Effect of Accelerated Cooling on Linepipe Steel Mill Scale and Resulting Localized Corrosion Susceptibility

The structure and composition of mill scale on linepipe steel formed with and without accelerated cooling conditions (ACC) was investigated and correlated to localized corrosion susceptibility. The mill scale structure/composition was investigated using scanning electron microscopy equipped with x-ray energy dispersive spectroscopy and electron backscatter diffraction, as well as x-ray diffraction. Localized dissolution of the mill scale was investigated using electrochemical techniques including open-circuit potential measurements, electrochemical impedance spectroscopy, and electrochemical noise measurements in a corrosive phase solution. The various surface analytical and electrochemical techniques indicated that the mill scale formed without ACC consists of a relatively crack-free, thick inner wüstite layer with a thinner magnetite outer layer. However, the mill scale formed with ACC comprised a magnetite layer containing islands of retained wüstite, with some evidence of magnetite/iron eutectoid formation and which exhibited a relatively high density of through-scale cracks. These cracks can provide direct paths that connect the corrosive solution to the steel substrate, leading to a more rapid breakdown of the mill scale. Additionally, the cracks can form a crevice between the mill scale and the steel surface, providing sites for pit initiation and growth. Coefficient of thermal expansion mismatch thermal stress calculations indicate that a magnetite-based scale is more susceptible to cracking/spalling than a wüstite-based scale, resulting in the ACC plate being more susceptible to localized corrosion.