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What do sacrificial anodes do? |
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All metals immersed in an electrolyte (sea water for example) produce an electrical voltage. When two dissimilar metals are in contact (electrically connected) they produce a galvanic cell (like a battery), with the less noble metal (a bronze propeller for example) forming the anode and the more noble metal (stainless steel shaft) forming the cathode. |
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If you want to protect both metals you need to connect a third metal that is more active than the first two. The most active metal (zinc for example) becomes the anode to the others and sacrifices itself by corroding (giving up metal) to protect the cathode - hence the term sacrificial anode. |
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What factors increase corrosion? |
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The voltage difference between the two metals will affect the rate of corrosion. For example a stainless steel prop, which is a relatively noble metal, will cause more corrosion of a set of zincs than a bronze prop.
Corrosion will increase the saltier the water is. Increasing temperature will also increase the conductivity of water and the resulting corrosion. The corrosion rate doubles with every 10 degrees Celsius (18 degrees Fahrenheit) increase in temperature. Pollution can also increase corrosion. For example, many freshwater lakes have been contaminated by acid rain, which increases the conductivity of the water and therefore corrosion rates. |
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When should sacrificial anodes be replaced? |
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Anodes should be changed, at least, on an annual basis (including anodes in fresh water) or when they have corroded to half their original size. ------ Performance Metals “Premium” anodes include the exclusive patented “wear indicator.” When the red spot appears it is time to change your anode! |
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What precautions should I take when installing new anodes? |
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Make sure they make good electrical contact with the metal that is being protected. Remove any paint and clean the metal surface that will be in contact with the anode.
DON’T paint anodes! They can’t work if they are covered up. |
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What type of anodes should I use for my sterndrive? |
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Sterndrives and outboards are made from aluminum alloys, which are particularly prone to corrosion (less noble metal). The use of aluminum anodes is essential. Zinc anodes will NOT protect the outdrive in salt or fresh water. |
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What else should I do to help protect my sterndrive? |
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Keep paint (on engines, sterndrive units etc.) in good condition. A small scratch will corrode rapidly.
Leave the sterndrive unit immersed in the water. If you don’t the anodes can’t work.
Don’t use anti-fouling paint containing copper or mercury on a sterndrive unit. The metal in the paint will increase galvanic corrosion.
Don’t mix zinc anodes on the hull with aluminum anodes on the drive. The aluminum anodes will protect the zinc anodes in addition to the unit. |
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What anodes should I use in freshwater? |
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Where possible Navalloy™ (aluminum/zinc/indium alloy) anodes are recommended over zinc. Zinc anodes can become inactive after only a few months due to the build up of an insulating film of zinc hydroxide. Aluminum anodes will remain active.
Navalloy™ anodes should be changed on an annual basis, even if they look OK – remember the coating! |
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