All About Anodes

Would a "zinc" made of another material still work as well? Or perhaps better?

  When a boater says he’s putting new zincs on his boat, I’m reminded of the times we used to say “let’s go for a beer,” even though almost nobody in the group drank beer. Scotch or rum was preferred! It was merely an expression. Unfortunately for many boaters, when they have sacrificial anodes installed, the “zincs” are, in fact, made of zinc, even though there are sacrificial anodes made of a material that might well do a better job.

Sacrificial anodes are necessary to combat galvanic corrosion in metal structures. Boaters most often think of galvanic corrosion, and the problems it causes, in relationship to boats only. In fact pipelines and most other metal infrastructure projects also have problems with galvanic corrosion.

Galvanic corrosion occurs when two dissimilar metals are immersed in an electrolyte. Sea water is a great electrolyte. Fresh and brackish water also act as electrolytes, but not with the same vigor as salt water. When the two dissimilar metals are immersed in salt water, a flow of electrons moves from one to the other. Without getting into to the eye-glazing electro-chemical details, I’ll simply say this flow is what causes the corrosion.

A constant push by environmentalists has created a mindset among many folks that the metallic zinc, when it dissolves in water, is not environmentally friendly. Zinc, in large amounts, has, in fact, been found to be harmful to marine life, and that fact has led to pressure in some jurisdictions to ban the use of sacrificial zinc. Some zinc anodes also contain cadmium, a heavy metal that can be a serious health hazard.

The U.S. Environmental Protection Agency has a regulation in place that requires sacrificial anodes on all commercial vessels shorter than 80 feet to be environmentally friendly “… to the extent technologically feasible and economically practical and achievable.” While the wording of the regulation is, legally, nothing more than a “suggestion,” it does indicate the likely direction the matter of sacrificial anodes is heading. This regulation does not yet apply to pleasureboats.

The EPA also notes “…magnesium is less toxic than aluminum, and aluminum is less toxic than zinc.”

Generally speaking, sacrificial anodes made of zinc work well in salt water, magnesium anodes work best in fresh water and aluminum anodes work well in salt, brackish and fresh water. It appears the use of aluminum sacrificial anodes satisfies both the environmentalists and boat operators — something one rarely sees. The aluminum anodes are alloyed (often with very small amounts of zinc, iron, and indium) so they are more negative on the galvanic scale than zinc anodes and they, therefore, provide better corrosion protection.

In terms of longevity, an aluminum anode lasts between 30 and 50 percent longer than an anode made of zinc, depending on the manufacturer. A magnesium anode will last only about one-third as long as a similarly sized zinc anode. In some markets the aluminum anodes cost a bit more than those made of the more traditional zinc material, but because they last longer, overall cost is about the same or even less.

The aluminum anodes now on the market are considered by many the “best” type of sacrificial anodes available, for a number of reasons:

• They do a better job of protection than the traditional zinc anodes.
• They are accepted by various sterndrive manufacturers as the best material to use.
• They last longer.
• They can be safely used in salt, fresh or brackish water.
• They are environmentally friendlier than the alternatives.

Aluminum sacrificial anodes are readily available, but anodes made of zinc continue to “hold on” because boaters, and many small service yards, have simply not kept up with sacrificial anode technology. To them, “a zinc is a zinc.” Another reason is the manufacturers of sacrificial anodes have not properly informed the boating public of changes in the technology. Most of them make anodes of both aluminum and zinc, and they make as much selling one as the other, so they are not financially motivated to spend money on advertising or marketing.

The world’s largest manufacturer of sacrificial anodes for pleasure vessels is Richmond, B.C.-based Canada Metal (Pacific) Ltd. Its anodes are sold under the Martyr brand name.

Tyler Seebach, Canada Metal’s vice president of marine sales, said more boaters are becoming aware of the advantages of sacrificial anodes made of aluminum. As a result, the sale of pleasureboat sacrificial anodes made of aluminum has steadily increased 10 percent a year over the past six years.

I’ve limited this story to sacrificial anodes. Boaters who want a more detailed marine and technical electro-chemical corrosion discussion can consult a number of books on the market, two of which are most useful. “The Boatowner’s Guide To Corrosion” is written by Everett Collier and is published by McGraw-Hill. Nigel Warren’s “Metal Corrosion In Boats” is the other. The well-respected naval architect and marine writer’s book is published by Adlard Cole Nautical.

2 thoughts on “All About Anodes

  1. I have switched from zinc to magnesium when going from salt water to fresh water in the Great Lakes. When I go back to salt I will return to zinc. This is because I have an all aluminum boat and I believe my options are limited. Is there any other options for us aluminum yachties?

    • The alloying elements in the aluminum anode mean that the protection level is greater than that of zinc (position in galvanic series), i.e. aluminum anodes will protect aluminum better than a zinc anode (I know it sounds strange…)
      If you want to get technical, in seawater relative to an Ag/AgCl reference cell, aluminum has a freely corroding potential of approximately -750 mV, zinc is approximately -1000mv and aluminum anodes are approximately -1050 mV. The greater the negative difference in these values, the more protection the anode will have.
      I am a Senior Mechanical Engineer for a marine propulsion company that uses aluminum anodes exclusively for protecting aluminum equipment. We transitioned from zinc to aluminum in 2006 and have not looked back. As a bonus, we have shed significant weight as aluminum anodes are much lighter!

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