Steel - Aluminum corrosion

Just last week I picked up a Koga Miyata from 2002. Steel frame, but I think it has an aluminum stem with a steel shim between the stem and steerer. It's damaged, but not rusted, and due to its weight I think it will be fine with just a re-paint. It also has nasty looking rotten pedals. After a sanding down they'll look good again.

I also own multiple aluminum frames, and corrosion from sand/wind/tight cables is not the biggest issue with aluminum. What is an issue is aluminum fatigue. It doesn't happen quick or over time, but if it does, it can fail pretty badly. Weaker aluminum types used in gadgets like phone holders for example, will bend a bit the just break off as soon as it get to too much pressure, but until "too much" is reached, it can function just fine for a really long period of time. One of the frames has a big crack near the seatpost and it's ripe for the garbage. It's not fixable. For the rest, it forms a thin oxide film when the bare material is exposed to the atmosphere.

If you choose aluminum cassettes over steel ones, they will usually be under too much stress for the material and due to the shape and use chip off very slowly. You will need cassette replacements more often.