Forgot one thing in my previous post:
Your quote: How does the corrosion happen, is it present prior to assembly? That makes it a QA problem.
Corrosion generally happens over time in engine operation. As the article stated, the problem was found during engine testing. One of the things you do, or should do, is have an engine running on a test stand, accumulating cycles far advance of those engines in the field, it is a "Best Practice". At various points of cycling this engine, it is completely dis-assembled and reviewed for any problems that require attention so that corrective action can be taken on field engines before an "event" takes place. So I think this is how RR discovered the problem and decided it was best to change out the defective part/parts in ANA's engines ASAP to avoid another in service "incident".