25th Sep 2026
Galvanic corrosion is driven by the small natural voltage created when dissimilar electrically connected metals are immersed in an electrolyte such as seawater. Stray-current corrosion is driven by current from an external electrical source—commonly a fault in a boat’s DC wiring—leaving metal and passing through the water. Both can destroy underwater parts, but stray DC current can act far faster and is not cured simply by fitting more sacrificial anodes.
A shore-power earth connection can also link a boat’s underwater metals to neighbouring boats and marina structures, enabling low-voltage galvanic current. A correctly selected galvanic isolator is intended to block that low-level path while preserving safety-earth operation under a fault. It does not repair a damaged bilge-pump cable, leaking underwater light or other onboard DC fault.
Because electrical and corrosion tests can involve live AC/DC systems and safety earths, severe or unexplained corrosion should be investigated by a qualified marine electrician or corrosion specialist.
Do not disconnect protective earth to “test” corrosion
The shore-power earth is a life-safety conductor. Defeating it can expose people and metalwork to lethal fault voltage. Use standards-compliant isolation equipment and competent testing.
Contents
- The difference in one table
- How a galvanic cell forms on a boat
- How stray-current corrosion happens
- Is “electrolysis” the right word?
- Warning signs that point towards stray current
- Signs more consistent with galvanic loading
- What a galvanic isolator does—and does not do
- A safe diagnostic pathway
- Can a neighbouring boat cause corrosion?
- How to reduce both corrosion risks
- Frequently asked questions
- Technical references
The difference in one table
| Feature | Galvanic corrosion | Stray-current corrosion |
|---|---|---|
| Energy source | Natural electrochemical potential between dissimilar metals | Battery, charger, shore-powered DC supply or another electrical source |
| Required circuit | Two metals electrically connected and immersed in an electrolyte | Current leaves a conductor/metal into water and returns to its source by another path |
| Typical speed | Often gradual, though environment and area ratios can accelerate it | Can be extremely rapid—sometimes severe damage in days or weeks |
| Role of sacrificial anodes | Designed to become the active metal and protect connected components | May be overwhelmed; they do not remove the electrical fault |
| Role of galvanic isolator | Can block low-voltage galvanic current through a shore-earth conductor when correctly installed | Does not correct onboard DC leakage; may not address current entering through other paths |
| Typical evidence | Predictable anode consumption, corrosion at dissimilar-metal couples, berth-related changes | Sharp localised pitting, abrupt onset, intense anode loss or attack near faulty electrical equipment |
| Primary remedy | Correct materials, coatings, anodes, bonding design and shore-power isolation | Find and repair the electrical fault, then restore damaged protection |
How a galvanic cell forms on a boat
A galvanic cell needs four elements: an anodic metal, a more noble cathodic metal, an electrical connection between them and an electrolyte. A bronze propeller connected to a stainless shaft in seawater is one familiar example. Without protection, the less noble area can corrode. A sacrificial anode deliberately becomes the most active metal and supplies the corrosion reaction instead.
The area relationship matters. A small exposed aluminium area connected to a large stainless surface can suffer concentrated attack. Coatings reduce exposed area, while a stainless propeller or new metal accessory can increase the cathodic load. That is why a propeller change or damaged drive paint can make anodes disappear faster.
Read cathodic protection for boats for a fuller explanation of how hull, shaft, rudder and drive systems are protected.
How stray-current corrosion happens
Stray DC corrosion occurs when current escapes from its intended wiring into the water through a submerged or wet conductive part. It then returns to the power source through another underwater metal, bonding conductor, shore connection or vessel. Metal loss is concentrated where conventional current leaves the metal surface.
- A damaged positive pump cable lying in bilge water.
- A leaking underwater light, trim-tab actuator or transducer circuit.
- Incorrectly wired DC negative or bonding connections.
- Chafed wiring touching a metal through-hull or shaft system.
- A faulty charger or equipment installation creating unintended DC paths.
- A neighbouring vessel or marina installation leaking current into shared water.
AC leakage is primarily an electric-shock and fire hazard and must be treated urgently. It can also interact with corrosion systems, but its diagnosis and protection require different instruments and standards. Do not reduce a live-shore fault to an “anode problem”.
Is “electrolysis” the right word?
Boaters often use “electrolysis” for any rapid underwater corrosion. The term is imprecise in this context and can conceal the mechanism. Describe what is known: galvanic corrosion, suspected stray DC current, coating damage, dezincification or an unknown corrosion fault. Accurate language leads to the correct test.
| Phrase heard at the marina | Better question |
|---|---|
| “The marina has electrolysis.” | What measured current or potential shows an external electrical fault, and under what conditions? |
| “My zincs are working too well.” | Why has consumption changed, and are protected components within the correct potential range? |
| “Fit a bigger anode.” | Is anode capacity actually insufficient, or is a fault driving abnormal current? |
| “Disconnect the earth.” | Which compliant isolation device and test procedure preserves electrical safety? |
| “It must be the boat next door.” | Has our own DC, shore-power and bonding system been tested first? |
Warning signs that point towards stray current
- Severe metal loss over days or a few weeks rather than a normal season.
- Deep, sharply localised pitting near an electrical device or cable route.
- An abrupt onset after electrical work or installation of underwater equipment.
- Corrosion changes when a battery switch, charger or particular circuit is energised.
- Anodes are overwhelmed and protected metal still corrodes.
- Damage occurs on one boat or component in a way that does not match the rest of the galvanic system.
