This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
Stress Corrosion Cracking of Corrosion Resistant Alloys in
Halide Brines Exposed to Acidic Production Gas
L. A. Skogsberg
Consultant
Houston, TX 77019
2207 McDuffie
USA
P. H. Javora
Baker Hughes Corporation
11211 FM 2920 Rd
Tomball, TX 77375-8927
USA
P. R. Rhodes
Consultant
810 Myrtlea Ln.
Houston, TX, 77079
USA
E. A. Trillo
Southwest Research Inst.
6220 Culebra Rd.
San Antonio, TX 78238
USA
ABSTRACT
A program partly sponsored by the American Petroleum Institute (API) studying the interaction
of completion brines and corrosion resistant alloys (CRAs) has been underway for several
years. An earlier paper [1] addressed the effect of thiocyanate (SCN –) corrosion inhibitor on
promoting stress corrosion cracking (SCC) in CRAs exposed to halide brines, with a link to field
failures. The current paper addresses SCC risks upon exposure of brines to an ingress of acidic
production gas (CO 2 or CO 2 + H 2 S) or to air, both of which have resulted in field failures. Test
protocols are presented. Brine severity is related to halide ion concentration, including
formation of complex ions with Zn+2. Testing focused on a group of 13Cr martensitic stainless
steels alloyed with Ni and Mo, with more limited tests of duplex stainless steels (SS) and high-Ni
alloys. SCC results are presented in relation to alloy resistance and to test variables, including
temperature, pH, and additives to the brine.
Key words: corrosion resistant alloys, completion brines, brine, stress corrosion cracking,
martensitic alloys, O 2 , oxidants, CO 2 , pH, buffer, O 2 scavenger, corrosion inhibitor.
INTRODUCTION
The annular space between the production tubing and the carbon steel casing is filled with a
dense fluid, typically a halide salt. In wells with corrosion resistant alloy (CRA) tubing, SCC of
the CRA tubing OD or auxiliary components has occurred in a number of wells. Halide brines
have densities up to 2.2 g/ml (19.2 lb/gal or ppg) and contain a number of additives. A previous
publication linked some field failures to addition of SCN– corrosion inhibitor. [1]
Additional field failures in CaCl 2 brines have been linked to ingress of acidic gas containing
CO 2 and H 2 S and to exposure to air. SCC of martensitic stainless steel (SS), 13%Cr, 6%Ni,
2%Mo) was attributed to downhole leakage of acidic gas[2] whereas SCC of a duplex SS
tubing (25%Cr, 3%Mo) was attributed to air in the gas cap above column of brine.[3]
To understand the effects of brine compositions on the CRAs, a joint industry project was
formed under the auspices of American Petroleum Institute (API). It has been known as CRA in
1
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
Brine Testing Program. Under its auspices work has been underway for a number of years on
understanding the interaction of brine chemistry and CRAs.
The current paper evaluates SCC risks of a range of CRAs in a range of halide brine
compositions for the case of exposure to acidic production gas (CO 2 +H 2 S). Also evaluated are
SCC risks due to air exposure. However, the testing became focused on a group of martensitic
stainless steels alloyed with Ni and Mo, that are collectively referred to as modified 13Cr
martensitic SS, or alternatively in some publications as super (S13Cr) martensitic SSs. Most
tests evaluated the as-received brine, excluding proprietary additives such as corrosion inhibitor
or oxygen scavengers. For completeness and comparison, test results provided by member
companies in the API program or in the publications are cited.
EXPERIMENTAL PROCEDURES
Test results from the current API program follow a detailed protocol, outlined in Table 1 for the
test results presented.
Table 1
API Test Protocols in Autoclave Testing in Halide Brines
C-ring preparation
C-rings were cut from tubular with mill scale removed or machined from bar
stock.
Loading
C-rings were loaded to yield strength (YS) following TM0177-2005 method C
[4]. YS was obtained by mill cert or calculated by derating to test temperature
Creviced C-rings
In some high-Ni tests a strip of C-276 was spot-welded on the C-ring apex.
Insulation
C-ring insulation from the C-276 loading bolt used a PEEK insulating washer.
Isolation was measured prior to and after testing. Tests where isolation was
not maintained and with crack were rejected.
Brine preparation
Commercial grade brines were used, with analysis provided
Component brines were stirred before blending.
Autoclave testing
Testing used a 4 liter C-276 autoclave, with 20% volume head space.
Each test contained up to 8 C-rings, individually isolated from each other and
the autoclave wall.
Buffer addition
When noted in text, Na 2 CO 3 was added to NaBr, Ca(OH) 2 or CaO to CaBr 2 .
CaBr 2 brines were filtered before use.
Brine pH
pH was measured without dilution using SPE 8652 with a double junction
reference electrode, at room temperature, before and after testing.
Tests with O 2
Brine was purged with either air (3 psi O 2 ) or a N 2 -O 2 mixture at room
temperature.
Deaeration
After sealing, 3 vacuum evacuation cycles with N 2 .
Deaerated brine was transferred , then a final N 2 purge made.
Other additives in
When noted, (1) erythorbate or HSO 3 - O 2 scavenger or (2) a filming amine
some tests
corrosion inhibitor for carbon steel casing was added.
Tests with CO 2
Transfer of deaerated brine was followed by a CO 2 purge for 2+ hours.
After heating to test temperature, CO 2 purge was added to reach target
pressure.
Tests with CO 2 +H 2 S Transfer of deaerated brine was followed by a purge of a H 2 S / CO 2 gas
mixture for 2 hours, then locked in at ambient pressure. The autoclave was
then heated to the target temperature. The autoclave was then increased in
pressure with either CO 2 or H 2 S / CO 2 depending on the environment.
2
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
Draeger tubes were utilized to ensure target H 2 S was obtained. Adjustments
were performed for the first day, day three and on a weekly basis if needed.
Cross sectioning was often required. Crack is assigned if crack-like features
were greater than 25μm.
SCC resistance is based on 30 day tests.
14 day tests explored test variables. Some 90-day tests are included.
Test failure
Exposure time
Stress reduction at 265 F was about 7.5% from that at room temperature and was
approximately 11% for tests performed at 350 F
The API protocol of total saturation of CO 2 at P CO2 may represent a more severe test than gas
leakage under field conditions. The latter case would correspond to absorption of a volume of
CO 2 in the annular column of brine that represents gas leakage [5, 6] rather than full saturation.
