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I have a set of Rotor 3D cranks and I've just noticed that the non-drive-side crank arm is corroding at the end and around the pedal bore, beneath the surface finish. The drive-side arm is fine.

I would estimate that these cranks have done around 100000 km in their lifetime. Is it normal that they should start corroding like this?

I'm concerned that this pedal is never going to come out...

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    It might be the time you tried to get the pedal out. You can always put it back. But I guess with such a large mileage you had to take it out for service regularly and grease the threads anyway. If not it has to be stuck by now. There might be some effect from the contact of two different metals. I know aluminium rust mainly from caves end there once it starts it can eat the whole carabiner completely.
    – Vladimir F Героям слава
    Commented Apr 25, 2020 at 9:32
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    The large patch on the crank is not corrosion, it's wear, possibly from sand hitting the crank at the bottom of the stroke. Can't tell about the discoloration around the pedal shaft -- looks like the crank surface may have flaked a bit (not unusual considering the stress at that point), or else it's "salts" that migrated from the steel pedal shaft.
    – Daniel R Hicks
    Commented Apr 25, 2020 at 11:58
  • Do you live in a place where salt is applied to the road in winter or icy conditions? Or do you live near the ocean? All I see is rust on the pedal's axle shaft, not on the crankarm
    – Criggie ♦
    Commented Apr 25, 2020 at 12:33
  • I'm concerned that this pedal is never going to come out... Remove, clean, grease threads, reinstall.
    – Andrew Henle
    Commented Apr 25, 2020 at 15:11
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    @Criggie Yes, I moved to Munich a year ago, where they're pretty diligent about gritting roads in the winter. Previously I lived the UK. I think it's happened since then. I guess I should be washing my bike more regularly in the winter.
    – Will Vousden
    Commented Apr 27, 2020 at 5:05

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Why have my crank arms started corroding?

In the content of the post concern about corrosion between the pedal and crank is discussed.

Summary: The crank arm has been abraded allowing the aluminum to corrode (oxidize). The pedal and crank connection is corroding due to exposure to water or a water/salt combination.

enter image description here

The issue in the orange circle is abrasion followed by corrosion.
Something has abraded the black coating off of that area of the crank, exposing it to air and allowing aluminum oxide to form. In aluminum - as Criggie says in the comments - the oxidation forms a protective coating and does not eat into the metal over time like iron oxide, rust.

Causes for the abrasion seem to be identified in the comments as road grit. Something is rubbing on that spot. The corrosion is natural for aluminum exposed to the air.

You could

  • lightly sand the abraded area, removing the corrosion and any paint that is not adhering to the crank arm
  • Tape off the rest of the crank arm
  • Use an appropriate paint to cover the area

The issue in the red circle is corrosion between the pedal and the crank arm. Pedal axles are made of steel, the crank is aluminum and they will seize up if something isn't done.
During assembly the pedal threads should have been lightly greased to prevent seizing.
It would be best to:

  • Remove the pedal
  • Clean the pedal - use a wire brush or steel wool on the rusted surfaces
  • Clean the crank arm
  • Put a light coating of grease on the threads
  • Reinstall the pedal.

The cause of the pedal to crank arm corrosion is exposure to water. Water is a solvent.

We need to take the statement "Water is the universal solvent" with a grain of salt (pun intended). Of course it cannot dissolve everything, but it does dissolve more substances than any other liquid, so the term fits pretty well. Water's solvent properties affect all life on Earth, so water is universally important to all of us.
www.usgs.gov - Water, the Universal Solvent

Water and salt is especially deadly.

The galvanic corrosion of aluminum is usually mild, except in highly conductive media such as slated slush from road deicing salts, sea water and other salty electrolytes. The contact area must be wetted by an aqueous liquid or humidity in order to ensure ionic conduction.
Fixing Corrosion Between Anodized Aluminum and Steel

Bikes can get wet but should not be left wet. A wet bike needs to be able to dry out or to be dried off.

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  • Paint does tend to need primer. I wonder if the OP should either a) use clear coat after sanding off the oxide layer, which is what they normally do with polished aluminum parts, or b) leave it? Aluminum oxide layers, unlike with iron and steel, don't penetrate further into the metal.
    – Weiwen Ng
    Commented Dec 15, 2020 at 18:05

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