Adsorption of an organic corrosion inhibitor on iron and gold studied with a rotating EQCM

Adsorption of a carboxylic acid-based corrosion inhibitor was studied on gold and iron electrodes using a rotating electrochemical quartz crystal microbalance (rEQCM) electrode developed in our laboratory. The thin film electrodes were deposited on 10 MHz AT-cut quartz crystals. Concurrently, the adsorption was characterized with impedance. The inhibitor concentration was stepwise increased during the potentiostatic measurements. Addition of inhibitor resulted in an immediate increase of the resonant frequency up to a monolayer coverage. The apparent mass loss corresponding to the observed frequency increase can be explained by the replacement of water in the interfacial volume occupied by the adsorbing inhibitor molecules. Langmuir-Freundlich, multisite Langmuir, and Flory-Huggins isotherms were applied to the EQCM data. The experimental results correspond best to a Langmuir-Freundlich-type adsorption behavior of the inhibitor. Calculated standard free energies of adsorption show a slightly stronger interaction between inhibitor and iron than inhibitor and gold. The present study demonstrates that the EQCM is a unique tool for the study of substitution phenomena related to adsorption of corrosion inhibitors at solid electrodes not accessible by other methods. (C) 2001 The Electrochemical Society.