Paper No.
02055
2002
CORROSION
QUANTIFICATION OF DELETERIOUS PHASES IN DUPLEX STAINLESS STEEL BY
NOVEL ELECTROCHEMICAL PROCEDURES
M.A. Domínguez-Aguilar*, R.C. Newman*, J. Marsh** and J.W. Palmer**
*Corrosion and Protection Centre, UMIST
PO Box 88, Manchester, M60 1QD, UK
** CAPCIS Ltd
Echo Street, Manchester, M1 7DP, UK
ABSTRACT
Two novel electrochemical techniques have been developed for detection of sigma and related phases in
stainless steels. Both are based on anodic galvanostatic polarization. The first uses a mildly basic
solution and detects the phases themselves, while the second uses pure or mixed halide salt solutions and
detects alloy depleted zones around these phases. Both tests can be tuned to be more or less sensitive to
particular phases, depending on the application.
INTRODUCTION
Deleterious phases that precipitate in duplex stainless steels over the temperature range 600 to
1000 oC (σ, χ, Cr2 N) can be detected and quantified by microscopy. If necessary, this microscopy can be
done on fabricated or even installed components. However a sensitive and discriminating
electrochemical test would have major advantages: it can be rapid, it can use a minimally prepared
surface, and in principle it can be performed in a scanning or mapping mode. Such a test would be
complementary to magnetic methods, which are rapid and have some spatial resolution as well as ability
to look below the surface, but lack sensitivity and can be difficult to calibrate.
Electrochemical detection of chromium depletion in stainless steels is a well-developed field,
dominated by the Electrochemical Potentiokinetic Reactivation test (EPR) dating from the 1970s1,2. The
impetus for the development of this test was pipe cracking in Boiling Water Reactor (BWR) systems.
There are several versions of this test, but they are all based on passivating the steel in an acidic medium
and measuring the reactivation upon reduction of the applied potential. Published attempts to adapt the
Copyright
2002 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACE International,
Publications Division, 1440 South Creek Drive, Houston, Texas 77084-4906. The material presented and the views expressed in this paper are solely those of the
author(s) and not necessarily endorsed by the Association. Printed in U.S.A.
1
EPR test to detect the differing levels of Cr depletion associated with precipitation of σ phase in highalloy steels have been rather unconvincing3-8. There are numerous reasons why such a test should not
become a standard for this particular application: for example,
• Corrosion resistance and mechanical properties can be affected by precipitation that is
accompanied by quite small Cr depletion, e.g. 25 to 20%. This is a problem for all
electrochemical tests, but particularly so for the EPR test.
• The presence of Mo, and of phases such as χ that have very high Mo content and thus adjacent
Mo depletion, is not easily accommodated within the EPR method.
• The EPR test uses strong acids, a potentiostat, and a complex electrochemical procedure that
normally requires de-aeration of the solution. It is also slow.
• Performing the test at different temperatures would require complicate re-calibration.
If we ask “what is measured by the EPR test?”, it is clear that a pitting response would contain
essentially the same information, albeit in a more statistical form since the metal is not uniformly
activated. This leads to the suggestion that a chloride solution might be used, with the application of
oxidizing potentials. A drawback of such an approach might be that the high-alloy materials of interest
do not pit at ambient temperature. They might do so in a very concentrated chloride solution. In our
work we have used a bromide solution to eliminate the protective effect of Mo against localized
corrosion9-13 . This enables Cr depletion to be measured at ambient temperature, or alternatively the
effect of Mo can be tuned by varying the chloride to bromide ratio.
Alloy depletion is not adequate to obtain a complete description of the state of a steel containing
deleterious phases. For example, the precipitation of σ phase during quenching of a duplex steel may
occur at very high temperature, and thus results in relatively slight Cr depletion, but has a strong effect
on mechanical properties. We address this problem by complementing the halide test with a mild-alkali
test that detects the deleterious phases themselves. By judicious combination of the two tests, one can
avoid false negative results while maintaining a good level of quantification (recognizing that there is
more work to do on this aspect).
We emphasize that the testing approach outlined in this paper is designed, in the long term, to
accommodate all the concerns that arise from the precipitation of deleterious phases: avoidance of lowtemperature embrittlement, optimization of corrosion performance, and issues connected with stress
corrosion, fatigue and hydrogen embrittlement. Each of these is subtly and differently dependent on
alterations to the microstructure and composition of the material.
