Combatting sour corrosion

Sour corrosion is a serious issue that can cause material failure at stress levels well below the yield strength.

Slow strain rate tests as fast screening method

A major pipeline owner needed information concerning the sensitivity of different steel grades exposed to oil containing a significant amount of hydrogen sulphide. The purpose was to investigate the influence of different factors such as temperature, pH, H2S concentration, etc. on the material’s pick-up of hydrogen and to relate this to the degradation of material properties. By testing various environments, the customer will be able to define domains in which specific materials are applicable or not.

Sensitivity to hydrogen embrittlement is often investigated using constant load tests. However, this method only delivers a pass-or-fail result. In many cases, the results can be inconclusive: if all the experiments either failed or passed, no information is available to distinguish different materials. However, in the slow strain rate test, failure occurs almost always. The time or deformation moment when the failure occurs provides useful information about the severity of the testing conditions or the sensitivity of the material. Therefore, by using slow strain rate tests, the customer obtains more information while fewer experiments are needed.

Optimising material selection for sour service applications

In a next step, different experimental results are related and the material’s behaviour in the presence of hydrogen is modelled. The long-term goal is to optimise material selection for sour service applications. As each well is unique with its own H2S concentration, being able to provide an adequate material for a specific oil field has the advantage of reducing the CAPEX of the project without compromising the safety of the installation and its personnel.

Measuring & modelling to identify materials fit for customers’ service conditions

OCAS has a fully equipped sour corrosion laboratory that allows to test materials according to the latest standards (NACE TM0316, NACE TM0198, NACE TM0284, NACE TM0177, EFC16, etc.). In addition, OCAS also provides advanced proprietary test set-ups to its customers. Permeation testing under H2S environment gives greater insight into the amount of hydrogen going through a material, whereas fast extraction tests allow to study the amount of hydrogen trapped in a material as a function of the exposure time. Our instrumented double cantilever beam device can follow up real-time crack propagation.

Combining these techniques allows OCAS to model hydrogen distribution and transport as a function of the exposure time to the environment and the applied loads. Based on this data, the OCAS team can recommend the material that will best fit the customer’s service conditions.

“The customer selected OCAS for this project thanks to our combined competencies in sour testing, hydrogen concentration measurement and modelling.”

Marc-Antoine Thual, Research Engineer Metallurgy, OCAS