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In this study, the effect of grain size on the corrosion behavior of API-5L-X70 steel was investigate by using electrochemical methods. Two steel sheets with two different grain sizes were manufactured by using the thermomechanical method. In order to further investigate anneal and normalize heat treatment was performed on a fine grain sample. And another group of these samples were quenched in salt water and ice. By changing the time of heat treatment, a greater range of grain sizes were obtained. To investigate the structure and grain size, the electron microscopic images and MIP Cloud software were used. The average diameter of ferrite and polygonal ferrite grains and martensite packets due to the existence of different structures has been investigated. TOEFL linear polarization and electrochemical impedance tests were used to evaluate corrosion behavior, in simulated soil with neutral pH soil. The results showed that coarse-grained sample 2647 with a mean diameter of ferrite grains of 18.9 μm showed better corrosion behavior compared to the fine grain sample 6530 with a mean diameter of 13.4 μm. The results show the corrosion rate decreased from 1.533 mpy to 1.358 mpy. The same trend was observed in the heat treatment parts except for the air-cooled samples. In this samples by increasing the size of the polygonal ferrite grains from 23.62 μm to 31.79 μm the Corrosion current density was increased from 1.116 to 2.194 µA.cm-2 which can be due to depletion of impurities and the presence of a gradient of alloying elements.

Keywords: Grain size, API-5L-X70 Steel, Corrosion, Linear Polarization, Electrochemical Impedance, Heat treatment

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