Alloy Groups Tested
Test alloys are representative of alloys in field use. Martensitic stainless steels (MSS) are
extensively used as tubing and equipment. Distinction is made between standard 13Cr
martensitic alloys and modified 13Cr containing Ni and Mo. Tables 2 and 3 list the alloys used
in the API tests. Alloy designations follow API CRA specifications for CRA tubing. [7] A 13-5-1110 alloy has a composition of 13%Cr-5%Ni-1%Mo and minimum YS of 110 ksi (758 MPa).
UNS designation is used for alloys that do not fit this formula. A possible alloy variable in
modified 13Cr, the retained austenite fraction, is also listed in Table 3.
Table 2
Chemical Composition of Alloys Cited
Alloy
Chemical Composition, %
Mo
Ti
N
V
C
Fe
Ni
Cr
Cu
W
Nb
Co
0.21
86.7
0.14
12.91
--
--
--
--
--
--
--
--
0.01
0.20
0.01
0.01
0.02
0.01
86.4
81.9
80.0
80.0
79.0
0.01
5.20
4.3
5.9
5.9
4.68
6.9
12,89
12.8
12.1
12.1
13.46
12.00
0.60
1.0
1.90
1.90
1.67
2.90
0.06
0.05
0.08
0.10
-0.10
-0.08
--0.07
--
--------
-----
-----
0.04
--0.07
0.07
---
--
--
0.02
0.02
69.2
62.2
5.01
6.86
22.05
25.0
3.10
3.15
---
0.18
0.29
---
0.50
0.50
-2.22
---
---
0.02
0.02
0.02
0.02
0.02
0.01
0.01
0.01
0.01
4.0
39.1
36.0
23.2
27.0
27.4
7.5
16.7
15.1
Bal.
31.4
35.4
54.1
43.5
42.7
58.2
50.7
52.5
14.55
25.6
20.2
18.8
22.4
21.5
20.9
23.8
19.8
15.0
3.2
3.6
3.0
2.8
3.4
8.0
6.6
9.0
--2.04
0.96
0.93
2.13
1.52
0.30
--
-----------
----------
-0.71
-0.02
2,60
1.67
-0.75
0.10
----------
------0.3
3.6
---
2.50
--0.04
----1.10
Standard 13Cr
13Cr-L80
Modified 13Cr
13-5-0.6-110
13-4-1-110
13-6-2-95
13-6-2-110
S41426 (bar stock)
13-7-3-125
0.06
0.06
Duplex SS
22-5-3-125
S39274 (tubing)
High- Ni
15-60-16-135
25-32-3-125
935 (bar stock)
N07718 (bar stock)
21-42-1-120
N09925 (bar stock)
N07725 (bar stock)
25-52-11-125
20-54-9-130
3
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
Table 3
Mechanical Properties of Alloys Used in Testing
Alloy
YS (ksi)
UTS (ksi)
% Elongation
Hardness, HRC
13Cr-L80
13-5-0.6-110
13-4-1-110
13-6-2-95
13-6-2-110
S41426 (bar stock)
13-7-3-125
22-5-3-125
S39274 (tubing)
25-32-1-125
935 (bar stock)
N07718 (bar stock)
21-42-1-120
N09925 (bar stock)
N07725 (bar stock)
25-52-11-125
20-54-9-130
92.8
112.6
120.2
102.3
120.5
115.5
134.7
142.3
136.7
134.4
111.7
131.2
121.0
115.0
126.0
132.0
131.0
118.4
122.5
127.9
122.0
126.2
137.8
137.2
148.1
150.0
141.7
159.3
173.6
136.0
164.0
164.0
142.0
143.0
23
20
34
24
23
21
30
19
21
19
25
32
15
29
29
17
19
23
26
27
26
27
30
31
33
34
30
30
37
25
33
37
32
--
% Retained Austenite
5.2 ± 0.2
2.8 ± 1.0
10.0 ± 0.7
5.2 ± 0.2
15.5 ± 0.2
Brines Tested
Brines were obtained from commercial suppliers, and were normally tested as-received, without
the proprietary additive package (corrosion inhibitor, oxygen scavenger, and sometimes biocide)
that are normally added prior to application. The additives used in API tests are generic, and
identified in the tables. Table 4 lists brines used in API testing or cited in other reports. Trisalt
refers to brines containing CaCl 2 , CaBr 2 and ZnBr 2 .
Included in Table 4 is the pH in the as-received brine pH, and the lower pH measured after
saturation with 100 psi CO 2 . Zn-containing brines contain buffer due to blending with 19.2 ppg
ZnBr 2 -CaBr 2 , which contains the equivalent to 0.04 M OH–.
Table 4
Brine Compositions Cited
Density
ppg
g/ml
11.6
1.39
12.4
1.49
13.0
13.7
14.2
1.71
14.2
14.2
14.2
14.3
14.7
15.1
15.5
1.86
15.5
17.0
18.0
18.0
19.2
2.31
Brine
NaBr
CaCl 2
NaBr
CaBr 2
CaBr 2
(a)
CaBr 2
CaBr 2 -CaCl 2 (low)
CaBr 2 -CaCl 2 (high)
CaBr 2 -ZnBr 2
trisalt
CaBr 2
CaBr 2 -CaCl 2
CaBr 2- ZnBr 2
trisalt
trisalt
ZnBr 2 -CaBr 2
trisalt
ZnBr 2 -CaBr 2
46
----------------
Composition, %
CaCl 2
CaBr 2
39.1
---44.9
-49.1
-51.9
11.4
42.6
23
32.2
-41
9.6
31.6
-54.5
18.3
35.2
-39.9
17.6
41.1
7.4
27.6
-28.3
4.9
27.9
-22.8
4
ZnBr 2
------16.4
13.4
--13.2
6.4
22.3
42.8
39.8
52.8
initial
~8
6-9
3 6 - 6.7
5.6
2.8
6.5
6.5
5.3
--7.5
-5.8
--1 to 2
3.9
1.5
pH
+ 100 psi CO 2
~6
2.8
2.5
2.5
2.3
2.3
-2
-3.2
1.5
-----
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
(a)
0.26 wt% Cl–
Brine severity resulting in SCC of CRAs can be viewed from two different parameters (1) acidity
and (2) total molar halide ion concentration, [Cl–]+[Br–]. Table 5 compares a wide range of
brines, but distinguishes between total and free halide concentrations. Total concentration refers
to simple halide anion formation based on the chemical formula. Free concentration accounts
for complex ions formed between Zn+2 and halide anions, which reduce the free concentration of
Cl- and Br-. Free anion concentrations were calculated by the Automatic Chemical Model at
2650F. [8]
Table 5
Free Halide Anions in Concentration in Brines
Brine
ppg
11.6 CaCl 2
14.2 CaBr 2
15.1 CaCl 2 -CaBr 2
15.5 ZnBr 2 -CaBr 2
15.5 trisalt
18.0 ZnBr 2 -CaBr 2
19.2 ZnBr 2 -CaBr 2
Total Concentration
[Cl ], M
[Br ], M [Cl–]+[Br–], M
9.51
-9.51
-8.85
8.85
6.1
7.7
13.8
-10.7
10.7
5.9
8.7
14.6
-4.9
4.9
-15.8
15.8
Free Concentration
[Cl ], M
[Br ], M
--8.8
----6.4
5.4
7.9
-3.5
-2.5
pH
6
6
6
5.5
5.5
2.5
1.5
RESULTS
Test results are segregated according to alloy group and exposure conditions. Exposure to the
acidic gas environments were either (1) CO 2 saturation (100 or 500 psi), or (4) saturation with
CO 2 +H 2 S (0.15 or1.5 psi). Saturation to specific partial pressures of CO 2 and H 2 S represents
halide brine in equilibrium with the partial pressure of these acid gases leaking into the brinefilled annulus. Additional N 2 tests were made with a N 2 purge and gas cap, or and an air gas
cap (0.15 or 3.0 psi O 2 after purging).
