Environmentally Benign Corrosion Inhibitors

Paints are used extensively for the corrosion protection of aircraft. A paint system typically comprises of an underlying primer and a topcoat with the primer containing pigments that act as corrosion inhibitors. The most successful inhibitors in aircraft paints are the chromates (e.g., zinc yellow or strontium chromate). Since chromate corrosion inhibitors represent a significant environmental and health liability, more environmentally benign inhibitors are required. We propose a new class of corrosion inhibitors based on reduction-oxidation (redox) active, electrically conductive oxides. When galvanically coupled to a metal, they will maintain the mixed potential of the couple within the passive range of the metal preventing active corrosion. Unlike conventional chromate inhibitors, these metal oxides autocatalytically reduce 02 or H2O allowing them to sustain anodic protection without themselves discharging to active potentials or being consumed in the films (probably <20 nm thick) directly on metal surfaces or on carrier pigments, e.g. TiO2, SiO2, or Al flake, by a solution process is also proposed. The process should allow inexpensive production of large quantities of coated pigment material. The Phase I objective is to demonstrate the mechanism of corrosion inhibition using galvanic coupling studies, chronopotentiometry, and other electrochemical characterization techniques. The oxides will be applied on metal substrates such as Al alloy 2024, stainless steel (316), and Ti as well as on selected carrier pigments. The role of H2O, oxygen, and pH on the inhibition mechanism will be studied.