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Quantitative evaluation of material degradation of thermally aged duplex stainless steels using chemical immersion test
ELSEVIER
Journal of NuclearMaterials 240 (1996) 62-69
Quantitative evaluation of material degradation of thermally aged duplex stainless steels using chemical immersion test Y.S. Yi *, T.
Shoji
Division of Fracture Physics and Chemistry., Research Institute for Fracture Technology, Faculty of Engineering, Tohoku University, Sendai 980, Japan
Received 31 May 1996; accepted 27 July 1996
Abstract In order to develop a non-destructive evaluation technique for detection of thermal aging embrittlement of duplex stainless steels, corrosion tests on unaged and aged specimens of cast duplex stainless steels were performed in 5 wt% HCI solution. After the immersion test, the dissolution rate of specimens was obtained by a dissolved depth measurement with an AFM. In the measurements of dissolved depths, a replica technique was used for easier handling and also for a possible field application of the AFM analysis method. Changes in corrosion properties by aging measured in terms of the dissolved depth after the immersion were compared with the changes in mechanical properties by aging embrittlement. The changes in corrosion properties of unaged and aged specimen were analyzed in relation to the microstructural change by thermal aging. Based upon insights on the immersion test results and the comparison of the changes in corrosion properties and mechanical properties, a possible non-destructive detection and evaluation technique for thermal aging embrittlement by spinodal decomposition is proposed.
1. Introduction In a previous report [1], the detection possibility of the thermal aging embrittlement by means of an electrochemical polarization measurement was suggested and a non-destructive evaluation method was proposed. However, the evaluation method developed in the previous report has a limitation of application as follows. Since the material degradation is evaluated by comparing the dissolved depths of aged materials with those of unaged materials, the evaluation procedure requires dissolution data on unaged material, although sometimes it is not possible to find unaged material in rather old plants. In this study, the dissolution behavior of cast duplex stainless steels in immersion tests in hydrochloric acid solutions is evaluated by measuring dissolved depths on
* Corresponding author. TeL: + 81-22 217 6897; fax: + 81-22 217 6895; e-mail: [email protected].
the specimen surface tested. Measured dissolution rates of unaged and aged materials are compared with each other and a possibility of use of changes in dissolution rates with aging was investigated as a representing parameter of the degree of material degradation. In the previous report, it was shown that, in duplex stainless steels, austenite phase dissolved faster than ferrite phase during polarization in HNO 3 solution. On the other hand, hydrochloric acid was used to enhance the detection sensitivity of spinodal decomposition by increasing a preferential dissolution of ferrite. Many researches in the past have observed a preferential dissolution of either the ferrite or austenite phase depending on the electrochemical potential and the composition of the solution where the duplex stainless steel is exposed. It was also reported that preferential dissolution of ferrite phase occurred in reducing acids such as sulfuric and hydrochloric acids in the vicinity of corrosion potential while preferential dissolution of austenite phase is encountered at higher electrochemical potentials or in more oxidizing media [2]. Concerning the mechanism of the preferential dissolution of ferrite phase of duplex stainless
0022-3115/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3 1 15(96)00459-X
ES. Yi, T. Shoji / Journal of Nuclear Materials 240 (1996) 62-69 steel in acids, it has been suggested [2-4] that a galvanic action between two phases can result in an accelerated dissolution of the ferrite phase.
63
Table 2 Aging conditions of materials Specimens
Aging temperature (°C)
KH-S a KH-HT
284 550
Aging time
M-A M-C M-D
400 400
300 h 3,000 h
SK-1 SK-2 SK-3
400 400
1,100 h 8,000 h
F-I F-2 F-3
350 400
1,000 h 1,000 hrs
SH-I SH-2 SH-3
400 400
1,100 h 3,000 h
D-1 D-2
400
1,000 h
12 y 1h b
2. Experimental 2.1. Materials The chemical compositions and ferrite content of cast duplex stainless steels used in this study are listed in Table 1 and the temperature and time of the aging are presented in Table 2.
2.2. Immersion test Immersion tests were performed in 5 wt% HC1 solutions. As previously described, the preferential dissolution of ferrite phase by a galvanic action with austenite phase occurs in the vicinity of corrosion potential of duplex stainless steels. Taking into consideration the fact that electrochemical reactions on a specimen surface occurs at its corrosion potential during immersion tests, a chemical immersion test method was adopted so that the ferrite phase might be preferentially dissolved. The test samples were prepared by the same method as described in Ref. [1]. The specimen surface was polished with diamond paste down to 6 Ixm. After polishing, the surface of the specimen was cleaned with ethanol and then dried. The area for immersion tests for dissolution measurements was 0.2-0.3 cm 2 and the remainder was insulated with non-conducting paint. The test solution was prepared with hydrochloric acid (36%) and distilled water and its temperature was maintained at 251 °C in a water bath during tests. The tests were conducted in this electrolyte of about 200 ml for 1 h and 2 h.
a KH-S is a service exposed material and the others are laboratory aged materials. b Heat treatment for recovery of spinodal decomposition.
A F M measurements, the replica of the etched surface was made and used so that the evaluation of corrosion rates by immersion tests can be made as one of the non-destructive tests. The replica was obtained by pressing Transcopy Replica (Struers, Denmark) on the etched surface for 60 s, For the A F M measurements, replica samples should be flat and thin ( < 4 mm) and therefore, the obtained replicas were adhered to a thin glass plate.
