In-situ characterization of initial marine corrosion induced by rare-earth elements modified inclusions in Zr-Ti deoxidized low-alloy steels

In the present research, the effects of rare-earth (RE) elements modified inclusions on initial marine corrosion of Zr-Ti deoxidized low-alloy steels were studied by first principle modeling and a variety of analytical methods including in-situ scanning vibrating electrode technique (SVET), scanning electron microscopy with x-ray microanalysis (SEM/EDS), and confocal Raman microscopy (CRM). The obtained results revealed that two types of complex inclusions with intertwined components of RExZryOz-RE2O2S and RExZryOz-RE2O2S-TiN were formed. RE2O2S preferentially dissolved in 0.5 wt% NaC1 solution, whereas the dissolution of RExZryOz propagated from the periphery to the middle of the inclusions and subsequently, formed stable pits on the steel surface. TiN was stable in the marine environment and finally, became detached from the substrate. Moreover, in-situ SVET results demonstrated the local current density change during corrosion process was influenced by single and multiple inclusions, indicating that the dissolution of inclusions induced the electrochemical activities of pitting corrosion and further inhibited the propagation of local corrosion. Furthermore, the corrosion tendency of inclusions and the Fe matrix was examined by first principle modeling. Finally, two schematic models were proposed to demonstrate the effects of inclusions on initial marine corrosion of low-alloy steels. (C) 2019 The Authors. Published by Elsevier B.V.