Sensitivity To Intergranular Attack Kinetics of High-Alloyed Austenitic Stainless Steels With Copper
Sensitivity To Intergranular Attack Kinetics of High-Alloyed Austenitic Stainless Steels With Copper
Sensitivity To Intergranular Attack Kinetics of High-Alloyed Austenitic Stainless Steels With Copper
Abstract
The sensitivity to intergranular attack of stainless austenitic steels is caused by gradient of chromium
concentration in the structure. This is caused by precipitation of carbides on the grain boundaries and thus
drop of chromium concentration in the adjacent region.
The degree of sensitization was studied using double-loop electropotentiokinetic reactivation method in 0.5M
H2SO4 with addition of NH4SCN. The samples of austenitic stainless steel AISI Super304H were aged at
different temperatures (620-720°C) and times (1-1000h). The structure of the samples was studied using
optical and scanning electron microscopy. The results showed that even the steel stabilized with Nb is prone
to sensitization; the experimental rate of sensitization fits the Larson-Miller equation.
Key words:
steel, intergranular, corrosion, precipitation
1. INTRODUCTION
Intergranular corrosion is a localized attack caused by structural inhomogeneity. The inhomogeneity can be
either local depletion of certain alloying element, precipitation of secondary phase or segregation. All of
these result in concentration gradient of certain element in the structure of the material. If the local
concentration of chromium in the case of stainless steels, drops below 14% wt., the material is unable to
form passive layer in this area the surface becomes active and it corrodes with high corrosion rates. Material
is therefore “sensitized”.
1.1. Sensitization
During the thermal or thermo-mechanical processes, precipitates can form in the steel structure. In the
austenitic steel, the precipitated phase causing sensitization is the chromium carbide (Cr23C6). The
precipitation of the carbide causes transport of chromium from surrounding area to the nucleation site.
The combination of precipitates varies with steel compositions, exposure length and temperature. There can
be large variety of carbides (M23C6, M6C), nitrides (Cr2N, TiN), carbonitrides (MX), Laves-phases (Fe2Mo,
Fe2Nb), δ-phase (FeNi)x(CrMo)y, χ-phase Fe36Cr12Mo10, G-phase (Ti4C2S2) and in the case of copper alloyed
steels even the ε-phase [1-9].
The composition of the steel does not only affect the composition of the secondary phases, but mainly their
solubility in the solid solution. This is more affected by lattice structure than the element composition. This
explains the difference in the sensitization temperature range between ferritic and austenitic steels. The
austenitic steels sensitization temperature range is 450-850°C [10, 11], and the range for the ferritic steels is
430-930°C [12]. The differences between the two are different diffusivities of carbon and other elements in
the given lattice. The higher carbon diffusivity in the ferritic lattice allows nucleation of the precipitate at lower
temperature while the low diffusivity of carbon in austenitic lattice requires higher temperatures to start the
nucleation.
23. - 25. 5. 2012, Brno, Czech Republic, EU
The aim of the work was to determine the parameters of sensitization of steel AISI Super 304H using the
double-loop electro potentiokinetic reactivation. The goal is to determine the maximum sensitization for each
temperature and length of exposure.
2. EXPERIMENT
2.1. Samples
The material used for all the experiments was creep resistant austenitic stainless steel AISI Super 304H. The
composition is summarized in Tab. 1.
C Si Mn Cu Cr Ni Nb N
Element
0.08 0.2 0.8 3 18 9 0.1 0.1
23. - 25. 5. 2012, Brno, Czech Republic, EU
Qr (1)
DOS
Qa
101
3. RESULTS AND DISCUSSION
100
The sensitization of the samples with different thermal
history is visible from Fig.-Fig. . It is visible, that the
10-1
sensitivity results are affected by the composition of
testing electrolyte. In the beginning we even used the
10-2
0.1M NH4SCN, but the forming oxygen made the
sensitization curves unreliable in most cases. The
10-3
curve support show the previously mentioned
phenomenon of sensitization and subsequent “self-
10-4
-0.8 -0.4 0 0.4 0.8 healing”. It is also visible, that the sensitization time
and thus the time to begin nucleation are higher for
Potential [V/ACLE]
lower temperatures. The chromium diffusion, which is
the limiting process of M23C6 forming, is temperature
Fig. 1: Integration of active and repassivation
driven and can be fitted by Larsson-Miller parameter:
peak of the potentiodynamic curve
(2)
23. - 25. 5. 2012, Brno, Czech Republic, EU
60
50
0.5M H2SO4 0.01M NH4SCN
0.5M H2SO4 0.001M NH4SCN
40
40
30
20
20
10
0.5M H2SO4 0.01M NH4SCN
0.5M H2SO4 0.001M NH4SCN
0
0
0 200 400 600 800 1000
0 200 400 600 800 1000
Aging time [h]
Aging time [h]
Fig. 2: Sensitization as function of aging time; Fig. 3: Sensitization as function of aging time;
temperature 620°C temperature 670°C
100
Fig. 5: Structure of the sample aged at 620°C for Fig. 6: Structure of the sample aged at 670°C for
1000h - detail of the grain boundary 100h
4. CONCLUSION
We have shown the effect of thermal aging on
austenitic stainless steel AISI Super304H.
Our electrochemical experiments using double-loop
potentiokinetic reactivation method showed that the
material is prone to sensitization - the time to reach
the maximum sensitization can be fitted by Larsson -
Miller parameter. The maximum degree of
sensitization differs depending on the electrolyte
composition - however it seems that electrolyte with
the higher activator content (0.01M NH4SCN) is too
aggressive for the material and thus yields unlikely
results (95% DOS). The maximum degree of
sensitization in the other electrolyte was
37.5/50/61% for samples aged at 620/670/720°C.
