Adsorption of a bromine labeled carboxylic acid corrosion inhibitor on iron measured with EQCM, EIS and XPS

Adsorption of bromine labeled aromatic carboxylic acid corrosion inhibitor on iron, polarized in the active region, was studied with the rotating electrochemical quartz crystal microbalance (rEQCM), electrochemical impedance spectroscopy (EIS) and X-ray induced photoelectron spectroscopy (XPS). The EQCM quantifies adsorption via the mass loss resulting from the fact that the inhibitor replaces water molecules in the interfacial volume occupied by the adsorbent. EIS permits to characterize adsorption by determining changes in the double layer capacity resulting from addition of the inhibitor to the solution. Finally, XPS provides an independent ex situ analysis of the electrode surface after completion of an adsorption experiment. Results obtained with EQCM and EIS by adding increasing amounts of the model inhibitor to 0.1 M NaClO ₄ suggested that monolayer coverage was reached at the surface at a concentration of 2.25×10 ⁻² M. XPS data indicate that the surface concentration approaches a limit in this concentration range. The EQCM adsorption data fitted best the Langmuir–Freundlich isotherm. Compared to a previously studied similar inhibitor without Br substituent, the present inhibitor replaced more water from the surface layer and it exhibited a higher adsorption constant. Consistent with these observations, linear sweep experiments showed it to be more effective in suppressing the anodic dissolution in the active region. The different results of this study confirm the usefulness of the EQCM for the characterization of competitive adsorption of corrosion inhibitors involving replacement of the solvent from the electrode surface.