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Corrosion can feel like black magic, but it’s only chemistry. Here are common misconceptions and real-world explanations for what’s really happening.
Corrosion can take many forms and comes in many appearances, but all corrosion follows the same principles. Photo: Kelsey Bonham Bailey (3), Getty images/Demien Vasilin, BoatU.S. File Photo
Most boats have a lot of metal aboard, so every boater needs to be concerned with corrosion. It deteriorates electrical connections, weakens fasteners, stops our electronics from working, and in worst-case scenarios, can jeopardize important below-water fittings – and we normally don’t discover the damage until the piece fails.
While misleading advice can run rampant around the docks, from the metal’s perspective it’s simple: We dig up some metal ore and manufacture it into something useful, and from then on that metal wants to return to its naturally occurring, most stable chemical state. We need to prevent that, and so it helps to understand properties of different metals we use on our boats.
When we process metal ores into useful metal products, we usually do so using a lot of heat, and that energy gets stored inside the metal’s new chemical structure. To return to its original form, the metal wants to release that energy by giving up electrons, which it can do when there is another metal willing to accept them (a cathode), a common electrolyte (a liquid solution that can conduct electricity, such as seawater), and a conductive pathway (a means for those electrons to travel, like direct contact or a wire).
When those three ingredients are present, our metal in question will give up its electrons and, as it does, its chemical structure degrades and the process of galvanic corrosion begins. With that in mind, here are some common myths about how this chemical reaction happens in the real world.
Myth 1: Two metals need to touch for one to corrode
Galvanic corrosion requires two different metals to be present, but they don’t need to be directly touching as long as there is still some way for electrons to travel between them. One example is the green wire in the AC electrical system of a marina. Through that wire, there is a conductive pathway among all the boats that are plugged in, so the metal on one boat could be the anode to – and give up its electrons to – the metal on another boat, which is why installing a galvanic isolator to interrupt this pathway is a good idea!
Galvanic corrosion can also occur with only one metal, because many of the metals we use on boats are alloys made up of multiple metallic elements. Bronze, for example, is mostly copper and tin but can include small amounts of other elements, sometimes trace amounts of zinc. Copper and tin are relatively stable together; but over time, zinc will give up its electrons to copper, and the bronze will corrode (and turn pink). This is sometimes called “self-corrosion,” but it still falls under the umbrella of galvanic corrosion and happens for the same reasons.
Myth 2: Corrosion is only a problem in saltwater
Corrosion is a much more prominent issue in saltwater environments because saltwater is a good conductor and encourages electrochemical reactions – of which corrosion is just one example – to occur. But that doesn’t mean that corrosion can’t happen in freshwater, albeit usually much more slowly. Freshwater is still conductive because it’s not pure. Even without salt, there are still plenty of dissolved materials floating around that can help conduct electricity.
Myth 3: Some metals are immune
All metals can corrode; some just corrode more easily than others. Mild, ungalvanized steel without any protective paint, for example, can corrode into rust within a matter of days. Stainless steel, however, is more resistant because it contains chromium. The chromium forms a tightly adhered chromate oxide layer when in contact with oxygen – essentially, stainless steel makes its own “paint” to shield the rest of the steel from the outside environment. Rust is also an oxide layer – iron oxide – but because it’s not tightly adhered, it flakes off, exposing more bare steel to the environment and allowing more rust to form, until it’s all rust.
When that chromate oxide barrier is damaged or not allowed to form, however, stainless steel will still rust. The oxide barrier can be damaged by dings and scrapes, when salt is allowed to sit on it for too long, or if kept in an oxygen-deprived environment – think of a screw left under a wet towel, or a fitting that goes through the deck without proper sealant to prevent water intrusion, or a prop shaft sitting idle inside a narrow bearing without adequate water flow.
Photo: Getty Images/Slobodan Miljevic
What’s The Big Deal About Brass?
Bronze has been used aboard boats for centuries due to its natural resistance to corrosion (as well as its attractiveness). Unfortunately, while bronze and brass can appear identical on the shelf of a chandlery and are sometimes even labeled interchangeably in home hardware stores, the two metals are very different.
Both bronze and brass are copper alloys, meaning they are majority-copper. “True” bronze technically contains no zinc, but many copper alloys are still considered bronze when their zinc content is in the low single digits of percentages.
Brass, on the other hand, contains much more zinc. Aluminum brass is 22% zinc, admiralty brass is 28% zinc, and naval brass (which one might assume to be marine grade, given the name) contains a whopping 39% zinc. Zinc readily gives up its electrons to copper, meaning that a brass component on a boat may only last a few years before it corrodes to the point of failure.
The best way to avoid accidentally using brass when you’re looking for bronze — as well as the best way to ensure the quality of any metal going on your boat — is to go to a marine-specific store and buy from a trusted, American-made brand (European manufacturers of metal components for boats often have more relaxed standards for what constitutes bronze vs. brass). Groco, which manufactures in Maryland, is one such example of an American-made brand.
For nonintegral metal items aboard, like decorative barometers, ships’ bells, or other instruments, the use of non-marine-grade metals like brass isn’t as big of a deal. Use brass polish to fend off any tarnish. — K.B.B.
Myth 4: Electrolysis is a kind of corrosion
Nope! Believe it or not, electrolysis is not a kind of corrosion, according to the American Boat and Yacht Council. Electrolysis is a scientific technique that involves using DC electricity to drive chemical reactions for a variety of purposes: It’s used in DNA analysis, hair removal, electroplating metals, and even to split water molecules into hydrogen and oxygen to maintain the air supply in submarines.
When boaters talk about electrolysis, usually they’re referring to electrolytic corrosion, or corrosion that occurs via the electrolytic process. The next time your buddy down the dock says, “I think electrolysis is eating up my propeller,” feel free to pull out a, “Well, actually it’s electrolytic corrosion!” I’m sure he’ll really appreciate that!
Myth 5: Stray current is wrecking my boat
Electrolytic corrosion is a truly terrible kind of corrosion. Also called stray current corrosion, and sometimes mistakenly called electrolysis (see Myth 4), electrolytic corrosion occurs when there is an active electrical current being applied, like if a live wire became disconnected and dunked into the bilge water, as opposed to there simply being an available pathway for electrons to travel. But despite the fear it justifiably warrants, in reality it’s not too common.
Electrolytic corrosion occurs significantly faster than galvanic corrosion. And by “faster,” that can mean days. You can go from a perfectly healthy propeller or thru-hull on Monday to a crumbling mess on Thursday. The chemical reaction can happen so aggressively that it creates bubbles and discolors the water. If you put a healthy propeller in the water in May and it comes out with some light discoloration or pitting in October, it’s highly likely that was run-of-the-mill galvanic corrosion and not a stray current issue.
Myth 6: Carbon fiber is irrelevant to corrosion
With the increased use of carbon fiber components and even carbon fiber hulls, it’s worth mentioning that carbon fiber can be a part of the corrosion equation. Carbon fiber is made from graphite, which is a cathode to and potential corrosion cause of stainless steel, titanium, bronze, brass, copper, aluminum, and pretty much any other kind of metal you might find on a boat.
As long as carbon fiber is isolated from other metals, nothing will happen. For this reason, carbon fiber hulls and components are generally sealed with vinylester resin, which isolates the graphite from the outside environment. However, fittings and anything that goes through a carbon fiber structure require special attention; ABYC advises insulating all power sources and electrical components in a carbon fiber hull, given the material’s conductivity. If you have a carbon fiber hull or component, it’s worth consulting an expert before drilling any holes or making any modifications, especially electrical ones.