2003 ST 4 Stroke 9.9/15HP Johnson outboards Service Manual, Page 39Get this manual
PERIODIC MAINTENANCE ANODES AND BONDING WIRES
ANODES AND BONDING WIRES INSPECT EVERY 50 HOURS (3 MONTHS)ANODES If 23 of anode has corroded away, replaceThe anode should be periodically cleaned with wire brush to ensure maximum effectiveness
IACAUTION I
Never paint the anode
NOTE: The anode securing bolt should be covered with silicone seal