No I never have seen a such picture Wow ! Thanks for my education! Do you know how old this pump was? The owner must have used acid for coolant. Yet I still think the thermostat is the culprit.
It's caused by lack of coolant maintenance. Once the coolant breaks down the engine starts to rust from the inside out. The severe erosion on the water pump is usually a combination of corrosion and abrasion from the rust particles in the water (they act like sandpaper).