface Oxide Observation Figures 5 (a) and (b) show the FIB cross-sectional view of the oxidized specimens 2 (OPS polished) and 6 (600-grit SiC ground). Both specimens were oxidized in a 1% CO / CO 2 environment at 550 ºC for 500 h. The formation of duplex oxide layers can be found in OPS polished specimen 2, Figure 5 (a), where the outer oxide layer exhibited crystallographic orientation features. However, only a thin oxide layer formed for the 600-grit ground specimen 6, Figure 5 (b). The presence of a deformation layer is associated with the 600-grit ground surface finish, which has been indicated in Figure 5 (b). Figures 5 (c) and (d) show the FIB cross-sectional view of the oxidized specimens 4 (OPS polished) and 8 (600-grit SiC ground). The addition of water vapor into the oxidation environment tended to increase the thickness of the oxides formed on the OPS polished samples as shown in Figures 5 (a) (dry gas) and Figures 5 (c) (3% water). The effect of water addition on the increase in the oxide thickness can also be seen for 600-grit SiC ground specimens 6 and 8 as shown in Figures 5 (b) and (d), respectively.

Figure 5: Secondary electron images of the FIB cross-sections revealing the sub-surface features of the specimen oxidized at 550 ºC for 500 h: (a) OPS polished specimen 2, subjected to a 1% CO / CO2 environment; (b) 600-grit SiC ground specimen 6, subjected to a 1% CO / CO2 environment; (c) OPS polished specimen 4, subjected to a 1% CO / CO2 + 3% H2O environment; (d) 600-grit SiC ground specimen 8, subjected to a 1% CO / CO2 + 3% H2O environment.

Compositional Microanalysis To understand the FIB revealed channeling contrast for the inner oxide layer formed on OPS polished specimens, such as specimen 2 tested in dry gas shown in Figure 5 (a) and specimen 4 tested in 3% water shown in Figure 5 (c), a FIB prepared TEM thin foil from specimen 2 (OPS polished) was examined by analytical STEM. The STEM image in Figure 6 (a) reveals the multilayered structure for the inner oxide together with the outer oxide. From the O element distribution, Figure 6 (b), it is confirmed that they are rich in O element. In addition, the inner oxide is shown to be rich in Cr element when compared with the outer oxide, Figure 6 (c). It has been also observed that the outer oxide contains higher Fe element compared with the inner oxide, Figure 6 (d). The highest Fe element was observed in the stainless steel matrix, Figure 6 (d). More importantly, the Ni element distribution is consistent with the presence of the channeling contrast, initially revealed by the secondary electron image of FIB, Figures 5 (a) and (c). From both the STEM image and Ni element map, Figures 6 (a) and (e), it can be seen that the areas with a darker channeling contrast in the inner oxide are associated with the relative depletion in Ni element.

Figure 6: STEM image of the FIB prepared TEM thin foil and the EDX compositional maps for both the outer and inner oxides formed on specimen 2 which was subjected to a 1% CO / CO 2 oxidation at 550 ºC for 500 h: (a) STEM image; (b) O map; (c) Cr map; (d) Fe map; (e) Ni map.

Residual Stress X-ray diffraction residual stress measurements were undertaken for all specimens 1 to 8. Figure 7 summarizes the residual stresses along both the x and y directions, as defined in Figures 2 (a) and (b). OPS polished specimen 1 (non-oxidized) showed little surface residual stress, whereas 600-grit ground specimen 5 (non-oxidized) showed a significant level of compressive residual stress, Figure 7 (a) for the x direction and Figure 7 (b) for the y direction. Residual stress was measured to be -120 MPa along x direction and -250 MPa along y direction for specimen 5 with a 600-grit SiC ground surface finish. Since the 600-grit grinding marks were all intentionally left along the y direction, the magnitude difference between the x and y directions could be associated with the grinding direction. After oxidizing the OPS polished specimens, the presence of compressive residual stresses can be observed, see OPS polished specimens 1 to 4 in Figures 7 (a) and (b). The highest compressive residual stress occurred in specimen 4 which was oxidized in a 1% CO / CO2 + 3% H2O environment. This indicates that the formation of thick oxides on an OPS polished specimen changes the surface residual stresses. In contrast, the compressive residual stress formed on the 600-grit ground specimens did not change as a result of oxidation; see 600-grit SiC ground specimens 5 to 8 in Figures 7 (a) and (b). This seems to be consistent with the relatively thin oxide formed on these 600-grit specimens, as revealed by FIB cross-section, Figures 5 (b) and (d).

1 1 3

5 6 7 8

2

3

2

6 4

4

5

7 8

(a) (b) Figure 7: X-ray diffraction measured surface residual stresses for all specimens 1 to 8: (a) x direction and (b) y direction. Numbers indicate specimen IDs given in Table 2.

