Mechanism Understanding of the Role of Rare Earth Inclusions in the Initial Marine Corrosion Process of Microalloyed Steels

In this study, the corrosion behavior of rare earth (RE) microalloyed steels was first evaluated through potentiodynamic polarization tests and corrosion weight loss experiments, and then the corrosion morphologies were observed by scanning electron microscope (SEM). After immersion in a NaCl solution, the sulfides (or oxygen sulfides) dissolved preferentially, followed by corrosion at the boundary between the Fe matrix and oxides. Afterwards, the inclusions fell off as a whole, which promoted pitting nucleation. The first principle modeling demonstrated that the work functions of various kinds of inclusions descended in the following order: La(2)Zr(2)O(7) > LaAlO(3) > (La(2)O(3) ≈ Fe ≈ La(2)O(2)S) > La(2)S(3), which provided a theoretical explanation to the dissolution behaviors of inclusions. That is, inclusions containing sulfur tend to dissolve preferentially, whereas the oxides do not dissolve easily. Subsequently, the surface current distributions were detected by the scanning vibrating electrode technique (SVET), which provided more microscopic insight into the role of inclusions in the corrosion propagation. Results showed that the active sites of pitting nucleation accelerated the transverse propagation of corrosion. Finally, local corrosion spread to the whole surface as uniform corrosion.