1. TESTING AND SELECTION OF DUPLEX STAINLESS STEELS
FOR SOUR ENVIRONMENTS
Hans Eriksson, J. Michael Nichols (Sandvik sweden)
Karan Jain (Sandvik India)

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3. INTRODUCTION
Deeper exploration and drilling necessitates development and selection of higher steel grades that can survive harsh conditions and maintain productivity.
The measure of H2S and CO2 in the formation often form the preliminary basis of selection followed by temperature, pressure and other important factors like chloride content and other elements.
Different CRA with various chemical compositions and microstructures display varying resistance to stress corrosion cracking (SCC) which is the main type of corrosion for CRA in sour service. Although duplex steels have inherently high strength, their sour corrosion resistance has been debated.
The sour resistance of Duplex stainless steel is discussed here. Laboratory test results of different grades are presented and evaluated regarding their severity.

4. SCC TEST METHODS
Test Technique
Relaxation during testing
Specimens in corrosive environment
Plastic deformation in corrosive environment
Stress level
Duration
Constant strain
Yes No
<YS
Several weeks
Constant load
Yes/No3
Yes/No
<UTS
SSRT
Several hours
Since stress corrosion cracking (SCC) is the most important corrosion type of CRA’s in sour environments, it is of great importance to select proper methods to evaluate the suitability of different CRA’s.

5. CORROSION MECHANISM
Sour environments are virtually oxygen free. This obviously means that no oxygen can take part in any corrosion process appearing in sour environments. In de-aerated (oxygen-free) environments, H2S reduces the ability of CRA’s to passivate.
The most significant type of corrosion in sour environments is cracking.
There are two mechanisms of cracking
Anodic stress corrosion cracking.
Hydrogen embrittlement.
SCC is also promoted by increasing temperature & this causes further increase in corrosion types like pitting and general corrosion.
In effect, higher temperatures and H2S add to the complexity of the corrosion mechanism.

6. PROPERTIES OF dUPLEX STAINLESS STEELS IN SOUR ENVIRONMENTS
Effect of partial pressure and bicarbonate ions.
Effect of temperature.
Effect of cold work.
Effect of Chloride content.
Service experiments Vs Laboratory tests.
Grade
C (Max.)
Si (Max)
Mn (Max) Cr Ni Mo N
SAF 2205
0.03
1.0
2.0 22
5.5
3.2
0.18
SAF 2507
0.8
1.2 25
7.0
4.0
0.3

7. Effect of partial pressure & bicarbonate ions.
The tested material emanated from extruded bar (15 mm diam.) which was annealed at 1020°C/ 20 min (SAF 2205) and 1060°C/20 min (SAF 2507) and water quenched.
The tests were performed at 80°C, since this temperature is considered to be within the most critical range [2, 12, 13]. Both constant-load (test duration 720 h) and SSRT (strain rate 3 • 10-6 S-1) were performed in order to reveal result differences between these two methods.
The applied load in the constant-load tests was 100% of the YS at the test temperature (80°C). This implied 465 MPa in the case of SAF 2205 and 477 MPa for SAF 2507.

8. Effect of partial pressure & bicarbonate ions.
At least two specimens of each grade were tested in each of the 18 conditions determined by the different combinations of PH2S, concentration of bicarbonate ions and test method (constant load and SSRT). In addition, reference SSRT’s were performed in distilled water at 80°C to facilitate comparisons with the tests in the corrosive environments.
The partial pressure of hydrogen sulphide (PH2S) was 2, 4 and 8 psi (corresponding to 0.014, and MPa, respectively). The test solution was 5% NaCl acidified to pH 4 and with additions of 0, 10 and 20 mmol/ l sodium bicarbonate (NaHCO3) corresponding to 0.84 and 1.68 g/ l, respectively.
The purpose of adding NaHCO3 was to prepare solutions which were more similar to actual formation waters than the plain chloride solutions normally applied in laboratory tests. Bicarbonate ions are present in formation waters with varying concentrations, with about 30 mmol/ l (1.8 g/ l) being a normal value [14, 15].

9. Effect of partial pressure & bicarbonate ions.
Grade
NaHCO3
g/l mmol/ l
Constant Load1) PH2S,psi
SSRT2) PH2S, psi
SAF 2205
SCC/SeC
SCC/Se
SCC/SC
SAF 2507
SCC
0.84 10
No corr.
1.68 20

10. Effect of partial pressure & bicarbonate ions.
IMPORTANT OBSERVATIONS
- The addition of bicarbonate ions results in a markedly reduced SCC susceptibility. As an example can be mentio­ned that both SAF 2205 and SAF 2507 suffered SCC at PH2S 2 psi and no NaHCO3 addition. At 20 mmol/ l of NaHCO3 both grades were fully resistant at PH2S 2 psi. Only slight attacks occurred at PH2S 8 psi and 20 mmol/ l NaHCO3.
- Increasing PH2S from 2 to 8 psi increased the SCC susceptibility of the tested grades, although the tendency was not especially pronounced.
- SAF 2507, proved to be slightly more resistant than SAF This effect was best observable in the tests where 10 and 20 mmol/ l NaHCO3 were added to the test solu­tion.
- The two test methods employed, constant load and SSRT, somewhat surprisingly indicated fairly similar borderline environmental limits. It was expected that SSRT to some extent should provide more conservative results than constant load. However, as discussed under "SCC test methods" in this report, constant load tests initially imply plastic strain at a rate which at least for a limited time is in the range applied in SSRT.

