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.
- Published in Articles, Educational, Scientific
Cathodic protection of offshore structures
Offshore structures
Offshore structures include platforms, semi-submerged structures in water or wells, pipelines, and underwater equipment. Offshore steel structures are designed based on two criteria of strength coefficient and strength. Its strength coefficient depends on the amount of surface material that is lost during the corrosion process. Its strength is affected by pitting corrosion. Therefore, a proper and accurate design of a corrosion control system is essential.
The outer area of offshore fixed steel structures is divided into two categories according to its surroundings.
The part of the structure that is adjacent to the air and above the water surface and must be protected by a suitable coating. Another part that is under water and to protect it, a precise cathodic protection system is required.
Cathodic protection system
When the structure is inside the electrolyte (water), the potential difference between the different points of the structure causes a current to flow between these points. This current causes the part that has a more negative voltage to corrode. The principle of cathodic protection by creating a more negative potential around the structure leads to the potential of different parts of the structure being the same. Therefore, the steel structure is protected against corrosion. This current may be supplied by various sources such as transformer rectifiers or sacrificial anodes.
Cathodic protection by sacrificial anode
The sacrificial anode is usually a metal that has a more negative potential than the structure. The connection of the anode to the structure causes a current to flow inside the electrolyte from the anode to the structure. For offshore structures, the sacrificial anodes are made of aluminum or zinc. Aluminum sacrificial anode is used more than zinc sacrificial anode due to its “better current to weight ratio” feature.
Types of sacrificial aluminum anodes widely used in offshore structures
Important types of sacrificial aluminum anodes designed for offshore applications include bracelet anode, stand-off anode, and welding anode.
Core anodes are connected to the bases by means of welds or bolts before installing the structure in the sea. These anodes are generally installed on offshore jackets.
Aluminum bracelet anodes are specially designed for cathodic protection of oil and gas pipelines. These anodes are produced from alloys with high purity and very low percentage of iron and are designed with diameters appropriate to the desired pipe diameter. These anodes usually consist of two parts that are connected to each other by an internal iron belt.
- Published in Articles, Educational, Scientific
Cathodic protection of pipelines
The role of cathodic protection of pipelines in the oil and gas industry
One of the main and very costly problems in large industries such as gas, oil, petrochemical, shipping, etc., is the corrosion of metal structures.
In the oil and gas industry, carbon steel pipelines are used to transport hydrocarbons from jackets to production platforms and storage tanks, and enter refineries through pipelines. Carbon steel pipelines are the safest and least expensive way to transport large volumes of hydrocarbons over long distances.
Fuel and water transmission pipelines and other structures buried in soil or submerged in water, due to their metallurgical conditions, are in a state of corrosion and after a while, the performance of the structure is disrupted.
Corrosion in the oil and gas industry dramatically increases the overhead costs of plumbing operations due to the replacement of worn equipment and damage to adjacent equipment. Therefore, it is necessary to use a proper protection system to prevent corrosion.
Corrosion control mechanism in cathodic protection of pipelines
Corrosion control mechanisms in oil and gas pipelines are mitigation measures to eliminate or reduce corrosion to prevent pipeline failure and related consequences such as crop failure, environmental pollution and potential accidents.
Five control mechanisms to minimize pipeline corrosion and cathodic protection of pipelines that must be considered during the design and construction of pipelines include design, material selection, coating, cathodic protection, and chemical protection.
Designing
One of the important factors in corrosion control is design. Proper design of a steel pipe increases the life of the structure. Gaps, steel geometry, sharp edges, welded joints, and pipeline connections such as flanges, valves, and pipe supports are often subject to local corrosion due to fluid penetration and dissolution.
In addition, these environments have poor coverage due to inaccessibility or sensitivity to coverage failure. These factors contribute to corrosion and should be carefully considered during the design phase to limit corrosion.
Material selection
The choice of materials is very basic in terms of engineering design to reduce corrosion. The choice of material depends on the environmental conditions such as the fluid being transferred, the temperature, the pressure and the electrolyte in which the pipe is placed (water or soil). Also in flange joints, piping and general joints where it is possible to pair metals of different materials, to prevent galvanic corrosion, care must be taken in determining the material.
Due to its good mechanical properties, availability and low cost, carbon steel, despite its low corrosion resistance compared to other corrosion-resistant alloys, is the main material for pipeline transmission.
coating
One of the main and simplest methods of protecting pipelines is coating. A suitable coating prevents direct contact of the outer surface of the tube with the surrounding environment and thus prevents the flow of ions through the electrolyte, thus limiting corrosion.
There are different methods and types of coating and the choice of its type depends on the type of material selected:
- Painting
Painting is used in environments where the equipment is only exposed to air and no electrolyte is available.
- Coating
The main types of coatings for coating structures that are exposed to electrolytes are strong coatings such as polyethylenes, bitumen and polypropylene.
- Lining
Lining the surface of the structure with rubber sheets.
- Cladding
coating one metal with another.
- Cathodic Protection
Using a cathodic protection system along with coating is an effective way to prevent corrosion of pipelines.
There are two general methods for cathodic protection of pipelines:
- Sacrificial anode system
- Impressed current anode system
In cases where the length of the pipeline is very long and in the presence of stray currents, an impressed current system is used to protect the pipeline.
Chemical protection
Chemical protection includes corrosion inhibitors, Oxygen purification system, and biocide in a chemical process.
In the chemical process, biocide is injected into the system, liquid or water to kill microorganisms such as reduced bacteria that increase corrosion under microbiological effects with sulfide. Oxygen scavenging chemicals are injected into the water to deoxygenate to reduce the severity of the corrosion and ultimately achieve adequate cathodic protection of the pipelines.
