Cathodic protection of the vessel with sacrificial anodes
Vessel corrosion – cathodic protection and sacrificial anodes
To achieve cathodic protection of the vessel with a sacrificial anode system, certain tools and techniques must be used. Vessels are among the structures that are most exposed to corrosion due to the environment in which they operate. Seawater is a very corrosive environment because the salt in it makes it a very good conductor of electricity and leads to the oxidation of iron by releasing many ions. Almost every part of the vessel is subject to corrosion of varying severity.
Parts of the vessel that are underwater or exposed to water, some of which are:
Exterior hull, rudder, impeller shaft, ballast tanks, ballast pipes
There are several ways to protect the hull and other parts of the vessel from corrosion. The method chosen depends on the area to be protected, its shape and environment. There are three major ways to protect against corrosion:
- Coating with anti-corrosion paints
- Cathodic protection system of sacrificial anodes
- Impressed current system
In this article, we will discuss the method of cathodic protection system of sacrificial anodes.
Classification of anodes
Anodes can be classified according to their shape, size, type of raw materials and method of installation at the protected level. Anodes can be used in small or large sizes, and this affects their weight and the overall weight of the protected structure. The size and shape of the area to be protected, availability and access to space, and structural strength considerations are factors that affect the size and weight of the anode.
Anode shape
Anodes can take many forms depending on their application. Sacrifice anodes are produced in various forms such as flat or block shaped, cylindrical or semi-cylindrical, tear anodes, bracelet anodes, disc anodes and tubular anodes.
The choice of anode shape depends on several factors. Some of these factors are:
- Shape of the protected surface,
- Availability of sufficient space to install the anode,
- Availability
- Easy installation.
For example, flat anodes are mostly used for large, flat surfaces such as the hull. In high-speed boats, where flowing water is very important, tear anodes are used because flat anodes increase the boat’s strength. Bracelet anodes are used for pipelines and impeller tentacles, while tubular anodes are used for cables. Of course, in some cases the choice depends on the availability, cost and flexibility in the design. For example, if the cost is high, cylindrical anodes can be used to protect pipelines instead of bracelets.
Anode type
Zinc or aluminum sacrificial anodes are commonly used for marine applications. Two features of anode performance measurement are listed below.
- Closed circuit potential indicates ease of anode corrosion. The more negative the value, the corrosion is easier.
- Electrochemical capacity (Amp-hr / kg) indicates the amount of anode material consumed.
From the table above it can be seen that aluminum has a higher closed circuit potential, so it starts working faster than zinc. It also has a higher electrochemical capacity than zinc and will last longer at the same weight.
In addition, zinc anodes in freshwater tend to form a calcareous coating on the anode surface that prevents them from working effectively. However, zinc anodes are sometimes more reliable in low-oxygen environments, such as marine sediments or areas with high bacterial activity. Therefore, while aluminum is more efficient, the use of zinc may be more effective in some cases.
Aluminum anodes can also ignite if they fall from a height on oxidized steel. Therefore, their use inside cargo tanks is not recommended. Therefore, the choice of materials depends on the type of environment in which the anode is placed and must be done carefully.
Anode installation methods
There are three basic ways to install an anode on a protected structure:
- Welded connection
- Connection by bolts
- Gutter connection
Welded connection ensures the closest and safest contact between the anode and the body, and electrical conduction through the insert is well done. But sometimes welding is not possible due to lack of access or favorable conditions. Therefore, the connection can be used with bolts or studs. In addition, it is easier to replace anodes connected with bolts than to weld.
Tavana Zob Sana’ati Kavir Company, in the field of cathodic protection of vessels with sacrificial anodes and similar tools, is able to produce sacrificial aluminum and zinc anodes in different dimensions and shapes according to the client’s request. You can contact the sales department for more information.
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Applications of sacrificial anode systems
The method of sacrificial anodes and its applications
In the sacrificial anode method, the current required to combat the corrosion of the structure is provided by another metal, which is usually made of zinc, aluminum and magnesium. Because these sacrificial anodes do not have the ability to provide high current, sacrificial anodes are used where low current is required to protect against corrosion.
Applications of sacrificial anodes include the following:
- Where the protected structure is small.
- Items that do not have access to electricity, such as offshore platforms or pipelines.
- Items where the use of electricity is dangerous, such as inside oil storage tanks.
- Where soil specificity is low. The use of sacrificial anodes in high-strength soils is not recommended due to reduced output current.
- To fix the wandering currents.
According to the above, you can get the necessary information about cathodic protection from the sales and development unit of Tavanazob Sana’ati Kavir Company regarding the applications of the sacrificial anode system, and follow other questions about the applications of the sacrificial anode system from the sales support department.
In this regard, you can receive the contact numbers of the sales and development unit from the relevant department or send questions related to cathodic protection or sacrificial anodes via e-mail.
