In order to clarify the suitability of aluminum with hydrated oxide film as a base material for painting, the strength of adhesion of epoxy-phenolic resin to aluminum hydrated using several kinds of baths was evaluated by measurement of the peeling strength, and the mechanism of adhesion for each type of treatment was observed by SEM and ESCA.
(1) In every method of treatment, resin adhesion rose in the initial induction stage (a), dropped extremely for a time at the beginning of film growth (b), and then rose again depending on the time of treatment (c). The mechanisms were considered to be as follows:
(a) The increase in specific surface area depending on the formation of lumps,
(b) The decrease in the adhesion of the lumps to the aluminum base depending on the formation of needle-like or petal-like hydrated oxide film,
(c) The increase of specific area depending on the growth of petal-like hydrated oxide film having good adhesion to the aluminum base.
(2) The films that were treated in waterglass solution after hydration promoted in sodium hypochlorite solution showed better resin adhesion than did films without waterglass treatment. We considered the cause of the increased resin adhesion to be secondary improvement of the surface of the aluminum base depending on the formation of SiO
2-film.
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