A new project aims to quantify the material, stress and environmental factors inducing cracking in duplex and superduplex stainless steel fabrications subject to CP. This will make possible safe utilisation of these materials underCP, without risk of failure from hydrogen embrittlement, and consequently allow avoidance of unnecessary restrictions on offshore use of duplex and superduplex stainless steels.
Increasingly, industry is using cathodic protection (CP) offshore for prevention of pitting corrosion of duplex and superduplex stainless steels used for subsea components on high temperature wells. However, despite earlier good experience, there have recently been some expensive failures. These are apparently related, in that all used aluminium alloy anodes for sacrificial CP without external control of the applied potential. Therefore, TWI has just commenced a Group Sponsored Project (GSP or Joint Industry Programme) to provide data to assist industry in assessing design and safety of subsea cathodically protected equipment manufactured in these materials.
The project has just started and will run until summer 2000. It is open to additional sponsors and is being supported by:
- The Health & Safety Executive
- Shell UK Expro
- Weir Materials
The programme is studying 22%Cr duplex and 25%Cr superduplex, stainless steels, in both wrought and forged product forms. The work will compare performance of alternative materials, for example 13%Cr weldable martensitic steels.
Two phases of work are running concurrently. The first phase concerns the application of aluminium alloy anodes on duplex and superduplex stainless steel. Samples will be tested in seawater with the CP applied by potentiostats to -1100mVSCE and primarily subjected to constant tensile load. Application of constant load is essential as an appreciable degree of low temperature creep is anticipated. A range of applied load will be used so a threshold stress and strain condition can be identified for each parent material. Notched bend tests on two materials will assess the engineering significance of localised yielding.
The second phase will determine safe operating CP potentials for microstructures which may be susceptible to cracking. Polarisation will be applied to potentials less negative than -1100mVSCE on a range of microstructures, to extend the tensile and bend testing in Phase I above. Phase II will also include welds deliberately produced to contain intermetallics and high ferrite levels.
Further work in Phase III is subject to the results from the above. If you are interested in participating in this project, further information is available from Paul Woollin at TWI. For more information, please contact us.