Adsorption of organic corrosion inhibitors on iron in the active and passive state. A replacement reaction between inhibitor and water studied with the rotating quartz crystal microbalance

Adsorption of a carboxylic acid model compound and of benzoic acid on iron from the liquid phase, in presence or not of an amine base, was studied usi ng the rotating electrochemical quartz crystal microbalance (rEQCM). The in vestigated acid,. base couples are models for corrosion inhibitors in fully formulated protective coatings for mild steel. Adsorption resulted in a de crease of mass on the quartz electrodes because water is replaced in the vo lume occupied by the adsorbing organic molecule. A simple model taking into account the molecular volume of the adsorbent and of water molecules could explain the measured mass loss. The effect of the organic base was studied by characterizing the individual adsorption behavior of the acid and base parts. From the measured mass loss due to the water replacement upon adsorp tion, the adsorption coverage was calculated. The resulting isotherms follo wed a LangmuirFreundlich behavior from which the adsorption constant, the s tandard free energy of adsorption and the heterogeneity parameter could be calculated. A co-adsorption with the carboxylic acid was found in the prese nce of the organic base. Indeed, the organic base showed the highest adsorp tion constant on metallic iron. Both acid and base adsorbed more easily on metallic than on oxidized iron. A good correlation between the adsorption b ehavior from EQCM data and polarization curves was found. (C) 2001 Elsevier Science Ltd. All rights reserved.