The picture shows your corrosion look at where the copper pipe is attached to pipe bracket, look at bracket corrosion, the bracket is not insulated from the copper pipe. Dissimilar metal corrosion is what is showing in that picture.
But impellor wear is caused by cavitation, at one of the water plants I worked we had a wall of shame with many impellors that looked exactly like that.
Link shows pictures of cavitation on pump impellors
