Galvanic corrosion is the silent destroyer of boats. When dissimilar metals are immersed in seawater (an excellent electrolyte) and electrically connected, the less noble metal corrodes preferentially while the more noble metal is protected. Without cathodic protection, underwater metals on a boat (propellers, shafts, through-hulls, trim tabs, rudders) can suffer accelerated corrosion that pits, weakens, and eventually destroys them. The cost of replacing a corroded propeller shaft far exceeds the cost of the zinc, aluminum, or magnesium anodes that would have prevented the damage.
Cathodic protection on recreational boats uses sacrificial anodes, which are pieces of a less noble metal bolted to the underwater hardware or the hull. The anode corrodes preferentially, "sacrificing" itself to protect the more noble metals. Understanding the galvanic series, choosing the correct anode alloy for your water type, sizing anodes appropriately, and inspecting them regularly is essential knowledge for any boat owner or marine technician.
The Galvanic Series and How Corrosion Occurs
The galvanic series ranks metals by their electrochemical potential in seawater. Metals at the anodic (active) end corrode preferentially when coupled with metals at the cathodic (noble) end. At the noble end are graphite, platinum, titanium, and passive stainless steel. At the active end are magnesium, zinc, and aluminum alloys. In between fall common marine metals: bronze, brass, copper, and mild steel.
For galvanic corrosion to occur, four conditions must exist simultaneously: two dissimilar metals, an electrolyte (seawater), an electrical connection between the metals, and an oxygen supply at the cathode. Remove any one and corrosion stops. On a boat, all four conditions exist naturally: the bronze propeller connects through the shaft to the engine, seawater is the electrolyte, and dissolved oxygen is abundant.
Stray current corrosion is a related but distinct problem caused by DC current leaking from the boat's electrical system into the water through underwater metals. Stray current corrosion is far more aggressive than galvanic corrosion and can destroy a propeller in weeks. An isolation transformer or galvanic isolator on the shore power connection, combined with proper wiring per ABYC E-11, prevents stray current from shore power.
Anode Alloy Selection by Water Type
The three sacrificial anode alloys are zinc, aluminum, and magnesium. Choosing the wrong alloy for your water type is the most common cathodic protection mistake.
Zinc anodes (MIL-A-18001K) are the traditional standard for saltwater. Zinc has a driving potential of about -1.05V vs. Ag/AgCl reference. Zinc works well in seawater but becomes passivated and ineffective in fresh water. Aluminum anodes (MIL-A-24779) provide higher driving potential (-1.10V), deliver more amp-hours per pound than zinc, and work across salt to brackish water. Magnesium anodes have the highest driving potential (-1.55V) and are designed for fresh water, where high resistivity requires more voltage.
Magnesium corrodes too rapidly in saltwater and will be consumed in months rather than the intended 1-2 year service life. Conversely, zinc passivates in fresh water and provides no protection at all. Aluminum is the most versatile choice for boats that operate in varying water conditions.
Sacrificial Anode Weight Calculator
Size zinc, aluminum, or magnesium anodes for hull, shaft, and propeller corrosion protection by water type and surface area.
Anode Sizing and Placement
Anode sizing is based on the surface area of protected metals and desired service life. The calculation uses current density requirements (milliamps per square foot) and anode capacity (amp-hours per pound). For example, 10 sq ft of bronze at 5 mA/ft² needs 50 mA of protection. With zinc at 370 Ah/lb for 1 year: (0.050A × 8,760h) / 370 = 1.18 lbs minimum.
In practice, recreational boat anode sizing follows manufacturer guidelines. Replace shaft anodes, trim tab anodes, and hull anodes with the OEM-specified size. If anodes are consumed more than 50% in a single season, they may be undersized or there may be a stray current problem. If anodes show less than 10% consumption, they may be passivated, electrically isolated, or coated with paint.
Each anode must be in direct electrical contact with the metal it protects (no paint or isolation between the anode and mounting surface). The vessel's bonding system (green wire connecting all underwater metals per ABYC E-11) ensures protective current can flow from the anode to all metals, not just the one the anode touches directly.
Sacrificial Anode Weight Calculator
Size zinc, aluminum, or magnesium anodes for hull, shaft, and propeller corrosion protection by water type and surface area.
Inspection Schedule and Replacement Criteria
Anodes should be inspected at every haulout and at least annually. Check consumption percentage, surface condition (active corrosion = working; smooth or painted surface = passivated or isolated), and physical integrity (cracked or loose anodes must be replaced immediately).
The standard replacement threshold is 50% consumption. At this point the anode still has capacity but is approaching end of life. Anodes consumed to 75% or more should be replaced immediately. An anode that looks barely touched after a full season is not working: check for paint on the surface, broken bonding wires, or wrong alloy for the water type.
Also verify bonding system continuity with a multimeter. Resistance between the anode mounting surface and each underwater fitting should be less than 1 ohm. If you have a silver/silver chloride reference electrode, protected metals in seawater should read between -0.80V and -1.10V. Readings more positive than -0.80V indicate insufficient protection.
Sacrificial Anode Weight Calculator
Size zinc, aluminum, or magnesium anodes for hull, shaft, and propeller corrosion protection by water type and surface area.