This work was carried out to have a better understanding of the synergistic effect between phosphonocarboxylic acid salts (PCAS) and fatty amines ( FA) for the corrosion protection of a carbon steel in a 200 mg l(-1) NaCl solution. These compounds, combined with a biocide at low dosage, are used for water treatment in cooling circuits. First, electrochemical impedance measurements were performed to verify the inhibitive properties of the mixtures. Among the different concentrations tested, maximum efficiency was reached for the mixture containing 200 mg l) 1 PCAS + 50 mg l(-1) FA. Then, electrochemical measurements ( current - voltage curves and impedance diagrams) were combined with surface analysis (X-ray photoelectron spectroscopy) for each compound at the optimized concentrations and for the mixture. In the presence of amines, the iron oxide/hydroxide layer remained porous and the inhibition efficiency was relatively low. The FA acted by repelling the water molecules from the interface. PCAS sealed the pores of the oxide layer by forming a chelate. Thus, the growth of the inhibitive layer was limited but the efficiency was higher than for the FA alone. For the optimized mixture, the formation of the chelate was enhanced and the protective properties of the layer improved. A competitive adsorption mechanism was proposed to account for the synergistic effect observed in the presence of both compounds.