11. Effect of TEMPERATURE.
SAF 2205 and SAF 2507 were also tested in different H2S/ CO2/ Cl- environments at 80°C and 140°C by Cortest Labs. C-rings, prepared from annealed tubes (OD 60.3 mm, WT 2.77 mm), were tested in 15% NaCl under the following four conditions:
Test 1 : PH2S 14 psi, PCO2 686 psi, 80°C
Test 2 : PH2S 08 psi, PCO2 947 psi, 80°C
Test 3 : PH2S 40 psi, PCO2 760 psi, 140°C
Test 4 : PH2S 62 psi, PCO2 963 psi, 140°C
Four C-rings, two loaded to the 0.2% YS and two loaded to the elastic limit, of each grade were all tested under the four different conditions.

12. Effect of TEMPERATURE.
As can be seen, the temperature has a dramatic influence.
- At 140°C, both grades were completely resistant at PH2S 40 psi, while both grades displayed some attack at 80°C and PH2S 8 psi.
- The tolerable PH2S was apparently fairly similar for the two grades.
- However, it should be emphasized that selective corrosion (denoted as pitting in figure 5) was more frequent and pronounced in SAF 2205 than in SAF 2507.
- There was no observable effect of the two slightly different stress levels (0.2% YS and elastic limit).

13. Effect of COLD WORKING.
SCC tests of SAF 2205 tubes with different levels of cold work in 5% NaCl + 0.5% HAc at temperatures up to 90 °C and varying PH2S. Stress level: 0.2% YS at test temperature.
Test time: 500 hours; Closed symbols – failure, open symbols – no failure
Interesting point :
The ratio σth/ YS is reduced by an increasing degree of cold work. However, it should be born in mind that the YS is increased by cold work which means that the reduction of the threshold stress in absolute figures is limited.
In some cases hardness specifications go hand in hand with yield strength specifications for CRA selections in sour environments.

14. Effect of CHLORIDE CONTENT.
Increasing chloride concentration has a strong influence in reducing the tolerable H2S. The image illustrates this and also the area of selective corrosion of the ferrite phase, between complete resistance and cracking.

15. Effect of CHLORIDE CONTENT.
Tests of SAF 2205 cold worked (about 50%) tubes in 5% and 15% NaCl, PCO2 70 bar, varying PH2S and temperatures from room temperature to 260 °C.
Stress level: 0.2% YS at test temperature.
Test time: 500h
As can be seen, at least PH2S 0.1 bar can be tolerated. At high temperatures, above about 150°C, higher PH2S can be tolerated, which supports the opinion that 80°C lies within the most critical range.

16. SERVICE EXPERIENCE Vs LAB TESTS
Service experience of DSS in sour environments is gene­rally good with very few failures. Cases exist in which the PH2S are considerably higher than what could be expected as possible according to laboratory results. Obviously, practical situations include some beneficial factors. Laboratory tests almost always involve one or more accelerating factors.
As shown above the applied load is important. A much higher PH2S can be tolerated provided the stress is relatively low. A production tubing string is normally constructed with a design factor of at least 2. Although the overall stress is low, higher stresses can be expected locally. However, an analysis comprising several critical situations, demonstrates that stresses in excess of the YS are unlikely to occur when the material is in contact with the process fluid.
Furthermore, service environments contain hydrocarbons which can form a protective layer on the metal surface. This also means that the conditions are considerably milder when the process fluid has low water content. As pointed out above and reported in the literature formation waters contain bicarbonate ions which increase the tolerable PH2S, mainly by increasing the pH-value.

17. GUIDELINES FOR TESTING & selection
It is very important to take into account that the conditions in actual applications are generally much milder than in laboratory tests which normally are considerably conservative. The following factors provide milder conditions in service:
- Lower stresses, relaxation often possible
- Presence of protecting hydrocarbon films
- The water content can be low
- The presence of bicarbonate ions in formation waters
Among the test methods SSRT provides especially severe conditions with continuous plastic deformation, monoto­nically increasing load (<UTS) and no possibility of relaxa­tion. Since the test time of SSRT is short, also tests with longer duration are recommended. Therefore, a preferred test procedure can be as on the next slide.

18. GUIDELINES FOR TESTING & selection
1. Long-term (> 1 month) tests of a relevant specimen type (e.g. C-rings in the case of tubes) in an environment with the maximum expected PH2S applied. The solution should contain the expected chloride content. The actual or a slightly lower pH value should be obtained by applying the expected PCO2 and adding a reasonable amount of bicarbonate ions, depending on the composition of the formation water.
The test temperature should be the bottom hole temperature and/ or 80°C. The latter tempera­ture is known to be within the most critical range.
2. SSRT in formation water (actual or synthetic). Partial pressures of H2S and CO2 should be close to the maxi­mum expected ones. The test temperature should be the bottom hole temperature and/ or 80°C. Off course, service experience of the considered alloys provides the most helpful information if such experience is available from conditions reasonably close to those expected.
A procedure according to the above recommendations will in many cases lead to the conclusion that DSS can be safely used in conditions considerably more severe, such as PH2S >0.1 bar, than would be expected from conservative laboratory tests. However, since failures are expensive and hydrogen sulphide is extremely harmful to human life, a certain degree of conservatism in material selection is recommended.