General term declaration

The sacrificial anode, also called anode protection, has its name thanks to its functionality. This sacrifice is the anode literally to the subject to protect, to which you attached. Such a subject can be for example a vessel or a hot water storage, what damage of any kind associated with high costs. Purpose of a sacrificial anode is to protect from damage caused by the so-called corrosion.


Corrosion is a chemical reaction of a material with its environment. This sounds all harmless, however, when one looks at the concept of origin of corrosion, you will find that the corrosion process not so harmless. In Latin, “corrodere” means decomposing, decomposing or gnawing.

Corrosion exists in different species, but in the context of sacrificial anodes, you always speaks of electrochemical corrosion. This oxidized the anode, which in concrete terms means that you free electrons. It is in a voltage range, in the electrons is always in the direction of the anode flow. In contrast, a cathode – the counterpart to the Anode – reduced and not oxidized, since them electrons.

The corrosion product a final oxidation leads to the above already implied destruction of a metal and can be described as a rust, unless the oxidizing metal iron or steel. Otherwise is a corrosion is not less harmful, but not just with the term rust titled.

Water types

In general it can be said that water is not equal to water in relation to the subject of corrosion, since it has different aggression potentials as an electrolyte depending on the type of water. Accordingly, different types of sacrificial anodes are required for different types of water bodies. Water types are generally divided into fresh water, brackish water and salt water.

Fresh water has low electrical conductivity since it contains very few salts necessary to produce a series of stresses. For this reason, one uses for fresh water aluminum anodes or sometimes magnesium anodes. In the case of the magnesium anodes, however, it must be ensured that the waterway is very low in salt, since otherwise an unnecessary overreaction of the anode can be imminent.

Brackish water stands out by virtue of its slightly higher salt content. Other than fresh water, it is not drinkable. Although it does not have such a high salt content as salt water, in brackish water zinc anodes are usually used anyway. The use of aluminum anodes is also possible.

Salt water has his name according to the highest salt content. It is as well as brackish water unfit for human consumption, has the best conductivity and can therefore be best to create a voltage range for the movement of electrons is necessary. This characteristic makes the salt water to the most dangerous waters types because it is the fastest way to corrosion. In Salt Water use also zinc anodes.

In order to be able to use his boat carelessly both in inland navigation and in the maritime carefree, our experts recommend using aluminum sacrificial anodes. 
The metal in this sacrificial anodes has a high potential difference and is normally used in fresh water which has a low electrical conductivity. But you can still use easily these anodes in salt water, as salt water has a very good electrical conductivity. It is important to note that the sacrificial anodes will probably be faster dissolve in salt water because. Therefore, we recommend to replace the sacrificial anodes more often within the season in order to have risk-free fun in both types of water. However, this effort can also be avoided by using larger sacrificial anodes, which leads to the same result as the more frequent replacement of the sacrificial anodes.

Sometimes refers you to a water types in more detail to classify its water hardness. This is the general statement that the harder the water, the better the electrical conductivity. On the hardness of a water type can be based on the contamination. It is generally so that the cleaner the water, the more it corresponds to the drinking water from the water tap and will therefore be equated with fresh water. In this context it is called soft water. In contrast to this is hard water more contaminated, because it corresponds to unprocessed and brackish or salt water.

Thus, zinc anodes should be used for hard water and aluminum anodes for soft water. In special cases, if necessary, magnesium anodes can be used. 

Do you need sacrificial anodes at all?

As a conclusion to the types of water, what a sacrificial anode is to be used, you can now call into question whether sacrificial anodes are required.

In brackish water and salt water, this is out of the question, because of their good electrical conductivity an oxidation of the sacrificial anode favored. Thus in this case it was very likely cause irreparable damage to the object to be protected – for example boat/ship or hot water storage – if you do not have a sacrificial anode.

Fresh water is often incorrectly assessed the situation. Although the electrical conductivity is not as high as that of the brackish or salt water, there is still a real danger that corrosion occurs and thus the object to be protected is irreparably damaged.

