Roy, this is a good explanation from the Defender web site without all of the technical issues to confuse things
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All over the world you will find that there are many different water environments which require different protective solutions for your boat. However, there are really only three basic water types, which are salt, fresh and brackish waters. Martyr™ zinc and aluminum anodes protect your boat very well in salt and brackish waters whereas magnesium anodes protect the best in fresh water.
NEVER MIX Zinc and Aluminum Anodes on the same vessel.
- Zinc Alloy Anodes = Salt water only
- Not recommended for use in fresh water
- Alloy is manufactured to meet or exceed US Military Specification (MIL-A-18001K)
- Aluminum Alloy Anodes = Salt or Brackish water
- Not recommended for use in fresh water
- Proven to last longer than zinc due to increased capacity
- Alloy is manufactured to meet or exceed US Military Specification (MIL-A-24779(SH))
- Magnesium Alloy Anodes = Fresh water only
- Not recommended for use in salt or brackish water
- The only alloy proven to protect your boat in fresh water
The protection of the underwater metal parts of a boat or motor from corrosive electrolytic action is a very real and necessary concern. The use of an activated alloy anode to distract the corrosive action to itself and away from the parts of the boat you want to protect is a simple and economical answer. Many people refer to these types of anodes simply as " ZINCS. "
The best anodes are made from an "activated alloy" in which catalysts have been added to ensure that the anode gives 100% of itself in the purpose of protecting your boat from corrosive action. It is this total giving of itself that is the root of the name "Martyr". The complete and active 'giving' of a Martyr anode is your assurance of the finest protection you can buy for your boat.
What is Galvanic Corrosion? If you've noticed corrosion on the metal parts of your boat located below the waterline, you are the victim of "galvanic corrosion". The scientific term "galvanic" corrosion describes the type of corrosion that anodes are intended to absorb. This corrosion is normally caused by different metals being near each other in salt water. Galvanic corrosion, an electromechanical action, causes metal parts to decompose. This destructive process is caused by electrolysis, an electric current set up between the metal parts of your boat, with salt water as the electrolyte. The effect is like a flashlight battery -- an electrical current is created and continues until one of the metals is eaten up -- the battery goes "dead".
What is Electrolytic Corrosion? If you see an accelerated corrosion on the metal parts of your boat located below the waterline, you are the victim of "electrolytic corrosion". The scientific term "electrolytic" corrosion describes an accelerated type of corrosion that occurs when an electric current is added to the water surrounding your boat (usually at a dock). This corrosion is typically caused by faulty wiring that permits an electric current to enter the water. This corrosion, combined with galvanic corrosion, is also an electromechanical action which causes metal parts to decompose, but at a very accelerated level. This destructive process is also caused by electrolysis, which is an electric current set up between the metal parts of your boat, with salt water as the electrolyte, but it can be much more damaging in a very short time.
You can see the eating away on shafts, propellers, rudders, and other fittings and drive parts under the water as a pitting on the surface. The results are unattractive, affect operating performance, and cause excessive maintenance of hull, propellers, engine, accessories and fittings. Some metals react more actively than others and will be eaten up first. This activeness is the reason that a steel fitting can corrode away while a brass one next to it will not (at first). Both these parts could be protected, however, by a properly installed alloy anode. The anode will be eaten away and the steel and brass fittings will be untouched. The sacrificing action of the anode is the reason it is called a sacrificial anode, and the source of the name "Martyr".
The protection of the underwater metal parts on your boat or motor from corrosive electrolytic action is a very real and necessary concern. The use of an activated alloy such as an “anode” will help. Anodes will distract the corrosion action to itself and away from expensive metal parts. Fortunately anodes are relatively inexpensive and are readily available. Factors such as warm water temperatures, polluted water and stray current corrosion can cause your anodes to waste away at an accelerated rate so it is wise to check them on a regular basis.
An important thing to remember is that you must complete a "circuit" for the anode to be effective, which means that the zinc must be in direct contact with the metal components of your boat located below the waterline. This is the reason that a hanging grouper-style of anode comes with a wire and metal clip that is intended for attachment to a "ground" within your boat. When attaching a zinc anode to a prop or shaft, please be certain that the point of surface attachment is clean and "bright" so a good contact is made, completing the circuit. Also, never paint over a zinc with bottom paint, which will only isolate and protect the anode from its intended purpose.
A Special Note on Magnesium Anodes for Fresh Water… Essentially, fresh water is a much less conductive environment than salt water, therefore magnesium anodes are your best choice as they are much more active (less noble) than zinc or aluminum anodes. The result is increased efficiency thus superior protection for your underwater metal components.
Magnesium Anodes can be special ordered.
* It is not recommended to use magnesium anodes in salt or brackish water. The result may be an accelerated corrosion rate, which may leave you with no anode protection in a short period of time and damage the metal parts of your boat.
