The anticorrosion characteristics of sulfachloropyridazine (SCP) as an eco-friendly inhibitor for the corrosion of mild steel in 0.5 M H2SO4 solution has been studied, for the first time, using potentiodynamic measurements and electrochemical impedance spectroscopy (EIS). The possible synergistic effect of halide ions on the inhibitive effect of SCP has been investigated. Thermodynamic calculations indicated that SCP adsorption on steel surface obeyed Langmuir adsorption isotherm. Electrochemical measurements showed that the SCP acted mainly as an anodic inhibitor. Halides significantly promoted the inhibition performance of SCP through the pre-adsorption on the electrode surface which is positively charged under the present conditions.