Is this aluminum "rust" in my frame? I found an aluminum bike in the trash and it seems like it has been exposed to the rain, especially because some of the metal is slightly rusty.

The frame is a different thing, because altough it is made in aluminum, it has worm-like bright shapes, with a small relief (when I pass my fingers it isn't as smooth as the rest of the frame).

This:



So, my questions:


Is this "rust"? (I know aluminum doesn't have corrosion like steel does, but this is IMO somekind of corrosion).
Is this a damaged in a point that it is dangerous to use it again?
Can I solve it? (I tried sanding a small bit and it takes care of it, of course the shine is gone in those bits).
Yep, "raw" aluminum develops a layer of corrosion fairly rapidly.  Thankfully, unlike with iron/steel, the corrosion is "tight" and eventually develops to a thickness where additional corrosion is prevented (in normal circumstances).

The corrosion can be prevented by either coating the aluminum with a lacquer-like coating or "anodizing" it.  Anodizing is a process where the aluminum is dipped in an electrolyte solution and a current is run through it, to develop a layer of "super corrosion" that is smooth and fairly hard/scratch resistant.

I'm guessing the bike was lacquered and the coating is breaking down.  Once one little spot of breakdown appears, the corrosion below will push up the adjacent areas, creating a growing spot that turns into the "worms" you see.  There is no structural problem as a result of this -- it's purely cosmetic.

The thing to be concerned about would be if there were any spots (particularly around the joints) where a salt-like crust was forming.  This would indicate that the aluminum is "crystalizing" in the weld, and fractures, if not already present, are imminent.