3. Results 2.3. Surface observation and dissolved depth measurement after immersion tests The corrosion rates of specimens during immersion tests were calculated from the measurements of the dissolved depths of a specimen surface with an AFM. In the
In the previous report [1], the dissolved depth of the etched surface was measured from the height of the original surface. The examination of the etched surface during the immersion in HC1 solution showed that crevice corrosion occurred under the paint coating and hence the dis-
Table 1 Chemical compositions and ferrite content of materials Specimen
KH M SK F SH D
Grade
CF-8 CF-8M CF-8M CF-8M CF-8M CF-8M
Composition (wt%)
Ferrite content
C
Si
Mn
P
S
Cu
Ni
Cr
Mo
N
0.062 0.059 0.05 0.032 0.06 0.04
1.17 0.94 0.89 1.27 1.02 1.12
0.31 0.77 0.83 0.63 1.04 0.73
NA 0.028 0.027 NA 0.015 0.033
NA 0.013 0.003 NA 0.013 0,012
NA NA NA NA NA 0.26
8.03 9.31 9.70 9.97 12.05 9.20
21.99 20.53 18.51 19.62 18.68 20.67
0.17 2.16 2.27 2.50 2.56 2.11
0.038 0.044 NA 0.027 NA 0.08
35.5% 17.5% 13.9% 24.0% 5.6% 13.5%
64
Y.S. Yi, T. Shoji /Journal of Nuclear Materials 240 (1996) 62-69
Fig. 1. Example of the measurement of dissolved depth of M-A (unaged specimen).
KS. Yi, T. Shoji / Journal (~fNuclear Materials 240 (1996) 62-69
solved depth of each phase cannot be exactly measured in the etched surface boundary. Austenite phases seem to be scarcely dissolved in the present condition of immersion.
65
In the measurements of dissolved depth of the ferrite phase, therefore, the relative depth was measured with the austenite phase as a reference in height measurements.
Fig, 2. Example of the measurement of dissolved depth of M-C (aged specimen).
ES. Yi, T. Shoji / Journal of Nuclear Materials 240 (1996) 62-69
66
Figs. 1 and 2 show examples of the measurements of the dissolved depths for the unaged and aged specimens of M materials, respectively, which were immersed in the solution for 1 h. Since the measurements were performed on the replica surface, the undulation of the etched surface in these figures is shown reversely. A large difference of dissolved depths of ferrite phases is found in these figures for the unaged and for the aged specimens. The dissolved depths of ferrite phase during immersion for 1 h and 2 h are summarized in Table 3. In the data of SH materials for 1 h immersion, negative values are shown in this table. These negative values are due to undulation which has polished the surface before testing. As discussed in the previous report, during mechanical polishing, the ferrite phase with higher hardness is polished much less than the softer austenite phase. Since the ferrite phase is higher than the austenite phase on the polished surface and the dissolved depth measurements were conducted with the height of the austenite phase as a reference, the dissolved depth of the ferrite phase can be negative at the stage of a small amount of dissolution. The dissolved depths of ferrite phases of unaged specimens are much larger than those of aged specimens and the dissolved depth of the ferrite phase consistently decreases by large amount as aging time increases as can be seen clearly in Table 3. Fig. 3 shows the relation between the dissolved depth of the ferrite phase and the immersion time for M materials. In this figure, the dissolved depths of the ferrite phase in all the specimens increase in proportion to immersion time up to 2 h. From these results, we can see that the
Table 3 Dissolved depths of ferrite phases during immersion Specimen
Dissolved depth (mm) 1 h immersion
2 h immersion
KH-HT KH-S
250.0 142.8
NA NA
M-A M-C M-D
279.6 +__33 ! 29.7 ___29 33.1 5:14
545.8 + 72 229.0 5:23 103.0+ 19
SK- 1 SK-2 SK-3
NA 107.3 + 27 30.9+9
NA 192.2 __+25 115.2 _+19
254.9-1-49 105.8+ 17 32.6+8
511.9+57 198.0+30 122.4+24
312.9 -I-36 - 12.7+ 16 -43.5+9
629.0 + 52 65.74-_22 18.0+ 11
247.1-1-29 139.3 +__12
464.34-71 330.7 +_29
F-I F-2 F-3 SH- 1 SH-2 SH-3 D-I D-2
t
M-A
5°°°t I " .~ 400.0
El
M-C
.
.
.
.
"
~ 300.0
0.5
1 1.5 Immersion time (hr)
2
2.5
Fig. 3. Variation of dissolved depth of the ferrite phase with immersion time.
dissolution of the ferrite phase in this immersion condition occurs as an activation controlled process. Dissolution rates of specimens were obtained by averaging dissolution rates for the 1 h and 2 h immersion results and adopting this as a property of materials which may represent a state of degradation. This property will be compared with mechanical properties of materials.
