What is a Sacrificial Anode?
A Sacrificial Anode is simply a disposable metal attached to your boats underwater metals, that will sacrifice itself (corrode) first in order to prevent the important metals corroding.
If two dissimilar metals are touching underwater, such as a stainless drive shaft and bronze propeller, a naturally occurring transfer of electricity carries through the water from one metal to the other and causes the more active of the two metals to corrode. A sacrificial anode when secured to the propeller or drive shaft is now the most active metal in the system and corrodes before the expensive metals. When the anode reaches 40%-50% of its original size, it is replaced, to continue its protection cycle.
A Sacrificial Anode is commonly referred to as Zinc; however Zinc is not the only anode that protects underwater metals, Aluminum and Magnesium are also used.
Ask your local area dealer, boat manufacturer and certified marine electrician for their professional opinions on which anode to use for different environments.
We recommend the installation of a galvanic isolator to all boats in marinas. This prevents you sharing your anodes with any under protected boats or marine structures on the dock. In most cases we have seen the life of anodes increase significantly after a galvanic isolator has been installed by a certified marine electrician.
[Technical: A galvanic isolator blocks low voltage DC currents coming on board your boat on the shore power ground wire. These currents could cause corrosion to your underwater metals; through hulls, propeller, shaft etc. Marina boats, all plugged into shore power act as a giant battery. They are connected together by the green shore power ground wire, which is (or should be) connected to their DC grounds, engine block, and bonded underwater metals. If the boats are in salt water then that forms an electrolyte and the dissimilar metals connected together act as a battery, causing corrosion.
The galvanic isolator has two pairs of diodes set up so that a voltage of about 1.2 volts is required to cause them to conduct. As most DC voltages caused by galvanic action will be less than this, they are blocked. Good quality isolators also contain a capacitor, which only conducts AC current, as a backup.
Normally no AC current is carried on the shore power ground wire, but it has to be able to carry the full load of the circuit in the event of a fault. Therefore it is important to have a good quality unit that will not overheat when required to carry the rated load. Some heat will be generated by the voltage drop and the unit must be able to withstand this.
As the galvanic isolator fulfills such a key function in the AC circuit it is only prudent to use the best quality unit available].
Generally these are the rules for each body of water:
Salt Water: Use Zinc or Aluminum Anodes.
Note: If you are plugged into shore power and do not have a galvanic isolator, anodes may be consumed at a rapid rate, especially aluminum.
Brackish Water (Salt and Fresh mixed): Use Zinc or Aluminum Anodes.
Note: The fresher the water the less electrical current is transferred, requiring the more active aluminum anode.
Fresh Water: Use Magnesium Anodes ONLY.
Magnesium anodes are ideal for fresh water, as they are both economical and highly effective as sacrificial anode. They have excellent properties for corrosion protection in higher resistive, aqueous electrolytes.
It is to be noted that magnesium anodes are higher on the Galvanic Scale than aluminum anodes and zinc anodes, and produce more driving current between anode and cathode than the latter.
A wide variety of magnesium anodes are available today as per the demands of individual industries.
Our Mag 22 magnesium anodes are used in submerged structures such as barges, ships, dams and tanks. The installation involves welding or bolting the flush-mount magnesium anodes (with two cast-in galvanized steel straps – ¼” x 1-1/2” x 18” on 10” centers)) to the structure. Likewise, these magnesium anodes can provide protection to submerged or semi-submerged structures such as pilings, and piers.
Magnesium anodes are also suited to direct underground earth burial applications, such as a buried propane tanks, containers, pipelines, or similar structures. The buried Magnesium anodes attached with a copper wire to the structure makes it cathodic, preventing corrosion.
Mag Anode – Underground type 17D3 – 32D5 – 48D5 cathodic protection anodes are available packaged or unpackaged, with or without copper leads, per customer requirements.
Copper leads carry the electrons and form continuity between the magnesium anodes and the structure requiring protection.
The packaged option includes the magnesium anodes with backfill material pre-packaged.
Backfill is 75% Gypsum, 20% Bentonite and 5% Sodium Sulfate, used with magnesium anodes to lower the anode-to-earth resistance and to help retain moisture around the anode; the mixture is a low cost, non-hazardous, electrically efficient ground bed conductive earth backfill. A cotton bag holds the backfill around the inner magnesium anode. It is recommended that 5 gallons of water be poured on the anode once placed in the earth to wet the backfill. This lowers the resistance between the magnesium and the earth causing more electric current to flow than would be possible in dry soil.
Mag Anodes - 2R5 – 75S – 15S – are magnesium anodes used in Heat Exchanger and Condensers, they are specifically designed for use on heat transfer equipment. All sizes have 1” long x ¾” galvanized steel core with a 1-1/2” counter bore hole.
Whether you are using magnesium anodes, aluminum anodes or zinc anodes it is recommended that you inspect them periodically and replace them when there is 40% - 50% of the original anode remaining.
NOTE: While statements contained herein are believed to be accurate, they are offered as suggestions only and no warranty or representation is intended.
Remember, ask your local area dealer, boat manufacturer and certified marine electrician for their professional opinions on which anode to use for different environments.
Never paint a sacrificial anode, or the surface it mounts to. There needs to be a solid physical electrical contact between the anode and the metal requiring protection. If there is a break in this electrical continuity, the protection stops, resulting in corrosion of the important metals.