Corrosion behavior of ultra-high strength steel 300M in different simulated marine environments

Corrosion behavior of 300M in neutral corrosion environments containing NaCl simulated by total immersion (TI), salt spraying (SS) and periodic immersion (PI), was investigated by surface analysis techniques, corrosion weight-loss method, and electrochemical measurements. In total immersion environment, rust on the steel consisted of a porous outer rust layer with main constituent of γ-FeOOH, and an inner rust layer of dense Fe₃O₄ film with network broad cracks. In salt spraying environment, outer rust with main composition of γ-FeOOH/α-FeOOH/Fe₃O₄ was compact, and inner rust showed dense Fe₃O₄ film. Rust formed by periodic immersion exhibited a compact outer rust layer with constituent of α-FeOOH/γ-FeOOH/Fe₃O₄ and an inner rust layer with composition of α-FeOOH/α-Fe₂O₃; inner rust showed a ultra-dense film adherent to the steel. The corrosion rate showed a rule of v_ss(salt spraying)>v_ti(total immersion)>>v_pi(periodic immersion) in 0–240 h, and v_ss≈v_ti»v_pi in 240–720 h. The rust formed by periodic immersion was dense and compact, with stable electrochemical properties, and had excellent protection on the steel. Humidity and oxygen concentration in all the environments played major roles in rust formation.