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Why Use Aluminum Anodes Instead of Zinc Anodes?

For most ships, the annual maintenance work is roughly the same, including towing, power unit cleaning, repair and scratching, etc. In addition, it is an indispensable step to apply a new layer of antifouling paint on the hull shell and replace the zinc block (scientific name is zinc alloy sacrificial anode, referred to as zinc anode, commonly known as “zinc block”).

Sacrificial Anode
Generally speaking, during regular maintenance, you basically repeat the last work: repeatedly purchasing the same brand of paint, which can reduce maintenance time and simplify the work process. Regarding the maintenance of boats, shell anti-corrosion is the top priority. Here are several issues related to ship anti-corrosion that all ship owners need to know.

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01 What materials are more environmentally friendly?
Nowadays, science and technology are developing more and more rapidly, and various new materials are emerging one after another. However, different materials have different impacts on the environment. As a member of the earth, shouldn’t we choose environmentally friendly materials and make our own contribution to the earth’s environment on which we live.
The water may look blue, but we need to keep it “green”.
As we look for ways to maximize the protection of ship hulls from marine life, reducing the infiltration of toxic substances into the ocean should also be considered, so the chemical composition of primers is constantly being optimized to reduce harmful emissions.

02 Sacrificial anodes also need to be reconsidered.
Recent studies have shown that toxic materials from zinc anodes, the most common anode material, can accumulate under docks when placed densely. Because conventional zinc anodes contain a small amount of cadmium, which is a toxic heavy metal, even a small amount is toxic. Cadmium is actually banned in some countries. The anodes are consumed sacrificially, and their products are metal ions, which will dissolve into the sea and therefore enter marine animals and plants, poisoning them.
Environmental regulators everywhere are concerned about the impact of these metals – do we need to pollute our home planet with these toxic substances simply for corrosion protection?

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03 Why is a sacrificial anode needed?
We need sacrificial anodes to protect ships from corrosion, which occurs when dissimilar metals are in close proximity to each other and surrounded by an electrolyte, such as salt water. This is common galvanic corrosion. Most boats’ underwater equipment contains at least two different metals, mostly stainless steel components, but there are also many bronze components. Both outboard and stern drives have stainless steel and aluminum components. Any two dissimilar metals can form a galvanic cell and produce an electric current, which can cause one of the metals to corrode. This current is not always harmful and can be harnessed if properly designed. We often design this as a special battery to protect the hull from corrosion. But how should it be designed? Let’s briefly introduce it below.

Ship anti-corrosion
Any two metals coming into contact in seawater will form a low-voltage battery, in which one of the metals will slowly be destroyed. Electric current flowing out of one metal causes that part of the metal to corrode, while the incoming metal is protected. To avoid corrosion of the parts we want to protect, additional metal needs to be installed, making it an anode or cathode, connected to the hull and the underwater parts of the boat. This new metal anode is conductive, and it becomes the part through which the current flows, corrodes itself, and the parts we want to protect are protected. This extra metal displays an act of selfless heroism, sacrificing itself to protect the other metals. Unlike bronze or stainless steel equipment parts, these anodes are inexpensive and easy to replace.
Anodes have been used in the marine industry for decades, and almost all ships require sacrificial anodes.

04 Alternatives to zinc anodes
The sacrificial anode is generally a special metal alloy material that has very high activity. Now there is a new aluminum alloy anode that is more environmentally friendly than the common zinc/cadmium alloy anodes that have been around for decades. This new aluminum alloy has better performance and environmental friendliness, and does not contain any toxic ingredients.
Aluminum is a metal friendly to nature. More importantly, its performance is also excellent, with its electrical capacity in kilograms being almost three times that of zinc. Tests have shown that aluminum anodes provide at least 50% longer protection than equivalent weight zinc anodes, and aluminum anodes have no toxic components.
The toxicity of zinc anodes is drawing the attention of regulators as environmental concerns grow and safer alternatives are now available. According to reports, the US state of Maryland will ban the use of zinc anodes, and other coastal states are also considering similar measures.

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05 Anodes used in fresh or brackish water environments
Vessels that frequently sail in low-salinity seawater and freshwater must use aluminum anodes when anchored in low-salinity waters, because zinc anodes are far less effective than aluminum anodes at low salinity. Boats that sail in fresh water for a long time must use magnesium anodes, as other metals simply cannot function effectively in fresh water.
Aluminum anodes are also lighter and can provide the same anti-corrosion effect at half the weight of zinc anodes.
Aluminum anodes used to be more expensive, but in recent years aluminum prices have fallen below zinc prices, giving aluminum anodes a cost advantage. With aluminum anodes, the boat is better protected, the anode life is longer, and since it is cadmium-free, both the boat and the environment are better protected.

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06 Summary

  1. The anode should be checked regularly: consumption too fast or too slow means there must be a problem. Slow consumption may mean an incomplete connection system, or an improper anode metal selection.
    Remember: only magnesium anodes are available in fresh water, while aluminum anodes can work in both salt water and brackish water.
  2. Clean the installation studs frequently and use sandpaper or wire brush to smooth them.
  3. Complete consumption of anodes within one year indicates that there is too little anode material and more anodes need to be installed.
  4. The anodes can be replaced underwater, but most shipbuilders recommend replacing the anodes on land during dock repairs to ensure safe anode installation and cleanliness.
  5. The anode must be reliably connected to the protected structure to ensure that all metal parts are protected. Since the current is smaller, heavier wire is needed instead of lighter wire.

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