Mechanical and Electrochemical Evaluation of HCRAs for Oil and Gas Applications in Sour Environments

Increasingly, new systems are installed and used in harsh conditions such as installations with high expected ground movements, reeled offshore pipe lays, high pressure/high temperature (HPHT) service conditions and pipelines with occasional pressure spikes above the material yield strength. As a result of these demanding applications, the utility and necessity of strain-based design (SBD) has increased. To evaluate SBD components, testing methodologies need to correlate real material performance data to calculated design limits. One realistic test method for  simulating the corrosion/mechanical failure conditions of SBD components is the cyclic slow strain rate test (CSSRT), alternatively named the ripple strain rate test (RSRT).
This paper documents the RSRT evaluation of UNS N06625 weld overlay material applied to low grade carbon steel for potential applications in an SBD oil and gas production situation containing high H₂S and CO₂ partial pressures as well as very oxidizing environments. The test methods (involving liquid H₂S loading) are discussed in more detail. In addition, RSRT results will be compared with electrochemical testing in highly oxidizing environments containing ferric chloride at high temperatures (critical pitting and crevice temperatures following a modified version of the ASTM G48). Overall, the goal is to develop a rapid pre-screening, ranking and suitability process for Highly Corrosion Resistant Alloy (HCRA) or HCRA-clad materials that can take into consideration their corrosion resistance to highly corrosive environments and include mechanical strength testing of the desired material.
These main objectives help qualify suitable materials and processes that can be used in the Oil and Gas industry. This is particularly true for highly corrosive and high stress environments where the current international standards do not contemplate the high levels of H₂S, high CO₂ partial pressures and oxidizing environments together with strain-based loads.
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