Mercury Mariner 25HP 4-Stroke Outboard Service Manual 1997, Page 18Get this manual
Corrosion Control Anode Your outboard has four corrosion control anodesAn anode helps protect the outboard against galvanic corrosion by sacrificing its metal to be slowly eroded instead of the outboard metalsEach anode requi res periodic inspection especially
2Two anodes (c) are installed in the engine blockRemove anodes at locations shownInstall each anode with rubbe seal (d) and cover (e)Tighten bolts to 70 lbin(8 Nm) torque
in salt water which will accelerate the erosionTo maintain this corrosion protection, always replace the anode before it is completely erodedNever paint or apply protective coating on the anode as this will
reduce effectiveness of the anode