Tropical Corrosion Control: Anodes, Bonding and Stray Current in Bali

Written and maintained by the Bali Shipyard Build Desk · last reviewed 19 August 2026

The three mechanisms, kept separate

They are routinely confused, and the fix for one does nothing for the others.

Mechanism	Cause	Typical symptom
Galvanic corrosion	Dissimilar metals connected in an electrolyte.	Steady anode consumption; pitting on the less noble metal.
Stray-current corrosion	External current leaking through the vessel to water.	Very rapid, localised damage; anodes gone in weeks.
Crevice / poultice corrosion	Oxygen starvation under deposits, fittings or growth.	Hidden pitting under fittings, blocks and fouling.

Anodes: chosen, sized and measured

Material

• Zinc — the traditional choice for salt water and still standard for most vessels in Bali.
• Aluminium — works in salt and brackish water, generally lasts longer, and suits vessels moving between salinities.
• Magnesium — fresh water only. In salt water it is consumed extremely fast and can over-protect, damaging paint on aluminium hulls.

Mixing anode materials on a single vessel undermines the protection scheme. Pick a system and stay with it.

Sizing and placement

Anodes must be electrically connected to what they protect, adequately sized for the surface area involved, and distributed sensibly — shaft, propeller, rudder, trim tabs, and hull anodes where the bonding system requires. Anodes painted over, or bolted to a corroded surface with poor contact, protect nothing while appearing to be present.

Bonding continuity

The bonding system connects underwater metals so a single anode set can protect them all. It is also the most commonly broken system on a boat, because connections corrode in exactly the humid, salty environment they live in.

Testing is straightforward: with the vessel ashore and isolated, measure resistance between each underwater fitting and the bonding bus. Low resistance means connected; high or open means that fitting is unprotected regardless of how many anodes are fitted.

A broken bond typically shows up as an anode consuming abnormally fast while some other fitting quietly corrodes. That exact pattern is described in the first case in the project portfolio.

Stray current: the fast destroyer

Stray-current corrosion can destroy a propeller or a shaft in weeks. Sources, in rough order of frequency:

1. Shore power faults — the most common. A wiring fault aboard, or on the dock, allows current to find a path through the water.
2. Neighbouring vessels with their own faults, connected to the same shore supply.
3. DC faults aboard — a chafed positive conductor touching a bonded metal component.
4. Bilge pumps and submerged equipment with degraded insulation.

Practical protections: a galvanic isolator or isolation transformer on shore power; regular insulation-resistance testing of DC circuits; and simply disconnecting shore power when the vessel is unattended for long periods, which removes the largest single source.

Diagnosis

Hull-potential measurement with a reference electrode is the tool. Readings taken with shore power connected and disconnected, and with neighbouring vessels connected and disconnected, isolate the source quickly. It is a measurement job, not a guessing job — and it costs far less than replacing running gear.

Materials in a tropical marine environment

• Stainless steel depends on oxygen to maintain its passive layer. Immersed, in a crevice, or under a deposit it becomes vulnerable — which is why stainless fasteners fail where they are hidden, not where they are visible.
• Aluminium hulls and outdrives require copper-free antifoul and careful isolation from more noble metals, including yard blocking materials.
• Bronze fittings can dezincify, losing zinc and leaving a weakened pink copper structure that looks acceptable and is not. Old seacocks are the classic case.
• Steel hulls depend primarily on coating integrity; anodes are a backup for coating damage, not a substitute for coating.

Annual corrosion routine

1. Photograph and measure every anode; record percentage consumed and elapsed months.
2. Renew anodes; ensure clean metal-to-metal contact at the mounting.
3. Test bonding continuity to every underwater fitting and record the readings.
4. Inspect all bronze seacocks and through-hulls for dezincification; test rather than assume on anything over a decade old.
5. Check shore-power isolation equipment is present and functioning.
6. Insulation-test DC circuits, particularly bilge pumps and submerged equipment.
7. Record hull potential as a baseline to compare against next year.

All of this belongs in the annual haul-out scope — see the 12-month maintenance calendar — and the readings belong in the job file described in documenting repairs for insurance and survey.

Reading the anodes as a diagnostic

Observation	Likely meaning	Next step
Even consumption, roughly annual life	System working as designed	Record the rate; renew and continue.
Anodes barely consumed	Not electrically connected, or painted over	Test continuity to the fitting they protect.
One anode consumed far faster than others	Localised galvanic couple or a bonding break elsewhere	Continuity test across all underwater fittings.
All anodes gone within weeks	Stray current	Hull potential measurement with and without shore power.
Anodes intact but fittings corroding	Fittings not bonded into the system	Trace and repair the bonding conductor.
Paint blistering near anodes on an alloy hull	Over-protection	Review anode material and total mass.

Habits that prevent most of it

• Disconnect shore power when the vessel is unattended for extended periods; it removes the largest single stray-current source.
• Never paint an anode or its mounting face; contact resistance defeats the whole system.
• Keep a single anode material across the vessel rather than mixing zinc and aluminium.
• Photograph anodes at each inspection alongside a scale, so consumption can be judged rather than recalled.
• Treat a fast-consuming anode as a fault report, not as a reason to fit a larger one.