The inhibition efficiency of a phosphonic acid, Diethylene Triamine Pentamethylene Phosphonic acid (DTPMP) in controlling corrosion of carbon steel immersed in an aqueous solution containing 60 ppm of Cl- has been evaluated by weight loss method in the absence and presence of Zn2+. The formulation consisting of DTPMP and Zn2+ has excellent inhibition efficiency (IE). A synergistic effect is noticed between Zn2+ and DTPMP. Addition of sodium gluconate (SG) enhances the IE of Zn2+ and DTPMP system. The DTPMP-Zn2+-SG system function as a mixed inhibitor as revealed by polarization study. AC impedance spectrum, optical and atomic force micrographs reveal the formation of a protective film on the metal surface. FTIR spectra reveal that the protective film consists of Fe2+-DTPMP complex, Fe2+-SG complex and Zn(OH)2.