Operating principle of cathodic protection of pipelines. Schematic diagram of tread protection for an underground gas pipeline. Features of cathodic protection schemes

No matter how popular plastic is, most highways laid in the ground (buried) are mounted from steel or cast iron samples. A significant disadvantage of such pipelines, despite all their undeniable advantages, is the susceptibility of the materials to corrosion. Regardless of the type (electronic/chemical, caused by stray currents or another factor), it significantly reduces the service life engineering communications or a separate part thereof.

Depending on local conditions and economic feasibility, several pipeline protection methods are implemented in practice. All of them are divided into 2 groups - active and passive. Cathodic protection belongs to the first. This material is devoted to its features, arrangement technology, and operating principle.

Scheme of cathodic protection of pipelines

Compound

• Industrial/voltage source.
• Current converter (AC/DC).
• Anode grounding conductor (single or combined).
• Circuit connecting elements (metal conductors).

Additionally

• Voltmeter.
• Control electrode (copper sulfate).

Operating principle

Connection

The role of the cathode in this scheme is played by the pipeline itself. It connects to the "-" rectifier. Accordingly, the anode is to its “+”.

Operating condition

The presence of an electrolytic environment (in in this case– soil) and an anode made of conductive material. It doesn't have to be metal.

Protection operating procedure

When voltage is applied to the circuit, a electric field, creating cathodic polarization in the pipeline section. Without going into the intricacies of the ongoing processes, it is enough to say that as a result, it is not the pipeline that is destroyed, but the anode, since it is formed precisely in the “+” voltage region. After a certain time, it is much easier and cheaper to replace a ground electrode than one or several pipes on the route.

Features of cathodic protection schemes

• Both fixed lines and mobile generators can be used as a power source.
• The maximum potential of the protective field for pipelines that do not have a special coating is not regulated. In other cases (for example, if the route elements have polymer insulation) it is calculated individually for each circuit.
• Depending on the specifics of the pipeline, anode grounding electrodes may differ in the method of placement (distributed, concentrated) and position relative to the ground level (extended, deep).
• The anode material is selected for a specific soil based on operation without replacement for at least 15 years. This period can be artificially increased if the ground electrode is placed in any environment. For example, into crushed coke.

Electrochemical corrosion protection consists of cathodic and drainage protection. Cathodic protection of pipelines is carried out by two main methods: the use of metal anodes-protectors (galvanic protector method) and the use of external sources direct current, the minus of which is connected to the pipe, and the plus to the anode grounding (electrical method).

Rice. 1. Operating principle of cathodic protection

Galvanic tread protection against corrosion

The most obvious way to implement electrochemical protection metal structure having direct contact with the electrolytic medium is a method galvanic protection, which is based on the fact that different metals in the electrolyte have different electrode potentials. Thus, if you form a galvanic couple from two metals and place them in an electrolyte, then the metal with more negative potential will become an anode-protector and will be destroyed, protecting the metal with a less negative potential. Protectors essentially serve as portable sources of electricity.

Magnesium, aluminum and zinc are used as the main materials for the manufacture of protectors. From a comparison of the properties of magnesium, aluminum and zinc, it is clear that of the elements under consideration, magnesium has the greatest electromotive force. At the same time, one of the most important practical characteristics of protectors is the coefficient useful action, showing the proportion of the tread mass used to obtain useful electrical energy in the chain. Efficiency protectors made from magnesium and magnesium alloys rarely exceed 50% in, in contrast to protectors based on Zn and Al with efficiency. 90% or more.

Rice. 2. Examples of magnesium protectors

Typically, protector installations are used for cathodic protection of pipelines that do not have electrical contacts with adjacent extended communications, individual sections of pipelines, as well as tanks, steel protective covers(cartridges), underground tanks and containers, steel supports and piles, and other concentrated objects.

At the same time, tread installations are very sensitive to errors in their placement and configuration. Incorrect choice or the placement of tread units leads to a sharp decrease in their effectiveness.

Cathodic corrosion protection

The most common method of electrochemical protection against corrosion of underground metal structures is cathodic protection, carried out by cathodic polarization of the protected metal surface. In practice, this is realized by connecting the protected pipeline to the negative pole of an external direct current source, called a cathodic protection station. The positive pole of the source is connected by a cable to an external additional electrode made of metal, graphite or conductive rubber. This external electrode is placed in the same corrosive environment as the object being protected, in the case of underground field pipelines, in the soil. Thus, a closed electrical circuit is formed: additional external electrode - soil electrolyte - pipeline - cathode cable - DC source - anode cable. As part of this electrical circuit the pipeline is the cathode, and an additional external electrode connected to the positive pole of the DC source becomes the anode. This electrode is called anode grounding. The negatively charged pole of the current source connected to the pipeline, in the presence of external anodic grounding, cathodically polarizes the pipeline, while the potential of the anode and cathode sections is practically equalized.

