The existing state of hydrogen in electrochemically charged commercial purity aluminum and its effect on the tensile properties

Hydrogen is introduced in commercial (99% pure) aluminum by electrochemical charging to study the existing state of hydrogen and its effect on the mechanical properties of aluminum. Electrochemical charging is conducted in an aqueous solution of H₂SO₄ with 0.1 mass% NH4SCN as a hydrogen recombination poison. The potential and pH during the charging are chosen from the immune, passive, and corrosive regions on Pourbaix diagram to determine the optimum conditions for the charging. The maximum amount of hydrogen absorbed is obtained in the immune region. The amount of hydrogen and its existing state are examined using hydrogen desorption curves, which are obtained by thermal desorption spectroscopy. The curves show distinctive peaks that correspond to trapping sites of hydrogen in the material. One of the peaks is observed at approximately 1001C and it corresponds to vacancies and dislocations in the material; another peak is observed at approximately 400°C and it corresponds to molecular hydrogen in blisters. It is presumed that charged hydrogen diffuses into the bulk of the material to form hydrogenvacancy pairs, and then these pairs cluster to form blisters. The fracture strain of charged aluminum in the immune region decreased with a slower strain rate, showing an inverse dependence on the fracture strain of the uncharged material. This phenomenon is considered to be caused by the transport of hydrogen by dislocations through the interaction between hydrogen and the dislocations. The phenomenon is further confirmed by the observation of hydrogen release during tensile deformation, where the amount of hydrogen is higher in the strain rate region where the interaction between the dislocations and hydrogen is more prominent.