Organic corrosion inhibitors: where are we now? Part I. Adsorption

Abstract

A survey of studies on the adsorption of organic corrosion inhibitors (CIs) on metals from acid and neutral solutions for the period 2005–2015 is presented. It is noted that in order to estimate the adsorption of CI in corrosive acid solutions, the electrochemical impedance spectroscopy method (EIS) remains the most valuable. However, even the use of EIS for building CI adsorption isotherms is an approximation, which demands selecting the optimum conditions to reduce errors (control electrode potential, minimizing the rate of electrode dissolution, etc.). Besides, the conclusions about the state of the adsorption system and mechanism of adsorption of chemical substances can be made, at least, based on results of determining both the adsorption energy and the adsorption process kinetics. Addition of such a study to measure the aftereffect and XPS studies of the protective film formed on the metal by a CI provides valuable information about the nature of its adsorption. In reviewing studies of CI adsorption in neutral aqueous solutions, much attention is paid to the in situ ellipsometry method. It is noted that the best conditions for such investigations may be produced on the surfaces of electrodes at potentials corresponding to the absence of intense cathodic hydrogen evolution or anodic dissolution of the metal. Examples of CI adsorption measurements by other in situ methods (EIS and electrochemical quartz microbalance, EQMB), the results of which are consistent with those of ellipsometry, are discussed. Results of successful application of ellipsometry in combination with the XPS studies of protective layers on various metals in the study of not only individual CIs but also mixtures thereof are provided. Ellipsometric studies have played an important role in the discovery of a new possibility of using CIs, which consists in successive formation of adsorption layers of different CIs on a metal (layer-by-layer coating).