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New test assesses cracking of corrosion resistant alloys

Slow strain rate testing (SSRT) has been used for the assessment of environmentally-assisted cracking of corrosion resistant alloys (CRAs) in the oil and gas industry for environments containing chloride ions and hydrogen sulphide (H2S). However, this process was not only deemed too severe but also unrealistic in terms of applied stress.

 

To overcome this, a modified SSRT method was developed involving cyclical loading below 0.2% proof stress in order to reduce the severity of the process.(1) This new cyclic SSRT test method enabled the stress to occur only in the elastic region, and was further developed by Nisbet et al(2) and then further refined by NACE Task Group TG544(3) to create the ‘ripple load test’ (RLT).

Figure 1 Photograph of RLT equipment (ie a modified SSRT rig, with an autoclave used as the environmental chamber)
Figure 1 Photograph of RLT equipment (ie a modified SSRT rig, with an autoclave used as the environmental chamber)
Figure 2 RLT plots recorded during testing: a) Stress vs time
Figure 2 RLT plots recorded during testing: a) Stress vs time
Figure 2 RLT plots recorded during testing: b) Stress vs strain
Figure 2 RLT plots recorded during testing: b) Stress vs strain

The RLT applies slow and repetitive tensile load cycles on a test specimen while exposing it to worst-case operating conditions to result in an acceleration of sulphide stress cracking or stress corrosion cracking on susceptible materials. This, in turn, creates a relatively short test duration, with RLT considered to be an intermediate test method between constant load/strain and SSRT, making it ideal for the assessing of the resistance of CRAs to environmentally-assisted cracking. Cyclic loading and unloading is typically between 100 and 80% of 0.2% proof stress of the material in the test environment.

As a result of this work, TWI has modified SSRT rigs and developed RLT capabilities that can be used by Industrial Members, some of whom have already taken advantage of the new capabilities with tests undertaken to assess stress corrosion cracking resistance on stainless steels in sour environments.

Avatar Qing Lu Principal Project Leader, Metallurgy, Corrosion and Surfacing Technology Group

Qing Lu graduated with a BSc in Materials Science and Engineering from Harbin University of Science and Technology, China, in 1984. She obtained a Ph.D in Corrosion Science and Engineering at University of Manchester Institute of Science and Technology (UMIST) in Manchester in 1999. This was followed by working on a number of industrial post-doctorate projects at this institute, including corrosion in chemical processing plants and surface engineering studies for automotive applications, between May 2000 and April 2005.

Prior to re-joining TWI in August 2012, she worked for DNV as a senior engineer between November 2011 and August 2012, and for Wood Group Integrity Management as senior materials and corrosion engineer between February 2011 and October 2011 after leaving TWI in February 2011. Prior to joining TWI initially in October 2006, she was appointed as senior research scientist at Westmoreland Mechanical Testing and Research Ltd in UK where her main area of expertise was in failure investigations, primarily related to corrosion.

Her role at TWI carries a particular emphasis in Corrosion Resistant Alloys, which are studied through various types of projects addressing failures, corrosion testing and evaluation, mitigation of corrosion, especially in structures and components joined by welding or other joining techniques.

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