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

Adsorption of a carboxylic acid-based corrosion inhibitor was studied on go ld and iron electrodes using a rotating electrochemical quartz crystal micr obalance (rEQCM) electrode developed in our laboratory. The thin film elect rodes were deposited on 10 MHz AT-cut quartz crystals. Concurrently, the ad sorption was characterized with impedance. The inhibitor concentration was stepwise increased during the potentiostatic measurements. Addition of inhi bitor resulted in an immediate increase of the resonant frequency up to a m onolayer coverage. The apparent mass loss corresponding to the observed fre quency increase can be explained by the replacement of water in the interfa cial volume occupied by the adsorbing inhibitor molecules. Langmuir-Freundl ich, multisite Langmuir, and Flory-Huggins isotherms were applied to the EQ CM data. The experimental results correspond best to a Langmuir-Freundlich- type adsorption behavior of the inhibitor. Calculated standard free energie s 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 adsorpt ion of corrosion inhibitors at solid electrodes not accessible by other met hods. (C) 2001 The Electrochemical Society.