Marine Metals and Galvanic Corrosion

The Galvanic Series — A Practical Ranking of Marine Metals

The galvanic series is the ranked list of metals and alloys arranged by their electrochemical potential when immersed in seawater. It is the single most important reference for anyone who installs, repairs, or maintains metal hardware on a boat. The ranking runs from most noble (least likely to corrode) at the top to least noble (most likely to corrode) at the bottom. When two metals from different positions in the series are connected in seawater, the less noble metal sacrifices itself — it corrodes — to protect the more noble one. This isn't a defect or a failure. It's physics, and it's happening right now on every boat in the water.

The practical galvanic series for marine metals, from most noble to least noble, reads: titanium, stainless steel 316 (passive), Monel (nickel-copper alloy), silicon bronze, copper, brass (red and yellow), tin, lead, stainless steel 316 (active), cast iron, mild steel, aluminum alloys, zinc, and magnesium. The critical detail here is that stainless 316 appears twice — in its passive state, where the chromium oxide layer is intact and it behaves like a noble metal, and in its active state, where that protective layer has broken down (from crevice corrosion, oxygen depletion, or chloride attack) and it behaves like a much less noble metal, closer to mild steel. This dual nature of stainless steel is the root of more corrosion failures on boats than any other single factor.

The farther apart two metals are on the galvanic series, the faster the less noble one corrodes. A bronze through-hull connected to a stainless steel bolt is a modest galvanic couple — the metals are relatively close in the series. But a zinc anode bolted to the same bronze through-hull represents a large potential difference — the zinc corrodes rapidly and preferentially, which is exactly what you want it to do. Conversely, an aluminum hull fitting connected directly to a bronze seacock represents an aggressive galvanic couple that will eat through the aluminum at a rate that surprises people who think corrosion takes years. In warm tropical water, significant aluminum corrosion from galvanic coupling can occur in a single season.

Understanding this ranking isn't academic — it's the decision framework for every fastener choice, every hardware installation, and every repair on your boat. When a surveyor finds white powder on an aluminum fitting, wastage on a bronze prop, or pitting on a stainless chainplate, they're looking at the galvanic series in action. When you choose to use a silicon bronze bolt instead of a stainless one, or insist on a nylon isolating bushing between two dissimilar metals, you're making decisions based on this same ranking.

The Three Requirements for Galvanic Corrosion

Galvanic corrosion doesn't just happen because two different metals are near each other. It requires three specific conditions to be present simultaneously: dissimilar metals, an electrical connection between them, and an electrolyte that completes the circuit. Remove any one of these three, and galvanic corrosion stops. This is the basis of every corrosion prevention strategy on a boat — you can't change the fact that you're floating in an electrolyte, so your options are to avoid dissimilar metal contact or break the electrical connection between them.

Dissimilar metals means two metals at different positions in the galvanic series. The greater the separation, the greater the driving voltage of the galvanic cell, and the faster the less noble metal corrodes. A bronze through-hull paired with a bronze seacock isn't a galvanic couple — they're the same material. But replace that seacock with a stainless steel ball valve and you've created a dissimilar metal pair. In practice, almost every boat has dissimilar metals in contact somewhere: stainless fasteners in aluminum masts, bronze seacocks with stainless hose clamps, Monel shafts with bronze props. The goal isn't to eliminate all dissimilar metal contact — that's impossible. The goal is to manage it.

The electrical connection is the path for electron flow from the anode (corroding metal) to the cathode (protected metal). Direct metal-to-metal contact is the most obvious electrical connection, but it can also occur through the boat's bonding system, through conductive antifouling paint, through carbon fiber structures (which are electrically conductive), or through the boat's wiring. Breaking this electrical connection with insulating materials — nylon bushings, rubber gaskets, non-conductive sealants, Tef-Gel on bolt threads — is one of the two primary strategies for preventing galvanic corrosion.

The electrolyte is the liquid medium that carries ions between the two metals, completing the electrochemical circuit. Seawater is an excellent electrolyte — its dissolved salts make it highly conductive. Brackish water is a moderate electrolyte. Fresh water is a poor electrolyte, which is why galvanic corrosion proceeds much more slowly in freshwater lakes than in the ocean. But freshwater isn't zero risk — any dissolved minerals or pollutants increase conductivity. Bilge water, especially if it contains salt spray, detergent, or battery acid, is also a competent electrolyte. The inside of your boat can have galvanic corrosion problems if dissimilar metals are in contact in areas where bilge water collects.

The practical implication is straightforward: since you can't eliminate the electrolyte (the boat is in the ocean), every dissimilar metal joint must either be electrically isolated or cathodically protected with sacrificial anodes. There is no third option. Hoping that two different metals won't corrode because they're above the waterline ignores the reality of salt spray, condensation, and rain that creates electrolyte on deck hardware, mast fittings, and rigging terminals. Above-waterline galvanic corrosion is slower than below-waterline, but it's relentless.

