What causes this familiar finding on watch movements?

I see this quite often, and I never know if it's likely due to water damage, corrosion, or maybe years of overhauls and plating wear from the cleaning solutions. Any insights?

Sometimes there is wear to the rotor from rubbing against the case-back, but that has a different look since the worn areas are more localized.

Exactly.

Can you please explain what you mean by this? Maybe give examples of what solutions you are referring to?

I guess Dennis didn't have time to elaborate, but I think I found the cleaning solution he was referring to:



At least it was better than this other one I saw for sale:



😉

So let's really look at what's going on with that specific movement:



There are three areas of concern that I have pointed out in regards to the movement finish. The top left area on the balance cock is from rotor wear - bottom edge of the rotor scraping the outer edge of the balance cock. You can see the yellow brass showing through.

The area on the right is odd - it's very copper coloured, and since the plates are rhodium plated brass, it's strange to see such a copper colour there, but the pattern of the marks looks again like some sort of abrasion.

The third area is on the rotor's weight. Although I believe the squirrel meant to say this was "brassing" the problem with that theory is that the underlying material on the weight is not brass. Most of the masses used on oscillating weights are made of sintered tungsten, and on more expensive watches they are often made of gold, like my old Blancpain was with an 18k gold rotor:



So as you can see here with the Cal. 750 series oscillating weight that I replaced the pinion on, the plating had partially come off the underside of the weight, and it's certainly not brass showing underneath:



So the marks that are on the rotor of the Rolex above are most likely just dirt/tarnish rather than missing plating.

Plating does come off, but to suggest it's because of the use of harsh chemicals used for cleaning is far fetched. Although excessive cleaning can cause the failure of plating, it's spending too much time in an ultrasonic cleaner than can do the damage, rather than the cleaning solutions. In addition, plating is certainly not permanent, so the older the watch is, the more likely it is that the plating bond will fail. I have also encountered patterns in plating loss than can only come from one part sitting on top of another part during the cleaning process prior to plating, leading to a poor bond of the plating to the base material.

Unless someone is using a very non-standard cleaning solution to clean movements, this not likely related to the solutions at all.

Cheers, Al