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New test assesses cracking of corrosion resistant alloys

Published by , Senior Editor
World Pipelines,

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).

New test assesses cracking of corrosion resistant alloys

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. This new cyclic SSRT test method enabled the stress to occur only in the elastic region, and was further developed by Nisbet et al and then further refined by NACE Task Group TG544(3) to create the ‘ripple load test’ (RLT).

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. 

Image caption: Photograph of RLT equipment (i.e. a modified SSRT rig, with an autoclave used as the environmental chamber).

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