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 anodes on your boat. When an anode is missing or largely wasted away, 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 makeup 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 connecting 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.
Sacrificial anodes cannot perform their function unless they are exposed. Putting paint on an anode smothers it, rendering it useless. Never coat 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.
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.
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.
Heat exchangers, because they are typically a copper alloy, are at risk of galvanic corrosion. To combat this, many 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.
In recent years cadmium in zinc has become an environmental concern, leading to a movement in the direction of aluminum anodes. Such anodes are effective even for protecting aluminum components — lower end cases, for example — because the aluminum used in the anode is a more anodic alloy. Aluminum alloy anodes are almost certainly to become more common. It has not happened already only because the cost of aluminum anodes has been higher than zinc without any discernible benefit to the boat owner. Today aluminum is actually cheaper than zinc. In addition, aluminum anodes tend to last longer, they work better than zinc in brackish water (and maybe in salt water as well) and they appear to be better for the environment. When making the switch from zinc to aluminum, ALL of your anodes must be aluminum. This can be a problem in some locales as many local marine suppliers still do not stock a wide selection of aluminum anodes. That will eventually change.
In freshwater, magnesium anodes protect underwater metals better, particularly underwater aluminum. However, magnesium is a good choice for freshwater only. If any of your boating is also in brackish or salt water, fit aluminum anodes.