The properties of an adsorbed corrosion inhibitor—tall oil fatty acid (TOFA) imidazolium chloride—on mica, gold, and X65 steel were studied using in situ atomic force microscopy (AFM). Topography images and thickness measurements show that the structure of inhibitor film changes from monolayer to bi-layer as inhibitor concentration exceeds its critical micelle concentration (CMC). Further kinetic study indicates that the developing of a full film took about 6 h. Quantitative force measurements were performed to evaluate the forces associated with adsorption of inhibitor films. Results show that the mechanical stress needed to physically remove adsorbed inhibitor molecules is of the order of MPa.