Corrosion inhibition by carboxylic acids—an overview

Corrosion of metal is a natural, spontaneous, and thermodynamically stable process during which a metal, when it comes in contact with environments such as oxygen and water, is oxidized to its metal oxide, a stable product having low energy compared to the pure metal. There are many methods to control the process of corrosion. One such method is the use of corrosion inhibitors. The use of a chemical substance in a very low concentration is very much used in cooling water systems and the pickling process. Many compounds—organic, inorganic, and natural products—are used as corrosion inhibitors in very small amounts. Usually, these inhibitors contain polar atoms such as oxygen, sulfur, phosphorous, nitrogen, etc. During the corrosion inhibition process, these inhibitors coordinate with metal ions such Fe2+, through the polar atoms resulting in the formation of Fe2+ inhibitor complex formed on the anodic sites of the metal surface. Thus corrosion of metals is controlled. Usually, in the corrosion inhibition evaluation process, the weight loss method is employed. The mechanistic aspect is studied by electrochemical studies such as polarization study and electrochemical impedance spectra. The protective film is analyzed with the help of ultraviolet-absorption spectra, fluorescence spectra, and Fourier-transform infrared spectroscopy. The surface morphology has been analyzed with the help of scanning electron microscopy, energy dispersive X-ray analysis, and atomic force microscopy. Contact angle measurement is made to determine the hydrophobicity of the metal surface. The recent trends in making use of carboxylic acids as corrosion inhibitors are discussed. The metals used, the inhibitors used, the medium, the methods, and important findings are discussed in this review. As a case study, the inhibition efficiency of malonic acid in controlling corrosion of mild steel in low chloride medium water is presented in this study.