The problem in one paragraph

Put two different metals in water that conducts (salt especially) and you've built a battery: the less noble metal dissolves to protect the more noble one. Your customer's aluminum outboard or sterndrive is often the least noble big thing in the marina — unless sacrificial anodes volunteer first. Anodes exist to corrode so the drive doesn't.

Choosing anode material

• Zinc: the salt-water traditional choice; passivates and stops working in fresh water.
• Magnesium: fresh water only — dangerously overactive in salt (can overprotect and damage paint/aluminum).
• Aluminum alloy: works in salt, brackish, and fresh; the versatile modern default and what most drive manufacturers now recommend. When in doubt, aluminum-alloy anodes.

Never paint an anode, and never install one over paint or slime — it needs bare-metal electrical contact. Replace at roughly half consumed.

Reading the drive like a story

• Anodes vanish fast (a season or less): hot marina, stray current, or a bonding problem. Investigate, don't just fit bigger anodes.
• Anodes look new after years: they aren't connected (paint or corrosion under the mount) or the wrong alloy passivated. "Like new" anodes on a pitted drive is the worst sign on the checklist.
• Paint blistering and white powder (aluminum oxide) on the drive: protection lost — act now; drives are expensive and corrosion is compounding.

Stray current: the drive-killer

Galvanic corrosion is slow; stray DC current (a chafed bilge pump wire, a miswired dock neighbor) eats a drive in weeks. If damage looks fast and localized: check the boat's DC system for leakage, and consider the shore power system — recommend a galvanic isolator or isolation transformer for marina-kept boats. A silver/silver-chloride reference electrode and a multimeter can measure hull potential — the ABYC-style corrosion survey is a billable, valuable service.

Common mistakes

• Magnesium anodes left on when the boat moved to salt water
• Replacing anodes without cleaning the mounting surface to bright metal
• Ignoring the internal engine anodes (many engines carry pencil anodes in the cooling system — check the service manual for locations)
• Blaming "electrolysis" without checking for stray current

When to walk away

A drive with deep intergranular corrosion (metal flaking like pastry) is structurally suspect — replacement conversation, not a paint job.

Safety: never disconnect shore power grounding conductors to "fix corrosion" — that protective conductor is what keeps a fault from electrifying the water around the boat.