Effect of Fatigue on Pitting Corrosion Susceptibility of CRA SCRs in Sour Environments

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1 Effect of Fatigue on Pitting Corrosion Susceptibility of CRA SCRs in Sour GROUP SPONSORED PROJECT OUTLINE PR August 2013 Summary There is strong experimental evidence that stainless and clad pipes corrode more readily under sour fatigue loading than under static conditions, with significant consequences for CRA steel catenary risers. This project will further develop a technique which will enable accurate data on pitting susceptibility under corrosive and high pressure and temperature environments to be produced using short-term tests to investigate influence of cyclic loading upon pit initiation. Concern exists that cyclic loading may reduce pitting corrosion resistance which may impact integrity or fatigue performance. TWI has observed pitting in endurance fatigue testing in corrosive conditions in which the corrosion resistant alloy (CRA) should have exhibited good pitting resistance. It is proposed to investigate pitting corrosion under cyclic loading using electrochemical testing in an autoclave, which once the technique has been validated, will allow pitting tests to be carried out quickly on specimens under load. This technique has already been developed for use on unstressed samples during TWI JIP and CRP (Core Research Programme) work, and results produced gave confidence in the basic approach, but it requires further validation to improve the test method for cyclic (fatigue) loading. Initially, Selection of corrosive or high pressure and high temperature (HPHT) test conditions will be made based upon TWI and Sponsor Group input to consider environments where unexpected pitting had occurred in endurance fatigue. The tests to be carried out would include confirmation of pitting resistance of the selected alloys in the test conditions without cyclic loading in both long term exposure and short-term electrochemical testing, electrochemical assessment of the effect of cyclic loading on pitting resistance of the alloys in short-term testing and long term pitting resistance with or cyclic loading to derive comparative data.

2 Project Concept In previous work performed by TWI, corrosion fatigue performance of pipes had been affected by pitting in conditions well within the normal operating range for the material. Testing had been carried out in an environment that was deliberately severe, but believed to be acceptable for this material, on the basis of industry advice, and very limited published information (Kopliku et al, 2002) for parent materials. Tests were undertaken on alloy 825 and type 316 stainless steel and endurance tests of about 3 weeks did not induce pitting in these specimens with high stress range conditions. However, medium and low stress range conditions extending over some weeks testing led to pitting. For comparison, unstressed material in these tests did not pit in the case of type 316 austenitic stainless steel (10 weeks exposure), whereas some degree of pitting was present in the welded 825 (22 weeks exposure) but this was more extensive in the endurance specimen than the unstressed specimen under the same conditions. Concern therefore exists regarding the reduction in pitting corrosion resistance (long term) when subject to cyclic loading which would not have been evident from short term tests, and may subsequently impact upon corrosion-fatigue performance. (Full details of this work are under embargo until October 2016). If, as suspected, cyclic loading is encouraging pitting, which could then induce fatigue failure, in what would have been considered to be an acceptable environment under static or non-loaded conditions, this could have serious design implications. Presently most CRA usage in SCRs has been restricted to conditions deemed corrosive to carbon steels which can be controlled via inhibition, but with clad pipe increasingly used in fatigue sensitive locations such as the touch-down region. In essence, these are acceptable operating environments for carbon steels from a corrosion perspective, but a conservative approach is taken of zero material loss in such locations resulting in the specification of CRA. Occurrence of pitting in such conditions is manifestly a concern in itself but also raises questions over performance that may be expected in more aggressive production environments where CRA would perforce be stipulated in future developments. It is therefore important to have confidence in whether the material/environment combinations would have been deemed resistant to pitting by standard tests, and if so, it is then necessary to know whether they would have proved to be resistant under longer term exposure with the additional consideration of the influence of cyclic loading on material performance. Whilst the problems have been observed for type 316 stainless steel and alloy 825, testing should extend to alloy 625 which is frequently selected for such components, to be certain that it does not suffer this same degradation. Typical pitting tests used to confirm material suitability for a particular environment would involve long term exposure (60-90 days), followed by surface examination, weight loss measurements, and possibly sectioning. If the results are clear, this is satisfactory, but if a limited number of small pits are found, it is uncertain whether these are indicative of an incipient problem, or whether they are early pits which have re-passivated, and are now stable. Passive electrochemical techniques overcome this problem by monitoring activity throughout the exposure and thus provide a possible means of monitoring behaviour and changes in response, for example when cyclic loading is applied. The electrochemical noise (EN) technique is particularly suitable for such applications as it does not use any external potential perturbation. Two nominally identical specimens are immersed in the environment and connected through a zero resistance ammeter (ZRA). Any pits will be anodic, so that pitting on one sample will cause a current to flow between that and the more cathodic sample which has not started to pit. The current transient will give indication of the localised material loss which can also be approximated to a pit dimension (assuming pit geometry). Thus, long term exposure using electrochemical current measurements will give good confidence in the pitting resistance of the material.