4. Discussion 4.1. Dissolution behavior of duplex stainless steels The immersion tests and subsequent AFM analysis results clearly showed the preferential dissolution of the ferrite phase in this immersion condition. This result is well consistent with other investigation results. Yau and Streicher [4] reported that the corrosion rate of the F e 32%Cr-10%Ni alloy with a pure ferritic structure is always lower than that with duplex structure in 1.2 N HCI solutions. The higher corrosion rate of a duplex structure resulting from preferential attack of a ferrite phase can be attributed to a galvanic action between ferrite and austenite phases. If two corroding metals are galvanically coupled, the corrosion rate of the metal with the most active corrosion potential is accelerated and that of the other metal is retarded. The metal with the active corrosion potential becomes an anode and the one with the noble corrosion potential, a cathode [5]. Hence, whether it can be an anode or a cathode is determined on the basis of corrosion potential of the phases in the solution. The galvanic action between ferrite and austenite phases in duplex stainless steel occurs because the corrosion potentials of two phases are different, which is caused by a difference in chemical composition between the two phases. The analysis of chemical composition of duplex stainless steels shows that the ferrite phase contains more chromium but less nickel than the austenite phase [6,7]. The higher chromium and lower nickel contents in the ferrite phase
Y.S. Yi, T. Shoji / Journal of Nuclear Materials 240 (1996) 62-69 should make it anodic to the corresponding austenite phase. The resulting galvanic effect would be expected to accelerate the corrosion rate of the ferrite phase and perhaps, to decrease the dissolution of the austenite phase. The corrosion rate is generally determined by the measurement of weight loss of specimens or the chemical composition analysis of the test solution [8,9]. The determination method of corrosion rates using the measurement of weight loss during immersion is not a suitable corrosion test as a non-destructive method. In a test solution analysis, not only the total corrosion rate but also the dissolved amount of each alloyed element can be obtained. However, the analysis results using this method may be strongly influenced by crevice corrosion. On the other hand, electrochemical polarization measurements and immersion tests in hydrochloric acid solutions have the possibility that crevice corrosion can occur. Crevice corrosion may be inevitable in solutions including C1-. In this study, the crevice corrosion was also observed at the insulated boundary. The undesirable crevice corrosion should make it difficult to measure exact corrosion rates by the solution analysis or measure the weight loss. These two measurement methods for the corrosion rates are not suitable to the purpose of this study. Therefore, the dissolved depth was measured with an AFM and also a replica technique was used. The AFM and replica should make it possible to use the evaluation method using the immersion test as a non-destructive method. 4.2. Detection mechanism of spinodal decomposition in immersion test As has been shown in Table 3, the dissolved depths of the ferrite phase of materials decreased with aging time. The dissolution rate of the ferrite phase in aged duplex stainless steels is expected to be largely dependent upon the Cr concentration fluctuation formed by spinodal decomposition. In many investigations [6,10,11], the concentration distribution of Cr, Fe, Ni, etc. in spinodally decomposed ferrite phases were obtained using APFIM. Those concentration profiles show that spinodal decomposition in ferrite phases occurs on a very fine scale, a few nm, by thermal aging and hence the ferrite phase decomposes into the Cr rich zone and the Cr depleted zone. In this study, to explain the dissolution behavior of the ferrite phase composed of regions with different Cr contents, it was assumed that the ferrite phase was quantitatively divided into small zones with only two Cr concentrations, that is, Cr rich zones and Cr depleted zones, although it is not yet clear which Cr level can clarify the area as high and low chromium. When atoms on the ferrite surface are dissolved into an electrolyte, the two zones appears repeatedly, Then, an overall dissolution rate of the ferrite phase should be determined by the zone which is dissolved at a slower dissolution rate between two zones. As the Cr content of an alloy increases, the corrosion
67
and passivation potentials decrease [12,13]. Sugimoto et al. performed polarization measurements for F e - x C r alloys in 1 N HCI solution and their results showed that an alloy with high Cr content begins to passivate in more negative potential than that with low Cr content and its passivation state is maintained to a high potential region [14]. It is evident that the corrosion behavior of C r - F e alloys at a potential will be strongly dependent upon the Cr content. The zones formed in the ferrite phase by spinodal decomposition have different chemical compositions and their corrosion potentials are different from each other. The Cr rich zone with a more active corrosion potential is expected to become the anode due to its high Cr and low Ni contents. Since the electrochemical reactions on an anode and a cathode occur at the mixed (galvanic) potential, the anodic dissolution reaction on the Cr rich zone as an anode in the galvanic coupling occurs at a much nobler potential than its corrosion potential. If the potential where electrochemical reactions by galvanic reaction occur on a high Cr alloy is located in its passivation potential range, its corrosion rate decreases abruptly. From this consideration, Cr rich zones probably lie in a passivation state, while Cr depleted zones are active. Then, the corrosion rate of the Cr rich zone should be extremely small, since it corresponds to passive current density. This slow corrosion rate of the Cr rich zone is expected to decrease the corrosion rate of the ferrite phase in aged materials. Because of insufficient experimental data for alloys with various chemical compositions, it is very difficult to estimate the corrosion rates of very small regions with different chemical compositions formed by spinodal decomposition. However, based on the corrosion data of F e - C r alloys, if it can be assumed that the Cr rich zone should be passive or lie in an active-passive transition region in the present condition of immersion, a dissolution rate of the Cr rich zone can be extremely small. Fig. 4 shows the schematic diagram for this interpretation. It can be summarized that the smaller dissolved depth of the ferrite phase in aged specimens opposed to that of unaged specimen is attributed to the slow dissolution rate of Cr rich zones,
Ferrite in unaged marl
,,""
~.
-"
i ~r depleted . . . . .."Ferrite in
~
~
,
!
.
i, ~ " -
Cr. ~ ....
