All corrosion is chemical. Corrosion changes the oxidation state of the metal. Many metals will react with a variety of materials to form other compounds. In phoebeisis case, the iron and aluminum reacted with the chloride in HCl to form iron and aluminum chlorides. Your frame's corrosion is probably due to reaction of the aluminum with chloride from road salt. This is worse than I've seen before but it's not outside of what's possible if aluminum is severely abused. Other possibilities are that the frame was stored in an environment that had a lot of ammonia around, like a barn.

I'd not waste too much time on the frame. Even from your fuzzy pictures, you can see some very deep pits in the metal. I don't think the frame is worth much of anything over what you can get for recycling it.