Formation of iron carbonate layers on mild steel is an important factor in CO2 corrosion as they provide a protective barrier that can help preserve pipeline integrity. However, the protectiveness conferred by such layers can be compromised due to their mechanical removal.The main objective of this work was to evaluate the mechanical integrity of an iron carbonate layer, grown on an API 5L X65 steel, by nanoindentation and scratch test methods. Berkovich and Vickers-type indenters were used to determine the hardness of an iron carbonate layer and its X65 steel substrate. A scratch tester with a conical indenter, 120° cone angle and 20 μm diameter, was used to determine the critical force to remove the iron carbonate layer. Nanoindentation results indicated that the hardness of the iron carbonate layer was 11.6 ± 3.5 GPa and the hardness of the steel was 2.4 ± 0.2 GPa. According to the failure map (hardness of the substrate vs. hardness of the layer), the failure mode of the iron carbonate on steel (whose hardness is 5 times higher than the substrate) is by chipping. In order to corroborate this postulate, scratch testing was used to determine the minimal normal force to detect superficial removal and total delamination of the iron carbonate layer from the steel. The required forces were determined to be of the order of 40 mN and 400 mN, respectively. The presence of chevron-type cracking patterns confirmed cohesive failure of the layer at low applied forces, while the chipping pattern at higher forces was indicative of the adhesive failure mode. These results were further corroborated by profilometry and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) analyses. Finally, the shear stresses associated with the partial and total removal of iron carbonate were determined. The results indicate that the shear stresses required for partial and total delamination are of the order of 300 MPa and 2 GPa, respectively.

Subject
Scanning electron microscopy, Constant load, Energy dispersive X-ray spectroscopy, Materials, Substrates, Iron carbonate, Surface hardness, Stress, Force, Mathematical models, Mechanical failure, Shear, Steel
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2019
Association for Materials Protection and Performance (AMPP)
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