Electrochemical Corrosion Study of Metallic Materials in Phosphoric Acid as Bipolar Plates für HT-PEFCs

This work investigates various commercially available austenitic stainless steels and Ni-based alloys as possible metallic bipolar plates for high-temperature polymer electrolyte fuel cells (HT-PEFCs). The dynamic formation and depletion of passivation layers that depends strongly on applied potential was analyzed in 85 wt% phosphoric acid at RT and 130°C by means of cyclic voltammetry and steady-state polarization. All materials showed a beneficial passivation in the potential window 0.3–1.2 V, which is mainly based on a stable inner Cr2O3 layer, and a reduction of the passive layers at cathodic polarization. Alloy 2.4869 (Cronix 80) with 80 wt% Ni and 20 wt% Cr reveals the lowest corrosion rates of 16.1 μA cm−2 at 130°C in the passive region at 0.6 V and a free corrosion potential Ecor of 235 mV. The improvement of passivity was achieved by the dominant superficial Ni3(PO4)2 layer. Alloying additions Mo and Ti revealed a characteristic increase of passive current densities due to instability of these passivation components at 130°C. Passivation based on Fe oxides and phosphates in stainless steels shows to be beneficial at RT, but less efficient at 130°C