Anodes

By Martin Wigg

Determining which sacrificial anode will fully and safely protect a boat depends on a number of factors. As previously discussed, it is important to understand the physical properties of each anode material. In addition, boaters should also understand how each anode works with different hull compositions, sterndrives and types of water.

The best way to determine whether a boat is fully protected is by measuring its cathodic protection voltage or hull potential using a voltmeter. But, if one isn't available, here are some simple guidelines for selecting the right anode.

The hull material of a boat determines, in part, which anode material to use. A fiberglass boat having an inboard engine with bronze and stainless metal parts needs less protection than an aluminum hull or a boat with an aluminum sterndrive. Zinc or aluminum alloy anodes will work well for these types of boats. The voltage generated by these anodes cannot overprotect, i.e., they cannot cause any damage no matter how much anode material is added. The maximum voltage generated is the voltage of the anode itself. Magnesium would also work with a fiberglass boat but only in freshwater. However, aluminum or wooden hulled boats can be overprotected by very active magnesium. Steel hulls can also be overprotected and the excessive protection voltage will rapidly lift the paint off the hull.

Sterndrives and outboard motors, because of their very active aluminum assembly, are hard to protect. Initially, the anodes for these units were made of zinc. But corrosion problems in the early 1990s sparked the major engine manufacturers to start selling aluminum alternatives. The increase in protective voltage ensures that the sterndrive is protected. Today, in some cases, using zinc may invalidate an engine's warranty. Again, caution is needed when using magnesium anodes as they can overprotect.

Water type is the final influential factor when choosing an anode. When used in freshwater, a zinc anode forms a coating of zinc hydroxide that insulates it and stops it from working. Magnesium used in saltwater can disappear very quickly, and if it is used on an aluminum sterndrive or outboard motor, it can be very dangerous. If piloting a boat with an aluminum hull or sterndrive down river and into the ocean, the overprotection of the magnesium anodes would cause hydrogen bubbles to form under the paint on the hull or drive resulting in it being literally blown off. Acid rain and pollution can also increase the conductivity of freshwater to the point where the same thing will happen.

Aluminum alloy is the only anode material that is safe for use in all types of water and accepted by the major sterndrive manufacturers as the best material to use. It is lighter and protects better than zinc and is not so active that it becomes dangerous like magnesium. Aluminum alloy is also environmentally friendly, unlike zinc, which is considered a pollutant.

Editor's note: Martin Wigg is the president and founder of Performance Metals. He holds an MBA and a Bachelor of Science degree in mechanical engineering. Paul Fleury is certified by the American Boat and Yacht Council as an Electrical Technician and the National Association of Corrosion Engineers as a Corrosion Technologist.

Sacrificial Zincs
by Don Casey

Any time you have two different metals that are physically or electrically connected and immersed in seawater, they become a battery. Some amount of current flows between the two metals. The electrons that make up that current are supplied by one of the metals giving up bits of itself-in the form of metal ions-to the seawater. This is called galvanic corrosion and, left unchecked, it quickly destroys underwater metals.

The most common casualty of galvanic corrosion is a bronze or aluminum propeller on a stainless steel shaft, but metal struts, rudders, rudder fittings, outboards, and stern drives are also at risk. The way we counteract galvanic corrosion is to add a third metal into the circuit, one that is quicker than the other two to give up its electrons. This piece of metal is called a sacrificial anode, and most often it is zinc. In fact, most boaters refer to sacrificial anodes simply as zincs.

It would be hard to overstate the importance of maintaining the zinc anodes on your boat. When a zinc is gone, the metal component it was installed to protect begins to dissolve-guaranteed.

How much zinc
The amount of protection a zinc anode provides depends on its surface area. The zinc surface area needed varies with the kind of metal being protected and with the chemical make-up of the water, but you can use 1% of the surface area of the protected metal as a starting point. Check the protected metal frequently. If it shows signs of corrosion despite the zinc, you need more surface area.

Zincs should be replaced when about half of the anode has been lost to corrosion. Ideally we want that to occur not more frequently than annually. The longevity of a sacrificial zinc anode is a function of its weight. When a zinc lasts less than a year, you need one with more weight.

Normally, however, you are not faced with determining the appropriate anode size (other than diameter for a zinc shaft collar). Rather, you are simply replacing depleted zincs with new ones of the same size. Check all zincs at least annually and replace all that are half depleted. Here are some replacement guidelines.

Electrical contact is essential
There is an unfortunate misconception that a sacrificial anode can be mounted anywhere, even hung over the side on a string, and it will still perform its appointed duty. That is dead wrong!

For a zinc anode to provide any protection, it must be in electrical contact with the metal being protected. The conductivity of the water is not adequate. We need low-resistance, metal-to-metal contact-either by mounting the zinc directly to the metal being protected or by con- necting the two with a wire. A hanging anode can provide protection if it is connected by a wire to the metal being protected.

Where the zinc is mounted directly to the protected metal-bolted to the side of a metal rudder, for example-it is essential to make sure the surface under the zinc is bare and bright before the anode is installed. This is to ensure good electrical contact.

No paint
Zinc anodes cannot perform their function unless they are exposed. Putting paint on a zinc smothers it, rendering it useless. Never coat zinc anodes with bottom paint, or anything else.

Props and rudders
Propellers are normally protected by a zinc collar fashioned in two pieces and bolted together around the shaft forward of the propeller. It is essential to make sure the shaft is clean and bright before clamping the collar to it. Corrosion protection for outboard and outdrive propellers is typically provided by a bolt-in-place zinc ring or a zinc prop nut.

Metal rudders and struts are most easily protected with zinc disks bolted directly to the metal. Rudder zincs have a shallow dome shape to streamline them and minimize their drag and turbulence.

Hull plates
Bonding is a different subject altogether, but boats with all underwater fittings bonded together electrically are typically fitted with one or more zinc plates bolted to the hull. The mounting bolts for these anodes are connected by heavy-gauge electrical cable to the bonding circuit. If these anodes are allowed to deplete or if the electrical connection deteriorates, other underwater metal, such as bronze through-hull fittings, will begin to corrode.

Zinc hull plates are also fitted to metal boats to protect the hull. Needless to say, such anodes must be carefully monitored.

Outdrives
The mix of immersed metals makes stern drives and outboards particularly prone to galvanic corrosion. Many are fitted with multiple anodes. Typically, these include at a minimum a sacrificial trim tab (intended to warn you of depletion by a change in steering), a zinc plate or two attached to the gear case or the anti-ventilation plate, and perhaps anodes in the exhaust cavity and in the cooling-water jacket. It is a good idea to consult your engine manual to be sure you know where every anode is located. Then check all of them and renew any that are more than half depleted.

Zinc pencils
Heat exchangers, because they are typically a copper alloy, are at risk of galvanic corrosion. To combat this, most heat exchangers are fitted with a zinc "pencil" anode. You will find it (or not) under a brass plug in the exchanger. The pencil is unscrewed from the plug for replacement. Some engines have a similar zinc pencil inside the cooling-water jacket to protect dissimilar metals in the engine. Determine if your engine and heat exchanger are fitted with internal anodes, and if so, check them at least annually. If they are half depleted. . .well, you know.