Galvanic & Stray-Current Corrosion in Tropical Waters

Most owners meet corrosion at a haul-out. Manganese bronze that has lost its zinc crumbles pink under a screwdriver. A stainless shaft comes out of its bearing with a row of pits down one side. Anodes meant to last a season vanished in ten weeks.

None of that is bad luck. Corrosion is an electrical process, and electrical processes can be measured and interfered with. This guide covers the three failure modes behind nearly all underwater metal loss — galvanic corrosion, stray current, and the shared-earth “hot dock” — for Andaman conditions: warm water all year, brackish moorings, relentless fouling, a long layup.

On this page

1. The cell: three ingredients and nothing else
2. The galvanic series in seawater
3. Why the tropics are harder on everything
4. Brackish water: the section that saves propellers
5. Stray current: a genuinely different animal
6. The hot dock
7. Bonding: for, against, and the worst of both
8. Measuring it: the reference-electrode survey
9. Choosing, sizing and fitting anodes
10. An inspection year for the Andaman coast
11. Troubleshooting
12. Where to start

The cell: three ingredients and nothing else

A galvanic cell needs exactly three things: two dissimilar metals, an electrolyte joining them (seawater is an excellent one), and an electrical path between them — usually a shaft, a bonding wire, or the fact that both are bolted to the same bracket. Remove any one and the cell stops.

Given all three, current flows. The more active metal becomes the anode, its atoms going into solution as it wastes away; the more noble metal becomes the cathode and is protected. The metal is not eaten by the water — it is consumed to power a battery you did not know you had built.

Two things set the rate. Potential difference decides how hard the cell is driven. Area ratio decides where the damage lands, because current density destroys metal, not total current: a small anodic part wired to a large cathodic surface takes all of it. Hence a stainless skin fitting on bronze bolts is a slow-motion disaster, while the reverse is fine.

The galvanic series in seawater

Marine work measures potential against a silver/silver-chloride (Ag/AgCl) reference. More negative means more active — more willing to corrode, and to protect its neighbours.

Metal	Approx. V vs Ag/AgCl
Magnesium	−1.60 to −1.63
Aluminium anode alloy (Al-Zn-In)	−1.05 to −1.10
Zinc	−0.98 to −1.05
Aluminium alloys (5000/6000 hull, outdrive castings)	−0.76 to −1.00
Mild steel, cast iron	−0.60 to −0.71
304 stainless — active	−0.46 to −0.58
316 stainless — active	−0.43 to −0.54
Aluminium bronze	−0.31 to −0.42
Copper	−0.30 to −0.36
Naval brass, manganese bronze	−0.27 to −0.40
Monel 400	−0.04 to −0.14
304 stainless — passive	−0.05 to −0.10
316 stainless — passive	0.00 to −0.10
Titanium	−0.05 to +0.06
Graphite	+0.20 to +0.30

These are typical ranges for clean metal in aerated, moving seawater at ambient temperature, and they are best read as a ranking rather than as precision data. Published tables differ, and many quote against a saturated calomel or copper/copper-sulphate reference instead, which shifts every figure by tens of millivolts — check which reference a table uses before comparing it with a meter reading.

Why stainless appears twice

Stainless is only stainless because a thin chromium-oxide film forms on it, and that film needs dissolved oxygen to maintain itself. Clean stainless in moving, aerated water is passive and sits near titanium. Starve it of oxygen and the film breaks down: the metal goes active, some 400 to 500 mV more negative, and now sits below bronze.

That swing explains almost every stainless failure on a boat. The dangerous places are oxygen-starved crevices — under a cutless bearing, inside a stern tube, beneath a rope-cutter boss, and very often under a wrap of tape or a smear of sealant applied with good intentions. Inside the crevice the metal is active; outside, it stays passive. A tiny anode against a large noble cathode is the worst possible geometry, and the result is deep, narrow pitting that can perforate a shaft while the rest still looks showroom-bright.

Why the tropics are harder on everything

Corrosion is a chemical reaction, and reaction rates broadly follow an Arrhenius relationship: rate climbs steeply with temperature. The workshop rule of thumb is a rough doubling per 10°C — treat it strictly as that, because real corrosion depends on oxygen supply, flow, salinity and surface condition as much as on temperature. It is a teaching rule from general chemistry, not a measured figure for marine corrosion.

