Fig. 1 Microstructures of the tested steels with different Cr contents: a 0.10Cr; b 0.20Cr; c 0.50Cr

Fig. 2 Schematic diagram of the wet-dry acid corrosion test device

Fig. 3 Corrosion kinetics of the tested steels under wet-dry acid corrosion environment: a corrosion loss-corrosion time; b corrosion rate-corrosion time

Fig. 4 Potentiodynamic polarization curves of the tested steels with increasing corrosion time: a bare steels; b 7 days; c 21 days; d 35 days; e 49 days; f 98 days

Fig. 5 EIS plots of bare steels in 0.1 mol/L Na2SO4 solution with pH 3.0 and at the temperature of 25 °C: a Nyquist plots; b Bode plots

Fig. 6 Equivalent circuit for simulating the EIS plots of bare steels

Fig. 7 EIS plots of the rusted steels in 0.1 mol/L Na2SO4 solution with pH 3.0 and at temperature of 25 °C: a, b 7 days; c, d 21 days; e, f 35 days; g, h 49 days; i, j 98 days

Fig. 8 Equivalent circuit of EIS plots for the rusted steels

Fig. 9 Quantitative rust analysis of the tested steels at different corrosion time: a 21 days; b 98 days

Fig. 10 SSA of the rust of the tested steels in wet-dry acid corrosion: a 21 days; b 98 days

Fig. 11 Surface morphologies of the rusted steels at 98 days in wet-dry acid corrosion environment: a 0.10Cr; b 0.20Cr; c 0.50Cr

Fig. 12 Cross-sectional morphologies of the rusted steels at 98 days in wet-dry acid corrosion environment: a 0.10Cr; b 0.20Cr; c 0.50Cr

Fig. 13 TEM morphologies and corresponding SAED patterns (insets) of the rust for 0.10Cr and 0.50Cr steel at the corrosion time of 98 days: a 0.10Cr α-FeOOH; b 0.50Cr α-FeOOH; c 0.10Cr amorphous phase; d 0.50Cr amorphous phase