EXPERIMENTAL PROCEDURE
Material
The experimental material was UNS S32760* , a 25Cr super-duplex stainless steel, with the nominal
composition shown in Table 1. It was delivered in the form of a 16 mm diameter round bar, solutionannealed at 1060 °C and quenched, followed by isothermal heat treatment at 675 °C or 825 °C for 100,
*
Trademark ZERON 100®
2
300 and 1000 s to induce precipitation of σ, χ and nitrides depending on the temperature and time of
heat treatment. The test samples exposed the cross-section of the bar.
Table 1. Chemical composition (wt%) of UNS S32760.
Element
C
Si
Content
0.017 0.24
Mn
P
S
Cr
Mo
0.54
0.021 0.001 24.97 3.58
Ni
Cu
W
N
Fe
6.97
0.52
0.54
0.22
Bal.
Mild-alkali test
The solution used for this test was 0.1M Na2CO3 at room temperature, open to the air. The results
were weakly dependent on normal variations in the ambient temperature (this might be more of a
concern for the halide tests).
There is no reason why this test should not be developed using coarse grit finishes. However the
results obtained using polished (1 µm alumina) finishes were so convincing that they alone are presented
in this paper. Probably the polishing facilitates to some extent the exposure of very small particles or
grain-boundary films of deleterious phases.
The results presented were all obtained with a procedure very similar to the following:
• Polish the sample to a 1 µm alumina finish.
• Immerse in a beaker containing the test solution, a platinum counter-electrode and a reference
electrode (SCE).
• Passivate potentiostatically at a potential of 0.20 to 0.30 mV (SCE). This step is optional but
gives cleaner results.
• Apply a low anodic current and measure the potential for up to 10 minutes.
In the absence of deleterious phases, the potential would rise almost linearly to a value where the
alloy as a whole (with a bias towards the ferrite phase) was dissolving transpassively. With deleterious
phases present, the potential would arrest at particular values depending on the particular phases.
Halide test
This test was performed at controlled temperatures up to 50o C using pure chloride solution (0.85M
NaCl) and at room temperature using 0.85M NaBr and various mixtures with 0.85M NaCl.
The sample was simply immersed after polishing or grinding (the test was less sensitive to surface
treatment than the alkali test), then an anodic current was applied and the potential was monitored. A
“perfect” response was indicated either by no pitting (potential rising to the transpassive region) or by
pitting at a defined high anodic potential, depending on the solution used. A “bad” response was
indicated by a drop in potential and by potential fluctuations indicative of pitting events (these might be
more correctly characterized as intergranular or inter-phase corrosion in some cases).
3
RESULTS AND DISCUSSION
Mild-alkali solution
As a baseline, Figure 1 shows the anodic polarization behavior of UNS S32760 treated at different
temperatures and times. The precipitation of Cr[Mo]-rich phases generates electrochemical activity in
the range 250 to 700 mV (SCE), associated with transpassive dissolution of the Cr and Mo components.
The purpose of the galvanostatic test is to make a horizontal cut through these features (whose heights
by the way are time-dependent) and if possible to detect them one by one, i.e. to discriminate the
different deleterious phases.
10
10
-2
-3
AS RECEIVED MATERIAL
AGED 675°C-300 s
AGED 675°C-1000 s
10
-4
AGED 825°C-300 s
i (A/cm 2 )
AGED 825°C-1000 s
10
10
10
10
10
-5
-6
-7
-8
-9
-0.5
0
0.5
1
E (V) vs SCE
Figure 1
Polarization curves for UNS S32760 with various heat treatments in naturally aerated
0.1M Na2CO3 at room temperature, showing an increase in electrochemical activity with
ageing time and temperature. Scan rate recorded at 0.25 mV/s
Figure 2 shows the response of this test to variations in the condition of UNS S32760, using a current
density of 35 µA/cm2 and a prepassivation treatment of 250 mV for 1 minute. Clearly there are multiple
arrests in the potential depending on the particular phases present. The lowest arrest is due to Cr2 N, the
next to χ and the highest to σ.
4
0.8
α+γ
0.7
E (V) vs SCE
σ
0.6
χ
AS RECEIVED MATERIAL
AGED 675°C-300 s
0.5
AGED 675°C-1000 s
Cr2N
AGED 825°C-100 s
AGED 825°C-300 s
0.4
AGED 825°C-1000 s
0.3
0
Figure 2
100
200
300
400
Time (s)
500
600
700
Potential of UNS S32760, subject to various heat treatments, during anodic galvanostatic
polarization at 35 µA/cm2 in 0.1M Na2CO3 at room temperature, following prepassivation at 250 mV for 1 minute.
