An investigation was conducted to improve the corrosion inhibition efficiency of molybdate-based inhibitors for mild steel which is the main construction material of cooling water systems, using nitroethane as an organic compound beside zinc. In this study a new molybdate-based inhibitor was introduced with the composition of 60？ppm molybdate, 20？ppm nitrite, 20？ppm nitroethane, and 10？ppm zinc. Inhibition efficiency of molybdate alone and with nitrite, nitroethane, and zinc on the uniform corrosion of mild steel in stimulated cooling water (SCW) was assessed by electrochemical techniques such as potentiodynamic polarization and electrochemical impedance (AC impedance) measurements. Weight loss measurements were made with coupon testing specimens in the room temperature for 48？h. Studies of electron microscopy, including scanning electron microscopy (SEM) photograph and X-ray energy dispersive spectrometry (EDS) microanalysis, were used. The results obtained from the polarization and AC impedance curves were in agreement with those from the corrosion weight loss results. The results indicate that the new inhibitor is as effective as molybdate alone, though at one-ninth of the concentration range of molybdate, which is economically favorable. 1. Introduction The protection of cooling water systems as well as heat supply water has become one of the great important issues in the world economy. The application of corrosion inhibitors especially in closed systems holds a prominent place amongst other methods of corrosion control [1]. The actual trends in the environmental protection essentially have changed the traditional approach to corrosion inhibition. Since the toxicity of chromate-based inhibitor is a limiting factor in its use as a corrosion inhibitor, the changes in formulation of corrosion inhibitors are prompted primarily by an increasing demand to reduce environmental impact [2]. Molybdate-based inhibitor has long been known as an inorganic and anodic type of corrosion inhibitor, which is effective for protecting mild steel in the pH range 5.8–8.5 [3, 4]. Lizlovs has observed that in the aqueous system containing aggressive ions, molybdate has corrosion inhibition only in the presence of oxygen [5]. In fact, the presence of aggressive ions such as chloride (Cl？) and sulfate anions reduces the efficiency of , so higher concentrations are necessary for corrosion inhibition [6, 7], which is not economically favorable. In order to achieve better efficiency and reduce the quantity of molybdate？ other oxidizing agents such as nitrite ( ) and organic
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