Curbing galvanic corrosion
Strange things happen when metals meet water, so be prepared with sacrificial anodes.

Scrunched up under the galley sink, the insulated panel removed to provide access to the engine compartment, I gave the brass nut on the heat exchanger a turn with a wrench. Slowly, I backed the nut out, the white Teflon tape on the threads shredding in long, stringy lengths. To my utter surprise and annoyance, the long pencil zinc that should have been there, albeit in a quasi-consumed state, was gone.

Two important points became quite clear. The first: Always double-check any job you pay a boatyard to do. I’d specifically asked the yard’s crew to replace the pencil zinc when they winterized the engine. Evidently, they hadn’t, and I’d only discovered it because I looked halfway through the sailing season. The second point was obvious, but vividly reinforced: Zincs are there for a reason, and it’s vital to keep an eye on them to prevent damaging galvanic corrosion from attacking the dissimilar metals that live together in engines, saildrives, water heaters, props and shafts and through hulls.

That missing zinc sure sacrificed itself. Hence the proper name for it and others of its kind is sacrificial anode. The anode part of the term will shed some light on just how the zinc disappeared. It gets complicated, but the simplest way to explain what happened to it is to think of how a battery works.

To make a battery, you need a positive electrode to receive electrons (current) from the external circuit when the cell is discharged, and a negative electrode to supply the electrons. You also need an electrolyte to facilitate the flow of current between the two. When two different metals are joined and submersed in water, an electrical circuit is created. Essentially, you’ve made a battery. The water serves as the electrolyte and carries the current between the two metals. The anode, or the least noble metal, gives up bits of itself (corrodes) in the form of metal ions as the current flows from it. The cathode, or the more noble metal, doesn’t corrode. A more noble metal simply means it has a neutral or negative electrical potential; it won’t as readily generate a positive flow of ions, whereas a less noble metal like zinc will.

The process I’ve just described is called galvanic corrosion. It’s often confused with electrolysis, especially when talking about corrosion problems rightly or wrongly associated with marina shore power systems, but they’re not the same. Electrolysis is strictly defined as the production of chemical changes by the passage of an electric current through an electrolyte. The better term for what may be happening in a marina or in a boat’s AC or DC system is stray current corrosion, which I’ll get into later.

A diet of zinc
My pencil zinc disappeared because it served as the anode in the electrical circuit created inside the engine due to the presence of dissimilar metals joined together in an electrolyte, or seawater. That’s why its proper name is sacrificial anode. By placing in the circuit a metal that’s less noble than, say, a stainless steel prop shaft and a bronze propeller, the more noble metals don’t release the electrons and therefore don’t corrode. The less noble metal is more active electrically and does corrode.

Take that zinc out of the equation and the process continues. Only now it’s expensive bits of the engine’s inner works that are being gradually eaten away by galvanic corrosion. In the case of a bronze prop on a stainless steel shaft, bronze is less noble than stainless steel, so it’ll be the prop that serves as the anode and will suffer the effects of galvanic corrosion if a sacrificial anode isn’t present.

Corrosion problems are more common in warmer water because higher water temperature increases its conductivity. Salt water is also a better conductor than fresh water. That’s why boats in the tropics tend to suffer more from the effects of galvanic corrosion than those in places like Maine.

The amount of protection a sacrificial anode can give is a matter of its surface area and weight. Factors such as the types of dissimilar metals it’s meant to protect, whether the water is fresh or salty, the water temperature, and the composition of the hull all come into play. Generally speaking, an anode should last a year before it needs replacing. When it’s half gone, that’s the time to install a new one. Zincs that disappear quickly and look shiny, instead of being covered with a layer of zinc oxide, indicate possible trouble with stray current.

Sacrificial anodes needn’t be zinc, though zinc is most common because it’s cheap and one of the least noble metals around. However, in fresh water, zincs develop a coating of zinc hydroxide that insulates it and stops it from supplying those electrons. In other words, it no longer serves as an anode and thus stops sacrificing itself. This can happen in just a couple of months.

Naturally, painting a sacrificial anode will do the same thing. Another good way of stopping the anode from working is to install a new one on a dirty prop shaft. Dirt or paint on the shaft will interrupt the electrical connection. The same goes for anodes mounted on rudders. To work, they’ve got to have a good electrical connection to the metal they’re protecting.

There is an alternative to zinc sacrificial anodes. Performance Metals manufactures sacrificial anodes made from an aluminum-indium alloy that’s less noble than zinc, making it an excellent material to fight galvanic corrosion. According to the company, it has more current capacity for the same volume, which means a given anode made from aluminum alloy will last 50 percent longer than a comparable one made from zinc. It’s also one-and-a-half times lighter than zinc.

Magnesium is less noble than either zinc or aluminum alloy and can provide superior protection in fresh water, which is why it’s used as sacrificial anodes in water heaters. However, in salt water it’ll rapidly dissolve and can actually cause damage.

Stray currents
It would be great if that was all there was to think about. You just make sure your sacrificial anodes are in good shape and have a good electrical connection to the metals they’re protecting, and that’s that. However, it gets a lot more complicated in a big hurry if you start considering the effects of stray electrical current either coming from your boat’s AC or DC electrical system, from that of a nearby boat, or from a marina’s shore power system. Stray current can quickly accelerate the process of galvanic corrosion and rapidly erode
sacrificial anodes.

Stray current can come from a variety of sources. A damaged wire lying in bilge water will emit current that will seek a path to an anode, then to ground through the water around the boat, thereby introducing an external electrical charge to set up the corrosive reaction in metals. If a boat at the dock has faults in its electrical system, it can damage other nearby boats since all vessels are linked together via the ground in the shore power system. Current can flow between the AC ground and the boat’s DC bond. It’s also possible to create a galvanic cell between two or more boats. The one with the less noble metals will serve as the anode and suffer damage from corrosion.

Fortunately, there is a simple solution to fighting stray current corrosion at the dock: a galvanic isolator. Installed between the AC safety ground and the DC bond, these transformers block most of the stray low voltage that may be coming from electrical faults in nearby boats or from the marina’s shore power system. They must be rated to carry 135 percent of the shore power current in case the AC circuit breaker fails. In other words, don’t install a galvanic isolator rated for 30 amps on a 50-amp shore power system just to save money.

Not all boats are equipped with galvanic isolators. Sonata, our 36-foot Pearson cutter, was built in 1981, and it didn’t have one. Before we began living aboard, the boat remained mostly at moorings, where the only stray current I had to worry about was my own. But when we decided to move onto Sonata I had a certified marine electrician go over the AC system, which we’d hardly ever used. He strongly recommended that we spend the couple hundred dollars to buy and install a galvanic isolator, which we did, of course. If your boat isn’t equipped with one and it’s at a marina, you’re opening yourself up to some potentially very costly problems.

Zinc fish are another good idea when staying for long periods in marinas while connected to the shore power system. These are simply large pieces of zinc with a heavy gauge wire embedded in the metal. The other end of the wire is fitted with alligator clips. For it to work, the fish must be hung over the side in the water and the alligator clips must be attached to clean metal that’s wired into the boat’s bonding system. Clipping to the rigging may not guarantee a good electrical connection, and clipping to a stanchion definitely won’t work since stanchions aren’t usually bonded. On Sonata, I clip directly to a stainless steel strut mounted on the engine. That way I know the zinc is protecting everything attached to the engine, including the zincs on the prop shaft.

It’s easy to forget about things like sacrificial anodes. They’re out of sight and therefore mostly out of mind during the sailing season. However, they play a key role in protecting the boat from damaging galvanic corrosion. Keeping an eye on them just makes good sense.