Steve D’Antonio Stray-current corrosion can destroy unprotected metal components like this propeller in a brief exposure period.
There’s a conversation I find myself having on too many occasions that goes something like this:
Jim: “Hey Steve, I’m interested in purchasing/building this vessel. It’s XX feet, does XX knots with a range of XXXX miles…Oh, and it’s aluminum/steel.”
Me: “That sounds great Jim. Have you ever owned a metal boat before?”
Jim: “No. Why, is that a problem? What do I need to know?”
Me: “You need to know everything. When it comes to corrosion, you’ve got to educate yourself to be your own expert, because the guidance you get from other boatowners, and even industry professionals, is almost always incorrect.”
While that advice to owners is intended to steer them on a path to technical education and self-reliance, I worry that often they turn directly to the internet where the volume of corrosion misinformation available seems infinite. My intent with this two-part article is to provide a solid primer for boatyard technicians and boat owners who should know the essentials of corrosion prevention.
It’s a rare day when I don’t hear from someone trying to solve some vexing corrosion problem. No surprise: Almost every modern boat includes a wide range of metallic, corrosion-prone components—bronze seacocks, iron engine blocks, stainless steel and aluminum deck hardware, and copper wiring. Add water or electricity or both to this mix, and the results can be crippling and costly.
While there are scores of different corrosion variations specific to different metals—poultice corrosion in aluminum, crevice corrosion in stainless steel—there are just two overarching mechanisms that cause most corrosion on small craft: galvanic or dissimilar-metal, and stray-current corrosion.
Galvanic
Galvanic corrosion is electrical in nature, occurring when dissimilar metals are placed in direct contact with each other, or are electrically connected via a wire or other conductor, while simultaneously exposed to an electrolyte such as fresh or seawater, or even high humidity. (Note that seawater’s enhanced conductivity accelerates the process.) In this case, electricity is created by the dissimilar metals sharing an electrolyte. It functions much like a battery, albeit at a very low rate typically measured in millivolts.
While virtually any two metals will interact with each other in these conditions, the farther apart they are on the galvanic series or scale (see Table 1), the more dramatic their encounter will be. Metals located at the most noble and most corrosion-resistant end of the galvanic series include exotics such as graphite (including carbon fiber), gold, and titanium as well as more pedestrian 316 stainless steel, and nickel-chrome alloys used for propeller shafts. Those at the least noble end of the series, such as magnesium, zinc, and aluminum alloys, are significantly less corrosion resistant.
Some especially problematic combinations of dissimilar metals include copper (and copper alloys such as bronze and brass), aluminum alloys, and to a lesser degree, stainless steel and aluminum. Because of its extreme location on the galvanic scale, virtually any metal placed into contact with aluminum in the presence of moisture, will cause the latter to corrode. In 1895, before this phenomenon was thoroughly understood from a boatbuilding perspective, the Herreshoff-designed and built America’s Cup contender Defender, was assembled using nickel-aluminum alloy hull plating above the waterline and bronze plate below the waterline, all built over steel frames and attached with bronze rivets throughout. The result was a battery-like hotbed of galvanic activity. Predictably, the victorious sailing yacht was short-lived. By 1901, the hull plating had pitted so heavily the boat was no longer seaworthy and had to be scrapped, but not before fulfilling her successful Cup defense.
Steve D’Antonio Textbook galvanic corrosion set up with this mix of a bronze seawater strainer, an aluminum bracket, and a stainless bolt.
Examples of common onboard combinations prone to galvanic corrosion include brass hydraulic steering cylinders or bronze seawater strainers supported by aluminum brackets, bronze sail track installed on aluminum spars, aluminum hydraulic cylinders that utilize brass hydraulic fittings, and stainless fasteners married to aluminum substrates such as spars, cranes, arches, radar masts, etc.
Steve D’Antonio Aluminum sail; and turning-block track on masts and deck are frequently attached with stainless fasteners, leading to mild galvanic corrosion.
The best examples of galvanic corrosion on a boat are intentional: sacrificial anodes attached to underwater metals such as propeller shafts, through-hull fittings, struts, rudders, and heat exchangers. Known as cathodic protection, the generic “zinc” anode is commonly made of one of three possible sacrificial metals well suited to the purpose: zinc, aluminum, and magnesium. Zinc should only be used in seawater, magnesium is suited exclusively for fresh water, while aluminum can be used in fresh, brackish, or sea water (see Table 2). Whatever its material, the anode’s function is to corrode, while protecting the metal to which it’s attached.