Immersion time
Fig. 4. Schematic diagram showing the dissolution behavior of ferrite phases in unaged and aged specimens during immersion in 5 wt% HC1 solution.
68
Y.S. Yi, T. Shoji / Journal of Nuclear Materials 240 (1996) 62-69 6.0
L__Z • KH
'
~ 5.0
6.0
• o i
4,0 •
SH
3,0
E
I
4.0
'",, ',
"~
310
'
-x =
1.0
0.0
-z
0.0
....
i .... 50
L.... 100
i .... 150
J .... 200
~ .... 250
i .... 300
-t.o
350
Microhardness increase of ferrite phase (Hv)
Fig. 5. A relationship between dissolution rate and increase in microhardness of the ferrite phase.
4.3. Relation between dissolution rate of ferrite phase and change in mechanical properties Fig. 5 shows the unique relation between the dissolution rate of the ferrite phase and the increase in microhardness for different materials and aging time. The dissolution rate of the unaged materials shows a range of 4 - 5 n m / m i n . The dissolution rate of aged materials decreases consistently with the increase in microhardness. As can be clearly seen in Fig. 5, a single correlation for six different materials can be drawn between the dissolution rate and the increase in microhardness with aging. In Figs. 6 and 7, the dissolution rate of the ferrite phase have been plotted against the change in upper shelf energy (USE) and transition temperature in Charpy impact tests normalized with ferrite contents of each materials. These relations were obtained from the correlation curves between the microhardness increase and the change in Charpy impact properties in the previous paper. In these figures, there is a good correlation between the decrease in dissolution rate and the changes in impact properties because both of these quantities are reflecting a chromium concentration variation by spinodal decomposition.
"Z 6.0 ~
5.0 4.0 3.0
"'o. 2.0 1.0
g o.o ?R -1.0 2
4 6 8 AUSE/ferrite content 0 / % )
10
Fig. 6. A relationship between dissolution rate of the ferrite phase and reduction in USE.
,,
,sK
'*~ 2.0 =
1.0
-i.0
•
.~
,aa 2.0
D [7 •• MSH
5.0'
~r..... m ~-.... •1•..4 5 10 15 k DBTT/ferrite content (C/%)
Fig. 7. A relationship between dissolution rate of the fen'fie phase and shift in DBTT.
Therefore, the dissolution rate is considered to be a parameter which indicates the degree of the aging embrittlement of materials. The reduction in USE and the shift in transition temperature can be estimated using the evaluation diagram and based on these results, the evaluation of the integrity of structures and the estimation of the remnant life can be possible.
5. Summary
In this study, the dissolution behaviors of unaged and aged materials in 5 wt% hydrochloric acid solution were investigated and the possibility of non-destructive evaluation of thermal aging embrittlement due to spinodal decomposition of cast duplex stainless steels using immersion technique and AFM analysis was discussed. The main conclusions in this study can be summarized as follows. (1) After immersion in 5 wt% HCI solution, it was observed that the ferrite phase was preferentially dissolved, which is caused by the galvanic action between ferrite and austenite phases in duplex stainless steels. By galvanic action with the austenite phase, the ferrite phase becomes an anode because of its higher chromium content and lower nickel content. (2) The dissolved depth of the ferrite phase in immersion increases in proportion to immersion time, which means that the dissolution of the ferrite phase in this immersion condition occurs as an activation controlled process, (3) The dissolved depth of the ferrite phase in the aged materials decreased with the increase of aging time. It is analyzed that the decrease of the dissolved depth of the ferrite phase in aged specimens is due to the slow dissolution rate of the Cr rich zone formed by spinodal decomposition. (4) The dissolution rate of the ferrite phase of duplex stainless steels showed a good correlation with the changes in mechanical properties such as USE, transition temperature and microhardness of the ferrite phase.
Y.S, Yi, T. Shoji / Journal of Nuclear Materials 240 (1996) 62-69
References [1] Y.S. Yi and T. Shoji, J. Nucl. Mater. 231 (1996) 20. [2] E. Symniotis, Corrosion 46 (1) (1990) 2. [3] G.O. Davis, J. Kolts and N. Sridhar, Corrosion 42 (6) (1986) 329. [4] Y.H. Yau and M.A. Streicher, Corrosion 43 (6) (1987) 366. [5] M.G. Fontana and N.D. Greene, Corrosion Engineering (McGraw-Hill, New York, 1967). [6] P. Auger, F. Danoix, A. Menand, S. Bonnet, J. Bourgoin and M. Guttmann, Mater. Sci. Technol. 6 (1990) 301. [7] H.D. Solomon and Lionel M. Levinson, Acta Metall. 26 (1978) 429.
69
[8] S.H. Sanad, A.A. Ismail and N.A. Mahmoud, J. Mater. Sci. 27 (1992) 5706. [9] A.P. Bond and H.H. Uhlig, J. Electrochem. Soc. 107 (6) (1960) 488. [10] M.K. Miller and J. Bentley, Mater. Sci. Technol. 6 (1990) 2852. [11] H.M. Chung and T.R. Leax, Mater. Sci. Technol. 6 (1990) 249. [12] D.A. Jones, Principles and Prevention of Corrosion (Macmillan, New York, 1992). [13] A.J. Sedriks, Corrosion 42 (7) (1986) 376. [14] K. Sugimoto and Y. Sawada. Corros. Sci. 17 (1977) 425.
