Kristian Haraldsen
2005-09-05
HySafe conference, Pisa 2005-09-08
Kristian Haraldsen and Håkon Leth-Olsen, Norsk Hydro ASA, Corporate Research Centre
Stress Corrosion Cracking of Stainless Steels in High Pressure Alkaline Electrolysers
KHAR • Date: 2005-09-05 • Page: 2
Content
Background Stress Corrosion Cracking (SCC) SCC in Alkaline environments Experimental setup
- Materials- Test conditions
Results- Austenitic stainless steels- Duplex stainless steels- Welded Duplex stainless steels
Conclusions and recommendations
KHAR • Date: 2005-09-05 • Page: 3
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: 4
Examples of leaks
Separator
lye
Penetrant
KHAR • Date: 2005-09-05 • Page: 5
Stress Corrosion Cracking (SCC)
Failure of engineering materials by slow, environmentally induced crack propagation
Combined effect of tensile stresses and corrosion reactions
Tensile stresses- Externally applied loads (static)- Residual stresses from welding, machining etc.
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
Tensile
stresses
MaterialEnviron-
ment
SCC
KHAR • Date: 2005-09-05 • Page: 6
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: 7
Experimental Resistance to SCC tested by autoclave exposure of C-ring specimens
Test specimens tensioned to plastic deformation
Test conditions:- Solution: 30 and 40% KOH- Temperature: 90, 100 and 120°C- Oxygen pressure: 15 and 30 bar- Exposure time: 3 months
Results analysed by- Visual examination- Optical microscopy- Scanning electron microscopy (SEM)- General corrosion by weight-loss
KHAR • Date: 2005-09-05 • Page: 8
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: 9
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: 10
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: 11
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: 12
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: 13
Welded Duplex 2205
Duplex 2205 chosen for further qualification- Good resistance to SCC- Rel. low Price, good availability
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- 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
KHAR • Date: 2005-09-05 • Page: 14
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- Metallographic analyses did not reveal any
deviance of the weld- No cracking in HAZ
No SCC in tensioned weld at 120°C- 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: 15
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: 16
Summary of test results
Material CompositionTest results
(3 months exposure)
Trade name Other name W.nr.UNS/ASTM
%Cr %Ni %Mo90°C
30%KOH15 bar O2
100°C40%KOH30 bar O2
120°C40%KOH30 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 Std. weld 600 weld ***
625 weld ***
SAF 2906 - - S32906 29 6 2
* Bolt of AISI 316L broke ** General corrosion attacks *** Local corrosion attacks
KHAR • Date: 2005-09-05 • Page: 17
Summary and Conclusions
Austenitic stainless steels vulnerable to SCC on oxygen side of high-pressure 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.