Study on the corrosion behavior of three types of steel in marine atmospheric environment

Q235, Q345, and 921 steels were selected as study materials for a one-year exposure test in the marine atmospheric environment of Qingdao. The corrosion behavior and mechanisms of the three steels were systematically investigated using weight-loss measurements, electrochemical tests, pitting morphology observations, and rust layer phase analyses. The results indicated that the corrosion rates followed the order Q345 > Q235 > 921. Q345 steel exhibited MnS inclusions that acted as pitting-sensitive sites, resulting in lower corrosion resistance than Q235 steel, while 921 steel, containing Ni, Cr, and Mo, showed significantly reduced corrosion rates. XRD analysis revealed that the corrosion products of all three steels mainly consisted of γ-Fe₂O₃/Fe₃O₄, α-FeOOH, and γ-FeOOH, with 921 steel exhibiting a higher proportion of α-FeOOH and greater amorphization, forming a dense protective layer that effectively slowed electrochemical corrosion. XPS results showed that the formation of NiFe₂O₄ in 921 steel conferred cation selectivity to the rust layer, enhancing its protective effect. Raman spectroscopy indicated that the rust layers of all three steels displayed an outward-to-inward stratified structure, but the inner α-FeOOH layer of 921 steel was richer and more stable, providing the basis for its superior corrosion resistance. Electrochemical tests further demonstrated that 921 steel had significantly higher polarization resistance and impedance than Q235 and Q345 steels, which increased with exposure time, indicating a dense and stable rust layer that strongly hindered charge transfer and ion diffusion.