The most effective means of preventing galvanic corrosion is to avoid using dissimilar metals in applications where they will be in direct contact with each other or be otherwise electrically connected. The practical definition of dissimilar in this context is any two metals whose resting voltage differ by more than 200 mV in the galvanic series. Where this is unavoidable, another effective method of corrosion protection is to insert a layer of non-conductive material (or in some cases such as aluminum fuel tanks, a metal that is benign to both layers) as insulation between them. Appropriate nonconductive materials include prefabricated fiberglass or epoxy-based sheet, also known as GPO3 or G10, but you should avoid using non-reinforced plastics such as UHMW or rubber sheeting in highly loaded structural applications—under cleats, sail tracks, and steering rams for instance. Stainless steel is often installed as an insulator between aluminum and copper-based alloys—aluminum fuel tanks and brass (a copper alloy) plumbing fittings and valves for instance. In this assembly, the dissimilar materials are technically still connected, but the stainless-steel bushing provides the necessary degree of isolation to inhibit galvanic corrosion. This approach, which works only because the fuel tank is not immersed in water, would not be acceptable for submerged, or continuously wetted components.
While galvanic corrosion is typically a localized phenomenon caused by interaction between the dissimilar metals on a single vessel, it can also occur between vessels. This interaction can be confusing and is undoubtedly the source of a great deal of misinformation, including most “hot marina” myths. Inter-vessel galvanic corrosion occurs when, for instance, two or more nearby vessels plug into shore power, and the green AC safety grounding wiring on each vessel common with its bonded underwater metals—seacocks, struts, shafts, and anodes—becomes connected whether or not the shore power is energized. In short, the moment a shore-power cord is plugged in, the AC safety grounds and bonding systems on the vessels are interconnected. When this occurs, intact sacrificial anodes, or less noble underwater metals such as aluminum stern drives, on one vessel may begin protecting underwater metals on other connected vessels, ones whose anodes are depleted. Except for the fact that the shore-power cord must be connected, this phenomenon has little if anything to do with the marina, or AC shore power per se. It remains a galvanic, and hence DC rather than AC, interaction.
Such multi-vessel galvanic action is easily thwarted with either a galvanic isolator, or an isolation transformer. The former block, up to 1.4V (above the typical galvanic corrosion voltage threshold) of DC voltage on the AC shore-power safety grounding wire, while still allowing AC fault current to flow freely, as it is critical to safety. Because galvanic corrosion is DC in nature, the electrical interconnection of adjacent vessels is prevented by the galvanic isolator.
Isolation transformers take this a step further by isolating all direct shore-power connections to the vessel, including the AC safety ground, between the vessel and dock power, thereby blocking any level of inter-vessel interaction.
Every boat equipped with a shore-power system should utilize one of these devices to prevent corrosion induced by connection through the shore-power safety ground. Galvanic isolators are relatively affordable, while transformers are more costly, the latter, however, offer benefits in addition to absolute isolation. (For more on isolation transformers and galvanic isolators see “The Complexity of Plugging In” in Professional BoatBuilder No. 181, page 100.)
While it is a potentially serious and costly phenomenon, galvanic corrosion occurs at a stately pace, typically over the course of weeks, if not months and years. With proper alloy selection, isolation, and cathodic protection, it can be minimized if not eliminated.
Stray Current
This form of corrosion differs from the galvanic variety in that it only occurs in the presence of an outside source of electricity, usually a vessel’s own DC electrical system, typically a battery or battery charger. AC voltage from shore power doesn’t cause stray-current corrosion except in some very rare circumstances. If it did, the DC voltage blocking ability of a galvanic isolator would be ineffective. In those few instances where AC-induced stray-current corrosion does occur, it is of greatest concern to aluminum-hulled vessels or those equipped with aluminum drives.
Steve D’Antonio Aluminum lower units on outboards or pod drives are particularly vulnerable to stray-current corrosion.
The typical stray-current corrosion scenario involves a faulty electrical connection that is located in, or close to, bilge water or that makes contact with submerged metal. Contrary to popular belief, electricity does not seek ground; it seeks a return path to its source. In the case of stray current corrosion, that’s usually the vessel’s battery. Current leaking into bilge water can travel to a through-hull fitting, into the surrounding water and then to the propeller and shaft, which are grounded to the DC negative system via the engine block, providing a route back to the battery. (Often shafts/props do not represent a low-resistance connection to the vessel’s DC negative or grounding/bonding system because the oil-filled transmission is a poor conductor, however, a bonded stuffing box can provide this path.) In this example, the propeller will almost certainly suffer from severe and rapid corrosion. Unlike galvanic corrosion, which occurs comparatively slowly, stray current corrosion often moves with startling rapidity, potentially destroying a propeller, shaft, or sterndrive in a matter of days.
