Corrosion Inhibition of Iron in Acidic Solutions by Monoalkyl Phosphate Esters with Different Chain Lengths

The inhibition effect of three amphiphilic monoalkyl phosphate esters with different chain lengths-mono-n-butyl phosphate ester (BP), mono-n-hexyl phosphate ester (HP), and mono-n-octyl phosphate ester (OP)-on the corrosion of iron in 0.5 M H2SO4 solutions was investigated by using electrochemical impedance spectroscopy (EIS) and polarization curve methods. The electrochemical results indicate that BP, HP, and OP all acted as mixed-type corrosion inhibitors with a dominant cathodic effect. BP shows the lowest inhibition efficiency as compared to HP or OP. However, the inhibition efficiency of HP is almost the same with that of OP under similar conditions because the molecular aggregation state at the iron/solution interface plays a more important role. X-ray photoelectron spectroscopic (XPS) characterization demonstrates that three alkyl phosphate esters adsorbed on the iron surface through the un-ionized P-OH groups. The adsorption of BP, HP, and OP fitted well with the Langmuir model.