Inhibitor or promoter: Insights on the corrosion evolution in a graphene protected surface

Corrosion is a fundamental electrochemical problem that results in significant material degradation and failure and affects the performance of many engineering components. Additionally, corrosion associated material losses often produce contamination of surrounding media. Discovery of two-dimensional (2D) materials has opened a venue for multiple attempts in designing atomically thin anticorrosive coatings for prevention and control of corrosion propagation in metals. It was shown that graphene, for example, when uniformly coating a metal surface can inhibit corrosion-initiated degradation of copper and nickel. Here we demonstrate that graphene plays an important role in oxidation-based corrosion processes. Non-uniform graphene coverage has an enormous effect on corrosion protection and may lead even to acceleration of corrosion. We demonstrate that when immersed in sodium chloride solution the exposed edges of graphene become the centers for iron oxidation and propagation. Beneficially, high mechanical stability of graphene substantially eliminates metal losses and leads to only oxidation-based mass increase. Our extensive characterization analysis indicates the reason for this is high concentrations of trapped chlorine atoms on the edge of detached from the surface graphene flake. Chlorine increases metal dissolution and results in significant oxidation sites along the graphene edge line.

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