Sacrificial anodes, galvanic isolators, and isolation transformers offer little if any protection against this electrical scourge. Note that isolation transformers can be beneficial for preventing stray-current corrosion that originates on other vessels.
The most effective means of preventing stray-current corrosion is observing sound wiring practices and adhering to American Boat and Yacht Council (ABYC) Standards everywhere onboard, particularly in and around bilge areas.
Steve D’Antonio A common source of corrosive stray current in the bilge is pump wiring connections that are too close to the water and unprotected by waterproof heat-shrink tubing.
To that end, I insist that electrical connections to bilge pumps and float switches be made no less than 18” (457mm) above the base of the pump (my personal standard, not ABYC’s). The primary reason is to improve reliability; however, this approach also reduces the likelihood of stray current leaking into bilge water and avoids the resulting corrosion. In applications where these connection geometries are impractical, connections should be made completely waterproof using heat-shrink butt splices or simple heat-shrink tubing and, if necessary, application of silicone sealant.
No electrical connections should ever be allowed to lay in bilge water, regardless of water resistance. A detail as seemingly innocuous as improperly crimping, and thereby piercing a heat-shrink butt splice, can create a path through bilge water for stray current. I routinely encounter pierced heat-shrink insulation on a range of boats. It’s usually caused by a technician selecting a crimping die for uninsulated terminals and applying it to the insulated variety, but it can also occur when over-aggressively using the correct insulated-terminal die. Check your connections closely, and seal them with silicone or polyurethane sealant if they are breached.
A bonding system is another strategy to prevent or diminish the effects of stray-current corrosion. Performing as one segment of a vessel’s overall grounding system, it encompasses the DC negative, AC safety ground, and lightning-ground systems, all of which are interconnected in an ABYC-compliant design.
In brief, a bonding system electrically connects underwater metals and many metallic hardware and machinery components, including through-hull fittings, seacocks, rudders, propeller shafts, struts, and strainers. Note that bonding of shafts must be via shaft brushes capable of achieving the ABYC-mandated 1 ohm or less of resistance between shaft and anode, which is typically only achievable using silver slip-ring brush assemblies. Common and inexpensive wand/carbon or bronze brush styles will not meet the maximum resistance standard.
Steve D’Antonio A bonding system connected to all underwater metal hardware and machinery protects them from stray-current corrosion by returning any fault current from onboard systems directly to the battery.
There are two primary benefits to bonding. The first is mitigation of stray-current corrosion. In the scenario I described above—voltage leaking into bilge water from a defective bilge pump connection—if the seacock through which the fault current flowed was bonded, all or most of the current would return through the bonding system directly to its source, the battery. It would not run through the water the vessel is floating in, thereby eliminating or minimizing the stray-current damage to the propeller.
The second benefit of a bonding system relates back to galvanic corrosion. The metals that comprise a bonded system are nearly always dissimilar—silicon bronze seacocks, stainless-steel alloy shafts, and manganese-bronze propellers for instance. This necessary mix of metals with different potentials violates the guidelines on galvanic corrosion. Bonding systems provide an exception to the rule because they contain one additional ingredient in the seawater-metal cocktail: a sacrificial hull-mounted anode. These are commonly installed on the transom of a planing vessel, and on the hull bottom of a displacement hull. Connecting underwater metals to each other, then to an anode, follows the bond-and-protect protocol, a proven approach so long as you follow a handful of guidelines. Chief among these is ensuring low-resistance connections are made between all bonded components and hull anodes. ABYC’s resistance standard for all these connections is a demanding 1 ohm maximum.
Steve D’Antonio On a planing hull, the anode for an onboard bonding system mounts to the transom below the waterline.
Where fiberglass vessels are concerned, bonding systems are strongly recommended but not mandatory for compliance. Their installation guidelines are detailed in ABYC Standard E-2, “Cathodic Protection.”
On most of the boats I inspect, bonding systems and their connections are in abominable condition—green, crusty, loose, or broken all together. They are a reminder that bonding systems need to be periodically inspected and maintained. Corroded or otherwise poor connections should be cleaned or replaced. If you have doubts about the integrity of the system, check resistance between components using an ohm meter while the vessel is hauled out, using an ohm meter. (For more on bonding systems see “Bonding Basics” in Professional BoatBuilder No. 138, page 18.)
Corrosion Subsets
With a firm understanding of the most common forms of metal corrosion on boats, it’s time to look closely at some of the applications, behaviors, and peculiarities of different metals you’re likely to encounter.