3 Project Concept continued To explore the effect of cyclic stress further, an environment in which pitting does not occur in the very long term on unstressed specimens, but which is still aggressive, needs to be identified, and electrochemical current measurements need to be made on endurance test specimens during testing. There are various ways in which an environment can be made more aggressive, but the only way which permits convenient variation throughout a test is to increase the temperature. Thus a suitable sour environment would be selected, and tests would be carried out to determine and confirm the critical pitting temperature. Generally, ASTM G150 is used to determine the critical pitting temperature (CPT), but as the standard uses 1M NaCl solution and +700mV which might alter the pitting behaviour the proposed study would determine CPT using electrochemical noise (EN) in the suitable environment with no external potential. Fluid flow can also affect pitting resistance of materials and will be investigated under cyclic and mechanical static loading if there is sufficient support from the Sponsor Group to address this activity. Objectives The objectives are to assess whether pitting may occur during longer term exposures than are usually specified for qualification tests, and to assess whether mechanical loading and flow affect this. To this end, a short-term electrochemical test will be used and validated with long-term (up to 90 days) tests. Determine the critical pitting temperature (CPT) in short-term electrochemical tests (less than 2 days) Validate the measured short-term CPT values by performing long term tests (of at least 30 days) on unstressed specimens at constant temperatures, at 5ºC above and below the CPT Assess impact of cyclic or static loading upon pitting resistance in long term tests. Determine the effect of fluid flow on the CPT (if viable) Benefits The Sponsors would derive the following benefits from the project: Guidance on the safe selection and qualification of CRA pipes for fatigue applications such as SCRs. Quantitative knowledge of the effect of cyclic and/or static loading on pitting susceptibility and potential reduction in material pitting resistance to assist initial materials selection considerations Validated short-term electrochemical test to provide pitting resistance data under high pressure and temperature and under load. Knowledge of the effect of flow on pitting susceptibility in HPHT environments which may influence material selection decisions. Availability of extensive real-time data for a particular system, which can be used directly in experimental work and indirectly for modelling. Copyright TWI Ltd 2013

4 Approach To establish the pitting limits of the materials under mechanical loading, it is proposed to conduct the following tasks: State-of-art review of technology gaps and industrial needs to assist in selecting suitable borderline corrosive and fatigue environments for testing, with input from TWI and the Sponsor Group. Prepare equipment for testing and obtain materials. Measure the critical pitting temperature (CPT) in short-term tests (less than 2 days) conducted in environments currently used in typical corrosion fatigue test programmes. Long term tests (of at least 30 days) will be carried out at constant temperatures, at 5ºC above and below the CPT to confirm the measured CPT values under dynamic and/or static loading Tests to investigate the effect of fluid flow on pitting resistance under dynamic/static loading conditions. The CPT will be determined by using electrochemical monitoring (current and potential) of samples as the temperature of the test environment is raised. Pitting events will be registered by changes in current and potential of coupled samples. Following from this, longer term tests will be carried out at constant temperatures, at 5ºC above and below the CPT to confirm the measured CPT values. Samples in the long term tests will also be electrochemically monitored throughout the test. Task 1: State-of-Art Review This will focus on verifying the technology gaps and industrial needs related to the effect of cyclic loading on pitting resistance. The review results will enable borderline corrosive conditions and/or fatigue conditions to be confirmed. The equipment that will be used for testing can then be set up with any modifications being undertaken that may be necessary to assist in fulfilling these conditions. Task 2: CPT Tests A configuration with a two working electrode electrochemical set-up will be used, to detect the onset of pitting in unstressed samples. One electrode will be the parent material and the other sample will contain a weld. Current and potential monitoring will be carried out during the test at the rate of approximately 1Hz as the temperature of the solution is increased from ambient to approximately 90ºC at the rate of 5 C/hr. This test, often termed thermometry, will be used to determine CPT. When using EN the CPT can be taken as the temperature at which significant current and potential transients are observed with steady current increase over 60 seconds. Once the test is completed test samples will be visually examined and photographed after testing and further examination under a scanning electron microscope (SEM) to confirm the presence of pitting may also be used. Task 3: Long term CPT confirmation tests These tests will be conducted to validate the CPT obtained from the pitting limit tests in the selected test environments. The same two working electrode electrochemical set-up and same material combinations will be used as for the CPT test. Unstressed and stressed (both fatigue and static loads) samples will be exposed in the test environment for a period of at least 30 days, and up to 90 days in the event of no indications of pitting, at a constant temperature during which the current and potential of the coupled samples will monitored. Tests will be conducted at two temperatures; at 5 C below the measured CPT and 5 C above the CPT. The samples will be visually examined after the test and further examination under a SEM may also be used. Task 4: Pitting Resistance Limit Test Programme The material/environment combinations to be used are to be confirmed, depending on the interests of the Sponsor Group, but are likely to include 316 and 825 clad and solid materials. Critical pitting temperature tests will use knowledge/input from Sponsors on cases of pitting in corrosion fatigue testing and also borderline environment chemical conditions (for normal pitting envelope), and cover a range of temperatures, as indicated above. The results of these tests will perform the dual purpose of indicating the severity or otherwise of the current material environment combinations, and identifying a suitable near threshold environment for fatigue endurance testing both with and without flow effects.

5 Deliverables The programme will be reported throughout the duration via regular updates, and scheduled progress reports reviewed with the Sponsors at six month intervals. On completion, the project will deliver a full report on the work undertaken, findings and recommendations. This will include; quantitative data on the effect of cyclic and/or static loading on pitting susceptibility and potential reduction in material pitting resistance to assist initial materials selection considerations; validated pitting resistance data under high pressure and temperature and under load; and details of the effect of flow on pitting susceptibility in HPHT environments which may influence material selection decisions. Price and Duration The overall estimated price for the work is 450,000 (excluding VAT), which requires 30,000 per company per annum for 3 years ( 90,000 total) from each of the 5 Sponsors. It is anticipated that the project will commence with an agreed scope of work with a minimum of 3 Sponsors. References Kopliku A, and Cavassi P; Selection of UNS N08825 as Cladding Material for Offshore Pipelines Transporting Sour Multiphase Hydrocarbon, NACE Corrosion 2002 Conference, Paper No.02042, NACE International Further Information For further information on how a Group Sponsored Project (GSP) runs please visit: GSP Co-ordinator: Lizanne Capstick gsp@twi.co.uk Project Leader: Stuart Bond stuart.bond@twi.co.uk Copyright TWI Ltd 2013