The direction is not in doubt. Andaman surface temperatures sit around 28–30°C year-round, against perhaps 10–15°C in a temperate European marina — and most boating literature on corrosion was written somewhere colder.

Warm water also carries less dissolved oxygen, which makes passive films on stainless and aluminium harder to maintain and slower to re-form after damage; marginal passivation is exactly the condition in which pitting starts. Then the biology: no winter means fouling never stops, and biofilm is not cosmetic. It consumes oxygen locally and creates precisely the depleted micro-crevice that turns passive metal active — which is why a fouled boat on a monsoon mooring is in a far more aggressive environment than the same boat working in clean flowing water.

Brackish water: the section that saves propellers

The water here is not consistently seawater. River outflows, mangroves and heavy monsoon rain mean salinity swings by season, by tide, and by how far up a river you keep the boat.

Salinity sets conductivity, and conductivity sets the size and reach of a galvanic cell. In full seawater the electrolyte conducts well, so an anode protects metal a useful distance away — including, unhelpfully, metal on a neighbouring boat. In low-salinity water it is resistive, current struggles to travel, and an anode may only protect what is very close to it, leaving a rudder or P-bracket effectively bare.

The bigger problem is what low salinity does to zinc. Zinc passivates in low-conductivity water, building an insulating film of zinc hydroxide and carbonate over itself and ceasing to deliver current. An anode showing no wastage is not good news: it is either passivated or not electrically connected, and both mean no protection while the propeller behind it corrodes freely.

Indium-activated aluminium-zinc alloy does not do this. It stays active across salt and brackish conditions, sits slightly more negative than zinc so it drives protection a little harder, and carries roughly three times zinc’s charge per kilogram — around 2,500 Ah/kg against zinc’s 780 Ah/kg, though a same-sized anode gains rather less than that, aluminium being the lighter metal. Magnesium, at roughly −1.6 V, is what fresh water needs because nothing else has the push to overcome the resistance; in seawater it is far too much, wasting itself at a ludicrous rate and over-protecting aluminium components on the way.

So the recommendation is simple. For a boat moving between open sea and brackish moorings, aluminium alloy anodes are the correct default — they work in both. We have covered the alloy choice in more depth for local conditions in zinc versus aluminium anodes in Thai waters. Zinc is defensible on a vessel permanently in full-salinity water, but it has no advantage over aluminium and one serious weakness. Magnesium belongs in fresh water and nowhere else.

Stray current: a genuinely different animal

Everything above is driven by a few hundred millivolts. Stray-current corrosion is driven by your own electrical system at 12 or 24 volts, and it is a different order of severity.

DC current escapes the wiring and returns through the water and the underwater metal. Wherever it leaves metal and enters the water, metal is consumed. The usual sources are a bilge pump with a corroded connection sitting in bilge water, a pump wired without a proper return so it uses the bonding system as a ground path, a badly installed inverter or charger, or a poorly earthed shore-power installation.

The severity gap matters. Galvanic currents in the microamp-to-milliamp range eat an anode over a season. A stray-current fault passing a few amps can destroy a propeller or deeply pit a shaft in days to weeks. Owners routinely misread this as bad anodes and fit more, which achieves nothing.

Tell them apart by pattern: anodes disappearing in weeks with no change of berth or use; pitting where galvanic logic says it should not be, such as mid-shaft, on a rudder stock or on a single bolt; and loss that stops when the shore lead is unplugged (shore earth or charger) or continues with it out (your own DC system).

Then measure. Clamp meter round the bonding conductor, or a leakage test between bonding system and water with everything off. Shore lead out, then isolate DC circuits one at a time until the leakage disappears. That circuit has the fault. Fix the wiring; do not fit more anodes — the cure is sound cable, terminals and bonding hardware, not more sacrificial metal.

The hot dock

Shore power is where one owner’s problem becomes everyone’s.