Test results in the tables are identified either as a pass, ”O” or a failure, “X”. The “X”
designation includes crack- like features at least 25 μm deep that were observed on
metallurgical mounts. For the case of multiple specimen tested, the designation X[3] or O[4]
indicates the number failures or passes in a group of identical specimens. Projection of
acceptable field performance requires no cracking in a 30 day test, which is consistent with CRA
test requirements for use in exposure to produced gas containing H 2 S. [9] The 14-day tests,
which were intended to explore test variables, resulted in many failures in tests of modified
S13Cr, and are therefore included in the tables. Test results from other laboratories, except as
noted, met the following criterion: (1) free of SCN– or titratable oxidant and (2) insulation from
other alloys. Test results provided by member companies are cited [10] as Company A, B, C, D
and F.
Standard 13Cr-L80 was found to be fully resistant to SCC, and is therefore not included in
martensitic SS test results. In addition, severe tests containing SCN– were reported earlier.[1]
5
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
No SCC was observed in 8 additional API tests following brine exposure to air or acidification
with CO 2 .
Modified 13Cr Martensitic SSs
Exposure to CO 2
Table 6 summarizes API tests in a range of brines saturated with CO 2 . Addition of CaO as
buffer and erythorbate O 2 scavenger was made in a number of tests. Addition of a generic
amine corrosion inhibitor (for carbon steel) was included in Test #80. The most severe brine,
based on Table 6, is 15.5 ppg trisalt, with cracking in all specimens after 14 days, and the final
pH 1.7.
Table 6
API Tests of Modified 13Cr in Halide Brines + CO 2
Test
#
Brine
Conditions
Additive
ppg
Te
mp
o
F
YS
Time
calc
days
265
265
265
@RT
@RT
@RT
265
265
265
265
265
265
Results
(a)
BS
13-6-2
-110
-95
13-5-0.6
-110
13-4-1
-110
13-6-2
-110
13-7-3
125
14
30
30
X,X
O,O
O.O
X,X
-O.O
X,X
-OO
X,X
O,O
--
X,X
O,O
O.O
----
@T
@RT
@RT
@RT
@RT
@RT
@T
@T
@RT
90
30
30
30
30
30
-O,O
O,O
X,X
O,O
O,O
O,X
-X,X
-O,O
O,X
O,O
----
-X,X
O,O
O,X
O,O
O
O
O[4]
------
O,O
O,O
O,O
O,O
O.O
--
--
-X,X
O[4]
X,X
O,O
-----O
O[4]
--
-X,X
30
X
X
--
X
X
X, O[3]
--O
-----
X
O
O,O
O
O
O,O
CO2 Saturation (100 psi)
24
45
67
15.5 trisalt
12.4 NaBr
12.4 NaBr
78
50a
50b
51
52
70
13.7 CaBr2
14.2 CaBr2
14.2 CaBr2
14.2 CaBr2
14.2 CaBr2
14.2 CaBr2
79
23
80
14.7 CaBr2
14.2 CaCl2-(11%)
-CaBr2 (43%)
14.2 CaCl2-(11%)
(b)
-CaBr2 (43%)
14.2 CaBr2
68
14.2 CaBr2
62
74
81
83
none
erythorbate
+ buffer
buffer
buffer
buffer
buffer
265
265
30
14
erythorbate
+ inhibitor
buffer
265
@RT
@T
@T
300
@RT
30
X,X
O,O
X
--
O,O
--
14.2 CaBr2
13.0 CaBr2
buffer
350
350
30
30
13.0 CaBr2
25 g/l Cl
@RT
@RT
@T
@RT
@T
O,O
-----
O,X
O
O
O
O
X
O
O
---
------
O,O
O
O
O,O
O,O
-O
O
O
O
@RT
30
--
O
--
--
X
X
–
350
30
30
CO2 Saturation (500 psi)
75
17.0 trisalt
350
6
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
77
17.0 trisalt
(a)
(b)
350
@T
@RT
@T
----
30
O
---
-O
O
-O
O
X
O
O
O
O
O
bar stock, UNS S42426
improved de-aeration protocol used
SCC in Table 7 compares the effect of 11.0 ppg brine composition in tests varying mixture of
NaBr and CaCl2. Test conditions were 13-5-2-95, 284oF (140oC), 645 psi CO2 + 0.1 psi H2S.
Tests were 14-day exposure of U-bends, with full de-aeration, addition of 0.03 N NaOH, HSO3–
O2 scavenger, a biocide and a filming amine type of corrosion inhibitor. The positive shift in the
pitting potential, measured during an anodic scan, is consistent with the increase in the halide
ion concentration. [11].
Table 7
Effect of Molar Halide Concentration on SCC in 11.0 ppg Brines + CO2
Brine
vol% CaCl2
100
70
50
30
10
0
pH
vol% NaBr
0
30
50
70
90
100
initial
9.0
9.3
10.0
10.2
11.0
11.6
Results
final
4.0
4.4
4.6
4.8
5.2
6.2
X,X
X,X
O,X
O,O
O, O
O, O
Halide
Concentration
M (= mol/l)
7.7
6.6
5.9
5.2
4.6
4.2
Pitting
Potential
-- 0.10 V vs. SCE
+ 0.30 V vs. SCE
.