In the alkaline solution, Mo and N are particularly reactive, and there is a concern that the test might
overstate the importance of phases rich in these elements, at the expense of the σ phase (at least for
mechanical properties – not for corrosion properties which are severely degraded by Mo depletion due to
χ phase). This might represent a problem if this was the only solution used for evaluation. However the
nitride phase can be substantially pre-dissolved during the pre-passivation phase if the potential is
selected correctly, and the halide test can be tuned to be less sensitive to χ phase as shown below.
Halide test
Using 0.85M NaCl at room temperature, only the most grossly treated material showed evidence of
pitting – Figure 3. If the temperature was increased to 50o C, a better discrimination was obtained –
Figure 4. But the best results were obtained using NaBr or mixed NaBr/NaCl solutions, where the
protective effect of Mo was reduced or eliminated by formation of soluble complexes with Br14. Figure 5
shows the very high sensitivity of the test with a NaBr solution: even the minimal heat treatment at
675°C for 100 s gave a perceptible response in the potential. Such a treatment scarcely affects the
mechanical properties, but degrades the corrosion resistance in hot chloride solutions.
5
Data 1
1.20
1.00
E (V) vs SCE
0.80
0.60
0.40
0.20
AS RECEIVED MATERIAL
AGED 675°C-300 s
0.00
AGED 675°C-1000 s
AGED 825°C-300 s
-0.20
AGED 825°C-1000 s
-0.40
0
100
200
300
400
500
600
700
Time (s)
Figure 3
Response of UNS S32760 with various heat treatments to anodic galvanostatic
polarization at 10 µA/cm2 in 0.85M NaCl at room temperature, showing poor
discrimination.
Data 1
1.00
0.80
AS RECEIVED MATERIAL
E (V) vs SCE
0.60
AGED 675°C-300 s
AGED 675°C-1000 s
0.40
AGED 825°C-300 s
AGED 825°C-1000 s
0.20
0.00
-0.20
-0.40
0
Figure 4
100
200
300
400
Time (s)
500
600
700
Response of UNS S32760 with various heat treatments to anodic galvanostatic
polarization at 10 µA/cm2 in 0.85M NaCl at 50oC, showing improved discrimination.
6
Data 1
1.20
AS RECEIVED MATERIAL
AGED 675°C-300 s
1.00
AGED 675°C-1000 s
AGED 825°C-300 s
E (V) vs SCE
0.80
AGED 825°C-1000 s
0.60
0.40
0.20
0.00
-0.20
Figure 5
0
100
200
300
400
Time (s)
500
600
700
Response of UNS S32760 with various heat treatments to anodic galvanostatic
polarization at 10 µA/cm2 in 0.85M NaBr at room temperature, showing excellent
discrimination.
1.20
1.00
E (V) vs SCE
0.80
0.60
0.40
20°C
0.20
30°C
40°C
0.00
-0.20
50°C
0
50
100
150
Charpy Impact toughness
Figure 6
200
250
300
(J/cm 2 )
Example of a correlation between electrochemical results in the NaCl test (0.85M) from
20 to 50°C and Charpy impact toughness at –50°C. Potential was recorded after 500 s.
7
Correlation with mechanical and corrosion properties
A large number of correlations have been made in this program. Figure 6 shows one example, using
the NaCl solution and Charpy impact toughness determined in samples with an increasing content of
secondary phases. An increase of temperature in the halide test reduces noticeably the pitting resistance
whenever depleted zones are present that have a critical pitting temperature (CPT) below the test
temperature. The CPT of the base alloy in chloride solution is above 50°C, and the degree of
discrimination of secondary phases will depend on the difference between the CPT and the test
temperature. This is an advantage of using a chloride solution; on the other hand the bromide solution
(CPT < 20°C) is very convenient when tests have to be performed in the field or at “room temperature”.
The bromide solution is less sensitive to the identity of the deleterious phase, since it detects mainly Cr
depletion.
