How to fix water damage phone with corrosion

First thing to try is a battery replacement. However water damage in most cases requires lots of work and suitable professional tools to be taken care of properly. You should take off the logic board, remove EMI shields, decontaminate your board with ultrasonic bath, troubleshoot possible damaged components, solder everything back and reinstall the logic board checking that all the rest of components work as they should.

If battery replacement solves your issue without all of the above steps you might have been lucky but don't be surprised if the phone suddenly dies in a matter of a few weeks or months.