Water damaged iPhone 6 won't restore to factory settings

I'm doing my first ever water damage repair for a client and I'm having some trouble. Apparently the phone worked for the first two weeks, and he had the screen replaced by another repairer but then it stopped working completely. I changed the battery, and when I tried to power the phone on, the Apple logo appears for about 4 seconds then the phone powers off. I tried to do a hard reset, and the connect to iTunes logo appeared but plugging it in did nothing, so I replaced the charging port. Restoring the phone to factory settings appears to work, but then when the phone restarts after restoring, I just get the apple logo for a few seconds and then iTunes says that it has found an iPhone in recovery mode.