Reading this link compelled me to respond with more information on anode. One link that's attached above is from a company that only makes aluminum anodes and they're pushing these anodes for ALL bodies of water. While aluminum does perform well in salt water, there's drawbacks in certain cases and anode mfr's aren't either don't know or aren't sharing this info. As a leading mfr of zinc (ZN), aluminum (AL) and magnesium (MG) anodes, this year we are receiving a substantial increase of inquiries in wanting to switch from ZN to AL anodes. This has been induced partly by a couple anode mfrs pushing Aluminum. We take the stance of not pushing any type of them, but giving out information so customers can make an informed choice of protection.
AL anodes are being marketed as an anode that performs well in all situations, is cheaper, is lighter and lasts longer. Is there truth in this? Yes, and NO.
Al is more negative (electrochemically) than ZN by 50mv. In saltwater and upper levels of brackish waters "both" anodes will protect the basic metals that vessels use. In the lower brackish levels and fresh water, AL out performs Zn electrochemically. While ZN looses an edge here, it makes this up in the sloughing dept. Zn sloughs it's consumed metal much better than AL. Without sloughing, the consumed metal can film over and form a crust (barrier) which can cause passivation in the anode. The "partial" protection of ZN is then a welcomed advantage over the now reduced or inactive AL anode. Both of these anodes bow to magnesium (MG) anodes in freshwater. The high current output of MG is needed in the high resistivity fresh waters.
In salt water the same scenario can also occur. As a vessel increases in speed, corrosion rates are increased and anodes work harded. When the boat is moored, the anodes settle down (without a current demand on them) they can film over. This occurs when a vessel is moored and the longer it sits, the higher the chance of anode passivation. This however, occurs predominantly in AL anodes. When the vessel moves again, anodes react by once again putting out more current, therefore re-activating themselves. Cutting through the water also cleans any soft film off the anode, exposing new anode material to be consumed. The longer a vessel sits, the bigger the chance of filming over, followed by having a crust form. If a vessel remains active in salt water, both ZN and AL work. If a vessel sits for periods of time, it's at risk of loosing anode protection with AL.
While some claim never to use MG in saltwater, it works. The cautions are over-voltage because of the high voltage it puts out. If done correctly, there's no issues, however, Joe Boater likely doesn't know the in's n out's of cathodic protection, so beware. Another drawback is the lifespan of MG. Very short to ZN and AL, with AL lasting about 30% longer than ZN.
Bottom line: look at vessel habits before making a choice. If the vessel sits for periods of time, either use zinc anodes and / or do maintenance on the anodes, especially aluminum anodes which involves cleaning them.
On Aluminum hull vessels: Over-voltage is a concern. Both zinc and aluminum anodes have enough capacity to protect aluminum hulls which are generall made from alloy 5086 series. Installing excessive amounts of anodes likely will create a release of hydrogen from the metal's surface, which causes deep pitting. This over-voltage easily occurs with magnesium anodes high current output of nearly 50% high capacity and zinc and aluminum. Only use magnesium to protect aluminum if you know what your're doing.
Sometimes Magnesium is the correct choice, sometimes Aluminum, sometime Zinc. There's a place in corrosion protection for all three of these anode types, depending on the many variable factors.