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Johnson/Evenrude What is Corrosion?Corrosion is an electrochemical process of deterioration of metal components when exposed to an aqueous environment water. This occurs both underwater and in the atmosphere. The deterioration is the process of the metal changing into its oxide form. Steel, for example, will degrade oxidize back to its natural stable state - rust iron ore. You need anodes on your engine because when two different metals are in contact, electrons will flow from the more negatively charged metal anode to the more positive metal cathode. If you want to protect both types of metal you must add a third metal. The most common metal is zinc although magnesium and aluminum are also used. This active metal becomes the anode for both metals. The zinc or aluminum sacrifices itself to protect the other two metals, hence the term "sacrificial anode." Zinc is the most common material used. Zinc anodes are not very effective in freshwater and can stop working after only a few months if not made to mil. specifications. It is a good policy to change them regularly, even if they look OK. Remember, if an anode doesn?t wear away it is not working! The aluminum alloy used in anodes is very different from normal aluminum. It includes about 5% zinc and a trace of Indium, which prevents the build up of an oxide layer. Aluminum Anodes last longer and will provide better protection that Zinc. Aluminum anodes can be used in salt and brackish mix of fresh and salt water water environments. However in freshwater, due to the increased resistively of the water and reduction of salts, Aluminum and Zinc Anodes will start to passivate, ultimately renders them ineffective. In freshwater it is recommended to use Magnesium Anodes. Magnesium Anodes have a higher driving voltage than Zinc or Aluminum, and therefore are more effective in freshwater. It is not recommended to use Magnesium in saltwater for that very reason.Area and Weight of AnodesThe surface area of the sacrificial anodes determines how much protection amperage you get. The weight determines how long they will last. Different anodes have different capacities measured in Amp Hours per Pound. The ratio of the area of the cathodic protected surface to the anodic corroding surface is critical in galvanic corrosion. The smaller the area where the anode is giving up material, the faster it will take place. Ideally the anodic area should be much bigger than the cathodic area. This ratio can be improved by painting the cathodic surface. Never paint the sacrificial anode.Several Factors Affect Corrosion Seawater is a good conductor and freshwater a bad conductor, so corrosion is worse in seawater. Generally, corrosion rates increase in proportion to the amount of oxygen in the water. However, cracks and crevices, which are areas starved of oxygen, become anodic and corrode also. Higher temperature increases corrosion rates - doubling for every degreeC 55 degrees F. There are various. FREE Shipping Over $100, No Sales Tax