Aluminum doesn't rust. But it can corrode to the point it becomes irreparably damaged. Usually anodizing helps protect aluminum - to some extent. Bare aluminum is most readily corroded. The culprit in aluminum corrosion is usually salt. Salt air if you live near the ocean. Salt from roads that are salted in the winter. Salt from sweat that is secreted out of your body.
What can you do to reduce corrosion and protect your frame and aluminum parts? Number one, clean your bike periodically before corrosion sets in. Number two, clean your bike periodically before corrosion sets in. Once corrosion sets in, more drastic measures are required. Here's a process recommended by Charlie Cunningham to protect your bare aluminum frame.
It's much easier to work on a frame once all the parts have been stripped off it. Trying to work around parts takes longer and in the case of leaving your crank installed, you are guaranteed to get cut by a chainring at least once. With the frame stripped down, wipe off any excess dirt or grime before starting. Then with a 3M #7447 Scotch-Brite™ pad apply Fluid Film. I use the brush can because it doesn't take a lot and the brush top makes it easy to apply a bit to the pad. I also use nitrile work gloves, because your hands will get black from working with aluminum and even through the info on it says it's non-toxic, it's probably wise. There are also several other applications for a bicycle, but I'm not sure I'd use it for headsets. Could be a good option for seat posts if yours tend to become frozen.
I cleaned up three Cunningham frames recently. One frame took only one application of Fluid Film to clean it up. The other two took two and three applications respectively. I've also used it on hub shells that got a new wheel build. Sometimes corrosion builds up under the spoke elbow as it passes over the flange. A little Fluid Film and it will resist further corrosion.
Once the frame is cleaned up, the fluid film leaves behind a treatment that will help prevent further corrosion and with the #7447 Scotch-Brite™ pad, leaves the bare aluminum an nice buffed out appearance - not too polished looking. The next step is an application of Nu-Finish car polish (liquid in this case). This is applied with a finer Scotch-Brite™#7445 pad. This seals and gives a great bare aluminum look. This finish will last longer than if it's been polished to a mirror finish.
Here's some before and after shots of the worst of the three Cunninghams.
It will get new decals
There's still some deep damage, but the Fluid Film and polish are protecting the surface from further damage.
Some riders sweat profusely and some of them have what I call caustic sweat. It's just gnarly, damaging sweat. I've seen carbon headset spacers fused to steerer tubes. Top tubes on steel bikes that get eaten up resulting in rusted out cable stops. And in this case, aluminum handlebars with tiny holes that are eaten away by sweat. In this case, it's time for a new bar and instructions to remove bar tape, clean, and retape every 6 months or so. Easy to picture the end of a bar folding over because your sweat ate away at the bar.
(What's playing: Bob Dylan Freight Train Blues)