Actually, a number of current engines, although I do not know if our engine is one of them, have a problem when the engine is "washed". In recent years, many people have noted (and complained) about rust on the spark plug threads. You won't notice it until you change your plugs. On some cars with 100,000 mile plug change intervals, you need to use an air compressor to remove the plugs. They sort of rust to the head. Of course, the problem is stripping the insert threads, requiring them being drilled out and an insert installed to receive the plugs.

 

Our engines have aluminum heads - so likely not a worry. Aluminum doesn't rust like iron.

 

It's not the head, it's the plugs that rust.