According to the literature15-17, material in the ranges of 27-49 J, 50-100 J, and above 100 J may be
considered as having low, intermediate and high quality respectively; our tests with suitable calibration
can clearly make such a discrimination. Extreme toughness problems are likely to be less prevalent in
future due to improvements in manufacturing; the prediction of corrosion resistance will be equally
important. The tests developed in this project are particularly useful for quantifying corrosion resistance.
CONCLUSIONS
1
2
3
4
The aim of the EPR test (quantification of Cr depletion) can be met for 25Cr duplex stainless
steels using a halide solution with anodic galvanostatic polarization. Such a test also has the
ability to emphasize or de-emphasize the role of Mo, by varying the bromide content.
A mild-alkali test gives complementary information by detecting the deleterious phases
themselves.
Cr nitrides can be pre-reacted in the alkaline test by a prior polarization treatment, so that
they do not interfere with the quantification of the more damaging phases.
There are promising correlations with mechanical and corrosion properties.
ACKNOWLEDGEMENTS
The authors would like to thank Materials Engineering Ltd of Aberdeen, who kindly performed the
mechanical testing. We would also thank to Weir Materials and Foundries, Manchester, for supply of
material. Acknowledgments are also due to the Instituto Mexicano del Petróleo for sponsorship.
REFERENCES
1. W. L. Clarke, V. M. Romero & J. C. Danko, “Detection of Sensitization in Stainless Steels Using
Electrochemical Techniques”, Corrosion/77, paper no. 180, Houston, TX: NACE International,
1977.
8
2. W. L. Clarke, R. L. Cowan & W. L. Walker, “Comparative Methods for Measuring Degree of
Sensitization in Stainless Steel”, in Intergranular Corrosion of Stainless Alloys, STP 656, pp. 99132, ASTM, Philadelphia, 1978.
3. I. Garz, S. Schultze & J. Gollner, Werkstoffe und Korrosion 48 (1997), p. 165.
4. E. Otero, C. Merino, C. Fosca & P. Fernandez, “Electrochemical Characterization of Secondary
Phases in a Duplex Stainless Steel by EPR Test”, in Duplex Stainless Steels’94, Vol. 3, paper no.
56, Abington Publishing, 1995.
5. J. R. Scully & R. G. Kelly, Corrosion 42 (1986), p. 537.
6. S. Schultze, J. Gollner, K. Erick, P. Veit & I. Garz, “The modified EPR-test: A New Tool for
Examination of Corrosion Susceptibility of Duplex Stainless Steels”, in Duplex Stainless Steels
’97, Vol. 2, pp. 639-647, KCI publishing, 1997.
7. P. Merino, X.R. Novoa, G. Pena, E. Porto & L. Espada, “Study of Intergranular Corrosion
Susceptibility of Duplex Stainless Steels Using Potentiokinetic Reactivation Tests PRT”, in
Duplex Stainless Steels ’91, Vol. II, pp. 871-878, Les Editions de Physique, 1991.
8. M. Vernau, C. Lojewski & J. Charles, “Modified EPR Test for Duplex Stainless Steels Surface
Contamination and Microstructural Investigations”, in ref. 7, pp. 863-870.
9. A. P. Bond, J. Electrochem. Soc. 120 (1973), p. 603.
10. E. Wallis, Werkstoffe und Korrosion 41 (1990), p. 155.
11. R. Guo & M. B. Ives, Corrosion 46 (1990), p. 125.
12. E. A. Abd El Meguid, Corrosion 53 (1997), p. 623.
13. P. Ernst, M.H. Moayed, N.J. Laycock & R.C. Newman, “Prediction of Pitting Potentials and
Critical Pitting Temperatures”. Passivity of Metals and Semiconductors, eds M.B. Ives, J.L. Luo and
J.R. Rodda, pp. 665-674, The Electrochemical Society, Proceedings Volume 99-42 (2001).
14. D. Tromans & A. Sato, Corrosion 57 (2001), p. 126.
15. T. Havn, A Morini, H. Salbu & O. Strandmyr, “Quality Improvements on Duplex and
Superduplex Cast and Forged Products for Offshore Applications: Producer and User
Viewpoint”, in ref. 6, Vol. 1, pp. 191-199.
16. P. C. Gould & J. C. M. Farrar, “Fracture Toughness of Duplex and Superduplex Stainless Steel
Welds”, in ref. 6, Vol. 1, pp. 483-490.
17. C. S. Wiesner, “Toughness Requirements for Duplex and Super Duplex Stainless Steels”, in ref.
6, Vol. 2, pp. 979-990.
9