Dynamic Electromotive Mechanism for the Galvanic Corrosion of 2D Coatings on Metals

The attractive application of two-dimensional (2D) materials as nanoscale corrosion-resistant coatings for metals in realistic environments is being challenged by ubiquitous galvanic corrosion, for which the key electromotive mechanism still lacks reliable clarification. In this work, four representative heterostructures based on the two most robust 2D coatings (graphene and hexagonal boron nitride) and two prototypical metal substrates (Cu and Ni) are comparatively studied by first-principles calculations. The obtained work functions are combined with available experimental results to confirm that the previously supposed electromotive force based on the static electronic-potential difference cannot rationalize the expected metal → coating electron transfer. Alternatively, the cathodic oxygen-reduction reactions (ORRs) on coating/metal surfaces, as well as the hydrogen-evolution reactions (HERs) in certain acidic conditions, are found able to provide a reasonable dynamic electromotive force to drive the electronic depletion on metals. The yielded corrosion potentials accurately unify the measured values in various neutral and acidic solutions, and the stability of O2 adsorption (i.e., the starting step of ORR) closely explains the experimental corrosion current density. The joint electronic-structure and electrochemical mechanisms underlying the surface-reactivity trends are revealed by both quantitatively portraying the free-energy profiles (plus kinetic corrections) for the cathodic reactions and systematically analyzing the multibody couplings between metal surfaces, 2D coatings, and adsorbates. The dynamic electromotive mechanism discovered here precisely confirms the realistic electrochemical reactions on coating/metal surfaces and the associated interfacial electron-transfer behaviors and can motivate more effective corrosion-control strategies.