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 an Fe sheet was analyzed employing a hydrogenochromic sensor and electrochemical hydrogen permeation tests. Large crystallographic pits formed in the initial stage barely promoted hydrogen entry, because the pits initiated in weak-acid and the initiation potential is higher than the potential of hydrogen evolution reaction. Hydrogen entry was accelerated by the acidification under an island-like rust layer. The rust layer became thick in the outer regions due to the alkalization around the rust-formed area. The hydrogen entry was prominent under the thick rust layer in drying process since it contains NaCl solution.