The corrosion inhibition mechanism of sodium nitrite on carbon steel has been carefully studied in different simulated cooling water solutions by using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The obtained results showed that the corrosion inhibition increases by increasing the nitrite concentration up to 9 mM in 5 mM chloride solution at 25 °C. The corrosion state and corrosion current density are strongly depended upon the nitrite and chloride concentration, solution temperature, and applied potential. The interaction of nitrite with metal surface follows the Langmuir adsorption isotherm and the adsorption is as a physisorption process in a neutral solution.