tdracer,
The engine companies try to prevent this with various coatings - I suspect whatever coatings Rolls is using on the Trent 1000 are not working as well as they'd planned...
I also suspect the alloy being used for the turbine blades may be more prone to corrosion than perhaps other alloys required by other engine manufacturers. In other words, if the coating deteriorates, the bare alloy needs to provide a margin for a period of time in terms of strength and corrosion resistance.