DISCUSSION The oxidation behavior of UNS S31609 austenitic stainless steel in a simulated AGR coolant gas environment has been investigated. It is found that OPS polished stainless steel specimens had formed thick duplex oxide layers, Figures 5 (a) and (c). This is in contrast with the thin oxide layer formed on the 600-grit SiC ground stainless steel specimens, as shown in Figures 5 (b) and (d). The OPS surface finish is representative of a deformation-free surface, and thereby is similar to an electropolished surface finish. UNS S31609 stainless steel contains ~17% Cr, which is in the marginal content of the so-called high or low Cr steels by Caplan.24 When the oxidation rate of Cr-containing steels (ranging from 10% to 26% Cr) in ArH2O environment at 600 ºC was studied for up to 20 h, it was found that cold work increased

the oxidation of the 10%, 15% and 16% Cr steels but decreased that of the steels containing 24% and 26% Cr.24 Thus the lower oxidation rate in 600-grit ground UNS S31609 stainless steel specimen is consistent with the oxidation behaviour of high Cr steels by Caplan. 24 However, the present results are inconsistent with the work undertaken by Ostwald and Grabke4, where five surface finishes and two types of Cr-containing steels (both the 9% Cr-1% Mo and 20% Cr-32% Ni steels) were examined. Oxidation tests were undertaken at 600 ºC for up to 100 h in three gas environments: H2-2.5% H2O, N2-20% H2-0.49% H2O, and N2-20.5% O2.4 It is likely that the duplex oxides formed on OPS polished UNS S31609 stainless steel specimens were poorly protective and thereby grew faster, whereas the oxides formed on 600grit ground specimens were highly protective. In fact, higher porosity was observed in the outer oxide of those OPS polished specimens and shown in Figure 5 (c). In addition, Figure 4 (a) reveals that the oxides formed on OPS polished specimen tend to form preferentially at grain interiors when compared with grain boundaries. This indicates that the elemental diffusion coefficients, in particular for Fe and Cr, are different for the grain interiors and grain boundaries. Although no direct measurement of the diffusion coefficients was given here, it is likely that Cr element diffused slowly compared with the Fe element for the grain interiors. When the specimen was cold deformed by 600-grit SiC grinding, the presence of dislocations in the grain interiors led to a faster diffusion rate for Cr element and thus thinner oxide was observed in Figures 5 (b) and (d). It has been demonstrated that addition of H2O into 1%CO / CO2 environments at 550 ºC accelerated the oxidation rate of UNS S31609 stainless steel specimens. This applies for both the OPS polished surface finish and 600-grit SiC ground surface finish, Figures 5 (a) to (d). In fact, the oxides gradually formed on the grain boundaries of OPS polished specimens by increasing the H2O content, Figures 4 (a), (c) and (e). By undertaking STEM EDX compositional microanalysis, the presence of the multi-layered structure for the inner oxide of OPS polished specimen has been observed. The multi-layered structure consists of Ni enriched locations and depleted locations, Figures 6 (a) and (e). The relative thermodynamic stability of the NiO, Cr2O3 and Fe3O4 oxides were compared based on Ellingham diagram.15 It shows that NiO is the least stabilised one among the three. This explains for the preferential oxidation of either Fe or Cr elements compared with Ni element. However, since the O map in Figure 6 (b) does not show any indication of inhomogeneous distribution of O element in the multi-layered structure for the inner oxide, this might suggest that the channelling contrast revealed in Figures 5 (a) and (c) and 6 (a) is associated with the different chemical composition for the inner oxide. In fact this type of inner oxide has been also observed for D9 stainless steel in a lead bismuth eutectic coolant environment.25 CONCLUSIONS The conclusions of this work can be summarized: I.

II.

A colloidal silica OPS polished surface finish promotes the formation of thick duplex oxide layers on UNS S31609 stainless steel, where both the inner and outer oxides can be observed. However, a 600-grit SiC ground surface finish leads to the formation of a relatively thin oxide layer. The outer oxide is rich in Fe element when compared with the inner oxide. On the other hand, the inner oxide is rich in Cr element when compared with the outer oxide. The

III.

IV.

multilayered structure for the inner oxide is associated with the inhomogeneous distribution of Ni element. Addition of H2O into a 1% CO / CO2 gas environments enhances the oxidation rate for both the OPS polished and 600-grit SiC ground surface finished UNS S31609 stainless steel specimens. The formation of the oxides on OPS polished UNS S31609 stainless steel specimens leads to a significant change of residual stress. However, there is little change of the magnitude of the residual stress on 600-grit ground ones. ACKNOWLEDGEMENTS

