Zinc anodes, also known as zinc alloy sacrificial anodes, play a key role in the cathodic protection of Marine sacrificial anodes. "Zinc" was once synonymous with sacrificial anodes, because the most sacrificial anodes made were zinc blocks. Currently, other metals, such as aluminum and magnesium, can also be used as sacrificial anode materials.
Sacrificial anodes are made of a relatively "cheap" metal that protects more expensive metal parts on a boat or yacht, such as shafts, propellers, rudders, stern drives, and other important parts. The main principle of action is that the metal in the anode is more "noble" and will "sacrifice" itself, it will be corroded before the protective body, and protect other metal components from corrosion in water for a long time.
Zinc anodes, aluminum anodes and magnesium anodes are three common sacrificial anodes, and these three anodes have their own place in different application fields.
Advantages of aluminum anodes
Many offshore equipment uses aluminum anodes instead of zinc or magnesium anodes for the following reasons:
1, in recent years, aluminum prices have fallen, making it more cost attractive to install aluminum anodes in storage tank containers, engines, filters, heat exchangers and other equipment.
2, zinc anode can play a good protective effect in salt water, and can also work in brackish water, but the effect is poor in fresh water. Aluminum anodes have a good protection effect in salt water, and the effect is good in salty water with slightly lower salinity, but the effect is not good in salty water with higher salinity, and the effect is far better than zinc anodes in fresh water. However, the passivation of the aluminum anode (surface oxide film) is faster than that of the zinc anode, and the aluminum anode must be cleaned regularly to work properly. If this is not done, the aluminum anode may be "passivated" and thus fail.
3, the U.S. Environmental Protection Agency reports that magnesium and aluminum anodes are more environmentally friendly than zinc anodes, and in the various environments tested, zinc anodes are more environmentally damaging than magnesium and aluminum anodes. Therefore, if the ship frequently shuttles back and forth between saltwater and freshwater environments, it should be considered to use an aluminum anode instead of a zinc anode. Aluminum anodes are cheaper, last longer, and protect better than other metal materials in different water environments. If the ship is sailing in an area with higher salinity, and in very warm tropical waters, the higher water temperature will produce more dissolved oxygen, which will increase the rate of anode corrosion, resulting in a shorter life of the sacrificial anode.
When to use magnesium alloy sacrificial anodes - King of fresh water
Although aluminum and zinc anodes can be used in fresh water, both aluminum and zinc anodes are inferior to magnesium anodes in fresh water. In fresh water with high resistivity, the high output current of magnesium is required to ensure adequate protective current. Although magnesium anodes are more expensive and have a shorter life than zinc and aluminum anodes, only magnesium anodes can be adapted to the high resistivity of fresh water environments, especially if the ship will sail in fresh water for a long time, and magnesium anodes should be used.
The high current output of magnesium in brine has the risk of overpotential protection (hydrogen embrittling), and the lifetime in brine is usually too short, so magnesium anodes are generally not used in brine environments.
Advantages of zinc anodes over aluminum anodes
The corrosion products of zinc anodes are more likely to "fall off", exposing the fresh zinc alloy anode to water, allowing the anode to remain electrochemically active and release current. Aluminum anode is not, if in a static environment, its corrosion products are not easy to fall off, the metal consumed by the aluminum anode will form a barrier, so that the anode passivation, resulting in aluminum anode can not continue to corrosion. The longer it is placed in a static environment, the higher the chance of anode passivation.
If a ship regularly sails in saltwater, both zinc and aluminum anodes will work; However, if you are sailing in salt water for a long time, zinc anodes are the best choice.
How does a sacrificial anode work?
Whenever two different metals form an electrical connection and are placed in water or other electrolytes, they technically constitute a galvanic cell, at which time there is a certain current flowing between the two metals. The ions that make up the current are supplied by one of these metals, and it releases part of itself into the seawater in the form of metal ions, which is the process of corrosion. If left unchecked, the ship's equipment will slowly erode away.
When to replace the sacrificial anodes?
No matter what kind of sacrificial anode material is chosen, it should be replaced in time to protect the ship from corrosion. When about half of the anode is consumed due to corrosion, it should pay close attention to the corrosion of the anode and decide whether to replace it according to the specific situation. If the anode is more than half consumed in less than a year, it should be replaced with a larger anode. Never cover the anode with paint or anything else, this will make it not expendable due to occlusion, so it will not be protective.
Different types of anodes can be used, but it is best not to mix different anodes for the same part. Before installing the anode, it is necessary to ensure that the hull beneath the sacrificial anode conducts electricity well, this is to ensure good electrical contact.
What does a sacrificial anode do?
It is better to spend little time and money to replace anodes regularly than to spend huge sums of money to repair the ship!! After all, anodes cost far less than repairing ships.