- A tingling sensation, repeated shore-power trips or unexplained current on bonding conductors—these are immediate electrical-safety warnings.
Electric-shock warning
Never enter the water around a marina or boat where electrical leakage is suspected. Isolate the supply through safe procedures and involve the marina and a qualified electrician.
Signs more consistent with galvanic loading
- Anodes waste progressively and protected metal remains sound.
- Consumption increases after fitting a larger stainless or bronze component.
- Wear changes after moving from fresh to brackish or salt water.
- Anode life shortens when continuously connected to a marina earth network.
- Corrosion centres on known dissimilar-metal couples or damaged coatings.
- A wrong anode alloy becomes passive or excessively active for the water.
- The pattern develops over a repeatable seasonal timescale rather than suddenly.
These signs are not proof. A vessel can have both galvanic loading and a stray-current fault. Testing should examine the complete system rather than choosing one explanation from appearance alone.
What a galvanic isolator does—and does not do
When a shore-power cable connects the boat’s protective earth to the marina earth, underwater metalwork can become electrically connected to other vessels and steel structures. The natural voltage between those metals is low. A galvanic isolator uses semiconductor elements to block low-voltage galvanic current while allowing higher fault current to pass through the protective conductor.
| A galvanic isolator can help with… | A galvanic isolator does not fix… |
|---|---|
| Low-voltage galvanic current using the shore-earth path | A positive DC cable leaking into bilge water |
| Anode loss associated with otherwise correct shore connection | Broken bonding straps on a drive |
| Maintaining a safety-earth path when correctly rated, installed and monitored | Wrong anode alloy or painted anodes |
| A defined part of a compliant shore-power corrosion strategy | External current entering through another cable, metal pipe or direct water path |
The isolator must suit the electrical system and applicable standards, be installed in the correct protective-earth conductor and be tested. An isolation transformer provides a different form of shore separation and may be chosen for more complex installations.
Review shore-power galvanic protection
Browse galvanic isolators, then use qualified installation and verification.
A safe diagnostic pathway
- Preserve evidence. Photograph anodes, metal loss, coating failure and electrical installations before cleaning.
- Establish timing. Determine when the damage began and what changed immediately beforehand.
- Verify anode system basics. Confirm alloy, part, immersion, remaining mass, contact and bonding.
- Review every powered underwater or bilge-adjacent device. Include pumps, lights, sensors, trim systems and chargers.
- Compare connected and disconnected shore-power conditions safely. This is a controlled electrical test, not permission to defeat earth conductors.
- Measure in-water potentials. A corrosion specialist uses a suitable reference electrode at defined locations and operating states.
- Measure leakage and conductor currents with appropriate instruments. Tests may include DC current, AC leakage, insulation and galvanic-isolator checks.
- Repair the cause before increasing anode capacity. Then renew damaged anodes, coatings, straps and components and retest.
A multimeter continuity check ashore can reveal broken connections, but it cannot independently distinguish a healthy galvanic system from an in-water stray-current fault.
Can a neighbouring boat cause corrosion?
Yes, an external vessel or marina installation can influence shared water, particularly where shore-earth conductors and underwater metals are interconnected. However, blaming the adjacent boat without measurement delays diagnosis. Your own boat may have the fault, or both systems may contribute.
Record whether symptoms change after moving berth, disconnecting through approved shore-power procedures or when nearby equipment operates. Provide the marina with measured evidence from a competent investigator. Do not board or disconnect another vessel.
How to reduce both corrosion risks
- Use correct, traceable hull anodes and component anodes for the water and equipment.
- Maintain drive and hull coatings so anodes are not protecting unnecessary bare area.
- Keep bonding and continuity straps exactly as designed and inspect after service.
- Install shore-power systems, galvanic isolation and RCD/ELCI protection to applicable standards.
- Route and support DC wiring away from wet bilges and sharp metal edges.
- Use correct overcurrent protection close to power sources.
- Inspect underwater electrical equipment and seals on schedule.
- Track anode wear with dates and photographs so sudden changes stand out.
- Commission in-water potential and leakage testing for complex, metal-hulled or persistently affected vessels.
Correct corrosion protection is not “more anodes everywhere”. Overprotection can also damage coatings and some materials. Design and measurement matter.
Frequently asked questions
Can sacrificial anodes stop stray-current corrosion?
They may absorb some current temporarily, but can be consumed rapidly and do not repair the electrical fault. Find and eliminate the source.
Does rapid anode wear always mean stray current?
No. Wrong alloy, increased metal area, coating damage, salinity and shore-earth galvanic current can also accelerate wear. The severity and timing guide the investigation.
Will a galvanic isolator stop corrosion from the boat next door?
It can block low-voltage galvanic current travelling through the shore-earth conductor when correctly installed. It cannot block every possible water or wiring path and does not fix a neighbour’s DC fault.
What is the difference between galvanic corrosion and electrolysis?
Galvanic corrosion is a defined electrochemical process between connected dissimilar metals. “Electrolysis” is often used loosely by boaters for electrically accelerated corrosion and should be replaced with a more precise diagnosis.
Can I find a DC leak by turning circuits off one at a time?
Circuit isolation can be part of professional diagnosis, but live-system testing around water and shore power carries shock and fire risks. Use proper instruments and a qualified marine electrician.
Why do my anodes wear only when plugged into shore power?
The shore-earth connection may be creating a galvanic path, the isolation equipment may be absent or faulty, or a charger/electrical fault may be involved. Have the complete installation tested.
