When should I replace the anodes on my outboard?

Outboard anodes: when to check and replace them · Symptom: The gray metal blocks on your lower unit or engine bracket look eaten away, crusty, or white and chalky, or you're seeing new corrosion and paint bubbling on the motor itself.

Quick answer: Sacrificial anodes are the dull gray metal blocks on your outboard (usually on the gearcase just above the prop and on the bottom of the engine bracket). They corrode on purpose so your motor doesn't. Replace them when they're about 50% gone. Mercury recommends checking them every time you pull the boat out, or at least monthly if it stays in the water. Never paint them, and clean off white chalky coating or growth, because a coated anode stops protecting the motor.

Likely causes (most to least common)

Normal wear: the anode is doing its job and is now about half consumed
Anode coated with a white chalky (calcified) layer, marine growth, or a thin film of scum, so it can't work
Anode painted over (paint stops it from working)
Loose anode or a broken ground connection, so it no longer protects the motor
Wrong anode metal for your water (magnesium is for fresh water only; aluminum or zinc for salt or brackish water, per your manual)
Boat kept in the water, or stray electrical current at a marina, which wears anodes much faster

Safe owner checks & fixes

Find the anodes. Yamaha says they're usually on the lower unit just above the prop and on the bottom of the engine bracket; some motors also have one on the trim tab or trim rams. Your owner's manual shows the locations.
Check them every time you haul out. Mercury says to check boats kept in the water at least monthly during the season.
Replace any anode that is about half gone, cracked, or loose. Buy factory-recommended replacements in the right metal for where you boat.
Clean coated anodes. Mercury's FAQ says to use sandpaper, not a wire brush, to remove a white chalky coating; Yamaha suggests a brush and soap during your regular washdown to remove scum.
Never paint or cover anodes, and don't paint around them so that they're covered.
When installing a new anode, clean the mounting surface to bare metal so it makes good contact, and tighten the screw firmly. Check any ground wires attached to the anodes (Yamaha's manual says to inspect them on equipped models).
If your anodes wear away very fast, or not at all, ask a dealer or marine electrician to check for stray current or a bad ground.

Parts / tools

Correct replacement anodes for your model and water type, screwdriver or socket set, sandpaper or a scuff pad, owner's manual.

Call a marine mechanic when

Anodes are disappearing within weeks; the motor has bubbling paint or pitting even with good anodes; you can't get an anode bolt out; or the anode is internal (inside the power head). Yamaha says internal anodes should be inspected and replaced by a dealer.

Sources

Values not stated here are model-specific; check your owner's manual.