Effect of Phosphate Concentration on the Corrosion of Iron in Water—Polyethylene Glycol Mixtures

Various electrochemical measurement techniques (polarization, electrochemical impedance spectroscopy [EIS], and electrochemical quartz microbalance [EQCM]) and accelerated corrosion tests were used to show that phosphates can be used as iron corrosion inhibitors in 20% polyethylene glycol (PEG 400) solutions. Their action mechanisms were found to modify the anodic behavior of the iron. In solutions with low-phosphate concentrations (<5 × 10^–3 M), an iron oxide film formed, as in the case of aqueous solutions. But this oxide film did not provide protection and pitting occurred. At a phosphate concentration of 5 × 10^–3 M or more, the iron was protected but by a film that was different from the one observed in aqueous solutions. A deposit, composed of PEG and phosphates, adsorbed preferentially on the anodic sites of the iron.