Stress Corrosion Cracking of Stainless Steels in High Pressure
Alkaline Electrolysers
HySafe conference, Pisa 2005-09-08
Kristian Haraldsen and Håkon Leth-Olsen,
Norsk Hydro ASA, Corporate Research Centre
Kristian Haraldsen
2005-09-05
Content




Background
Stress Corrosion Cracking (SCC)
SCC in Alkaline environments
Experimental setup
-

Results
-

Materials
Test conditions
Austenitic stainless steels
Duplex stainless steels
Welded Duplex stainless steels
Conclusions and recommendations
KHAR • Date: 2005-09-05 • Page: 2
Background

Atmospheric electrolysers operate safely with austenitic stainless steel of
type AISI 316L or similar.

Newly developed high pressure electrolysers operate at 70 – 80°C with 30 –
40% KOH solution and an O2 pressure of 15 – 30 bar.

A few occasions of leakages have been observed at the oxygen side of high
pressure electrolysers.
-
Construction material AISI 316L
Located in weld or close to the weld (heat affected zone – HAZ)
Confirmed to be intercrystalline stress corrosion cracking (SCC)

Commonly referred SCC temperature limit for AISI 316L in high concentrated
caustic solution is about 100°C

Need for alternative materials for safe use in high pressure electrolysers
-
Nickel base alloys resistant, but high price materials
KHAR • Date: 2005-09-05 • Page: 3
Examples of leaks
Separator
lye
Penetrant
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Stress Corrosion Cracking (SCC)

Failure of engineering materials by slow,
environmentally induced crack propagation
Tensile

Combined effect of tensile stresses and corrosion
reactions

Tensile stresses
-
stresses
SCC
Externally applied loads (static)
Residual stresses from welding, machining etc.
Environ-

May occur in materials resistant to general corrosion
in the same environment
-

Initiates at local defects in oxide layer
Difficult to predict and identify before leakage
-
No fore-warning
NDT (non-destructive testing) methods not very informative
Inspection in small size piping difficult
KHAR • Date: 2005-09-05 • Page: 5
ment
Material
SCC in Alkaline Environments

Commonly referred temperature limit for
AISI 316L in high concentrated caustic
solution is 100°C

Temperature limit lowered by the high
oxygen pressure
-
Oxidising conditions may cause trans-passive
dissolution of Cr as CrVI
KHAR • Date: 2005-09-05 • Page: 6
Experimental

Resistance to SCC tested by autoclave exposure of C-ring specimens

Test specimens tensioned to plastic deformation

Test conditions:
-
-

Solution:
Temperature:
Oxygen pressure:
Exposure time:
30 and 40% KOH
90, 100 and 120°C
15 and 30 bar
3 months
Results analysed by
-
Visual examination
Optical microscopy
Scanning electron microscopy (SEM)
General corrosion by weight-loss
KHAR • Date: 2005-09-05 • Page: 7
Materials

Materials selected based on expected resistance, availability and price.
-
Stainless steels with increased nickel (Ni) and chromium (Cr) content
Duplex stainless steels
KHAR • Date: 2005-09-05 • Page: 8
Results – Austenitic stainless steels

Susceptibility of AISI 316L/Ti confirmed, SCC at 100°C, 40% KOH and
30 bar O2
-

Bolts of AISI 316L (A4) fractured at 90°C, 30% KOH and 15 bar O2 (SCC)
2RE10 (AISI 310L): SCC at 100 °C, 40% KOH and 30 bar O2
KHAR • Date: 2005-09-05 • Page: 9
Results – Austenitic stainless steels (continue)

2RK65 (904L): SCC at 100 and 120 °C, 40% KOH and 30 bar O2

Alloy 28 showed no cracking at 90°C, 30% KOH and 15 bar O2
KHAR • Date: 2005-09-05 • Page: 10
Results – Duplex stainless steels

