The semiconducting behavior of No.20 carbon steel coated with different polymer coating in 5% sodium hydroxide solution was studied by utilizing potential-capacitance and Mott-Schottky analysis. Results showed that the phenol aldehyde resin and alkyd resin behaved as bipolar film, showed p-type semiconducting behavior at the potential range of -0.3V ~ -0.1V vs. SCE and as an n-type semiconductor at the potential range of 0V ~ 0.4V vs. SCE in 5% sodium hydroxide, whereas for the epoxy resin, it showed as an insulator in 5% sodium hydroxide solution at the same condition.