Stray Current Corrosion — The Silent Killer

Stray current corrosion is galvanic corrosion's far more destructive cousin, and the two are frequently confused. Standard galvanic corrosion is driven by the natural voltage difference between dissimilar metals — typically measured in tenths of a volt. Stray current corrosion is driven by voltage from the boat's electrical system (or the marina's shore power wiring) that has found an unintended path through the water. The driving voltage can be 12 volts or more — orders of magnitude greater than a galvanic cell — and the resulting corrosion rate is proportionally catastrophic. A bronze propeller that would last 20 years with normal galvanic activity can be eaten through in weeks by stray current.

Stray current corrosion occurs when DC current leaks from the boat's wiring into the water through underwater metals. The metal where current flows from the boat into the water becomes the anode and corrodes — rapidly. Common sources include: a chafed battery cable touching the hull or an engine ground strap with a corroded connection (forcing current to find an alternate path through the shaft, cutlass bearing, and into the water), a bilge pump with damaged insulation sitting in saltwater, or an improperly wired shore power connection that puts DC voltage on the boat's bonding system. The insidious part is that stray current corrosion can affect any metal, not just the less noble one in a pair. It can eat through bronze, stainless, and even noble metals if the current path forces them to act as the anode.

AC stray current from marina shore power is another vector. If the shore power ground wire provides a path between your boat's bonding system and the marina ground (which is connected to every other boat's bonding system through the water), you can become part of a massive galvanic cell where your boat's underwater metals sacrifice themselves to protect the neighbor's boat. This is why galvanic isolators and isolation transformers exist — they break the DC path through the shore power ground while maintaining the safety ground for AC fault protection.

Identifying stray current corrosion in the field involves looking for patterns that don't match normal galvanic behavior. If a bronze propeller is corroding but the zinc anodes next to it are intact, something is wrong — the zinc should have corroded first. If corrosion is rapid and localized rather than slow and even, suspect stray current. If corrosion appeared after a new electrical installation (new bilge pump, battery charger, or shore power hookup), suspect stray current. And if multiple boats in the same marina are experiencing unusual corrosion, the problem may be in the marina's wiring, not your boat.

Testing for stray current requires a silver-silver chloride reference electrode and a multimeter. With the reference electrode in the water near the boat's underwater metals and the multimeter set to DC millivolts, a reading more negative than -200mV below the normal galvanic potential of the metal being tested indicates impressed current (cathodic protection working correctly, if you have an ICCP system) or stray current (if you don't). Systematically turning off circuits one at a time while monitoring the reference electrode reading will isolate the source. This test should be performed with shore power connected and disconnected, as shore power ground faults are a common source.

Identifying Corrosion Damage on Underwater Hardware

Every haul-out is a corrosion inspection opportunity, and the ability to read the signs on your underwater metals tells you whether your cathodic protection is working, whether dissimilar metals are eating each other, and whether stray current is present. The inspection starts the moment the boat comes out of the water, while the bottom is still wet and marine growth is soft. Once everything dries and gets pressure-washed, some evidence is harder to see.

Through-hulls are the first priority. Bronze through-hulls in good condition have a uniform surface under the bottom paint — smooth, solid, with consistent wall thickness. Run your fingertips over the exterior flange and feel for pitting, thinning, or rough texture. A through-hull that feels like sandpaper is actively corroding. Dezincification on brass fittings that have been mistakenly installed below the waterline shows as a pinkish-copper surface when you scratch through the patina — the zinc has leached out, leaving a spongy copper matrix that looks solid but has lost all structural strength. Squeeze the fitting with channel-lock pliers; dezincified brass will deform or crumble where sound bronze won't.

Propellers and shafts show corrosion in characteristic patterns. Normal galvanic corrosion on a bronze prop appears as a gradual thinning of blade edges, mild surface pitting, and a rough texture. This is expected over years and is part of why you carry spare props. What's not normal is deep localized pitting, blade erosion concentrated on one area, or a prop that shows significant wastage while the shaft zinc is still 80% intact. These patterns indicate either stray current or a bonding system fault that's not allowing cathodic protection to reach the prop. Stainless steel shafts can develop crevice corrosion where they pass through the stern tube or inside the cutlass bearing — the oxygen-depleted zone between the shaft and bearing is a classic crevice corrosion site. Look for rust-colored weeping from the stern tube.

Rudder hardware — pintles, gudgeons, rudder post, and the rudder shoe if fitted — is often the most neglected corrosion inspection point because it's awkward to reach and partially hidden. Bronze rudder fittings should be inspected just like through-hulls: feel for pitting, check for thinning, and verify that there's no play or looseness from material loss. On spade rudders with stainless steel rudder stocks, check the bearing areas where the stock exits the hull for any sign of crevice corrosion — discoloration, rust staining on the hull surface around the rudder tube, or weeping. If the rudder stock is corroding inside the rudder tube, you won't see it until the rudder fails or is pulled for inspection.

