Long-term durability in water of aluminium alloy adhesive joints bonded with epoxy adhesives

Abstract

This paper begins with a review of some of the previous literature on the topic of durability of adhesive bonds to aluminium alloys. A number of papers published during the period 1977 to 1991 are discussed. The paper then details the current test programme using the wedge test to ASTM D-3762 and the adhesives and surface preparations chosen. A number of cold-setting epoxies were tested and a smaller number of hot-setting epoxy film adhesives. All testing was done on 2024-T3 clad aluminium alloy; specimens were immersed in distilled water at room temperature and readings of crack length taken at intervals. Readings were taken frequently for the first month and then at longer intervals. Fracture energy values were calculated after each set of readings. In the last 3years of the programme readings were taken at intervals of 2 or 3 months. After 5.5 years of total immersion the specimens were handed over to AEA Harwell for further testing and surface analysis of failures by scanning electron microscopy. Four surface preparation methods were compared. These were: (1) tank-type chromic acid anodise; (2) portable chromic acid anodise (Selectrons); (3) Deoxidine 202, a phosphoric acid paste etch; and (4) abrasion with 100 grit glass paper. From the literature a number of factors affecting durability are listed. A number of conclusions are drawn from the test programme and a further set from subsequent work on these specimens at AEA Harwell. Some relate to observations on electron microscope photographs of the surfaces of specimens after joint failure. Others show that, for a given type of surface preparation and a given type of adhesive chemistry, the durability of adhesive bonds depends on the permeability of the adhesive towards water.