Effect of deposition temperature on the formation of the corrosion-protective SnO_x:F coating layer on SUS 316 bipolar plates for PEMFC

Fluorine-doped tin oxide (SnO_x:F) films on SUS 316 were prepared as a function of substrate temperature using electron cyclotron resonance-metal organic chemical vapor deposition (ECR-MOCVD) in order to achieve corrosion-resistant and low contact resistance bipolar plates for polymer electrolyte membrane fuel cells (PEMFCs). The SnO_x:F films coated on SUS 316 substrate in the heating range from 200 to 500 °C were characterized by x-ray diffraction (XRD), Auger electron microscopy (AES) and field emission-scanning electron microscopy (FE-SEM). To simulate the aggressive PEMFC environment, all electrochemical experiments were conducted in 1 M H ₂SO₄+2 ppm HF solution at 70 °C. With increases in the heat treatment temperature from 300 to 500 °C, it was shown that both corrosion resistance and interfacial contact resistance (ICR) substantially increase. The AES data revealed that the amount of fluorine decreases with increasing temperature in our experimental range. The deposition temperature appears to be one of the critical process parameters on the formation of the corrosion-protective layer for PEMFC bipolar plates. It is probably caused by microstructural evolution before/after potentiodynamic corrosion tests under the PEMFC environment.