Insight into atmospheric corrosion evolution of mild steel in a simulated coastal atmosphere

The corrosion behavior of mild steel in a simulated coastal atmosphere environment has been investigated by the indoor accelerated wet/dry cyclic corrosion acceleration test (CCT), scanning electron microscopy (SEM), Raman spectroscopy and electrochemical measurements. During the CCT test of 60 cycles, the evolution of logarithmic (corrosion rate) vs. logarithmic (CCT cycles) presents a turning point at the 5th cycle, presenting a tendency to increase first and then decrease to gradually stabilize as the CCT cycle prolonged. Before the 5th cycle, γ-FeOOH and β-FeOOH and Fe3O4 were detected, respectively. And then, α-FeOOH as a new chemical composition was detected in the subsequent corrosion cycles. It is found that, after long term corrosion, the rust separated into a relatively dense inner layer rich with α-FeOOH and a loose outer layer rich with γ-FeOOH, both of which have poor electrical conductivity. The rapid increase of corrosion rate in the early stage since reducible corrosion products are involved in the reduction process of the cathode which promotes the dissolution of the anodic metal substrate. Afterward, as the rust layer thickens, the resistance of the rust increases, and the aggressive ions diffusion is blocked, gradually suppressing the electrochemical corrosion process. At last, when the composition and distribution of the rust layer remain stable, the corrosion presents a fluctuating speed around a certain value during the cracking and self-repairing process of the rust layer.