Effect of Microstructure and Texture Evolution on the Electrochemical Corrosion Behavior of Warm-Rolled API 5L X70 Pipeline Steel

Abstract

Thermomechanical treatments were used to improve the corrosion resistance of API 5L X70 pipeline steel materials. Successive warm rolling was performed at consistently reduced temperatures; 700 °C, 600 °C, and 500 °C. Steel plates comprised of different ferrite grain sizes were produced. However, the finest grain distribution was achieved at 700 °C rolling temperature. A combination of electron backscattered diffraction and X-ray diffraction (XRD) techniques were used to determine weak texture (i.e., preferred grain orientation) across all specimens. Grain orientation showed deviation toward the 〈111〉 direction at the surface of 700 °C rolled steel. After deformation at 600 °C, mostly 〈110〉 grains oriented parallel to the normal direction were obtained. Rolling at 500 °C resulted in random orientation of grains. Corrosion results show that anodic dissolution increased as the rolling temperature decreased in the order 700 °C > 600 °C > 500 °C for hydrogen-producing and non-hydrogen-producing test media. Also, molecular dynamics (MD) simulation confirmed that the adsorption energy of corrosive species interacting with the iron (Fe) surface increased in the order of Einteraction (111) < Einteraction (110) < Einteraction (100) for the two types of electrolytes. The relationships between the molecular species interacting in each corrosive media and selected crystal planes (i.e., (111), (110), and (100)) were established. X-ray photoelectron spectroscopy (XPS) confirmed that the adsorbed corrosion film on all tested steels are Fe2O3 (Fe2+) and hydrated ferric oxides such as FeOOH.