Mercury Optimax Models 135, 150, Direct Fuel Injection., Page 14Get this manual
MAINTENANCE
Corrosion Control Anode
The gear case has two corrosion control anodes (a)Another anode (b) is installed on the bottom of the transom bracket assemblyAn anode helps protect the outboard against galvanic corrosion by sacrificing its metal to be slowly eroded instead of the outboard metalsEach 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