[en] Electrochemical techniques have been used for investigating the inhibition of carbon steel by thiomorpholin-4-ylmethyl-phosphonic acid (TMPA) and morpholin-4-methyl-phosphonic acid (MPA) in natural seawater. The free corrosion potential was observed to the shift in noble direction which indicated that the phosphonic acids tested inhibit the corrosion of carbon steel in seawater. Potentiodynamic polarization curve shows clearly the fact that the addition of these molecules is associated with corrosion current density decrease and a corresponding reduction of the corrosion rate. The phosphonic acids tested as corrosion inhibitors of carbon steel in natural seawater are effective even with small concentration. Fourier transform infrared spectroscopy was used to obtain information on bonding mechanism between the metallic surface and the inhibitors. The morphology of the metal surface in the uninhibited and inhibited solution was examined using the scanning electron microscope coupled with an energy dispersive X-ray analysis system