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Summary
The influence of the etching stage of a pretreatment sequence for the adhesive bonding of aluminium was studied. The sequence consisted of etching, sulphuric acid anodizing, dipping in phosphoric acid and bonding with a rubber toughened acrylic adhesive. With the pretreatment kept unaltered except for the etching stage, the initial joint strengths of adhesively bonded 5251 aluminium alloy lap shear joints were determined for nine alternative etching solutions. Compared with the standard optimised Forest Products Laboratory (FLP) etch (sodium dichromate/sulphuric acid), solutions based on phosphoric acid and one solution based on sodium hydroxide with a strong oxidising additive resulted in marginally higher joint strengths, whereas most etching solutions based on sodium hydroxide resulted in lower joint strengths.
The surface topographies of the etched surfaces, examined by scanning electron microscopy, were scalloped. The FPL etch and those based on phosphoric acid gave the most consistently uniform etched surfaces with scallops 0.4-1.5μm across. The scallops produced in solutions based on sodium hydroxide were significantly larger, averaging between 7.2μm and 21.4μm across depending on the additives present. The uniformity of etch and the range of scallop sizes also depended on the additives.
The highest joint strengths were obtained with etching solutions which resulted in uniformly scalloped surfaces: these were the FLP etch, the solutions based on phosphoric acid, and the sodium hydroxide solution containing a strong oxidising additive.