Evaluation of Corrosion Inhibitors for Carbon Steel and Type 304 Stainless Steel in Nitrilotriacetic Acid Medium at High Temperature

At elevated temperatures, the corrosion of carbon steel and Type 304 stainless steel is high in media containing polycarboxylic acids such as nitrilotriacetic acid, which is a reagent used for the decontamination of nuclear reactor coolant systems. Hence, three commercial corrosion inhibitors (Philmplus 5K655, Prosel PC-2116, and Ferroquest LP7203) were evaluated for high-temperature applications. Preliminary screening of the inhibitors was done by electrochemical techniques, namely, polarization and impedance spectroscopy. Philmplus showed maximum corrosion inhibition efficiency and hence was used for high-temperature investigations. A concentration of 500 mg/L was found to be optimum. The high-temperature dissolution of corrosion product oxides such as magnetite and nickel ferrite that are relevant to nuclear reactors was also carried out in the presence of Philmplus. During the decontamination process, which involves the dissolution of corrosion product oxides, it is desirable to use an inhibitor that will alleviate the corrosion of the underlying base metal without compromising on the dissolution of the oxides present over it. Investigations were also carried out to evaluate hydrazine as a corrosion inhibitor for high-temperature applications; the results obtained were comparable to those of Philmplus.