Welcome to EnviroDIY, a community for do-it-yourself environmental science and monitoring. EnviroDIY is part of 
The Adafruit CP2104 Friend has the correct 6-pin layout to work perfectly with the FTDI interface header on all Mayfly versions. The attached photo shows one connected to a Mayfly 1.0 but you can connect it to the Mayfly v0.5b the exact same way. Give that a try and see if you’re able to communicate with the Mayfly board. If not, it’s possible the corrosion on the board is the cause of your problems. If that’s the case, you could try cleaning the corrosion off using acetone or 91% isopropyl alcohol and an old toothbrush. But if the corrosion caused a part on the board to be damaged when it was powered up, then it’s like unsalvageable. The Mayfly v.05b boards have been discontinued as have the v1.0 boards, but the v1.1 boards will hopefully be listed as single items on Amazon sometime soon, but I don’t know the date yet.