Additional tests conducted in other laboratories are listed in Table 8. The O2 scavenger is
typically an organic compound (erythorbate) or hydrosulfite, HSO3–.The corrosion inhibitor is
typically a high molecular weight morpholine-type amine added for corrosion control of the
carbon steel casing. [10,12-15] Test conditions are temperature ≥ 245oF, PCO2 ≥ 100 psi, PH2S
≤ 0.4 psi, and test times ≥ 30 days. In tests listed with low H2S added, the value of PH2S at the
end of the test is not known.
Table 8
Additional Tests of Modified13Cr + CO2
Ref.
Alloy
Brine (ppg)
T
F
"
245
"
275
"
"
295
"
"
O2 scv.
"
no
"
yes
"
no
no
"
"
Additives
Corr.
"
no
"
yes
yes
no
no
no
yes
other
"
none
"
none
none
none
none
"
"
"
275
"
338
"
"
yes
"
yes
"
no
yes
"
no
"
"
biocide
"
none
"
o
Company B
[10]
[12]
"
"
Company F
[10]
[13]
[14]
"
12-7-3-125
13-6-2-95
"
13-4-1-110
"
13-6-2-95
"
13-6-2-110
"
14.3 trisalt
14.3 CaBr2-ZnBr2
12.0 NaBr
12.0 CaBr2
16.6 ZnBr2-CaBr2
12.5 NaBr
14.2 CaBr2 (pH 3.1)
. 14.2 CaBr2 (pH
7.1)
15.5 ZnBr2-CaBr2
11.5 NaBr
11.5 CaCl2
11.2 CaCl2
14.7 CaBr2-CaCl2
7
Gas Cap
PCO2
PH2S
"
"
50
no
"
"
100
0
"
"
"
"
205
0
"
"
"
"
"
545
"
600
"
"
0.3
"
0.4
"
Results
X,O,O.
O
O
O
O
O [3]
X [3]
X [3]
O [3]
O
O
X
O
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
[15]
13-5-1-110
"
11.5 CaCl2
17.5 trisalt
350
"
no
"
no
"
none
"
100
"
0.4
"
X [3]
O [3]
.
Exposure to CO2 + H2S
Table 9 summarizes API tests 30-day tests at 265oF with 0.3 psi H2S added to the CO2. As
noted in the API protocol (Table 1), PH2S measurements were confirmed at the end of the test.
Every specimen failed by SCC, and a very adherent black scale was present on all surfaces.
Table 9
Tests of Modified 13Cr in Halide Brines + 100 psi CO2 + 0.3 psi H2S
Test
#
56
57a
57b
Conditions
Brine
ppg
12.4 NaBr
14.2 CaBr2
14.2 CaBr2
Results
Additive
YS calc
--Ca(OH)2 buffer
@RT
@RT
@RT
Time
days
30
30
30
13-4-1
-110
X,X
X,X
X,X
UNS S42426
-110
X,X
X,X
X,X
13-6-2
-110
X,X
X,X
X,X
13-6-2
-110
X,X
X,X
XX
Exposure to Air
Table 10 lists SCC test results for varying levels of O2 ranging from air saturation (3.0 psi O2) to
a fully deaerated 11.6 ppg CaCl2. Trace O2 refers to early tests when deaeration procedures
that were not sufficiently rigorous. Tests at 0.15 psi O2 were prepared by saturation with a gas
mixture of 1% and 99% N2.
.
Table 10
Effect of O2 on SCC of Modified13Cr Tests in 11.6 ppg CaCl2
Ref.
Test Conditions
Temp
Time
13-5-0.6
o
-110
F
days
265
30
-7
--
SCC Results
13-6-2
13-6-2
-95
-110
-X [4]
O [5]
---
Comp. A [10]
[16]
O2
psi
trace
3.0
#46
3.0
265
30
O
O
O
X
#20
#31
#32
#37
0.15
trace
"
none
265
265
350
350
14
14
14
30
O,O
O,X
O,X
O,O
O,O
X,X
O,X
O.O
-O,X
O,O
O,O
O,X
O.X
O,O
O,O
Comment
13-4-1
-110
severe 7-day test at varied levels
of stress [17]
½ the C-rings coupled to
carbon steel
new deaeration procedures used
Table 11 lists tests of modified 13Cr alloys with air (3 psi O2) in other brines. Tests are listed in
order of increasing test temperature.
8
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
Table 11
Tests of Modified 13Cr in Aerated Bromide Brines
Test
#
48
4
42
2
1
B
45
65
Brine
ppg
12.4 NaBr
15.5 ZnBr2-CaBr2
18.trisalt
15.5 trisalt
18.0 ZnBr2-CaBr2
14.2 CaBr2
12.4 NaBr
12.4 NaBr
30
A
71
12.4 NaBr
14.2 CaBr2
12.4 NaBr
72
12.4 NaBr
63
76
14.2 CaBr2
15.5 trisalt
Conditions
Additive
Temp
o
F
-225
-225
-225
-225
-225
-225
265
erythorbate
265
+ buffer
-265
-265
erythorbate
300
+ buffer
erythorbate
350
+ buffer
-350
–
10 g/l Cl
350
YS
calc
@RT
@RT
@RT
@RT
@RT
@RT
@RT
@RT
Time
days
30
14
30
14
14
14
30
30
13-5-0.6
-110
O[3]
-O.X
---O,O
O,O
13-5-1
-110
O,O
O,O
O,O
O,X
O[3]
O,O
-O,O
Results
(a)
BS
13-6-2
-110
-95
-O.O
-O,O
-O,O
-O[3]
-O,O
-O,O
-O,O
O
--
@RT
@RT
@RT
@T
@RT
14
14
30
O,X
-O
O
O,O
O,X
O,O,O
-O
O,O
--O
O
O,O
@RT
@RT
@T
90
30
O,O
---
O,O
O
O
X
O
O
30
13-6-2
-110
-O,O
O,O
O,O
O,X
-O,O
O,O
13-7-3
-125
----------
O,X
O,O
O
O
--
O,O
O,O
--O,O
-------
----
O,O
O
O
-O
O
(a) bar stock, UNS S41426
Company A test results of 13-6-2-95 and 13-6-2-110 in five aerated brines are listed in Table
12. Both U-bend and C-rings were tested at YS, with no significant difference noted. No
significant difference was noted between the two grades of 13-6-2. No SCC occurred in an
additional set of 20 tests with erythorbate O2 scavenger added.
