ANODES! AN ALTERNATIVE VIEW! pvShaun Wall MVS 108107
When two different metals are physically or electrically connected while at the same time being immersed in seawater, they become an electric battery and a measurable electric current flows between the two. Electrons 'supply' or permit this current-flow by one metal giving up bits of itself -in the form of metal ions- to the seawater and migrating to the other metal to complete the circuit. The recipient eventually becomes electro-plated with a skin of the donor metal.
One metal gives; the other takes and the give'n'take process is called "Galvanic Action". At its most sublime and in carefully controlled conditions it gives us electro-plating such as you find on cutlery marked EPNS. Click on the hyper-link to reveal all! But remember to come back here. The 'take' element doesn't matter a whit to sailors and boat drivers. The 'give' process can be catastrophic. A boat which lives permanently in sea-water can lose a bronze propellor, melted away by galvanic corrosion in less than two years.
Galvanic corrosion, if left unchecked on a boat, quickly destroys onboard underwater metals. The most common casualty of galvanic corrosion is a Bronze (basic alloy 9:1 Cu:Sn) propeller on a stainless steel (basic alloy Fe:C) shaft. The prop usually melts away quickest but metal struts, rudders, rudder fittings, outboards, and stern drives are all at risk. Galvanic corrosion may be counteracted by adding a third metal into the electrical circuit: one that is quicker at giving up its electrons than the component we want to protect. The third metal becomes a sacrificial anode. Zinc ingots are commonly used for the purpose. Zinc works and is not expensive. Sometimes Magnesium and its alloys will be found in special applications on well-minded boats. You can find some extra in Kempes if so inclined.
The importance of maintaining anode protection can not be overstated. Without a sacrifice willingly offered, Neptune will take for himself: starting with your props and working through every and any bit of metal in contact with sea-water, outside and inside the boat, provided the equation contains two dissimilar metals. This is important. Galvanic action relies on the presence of TWO or more dis-similar metals. So if your slinky yacht has only aluminium and no other metal there should be no deterioration. However, before you start preening yourself go to the very end of this article NOW and see the note about SHORE-POWER. Sometimes aluminium alloys have different compositions. They may look the same but galvanic sampling may discover that the stabard side is wolfing up the port side 'cause someone changed his sheet supplier half-way through the build. Neptune always has time on his side.
The amount of protection provided by an anode depends on its volume or more particularly its exposed skin area. The required surface area varies with the metal being protected and the chemical make-up of the water and there are no hard and fast rules in real life. Check the protected metal frequently for signs of corrosion. If you find no corrosion at all on your props thank your sacrificials! They are doing their duty: By Neptune!
The life of an anode is a function of its weight and shape is a factor. If free-hanging, weigh it every month. Determining the appropriate anode size is not a deal. Simply replace existing depleted anodes with new ones the same size and shape, on the basis that somebody earlier may have got it right. Check anodes regularly if possible. Replace any that are more than half -depleted; if depletion is happening a bit too fast, size upward!
Sacrificial anodes can be mounted anywhere, even hung over the side on a rope, but absolutely must have electrical contact with the metal/ component being protected. You need low-resistance, metal-to-metal contact-either by mounting the zinc directly to the metal being protected or by connecting metals and anodes by wire. A hanging anode can provide perfect protection if connected by wire to the metal being protected and you know the wire is not broken or cut. If you do find a break, put an ammeter set at milliamps across the break. If you register current you better solder that break now, but you can comfort yourself in the knowledge that the circuit is not broken elsewhere.
Where an ANODE is mounted directly to metal to be protected - bolted to the side of a metal rudder, for example - it's essential that the surface under the zinc is bare and bright before installation so as to ensure good electrical contact from the beginning. Do not paint it!
PAINT: Zinc anodes cannot perform unless exposed to the electrolyte, sorry! seawater! Painting an anode is a total no-no. Never coat zinc anodes with bottom paint, or anything else.
PROPS & RUDDERS: Propellers are normally protected by a collar fashioned in two pieces and bolted together around the shaft near the prop. It is essential to ensure the shaft is clean and bright before clamping the collar on. Corrosion protection for outboard props is typically a bolt-in-place ring or a prop-nut. Metal rudders and struts are most easily protected with zinc disks bolted directly to the metal.
HULL PLATES: Boats with all their 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. Anodic plates are fitted to metal boats for protection. 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 are typically copper or Cu alloy. Heat exchangers may be fitted with a 'pencil' anode. Look for it under a brass plug in the exchanger. [On L1 they held by three bolts associated with the oil cooler] 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. IF THEY ARE check them monthly or in rotation.
SHORE POWER: A very sneaky and oft-overlooked cause of galvanic corrosion can be your shore-power hookup, particularly if it is left on all the time. When you plug in, you link your tub electrically to other boats and maybe the jetty through the green earth lead. Your tub is now part of a large galvanic cell (a battery) interconnected with onshore metals well submerged in sea-water—as well as other boats attached to the same electric hook-up—and that is why on very quiet nights in a very large marina full of over-wintering metal yachts .......