Qualification of corrosion inhibitors for field deployment involves performance testing in the laboratory. Especially for sour service, a common set of test methods and associated protocols or guidelines used across the oil and gas industry, chemical companies and laboratories does, however, not exist. Development and testing of robust methodology were at the core of a Joint Industry Project (JIP) carried out at the Institute for Energy Technology (IFE). This contribution presents a selection of findings related to precorrosion, one of the aspects addressed in the JIP.
A series of benchmark experiments assessing the performance characteristics of various inhibitor chemistries mitigating corrosion of carbon steel under sweet (CO2) and sour (CO2:H2S 1:1) conditions at 25 °C and 60 °C without any precorrosion raised concerns related to uneven attack and risk for developing undesirable experimental artifacts, e.g. localized corrosion. Subsequent study of several precorrosion methods, including natural and electrochemically accelerated (anodic polarization) corrosion either in the absence or presence of H2S provided valuable insight into advantages and shortcomings of each approach.
From a methodology viewpoint the precorrosion approach and the selected conditions make it possible to generate corroding carbon steel surface either free of protective corrosion products, covered by nonprotective FeS, pre-load the test solution with suspended FeS particles etc., which could be useful for tuning the inhibitor tests to various scenarios relevant to sour corrosion.
Development of new oil and gas fields is likely to involve sour reserves to an increasing degree. Sour production often brings about difficulties in terms of asset integrity, related particularly to corrosion mitigation. Employing corrosion resistant alloys implies a considerable escalation in investment costs. On this basis the use of carbon steel and CO2/H2S corrosion inhibition remains a highly desirable option1.
The use of carbon steel and corrosion inhibitors is, on the other hand, not without its challenges in the presence of H2S. Inhibitor dose rates required to avoid localized corrosion may get very high, yet even so effectiveness may prove to be insufficient under certain conditions2. Several aspects known to create difficulties in sweet service, such as under-deposit corrosion (UDC) or top of line corrosion (TLC), become even more complex in sour service3-6. Particularly the presence of iron sulfide either suspended or within deposits is known to worsen the corrosion generally and make inhibition particularly challenging5, 6.
