Mercury Mariner 4 and 5HP 4-Stroke Outboards Service Shop Manual 1999, Page 16Get this manual
MAINTENANCE
Corrosion Control Anode
Your outboard has corrosion control anode installed to the gear caseAn anode helps protect the outboard against galvanic corrosion by sacrificing its metal to be slowly eroded instead of the outboard metalsThe anode requires periodic inspection especially 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
Spark Plug Inspection