Table 12
Aerated Brines: Effect of Brine and Temperature on SCC of 13-6-2
Brine
Temperature
o
11.6 ppg CaCl2
(a)
12.9 ppg CaCl2-CaBr2
14.2 ppg CaBr2
(a)
16.7 ppg CaBr2-ZnBr2
19.2 ppg ZnBr2-CaBr2
o
200 F
---O [1], X [3]
265 F
X [4]
X [4]
O [4]
O [2], X [6]
O [1], X [11]
o
350 F
-O [5], X [3]
O [8]
O [8], X [4]
O [2], X [6]
(a) 50 vol% mixture of CaCl2 and CaBr2 or CaBr2 and ZnBr2
As-received Brines
Two 90-day tests in as-received brines were made in deaerated brine with a N2 gas cap. Test
results demonstrate the increase in SCC susceptibility with increasing total molar halide
concentration. No SCC occurred in test #38. The 15.1 ppg CaBr2-CaCl2 brine was chosen to
9
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
test brine with maximum halide ion concentration (Table 5). Table 13 shows that SCC of
modified 13Cr SSs can occur in the absence of acidic gas or an oxidant.
Table 13
90-day Tests of Modified 13Cr, N2 350oF
Test
#
33a
73
Test Conditions
Brine
ppg
[Cl–]+[Br–]
14.2 CaBr2
15.1 CaBr2-CaCaBr2
8.85
13.8
YS
calc.
13-5-0.6
-110
13-5-1
-110
@RT
@RT
@T
O,O
---
O,O
X
O
M
Test Results
13-6-2
13-6-2
-95
-110
O,O
---
O,O
O
O
13-7-3
-125
S41426
-O
O
-O
X
Duplex and High- Ni Alloys
Exposure to CO2
Table 14 lists API tests of duplex SS saturated with 500 psi CO2. Also included are literature
citations with up to 0.4.4 psi H2S added
Table 14
Tests of Duplex SS + CO2
Ref
Brine
ppg
#38
#39
#34
[18]
[11]
"
"
[12]
"
"
[14]
14.2 CaBr2-CaCl2 (23%)
–
14.2 CaBr2 + 1% Cl
15.5 ZnBr2-CaBr2
11.2 CaCl2
12.0 CaBr2-CaCl2
14.8 CaBr2
18.0 trisalt
11.5 NaBr
11.5 CaBr2
"
11.2 CaCl2
14.7 CaBr2-CaCl2
12.7 NaBr
12.7 NaBr
14.2 CaBr2
"
[19]
"
"
(a) UNS S42426 tubing (25CrW)
Test Conditions
additive
temp
none
"
"
none
O2 scav.
+ inhib.
O2 scav.
+ inhib.
"
"
"
Inhib.
Inhib #1.
Inhib. #2
350
350
350
320
350
"
"
275
"
"
338
338
350
"
"
"
time
days
PCO2
psi
PH2S
psi
22-5-3
-125
30
30
30
30
30
"
"
30
30
60
30
30
30
"
"
"
500
500
500
168
100
0
0
0
0.45
0
"
"
0.3
"
"
0.4
"
4.4
"
"
"
X,X
X,X
XX
O [4]
---O
O
X
"
"
O,O
O,O
O,X
O,O
"
585
"
"
600
600
440
"
"
"
Results
(a)
S39274
X,X
X,X
O,O
-------O
X
O,O
O,O
O,O
O,O
The last two entries in Table 14 compare test results with two different amine-based inhibiters.
Differences in buffering capacity are shown by initial and final pH values. The pH drop using
amine #1 was 5.0 to 2.8, but 8.3 to 3.5 using amine #2.
Exposure to CO2 + H2S
10
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
Test results listed in Table 15 show cracking can occur in both duplex and some Ni-base CRAs
in CaBr2 brines at 350oF when exposed to acidic gas at 1.5 psi H2S. The SCC resistance of Nibase alloys increases with higher Ni content. The transgranular crack path of 25-32-3 high-Ni
alloy is shown in Figure 1.
Table 15
o
Tests of Duplex SS and High-Ni Alloys in 14.2 ppg CaBr2, 500 psi CO2 + 1.5 psi H2S, 350 F
Results
Test
#
YS
Calc.
@RT
@RT
@RT
@T
58
64
82
(a)
Duplex SSs
(a)
22-5-3
S38274
-125
-125
-X,X,X,O
X,X
X,X
-----
Ni-base
25-32-3
31% Ni
X,O
X,X
X
O
935
35% Ni
-O
---
25-52-11
51% Ni
O,O
O
O
O
(a)
N07718
52% Ni
-O
O
O
UNS No.
Figure 1 Crack features of alloy 25-32-3-125 (test #64)
Air Exposure
The effect of air or oxygen content in the duplex was not as straight forward as it is On the MSS
alloys (as seen in Table 10). Table 16 presents test results from three laboratories (two from
references and one from this test program). No SCC occurred in API tests in aerated 14.2 ppg
11
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
CaCl2 or 15.5 ppg trisalt. In contrast, in aerated 11.0 CaCl2, SCC occurred within 18 hours, but
not within 14 days when coupled to carbon steel. The third set of test results evaluated brine
composition, using brines that had been briefly exposed to air after N2 purging. Under these
test conditions, SCC was observed in CaBr2 brines containing more than 25 wt% CaCl2. No
cracking in 11.0 ppg CaCl2 occurred when an NaHSO3 O2 scavenger was added.
