Hydration of phosphoric-acid-anodized aluminum

Phosphoric acid anodization (PAA)( is a common method of surface preparation of aluminum adherends prior to adhesive bonding for aerospace applications. PA A surfaces are microscopically rough(6) and are more resistant to hydration than aluminum surfaces prepared by other treatments.(2,90) The microroughness provides for mechanical interlocking with the primer or adhesive, which results in a strong adhesive bond, while the environmental stability of the oxide (together with its porosity) results in excellent bond durability in hot, humid environments. 

FIGURE 17. Variation of the surface composition of PAA samples exposed to 100% relative humidity at 50°C. (From Reference 84.) 

FIGURE 19. Sputter depth-profiles of PAA samples before and after hydration. Note the small amount of P present in the hydrated sample, (from Reference 2.) 

These results demonstrate the synergism that is possible in a detailed, multitechnique approach to a problem where no one technique (XPS, AES, or SEM) would have given a complete understanding. They also illustrate the usefulness of specialized data analysis, such as that provided by SBDs. 

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