Important features in the adhesive bonding of aluminium

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Abstract
The effect of surface treatment of aluminium on the resultant bond strength and its durability upon exposure to warm/humid conditions has been examined. Mechanical testing revealed the superior bond strength of deliberately filmed substrates compared with mechanically abraded specimens. The poor performance of the latter is related to the limited wetting of the adherend by the adhesive and surface masking by embedded detritus. Exposure of the bonded joints to high humidity highlights the performance of substrates which have been anodized in phosphoric acid compared with chromic acid; chromate/sulphuric acid pickled substrates perform at an intermediate level. The observed behaviour is related to the wetting and penetration of the adherend by the adhesive as well as resistance to transformation of the alumina film to hydrated material. Partial penetration of the film formed in chromic acid by priming prior to bonding reveals a significant improvement in bond durability, which is thought to be largely due to the effective extension of the diffusion path of environmental species, i. e. moisture.

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