I recently purchased an M9 from a seller who described the sensor as replaced. Upon receiving the camera I checked the sensor info in the debug menu and was shown this info. I'm guessing the sensor has not in fact been replaced most likely?
The corrosion can be bad enough that it affects the sensor wiring, resulting in half the sensor failing but your sample image shows an issue at a quarter in. Not sure if that's due to the same issue.
Those are not corrosion, I'm sure the it's the glass that were replaced and those looked like tiny particles trapped under the glass during replacement process.
A lot of people including professional tried to DIY due to the process was well documented by the Chinese as reference, unfortunately most were not done in dust free chamber like Kolari or a well known shop in Beijing.
You can try to do a sensor cleaning (best to have someone experienced to do it), or just use it as it is like I did with my M9, which I enjoyed thoroughly until the glass eventually developed some kind of coating failure or separation...
Those spots look like regular dust. But as mentioned in the previous post, if it was a non-Leica sensor stack replacement, it could be dust trapped under the stack. Leica replaced the entire sensor rather than swap out the cover glass.
Corrosion spots when closely examined look more like donuts.
Here's what my M9 looked like before it was sent in just before the free replacement service was ended by Leica. The second and third are crops of the first, which I believe was the full sensor area:
In the last image (crop) you can see regular dust that is just black and fuzzy-edged compared to the line of 'donuts' that is cover glass corrosion.
The vertical line in your image, if it's dead down the centre of the image (it's unclear if your sample image is the full image or a crop of it), is potentially a sensor bifurcation issue due to how the FF sensor comprises of two halves when the chip was optically fabricated from the silicon wafer.
Normally this might only be visible when an image is pushed pretty hard but there were a few instances with my M9 where it became visible with less aggressive post production.
Unfortunately that's also indication of the fragile gold wires corroding resulting in irregular reading from both sides, which is worse than those tiny specs and render the sensor unusable...