It is therefore absolutely recommended for no matter what the circumstances sacrificial anodes to use. They are, so to speak, an ongoing insurance, at low cost, protection against unreasonably high costs in the case of real damage. Therefore should not be on the wrong end of savings, because a household contents insurance, for example, also has almost every house owner, to reduce the risk in the event of an emergency to minimize, instead of high cost to stay seated.

When should you replace a sacrificial anode?

The sacrificial anode is a wear part and must be replaced regularly as precisely as a result of the death of the sacrificial anode, the object to be protected remains intact. The wear of a sacrificial anode can be very easy to understand by the anode regularly look.

The anode is now already heavily oxidized, it makes sense to share, since it is with the time increasingly decomposes. A clear set of rules at what time you replace a sacrificial anode now, there were unfortunately not because the corrosion progress depends always on the types of water. However, it is customary for aggressive driving waters as brackish water or salt water, the sacrificial anode more than once per season. In safe waters types such as freshwater, this is not the case again. There is usually from the anode exchange once per season. Change of sacrificial anodes are mostly in the spring and autumn.

It may therefore be concluded that the exchange of a sacrificial anode not too long to wait, as otherwise there is a danger that the protected subject to serious corrosion of bears. Accordingly you should observe the oxidation of the sacrificial anodes and replace it as soon as less than fifty percent of their initial substance are left.

Which material is used for sacrificial anodes?

The raw materials for sacrificial anodes, there are clear differences. In principle are suitable for sacrificial anodes all metals in a voltage range of several elements under than the element to be protected. With regard to availability, efficiency and environmental compatibility, today we find but mostly anodes of zinc, aluminum or magnesium.

Add the material is coming in a sacrificial anode your potential difference of interpretation. This is related to the electrical conductivity of the types of water, what the sacrificial anode should be used. It is the higher the electrical conductivity of the water, the lower the potential difference of the metal to be used.

Accordingly, for example zinc anodes a very low potential difference, since it in brackish water or salt water should be used whose electrical conductivity is already very good. In contrast, sacrificial anodes made from aluminum a high potential difference and are within fresh water used as fresh water a weak electrical conductivity brings with it.

Magnesium sacrificial anodes even have an even higher potential difference than those of aluminum, but are not as durable as the latter, which increases the annual cost of anodes and anode assembly. Often the already increased potential difference of aluminum anodes is completely sufficient, so that magnesium does not have to be used.

Aluminum anodes are often alloyed with indium. Pure aluminum would form a passivating oxide layer on the surface in the natural oxygen-containing atmosphere. By adding indium, it is possible that this oxide layer does not form or tear continuously and thus the desired reactivity remains.

To sum up: Zinc for brackish or salt water. Aluminum for brackish or fresh water.

Attention! You should not attach two different anode materials on one ship. For example, equipping your ship with both aluminum anodes and zinc anodes will cause only the more active anode to dissolve (aluminum). At worst, this premature dissolution of the more reactive anode can result in passivation of the zinc anodes. Your boat would then no longer be protected against corrosion after the aluminium anode has dissolved.

Quality aspects in the production of sacrificial anodes

The basic method is to produce sacrificial anodes through a casting process. However, two casting methods can be distinguished. The quality of the sacrificial anode can be directly deduced from the choice of the casting process of the manufacturer.

The high pressure die-casting process for sacrificial anodes is widespread. It is a fully automated machine process. In order for the manufacturing process to run reliably and in a repeatable manner, a modification of the processed sacrificial anode alloy is required. For this requirement metallic additives are added exclusively on the basis of the manufacturing process. That means, that in order to ensure a favorable production, therefore, a compromise is made at the expense of the corrosion protection properties and at the expense of the environmental compatibility of the anode. In addition to the cost advantages of production, the high pressure die-casting process leads to a very smooth surface of the sacrificial anodes produced.

Alternatively, the gravity casting process is used for the production of the sacrificial anodes. This process is performed manually or partially auotmated. The alloy does not have to be adapted to the manufacturing process, but is designed for 100% optimum corrosion protection. The sacrificial anodes can be identified by a comparatively rough surface, partly orange-skin-like. A sacrificial anode with a rough / wavy surface has a larger functional surface than a smooth sacrificial anode of the same dimension. This larger functional surface has a positive effect on the anodic protection effect.

