Considerations for Boat Protection
Galvanic Corrosion: What It Is
Galvanic corrosion is an electrochemical reaction that occurs when two or more dissimilar metals are in contact and submerged in an electrolyte—such as seawater or brackish water. This creates a galvanic cell, where electrical current flows between the metals.
How It Works
In this system, the less noble (more electrically active) metal becomes the anode and sacrifices itself by corroding. It releases electrons that flow toward the more noble metal, the cathode, which is protected from corrosion.
Why It Matters for Boats
Boats often contain a mix of metals—steel hulls, brass fittings, aluminum drives, etc.—all of which are at risk when submerged. Without proper protection, the less noble components will corrode quickly.
Sacrificial Anodes to the Rescue
By installing sacrificial anodes —typically made of zinc, aluminum, or magnesium —the corrosion is redirected to the anode instead of valuable components.
Example:
A steel hull with brass fittings in water will cause the steel to corrode. But with a zinc anode installed (which is less noble than both), the zinc corrodes instead, preserving both the steel and brass.
Choosing the Right Anode Material
Each material has unique properties suited to specific marine environments.
- Zinc → Saltwater
- Aluminum → Salt or brackish water
- Magnesium → Freshwater only
Factors That Influence Corrosion
- Water salinity
- Pollution levels
- Flow rate or turbulence
- Oxygen concentration
- Temperature
- Metal surface area and coatings
Anode Size & Lifespan
- Surface area determines how much protective current the anode provides.
- Weight determines how long the anode will last.
Choosing the right size and number of anodes ensures long-term protection for your vessel’s components.
Understanding the Noble Scale
The Noble Scale ranks metals based on their electrochemical activity in an electrolyte (like seawater), using a silver/silver chloride half cell as the reference point.
Metals that are more negative on the scale are less noble and more likely to corrode. Those that are more noble (closer to 0 mV or positive) are less reactive and more resistant to corrosion.
NOBLE SCALELEAST TO MOST NOBLE | |
|---|---|
| MILLIVOLTS | METAL OR ALLOY |
| -1580 | Magnesium |
| -1100 | Aluminum with Indium |
| -1050 | Zinc |
| -860 | Cadmium |
| -790 | Mild Steel |
| -750 | Aluminum Stern Drive |
| -500 | Tin |
| -450 | Naval Brass |
| -340 | Copper |
| -240 | Lead |
| -80 | Silver |
| 0 | Gold |
How to Read It
- The more negative the value, the less noble the metal—and the more likely it will corrode.
- The anode should always be less noble than the metal it’s protecting.
Voltage Example
If you’re using zinc (-1050 mV) to protect brass (-450 mV):
Zinc (-1050) – Brass (-450) = -600 mV
This -600 mV differential is what drives cathodic protection, causing the zinc to corrode and the brass to remain untouched.
Target Voltage for Cathodic Protection
To effectively protect metal components from galvanic corrosion, a negative voltage shift of at least -200 mV (or -0.2 V) relative to the least noble metal in the system is generally required.
This ensures that current flows away from the protected metal and into the sacrificial anode.
The Risk of Overprotection
Too much voltage can be just as harmful as too little. Overprotection may lead to:
- Alkali corrosion in aluminum components
- Delignification in wooden hulls (breakdown of wood fibers)
- Blistering of protective coatings
One common cause is using a magnesium anode in brackish or saltwater, where it generates too high a voltage. Another is stray DC current from faulty wiring or equipment onboard or at the dock.
Recommended Cathodic Protection Ranges
| Hull or Component Type | Target Range (mV) |
|---|---|
| Wood Hull | –500 to –600 |
| Fibreglass Hull | –550 to –900 |
| Steel Hull | –800 to –1050 |
| Non-metallic Hull with Aluminum Drive | –900 to –1050 |
| Aluminum Hull | –900 to –1100 |
What Determines Voltage Shift?
Achieving proper protection depends on the amount of current the anode provides in relation to the area of metal exposed. This is affected by:
- Anode surface area (which determines amperage)
- Proximity to the protected metal
- Coating quality on the hull
- Water temperature and speed
Anode Capacity & Lifespan
Pure zinc has a theoretical capacity of 372 amp-hours per pound, but in practice it runs at about 95% efficiency giving ~353 Ah/lb.
- 25 lbs × 353 Ah = 8,825 Ah
- 8,825 Ah ÷ 2 amps = 4,412.5 hours
- 4,412.5 hours ÷ 24 = ~184 days
Example:
A 25 lb zinc anode (e.g., Z26) delivering 2 amps would last:
Rule of Thumb:
Be prepared to replace anodes when they reach 50% consumption to maintain effective protection.
Types of Anodes & Best Applications
Magnesium
- Most active on the Galvanic Scale
- Best for freshwater only
- Inboard drives on fiberglass or steel hulls
- Outdrives on wood, fiberglass, aluminum, or steel
- Freshwater with stainless steel props
⚠ May overprotect in salt or brackish water
Zinc
- Reliable in saltwater
- Preferred on wood hulls with inboards
- Compatible with outboards and outdrives
- Best when vessel stays in saltwater
✔ Traditional marine standard
Aluminum
- Versatile in salt, brackish & freshwater
- Ideal for aluminum sterndrives
- Recommended for aluminum props in freshwater
- Works with stainless steel props
✔ Long lifespan & wide compatibility
Docked Vessels & Shore Power Corrosion
When a boat is connected to shore power, the ground wire creates a shared electrical pathway between your vessel and others on the dock. If nearby boats aren’t properly protected with anodes, your boat’s system can end up doing the work—protecting theirs at the expense of your own.
Over time, this causes your anodes to deplete faster. Once they’re consumed, your boat’s underwater metals—especially those higher on the galvanic scale—are left vulnerable to corrosion.
How to Prevent It
The most effective solution is to install a galvanic isolator on the shore power ground wire. This device blocks low-voltage DC current, which is responsible for dockside galvanic corrosion, while still maintaining the essential safety grounding required for your electrical system.
With a galvanic isolator in place, your anodes will continue protecting your vessel—and only your vessel.
Stray Current Corrosion: A Hidden Threat
Stray current corrosion is one of the most aggressive and destructive forms of marine corrosion. Unlike galvanic corrosion, it doesn’t require two dissimilar metals—it happens when unintended electrical current enters the water and begins to attack any nearby metal.
Even a small DC voltage leak can completely override your anode system. In severe cases, stray current can destroy metal hardware in a matter of hours.
Common Sources of Stray Current
- Faulty or exposed DC wiring inside the boat
- Improperly grounded shore power systems
- Electrical leaks from neighboring vessels or marina infrastructure
How to Protect Against It
- Test your boat regularly for DC voltage leaks
- Keep DC wiring above the bilge to prevent water contact
- Ensure AC systems are properly insulated to reduce the risk of electric shock
- Use a proper bonding system:
All electrical and underwater metal components should be connected to the battery's negative terminal or its ground bus, equalizing voltage across the system and minimizing current flow into the water