Journal of NuclearMaterials 240 (1996) 62-69
Quantitative evaluation of material degradation of thermally aged duplex stainless steels using chemical immersion test Y.S. Yi *, T.
Shoji
Division of Fracture Physics and Chemistry., Research Institute for Fracture Technology, Faculty of Engineering, Tohoku University, Sendai 980, Japan
Received 31 May 1996; accepted 27 July 1996
Abstract In order to develop a non-destructive evaluation technique for detection of thermal aging embrittlement of duplex stainless steels, corrosion tests on unaged and aged specimens of cast duplex stainless steels were performed in 5 wt% HCI solution. After the immersion test, the dissolution rate of specimens was obtained by a dissolved depth measurement with an AFM. In the measurements of dissolved depths, a replica technique was used for easier handling and also for a possible field application of the AFM analysis method. Changes in corrosion properties by aging measured in terms of the dissolved depth after the immersion were compared with the changes in mechanical properties by aging embrittlement. The changes in corrosion properties of unaged and aged specimen were analyzed in relation to the microstructural change by thermal aging. Based upon insights on the immersion test results and the comparison of the changes in corrosion properties and mechanical properties, a possible non-destructive detection and evaluation technique for thermal aging embrittlement by spinodal decomposition is proposed.
1. Introduction In a previous report [1], the detection possibility of the thermal aging embrittlement by means of an electrochemical polarization measurement was suggested and a non-destructive evaluation method was proposed. However, the evaluation method developed in the previous report has a limitation of application as follows. Since the material degradation is evaluated by comparing the dissolved depths of aged materials with those of unaged materials, the evaluation procedure requires dissolution data on unaged material, although sometimes it is not possible to find unaged material in rather old plants. In this study, the dissolution behavior of cast duplex stainless steels in immersion tests in hydrochloric acid solutions is evaluated by measuring dissolved depths on
* Corresponding author. TeL: + 81-22 217 6897; fax: + 81-22 217 6895; e-mail: [email protected].
the specimen surface tested. Measured dissolution rates of unaged and aged materials are compared with each other and a possibility of use of changes in dissolution rates with aging was investigated as a representing parameter of the degree of material degradation. In the previous report, it was shown that, in duplex stainless steels, austenite phase dissolved faster than ferrite phase during polarization in HNO 3 solution. On the other hand, hydrochloric acid was used to enhance the detection sensitivity of spinodal decomposition by increasing a preferential dissolution of ferrite. Many researches in the past have observed a preferential dissolution of either the ferrite or austenite phase depending on the electrochemical potential and the composition of the solution where the duplex stainless steel is exposed. It was also reported that preferential dissolution of ferrite phase occurred in reducing acids such as sulfuric and hydrochloric acids in the vicinity of corrosion potential while preferential dissolution of austenite phase is encountered at higher electrochemical potentials or in more oxidizing media [2]. Concerning the mechanism of the preferential dissolution of ferrite phase of duplex stainless
0022-3115/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3 1 15(96)00459-X
ES. Yi, T. Shoji / Journal of Nuclear Materials 240 (1996) 62-69 steel in acids, it has been suggested [2-4] that a galvanic action between two phases can result in an accelerated dissolution of the ferrite phase.
63
Table 2 Aging conditions of materials Specimens
Aging temperature (°C)
KH-S a KH-HT
284 550
Aging time
M-A M-C M-D
400 400
300 h 3,000 h
SK-1 SK-2 SK-3
400 400
1,100 h 8,000 h
F-I F-2 F-3
350 400
1,000 h 1,000 hrs
SH-I SH-2 SH-3
400 400
1,100 h 3,000 h
D-1 D-2
400
1,000 h
12 y 1h b
2. Experimental 2.1. Materials The chemical compositions and ferrite content of cast duplex stainless steels used in this study are listed in Table 1 and the temperature and time of the aging are presented in Table 2.
2.2. Immersion test Immersion tests were performed in 5 wt% HC1 solutions. As previously described, the preferential dissolution of ferrite phase by a galvanic action with austenite phase occurs in the vicinity of corrosion potential of duplex stainless steels. Taking into consideration the fact that electrochemical reactions on a specimen surface occurs at its corrosion potential during immersion tests, a chemical immersion test method was adopted so that the ferrite phase might be preferentially dissolved. The test samples were prepared by the same method as described in Ref. [1]. The specimen surface was polished with diamond paste down to 6 Ixm. After polishing, the surface of the specimen was cleaned with ethanol and then dried. The area for immersion tests for dissolution measurements was 0.2-0.3 cm 2 and the remainder was insulated with non-conducting paint. The test solution was prepared with hydrochloric acid (36%) and distilled water and its temperature was maintained at 251 °C in a water bath during tests. The tests were conducted in this electrolyte of about 200 ml for 1 h and 2 h.
a KH-S is a service exposed material and the others are laboratory aged materials. b Heat treatment for recovery of spinodal decomposition.
A F M measurements, the replica of the etched surface was made and used so that the evaluation of corrosion rates by immersion tests can be made as one of the non-destructive tests. The replica was obtained by pressing Transcopy Replica (Struers, Denmark) on the etched surface for 60 s, For the A F M measurements, replica samples should be flat and thin ( < 4 mm) and therefore, the obtained replicas were adhered to a thin glass plate.