Conclusion: Sacrificial anodes that are produced in the high pressure die casting process look beautiful at first glance, but they have both qualitative and environmental disadvantages. The initially less aesthetically pleasing sacrificial anodes from the die casting process are clearly to be preferred for reasons of anodic protection. As so often in life – the inner values ​​count!

Just ask your sacrificial anode supplier what kind of casting process he uses for his anodes.

Which shape do sacrificial anodes have?

Sacrificial anodes are available in a variety of shapes and sizes. This diving designations such as shaft anodes , Block anodes , shaft anodes – rings, Hull anodes, bow thruster propeller anodes, anodes, shaft end Oden anodes, washer, pot anodes, Trim Tabs anodes, cap nuts, a – two – or even three-hole hats on. These names will depend on the form of sacrificial anode and thus the shape of the object to be protected from, at which you would like to attach the anode.

In the event of a ship has for example several ways to position the sacrificial anodes. Depending on the selected Ship part, different anodes are needed, which solely from the different form of the individual ship parts can be derived. This can be regarded as a good example of the propeller and the boat hull to compare. Not only the size of soft both ship parts strongly differ from one another, because the ship screw compared with the boat hull is rather small, but also in their form there are serious differences. The propeller is much delicate, if you the large boat hull compared. Therefore one needs for the boat hull is a block or fuselage anode, the rather elongated and covers a large area. On the other side is for the Ship screw a bow thruster propeller or anode used at the top of the propeller is attached.

In which areas are sacrificial anodes used?

Sacrificial anodes are used in different areas in which corrosion can occur. In shipping, you use most often. You should expensive ship parts such as the ship propeller – also referred to as the ship screw Hulk or the rudder blades protect a ship.

In addition, sacrificial anodes also in hot water storage is used, for example boilers in the boiler, zinc-plated rule from corrosion protection.

Even for larger tank bearings, underground pipelines and oil drilling towers used sacrificial anodes, to prevent corrosion. 

Installation of sacrificial anodes

Installation of sacrificial anodes as soon as you get the correct sacrificial anode on the basis of the types of water where the anode is to be used, has selected to come to the question of the assembly of a sacrificial anode. For a correct positioning of the sacrificial anode, it is important to ensure that an electrical conductivity between the sacrificial anode and the object to be protected exists. Otherwise the sacrificial anode is not correct and the protected subject matter is instead attacked. Therefore you should not have any seal made of rubber or other material between the sacrificial anode and the object to be protected position, but metal on metal. Sacrificial anodes are fitted depending on your model. You have either holes, so that you can tighten the screws or tabs, what is the anode to the object to be protected can be welded. In some sacrificial anodes, such as for example the shaft anode, you must click on the shaft diameter. Otherwise it is a smooth fit the sacrificial anode on the waves of the ship screw is not possible.

After an anode more than fifty percent is corroded, you should replace it. The exchange works usually easy because again, depending on the model, you need to set the sacrificial anode, as well as for their assembly, unscrew or refuse to remove it. To ensure that the dismantling of screwed sacrificial anodes is carried out as problem-free as possible, an allen key screw should be selected for mounting in advance, during the assembly of the new anode. By means of the inner hexagon head screw head, much higher torques can be transmitted than, for example, with cross screws or even slotted screws, so that the screw can be reliably detached from the corrosion products and the sacrificial anode can thus be easily exchanged.

Sacrificial anodes are in ships along the keel is attached. A suitable location for installation of anodes is especially the propeller as part of the bow thruster rudder at the rear of Steven of the boat. For smaller boats is the external border, what the sacrificial anode attached. In addition we also frequently uses the rudder blade and the propeller shaft to be sacrificial anodes to attach. The most common is the first named ship screw – also called the ship propeller, since the current intensity there is strongest and therefore the anti-corrosion protection at this point is absolutely necessary. How many sacrificial anodes you for a boot is now required, can be based on a rule of thumb to follow. This is the larger the surface of the underwater vessel, the more anodes are attached. You should prefer an anode fit too much than too little for the existing to minimize the risk of corrosion.

