A combination of surface enhanced Raman spectroscopy (SERS) and potentiodynamic polarization tests indicate that two corrosion inhibitors of carbon-steel adsorb on the surface of the steel's scale, rather than on the bare steel surface. The steel's scale consists of several iron compounds and the inhibitors preferentially adsorb on one. The identity of the compound on which the inhibitors adsorb has been partially determined. It consists of Fe(III) and exhibits a strong Raman peak at approximately 280 cm-1. The amount of the preferred compound is severely decreased at high potentials in the presence of chloride ions, which leads to a loss of inhibitor and pitting corrosion of the steel. Pre-polarization of the steel increases the amount of the preferred compound and improves the performance of the inhibitor.