Thus, the cathodic protection system consists of a protected structure, a direct current source (cathodic protection station), anode grounding, connecting anode and cathode lines, the surrounding electrically conductive medium (soil), as well as elements of the monitoring system - control and measuring points.

Drainage corrosion protection

Drainage protection of pipelines from corrosion by stray currents is carried out by directed drainage of these currents to a source or to the ground. Installation of drainage protection can be of several types: earthen, direct, polarized and reinforced drainage.

Rice. 3. Drainage protection station

Earth drainage is carried out by grounding pipelines with additional electrodes in places of their anode zones, direct drainage is carried out by creating an electrical jumper between the pipeline and the negative pole of a source of stray currents, for example, an electrified rail network railway. Polarized drainage, unlike direct drainage, has only one-way conductivity, so when a positive potential appears on the rails, the drainage is automatically turned off. In enhanced drainage, a current converter is additionally included in the circuit, allowing the drainage current to be increased.

Cathodic protection ta is the most common type of electrochemical protection. It is used in cases where the metal is not prone to passivation, that is, it has an extended region of active dissolution, a narrow passive region, high values ​​of passivation current (i p) and passivation potential (p p).

Cathodic polarization can be carried out by connecting the protected structure to the negative pole of an external current source. Cathodic protection is carried out external current. .

The cathodic protection diagram is shown in Fig. 4. The negative pole of the external current source 4 is connected to the protected metal structure 1, and the positive pole is connected to the auxiliary electrode 2, which works as an anode. During the protection process, the anode is actively destroyed and is subject to periodic restoration.

Cast iron, steel, coal, graphite, and metal scrap (old pipes, rails, etc.) are used as anode materials. The sources of external current for cathodic protection are cathodic protection stations, mandatory elements which are: a converter (rectifier) ​​that generates current; current supply to the protected structure, reference electrode, anode grounding conductors, anode cable.

Cathodic protection of factory equipment (refrigerators, heat exchangers, capacitors, etc.) exposed to an aggressive environment is carried out by connecting an external current source to the negative pole and immersing the anode in this environment.

Cathodic protection with external current is impractical in conditions of atmospheric corrosion, in a vaporous environment, in organic solvents, since in this case the corrosive environment does not have sufficient electrical conductivity.

Tread protection. Sacrificial protection is a type of cathodic protection. The pipeline protection scheme is shown in Fig. 5. A more electronegative metal, protector 3, is attached to the protected structure 2, which, dissolving in the environment, protects the main structure from destruction.

Once the protector has completely dissolved or lost contact with the structure being protected, it is extremely important to replace the protector.

Figure 5 Pipeline sacrificial protection scheme

The protector works effectively if the transition resistance between it and the environment is low. During operation, a protector, for example zinc, can become covered with a layer of insoluble corrosion products, which isolate it from environment and sharply increase the contact resistance. To combat this, the protector is placed in filler 4 - a mixture of salts, which creates a certain environment around it that facilitates the dissolution of corrosion products and increases the efficiency and stability of the protector in the ground.

In comparison with cathodic protection by external current, it is advisable to use sacrificial protection in cases where obtaining energy from the outside is difficult or if the construction of special power lines is not economically profitable.

Today, tread protection is used to combat corrosion of metal structures in sea and river water, soil and other neutral environments. The use of tread protection in acidic environments is limited high speed self-dissolution of the protector.

Metals can be used as protectors: Al, Fe, Mg, Zn. In this case, use pure metals as protectors is not always advisable.. To give the protectors the required operational properties Alloying elements are introduced into their composition.

Cathodic protection - concept and types. Classification and features of the Cathodic Protection category 2017, 2018.

The method of electrochemical protection (ECP) against corrosion has been used by engineers for many years to extend the service life of various metal devices and structures. However, it so happened that the most widely known technical solutions for the use of ECP for anti-corrosion protection of large metal-intensive structures and structures, such as underground pipelines in the oil and gas industry and in the housing and communal services sector or large steel tanks, although the operating principle of ECP is universal and can be successfully used in practice wherever there is contact between metal and aggressive electrolyte. In this article we would like to give, of course, very short review other possibilities for using electrochemical protection around us - in the industrial, public and even private spheres of life of a modern person.

