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You are here: Home » FAQ » Marine Anodes

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Marine Anodes

  • Q What Is That Hard Scale Covering My Zinc Anodes?

    A The hard scale commonly found on the surface of zinc anodes is Zinc Carbonate (ZnCO3).  Carbonate (CO3) occurs in all bodies of water and originates from atmospheric carbon dioxide gas (CO2) naturally dissolving into the water.  In general you do not need to be concerned about this scale as it only affects the maximum output amperage of the anode, not the anode’s required trickle output necessary to maintain effective corrosion protection.  Zinc Carbonate is water insoluble, but can be readily removed by either alkalies or acids.
  • Q What Is The Best Position For Anodes On My Propeller Shafts?

    A For corrosion protection, it makes no difference where you locate your shaft anodes along the length of a propeller shaft.  Any location is perfectly fine.  However, the manufacturers of propeller shafts recommend zinc anodes be placed the distance of "one hand width" (i.e., 4 inches) in front of the aft cutlass bearing.  This ensures adequate water flow through the bearing and, secondly, minimizes the shaft anode(s) weight contributing to shaft vibration on high performance boats.
  • Q How Do I Know If My Boat Lacks Corrosion Protection?

    A The best way is to measure it.  With a silver/silver chloride reference electrode it is relatively easy to get highly accurate voltage readings from all your underwater metals and determine if they are corroding or not.
  • Q Where Do You Generally Place Sacrificial Anodes?

    A Anywhere that a metal is submerged or subject to a harsh environment, and the medium that it is exposed to requires electrons. most common areas for sacrificial anodes are found on the ships hull, ships ballast tanks, steel structural wharfs, offshore structures, and even on modern automobiles.
  • Q What Is The Difference Between Zinc And Aluminum Anodes?

    A Both are low on the electro-negativity series chart. in comparison with steel. steel is the most common metal used for ships, wharfs, offshore structures, etc..., so where there is continuing electrolysis in water (especially salt water), a dissimilar metal or Impressed current has to be present to give up (sacrificial) electrons to neutralize the current created. zinc is a little cheaper metal than aluminum and does not last as long as in the same electrolysis conditions. aluminum is more expensive, but last longer. It generally works out to be the same cost over the same duration.
  • Q What Would Happen To My Vessel If I Never Used Anodes?

    A If there are no sacrificial anodes present, the electrolysis process would still continue and would attack the weakest areas which are exposed steel areas, bead welding, and any other areas with dis-similar metals, such as around the stern gland stern tube areas.
  • Q Why Can I Not Use Lead On A Steel Ship?

    A Lead as an anode should not be used on steel because it has a much lower number on the electro-negativity chart. If reversed, then you have created a problem whereas the ship acts as the anode to protect the lead.
  • Q What Precautions Should I Take When Installing New Anodes?

    A 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.
  • Q When Should Sacrificial Anodes Be Replaced?

    A 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.
  • Q What Factors Increase Corrosion?

    A 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.
  • Q What Anode Material Should I Use On My Boat?

    A
    The type of boat that you have will determine how careful you need to be. A fiberglass hull with an inboard engine will need much less protection than an aluminum hull or a boat with an aluminum sterndrive for example. Some simple guidelines:
    Inboard boats with mainly bronze and stainless metal parts can be protected using zinc or aluminum anodes. Don’t worry about overprotecting them. You are only overprotected when the weight of the anodes is so great that your boat sinks! The voltage generated by zinc or aluminum anodes will not cause any damage – no matter how much anode material is added, the maximum voltage that can be generated is the voltage of the anode itself. You could also use magnesium in freshwater locations on fiberglass-hulled boats. Be careful using magnesium on aluminum or wooden hulled boats since you can overprotect them. Steel hulls can also be overprotected to the point where excessive protection voltage rapidly lifts the paint off the hull.
  • Q What Metals Are Sacrificial Anodes Made From?

    A The three most active materials used in sacrificial anodes are zinc, aluminum and magnesium. They have different properties and uses.
    The first property to consider is their electrical potential. All metals generate a negative voltage (as compared to a reference electrode) when immersed in water. The lower – the more negative - the voltage, the more active the metal is considered to be.
    The second property that is important is the current capacity of the anode material. The anode generates a voltage difference and this drives a current between the anode and the protected metal and through the water. It’s like having a bigger battery, the more capacity you have the longer it will keep protecting. Incidentally, for a particular anode, the rate of current flow is dependant on the surface area of the anode and the longevity depends on the mass.
    The third property is Quality of the Anode Alloy.
  • Q What Do Sacrificial Anodes Do?

    A
    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.
    Aluminum anode alloy provides more protection and lasts longer than zinc. It will continue to work in freshwater and is safe for use in salt water. Aluminum is the only anode that is safe for all applications.
    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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