Here you can see a bearing housing register with fairly extensive corrosion damage.
Corrosion damage like this results in a loss of seal between the bearing housing and turbine housing, and can cause a series of knock on effects, including damage to the bearings, turbine, compressor wheel, and housing.
This kind of damage is really common on Porsche units, because the turbo is positioned so low down on the vehicle.
This means that the turbo ends up catching a lot more wet dirt and road debris thrown up from the back wheels than in other turbos, and over time, this causes corrosion damage to the surfaces of the bearing housing.
Whilst minor corrosion damage can sometimes be repaired, in the majority of cases, the parts will need to be replaced.