Effects of niobium and rare earth elements on microstructure and initial marine corrosion behavior of low-alloy steels

Abstract In the present paper, the effects of Nb and rare earth elements on initial marine corrosion behavior of low-alloy steels were investigated by first principle modeling as well as multifarious analytical techniques, such as scanning electron microscopy with X-ray microanalysis (SEM/EDS), transmission electron microscopy (TEM), scanning vibrating electrode technique (SVET), and electrochemical workstation. It was found that inclusions played key roles during the corrosion process. Spherified fine (Al, RE)-oxy-sulfide inclusions were formed in RE-bearing steel, which was dissolved preferentially in 0.5 wt% NaCl solution, and inhibited the propagation of initial corrosion. In contrast, coarse Al2O3 inclusions were formed in Nb-bearing steel, thus resulting in the selective dissolution of Fe matrix and further development of initial corrosion. RE-bearing steel demonstrated higher corrosion resistance as compared to Nb-bearing steel. Moreover, the work functions of low-indexed crystallographic planes of the inclusions and the substrate were calculated by first principle modeling, and the findings indicated consistent corrosion tendencies with experimental results, (RE)2O2S > REAlO3 > Fe > Al2O3. Finally, a schematic model was proposed to observe the influences of Nb and RE elements on corrosion initiation and propagation behavior of low-alloy steels.