The authors are grateful for financial support from the ENVISINC project, which has been funded through the Innovate UK (formerly Technology Strategy Board), part of UK Government Department for Business, Innovation and Skills. B.C. is partly supported by EDF Energy to undertake this research. We also thank Mr Teruo Hashimoto, Experimental Officer of School of Materials, The University of Manchester, for assistance in undertaking STEM compositional microanalysis. REFERENCES 1 M.P. O'Donnell, et al., "High temperature issues in advanced gas cooled reactors (AGR)," TAGSI/FESI Symposium: Structural Integrity of Nuclear Power Plant, (The Welding Institute, Cambridge: EMAS Publishing, 2013) 2 N.M. Smith, EDF Energy Report E/REP/BPKB/0093/AGR/11, "Summary of AGR Coolant Composition 2010" (Barnwood: EDF Energy plc., 2013) 3 R.A.W. Bradford, E/REP/BBAB/0014/AGR/10, "Current Status of Understanding of the Cracking in the Superheater Bifurcations and Tailpipe-Pintle Welds: A Summary of Structural Analysis Work Undertaken in the Period 2007 to 2009" (Barnwood: EDF Energy Nuclear Generation Ltd., 2010) 4 C. Ostwald, H.J. Grabke, "Initial oxidation and chromium diffusion. I. Effects of surface working on 9-20%Cr steels," Corros. Sci., 46 (2004): pp. 1113-1127. 5 G.H. Meier, et al., "Corrosion of iron-, nickel- and cobalt-base alloys in atmospheres containing carbon and oxygen," Oxid. Met., 17 (1982): pp. 235-262. 6 C.S. Giggins, F.S. Pettit, "Corrosion of metals and alloys in mixed gas environments at elevated temperatures," Oxid. Met., 14 (1980): pp. 363-413. 7 T. Gheno, et al., "Mechanism of breakaway oxidation of Fe-Cr and Fe-Cr-Ni alloys in dry and wet carbon dioxide," Corros. Sci., 64 (2012): pp. 222-233. 8 N.K. Othman, et al., "Water vapour effects on Fe-Cr alloy oxidation," Oxid. Met., 73 (2010): pp. 337-352.

9 G. Hultquist, et al., "Hydrogen in chromium: Influence on the high-temperature oxidation kinetics in H2O, oxide-growth mechanisms, and scale adherence," Oxid. Met., 54 (2010): pp. 110. 10 M. Michalik, et al., "Effect of water vapour on growth and adherence of chromia scales formed on Cr in high and low pO2-environments at 1000 and 1050oC," Mater. High Temp., 22 (2005): pp. 213-221. 11 S.K. Putatunda, "Corrosion behaviour of sensitised austenitic (AISI 304) stainless steel in a CO2 atmosphere," Mater. Sci. Eng. A, 91 (1987): pp. 89-95. 12 B. Chen, et al., "Microstructural sensitivity of 316H austenitic stainless steel: Residual stress relaxation and grain boundary fracture," Mater. Sci. Eng. A, 527 (2010): pp. 7387-7399. 13 Buehler, "Oxide polishing", Available from: http://www.buehler.co.uk/products/consumables/oxide-polishing.html. 14 J. Lindsay, et al., "Characterisation of stress corrosion cracking and internal oxidation of alloy 600 in high temperature hydrogenated steam," 16th Int. Conf. on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, (Asheville: NACE, 2013) 15 B. Gleeson, 1.09 - Thermodynamics and Theory of External and Internal Oxidation of Alloys, in: B. Cottis, M. Graham, R. Lindsay, S. Lyon, T. Richardson, D. Scantlebury, H. Stott (Eds.) Shreir's Corrosion, (Oxford, UK: Elsevier, 2010), pp. 182-184. 16 J. Sumner, N.J. Simms, CERT/0201, "9Cr-1Mo ferritic steel oxidation/carburisation mechanisms and wall-loss, Part 2: thermodynamic evaluation of the gas stream/heat exchanger surface" (Cranfield, UK: Centre for Energy and Resource Technology, Cranfield University, 2013) 17 Chapter 3.4.2. Vapour pressure of water at temperatures between 0 and 360 oC, In: Tables of Physical & Chemical Constants, 16th ed, (Kaye & Laby Online, www.kayelaby.npl.co.uk, 2005) 18 B. Chen, et al., "An improved method to identify grain boundary creep cavitation in 316H austenitic stainless steel," Ultramicroscopy, 111 (2011): pp. 309-313. 19

PDF-4+, (The International Centre for Diffraction Data, www.icdd.com, 2014)

20 M.J. Marques, et al., "X-ray diffraction characterisation of ion-implanted austenitic stainless steel," Surf. Coat. Tech., 195 (2005): pp. 8-16. 21 I.C. Noyan, J.B. Cohen, Residual Stress, Measurements by Diffraction and Interpretation, (New York: Springer-Verlag, 1987). 22 M.E. Fitzpatrick, et al., Determination of Residual Stresses by X-ray Diffration - Issue 2, A National Measurement Good Practice Guide No. 52, (Teddington, UK: National Physical Laboratory, 2005).

23 B. Chen, et al., "Quantifying internal stress and internal resistance associated with thermal ageing and creep in a polycrystalline material," Acta Mater, 67 (2014): pp. 207-219. 24 D. Caplan, "Effect of cold work on the oxidation of Fe-Cr alloys in water vapour at 600oC," Corros. Sci., 6 (1966): pp. 509-515. 25 P. Hosemann, et al., "Transmission electron microscopy (TEM) on oxide layers formed on D9 stainless steel in lead bismuth eutectic (LBE)," Corros. Sci., 66 (2013): pp. 196-202.