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Mechanical Characterization and Adherence of Iron Carbonate on an X65 Steel

Formation of iron carbonate layers on mild steel is an important factor in CO2 corrosion as they provide a protective barrier that helps preserve pipeline integrity. However the protectiveness conferred by such layers can be compromised due to their mechanical removal; a phenomenon that has hitherto been unexplored. The main objective of this work was to evaluate the mechanical integrity of an iron carbonate layer grown on an X65 steel by nanoindentation and scratch test methods. Berkovich and Vickers-type indenters were used to determine the hardness of the iron carbonate layer and the metal. A scratch tester with a conical indenter 120° cone angle and 20 μm in 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 11.63 ± 3.50 GPa and the hardness of the steel was 2.40 ± 0.21 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 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 the cohesive failure of the layer at low applied forces while the chipping pattern at higher forces was indicative of the adhesive failure mode of the iron carbonate on the X65 steel. 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 indicating that the partial delamination and total delamination shear stresses required are of the order of 300 MPa and 2 GPa respectively.

Product Number: 51319-13390-SG
Author: Claudia Prieto
Publication Date: 2019
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