- Published in Articles, Educational, Scientific
Cathodic protection of underground storage tanks
Corrosion of underground storage tanks
Underground storage tanks are generally in contact with materials such as soil and sand. The electrochemical reaction between the tank and the surrounding soil (electrolyte) leads to tank corrosion. The small voltage difference on the steel surface causes a current to flow from one part of the tank to another. Corrosion occurs where current flows into the soil (electrolyte). This part in the galvanic cell is called the anode. Where the flow from the soil enters the reservoir is the cathode and corrosion does not occur.
Prevention of tank corrosion (cathodic protection of underground storage tanks)
An outer cover as well as a cathodic protection system are used to protect the underground tanks from corrosion. A suitable outer cover can protect more than 99% of the tank surface. But since there is no complete cover, in addition to the outer cover, a cathodic protection system must be used to protect the tank from corrosion. The cathodic protection system converts the tank into a cathode by applying an external current and protects it from corrosion. In cases where the amount of current required to protect the structure is small, sacrificial anodes are used. For large structures such as large diameter pipelines, an impressed current system is used.
Magnesium sacrificial anode for cathodic protection of underground storage tanks
Magnesium sacrificial anode is the most common sacrificial anode used in cathodic protection of underground tanks. Magnesium anode can protect underground tanks in any soil. To reduce the electrical resistance of the anode to soil, a magnesium anode is usually used inside a linen bag with backfill material that is a combination of gypsum plaster, bentonite and sodium sulfate. Magnesium anode is produced in two types of standard and high potential. High potential magnesium anode is used in soils with a resistance of about 10,000 ohm-cm. Based on the dimensions of the tank and the soil resistance in which the tank is located, the number and dimensions of the required anodes are determined. The two most common sizes of magnesium anodes to protect underground reservoirs are 17-pound and 32-pound anodes. The anodes are installed at a suitable distance and depth from the tank. The anode wire is connected to the top of the tank with a low-resistance electrical connection. All connections are covered with waterproofing.
In terms of cathodic protection of underground storage tanks, the technical and engineering department of Tavanazob Sana’ati Kavir Company is ready to provide consulting services for which you can contact us.
- Published in Articles, Educational, Scientific
What is a sacrificial anode?
How does a sacrificial anode protect metal structures?
Corrosion of metal structures immersed in water or buried in soil will cause very dangerous and costly damage. One way to protect these structures is to use sacrificial anodes. The sacrificial anode sacrifices itself due to the electrochemical process of corrosion by destroying instead of the impeller, impeller branch, motor or other metal components in water or soil.
Sacrificial anodes are relatively inexpensive metal parts. Known as active metals, they are specifically designed to corrode structures instead of expensive metal parts. These anodes are made of metals that have a more negative voltage than the structural metal. Sacrificial anodes were usually made of zinc metal, hence they are also called “zinc anodes“, but magnesium or aluminum alloy can also be used to produce them. Each of these metals has different properties and applications. The electrical potential and flow capacity of anodic metals are two important features in determining the type and extent of sacrificial anodes used. Sacrificial anodes come in all shapes and sizes, but they all work the same way. They are electrically attached to the metal structure to protect it from corrosion.
How does the sacrificial anode work?
The potential difference between the sacrificial anode and the protected structure causes a current in the electrolyte to flow from the anode to the cathode. The anode and cathode must both be inside an electrolyte and have an electrical connection between them. The sacrificial anode is a metal that has a more negative potential than the structure and forms the negative pole of a corrosion cell. The positive pole of this corrosion cell is the metal (cathode) that protects it. The saltier or more polluted the water, the faster the sacrificial anode will erode.
Since the output current of the sacrificial anodes is much lower than that of the impressed current anodes, these anodes are used in places where a small current is required for protection. Also, in places such as submarine pipelines where we do not have access to electricity and current regulation, the sacrificial anode system is used.
- Published in Articles, Educational, Scientific
What is the cathodic protection?
Cathodic protection and its different types
Cathodic protection is a way to reduce corrosion by minimizing the potential difference between the anode and the cathode. This is achieved by creating a stream from an external source on the structure to be protected (such as a pipeline). When enough current is used, the whole structure will be at a potential. Therefore, there will be no anode and cathode sites. Cathodic protection is commonly used to protect numerous structures against corrosion, such as ships, sea vessels, submarine equipment, ports, pipelines, and tanks.
Types of cathodic protection systems
There are two main types of cathodic protection systems: galvanic system and impressed current system. Note that both types include an anode (from which current flows into the electrolyte), a continuous electrolyte from the anode to the protected structure, and an external metal connection.
• Galvanic system:
Galvanic cathodic protection system uses corrosion potentials for various metals. Without cathodic protection, there is one area of the structure with more negative potential than other areas and corrosion occurs. In this case, if an object with a higher negative potential (such as a magnesium anode) is protected near the structure (such as a pipeline) and a metal connection is made, the object becomes an anode and the whole structure becomes a cathode.
Therefore, the galvanic cathode protection system is called a sacrificial anode cathode protection system because the anode is sacrificed to protect the structure. Sacrificial anodes are usually made of metals such as magnesium, zinc and aluminum.
•Impressed current system
Impressed current cathodic protection system is a type of system that is usually applied in cases where there is a high flow requirements to protect against corrosion. The main difference between impressed current systems and sacrificial anodes is that the sacrificial anode system relies on the potential difference between the anode and the structure, while the impressed current system uses an external energy source to conduct the current. Impressed current anodes are usually high silicon cast iron anodes or mixed metal oxide anodes.
- Published in Articles, Educational, Scientific