3. Results 2.3. Surface observation and dissolved depth measurement after immersion tests The corrosion rates of specimens during immersion tests were calculated from the measurements of the dissolved depths of a specimen surface with an AFM. In the
In the previous report [1], the dissolved depth of the etched surface was measured from the height of the original surface. The examination of the etched surface during the immersion in HC1 solution showed that crevice corrosion occurred under the paint coating and hence the dis-
Table 1 Chemical compositions and ferrite content of materials Specimen
KH M SK F SH D
Grade
CF-8 CF-8M CF-8M CF-8M CF-8M CF-8M
Composition (wt%)
Ferrite content
C
Si
Mn
P
S
Cu
Ni
Cr
Mo
N
0.062 0.059 0.05 0.032 0.06 0.04
1.17 0.94 0.89 1.27 1.02 1.12
0.31 0.77 0.83 0.63 1.04 0.73
NA 0.028 0.027 NA 0.015 0.033
NA 0.013 0.003 NA 0.013 0,012
NA NA NA NA NA 0.26
8.03 9.31 9.70 9.97 12.05 9.20
21.99 20.53 18.51 19.62 18.68 20.67
0.17 2.16 2.27 2.50 2.56 2.11
0.038 0.044 NA 0.027 NA 0.08
35.5% 17.5% 13.9% 24.0% 5.6% 13.5%
64
Y.S. Yi, T. Shoji /Journal of Nuclear Materials 240 (1996) 62-69
Fig. 1. Example of the measurement of dissolved depth of M-A (unaged specimen).
KS. Yi, T. Shoji / Journal (~fNuclear Materials 240 (1996) 62-69
solved depth of each phase cannot be exactly measured in the etched surface boundary. Austenite phases seem to be scarcely dissolved in the present condition of immersion.
65
In the measurements of dissolved depth of the ferrite phase, therefore, the relative depth was measured with the austenite phase as a reference in height measurements.
Fig, 2. Example of the measurement of dissolved depth of M-C (aged specimen).
ES. Yi, T. Shoji / Journal of Nuclear Materials 240 (1996) 62-69
66
Figs. 1 and 2 show examples of the measurements of the dissolved depths for the unaged and aged specimens of M materials, respectively, which were immersed in the solution for 1 h. Since the measurements were performed on the replica surface, the undulation of the etched surface in these figures is shown reversely. A large difference of dissolved depths of ferrite phases is found in these figures for the unaged and for the aged specimens. The dissolved depths of ferrite phase during immersion for 1 h and 2 h are summarized in Table 3. In the data of SH materials for 1 h immersion, negative values are shown in this table. These negative values are due to undulation which has polished the surface before testing. As discussed in the previous report, during mechanical polishing, the ferrite phase with higher hardness is polished much less than the softer austenite phase. Since the ferrite phase is higher than the austenite phase on the polished surface and the dissolved depth measurements were conducted with the height of the austenite phase as a reference, the dissolved depth of the ferrite phase can be negative at the stage of a small amount of dissolution. The dissolved depths of ferrite phases of unaged specimens are much larger than those of aged specimens and the dissolved depth of the ferrite phase consistently decreases by large amount as aging time increases as can be seen clearly in Table 3. Fig. 3 shows the relation between the dissolved depth of the ferrite phase and the immersion time for M materials. In this figure, the dissolved depths of the ferrite phase in all the specimens increase in proportion to immersion time up to 2 h. From these results, we can see that the
Table 3 Dissolved depths of ferrite phases during immersion Specimen
Dissolved depth (mm) 1 h immersion
2 h immersion
KH-HT KH-S
250.0 142.8
NA NA
M-A M-C M-D
279.6 +__33 ! 29.7 ___29 33.1 5:14
545.8 + 72 229.0 5:23 103.0+ 19
SK- 1 SK-2 SK-3
NA 107.3 + 27 30.9+9
NA 192.2 __+25 115.2 _+19
254.9-1-49 105.8+ 17 32.6+8
511.9+57 198.0+30 122.4+24
312.9 -I-36 - 12.7+ 16 -43.5+9
629.0 + 52 65.74-_22 18.0+ 11
247.1-1-29 139.3 +__12
464.34-71 330.7 +_29
F-I F-2 F-3 SH- 1 SH-2 SH-3 D-I D-2
t
M-A
5°°°t I " .~ 400.0
El
M-C
.
.
.
.
"
~ 300.0
0.5
1 1.5 Immersion time (hr)
2
2.5
Fig. 3. Variation of dissolved depth of the ferrite phase with immersion time.
dissolution of the ferrite phase in this immersion condition occurs as an activation controlled process. Dissolution rates of specimens were obtained by averaging dissolution rates for the 1 h and 2 h immersion results and adopting this as a property of materials which may represent a state of degradation. This property will be compared with mechanical properties of materials.
