The Corrosion of a Stainless Steel in High Temperature Phosphoric Acid

The experiments described in this article were designed to evaluate 904L stainless steel as a candidate for replacing carbon composite in a phosphoric acid fuel cell (PAFC) bipolar plate application. We investigated the 904L steel in phosphoric acid purged with air or hydrogen gas at 170°C. Dynamic polarization showed that the steel experienced passivation with high currents. A depth profile obtained using X-ray photoelectron spectroscopy indicated that the surface film of 904L consists of an outer layer rich in Fe-oxide and an inner layer rich in Cr-oxide. After polarization, the Fe-oxide layer was selectively dissolved and Cr-oxide dominated the passive film. The Cr-oxide layer also experienced corrosion, but at a much lower rate. After 3 hours of polarization, the Fe-oxide was almost totally depleted, and phosphorus was incorporated into the passive film, changing its chemical composition. We concluded that 904L steel is an excellent candidate for use in PAFC applications