Effect of Different Substrate Pre-Treatments on the Resistance of Aluminum Joints to Moist Environments

The goal of this work was to evaluate and compare the performance of different surface pre-treatments applied to aluminium single lap joints exposed to warm moisture. Four different processes were considered: 1) degreasing-roughening; 2) degreasing-roughening and phosphoric acid anodising; 3) degreasing-roughening and conditioning with Picklex (R) 20 (a water-based product for metal surface preparation); 4) degreasing-roughening and phosphoric acid anodising and contaminating the adherend surface with CaCl2 to accelerate the tests. After each pre-treatment, lap-shear specimens were prepared with the open face technique. The half-joints were exposed to warm moisture (50 degrees C, 100% RH) for times up to 4 weeks, then they were completed and subjected to mechanical testing. The results show that anodising yields the best durability, as strength remained practically constant over time; all the other groups of specimens exhibited progressive reduction in strength and wider scatter of the results. A large role is played by interfacial damage of the adherends, since, under increasing exposure time, failure tends to occur as separation at the adhesive-metal interface. The strength decreased dramatically also for the anodised specimens when the surface was contaminated, which evidences the role of the interfacial damage. The interpretation is supported also by scanning electronic microscopy observation and energy-dispersive X-ray analysis.