Copper Alloys
While copper is naturally corrosion resistant, it has been best known to boatbuilders for centuries applied in sheets as metal cladding on hull bottoms to prevent teredo worms from digesting timbers. A natural antifoulant, copper is still the active additive in many modern bottom paints. Because it is an excellent biocide, on many vessels it is used in domestic water systems. Copper alloy tube is also used by many engine manufacturers for conveyance of seawater, for heat exchangers, and for wet-exhaust systems.
As corrosion-resistant as good copper hardware can be, it is susceptible to two forms of corrosion: impingement attack, or “erosion corrosion,” caused by fast-moving water in a copper pipe or tube. Because copper is relatively soft, it can literally be worn away by swiftly moving seawater. This is especially true at 90° turns and restrictions where turbulence and velocity are greater.
Copper can also fall prey to decay by exposure to hydrogen sulfide, a common constituent of polluted or onboard “black” water.
The terms brass and bronze encompass numerous copper alloys, the primary defining element of which is zinc. Located low on the galvanic scale, it corrodes very easily in the presence of other metals and an electrolyte, such as seawater, which is why it’s used in sacrificial anodes. Zinc, however, also imparts strength to copper, mixing the two, often in about a 40:60 ratio, yields brass suitable for clocks, lamps, and cabin hardware, but most definitely not, with few exceptions, for seawater plumbing.
Brass
It’s a metal that is almost synonymous with the sea, and resides in the family of copper alloys whose primary ingredients, in varying ratios, are copper and zinc. Some brasses comprise of as much as 50% zinc. The more common formulations are; red brass (85% copper, 15% zinc); leaded red brass (85% copper, 5% zinc, 5% lead, 5% silicon); cartridge brass (70% copper, 30% zinc); Muntz metal (60% copper, 40% zinc); admiralty brass (70% copper, 30% zinc); naval brass (60%copper, 40% zinc); aluminum brass (76% copper, 22% zinc, 2% aluminum); Tobin “bronze” (60% copper, 39% zinc and 1% tin); and manganese “bronze” (67% copper, 25% zinc, 3% Aluminum, 2.5% manganese, 2% iron). The last two in the list are called “bronze” but because of their high zinc content, they are in the brass family. In order to be considered a true bronze, a copper alloy must contain little (typically single percentage points) or no zinc.
Steve D’Antonio Brass is often inadvertently installed in pipe-to-hose adapters and pipe plugs in raw-water systems where it is vulnerable to dezincification.
While brass has many familiar uses aboard, from clocks and joiner work trim to lamps and electrical components, it must be avoided for use below the waterline or in raw-water plumbing. It is worth noting that many small (1/4″-3/8″ NPT) plumbing fittings, such as pipe-to-hose adaptors, pipe plugs, and bushings are made of brass, while their larger cousins are bronze. Most seacock and sea-strainer drains are fitted with 1/8″ or 1/4″ NPT plugs. Under no circumstances should these be replaced with brass. Where replacements are needed, 1/8” bronze plugs can often be obtained from the equipment manufacturer.
If you are unsure of a plug’s material, avoid using it below the waterline or for raw seawater. In my experience, you shouldn’t rely on chandlery clerks and many marine equipment vendors to provide accurate alloy information. Many are unaware of the important distinction between brass and bronze and the related prohibitions for use of the former.
Brass is often inadvertently substituted for bronze in water-injection ports on stuffing boxes, which frequently call for a threaded pipe-to-hose adaptor of 1/4” or 3/8”. I’ve seen this error lead to vessel loss.
If its primary constituent is copper, why is brass so susceptible to corrosion? It’s because most of the brass alloys also contain a substantial amount of zinc, which leaves them especially susceptible to a type of corrosion referred to as dezincification. This is a process whereby the zinc selectively corrodes from the alloy, leaving a porous copper shell that retains its shape but little strength.
Dezincified components can often be identified by splotchy reddish or pink coloring. A common example involves propellers, many of which are fabricated from a Manganese bronze, a brass alloy that’s especially susceptible to dezincification. It’s imperative to protect this often-substantial investment in essential propulsion hardware by monitoring the condition of sacrificial shaft anodes and renewing them when necessary. An anode is considered “depleted” when diminished to 50% of its original dimensions. In a conventional combination of proprietary shaft alloy and brass propeller, once the anode is gone, the next least-noble metal (or metal most likely to corrode) is the prop, which will begin to corrode, sacrificing itself for the shaft, which is more noble.
Steve D’Antonio The telltale pink hue of dezincification in this propeller suggests a need for replacement or addition of sacrificial shaft anodes (zincs).
