Saltwater is an electrolyte — a solution that conducts electricity. Any two different metals submerged in it and connected to each other (through the boat's hull, through bonding wires, through the water itself) form a battery. One metal becomes the anode and corrodes. The other becomes the cathode and is protected. This is galvanic corrosion, and it is not a possibility on a sailboat — it is a certainty. The only questions are which metal is losing material and how fast.
Understanding this requires understanding the galvanic series: a ranking of metals by their electrical potential in seawater. Metals near the active (negative) end of the series corrode preferentially. Metals near the noble (positive) end are protected. When two metals are in contact through an electrolyte, the active one always sacrifices itself.
The Galvanic Series for Sailors
The following is the portion of the galvanic series most relevant to sailboats, arranged from most active (corrodes first) to most noble (protected):
Most active — corrodes first: Magnesium → Zinc → Aluminum alloys → Cast iron / mild steel → Low-alloy steel → Stainless steel (active, in crevice) → Lead → Tin → Copper alloys → Naval brass → Bronze (silicon, aluminum, manganese) → Stainless steel (passive, in open water) → Titanium → Most noble — protected.
A few things on this list deserve attention. Zinc sits near the active end — this is why zinc anodes work. Aluminum is more active than steel, which is why aluminum outdrives and outboard lower units need aluminum anodes (or, in some designs, magnesium) to protect them. Bronze is relatively noble, sitting well above zinc and aluminum. Stainless steel appears twice on the list because its position depends on whether it has access to oxygen — a critical point explained below.
The farther apart two metals are on this list, the more aggressive the galvanic cell between them. Zinc next to bronze creates a strong cell — the zinc corrodes rapidly. Copper next to bronze creates a weak cell — the metals are close on the series and the corrosion rate is slow. This is why mixing metals that are far apart on the galvanic series is the primary thing to avoid on a boat.
What This Means on a Sailboat
Consider the typical sailboat underwater hardware inventory: a lead keel, bronze seacocks, a stainless steel propeller shaft, a bronze or stainless propeller, and possibly aluminum components in the rudder or keel attachment hardware. All of these are submerged in saltwater. If any of them are in electrical contact with each other — through the hull, through bonding wires, through the engine block, through the water itself — galvanic cells are forming between every pair of dissimilar metals.
The practical result: without protection, the most active metal in the circuit corrodes. In a typical installation with bronze hardware and a stainless shaft, any aluminum components are at serious risk. Bronze is significantly more noble than aluminum, so aluminum in contact with bronze will corrode to protect the bronze. This is why aluminum components near bronze through-hulls require careful attention and proper anode protection.
Lead keels are interesting because lead sits in the middle of the galvanic series — more active than stainless and bronze, but less active than aluminum. A lead keel in contact with stainless keel bolts is a classic galvanic pair, with the lead corroding slowly around the bolt holes over years. This is one reason why keel bolt inspection (looking for white powder around the bolt entry points, which is corroded lead oxide) is a standard part of any pre-purchase survey.
The Stainless Crevice Problem
Stainless steel behaves unusually compared to most metals on the galvanic series. It exists in two states. In open water with adequate oxygen, stainless forms a passive chromium oxide layer on its surface that makes it highly corrosion-resistant and electrochemically noble — it sits near the top of the galvanic series and is effectively protected. This is the stainless steel you're familiar with: the shiny, rust-resistant material used in standing rigging and deck hardware.
In a crevice — any situation where oxygen access to the metal surface is restricted — stainless cannot maintain its passive layer. The layer breaks down, and the steel reverts to an active state. In this active state, stainless steel drops dramatically on the galvanic series and becomes very vulnerable to corrosion. Worse, the active area (the crevice, which is small) is paired with a large cathodic area (the rest of the stainless fitting, which remains passive), and this geometry accelerates corrosion at the crevice to a rapid rate.
The practical consequence of this is chainplate failure. Stainless chainplates that pass through the deck and terminate in deck pockets — a common design on production boats — sit in a crevice environment. The chainplate is exposed to air at the top, but the section inside the deck pocket is oxygen-restricted, often wet, and frequently sealed against inspection by teak overlays or sealant. The active stainless in the crevice corrodes from the inside, and the failure mode is invisible until the rig comes down.
On boats with this chainplate design, the only real mitigation is periodic removal and inspection — pulling the chainplates, examining the section that lives in the deck pocket, and replacing them on a schedule (every 10–15 years on an actively sailed boat, sooner if the deck area is frequently wet). Stainless toggle pins and clevis pins in poorly-ventilated locations are subject to the same mechanism.
The Metal Mixing Rule
The most common mistake that accelerates galvanic corrosion is inadvertent mixing of metals that are far apart on the galvanic series. The most common version of this on sailboats is using stainless steel fasteners in aluminum.
Stainless is substantially more noble than aluminum. In saltwater, the aluminum around a stainless fastener becomes the anode in a strongly active galvanic cell. The aluminum corrodes, the corrosion products swell and lock the fastener in place, and within a season or two the fastener has effectively fused to the aluminum through a combination of mechanical locking and galvanic bonding. Attempting to remove it typically destroys the fastener, the surrounding aluminum, or both. This is a well-known problem on aluminum mast fittings, aluminum deck plates, and aluminum engine components where stainless fasteners have been used.
The fix is to use fasteners of the same alloy as the parent material, or to use a properly applied barrier (Tef-Gel, Duralac, or similar isolating compounds) between the fastener and the aluminum. These compounds interrupt the electrical contact between the two metals, preventing the galvanic cell from forming. They also prevent the mechanical locking that comes from corrosion products filling the thread engagement. Anywhere stainless hardware must be installed into aluminum, the threads should be treated with an isolating compound before installation.
The same principle applies to mixing copper with aluminum (the copper-aluminum couple is highly aggressive), bronze fittings on aluminum hulls, and stainless keel bolts in aluminum keels. Any hardware installation that joins two metals far apart on the galvanic series requires either an isolating barrier or a sacrificial anode nearby to absorb the corrosion current.
What Galvanic Corrosion Looks Like
On zinc anodes, galvanic corrosion is doing exactly what it should: the anode is consumed in place of your propeller and shaft. An anode that is actively working shows roughening and pitting on its surface and gradually shrinks over the season. An anode that's consuming faster than it should may indicate either a very active galvanic situation or stray current corrosion — a more serious problem covered separately.
On hardware, galvanic corrosion appears as pitting and roughening on the more active metal. Bronze hardware corroding galvanically with a more active metal nearby shows as dezincification — the zinc component of the bronze alloy corrodes out preferentially, leaving a porous, weakened copper structure that looks normal from the outside until it fails. Lead corrosion around keel bolts produces white powder (lead oxide and lead carbonate). Aluminum corrosion produces white powder (aluminum oxide) and eventually pitting that penetrates the parent metal.
The rate at which all of this happens depends on the electrical conductance of the path between the metals, the surface area ratio of anode to cathode (a small active area against a large noble area corrodes fast; a large active area against a small noble area corrodes slowly), and the conductivity of the water. Saltwater is a much better electrolyte than brackish water, which is better than freshwater — which is why the same boat can sail for years in a freshwater lake without the corrosion problems it would experience in a saltwater marina.