February 14, 2013
We use sacrificial anodes on our boats to lessen the electro-chemical reaction between the submerged metal of our boats and the water in which it is used. The difference in electrical potential exists between any two different metals submerged in an electrolyte. For boats, this electrolyte is fresh, brackish, or salt water. This electrical potential is similar to that of a battery, in that there is an electrical current that passes through the water from one terminal to another, or in our case, one metallic material to a different metallic material
An anode is the negative “terminal” and the other metals are the cathodes, or positive “terminal.” The anode is sacrificial because the anode slowly corrodes and dissolves in water, preventing the cathode from corroding and dissolving. Since the sacrificial anodes eventually disappear, they have to be replaced at regular intervals.
Traditionally the sacrificial anode was made of high purity zinc, which does a good job of protecting most of the submerged metals of boats. However, zinc is highly toxic, even in very small quantities, and all of the boat zincs dissolving in our lakes, rivers, estuaries and oceans has a significant effect on the natural environment. Zinc is highly toxic to plants, invertebrates, and vertebrate fish. It is also known to be toxic when inhaled during welding or soldering of galvanized metals, causing “zinc shakes,” “zinc chills,” “galvie flu” or “metal dust fever.”
Other materials traditionally used for sacrificial anodes are magnesium and aluminum. Magnesium has been used for boats located in fresh water, but should not be used on aluminum boats in salt water, as it will cause excessive voltage differences that can cause hydrogen bubbles to form on the metal that can lift paint off of the aluminum hull. Magnesium has a higher degree of anodic protection than zinc, but this higher protection is too great to be useful on aluminum boats or out-drives in salt water. Aluminum anodes have a slightly higher degree of protection than zinc, but without the harmful over-protection of magnesium. However, pure aluminum anodes will quickly form an oxide layer that electrically insulates the aluminum, defeating the purpose of the anode. (Zinc also forms oxides in fresh water, which insulates it and prevents it from working.) Toward the end of the twentieth century, certain aluminum alloys were developed to make them superior to other sacrificial anodes. One of these alloys is made of 95% Aluminum, 5% Zinc, and .02% Indium, and is marketed by Performance Metals Products under the trade name of “Navalloy®.”
Performance Metals Products manufacturers a complete line of sacrificial anodes for boats of all types, using their aluminum alloy Navalloy. Navalloy has many properties that make it ideal for sacrificial anodes on our boats. This alloy has all the advantages of aluminum, without the disadvantages of pure aluminum or the toxicity of zinc. Navalloy is lighter than zinc and lasts 30 to 50% longer than zinc anodes. It has a higher protection voltage than zinc, but not dangerously high as with magnesium.
Performance Metals Products anodes have built-in wear indicators in them. The more traditional anodes, such as hull anodes, shaft anodes, and rudder and trim tab anodes have a red dot cast under the surface of the anode, when the dot is visible; it is time to change the anode. Their pencil anodes, used on the inside of heat exchangers and other engine parts, have the anode cast around a stainless steel core, which allows the anode to dissolve without the remainder breaking off and potentially causing water blockage problems as it moves around loose in the engine. These pencil anodes also work well in hot water, typically found within the cooling circuits of marine engines, heat exchangers and refrigeration condensers. Zinc anodes provide greatly reduced protection in hot water. Zinc anodes form a non-porous layer when the boat is hauled out of the water, which must be cleaned before the boat is launched. Navalloy does not form this layer, and work immediately upon re-immersion in water.
Environmental agencies have determined that zinc anodes are a major cause of pollution in marinas. Maryland is currently considering phasing out the use of zinc anodes in favor of aluminum alloys to solve this problem with zinc in marinas and boatyards.
Navalloy anodes are made to military specification MIL-DTL-24779B(SH).
Mercury and Johnson/Evinrude/OMC started selling aluminum alloy anodes in the early 1990’s. Other outboard and out-drive manufacturers also are switching to aluminum anodes. Some of the manufacturers may void the warranty coverage of their products if zinc anodes have been used.
Since Navalloy anodes provide better protection, are less toxic, last longer, work in all waters and weigh less, are there any reasons to keep using zinc anodes?
Mark McBride – February 14, 2013