Inhibitive Properties of Sodium tungstate-Zn2+ System and its Synergism with HEDP

Sodium tungstate in the presence of Zn2+ ions is used to function as the corrosion inhibitors in the corrosion of mild steel. However, in view of the toxicity of Zn2+, focus is now shifted to low- Zn2+ or no- Zn2+ formulations. In order to reduce the concentration of Zn2+ in Sodium tungstate- Zn2+ formulations, a second substance, which is environmentally friendly, is desired. 1-hyroxyethane-1,1-diphosphonicacid (HEDP) is environment-friendly, forms complexes with metal ions, and possesses passivating properties. The ternary system containing 50 ppm of ST, 10 ppm of Zn2+ and 200 ppm of HEDP is quite effective. AC impedance studies indicate that surface films formed on mild steel exhibit high charge transfer resistance and low double layer capacitance, which suggests that the film is non-porous and hence protective. Potentiodynamic polarization studies show that this system works as an anodic inhibitor. From the results of reflection absorption spectral studies on the surface film, a mechanism for the inhibition of corrosion is proposed.