Sour (H2S –containing) environments present a major problem for oil and gas industries; of particular concern, is Underdeposit Corrosion (UDC), which is a type of accelerated localised corrosion observed under deposits, creating a risk to the asset integrity. Iron sulfides, a typical deposit in the sour environment, are considered critical due to their complex nature. The kinetics and controlling factors of sour UDC are not fully understood and there is no consensus on the methods used to evaluate it in pipelines. The efficacy of inhibitors may vary in such a scenario and so the amounts and efficiencies required cannot be a priori decided. There is, therefore, an urgent need for a protocol that addresses the characteristics of deposits to provide kinetic and mechanistic information on UDC phenomena.
In this work, both the general and localised corrosion kinetics of steel were studied in a simulated sour environment under ‘inert’ (sand) and ‘active’ (iron sulfide) deposits. Our multiscale investigation involved electrochemically measuring planar electrodes and 1- dimensional artificial pits to simulate actively dissolving interfaces with different geometries, followed by ex-situ surface analyses. In addition, the interfacial salt layer responsible for defining the stability of pits propagation was further investigated in-situ using synchrotron microfocus X-ray analyses.
The study showed that deposits stabilise a pitting-like corrosion in mild steel by altering the ionic diffusion process, which perturbs the local environment in a way that maintains an active chemistry for pit propagation. The salt films on the active interface have properties that are found to be dependent on the deposit type and the environment medium: sulfur-containing species, such as H2S or iron sulfide deposits, form an additional sulfide film at the metal interface, stabilising the kinetics for a further continued, yet slow pitting corrosion. Troilite presented the most aggressive form of deposit under the studied conditions. Therefore, based on the mechanistic understanding of UDC developed in this work, a protocol for the development of representative deposits for in-line corrosion sensing can be established, opening avenues towards researching effective and reliable inhibition protocols.