what is the most efective surface preparation for aluminum to prevent oxidation or corrosion.?

I know just enough to be dangerous (give misleading advice), so you should really ask an expert in this area or research it yourself.

Will the structure be in a marine or corrosive environment? It matters. I'm inclined to think that a solid, thick oxide layer will reduce corrosion compared to no anodization. Are you sure anodizing causes embrittlement? I hadn't heard of that. With regular anodizing, I think the oxide layer is fairly thin, isn't it? On the order of a micron? One website mentioned boiling the Aluminum in water after anodizing to seal the remaining pores.

There are also primers and paints intended for aluminum.

in order to achieve an acceptable bond, you need to prep the surface prior to application. the oxide layre is self protecting for the metal but may interfer with the bond.

try abrasion such as air abrasion using 20-30 micron al oxide or soda blast if you are bonding immediately.

if not bonding immediately, you can abrade and then prime with a suitable adhesive primer.

there are special adhesives for adhesive. try contacting hysol or master bond or prehaps ablestik

Nothing. When you cut a piece of aluminium, the freshly-cut surface very quickly oxidises so a protective coating of very hard Al2O3 is formed. This protects the metal underneath more effectively than any treatment you could give it. Anodising, painting and all the other treatments are just for appearance, not to protect the metal.

The very fact Aluminium is used is in itself proof nothing happens when oxidation takes place. To make it attractive best solution is to paint it.

Alodine

zee_prime is correct,

nothing. the oxide layer formed is impenetrable