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

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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.