Austenitic stainless steels in high temperature phosphoric acid

Austenite 316 L, 317 L, and 904 L stainless steels were investigated in 98% H₃PO₄ at 170 °C and they experienced passivation regardless of the purged gas. When polarized at 0.1 V (hydrogen) and 0.7 V (air) (phosphoric acid fuel cell (PAFC) environments), currents at the level of mA cm⁻² were observed. Compared to carbon composite under identical conditions, 904 L showed lower currents while 316 L and 317 L showed much higher currents.

X-ray photoelectron spectroscopy (XPS) depth profiles indicated that the surface film of the fresh steels consists of a Fe-oxide-rich outer layer and a Cr-oxide-rich inner layer. After being polarized in the PAFC environments, the Fe-oxide layer was selectively dissolved and Cr-oxide dominated the passive film. Phosphorus was incorporated into the film during the process, thus the chemical composition of the passive film differed from those formed in the polymer electrolyte membrane fuel cell (PEMFC) environments. The thicknesses of the stainless steels in the passive films in PAFC environments were estimated.