Corrosion inhibition of mild steel in stimulated cooling water by blends of molybdate, nitrite and picrate as new anodic inhibitor

1 Introduction

The protection of cooling water systems and heated water supplies 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 (Mustafa and Farr, 1992). The actual trends in the environmental protection have changed essentially 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 (Lahondy‐Sarc et al., 2000). 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 (Qian and Turgoose, 1987; Pryor and Cohn, 1953). Lizlovs has observed that in the aqueous system containing aggressive ions, molybdate has corrosion inhibition only in the presence of oxygen (Lizlovs, 1976). In fact, the presence of aggressive ions such as chloride (Cl⁻) and sulfate (SO₄⁻²) anions reduces the efficiency of MoO₄²⁻, so higher concentration is necessary for corrosion inhibition, which is not economically favorable (Burda, 1992; Stranick, 1984). In order to achieve better efficiency and reduce the quantity of molybdate other oxidizing agents such as nitrite (NO₂⁻) and organic compounds have been employed. At present, the best method to improve inhibitive capability is by using inhibitor in combination with others (Farr and Saremi, 1983; Farr et al., 1984; Mustafa and Shahinoor, 1995, 1996; Alexander and Moccari, 1993). As it has been observed previously, organic inhibitors usually designated as a film‐forming, protect the metal by forming a hydrophobic film on the metal surface. Therefore, natural organic molecules, containing one pair of electrons or π‐electron associated with multiple specially triple bonds or organic rings, can bond to metal surface by electron transfer to the metal to form a coordinate type of link which ultimately produces a barrier to the dissolution of the metal in the electrolyte (Sastri, 1998). As picrate contains a phenyl ring together with four substituents N and O atom, one can suggest that picrate has inhibition efficiency as an organic compound (Smith...