Early corrosion behavior of 35CrMo steel for high-strength bolt in simulated shallow and deep sea environments

Early corrosion behavior of 35CrMo low-alloy steel for high-strength bolt in the simulated shallow seawater and deep-sea environment was studied using mass loss measurement, electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), Raman spectrometer, X-ray photoelectron spectrometer (XPS) and 3D microscope. The results show that the corrosion of 35CrMo steel initiates mainly at the steel matrix around the inclusions of CaO–MgO–Al2O3, (Ca, Mn)S and their mixed composite. The corrosion rate in the simulated deep-sea environment is lower than that in the shallow seawater, which is declined with immersion time. The corrosion products formed in the two environments are composed of γ-FeOOH, α-FeOOH, Fe3O4 and little Cr(III) oxide or/and hydroxide, but the rust layer produced in the deep-sea environment is thin and uniform. The steel has pits in a shallow dish shape and tends to develop uniform corrosion in the deep-sea environment.