Keel bolts are critical and invisible. On an externally ballasted sailboat, steel or stainless keel bolts pass through the hull and secure the ballast keel. Corrosion of keel bolts is a structural emergency — keels have separated from hulls due to bolt corrosion, and the results are immediate capsizing. Unfortunately, keel bolts are difficult to inspect without removing internal joinery. Look for rust staining around the bolt heads inside the bilge sump, weeping moisture around the keel joint, or any sign that the keel-to-hull joint is opening. If keel bolt corrosion is suspected, ultrasonic thickness testing of the bolts is the definitive diagnostic, and it requires a qualified surveyor.

Isolation Techniques for Dissimilar Metal Joints

Since every boat inevitably has dissimilar metals in contact — stainless fasteners in aluminum extrusions, bronze fittings with stainless hose clamps, steel rigging on bronze turnbuckles — the practical skill is isolating those joints to break or impede the electrical connection. A perfect isolation stops galvanic corrosion entirely. A good isolation slows it dramatically. No isolation at all means you're running a battery with your boat's hardware as the electrodes and the ocean as the electrolyte.

Tef-Gel (manufactured by Sealey) is the standard anti-seize and anti-corrosion compound for marine fastener installations. It's a PTFE-based paste loaded with microscopic Teflon particles that prevent direct metal-to-metal contact between the bolt threads and the host material. Apply it to every stainless steel fastener that threads into aluminum, bronze, or any dissimilar metal. Tef-Gel does three things simultaneously: it prevents the direct electrical contact that drives galvanic corrosion, it lubricates the threads so the fastener can be removed later without seizing, and it prevents crevice corrosion in the oxygen-depleted thread interface. A $15 tube lasts for hundreds of fasteners and prevents thousands of dollars of damage. There is no excuse for installing a stainless bolt in aluminum without Tef-Gel.

Nylon bushings, washers, and sleeves physically separate dissimilar metals with a non-conductive barrier. When bolting stainless hardware to an aluminum mast or spar, use a nylon or Delrin bushing in the bolt hole, a nylon washer under the bolt head, and a nylon washer under the nut. This creates a complete electrical break between the stainless fastener and the aluminum structure. For through-deck fittings where stainless hardware passes through a cored deck and contacts aluminum backing plates, nylon isolation is essential. The bushings must be sized precisely — a sloppy fit that allows the bolt to touch the edge of the hole defeats the isolation.

Dielectric grease is used on electrical connections to prevent galvanic corrosion at wire terminals, bus bars, and battery connections where dissimilar metals (copper wire, tin-plated terminals, stainless screws) are joined. It's a silicone-based non-conductive grease that fills the air spaces around the connection, preventing moisture from creating an electrolyte in the joint. Dielectric grease does not impede the electrical connection at the actual contact surfaces — the mechanical clamping force of the terminal squeezes the grease out of the contact area — but it seals the perimeter of the connection against moisture intrusion. Apply it to every terminal, lug, and bus bar connection on the boat, especially in the engine compartment and bilge where salt-laden moisture is present.

Micarta, G-10, and other non-conductive gasket materials are used for larger-scale isolation — between stainless chainplate tangs and aluminum structures, between engine mounts and hull structure (when electrolysis between the engine block and the keel or bonding system is a concern), and between bronze bowsprit hardware and teak or aluminum substrates. These materials are strong, non-conductive, moisture-resistant, and can be machined to fit. For critical structural joints like chainplate tangs, the isolation material must be engineered to handle the loads involved — consult a marine engineer or rigger if you're designing an isolation scheme for a structural connection.

1. Identify all dissimilar metal joints

   During a haul-out or annual inspection, catalog every location where two different metals are in contact. Use the galvanic series chart to determine the severity of each couple. Prioritize joints where the metals are far apart on the series, are below the waterline, or are structural.

2. Clean all contact surfaces

   Remove existing corrosion products, old sealant, and contamination from both metal surfaces using a Scotch-Brite pad, wire brush, or appropriate solvent. Corrosion products can be conductive and will compromise new isolation materials.

3. Apply Tef-Gel to fastener threads

   Coat the full thread length and under the bolt head with a generous layer of Tef-Gel. Also apply it to the inside of the threaded hole if accessible. The goal is a continuous PTFE film between all contact surfaces.

4. Install nylon isolation hardware

   Place nylon bushings in bolt holes and nylon washers under bolt heads and nuts to create a complete electrical break. Verify that no metal-to-metal contact exists anywhere in the joint by checking with a multimeter set to continuity.

5. Verify electrical isolation

   Use a multimeter set to resistance/continuity. Place one probe on each metal component. If you get continuity (low resistance), the isolation is incomplete — find and eliminate the contact point. For joints connected to the bonding system, verify intentional bonding paths are maintained while galvanic couples are broken.