Corrosion behaviours of carbon steel coated with polyester and polyvinylester in a simulated flue gas desulphurisation (FGD) fluid (i.e., a mixture of H2SO4and 0.1 wt% Fe2(SO4)3at pH=2) have been investigated with electrochemical impedance spectroscopy (EIS). The protective nature of the coatings at 25°C has been observed in the following order: polyvinylester primer >> polyvinylester topcoat > polyester topcoat > polyester primer. Coating deterioration occurred and the coating deterioration rate accelerated with increasing temperatures upto 85°C. However, the corrosion resistance sequence has remained the same. Equivalent circuits are suggested for various coatings to discuss their mechanisms in this work.