Effect of inclusions on corrosion resistance of carbon steel

The marine environment significantly accelerates metal corrosion especially in underwater or undersea tunnels for high-speed rails, where the life of the metals will be shorter than normal life time; moreover, this type of corrosion critically affects the safety, promptness, and efficient operation of high-speed rails. Accordingly, this article systematically analyzes the behavior of inclusion-induced corrosion on steel by automatic scanning of inclusions, accelerated corrosion tests, and electrochemical testing. It focuses on the analysis of inclusions in steel used for high-rail signal connection equipment (Q235) and corrosive behavior in foggy environments. The results show that the types of inclusions in steel are oxides, sulfides, and composite inclusions, and each type of inclusion has different effects on pitting corrosion of the steel structure. Among the inclusions, the main types largely present are sulfide inclusions and oxygen-sulfur composite inclusions, which have a particle size smaller than 5 μm. The inclusions larger than 5 μm are oxide inclusions and they are present in small number. When components or steel structures are used in marine environment, sulfide inclusions in steel are easily dissolved and deformed to form pits, while oxide inclusions around the substrate get dissolved and fall off resulting in pitting corrosion. Composite inclusions also induce steel corrosion. Therefore, different types of inclusions induce corrosion in different ways. Sulfide inclusions and oxygen-sulfur composite inclusions have a greater impact on carbon steel. Electrochemical tests show that the self-corrosion potential is approximately −0.1 V, and Q235 steel itself is not resistant to corrosion. As inclusions participate in corrosion process, they cause fluctuations in the anodic polarization curve and accelerate the corrosion of Q235 steel. The research results are of great significance for understanding and improving the corrosion resistance of steel, corrosion resistance of metal equipment in the undersea tunnel environment, and improving the safety and efficiency of high-speed railways.