Localized and General Corrosion Characteristics of Carbon Steel in H2S Environments

Carbon steel remains the most commonly used material in oilfield applications. The susceptibility of carbon steel to various forms of corrosion is one of the major drawbacks to its economic and metallurgical advantages. The formation of Iron Sulfide (FeS) corrosion product is common during its application in H2S-containing environments. The plurality of mechanisms is a fundamental aspect of FeS formation that is still not clearly understood especially in H2S/CO2 environments. Localized and/or pitting corrosion of carbon steel occurring in oilfields is also an aspect of the corrosion process that is of increasing concern and linked to these environments. Pitting corrosion is known to be the most unpredictable form of corrosion and, as such, it is difficult to mitigate against. The nature of the environment is seen as a key contributor to the evolution of FeS corrosion product and the occurrence of localized and/or pitting corrosion, especially in complex oilfield environments containing H2S, CO2 and other acid gases. In addition, multiple cations such as Fe2+, Ca2+ and Mg2+ can further complicate the system. The presence of such cations in the brine chemistry could also impact the properties and chemistry of FeS films and the localized and/or pitting corrosion behavior of exposed carbon steel materials. This work focuses on understanding FeS formation kinetics and corrosion damage characteristics in different 10% H2S-containing environments in 1M NaCl brine and Forties brine containing Ca2+, Mg2+, Ba2+ and Sr2+. Linear polarization resistance (LPR) measurements combined with post-experiment surface analysis such as XRD and SEM are used to understand aspects of the localized and general corrosion behavior. The brine solutions are saturated with different sour corrosion systems; 10% H2S - 90%CO2 at 80°C. The relationship between the processes of FeS formation and evolution of localized and/pitting corrosion is established.