Developing a new method for corrosion and steel degradation studies

Sweet corrosion is a type of corrosion commonly occurring in oil and gas transport pipelines. Failure of steel pipelines from sweet corrosion is a costly consequence of general and localized corrosion (pitting). Historically, understanding sweet corrosion has largely driven by static ex-situ methods, which limits the provision of real time visualisation and mass-transport details responsible for film formation kinetics. In-situ studies are crucial to capture corrosion processes that inform safe pipeline operations because sudden failures, caused typically from localised corrosion, can lead to a major economic loss in addition to health and environment risks. A new method for corrosion and steel degradation studies has been developed in this research work including the design and manufacturing of a novel in-situ electrochemical flow cell (NX-DRT) as well as testing using neutron and X-ray radiation with the subsequent development in the methodology for data processing.

The custom-engineered electrochemical flow cell was developed to operate under relevant corrosion conditions. The compact design employed high transmission windows ensuring minimal radiation scattering, accommodating all three electrodes and connections. The flow cell allows the growth of thin corrosion films on a substrate from the continuous flow of electrolyte solution. The geometry, electrode and flow configurations are engineered to satisfy beamline (both neutrons and X-rays) requirements for imaging and diffraction. The integrated approach enables time-resolved 3-D visualisation of corrosion film growth over steel surface from the 360-degrees cell rotation along with electrochemical and crystallographic information, following the degradation of the material underneath the film.

Although neutron imaging has benefits over X-rays, limitation in resolution prevents from analysing thin corrosion scales formed on the steel surface. To improve the imaging capability at IMAT (UK), efforts were made to push the spatial resolution down to ~15 µm using fiber optics taper for white beam imaging and ~30 µm using Timepix-3 detector for energy resolved imaging. These works demonstrate that with advanced detector optics setup and by optimising beamline parameters including sample geometry, L/D ratio and working wavelength range, it is possible to reduce edge enhancement effects and enhance the resolution to resolve features that were previously below the limit.

From the 3D imaging and diffraction testing with neutron radiation, the method was proved successful to follow the evolution of corrosion, both general and localized, under semi in-situ conditions in which quantification of film thickness and volume of corrosion and degraded material can be derived with a spatial resolution of 60 µm for IMAT. Furthermore, using X-ray radiation proved to be advantageous to study 3D imaging and diffraction corrosion processes due to the capability to derive and quantify detailed information (spatial resolution of ~3 µm) such as film thickness, porosity, material degradation and kinetics with detailed information on supersaturation, CLAR, solubility limit and concentration of Fe ions. The developed method greatly contributes to the corrosion field because the existent characterisation methods up to date cannot directly provide all this information. In-situ diffraction on the other hand, allows the identification of corrosion composition of the film and at the interface between steel and the corrosion film.

IMAT has a spatial resolution of about 60 µm evidenced with the limitations of resolving features smaller than 60 µm in the flow cell corrosion experiments performed. On the other hand, DIAD with a spatial resolution of 3 µm demonstrated that the method developed in this project provided a foundation for advancing future in-situ 3D imaging and diffraction tests. Overall, this research work showcases comprehensive experimental and mechanistic study of corrosion film formation on steel enabling the development and optimisation of the next-generation NX-DRT flow cell technology. The developed method allows the provision of fundamental understanding of local chemistry under evaluated conditions and serves as an example for future corrosion studies and applications like testing inhibitors or customized industrial conditions.