This study examined the influence of phosphorus addition on the corrosion resistance of carbon steel for a flue gas desulfurization (FGD) system using electrochemical methods, such as potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) in 10 wt% sulfuric acid (H2SO4) solution at room temperature. The potentiodynamic measurements indicated that the addition of phosphorus increased the cathodic hydrogen evolution reaction. X-ray photoelectron spectroscopy (XPS) analyses of the corroded surfaces after the immersion testing indicated that the oxidized product of phosphorus-containing steel was composed mainly of iron(II) sulfate (FeSO4), iron(III) sulfate (Fe2(SO4)3), and iron(III) phosphate (FePO4). However, these products seem to have no protective property. The experimental results confirmed that phosphorus has a deleterious effect on the corrosion resistance of carbon steels in H2SO4 solution as a result of the higher hydrogen activity.