Table 16
Tests of Duplex SSs + Air
Ref
Brine
ppg
14.2 CaBr2
15.5 trisalt
11.0 CaCl2
"
"
11.0 CaCl2
11.0 CaCl2
11.7 CaBr2-- CaCl2 (32%)
12.3 CaBr2-CaCl2 (32%)
142.2 CaBr2-CaCl2 (25%)
12.3 CaBr2-CaCl2 (32%)
14.2 CaBr2-CaCl2 (9%)
11.7 CaBr2
9.0 CaCl2
12.4 NaBr
#5
#44
[18]
"
"
[6]
"
"
"
"
"
"
"
"
"
Test Conditions
PO2
addition
psi
3.0
3.0
3.0
none
3.0
coupled to steel
none
N2 purge
none
O2 scav.
trace
none
trace
"
trace
"
trace
"
trace
"
trace
"
trace
"
trace
"
trace
"
Time
days
30
30
0.75
14
17-125
30
30
30
30
30
30
30
30
30
30
Temp
o
F
350
350
290
320
30
325
325
325
325
325
325
325
325
325
325
Results
22-5-3
S39274
-125
O,O
O,O
O,O
O,O
X
O [4]
O [12]
O
X
O
X
O
O
O
O
O
O
Table 17 lists three tests of High-Ni CRA C-rings at high temperature (425 – 450 F). Test
severity was increased by using room temperature values of the YS. SCC was absent in all
tests. In Test #47, no corrosion was observed on the area exposed at the interface. The H2S
partial pressure in Company B tests was confirmed at the end of the test. Test conditions for a
second Company B test were modified by air exposure and addition of 0.24 cc/l 26% NaHSO3
solution, which is three times the recommended dosage. No SCC resulted.
Table 17
Tests of High-Ni Alloys at 425 to 450oF
Alloy
25-32-3-125
21-42-19120
25-52-11-125
15-60-16-135
20-54-9-130
(b)
N07718
(b)
N09925b
(b)
N07725
(a)
(b)
Comp B Test [10]
Conditions
Results
12.4 ppg NaBr
980 psi CO2
0.6 psi H2S
crevice for
½ half the C-rings
28 days
o
450 F
O
O
-O,O
-O,O
O.O
O,O
Test #47
Conditions
Results
15.5 ppg trisalt
N2 air purge
30 days
o
425 F
Intentional interface exposure
bar stock with UNS number cited
12
(a)
O ,O
(a)
O ,O
O,O
O,O
O,O
O,O
O,O
Test# 18
Conditions
15.5 ppg trisalt
N2 purge
30 days
o
425 F
crevice for
½ half the C-rings
Results
O,O
O,O
O,O
O,O
O,O
O,O
O,O
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
Other Variables
A number of variables were evaluated that could bear on brine formulations or evaluation of field
risks.
More Severe Field Conditions
In some tests of modified 13Cr martensitic, SCC occurred at (and was limited to) the area where
the C-ring apex was exposed at the brine-vapor interface. The wicking action resulted in brine
concentration and accelerated corrosion. This condition appears to replicate the conditions that
contributed the duplex SS failure in the air-filled gas cap.[3] This potential exposure condition
therefore represents a greater SCC risk than present using fully immersed test specimens. As
noted in Table 17, no SCC occurred during intentional interface exposure of high High-Ni alloys
in test #47.
Table 17 also list high-temperature tests of high-Ni CRAs using creviced C-rings. The intention
was to increase test severity by enhancing localized corrosion. All C-rings were free of SCC
and pitting corrosion.
Addition of Corrosion Inhibitor
Two studies have evaluated the contribution of high molecular weight amines, which are added
as corrosion inhibitor for carbon steel casing. Electrochemical measurements of 13-6-2 in 15.5
ppg trisalt at 225oF show no effect on the pitting potential.[8]. Company F tests [10] showed that
the buffer capacity was too low to be an effective buffer in 14.2 ppg CaBr2. After adding 1 vol%
inhibitor the pH increased to 7.1 but fell to 2.2 after testing at 205 psi CO2 and 295oF.
Presence of Titratible Oxidant
Some Company B tests of modified 13Cr SS in CaBr2 and CaBr2-CaCl2 brines were
compromised by presence of a titratible oxidant.[10] A concentration of ~2 meq/l bromate
(BrO3–) was measured in a thiosulfite-iodine titration. No SCC occurred in re-tests after adding
erythorbate O2 scavenger. The bromate source appears to be due to a combination of air
exposure and a pH decrease during long term exposure.
Addition of O2 Scavenger
The possibility that SCC of duplex SS could result from decomposition of bisulfite (HSO3 –) O2
scavenger was raised.[18] Our testing was limited to tests of modified 13Cr SSs in test #29.
Test conditions were 10 ppg NaCl, 265oF, but made more severe by testing at 3 times the
normal dosage of HSO3 –. No SCC or localized corrosion resulted.
13
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
Company B tests show SCC resistance in 28-day tests for both duplex and Ni-base CRAs in
12.4 ppg NaBr at (1) 450oC and (2) 390oC , 980 psi CO2, 0.5 psi H2S.[10] In test #29, no SCC
of modified 13Cr martensitic SSs occurred in 10.0 ppg NaCl at 265oF.
Successful use of erythorbate O2 scavenger has been demonstrated in a number of laboratories
tests cited. Elimination of SCC following addition of an O2 scavenger (Table 16) can be
attributed to removal of trace O2 in CaCl2 tests.
DISCUSSION
Alloy Composition and Strength
For each alloy group, higher alloy content resulted in higher resistance to SCC: Ni in high-Ni
alloys, Cr+W in duplex SSs, and Mo in modified 13Cr martensitic SSs.
In the modified 13Cr group the % retained austenite cited in Table 3 may be a variable affecting
SCC susceptibility. The bar product (1.7% Mo) with a higher % retained austenite was more
susceptible than tubular products with lower Mo content. No clear effect of strength (13-6-2-95
vs. 13-6-2-110) was observed, with the 95 grade having a higher % retained austenite than the
110 grade. Support for this conclusion was obtained in SEM studies, which showed high
retained austenite along the prior austenite grain boundary [20], which is the path of crack
propagation. [2]
Yield strength was not a significant variable.in modified martensitic SSs. In comparing test tests
results of 13-6-2, the 95 grade showed no higher SCC resistance, compared to the 110 grade.
The 13Cr-L80 grade of martensitic SS was resistant to SCC in current tests as well as the more
severe tests in brines containing SCN– reported earlier.[1] SCC of 13Cr did occur, however, in
the higher strength (95 grade) 13Cr martensitic SS. [6]
Stress
SCC was not highly sensitive to stress in tests with modified 13Cr martensitic SSs. This was
demonstrated by comparison of C-rings, with compression calculated using room temperature
values of the YS and Young’s modulus, with de-rated C-rings using test temperature values to
calculate the C-ring compression. In 14 tests, beginning with test #69, comparison was made
between paired C-rings, one de-rated C-rings and the other with higher stress, based on room
temperature values of the YS. In most cases, there was no difference. In 3 out of 6 pairs, SCC
occurred on only the de-rated C-ring.
