Mechanism of local hydrogen entry into Fe sheets induced by atmospheric corrosion: Significance of potential, pH, and rust layer thickness

Local hydrogen entry under a NaCl droplet on Fe sheets was analysed by employing a hydrogenochromic sensor and electrochemical hydrogen permeation tests. Large crystallographic pits barely promoted hydrogen entry, because crystallographic pitting proceeded at potentials higher than the potential of the hydrogen evolution reaction. Hydrogen entry was accelerated because of acidification under an island-like rust layer. The rust layer became thick in its outer regions, mainly owing to severe corrosion and alkalisation around the rust-formed area. Hydrogen entry was prominent under the thick rust layer after disappearance of the droplet, owing to the high concentrations of chloride ions and protons.