Corrosion inhibitor optimisation for application in high salinity and corrosion product forming CO2-containing environments

Carbon dioxide (CO2) corrosion of carbon steel is a degradation mechanism known for its manifestation within the energy sector, particularly in the geothermal energy production. Despite extensive research, the complexities of CO2 corrosion processes has still left aspects regarding the true reaction pathways and, in particular, corrosion product growth and localised corrosion mechanisms, unresolved.
Despite widespread research efforts related to enhancing the understanding of CO2 corrosion mechanism, as well as the development of a plethora of predictive corrosion models, there is a clear disparity in literature focus. The majority of effort is directed towards understanding and predicting uniform corrosion in the absence of protective corrosion products, with less attention afforded to localised corrosion in the presence of such surface deposits. On the one hand, this may be surprising given that localised corrosion constitutes one of the greatest failure modes within the energy sector. However, the localised corrosion mechanism in active materials such as carbon steel appears to be particularly difficult to both elucidate and mitigate. Such processes are not nearly as well understood or characterised compared to localised corrosion in passive materials, where the mechanisms are generally well established and accepted.
Numerous theories and influential parameters have been assigned to localised corrosion of carbon steel in CO2-containing environments. Many of those proposed revolve around the existence or generation of local heterogeneity in terms of either surface chemistry or surface texture/composition which invariably promotes galvanic/concentration cells or aids in inducing acidification processes which accentuate corrosion in discrete areas.
One factor which has been identified as a pre-cursor to localised corrosion is that associated with the precipitation of carbonate (FexCayCO3) corrosion products on steel surfaces. Such layers can be highly protective against general and localised corrosion when formed under appropriate conditions, suppressing corrosion rate by up to two orders of magnitude in laboratory studies. This is achieved through the layer acting as a diffusion barrier to electrochemically active species and/or blocking active sites on the steel surface.
Despite the clear benefits of corrosion products, many authors have suggested that the sporadic, non-uniform coverage of such layers is detrimental to pipeline integrity, as it is conducive to both the initiation and propagation of localised corrosion. In fact, a number of research papers refer to the underlying mechanisms of localised corrosion being associated with the local removal of corrosion products. Whilst this may be true, there are some intricacies associated with this statement which should be explored to gain a full appreciation of the initiation of such mechanisms, as well as consideration of possible alternative theories/explanations.
The actual driver for localised corrosion in systems under corrosion deposits or where there is partial coverage should be understood in order to have a full appreciation of the different modes, steps and mechanism of corrosion in such systems. Only by having an understanding of all eventualities, their complexities and the drivers for such reactions can these processes be effectively controlled to prevent failures.