Fluoride-Induced Corrosion of Stainless Steel: A Case Study for its Application as Proton Exchange Membrane Water Electrolysis Bipolar Plate Material.

Stainless steel is a promising material for bipolar plates (BPP) in proton exchange membrane water electrolysis (PEMWE) that could drastically reduce stack costs. However, dissolution of Fe from stainless steel BPP might trigger membrane degradation, which releases fluoride. Fluoride in turn could accelerate stainless steel corrosion. Therefore, the influence of fluoride contamination (namely 0, 1, 5, and 20 ppm in 0.5 mM H₂SO₄) on the dissolution stability of stainless steel (316L) is investigated utilizing a scanning flow cell coupled on-line to an inductively coupled plasma mass spectrometer (SFC-ICP-MS). Fluoride enhances the dissolution exponentially, resulting in enhanced dissolution efficiencies with increased fluoride concentration reaching ≈50% at 20 ppm. Complementary micro and nanostructure analysis (laser profilometry, scanning electron microscopy, and scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy) reveals pitting corrosion, whose severity and occurrence appear highly increased with higher fluoride concentration. The results suggest that fluoride impurities in combination with exposed stainless steel, e.g., due to coating imperfections, should be avoided in PEMWE application, as accumulation of impurities of both might lead to a self-accelerating degradation process.