Influence of pH value on the structure and electronic property of the passive film on 316L SS in the simulated cathodic environment of proton exchange membrane fuel cell (PEMFC)

The influence of pH value on the electronic property of the passive film on 316L SS in the simulated cathodic environment of the proton exchange membrane fuel cell (PEMFC), was investigated by potentiodynamic polarization curve, electrochemical impedance spectra and Mott-Schottky plot. The pH value effect on the composition and structure of the passive film on 316L SS was detected by X-ray photoelectron spectroscopy (XPS). The results showed that the passivity of 316L SS was enhanced, the transfer resistance of the film/solution interface significantly increased with the increase in pH value. Mott-Schottky analysis revealed that the passive film on 316L SS appeared a p-n heterojunction, the donor and acceptor densities within the passive film decreased, and the flat band potential moved to the positive direction with the increment of pH value. XPS results implied that the thickness of the passive film increased with increasing pH value. The passive film on 316L SS exhibited the duplex structure, in which the outer film was mainly composed of Cr-oxide and the inner film was dominantly consisted of Fe-oxides when the pH value was less than 3. In the case of pH 5H2SO4 solution, Fe-oxides were the major component throughout the inner and outer films.