Some of what you've posted is correct...and more correct than TreyWestgate posted...but you are also wrong several points.
Originally Posted by
SkyDog75
Corrosion is an irreversible process, and it can be the death of a frame if it progresses far enough. That's true whether the frame's steel or aluminum, as both can corrode. Take basic precautions, like making sure paint prevents bare metal from being exposed to the elements, and that'll help keep corrosion in check. If corrosion has started, clean it off and then seal it via touch-up paint or some other means.
While it is true that both metals can corrode...i.e. oxidize..., the mechanism for oxidation is very different in the two materials. When steel is exposed to oxygen, it forms the oxide (rust) continuously. Aluminum forms an oxide layer on the outside of the metal that is almost impervious to further oxygen infiltration and protects the underlying metal.
Chlorides from salt are damaging to both metals through the formation of either iron or aluminum chlorides on the respective metals. The difference is that aluminum strongly bonds to the chloride and goes, essentially, no where. Iron forms a chloride salt as well but the chloride ion is replace rapidly with oxygen which releases the chloride to pluck out more iron atoms. The process is catalytic so any chloride that is available will continue to eat away at the metal. This works in a moist environment and most chlorides that we use on the road will absorb enough moisture out of the air to keep the process going.
Originally Posted by
SkyDog75
In my experience, when a steel or aluminum frame fails, it'll crack somewhere, but not generally catastrophically.
I don't know how much experience that you have with frame failure but I've broken 4 frames...two of each material...along with a number of components made of either material. Aluminum, as you said, cracks and even tears. It makes a lot of noise while doing so. The frame or part will creak and groan a whole lot before failure. Failure of aluminum is generally a slow process.
Steel, on the other hand, tends to shear off rapidly with little to no warning. Axles, pedal spindles, spokes and even frames have all gone "ping" and are broken. No warning, no bending, no creaks or groans. When hub axles have broken, I didn't even know the axle was broken until I happened to take the wheel out to fix a flat or perform some other maintenance.
Originally Posted by
SkyDog75
A quality aluminum frame that's been treated well will generally last a long, long time.
Steel frames occasionally fail, too. You can find plenty of examples of cracked steel frames if you search.
Both very true.