Suitable corrosion inhibitors are of prime importance in order to prevent degradation of surfaces by oxidizing chemicals. In this work we studied ten symmetrical thiourea derivatives on aluminum and their efficacy in preventing oxidation by hydrochloric acid computationally and experimentally. We carried out DFT calculations of the inhibitors in both tautomer forms adsorbed on an aluminum-terminated α-alumina surface, focusing on the structure and energetics of adsorption as well as electronic properties. Chemisorption is dominated by electron transfer from the inhibitor S atom toward the surface as well as into the first few layers of the solid. We find that the aggregated amount of transferred charge is an important parameter of the system that correlates with the inhibition efficiency as determined with potentiodynamic measurements. The measurements indicate that the thiourea derivatives are cathodic-type inhibitors, which hinder the reduction of protons and thus indirectly the surface oxidation. This is rationalized with the formation of a positively charged layer on the surface that may repel protons. Our results may serve to further improve corrosion inhibitors on this technologically important surface.