The protective earth — green, or green-and-yellow — is a safety conductor and must never be lifted or omitted. It is also a low-resistance connection between the earthing systems of every boat on the pontoon, and since each of those boats also bonds to metal in the water, the result is one large galvanic circuit spanning the dock. Your anodes are now working for your neighbours too.

If every installation is sound this is merely inefficient. If one vessel has an unbalanced bonding system, no anodes or an actual DC fault, it drives current through the shared earth and that current returns through your running gear. That is a hot dock, and the classic symptom is anodes consumed several times faster in one berth than anywhere else. Three honest remedies:

A galvanic isolator sits in series with the shore earth conductor, blocking small DC potentials — typically around 1.2 V for the common arrangement of two silicon diodes in series in each direction, and up to about 1.4 V in some designs — while still passing AC fault current so the safety earth keeps working. Cheap, passive, effective against most hot-dock anode loss. Two non-negotiables: it must be fail-safe, so a failed unit fails conducting rather than breaking the safety earth, and rated at or above the shore breaker, so it carries full fault current without failing open. ABYC A-28, Galvanic Isolators — current edition 2024 — sets both requirements, specifying that the isolator’s current rating be no less than that of the main shore-power disconnect circuit breaker and that a failed unit still provide an effective ground-fault current path.

An isolation transformer is the proper answer on a larger vessel. Power transfers magnetically, with no metallic path between shore and boat, so the boat’s earth is genuinely independent. Expensive, heavy, and it must be installed and earthed correctly to be safe or useful — not a DIY job.

Unplugging is free, and works on any boat that does not need shore power to hold its batteries up. If it will sit for weeks, ask whether the lead needs to be in.

One point to be very clear about: a galvanic isolator does nothing against a stray-current fault on your own boat. It blocks a DC offset arriving down the shore earth; it has no effect on 12 V leaking out of your bilge pump wiring.

Bonding: for, against, and the worst of both

A bonding system ties underwater metals — shaft, engine, skin fittings, rudder stock, keel bolts — to a common point so they sit at one potential and share a common anode.

The case for is control: one anode set protecting the lot, and one reading that tells you where you stand. The case against is that bonding deliberately connects metals that would otherwise be isolated, creating galvanic cells that did not previously exist and offering stray current a route to every underwater fitting. Plenty of well-built GRP boats are deliberately unbonded, each fitting corroding alone at its own slow rate.

Both are legitimate. The middle is not. A partially or badly bonded boat is usually worse off than either extreme: some fittings connected to the anode, some not, and corroded or undersized conductors quietly removing protection from the far end. A fitting bonded to the shaft but not the anode is tied to a large area of noble metal with nothing sacrificial in the loop — the fastest way to lose a skin fitting. Decide which system you have and commit to it.

Measuring it: the reference-electrode survey

You need a silver/silver-chloride reference electrode on a lead, a multimeter with high input impedance (10 MΩ or better), and the boat afloat in the water it normally lives in. Meter on DC volts, 2 V range. Connect the negative lead to the reference electrode and the positive lead to the metal you want to read — bonding common point, engine block, or the shaft via a brush. Lower the electrode into the water a hull’s-thickness from the item of interest, touching nothing, and let the reading settle; it can drift for a minute or two. Repeat at bow, midships, stern and both sides. The pattern matters as much as the number: a stern reading far more positive than the bow says the running gear is under-protected.

For a bonded steel or iron hull: less negative than −0.80 V is under-protected and actively corroding; −0.80 to about −1.05 V is where you want to be; more negative than about −1.05 to −1.10 V is over-protected, risking coating disbondment and, on high-strength steels, hydrogen embrittlement.

For an aluminium hull or outdrive the window is narrower and hard at both ends: less negative than about −0.90 V is under-protected; −0.90 to −1.10 V is correct; more negative than about −1.10 V is over-protected, and beyond about −1.20 V the alkaline attack described below becomes active damage.

Watch which reference a published figure uses. The steel criterion is most often written as −0.85 V against a copper/copper-sulphate electrode, which is roughly −0.80 V against silver/silver-chloride — the same requirement, quoted two ways. Authorities also differ a little on the aluminium limits, so where a manufacturer specifies a window for your drive or hull, follow theirs.

