The coupling effect of coating and inhibition has been investigated in the present work. polypyrrole coating with adding coumarin was applied on carbon steel to protect it against corrosion. Electropolymerization process by cyclic voltammeter was carried out in 0.2M oxalic acid electrolyte containing 0.1M pyrrole monomer without and with 0.01M coumarin as an eco-friendly inhibitor. SEM/EDS, AFM and FTIR techniques were used to identify the coating film. Corrosion test using Potentiostat was achieved for uncoated and coated specimens and the results indicated that the corrosion potentials became nobler compared with uncoated specimen, this means that the anodic sites were covered by undoped and doped PPy film as illustrated from the deceasing of anodic and cathodic Tafel slopes. Protection efficiencies were acceptable and good (71.46% for PPy film and 77.47% for coumarin/PPy film). The polarization resistance was increased from 0.114 Ω.cm2 for uncoated C.S. to 0.176 and 0.404 Ω.cm2 for PPy coated and coumarin/PPy coated C.S. While the porosity percentage was 0.44% and 4.50 for PPy coated and coumarin/PPy coated C.S. due to increasing the roughness of coumarin/PPy film.