In the first two images, you can see corrosion damage to the seal area and inlet of the compressor cover. This type of damage is extremely common with Porsche units as, with the turbos sited so low down on the vehicle, they catch all the road debris that gets flung up from the back wheels. Over time, this causes the surfaces of the compressor cover to corrode.
The seal area is where the seal plate or bearing housing locates into the compressor cover, and when this becomes corroded, it ceases to be airtight and will leak.
The third image illustrates corrosion damage to the compressor cover profile, which can occur when the vehicle is sat idle for long periods of time. It is also very common on marine applications and on plant machinery that is open to the elements. Corrosion to the profile increases the gap between the compressor cover and the compressor wheel, which results in a loss of efficiency.
This image illustrates scoring out of the compressor cover profile, and occurs when excessive wear and movement in either the thrust bearing or journal bearing (or both) causes excessive movement in the rotor. In turn, this causes the compressor wheel to rub against the compressor cover, scoring out the profile. This makes the gap between the compressor cover and the compressor wheel too large, resulting in a loss of efficiency/power and excessive noise.