Sacrificial Anodes & Galvanic Corrosion

Printable checklist: Launch day

When dissimilar metals sit together in conductive water—especially salt water—the less noble metal can corrode to protect the more noble one. Discover Boating (NMMA) calls that galvanic corrosion (and, in older owner pages, electrolysis). A sacrificial anode, often nicknamed a “zinc,” is a replaceable piece of metal bolted to the hull, engine, shaft, sterndrive or prop so it wastes away first instead of the expensive hardware.

This page summarizes official Discover Boating / NMMA pages (and the NMMA semi-annual checklist PDF) only. Those pages talk about sacrificial anodes as zincs and about bonding and galvanic isolators. They do not publish aluminum-vs-magnesium alloy pick tables, amp ratings, DIY bonding diagrams or brand picks—so we do not invent those here. Use the engine, drive and anode makers’ published alloy for your water and hardware. Pair this guide with thru-hulls, seacocks and hose clamps, bilge pumps and high-water alarms, electrical and shore power safety, marine battery care, spring commissioning, winterizing, boat troubleshooting and when to call a pro, the printable Launch Day checklist, and the True Cost tool for yard and anode budgeting.

What galvanic corrosion does

Discover Boating’s saltwater-corrosion page: salt and minerals let salt water conduct electricity. Put dissimilar metals in that water—for example an aluminum outdrive next to a bronze propeller—and the water acts like a battery. The less noble metal starts to corrode to protect the other. Leaving salt crystals on metal after a rinse also holds moisture and speeds ordinary rust.

Discover Boating’s electrolysis page uses the same battery idea: current flowing between metals in conductive water removes metal from one piece. If that piece is your propeller, you lose hardware. The fix owners see on haul-outs is the ashen, pitted metal lumps on shafts, rudders or the transom—zincs.

What a sacrificial anode (“zinc”) is

Discover Boating glossary: zinc anodes are small pieces of zinc attached to metal boat and engine parts to help protect them from corrosion due to electrolysis when dissimilar metals sit in a saltwater solution.

Saltwater-corrosion page: a sacrificial anode, or “zinc,” is bolted onto the hull, engine and props. Because it is the “weaker” metal, it is eaten first—sacrificing itself to protect more expensive parts. Check them regularly and replace them when they are about halfway gone.

First-year maintenance page: unpainted metal anodes attached to sterndrives or inboard prop shafts and rudders prevent corrosion. Replace them when they are 50 percent deteriorated.

Electrolysis page: zinc is used because it has a higher voltage in the water, so current prefers to leave the zinc rather than the propeller. Zincs must be electrically connected to the items they protect. Often that is automatic because the zinc bolts directly to the shaft or underwater housing.

Alloy note (what sources do not publish)

Discover Boating / NMMA owner pages fetched for this guide call these parts zincs (zinc metal) or, once, “unpainted metal anodes.” They do not publish a zinc-vs-aluminum-vs-magnesium selection chart, freshwater-vs-saltwater alloy recipe or amp rating. Follow the engine, sterndrive and anode makers’ published instructions for the alloy that matches your water and underwater metals. We do not invent that chart here.

Bonding basics (owner-level)

Electrolysis page: on non-metal boats, a copper bonding wire inside usually connects underwater metal items so they share protection from the zinc anodes. Engines typically use the metal frame as the negative battery connection and connect to the prop shaft, so the engine and the negative side of the 12-volt system are part of that bonding path. Bonding to rigging, when present, is described there for lightning path into the water—not as the primary electrolysis protection.

Saltwater-corrosion page: a proper bonding system connects metal parts—from the engine to the thru-hulls—so they share the same electrical potential and current does not jump between them and corrode the boat.