Brasses are susceptible to other forms of corrosion in addition to dezincification. A common ingredient of household cleaners, ammonia, will readily attack brass, causing it to weaken and crack. Mercury is also corrosive to brass, as is the hydrogen-sulfide rich polluted water mentioned above.
Corrosion resistance requirements limit the range of materials from which seacocks and related components should be made. Only bronze, DZR brass, glass-reinforced plastic, and in some cases, stainless steel may be used.
While copper-zinc alloys used below the waterline are vulnerable to dezincification, that’s not true of true bronze alloys with primary elements of copper and tin. Other common alloying elements include silicon and nickel, technically making these alloys something other than bronze, but still acceptable for use in seawater plumbing.
Unfortunately, there’s a broad array of alloys between true bronze with no, or very little zinc and true brass, which contains a high percentage of zinc. Two common alloys often used in marine applications are 85-5-5 and DZR. 85-5-5 contains 85% copper, 5% zinc, 5% lead, and 5% silicon, and can be used below the waterline in seawater applications.
Some European manufacturers use the alloy referred to as DZR brass, a dezincification-resistant brass alloy with a higher zinc composition than many other copper alloys (30% or more). But it also includes trace amounts of other metals meant to retard zinc corrosion or leaching. To be used with confidence, DZR brass hardware must be embossed or de-bossed with the letters DZR, rather than simply a packaging description.
Leaded red brass is another copper alloy sometimes used in marine applications. Composed of copper, zinc, and lead, it is commonly used for the manufacture of pipe nipples and may be used in seawater applications provided the zinc content does not exceed 15%. (Note: I have never seen a pipe nipple marked with its alloy makeup, making it challenging to know whether a part is suitable or not.)
None of these alloys resists dezincification nearly as well as the zinc-free, or near-zinc-free, bronze alternatives. Because they are less expensive and unlikely to become a problem in the early years of a boat’s life, they can be an attractive alternative for more cost-conscious builders.
Bronze
Although it is a copper alloy, because bronze is free of any appreciable amount of zinc, it is not susceptible to dezincification. Its parent element is copper, its alloying element is tin, and possible trace amounts of zinc may be added to improve machinability. Pound for pound, it is often more expensive than brass, which contains less costly zinc.
Gunmetal is a bronze alloy commonly found in marine applications, although, as its name suggests, it was historically used for firearms manufacture. It is a bronze alloy consisting of 88% copper, 10% tin, and 2% zinc. While not especially strong, it is corrosion resistant and perfectly acceptable for below the waterline use. Gunmetal is often used to fabricate such cast items as cleats, chocks, and shaft logs.
Another useful bronze alloy, aluminum bronze, is strong but can be susceptible to de-aluminumification. This phenomenon is prevented with the addition of nickel to form an alloy commonly referred to as NI-BR-AL (nickel, bronze, aluminum). This strong corrosion-resistant bronze is often used for propellers and struts. Manganese is sometimes added to this already mixed soup of metals to further increase strength. Prop repair shops sometimes charge a premium for reworking NIBRAL props due to their tendency to take on a set or memory. This alloy should not be confused with manganese bronze.
Silicon bronze is by far the most popular alloy for the fabrication of underwater hardware. Most quality seacocks are made of this extremely resilient metal. Its common formulation is 96% copper, with the remaining 4% being silicon and other trace elements, sometimes including very small amounts of zinc. Bronze hardware such as nuts, bolts, and screws are often made of this alloy and can be expected to give long, corrosion-resistant service.
Phosphor bronze is typically made up of 85-95% copper and 5-10% tin, with the addition of a small amount of phosphorus, which improves hardening characteristics. It is a choice material for bearings and springs.
While the varieties of copper alloys tailored to very specific applications are so numerous that I haven’t the space to address them exhaustively here, the most important thing to remember on a boat is that most brass is ill-suited for most application where it’s called upon to convey or stem the flow of raw/seawater, regardless of whether it’s used above or below the waterline. This includes plumbing associated with raw-water strainers, stuffing boxes, seacocks, sanitation, and air-conditioning systems. Bronze alloys, on the other hand, are well suited to demanding service below the waterline.
In Part 2, we’ll continue our primer on corrosion prevention at sea by looking in depth at stainless steel and aluminum.
About the Author: For many years a full-service yard manager, Steve now works with boatbuilders and owners and others in the industry as Steve D’Antonio Marine Consulting. He is an ABYC-certified Master Technician and sits on that organization’s Engine and Powertrain, Electrical, and Hull Piping Project Technical Committees. He is also technical editor of Professional BoatBuilder.