Duplex 2507: SCC at 100 °C, 40% KOH and 30 bar O2

Duplex 2304: no SCC at 100 and 120°C , 40% KOH and 30 bar O2
-
General corrosion attacs occurred at 120°C
KHAR • Date: 2005-09-05 • Page: 11
Results – Duplex stainless steels (continue)

Duplex 2906 showed cracking at 120°C, 40% KOH and 30 bar O2

Duplex 2205 showed no cracking at 100°C, 40% KOH and 30 bar O2
KHAR • Date: 2005-09-05 • Page: 12
Welded Duplex 2205

Duplex 2205 chosen for further qualification
-

Need to test welded specimens
-

Welding procedure of duplex stainless steels important
SCC often occur in connection with welds
Welded C-ring test specimens prepared
-

Good resistance to SCC
Rel. low Price, good availability
Tested at 100 and 120°C, 40% KOH and 30 bar O2
Both tensioned and relaxed weld area
Weight loss coupons for general corrosion rate
Repeated test of base material
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Welded Duplex 2205 - results

General corrosion rate:
-
0.1 mm/year at 100°C
0.6 mm/year at 120°C

No SCC found in base material or in
connection with relaxed welds

SCC occurred in tensioned weld at 100°C
-
-

No SCC in tensioned weld at 120°C
-

Metallographic analyses did not reveal any
deviance of the weld
No cracking in HAZ
Protected by high general corrosion rate
Repeated tests did not reveal any SCC in
the weld or base material
KHAR • Date: 2005-09-05 • Page: 14
Duplex 2205 with alternative welds

Duplex 2205 with alternative weld filler materials tested at 100°C:
-
Nickel base alloy 600 (18-22% Cr, >67% Ni)
Nickel base alloy 625 (22% Cr, 64.5% Ni, 9% Mo)

No SCC occurred

Limited localised corrosion
-

Located in HAZ
50-100 µm deep (0.2-0.4 mm/y)
Local/general corrosion rate
-
Dependent on temperature
Expected to be lower at 70 - 80°C
Further testing needed
KHAR • Date: 2005-09-05 • Page: 15
Summary of test results
Material
Test results
(3 months exposure)
Composition
100°C
120°C
W.nr.
UNS/
ASTM
90°C
Other name
%Cr
%Ni
%Mo
30%KOH
15 bar O2
40%KOH
30 bar O2
40%KOH
30 bar O2
AISI 316L
-
1.4404
S31603
17
12
2.5
*

AISI 316Ti
-
1.4571
S31635
17
12
2.5

Sanicro 28
-
1.4563
N08028
27
31
3.5

2RK65
904L
1.4539
N08904
20
25
4.5

2RE10
AISI 310
1.4335
S31002
24.5
20.5
0.1

SAF 2507
25Cr duplex
1.4410
S32750
25
7
4


SAF 2304
23Cr duplex
1.4362
S32304
23
4.5
-
**
SAF 2205
22Cr duplex
1.4462
S32205
22
5
3.2




Trade name
Std. weld
***
***
600 weld
625 weld
SAF 2906

-
* Bolt of AISI 316L broke
KHAR • Date: 2005-09-05 • Page: 16
-
S32906
29
** General corrosion attacks
6
2
*** Local corrosion attacks

Summary and Conclusions

Austenitic stainless steels vulnerable to SCC on oxygen side of highpressure alkaline electrolysers at elevated temperatures

Alternatives
-
-
-
-
-

Use of alloy 28 (if available)
Use of Duplex 2205 with increased temperature safety limit
- Special focus on weld procedure (max. temperature)
Use of Duplex 2205 with nickel base alloy weld filler material
- Further qualification with regards to localised corrosion needed
Nickel base alloys in vulnerable pipes
Non-welded connections (without residual tensile stresses)
Practical modifications
-
Duplex 2205 piping with non-welded connections
Welded Duplex 2205 in tanks
Decreased temperature from 80°C to 60°C.
KHAR • Date: 2005-09-05 • Page: 17