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Cathodic protection of aboveground storage tanks
Protection of aboveground storage tanks against corrosion
Both sacrificial anodes and impressed current system are used for cathodic protection of aboveground storage tanks. In this method, the protection of the inner surface of the tank is done by the method of sacrificial anodes and the protection of the outer surface of the tank floor is done by ribbon MMO anodes. Because the tank has large dimensions and therefore high weight, to support the weight of the tank, ribbon anodes with a flat cross section are used. Also, flat anode will lead to better flow in the tank floor. Before placing the bottom and body of the tank in the desired location, first the anodes are installed in the correct way, then a special layer of sand is poured on it and finally the bottom of the tank is placed on the sand.
Cathodic protection of the tank floor
The impressed current cathodic protection system is placed in the bottom of the tank in such a way that first the ribbon MMO anodes are laid in the form of horizontal strips in the bottom of the tank and in order to connect the anodes to the rectifier, the conductor bar which are titanium strips without cover, are used perpendicular to ribbon anodes. The connection of titanium bar to ribbon anodes is done by spot welding. The negative phase of the rectifier is connected to several points of the tank body. The positive phase of the rectifier is connected to a junction box and then through it, titanium bars is attached to the anode. The job of titanium is to deliver current to the anode. These connections are well done in different places so that no interruptions occur. Also, a number of permanent reference electrodes are placed on the bottom of the tank so that the potential of different parts of the tank can be measured.
In the past, other methods were used to cathodically protect the tank around it, such as drilling wells and placing anodes around the tank. But these methods did not bring the flow evenly to the bottom of the tank.
Cathodic protection inside the tank
The ribbon anodes inside the soil only protect the outer surface of the tank floor from corrosion. Because there is a different electrolyte inside the tank, such as water or crude oil, other anodes must be used to protect the inside of the tank. Sacrificial anodes, and in some cases impressed current anodes, are commonly used to protect the inner surface of the tank. Inside the crude oil tanks, only the sacrificial zinc anode is used.
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Types of backfill in cathodic protection system
Types of backfill
The type of backfill used in the cathodic protection system in ground substrates depends on the type of cathodic protection system used. Due to the fact that the cathodic protection system exists in two types of sacrificial anode systems and impressed current anode systems, so two types of backfill materials are also used.
Sacrificial anode systems
The sacrificial zinc and magnesium anodes used to cathodic protection of structures buried in the soil are sometimes packed in a linen bag with a backing material. These materials reduce the local corrosion of the anode by preventing the anode from coming into direct contact with the soil. It also prevents the anode from becoming passive by reacting with salts in the soil. The backfill used improves the performance of the anode by creating a low-resistance environment around the anode. These materials are generally a combination of three types of gypsum gypsum, bentonite and sodium sulfate, and the percentage of their compounds depends on the environment used.
The following table shows the types of backfills according to the IPS-M-TP 750 standard:
Impressed current system
Impressed current anodes for cathodic protection of underground structures are usually surrounded by a carbon backfill. The purpose of using this support is:
- By reducing the resistance of the environment around the anode, it leads to an increase in the amount of current produced by the anode.
- As the anode level increases, the amount of current increases.
- Reduce anode consumption rate and thus increase anode lifetime.
Carbon backfill according to IPS-M-TP 750 standard is available in three types of 80% metallurgical coke, 90% petroleum coke and 95% special petroleum coke.
The chemical composition of these three types of backfill is specified in the table below:
Coal coke is generally used in shallow horizontal substrates. Petroleum coke or special petroleum is used for deep wells.
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Lifetime design of cathodic protection system
Necessity of designing the lifetime of the cathodic protection system
In order to protect metal structures immersed in water or buried in soil against corrosion, it is necessary to use a suitable cathodic protection system. The cathodic protection system must be designed to provide the required current for each part of the structure for the required lifetime. This requires determining the size, weight, number and type of anode. Also, the design of longevity is based on each of the cathodic protection systems, taking into account the specific conditions of those systems.
Design based on impressed current system
The calculations for impressed current cathodic protection systems are relatively simple. Since the impressed current anodes are weight stable, in this case it is only necessary to match the number of anodes with a certain output current with the amount of current required by the whole structure. The anode distribution should be such that it leads to a uniform and balanced distribution of current.
Design based on sacrificial anode system
The calculations for the sacrificial anode system are a bit more complicated. The amount of sacrificial anodes required to protect metal structures is directly related to the surface subject to corrosion. The better the initial coating, the less anodes are needed. The arrangement of the anodes on the surface of the structure is adjusted to ensure a uniform distribution of current throughout the structure.
By sacrificing its weight, the sacrificial anode provides the protection current needed. Therefore, in a sacrificial anode system, the number of anodes must not only provide the current required by the structure, but must also have sufficient weight to provide current for the designed life of the structure. The size and shape of the anode is determined by the following factors:
- Minimum and maximum output current requirements.
- Installation and connection requirements.
- Anode weight requirements required.
- To be economical.
Sacrifice anode used in the tank
Maintenance of sacrificial anodes for fixed offshore structures is generally very costly and sometimes impractical. Therefore, using the optimal number and weight for the structure under protection is very important.
In this regard, you can use the sales unit of the cathodic system and you can also buy the products of Tavanazob Sana’ati Kavir Company from the relevant postal address.
The question is, why is the design of the cathodic protection system important to us?
We will answer this question in other educational articles of Tavanazob Sana’ati Kavir Company to learn more about the cathodic protection system.
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