Investigation of Steel Alloy Chemistry, Microstructure, and Surface Finish on Oil field Corrosion and Scaling

This thesis presents the research work aimed at understanding the effects of metallurgical parameters of low alloy steel on the corrosion behavior in the CO2 atmosphere. The project is motivated by the increasing number of failures on the currently used materials and reduced lifetime of oil & gas wells. The acquired knowledge is crucial for the development of a framework to choose appropriate materials based on environmental factors, which can enhance corrosion prediction and mitigation of oil and gas wells.

Among the various parameters that affect corrosion behavior, the thesis focuses on changes in microstructure, changes in Cr content, the presence of calcium ions in the brine, and the effect of dynamic conditions. All these effects were evaluated against the CO₂ corrosion behavior of low alloy steel. Specifically, API L80 low alloy steel with varying Cr content (0%, 1%, and 3%) and/or varying microstructures were investigated as representative of production tubing material. The evaluation of the effects of these factors was carried out in accelerated conditions and in conditions that were pertinent to oil and gas operations in the Danish sector of the North Sea.

This investigation followed the corrosion process using electrochemical methods. To track the rate of corrosion as a function of exposure time, linear polarization resistance (LPR) was used. Potentiodynamic polarization (PP) was used to examine the material's electrochemical behavior while Electrochemical Impedance Spectroscopy (EIS) was used to track the progression of the processes taking place at the metal-electrolyte interface. The evaluation of the morphologies and longitudinal sections of the corrosion products and scale was aided by high-resolution scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS). These films were characterized by X-Ray Diffraction (XRD). Ex-situ depth-resolved phase identification of the corrosion scales that formed on steel surfaces with different initial microstructures was carried out using synchrotron X-ray diffraction in order to determine the depth of the corrosion products. ICP was used to analyze the corroding solution's chemical composition, and scale prediction software was used to model the results. To characterize the corrosion products found in samples with various Cr contents, X-Ray Computed Tomography (X-Ray CT) was used. Almost all the experiments were performed at ambient pressure, while the effect of flow was evaluated in a Flow Loop system at the Total Energies facility in Pau, France.
Overall, the results indicate that the microstructure influences both the corrosion behavior (resistance and propagation) as well as scaling (formation and scale adherence). The varying dissolution rate results in variation in protective scale formation, while, the morphology of the surface after dissolution affects the scale adherence. An increase in Cr content increases the inherent resistance of the material to corrosion due to the formation of Cr(OH)3 which develops prepassivation in the material. It is found that along with the environmental conditions, both microstructure and Cr content affect the scale composition. Samples with ferritic-pearlitic microstructure have a low dissolution rate initially and later form scale with low protective properties. While samples with martensitic structure show a high corrosion rate initially but form a protective layer within a short exposure time. Similarly, the 0Cr sample has a higher dissolution rate initially and thus forms a protective scale much faster than the slow dissolving 3Cr sample. The non-uniform corroded morphology due to selective corrosion enhances the scale adherence while high scale spallation is observed for samples with the uniform corroded surface. The 0Cr sample exhibits stronger corrosion resistance due to the faster production of protective scale and higher scale adherence under flowing circumstances, whereas the 3Cr sample exhibits lesser corrosion resistance due to scale removal. The erosion-corrosion generated by high flow velocity under high shear stress (145 Pa) results in some scale adhesion, which causes the corrosion resistance for 1Cr and 3Cr samples to slowly rise with exposure. Although the surface finish influences corrosion behavior in a solution with a low pH and no scale precipitation, its influence is less pronounced when protective scale precipitation occurs. In light of the existing knowledge, the combined results and observations discovered during this study are presented.