Phosphonic acids are effective chelating agents. They bind strongly to metal ions. Phosphonic acids have also been widely used as corrosion inhibitors along with various metal ions as Zn2+. The corrosion of various metals such as mild steel, copper, and aluminium have been prevented by phosphonic acids. Phosphonic acids exhibit good corrosion inhibition efficiency in acid medium, alkaline medium, and neutral medium. They can be used along with other inhibitors such as calcium gluconate, sodium molybdate, and trisodium citrate. Phosphonic acids show synergistic effects with these inhibitors. Various surface analysis techniques such as FTIR spectra, SEM, AFM, and EDAX have been used to analyze the nature of protective film formed on metal surface. Usually, the protective film consists of Fe2+- phosphonic acid complex along with Zn(OH)2 if Zn2+ is used. When phosphonic acids are used to prevent corrosion of iron, the adsorption process obeys Langmuir adsorption isotherm.
Key words: Corrosion inhibition, metals, surface analysis, AFM, SEM, adsorption isotherm
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