Thermostat Housing

I've just flushed the cooling system on my Tiger 885i and noticed whilst I was at it that the paint on water pump was flaking off. I flicked a bit and then a bit more and suffice to say, now all the paint has come off the water pump, its right back to metal. The reason I posted a message was because under the flaking paint there was a dry chalky reside that appears to be the reason behind the paint peeling off. Does anyone know what causes this to occur?

PS: the reason this message is titled 'thermostat housing' is because that is what I thought the water pump was

 

Cheers Pete