Electrical-system safety page: green or green-and-yellow wires bond metal hardware to help protect it from galvanic or electrical corrosion. Inspect those wires for corrosion or damage. Bonded hardware also serves as a backup earth ground for 120-volt AC. AC ground wires and bonding-system wires must be tied together in only one place. Shore-inlet ground ties into bonding through a galvanic isolator, which blocks tiny corrosion currents but passes fault current that could otherwise cause electrocution. Galvanic isolators can fail over time.

NMMA semi-annual checklist (corrosion prevention): keep zincs adequate; check through-hulls for proper bonding; grounds to zincs if required.

We do not publish DIY bonding diagrams, wire sizes or isolator wiring steps. How boat AC, shore AC and bonded hardware tie together belongs with a marine electrician (electrical and shore power safety). Discover Boating’s electrical shock-hazards brochure: at least yearly, have a qualified marine electrician verify the AC shore-power grounding system (including the connection to the DC negative), and verify that any galvanic isolators meet current ABYC standards and still work.

When to inspect and replace

Every few months (Discover Boating saltwater corrosion): inspect zincs and replace them if worn; check the propeller for pitting or damage.

At about half gone / 50% deteriorated (saltwater corrosion + first-year maintenance): replace the anode. Do not wait until nothing is left.

Semi-annual checklist: confirm zincs are still adequate and bonding at through-hulls looks proper (thru-hulls, seacocks and hose clamps).

Spring / haul-out / bottom work: include anode condition with other underwater hardware when you commission or paint (spring commissioning).

Normal life expectancy (electrolysis page): under normal circumstances zincs should last at least a year if they are working, and much longer if there are no problems. If they are sacrificed in a shorter period, find where external current is getting in.

Paint, wax and freshwater rinse after salt use help ordinary corrosion (saltwater-corrosion page) but are not a substitute for working anodes and bonding.

Stray current and marina neighbors

If other currents enter the bonding circuit, they can overpower the small voltage from the zincs (electrolysis page). The usual destructive clue: zincs eaten very quickly.

Shore power / stray current: docks with bad wiring may misuse the ground lead; shore power is a common source of external current. Persistent electrical oddities (flickering lights, blown fuses, a battery that will not hold a charge) can also signal a grounding or stray-current problem (saltwater-corrosion page)—see electrical and shore power safety and marine battery care.

Neighbor boats: boats on either side may share a dock ground path; current can take a shortcut through your underwater metal and exit via your zinc even when you are not the source of the fault (electrolysis page).

Zinc “fish” at the dock (electrolysis page): large lumps of zinc (often cast as a fish) hang in the water on a copper wire with an alligator clip tied to the boat’s negative bonding circuit. They are cheaper to replace than shaft zincs while you sit in a problematic slip. Clipping to shrouds or a rail only works if those parts connect to the negative bonding system.

Red flag (saltwater corrosion): if zincs installed a month ago are already halfway gone, call the pros—likely stray electrical current accelerating corrosion. Other red flags on that page: blistering paint (especially on aluminum), props/shafts/outdrives pitted like a golf ball or flaking metal.

When to call a yard or marine electrician

Stop owner DIY and get professional help when:

  • Zincs disappear far faster than a year (or are halfway gone within about a month of install)—suspect stray current or marina path (Discover Boating).
  • You see pitting, flaking underwater metal or blistering paint from corrosion underneath.
  • Bonding wires are corroded, broken or you cannot confirm how AC ground, shore inlet and bonding/galvanic isolator tie together → marine electrician (troubleshooting; electrical and shore power safety).
  • Through-hull bonding or fittings look doubtful—pair with thru-hulls, seacocks and hose clamps; flooding from a failed fitting is a separate emergency (bilge pumps and high-water alarms).
  • You need anodes fitted to a shaft/drive, a bonding repair or a galvanic isolator checked to ABYC as the electrical brochure describes.

Budget anode replacements, haul-outs and electrician visits in the True Cost tool and true cost of ownership guide. Put zinc/anode checks on the same seasonal pass as spring commissioning, winterizing and the Launch Day checklist.

Sources

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