Surface and corrosion properties of modified passive layer on 304 stainless steel as bipolar plates for PEMFCs

In this study, the relation between surface chemistry and corrosion properties of modified 304 stainless steels (304SS) was investigated. 304SS samples were submitted to plasma nitriding performed at two different temperatures: 420 degrees C (low) and 520 degrees C (high). Then, a thermo reactive deposition (TRD) was used in a mixture of ferro niobium, alumina and ammonium chloride. Finally, a pickling treatment was performed to access high corrosion resistant surfaces. Surface treated samples, both before and after pickling, were investigated by using Scanning Electron Microscopy (SEM) and Potentiodynamic (PD) techniques. X-ray Photoelectron Spectroscopy (XPS) was performed for the analysis of the surface layers of the samples after pickling. The surface layers were mostly comprised of iron and chromium oxides and hydroxides. XPS results proved the presence of a surface layer mostly constituted by iron oxides and oxyhydroxides for both the treated samples that resulted more homogenous for the sample nitrided at low temperature. Chromium was detected by XPS in the predominant form of oxide on the surface of the sample nitrided at low temperature.