Effect of trace rare earth elements (Ce) on corrosion resistance of high strength steel used for offshore platform

Abstract

In this study, a high-strength, offshore platform steel, EH420, was used as the original steel. By observing the original austenite morphology, the corrosion resistance of three experimental steels with different rare Earth contents in a simulated marine splash zone was compared and studied using cyclic immersion corrosion and electrochemical corrosion tests. The macro- and micromorphologies of three experimental steels in different cycles of corrosion, as well as the composition of rust layers, were observed and studied by employing an optical microscope, scanning electron microscope (SEM) and X-ray diffraction (XRD). The crystal microstructure of the three experimental of steels was analyzed by EBSD, and the electrochemical behavior of the three experimental of steels after corrosion was analyzed by electrochemical polarization curve and electrochemical impedance spectroscopy. The results show that the trace additions of Ce ( less than 20ppm ) in the steel can refine the grain size, reduce the grain boundary energy, promote the formation of the protective phase α-FeOOH and Fe3O4 of the rust layer, improve the compactness of the rust layer, and reduce the corrosion rate. In addition, the charge transfer resistance ( Rct ) and open circuit potential are increased, and the corrosion current density is reduced. Moreover, the higher the content of Ce is added in the steel, the more obvious its effect.Therefore, the trace additions of Ce improves the corrosion resistance of high strength offshore platform steel.