Effect of hydration pretreatment on tunnel etching behaviour of aluminium foil

Hydrous oxide films composed of pseudoboehmite and bayerite have been fabricated on the aluminium surfaces by hydration pretreatment in hot distilled water and ammonia solutions, respectively. After tunnel etching process, the specific surface area of aluminium foil with hydrous oxide film has been significantly improved, compared with that of aluminium foil with thermal oxide film. This effect can be attributed to the role that the hydrous oxide film contains plenty of water channels and defects, which allow the chloride ions to penetrate the film easily and reach the aluminium surface, thereby nucleating pits.

► The hydrous oxide film is composed of pseudoboehmite and bayerite.
► The hydrous oxide film provides water channels for pit nucleations.
► The hydrous oxide film may contain vast defects when adding ammonia.
► The specific surface area of etched foil with hydration pretreatment is improved.