Corrosion-resistant tantalum coatings for PEM fuel cell bipolar plates

Tantalum is a tough, corrosion resistant metal, which would be suitable for use as bipolar plates for proton exchange membrane (PEM) fuel cells, if it was not for its high weight and price. Relatively thin tantalum coatings, however, can be deposited on other inexpensive and lighter weight metals, such as aluminum and steel, providing a passive protection layer on these easily formed substrates. We have successfully deposited, high quality alpha (body-centered-cubic, bcc) and beta (tetragonal) phase tantalum coatings that were a few micrometers thick by dc magnetron sputtering on steel and aluminum. The growth of the thermodynamically preferred body-centered-cubic (bcc) tantalum phase was induced by a choice of deposition conditions and substrate surface treatment. The microstructure and corrosion resistance of the alpha-phase in an environment approximately simulating the electrochemical conditions used in a PEM fuel cell were investigated under potentiodynamic conditions. Preliminary potentiostatic measurements of a beta-phase sample are also presented.