BoatUS – BoatTech – Sacrificial Zincs by Don Casey
I too have always used zincs but would be interested in knowing if any mid and upper bay boaters use aluminum? Would there be any issue trying one zinc and one aluminum anode on a SS shaft?
In recent years cadmium in zinc has become an environmental concern, leading to a movement in the direction of aluminum anodes. Such anodes are effective even for protecting aluminum components--lower end cases, for example--because the aluminum used in the anode is a more anodic alloy. Aluminum alloy anodes are almost certainly to become more common. It has not happened already only because the cost of aluminum anodes has been higher than zinc without any discernable benefit to the boatowner. Today aluminum is actually cheaper than zinc. In addition, aluminum anodes tend to last longer, they work better than zinc in brackish water (and maybe in salt water as well) and they appear to be better for the environment. When making the switch from zinc to aluminum, ALL of your anodes must be aluminum. This can be a problem in some locales as many local marine suppliers still do not stock a wide selection of aluminum anodes. That will eventually change.
In fresh water, magnesium anodes protect underwater metals better, particularly underwater aluminum. However, magnesium is a good choice for freshwater only. If any of your boating is also in brackish or salt water, fit aluminum anodes
Aluminium anodes have several advantages, such as a lighter weight, and much higher capacity than zinc. However, their electrochemical behavior is not considered as reliable as zinc, and greater care must be taken in how they are used. Aluminium anodes will passivate where chloride concentration is below 1,446 parts per million.
One disadvantage of aluminium is that if it strikes a rusty surface, a large thermite spark may be generated, therefore its use is restricted in tanks where there may be explosive atmospheres and there is a risk of the anode falling.
Since the operation of a galvanic anode relies on the difference in electropotential between the anode and the cathode, practically any metal can be used to protect some other, providing there is a sufficient difference in potential. For example, iron anodes can be used to protect copper.
The rule is not to use different materials for annodes on the boat. Some of the Yanmars even have sacrificial zincs in them so I would be careful of crossing over to another material.
Lastly the purpose of the zinc would be defeated at its supposed to be a different noble matel, thus attracting the current and sacrificing its self. We dont have any allumininum in the water that I know of its all stanless steel like the shaft. but in the case some one would like a PB or a swim ladder, or even the prop of an outboard dinghy or whatever it would make it ineffective.
I am staying with traditional zinc.