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In general, several corrosion inhibitors have been used to prevent the inner corrosion of pipes. The inhibiting effects depend upon the materials used, corrosion environment, and temperature etc. Corrosion inhibitors can be divided into anodic passivating, cathodic precipitation, and adsorption organic inhibitors [1]; Anodic inhibitors increase anodic polarization to the critical protection potential of the metal or alloys. They are called passivating inhibitors because they drastically decrease the corrosion current. Cathodic precipitation inhibitors decrease the corrosion rate by increasing the cathodic polarization overvoltage (hydrogen and oxygen reduction). Thick deposits form in the presence of these inhibitors, and decrease depolarizer diffusion to the metal surface, inhibiting the cathodic reaction and corrosion. Organic corrosion inhibitors contain sulfur, nitrogen, or oxygen atoms and organic heterocyclic compounds containing polar groups [2-6]. These compounds adsorb and form a covalent bond on the metal surface [7]. Organic inhibitors cover the entire surface area of the corrosion metal with a thick film consisting of several monolayers and change the structure of the double layer at the metal interface, decreasing the depolarization rate. They may also act as a barrier film by blocking anodic and cathodic active sites or decreasing the electroactive species transport rate to or from the metal surface [7]. Nitrite, which is one of the representative anodic inhibitors, inhibits corrosion by forming a Fe2O3 oxide film on the metal surface [8-15]. However, to inhibit the corrosion of ductile cast iron (DCI) in tap water, the nitrite is necessary at 100 kppm [16]. This is not industrially practical or economical. Nevertheless, when two kinds of corrosion inhibitors co-exist, a synergistic effect is achieved. When a film forming inhibitor and an adsorption type inhibitor were co-existed, a synergistic effect of them has been reported [17-20]. Synergistic Effect of Molybdate and Monoethanolamine on Corrosion Inhibition of Ductile Cast Iron in Tap Water