This suggests that higher temperatures cause higher
DOS probably due to the higher short-range
chromium diffusion rates. The structure all the
samples with highest DOS show a ditch structure; in
case of the 620°C/1000h aged sample the grain
Fig. 7: Structure of the sample aged at 670°C for boundary is decorated with carbides.
4h Our data showed that even the material designed for
exposure in aggressive high temperature
environments is prone to sensitization and that the sensitization and desensitization is temperature and
23. - 25. 5. 2012, Brno, Czech Republic, EU
exposure time driven. This is especially important for practical application; the start-up or failures cause
overheating of the material. In the worst case scenario, the material can be sensitized during the start-up and
so-sensitized material can be then operated at much lower temperature - the desensitization (self-healing) in
this case can take up to several thousand hours.
The future work, currently in progress, will compare data from this and more conventional methods to
determine degree of sensitization (Streich, Strauss, oxalic acid test etc.) and will focus on further
mathematical modeling of such processes.
5. ACKNOWLEDGEMENT
This paper was created in the projects of MPO FR-TI1/086 and with financial support
from MSMT No 21/2012
6. LITERATURE
[1] MINAMI. Y.. KIMURA. H.. IHARA. Y. Microstructural changes in austenitic stainless steels during long-term aging.
Materials Science and Technology. 1986. vol. 2. p. 795-806.
[2] SOURMAIL. T. Precipitation in creep resistant austenitic stainless steels. Materials Science and Technology.
2001. vol. 17. p. 1-14.
[3] GONZALEZ-RODRIGUEZ. J.G.. CASALES. M.. ESPINOZA MEDINA. M.A.. ANGELES-CHAVEZ. C..
IZQUIERDO. G.. GUARDIAN. R. Microstructural evolution of Alloy 690 during sensitization at 700 °C. Materials
Characterization. 2003. vol. 51. no. 5. p. 309-314.
[4] SANDERSON. S.J. Sensitization times for intergranular corrosion in Alloy 800. Metal Science. 1977. vol. 11. no.
1. p. 23-30.
[5] BECKITT. F.R.. CLARK. B.R. The shape and mechanism of formation of M23C6 carbide in austenite. Acta
Metallurgica. 1967. vol. 15. no. 1. p. 113-129.
[6] ADAMSON. J.P.. MARTIN. J.W. The nucleation of M23C6 carbide particles in the grain boundaries of an
austenitic stainless steel. Acta Metallurgica. 1971. vol. 19. no. 10. p. 1015-1018.
[7] TRILLO. E.A.. MURR. L.E. A TEM investigation of M23C6 carbide precipitation behaviour on varying grain
boundary misorientations in 304 stainless steels. Journal of Materials Science. 1998. vol. 33. no. 5. p. 1263-
1271.
[8] GOLPAYEGANI. A.. LIU. F.. SVENSSON. H.. ANDERSSON. M.. ANDRÉN. H.-O. Microstructure of a Creep-
Resistant 10 Pct Chromium Steel Containing 250 ppm Boron. Metallurgical and Materials Transactions A. 2011.
vol. 42. no. 4. p. 940-951.
[9] ESCRIBA. D.M.. MATERNA-MORRIS. E.. PLAUT. R.L.. PADILHA. A.F. Chi-phase precipitation in a duplex
stainless steel. Materials Characterization. 2009. vol. 60. no. 11. p. 1214-1219.
[10] ENGELBERG. D.L.. Intergranular Corrosion. in: TONY. J.A.R. (Ed.) Shreir's Corrosion. Elsevier. Oxford. 2010.
pp. 810-827.
[11] MARSHALL. P.: Austenitic stainless steels: microstructure and mechanical properties. Elsevier Applied Science
Publishers. (1984)
[12] STREICHER. M.A.. Theory and application of evaluation tests for detecting susceptibility to intergranular attack in
stainless steels and related alloys - problems and opportunities. in. ASTM. Philadelphia. 1978. pp. 3-84.
[13] SINGH. R.. CHATTORAJ. I.. KUMAR. A.. RAVIKUMAR. B.. DEY. P. The effects of cold working on sensitization
and intergranular corrosion behavior of AISI 304 stainless steel. Metallurgical and Materials Transactions A.
2003. vol. 34. no. 11. p. 2441-2447.
[14] LIMA. A.S.. NASCIMENTO. A.M.. ABREU. H.F.G.. DE LIMA-NETO. P. Sensitization evaluation of the austenitic
stainless steel AISI 304L. 316L. 321 and 347. Journal of Materials Science. 2005. vol. 40. no. 1. p. 139-144.
[15] MATULA. M.. HYSPECKA. L.. SVOBODA. M.. VODAREK. V.. DAGBERT. C.. GALLAND. J.. STONAWSKA. Z..
TUMA. L. Intergranular corrosion of AISI 316L steel. Materials Characterization. 2001. vol. 46. no. 2–3. p. 203-
210.
[16] BRIANT. C.L.. MULFORD. R.A.. HALL. E.L. SENSITIZATION OF AUSTENITIC STAINLESS STEELS. I.
CONTROLLED PURITY ALLOYS. Corrosion. 1982. vol. 38. no. 9. p. 468-477.
[17] BRIANT. C.. RITTER. A. The effect of martensite on the sensitization of low carbon 304 stainless steel.
Metallurgical and Materials Transactions A. 1981. vol. 12. no. 5. p. 910-913.