The lowly engine zinc

How many power voyagers regularly check your engine zincs? Unfortunately, like Rodney Dangerfield, engine zincs just “don’t get no respect.” Nor any attention either. Typical replies to this questions go something like, “I don’t know where they are,” or, “I don’t know what they look like,” or even, “Was I supposed to?” I like this comment from a frustrated contributor to an Internet yachting forum, “I spend the better part of an hour reading the factory manuals and going over the engines with two friends and none of us could find any zincs. The manuals say ‘replace,’ but they don’t say where!”

As for the question, “Was I supposed to?” the correct answer is an emphatic “yes!” Engine zincs are essential in an engine’s raw-water circuits — heat exchangers, oil coolers — to protect them from galvanic corrosion. These devices are typically constructed of thermally-conductive metals — copper, brass, aluminum, steel — and in the presence of elevated temperatures and salt water, will experience rapid galvanic corrosion if not protected. Failures in heat exchangers and oil coolers can result in serious (read, expensive) engine damage.

Galvanic corrosion
Any time two or more different types of metals are immersed in the same electrolyte (water, especially salt water), and that are either in physical contact or electrically connected, one of these metals will corrode. Which metal corrodes first is determined by its place in the galvanic series of metals in seawater table. The metals at the bottom of the table are referred to as cathodic, or noble, while those at the top of the table are referred to as anodic, or less noble. The least noble (anodic) metals are sacrificed to protect the more noble metals — kind of like war in medieval times. When the least noble metal has been fully depleted, the next least noble metal will begin to corrode. Hence the use of sacrificial zincs and the importance of maintaining them.

All three of the least noble metals shown in the table are used as sacrificial anodes, depending on the metals to be protected, whether the water is fresh, brackish or salt, the degree and nature of any pollutants, temperature, etc. Generally speaking, zinc anodes will provide good protection in both salt and brackish water. Do not use zinc anodes in fresh water. When used in fresh water, the zinc tends to passivate, that is, it forms an insulating film of zinc hydroxide that prevents it from functioning as a sacrificial anode. Aluminum anodes will provide protection in both salt and brackish water. But aluminum should not be used in fresh water; it is not sufficiently active to overcome the lower electrical conductivity of the fresh water. Magnesium anodes should only be used in fresh water. If used in salt water, magnesium tends to deplete extremely rapidly and presents the danger of overprotection, causing hydrogen bubbles to form under paint coatings on sterndrives and outboards.

Aluminum and zinc are by far the most widely used in salt and brackish water; magnesium anodes are used exclusively in fresh water due to its lower conductivity. Bottom line — use the anodes specified by the engine manufacturer.

Outboard and sterndrive engines
The question of which anodes to use on outboard and sterndive engines answers itself. The lower unit is protected by one or more sacrificial anodes located below the waterline. These come in various shapes and sizes, most unique to individual brands and models of motors. Anodes may be combined with a trim tab attached to the cavitation plate, in which case, it not only provides protection against corrosion, but also may be adjusted to compensate for right-turning torque on a single motor installation with a clockwise-turning prop. In other cases, the anode may be built into the bearing carrier that mounts over the prop shaft.

What we’ve said here about the uniqueness in shape and size of outboard and sterndrive anodes applies to the protection of props and lower units. Sterndrives — also called inboard/outboards — which are exactly that, an inboard engine coupled to an outboard motor type drive and prop — require additional, but different, forms of anodic protection.

Inboard engines
With inboard engines, the problems are essentially the same — dissimilar metals, sea water, elevated temperatures, galvanic corrosion and potentially expensive engine damage. The solution — the anodes — are different. Called “pencil zincs,” they consist of a brass hex-head threaded plug cap with a removable/replaceable zinc cylinder roughly 2 to 3 inches long and about the thickness of your little finger that threads into the inside of the brass plug cap. The problem is that when installed, they look like drain plugs and just about every other hex-head bolt on the engine. However, we do have some clues as to their whereabouts.

Pencil zinc anodes are typically located in the engine’s raw water circuits of heat exchangers, oil coolers, and water-jackets on raw-water cooled engines, etc. They are usually located at, or near, the suction side of the raw-water pump, in the heat exchanger or oil cooler, typically mounted at the bottom and at either (or both) ends of the housing.

Making a list and checking it
So, how often should you check them and what should you look for? Opinions vary here, because much depends on the water your boat lives in, the engine itself, how often and how long it runs, etc. This is not a big problem with external zincs on outboards and sterndrives, just take a look whenever you put the boat on the trailer or lift it out of the water.

With internal engine zincs it’s a bit more work, but well worth the effort when you consider the relative cost of repairing a damaged engine versus the cost replacement pencil zincs — typically less than $10 each. Unscrew the hex-head cap gently, wiggle the zinc out gently, so as not break it off inside. Tap it gently, but firmly to remove the white, flaky depleted zinc. When the zinc appears to have been depleted to about one-half of its original size, replace it. Don’t wait until it’s almost gone, by then the protected parts will already have begun to corrode.

If your engine is newly installed, you should check the zincs monthly until you have an understanding of the rate at which they are corroding. Again, 50 percent depletion is the replacement point. You may find that once a season at your annual haul-out is sufficient. Or, perish forbid (as my sainted grandmother used to say) you may find that they are corroding much faster. In which case, you should check around with other owners in your marina or mooring field. If your zincs are corroding significantly faster than theirs, you may have a stray current problem on your boat, in which case, the services of a qualified marine electrician are in order.

If they are not corroding noticeably, they may not be doing their job. This could be because the pencil zinc is not in good electrical contact with its brass holder or the brass plug is not making good contact with its housing. Perhaps a well-intentioned installer used a thread lock or sealant (these are polymers and electrical insulators) and failed to torque the pencil zinc or the brass hex-head down sufficiently to ensure good electrical contact. A continuity check with a multimeter will tell the story. Be careful in tightening the pencil zinc in the brass plug; zinc is quite soft and is easily sheared off.

In any event, the number and location of zincs should be listed in your engine’s manual; failing that, you should spend a few moments with your motor’s dealer or mechanic and ask him to show you where they are and how to remove and replace them. While you’re there, pick up a few spares, they can be hard to find in the more remote voyaging destinations.

By Ocean Navigator