Enhancing corrosion resistance of Ti₂AlC MAX phase through Sn solid solution in harsh acidic environments

When proton exchange membrane fuel cells (PEMFCs) run in the harsh acidic environment at 60–80 °C, metallic bipolar plates are prone to dissolution and corrosion, leading to increased interfacial contact resistance (ICR) and a decrease in output power energy. In this work, high-purity Ti₂AlC MAX phase coatings, with and without Sn solid solution, were fabricated on 316 L stainless steel using a multiple sputtering technique followed by a subsequent heat treatment process. Effect of Sn modification on the corrosion resistance and electrical conductivity of coatings was particularly focused on under simulated PEMFCs conditions. The results indicated that the Ti₂(Al, Sn)C coating significantly improved electrical conductivity and corrosion resistance compared to the pristine Ti₂AlC coating. The observed phenomena in Ti₂(Al, Sn)C coating could be attributed to the unique appearance of passivation layer, where three layers—TiO₂, Al₂O₃ and SnO₂— were identified distinctly. One benefit was that the outermost SnO₂, resulting from the oxidation of Sn, partially inhibited the penetration of corrosive media. This enhancement improved the protective efficiency of the subsequent Al₂O₃ layer, which has a higher density than that of TiO₂ layer. On the other hand, the microstructure evolution of the passivation film was also well consistent with the capability of atomic diffusion, as indicated by the atomic vacancy formation energy and the migration energy by density functional theory simulation. As a result, the unusual layered oxides in the Ti₂(Al, Sn)C coating enhanced the corrosion resistance and ICR value of the coatings compared to the pristine Ti₂AlC coating.