4. Discussion 4.1. Dissolution behavior of duplex stainless steels The immersion tests and subsequent AFM analysis results clearly showed the preferential dissolution of the ferrite phase in this immersion condition. This result is well consistent with other investigation results. Yau and Streicher [4] reported that the corrosion rate of the F e 32%Cr-10%Ni alloy with a pure ferritic structure is always lower than that with duplex structure in 1.2 N HCI solutions. The higher corrosion rate of a duplex structure resulting from preferential attack of a ferrite phase can be attributed to a galvanic action between ferrite and austenite phases. If two corroding metals are galvanically coupled, the corrosion rate of the metal with the most active corrosion potential is accelerated and that of the other metal is retarded. The metal with the active corrosion potential becomes an anode and the one with the noble corrosion potential, a cathode [5]. Hence, whether it can be an anode or a cathode is determined on the basis of corrosion potential of the phases in the solution. The galvanic action between ferrite and austenite phases in duplex stainless steel occurs because the corrosion potentials of two phases are different, which is caused by a difference in chemical composition between the two phases. The analysis of chemical composition of duplex stainless steels shows that the ferrite phase contains more chromium but less nickel than the austenite phase [6,7]. The higher chromium and lower nickel contents in the ferrite phase
Y.S. Yi, T. Shoji / Journal of Nuclear Materials 240 (1996) 62-69 should make it anodic to the corresponding austenite phase. The resulting galvanic effect would be expected to accelerate the corrosion rate of the ferrite phase and perhaps, to decrease the dissolution of the austenite phase. The corrosion rate is generally determined by the measurement of weight loss of specimens or the chemical composition analysis of the test solution [8,9]. The determination method of corrosion rates using the measurement of weight loss during immersion is not a suitable corrosion test as a non-destructive method. In a test solution analysis, not only the total corrosion rate but also the dissolved amount of each alloyed element can be obtained. However, the analysis results using this method may be strongly influenced by crevice corrosion. On the other hand, electrochemical polarization measurements and immersion tests in hydrochloric acid solutions have the possibility that crevice corrosion can occur. Crevice corrosion may be inevitable in solutions including C1-. In this study, the crevice corrosion was also observed at the insulated boundary. The undesirable crevice corrosion should make it difficult to measure exact corrosion rates by the solution analysis or measure the weight loss. These two measurement methods for the corrosion rates are not suitable to the purpose of this study. Therefore, the dissolved depth was measured with an AFM and also a replica technique was used. The AFM and replica should make it possible to use the evaluation method using the immersion test as a non-destructive method. 4.2. Detection mechanism of spinodal decomposition in immersion test As has been shown in Table 3, the dissolved depths of the ferrite phase of materials decreased with aging time. The dissolution rate of the ferrite phase in aged duplex stainless steels is expected to be largely dependent upon the Cr concentration fluctuation formed by spinodal decomposition. In many investigations [6,10,11], the concentration distribution of Cr, Fe, Ni, etc. in spinodally decomposed ferrite phases were obtained using APFIM. Those concentration profiles show that spinodal decomposition in ferrite phases occurs on a very fine scale, a few nm, by thermal aging and hence the ferrite phase decomposes into the Cr rich zone and the Cr depleted zone. In this study, to explain the dissolution behavior of the ferrite phase composed of regions with different Cr contents, it was assumed that the ferrite phase was quantitatively divided into small zones with only two Cr concentrations, that is, Cr rich zones and Cr depleted zones, although it is not yet clear which Cr level can clarify the area as high and low chromium. When atoms on the ferrite surface are dissolved into an electrolyte, the two zones appears repeatedly, Then, an overall dissolution rate of the ferrite phase should be determined by the zone which is dissolved at a slower dissolution rate between two zones. As the Cr content of an alloy increases, the corrosion
67
and passivation potentials decrease [12,13]. Sugimoto et al. performed polarization measurements for F e - x C r alloys in 1 N HCI solution and their results showed that an alloy with high Cr content begins to passivate in more negative potential than that with low Cr content and its passivation state is maintained to a high potential region [14]. It is evident that the corrosion behavior of C r - F e alloys at a potential will be strongly dependent upon the Cr content. The zones formed in the ferrite phase by spinodal decomposition have different chemical compositions and their corrosion potentials are different from each other. The Cr rich zone with a more active corrosion potential is expected to become the anode due to its high Cr and low Ni contents. Since the electrochemical reactions on an anode and a cathode occur at the mixed (galvanic) potential, the anodic dissolution reaction on the Cr rich zone as an anode in the galvanic coupling occurs at a much nobler potential than its corrosion potential. If the potential where electrochemical reactions by galvanic reaction occur on a high Cr alloy is located in its passivation potential range, its corrosion rate decreases abruptly. From this consideration, Cr rich zones probably lie in a passivation state, while Cr depleted zones are active. Then, the corrosion rate of the Cr rich zone should be extremely small, since it corresponds to passive current density. This slow corrosion rate of the Cr rich zone is expected to decrease the corrosion rate of the ferrite phase in aged materials. Because of insufficient experimental data for alloys with various chemical compositions, it is very difficult to estimate the corrosion rates of very small regions with different chemical compositions formed by spinodal decomposition. However, based on the corrosion data of F e - C r alloys, if it can be assumed that the Cr rich zone should be passive or lie in an active-passive transition region in the present condition of immersion, a dissolution rate of the Cr rich zone can be extremely small. Fig. 4 shows the schematic diagram for this interpretation. It can be summarized that the smaller dissolved depth of the ferrite phase in aged specimens opposed to that of unaged specimen is attributed to the slow dissolution rate of Cr rich zones,
Ferrite in unaged marl
,,""
~.
-"
i ~r depleted . . . . .."Ferrite in
~
~
,
!
.
i, ~ " -
Cr. ~ ....