Other procedures for applying stress may yield test results similar to current C-ring tests.
Company A tests (Table 12) report no significant difference between C-ring and 4-point bent
beam tests.
Brine Composition
14
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
Rank ordering of brine severity was the object of several sets of tests, cited in Tables 12 and 15.
No clear pattern is apparent in API tests of modified 13Cr SS in 7 aerated brines (Table 11).
However, tests of modified 13Cr SSs acidified by CO2 does establish domains of acceptable
brine use under conditions of exposure to CO2. Test results listed in Tables 6 to 8 are plotted in
Figure 2, using Cl– concentration (g/l) vs. brine density (ppg) as coordinates. There is
remarkable consistency in test results from many laboratories. These results suggest that the
variables introduced in testing commercial formulations, containing corrosion inhibitors and O2
scavengers, did not introduce a significant bias toward SCC resistance.
15
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
Figure 2. Effect of Cl– and brine density on SCC of Modified 13Cr, ≥ 50 psi CO2, ≤ 0.4 psi
H2S, ≥ 225 oF
400
No Crack
350
Crack
Cl- (g/L)
300
250
200
150
100
50
0
8
9
10
11
12
13
14
15
16
17
18
Density (ppg)
Two SCC-free regions of Figure 2 are indicated. The left region in Figure 2 can be simply
related to total molar halide ion concentration, [Cl–]+[Br–], with brine severity reduced by dilution.
The right region, identified as “Zn effect”, incorporates two opposing trends – higher acidity and
but reduced free halide ion concentration due to formation of Zn+2 complexes. Brine severity can
be estimated within the SCC domain by the distance from the boundary lines.
Figure 2 is misleading, in that it implies that SCC susceptibility is linked to the Cl– concentration.
As emphasized by McKennis [21], total molar halide concentration in Zn-free brines (see Table
7) is a better indication of brine severity. This parameter is validated by the decrease in the
pitting potential with increasing halide ion concentration (Table 7). The high severity of 15.1 ppg
CaBr2-CaCl2 (Table 13) corresponds to the highest free-halide concentration (Table 5).
Some toleration of Cl– contamination in CaBr2 brines can be allowed. In Test #81, no SCC of
modified martensitic SSs resulted form addition of 25 g/l Cl– to 13.0 ppg CaBr2,100 psi CO2.
Though test results are limited, the SCC-free domain shown in Figure 2 appears to extend to
duplex SSs exposed to CO2 (Table 14). A borderline transition brine composition appears to be
11.0 ppg CaCl2 (22 w% CaCl2). Stevens et al. proposed 25 w% CaCl2 as a transition
composition for 22-5-3-125 duplex SS. [6]
16
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
Tests Protocols
No significant laboratory-based differences are observed in brine tests with modified martensitic
SSs acidified by CO2. However, differences emerged in comparing tests with ~ 0.3 psi H2S
added. Our test results (Figure 9) show high SCC susceptibility, while SCC was absent in some
tests from other laboratories listed in Table 8. Unclear, at this time, are differences in laboratory
protocols for reaching and maintaining the cited value of PH2S..
Test Reproducibility
Test reproducibility has been a challenged, both within the current test program and in
comparison with some published laboratory studies. Variable test results have been reported
both from use of multiple specimens (Table 12) and repeat tests (Table 18 below). One
variable, proposed by McKennis [19], is that surface deposits, such as CaCO2, may form,
leading to pit initiation, a precursor to crack initiation, as suggested in Equations 1 and 2.
HCO3– + Ca+2 → CaCO3 + 2 H+
(1)
Ca2+ + CO2 → CaCO3 + 2 H+
(2)
SEM examination of SCC in CaBr2 + CO2 tests found deposits of CaCO3/CaO near the sites of
SCC initiation. [20]
API tests identifed a borderline test condition for modified 13Cr martensitic SSs: 14.2 ppg
CaBr2 + 100 psi CO2. Table 18 presents 8 tests results for comparison, along with information
on the pH shift during testing. Test results are listed in order of increasing temperature.
Table 18
Modified 13Cr Tests of 14.2 CaBr2 + 100 psi CO2
Test
Conditions
Temp.
o
F
-265
-265
buffer
265
buffer
265
buffer
265
Additive
80
erythorbate
+ inhibiter
265
YS
calc
@RT
@RT
@RT
@RT
@RT
@T
@T
68
62
buffer
buffer
300
350
@RT
@RT
50a
50b
51
52
70
(a)
13-5-0.6
-110
O,O
O,O
X,X
O,O
O,O
O,X
--
13-4-1
-110
O,O
O,X
O,O
----X,OOO
O,X
X,X
O,O
O,O
O,X
barstock UNS S2426, with 1.7% Mo
Temperature Effect
17
Results
(a)
UNS
-110
X,X
O,O
O,X
O,O
O,O
O,X
O,O
O
X
X
pH
13-6-2
- 95
----O
O
--
13-6-2
-110
O,O
O,O
O,O
O,O
O
-O,O
---
O
O,O
O,O
start
final
4.6
3.6
5.2
2.8
2.3
2.6
6.5
2.9
6.7
3.1
6.5
6.7
?
2.7
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
The expectation was that SCC susceptibility would increase with increasing temperature. This
trend is confirmed in tests of duplex SS (22-5-3-125) in 11.6 ppg CaCl2 exposed to air. [18] For
modified 13Cr martensitic SSs, in contrast, no obvious detrimental effect of temperature
increase is clear within the temperature range of 225oF to 350oF for brines exposed to CO2
(Table 6) or air (Table12). Company A results (Table 12) indicate SCC severity may be greater
at 265oF than 350oF.
Oxidants
In 11.6 ppg CaCl2 tests, sensitivity to trace levels of O2 has been documented. Table 10 shows
that SCC of modified martensitic 13Cr occurred in the presence of trace O2, but was absent
(Test#37) when an improved de-aeration procedure was introduced. Bromate impurities in
bromide brines have been reported.
Effect of Additives
Buffer added to raise the pH has had mixed success in reducing SCC risks. Effective buffering,
if needed, is largely limited to NaBr brines, where soluble Na2CO2 is allows a pH raise to 9.9
(Company D). [10] Solubility limits in CaBr2 brines reduce the benefits of Ca(OH)2 or CaO
addition. In 14.2 ppg CaBr2 saturated with 100 psi CO2, final pH readings were 2.8±0.3,
independent of both the start pH and the addition of buffer. Amine carbon steel (casing)
corrosion inhibitors are also ineffective
In CaBr2 tests, Table 18 shows that there was no significant benefit from addition of buffer,
inhibitor. Although the initial pH varied from 3.6 to 6.7, the final pH was 2.3 ±0.3. Low solubility
limits the buffering capacity in CaBr2 (or CaCl2) brines.
