Abstract: With the objective of developing a new eco-friendly inhibitor formulation for the corrosion of carbon steel on neutral aqueous solution containing 60 ppm Cl-. The phosphonic acid {diethylenetriamine penta(methylene phosphonic acid)}(DTPMP) was chosen as the inhibitor, which gives the inhibition efficiency of 93%. Many phosphonic acids are used as good corrosion inhibitors like ATMP, HEDP and DTPMP etc. Among which we have used the DTPMP as a potent corrosion inhibitor for this study. Because it contains number hetero atoms like Oxygen and Nitrogen. The Sodium potassium Tartrate (SPT) which is an environment friendly and used as the first synergist, Zn2+ may be second synergist so it may used as a ternary inhibitor system. When DTPMP used alone shows some inhibition efficiency. If we combine with the formulation of 60 ppm Cl-, 50 ppm DTPMP, 25 ppm SPT, and 10 ppm Zn2+, it gives maximum inhibition efficiency. From FTIR spectra it was observed that the protective film is formed on the metal surface. EIS spectra also confirms the formation of protective film on the metal.