Immersion time
Fig. 4. Schematic diagram showing the dissolution behavior of ferrite phases in unaged and aged specimens during immersion in 5 wt% HC1 solution.
68
Y.S. Yi, T. Shoji / Journal of Nuclear Materials 240 (1996) 62-69 6.0
L__Z • KH
'
~ 5.0
6.0
• o i
4,0 •
SH
3,0
E
I
4.0
'",, ',
"~
310
'
-x =
1.0
0.0
-z
0.0
....
i .... 50
L.... 100
i .... 150
J .... 200
~ .... 250
i .... 300
-t.o
350
Microhardness increase of ferrite phase (Hv)
Fig. 5. A relationship between dissolution rate and increase in microhardness of the ferrite phase.
4.3. Relation between dissolution rate of ferrite phase and change in mechanical properties Fig. 5 shows the unique relation between the dissolution rate of the ferrite phase and the increase in microhardness for different materials and aging time. The dissolution rate of the unaged materials shows a range of 4 - 5 n m / m i n . The dissolution rate of aged materials decreases consistently with the increase in microhardness. As can be clearly seen in Fig. 5, a single correlation for six different materials can be drawn between the dissolution rate and the increase in microhardness with aging. In Figs. 6 and 7, the dissolution rate of the ferrite phase have been plotted against the change in upper shelf energy (USE) and transition temperature in Charpy impact tests normalized with ferrite contents of each materials. These relations were obtained from the correlation curves between the microhardness increase and the change in Charpy impact properties in the previous paper. In these figures, there is a good correlation between the decrease in dissolution rate and the changes in impact properties because both of these quantities are reflecting a chromium concentration variation by spinodal decomposition.
"Z 6.0 ~
5.0 4.0 3.0
"'o. 2.0 1.0
g o.o ?R -1.0 2
4 6 8 AUSE/ferrite content 0 / % )
10
Fig. 6. A relationship between dissolution rate of the ferrite phase and reduction in USE.
,,
,sK
'*~ 2.0 =
1.0
-i.0
•
.~
,aa 2.0
D [7 •• MSH
5.0'
~r..... m ~-.... •1•..4 5 10 15 k DBTT/ferrite content (C/%)
Fig. 7. A relationship between dissolution rate of the fen'fie phase and shift in DBTT.
Therefore, the dissolution rate is considered to be a parameter which indicates the degree of the aging embrittlement of materials. The reduction in USE and the shift in transition temperature can be estimated using the evaluation diagram and based on these results, the evaluation of the integrity of structures and the estimation of the remnant life can be possible.
5. Summary
In this study, the dissolution behaviors of unaged and aged materials in 5 wt% hydrochloric acid solution were investigated and the possibility of non-destructive evaluation of thermal aging embrittlement due to spinodal decomposition of cast duplex stainless steels using immersion technique and AFM analysis was discussed. The main conclusions in this study can be summarized as follows. (1) After immersion in 5 wt% HCI solution, it was observed that the ferrite phase was preferentially dissolved, which is caused by the galvanic action between ferrite and austenite phases in duplex stainless steels. By galvanic action with the austenite phase, the ferrite phase becomes an anode because of its higher chromium content and lower nickel content. (2) The dissolved depth of the ferrite phase in immersion increases in proportion to immersion time, which means that the dissolution of the ferrite phase in this immersion condition occurs as an activation controlled process, (3) The dissolved depth of the ferrite phase in the aged materials decreased with the increase of aging time. It is analyzed that the decrease of the dissolved depth of the ferrite phase in aged specimens is due to the slow dissolution rate of the Cr rich zone formed by spinodal decomposition. (4) The dissolution rate of the ferrite phase of duplex stainless steels showed a good correlation with the changes in mechanical properties such as USE, transition temperature and microhardness of the ferrite phase.
Y.S, Yi, T. Shoji / Journal of Nuclear Materials 240 (1996) 62-69
References [1] Y.S. Yi and T. Shoji, J. Nucl. Mater. 231 (1996) 20. [2] E. Symniotis, Corrosion 46 (1) (1990) 2. [3] G.O. Davis, J. Kolts and N. Sridhar, Corrosion 42 (6) (1986) 329. [4] Y.H. Yau and M.A. Streicher, Corrosion 43 (6) (1987) 366. [5] M.G. Fontana and N.D. Greene, Corrosion Engineering (McGraw-Hill, New York, 1967). [6] P. Auger, F. Danoix, A. Menand, S. Bonnet, J. Bourgoin and M. Guttmann, Mater. Sci. Technol. 6 (1990) 301. [7] H.D. Solomon and Lionel M. Levinson, Acta Metall. 26 (1978) 429.
69
[8] S.H. Sanad, A.A. Ismail and N.A. Mahmoud, J. Mater. Sci. 27 (1992) 5706. [9] A.P. Bond and H.H. Uhlig, J. Electrochem. Soc. 107 (6) (1960) 488. [10] M.K. Miller and J. Bentley, Mater. Sci. Technol. 6 (1990) 2852. [11] H.M. Chung and T.R. Leax, Mater. Sci. Technol. 6 (1990) 249. [12] D.A. Jones, Principles and Prevention of Corrosion (Macmillan, New York, 1992). [13] A.J. Sedriks, Corrosion 42 (7) (1986) 376. [14] K. Sugimoto and Y. Sawada. Corros. Sci. 17 (1977) 425.