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• Carbazoles used as corrosion inhibitors for mild steel. • Electrochemical measurements revealed mixed-type inhibitors. • SEM images reveal formation of protective film by inhibitors on steel surface. • Quantum chemical calculations are in good agreement with experimental results. Corrosion inhibition potentials of 3,6-dibromo-9-phenylcarbazole (DBPCZ) and 3(9H-carbazol-9-yl)-1,2-propanediol (CZPD) were investigated for 1 M HCl solution and microbiologically influenced corrosion (MIC) of mild steel using electrochemical and gravimetric methods, respectively. Potentiodynamic polarization results showed that DBPCZ and CZPD are mixed-type inhibitors (with predominant cathodic influence) of mild steel corrosion in 1 M HCl. DBPCZ and CZPD exhibited 81% and 87% inhibition efficiencies respectively as revealed by polarization measurements. Adsorption of DBPCZ and CZPD molecules on the steel surface in 1 M HCl was best defined by Frumkin adsorption isotherm. Uninhibited corrosion of steel by sulphate-reducing bacteria (SRB), which proceeded at the rate of 0.02 g cm−2 day−1 was subdued by 98% in the presence of 300 ppm of DBPCZ. Quantum chemical calculations suggested that the major molecular fragment in DBPCZ and CZPD molecules involved in donor-acceptor exchanges with Fe atom in mild steel is the carbazole ring. Monte Carlo simulations showed that the carbazole ring of the inhibitor molecules approaches Fe(110) in a near-flat orientation with predicted adsorption energies (E ads) of -114.24 kcal/mol and -119.85 kcal/mol for DBPCZ/Fe(110) and CZPD/Fe(110) respectively. [ABSTRACT FROM AUTHOR]