TC
You might indeed be correct in your thinking / what RR say but maintenance company has only washed them every 50 / 100 hours which in my case is every 4 months. They look after 20 RR engined machines and have never had a compressor let go in over 30 years. I would agree with you on the old compressors which didn't have coated wheels ( had to split the bloody thing every 6 months which in a 500 means engine out ) or a private owner flying once a fortnight.The problem with sticking distilled water through it means you have to light it off again to dry it out ( see Dick's post ) Now a turbine on a RR C20 B has on N1 1750 hours or 3000 starts so by washing after every day would almost half your starts, let alone N2 4500 hours or 6000 cycles and compressor 9000 cycles. My feeling is the less thermal shock ( start 0 to 800 degrees in a few seconds ) is better than trying to stop corrosion that in many peoples consideration does not exist or perhaps better is not what the manufacturer believes to be correct. Example would be my compressors, no corrosion in 25 years in over 10 different helicopters with about 6000 hours between them. Actually much more likely to have problems with the crap RR compressor case halves that I am sure are made of Swiss cheese, hence have gone to carbon fibre ones ( first in UK to have them ) which are streets better, only issue is sending the compressor to a RR AMC who hate seeing them as they don't need changing so often ( so no $ 10k bill ).
Only compressor problems I have had have been after overhaul where the AMC had not balanced the impeller properly , caused the anti ice seat flange to crack off the case completely n less than 300 hours. In other case the impeller cracked due to vibration due to guess what the manufacturer not making it properly !