If there is enough water damage that there was visible corrosion on the board, it is likely that there is similar corrosion under the EMI shields. Removing these shields is not a good idea, as you will likely break the logic board in the process.
Your best bet may be an ultrasonic cleaner. These are often sold as jewelry cleaners, but when used with a high purity isopropyl, will ensure that every microscopic bit of water vapor, and often the corrosion that goes with it, will be scrubbed away.
These can be a bit on the expensive side to get the high quality ones, but if they work, almost all of them are less expensive than a new phone.
Worth noting that I never suggest using these except as a last resort. While they can sometimes work miracles with phones that no longer work, they can just as easily ruin a phone that simply needed to be carefully dabbed out, and put into a bag of rice.
Hope this helps. I chose to invest in an ultrasonic cleaner for my cell phone repair shop, if there is a local tech repair shop around, it's possible they've invested in one as well.
Desmond
Mobile Rapid Response Unit