Analysis and Improvement of Trivalent Chromium Conversion Coatings for Aluminum Alloys

The trend toward replacement of highly toxic chemicals with less hazardous alternatives has created the demand for replacement of chromate conversion coatings for corrosion protection of aluminum alloys. Aluminum alloys are the predominate structural material used in aircraft because of their high strength-to-weight ratios, affordability and processability. Aluminum is subject to catastrophic failure due to stress-corrosion cracking caused by exposure to marine environments. A discovery by the Navy of trivalent chromium conversion coating (TCCC) has been identified as a promising replacement for the chromate process to impart corrosion protection to aluminum aircraft alloys. The TCCC process involves the deposition of a protective coating on the aluminum surface from an aqueous solution of trivalent chromium with additions of Na2SiF6 and NaOH. A post-deposition oxidation treatment with of H202 or KMnO4 improves the coating effectiveness. This TCCC procedure is promising but needs to be optimized before commercialization. American Research Corporation of Virginia proposes four technical objectives to improve the quality of the tccc process. In Phase I of the program, the composition and conditions of the deposition and oxidation steps will be improved, the elemental composition and structure of the films will be examined, corrosion due to salt water exposure will be tested, and the process will be monitored for generation of highly toxic chromates in solution. The significance of the proposed work will be the corrosion protection of aluminum surfaces by an environmentally favorable process.