Originally Posted by
cyccommute
Aluminum in aluminum won’t corrode either.
Not by galvanic action, but some corrosion will still happen, as you mention further:
For steel in steel and aluminum in aluminum, grease keeps another kind of corrosion down. We live in a world of salt...sweat, salt used for road clearing, and seawater being the main culprits..
I recently bought an aluminum bike with aluminum seat post, and when I pulled the seat post out it had some surface corrosion in the form of aluminum oxide on it. I suspect this was because the former owner used the bike in triathlons, and going from dripping wet from swimming to a bike caused water to seep down the seat post into the seat tube. Luckily there wasn't much corrosion which I was able to buff off with a wire brush, and I reinserted it after spreading a thin layer of grease on it.
I would assume the same would go for a steel seatpost in a steel frame. Let some water get in,and rust starts to become a problem. Putting some grease on the seat post helps prevent aluminum from becoming corroded to aluminum, and steel from rusting to steel.