Most corrosion inhibitors are high molecular-weight amines that have been shown to have low
buffer capacity. Comments following Table14 results indicate that inhibitors with higher buffering
capacity would be beneficial.
Organic O2 scavengers such as erythorbate and isoascorbate are effective in eliminating
residual O2 or titratable oxidants.
SCC Mechanism
As concluded earlier [1], the SCC mechanism for duplex SSs and high-Ni CRA’s is a variation of
the classic Cl– mechanism of austenitic SSs and Ni CRAs, with crack initiation in areas of
localized corrosion on a passive metal surface. Tests confirm the expected effects of increasing
SCC susceptibility with temperature, pH and oxidants, and the beneficial effect of cathodic
protection (on duplex SSs). In contrast, modified martensitic SSs showed no clear increase in
higher susceptibility at 350oF than at 265oF. The possible contribution of a H-assisted mode of
crack growth in duplex and modified 13Cr SSs was proposed in our earlier paper.[1]
Galvanic Effects
18
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
The above SCC mechanism predicts improved SCC resistance with a cathodic shift of potential.
This benefit was demonstrated when 22-5-3 duplex SS was coupled to carbon steel [18], and in
slow strain rate tests of 13Cr martensitic SS under cathodic polarization.[5]
Test Protocols
Most C-ring tests were conducted using YS values at room temperature. In tests where
comparison can be made with de-rated C-rings compressions using test temperatures values of
the YS, results were mixed. In seven tests where a comparison can be made, only the de-rated
C-ring passed.
Recommendations
Tests lasting 30 days are suitable for CRA- brine compatibility, and are consistent with CRAH2S test requirements for sour gas service.[9] Test variability is acknowledged. Sensitivity to
low levels of O2 requires a rigid method of O2 exclusion. Long-term storage of CaBr2 needs to
insure that the brine is free of BrO3– and pH > 5.
Conclusions
1.
2.
3.
4.
5.
6.
7.
8.
Current SCC tests replicate the two cases of tubing failures that during use of CaCl2
brines:
a) Duplex SS following ingress of acidic production gas and
b) Modified martensitic SS due to air exposure in gas cap of the annulus.
For brines free of SCN– additives, the primary variables promoting SCC susceptibility
are:
a) Brine composition,
b) Higher acidity from CO2 exposure, and
c) H2S in the production gas. In tests at 0.15 psi H2S, high SCC
susceptibility of modified 13Cr martensitic SSs occurred, coupled with
enhanced corrosion.
Figure 2, drawn from test results of modified martensitic SS acidified by CO2 in a
wide range of brines, presents domains that distinguish high and low SCC
susceptibility. The domain free of SCC corresponds to the beneficial effects of
dilution and Zn+2 complex formations. Parallel tests with duplex SSs (+ CO2) appear
to fit the format of Table 3, but with the SCC-free range expanded
SCC in CaBr2 brines cannot be attributed to Cl– impurities.
In CaBr2 tests with modified 13Cr saturated with CO2, buffer additions to CaBr2
brines were ineffective due low buffer capacity (solubility).
Addition of carbon steel corrosion inhibitors also had no beneficial effect, apart from
an initial increase in the pH.
Use of inorganic oxygen scavengers such as erythorbate was effective in improving
SCC resistance in brines containing O2, but no additional benefit was demonstrated.
No SCC risks using bisulfite (HSO3–) O2 scavenger have been found. Tests include
Ni-base alloys at 450oF and 0.5 psi H2S.
19
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
9.
10.
11.
12.
13.
14.
15.
SCC in in aerated brines, especially CaCl2, was observed in both modified 13Cr SSs
and duplex SSs.
The primary variable aiding SCC resistance is alloy content. Beneficial were higher
Mo in modified martensitic SSs, higher Cr+W in duplex SSs, and higher Ni content in
High- Ni alloys.
No SCC occurred in tests with standard (80 ksi grade) 13Cr martensitic SS
In the absence of H2S and SCN–, no SCC has been observed in Ni-base alloys.
Test variability, seen both in using multiple specimens and multiple repeat tests, is
an inherent consequence of SCC testing in halide brines.
.A more severe case resulting in SCC was observed in tests where evaporation
resulted in brine concentration on the stressed C-ring apex. These test observations
are consistent with the proposed field failure in the air cap region above the brine
column.[3]
Revision of API Test protocols addressed the following topics.
a) The need to check CaBr2 in storage for pH decrease, and possible
formation of BrO3– oxidant.
b) The need for a reliable procedure to insure complete deaeration, when
required.
c) The need to confirm C-ring isolation from the alloy C-276 bolt.
d) Some benefit of de-rating the C-ring stress by using test temperature
values to calculate the compression.
Two mitigating factors may be invoked which suggest reduced SCC risks under field conditions
than predicted in current tests. One is cathodic protection due to coupling to the carbon steel
casing. Another is that the leakage of acidic gas into the brine filled annulus may be less
extensive than the test protocol of complete saturation at equivalent partial pressures of CO2
and H2S.
Future test needs will be to extend the test envelop to address the increasing needs for halide
brines at higher temperatures and higher downhole partial pressures of H2S.
20
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
ACKOWLEDMENT
The authors acknowledge the unflagging sponsorship of this program by API membership of the
Joint Industry Project that made this effort possible as well as the contributions of member
companies, which have included Albemarle Corp., Baker Hughes, BP America, Cabot Specialty
Fluids, ConocoPhilips, Chevron-Texaco. ExxonMobile, Haliburton, Hamilton Metals, Haynes
International, M-I Swaco, Schlumberger, Statoil, ASA, Shell, Sumitomo, Tetra Technologies,
and Tenaris. The authors would also like to acknowledge the laboratory assistance of Steve
Clay, Justin Been, Jaime Garza, Dustin Noll and Christopher Wolfe.
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21
This document is not an API Standard; it is under consideration within an API technical committee but has not received
all approvals required to become an API Standard. It shall not be reproduced or circulated or quoted, in whole or in
part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction
and staff of the API Standards Dept. Copyright API. All rights reserved.
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22