Kristian Haraldsen HySafe conference, Pisa 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: 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: 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 O 2 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: Page: 4 Examples of leaks Separator lye Penetrant

KHAR Date: 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 Material Environ- ment SCC

KHAR Date: 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 Cr VI

KHAR Date: 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: 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: Page: 9 Results – Austenitic stainless steels Susceptibility of AISI 316L/Ti confirmed, SCC at 100°C, 40% KOH and 30 bar O 2 - Bolts of AISI 316L (A4) fractured at 90°C, 30% KOH and 15 bar O 2 (SCC) 2RE10 (AISI 310L): SCC at 100 °C, 40% KOH and 30 bar O 2

KHAR Date: Page: 10 Results – Austenitic stainless steels (continue) 2RK65 (904L): SCC at 100 and 120 °C, 40% KOH and 30 bar O 2 Alloy 28 showed no cracking at 90°C, 30% KOH and 15 bar O 2

KHAR Date: Page: 11 Results – Duplex stainless steels Duplex 2507: SCC at 100 °C, 40% KOH and 30 bar O 2 Duplex 2304: no SCC at 100 and 120°C, 40% KOH and 30 bar O 2 - General corrosion attacs occurred at 120°C

KHAR Date: Page: 12 Results – Duplex stainless steels (continue) Duplex 2906 showed cracking at 120°C, 40% KOH and 30 bar O 2 Duplex 2205 showed no cracking at 100°C, 40% KOH and 30 bar O 2

KHAR Date: 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 O 2 - Both tensioned and relaxed weld area - Weight loss coupons for general corrosion rate Repeated test of base material

KHAR Date: Page: 14 Welded Duplex results General corrosion rate: mm/year at 100°C 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: 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 µm deep ( mm/y) Local/general corrosion rate - Dependent on temperature - Expected to be lower at °C - Further testing needed

KHAR Date: Page: 16 Summary of test results MaterialComposition Test results (3 months exposure) Trade nameOther nameW.nr. UNS/ ASTM %Cr%Ni%Mo 90°C 30%KOH 15 bar O2 100°C 40%KOH 30 bar O2 120°C 40%KOH 30 bar O2 AISI 316L S * AISI 316Ti S Sanicro N RK65904L1.4539N RE10AISI S SAF Cr duplex1.4410S SAF Cr duplex1.4362S ** SAF Cr duplex1.4462S Std. weld 600 weld *** 625 weld *** SAF S * Bolt of AISI 316L broke ** General corrosion attacks *** Local corrosion attacks

KHAR Date: 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.