Over-protection is a real failure mode. Aluminium is amphoteric — attacked by strong alkali as well as acid. Drive it too negative and the cathodic reaction generates hydroxide at the surface, raising local pH; the alkaline film attacks the metal and lifts the paint from underneath. Warning signs: paint lifting in a halo around the anodes; a soft white chalky deposit on bare aluminium; heavy calcareous scale on adjacent metal.

Choosing, sizing and fitting anodes

Alloy. On the Andaman coast, aluminium alloy is the sensible default for hull, shaft, rudder and trim-tab anodes, for the brackish-water reasons above. Zinc remains valid for a vessel permanently in full seawater; magnesium is fresh water only. We are a specialist supplier of genuine MG Duff products, and stock anodes covering hull, shaft, rudder, engine and outdrive fittings in both zinc and aluminium within the same part-number families.

Never mix alloys on one hull. Zinc and aluminium anodes on the same system form their own couple: the aluminium protects the zinc, the zinc contributes little, and you finish worse off than with either alloy alone. Pick one and convert the whole boat.

Sizing follows the area to be protected and how much of it is bare — bare metal in warm flowing water demands far more current than well-coated metal. Work from the manufacturer’s guidance for your hull material and area, and err towards more frequent replacement rather than piling on anode mass, especially on aluminium, where more is emphatically not better.

Replacement interval. The familiar annual advice was written for temperate water and a winter layup. At 30°C, with year-round use and heavy fouling, it is optimistic. Inspect every two to three months and replace on condition rather than calendar.

Replace at 50% wasted. A wasting anode loses surface area and output with it, so the last half delivers far less protection than the first, and the remaining metal may lose contact with its fixing.

Fitting. The contact face must be clean, bright, bare metal on both sides of the joint — no paint, primer, antifoul, sealant or corrosion product. An anode that is not electrically connected is a decoration. And never paint an anode, or let it be caught by the antifouling roller: a painted anode delivers nothing at all.

An inspection year for the Andaman coast

Period	What to do
October, pre-season	Off the mooring or out of layup: full underwater inspection, replace all anodes regardless of condition, reference-electrode survey, check every bonding connection, leakage-test the DC system.
November–April, NE monsoon	Boat in use. Anode inspection every 8–12 weeks — a diver check is cheap next to a propeller. Watch for sudden changes in wastage rate, inspect the shaft where it enters the cutless bearing, check shore lead and isolator.
April–May, end of season	Haul, clean, replace all anodes, survey the running gear while it is dry and visible. Photograph everything for next year.
May–October, SW monsoon	Ashore: nothing corroding, but keep anodes off the paint job. On a mooring: unplug shore power unless genuinely needed, and get a diver on the boat at least twice — fouling and corrosion run hardest when nobody is looking.
Any new berth	Reference-electrode reading within two weeks, anode check at four. This is how you catch a hot dock early.

Troubleshooting

Symptom	Likely cause	First check
Anodes gone in weeks	Stray current, or a hot dock	Unplug the shore lead for two weeks. Unchanged means the fault is aboard.
Anodes barely touched, metal still corroding	Zinc passivated in brackish water, or anode not connected	Continuity, anode to bonding point; switch to aluminium.
Propeller pink and crumbling	Dezincification of manganese bronze	Protection-level survey; consider aluminium bronze next time.
Deep pits in a bright stainless shaft	Crevice corrosion in an oxygen-starved zone	Look under the cutless bearing, and under any tape or sealant.
Paint lifting around anodes on an alloy leg	Over-protection, alkaline attack	Reference-electrode reading; reduce anode area; check the alloy is not magnesium.
One skin fitting corroding, others fine	Partial or broken bonding	Continuity from that fitting to the bonding common point.
Everything worse in one berth	Hot dock	Reference-electrode survey there; fit or verify a galvanic isolator.
Corrosion started after a new inverter or charger	Installation fault or shore-earth issue	DC leakage test with that circuit isolated.

Where to start

Change to aluminium alloy anodes if your boat sees any brackish water, take one reference-electrode reading so you know where you stand, and when anodes vanish faster than they should, treat it as an electrical fault to trace rather than a reason to fit more metal. For help sizing anodes, get in touch.