Adjustment and modification of sacrificial anodes

It may happen that a sacrificial anode adapted or need to be modified so that the dimensions of the object to be protected. This should be in the rule is not insurmountable problem, since such a subsequent change to the sacrificial anode does not cause any interference with its function.

Is a sacrificial anode with holes, so you can see the existing drill holes as required to enlarge or add additional drill holes? You should however rather by a Specialist or Expert, because before you can use the drilling begins, some precautions must be taken. On the one hand one should take the sacrificial anode with a vice so that it cannot move and the other is the staking before the beginning of the Drilling very much, as it would otherwise with the drill may slip on the material. Add a drill must be coming to the strong enough to bring the material to penetrate.

If you want to subsequently in a sacrificial anode plates to secure to the Anode weld, instead you need to tighten the screws, this is also possible. This should also be an expert or experts. It is important to note that the tabs ideally should have the same metal as the sacrificial anode itself or at least with the same metal covered should be the same as the welding metals to less complications. A zinc plated steel bracket for example can be retrofitted without any major problems in a zinc anode welding.

In addition, sacrificial anodes also individually cast. This is usually connected with higher costs, because it must first of all be made a mold tool, then as a form of sacrificial anode is used. Questions related to individual anodes come mainly from business customers.

Testing the functionality of sacrificial anodes

Generally applies the statement that if a sacrificial anode observable is corroded, it can be assumed that their assembly has been carried out correctly and you do not have to worry about their functionality.

However, in order to also ensure that a sacrificial anode really works, you can make it to their functionality to test. This can be done by using a current measuring device are implemented, where related to the use in the case of warm water saving, the prerequisite is that the sacrificial anode isolated was installed. Then you can see the current measure the voltage range in which the sacrificial anode. If the current is strong enough now depends on the area in which the sacrificial anode their use. The hot water storage tank for example have their function limit at 0.3 mA. Only from this limit flows a stream of protection is sufficient to accommodate the corrosion on the sacrificial anode to steer and therefore the heating boiler to protect.

Possible reasons for the failure of sacrificial anodes

Possible reasons for the failure of sacrificial anodes is the surface of a sacrificial anode has not really been removed, so you can assume that the anode does not function correctly. A malfunction in a sacrificial anode can have multiple reasons. In the following you can see an overview of the most common reasons for a malfunction of sacrificial anodes:

Use of unsuitable anode material

The material of a sacrificial anode under must always be as the protected metal, since the anode is otherwise not to negative and that there is no corrosion can accommodate. Do you as a zinc anode on the aluminum hull of a boat, so is the metal zinc more noble than aluminum and thus there is no stream of electrons in the zinc anode ends.

The sacrificial anode was attached under electrical isolation

A sacrificial anode must always with its metal on the metal of the protected object, without rest in between any type of isolation. Sweeps you an anode for example with paint or varnish, this sets the function of the Anode except force. On the other hand, it should be noted that also the protective metal-free. There should be as little painted or by any object be sealed so that no current flow can arise. 

Influence of external power

External electrical currents , which is not directly related to the flow of electrons of the protected metal for sacrificial anode have to do can cause significant interference problems. For example, voltage drops of public passenger transport the current in the water and thus influence the effect of sacrificial anodes affect or even repeal. Foreign currents can be used for example in the case of boats with the help of field measurements identify.

Interference of other metals

Other metals, located close by the sacrificial anode in the water, can also cause the electron flow from the protective metal to the sacrificial anode to be distracted. For example a boat in the vicinity of another vessel in water, so the field can be disrupted in the water and the functional ability of the sacrificial anode is affected.

Cleaning of and caring about sacrificial anodes

To keep the sacrificial anode clean and match it to the well-maintained outside of the protected metal, one can apply different types of cleaning.

A wire brush is a good helper. It can free the sacrificial anode from algae and other debris. The bristles should be made of stainless steel to ensure in the use of the brush and the associated loss of bristles, to avoid rust.

A further method of care of sacrificial anodes is the use of a sand blasting gun. Due to the pressure of the sand blasting gun and the fine element concerns the resulting escaping the grains of sand, residues of any kind are very reliably removed.

Further information can be found in our Blog.