Fig. 1 Optical microstructures and the corresponding TEM images of Cu-added steel a, b; Cu-free steel c, d. Inset of b shows the EDS mapping image of Cu in b

Fig. 2 Variations of open circuit potential (EOCP) and linear polarization resistance (Rp) with immersion time for Cu-added and Cu-free steels: a EOCP; b Rp

Fig. 3 Electrochemical impedance spectra of Cu-free steel a, b; Cu-added steel c, d exposed to the SRB-incubated soil solution for different time

Fig. 4 Equivalent circuit models used for EIS fitting analysis: a equivalent circuit for one-time constant, b equivalent circuit for two-time constant, c time dependence of Rct + Rf derived from EIS date of SRB-containing SES

Fig. 5 Potentiodynamic polarization curves of Cu-added and Cu-free steels after 28-day immersion

Fig. 6 CLSM images of live/dead staining biofilms on the steel surface of Cu-free steel a, Cu-added steel b after 14 days incubation with SRB

Fig. 7 SEM images of surface morphologies of Cu-free steel a, Cu-added steel c after 28 days immersion in the SRB-incubated soil solution; b, d magnified images of a, c, respectively

Fig. 8 EDS elemental analyses of films formed on surfaces of Cu-free steel a, Cu-added steel b after 28 days immersion in SRB culture medium

Fig.9 EPMA mapping data of the cross section of corrosion products on Cu-free steel a, Cu-added steel b after 28 days immersion in SRB-incubated soil solution

Fig. 10 SEM images of surface morphologies of Cu-free steel a, Cu-added steel b after 28 days immersion in the SRB-incubated soil solution after removing biofilm and corrosion products

Fig. 11 Maximum pit depths and the corresponding data (red line) on the surface of Cu-free steel a, b, Cu-added steel c, d after 28 days immersion in the SRB-incubated soil solution

Fig. 12 Pit depths and pit diameters (${D}_{\mathrm{eq}}$) of two steels after 28 days immersion in SRB culture medium

Fig. 13 Schematic illustrations of SRB corrosion mechanisms for Cu-free steel a, Cu-added steel b