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Ok, so you are saying that if I were to place rust flakes or chips in a small sealed container of, say, PB Blaster, they would degenerate into sort of a rust powder? With steel in contact to aluminum, the aluminum will preferentially corrode, acting as a sacrificial anode for the steel.

Right. In that case, is the aluminum corrosion product aluminum oxide? That's pretty hard stuff, or at least the particles of it are....

Ok, so you are saying that if I were to place rust flakes or chips in a small sealed container of, say, PB Blaster, they would degenerate into sort of a rust powder? ...

I have never done that, What I do know is that if I have a rusty steel surface, most lubricants will help the rust break down into smaller particles if I apply some shear forces. Both this effect and the lubrication is what makes penetrating oils work.

I can think of several potential mechanisms. The study of surfaces interacting, wear and lubrication is called Tribology. The boys who study that can probably give you a better answer.

Use the acetone/ATF mix instead of PB Blaster. The results are superior.

...is the aluminum corrosion product aluminum oxide? That's pretty hard stuff, or at least the particles of it are...

You are correct. Aluminum oxide is the corrosion product and it is very hard. Aluminum oxide particles are commonly used as an abrasive. Corundum is one common name for the abrasive. Hardness does not necessarily mean high friction. Small aluminum oxide spheres are a standard item of commerce and I have used them as ball bearings and as spacers in various mechanisms. For mechanical uses, specify "synthetic ruby spheres". The red color makes them more visible and easier to assemble. Clear ones are known as "synthetic sapphire" are water white and crystal clear. If optical properties are important, they are the ones to use.

In any event, the material makes great ball bearings in the right application. So hardness is not necessarily a bad thing.