Electrochemical behaviour of sodium silicate and 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) on carbon steel corrosion control in neutral and aerated soft water solutions have been investigated using electrochemical impedance spectroscopy and Tafel polarization techniques. The impedance and polarization results revealed that inhibitors mixtures have shown synergistic effects at lower value of HEDP At 15 ppm silicate and 10 ppm HEDP showed optimum enhanced inhibition compared to their individual effects. The co-addition of 15 ppm HEDP and 15 ppm silicate has shown antagonistic behaviour. These observations have been supported by scanning electron microscopy (SEM) images, illustrating that the surface homogeneity increases in case of inhibitors mixture application and this provides good protection to carbon steel against corrosion in soft water solution. (c) 2007 Elsevier B.V. All rights reserved.