Electrochemical protection is based on the control of electrochemical corrosion currents, which always arise when any metal structure comes into contact with an electrolyte. With the help of ECP, the anodic deteriorating zone is transferred from the protected object either to a special anodic grounding (with cathodic protection), or to a separate product made of a more active metal (with sacrificial protection). You can read more about the physicochemical principles of cathodic and sacrificial protection against corrosion . The main thing that should be understood when deciding on the use of ECP is that mandatory contact between the protected object/system of objects and the external anode (anode grounding or protector) is required, both through a conductor of the first type (metal cable or direct metal contact) and through a conductor of the second type (electrolyte). The electrical circuit “structure - cable - anode - electrolyte” must be closed, otherwise a protective current simply will not arise in the system. A simple example is a pipeline or pile coming out of the ground to the surface. The ECP will operate only in the underground part. However, there are several examples where, at first glance, this rule does not work. For example, constant contact between the structure and the electrolyte is not ensured in zones of variable wetting, such as the tidal zone of piles on sea piers and moorings, the wave wetting zone of similar structures in freshwater bodies, etc. In these cases, it is necessary to use rather cunning ECP schemes that work only when the corrosion-hazardous zones are moistened. But how, for example, can you organize ECP against atmospheric corrosion of a metal structure in a humid sea or industrial air? It turns out that this is possible! But we will start with simpler cases.

A simple and obvious example of an object subject to electrochemical corrosion, which can be slowed down using ECP, is any metal structure buried in the ground or standing on the ground: a pile, a tank, a pipeline for any purpose. Of course, there is no need to use ECP everywhere, however, if the object is located in highly corrosive soil ( high humidity or salinity - obvious signs of such soil!), or this is an industrially significant and poorly repairable object - ECP will clearly not be superfluous. The design of such an ECP system is not very complicated. For example, if you need to protect pile foundation, then a cathodic protection station is sufficient low power(a battery may be enough) and several correctly located point anodes, or several small sections of an extended anode. Just remember that if the piles are made of pipes, they can also corrode from the inside, where the ECP will not work. A single, completely buried tank is also perfectly protected by point anodes along the perimeter of the structure, and the bottom of a tank standing on the ground is protected by a single point anode or a curved section of an extended anode. If it is possible to change the anodic grounding and the soil resistance is low, then instead of point anodes you can install protector installations, period efficient work which is usually 5-7 years.

Now let's move on to a not very common, but very productive method of electrochemical protection against corrosion of the internal surface of pipelines and tanks (vessels) of any capacity and purpose that have contact with an aggressive aqueous electrolyte (industrial wastewater or simply water with a high content of mineral salts and oxygen). In this case, the use of ECP allows you to extend the maintenance-free operation of the facility several times. A simpler case is the internal ECP of a tank, when during internal space Protectors or anode grounding are placed in the tank. The efficiency of ECP will increase significantly if inner surface the tank will be additionally protected insulating coating with good dielectric properties. More complex technical solution used for internal electrochemical protection of pipelines. In this case, it is most effective to introduce an extended flexible anode (PHA) made of conductive rubber into the internal cavity of the pipeline. The length of such an anode is usually equal to the length of the protected section of the pipeline. A certain technical difficulty arises from installing such an anode in an already operating pipeline, although this is also feasible in practice. Sometimes, to protect areas of limited length (5-30 m), it is sufficient to install a single point anode or protector into the internal cavity.

Internal ECP of a pipeline using protectors

Such internal electrochemical protection systems are extremely effective, even when nothing else helps in principle. For example, the service life of pipelines and various treatment plants - very corrosive Wastewater industrial enterprises- extended due to internal ECP by 5-20 times!

The next interesting case of using ECP systems is mooring structures, foundations of oil and gas platforms, bridge supports or any other metal structures in sea ​​water. By the way, the waters of some fresh water bodies in our “ecologically clean” country, especially near large cities and industrial enterprises, are close to sea water in terms of corrosiveness, so everything stated below applies to them with minor reservations.

Corrosion of a pile in the zone of variable wetting and splashing

So, metal structures in sea water are subject to active electrochemical corrosion, which cannot be stopped regular painting. According to the mechanism of the corrosion process at such objects, three main zones are usually distinguished:

• zone of variable wetting and splashing;
• zone total immersion in water;
• zone where the pile is immersed in the ground.

The greatest difficulty in implementing electrochemical protection systems is the zone of variable wetting, where there is no constant electrical circuit “structure - electrolyte - anode”. For these zones, anodic grounding (protectors) of a mesh or bracelet form are required, providing separate protection for locally moistened areas of the metal structure. In the most difficult cases it makes sense to provide forced constant moistening of the variable wetting zone of the structure, for permanent job ECP funds.

Electrochemical protection of the zone of complete wetting of metal piles in aquatic environment can be implemented depending on the design different ways, among which it makes sense to highlight the following:

• placement of several suspended point anodes, each of which protects the nearest piles surrounding it;
• in deeper areas it is possible to use long flexible anodes that are attached to cables, the ends of which are fixed to a metal structure and the bottom of the reservoir;
• if it is not possible to supply electricity to the structure being protected, then an acceptable method of electrochemical protection would be the use of large deep protectors with long design service life.

Magnesium protector for electrochemical protection of offshore structures

Now let's return to the announced ECP against atmospheric corrosion of a metal structure in humid sea or industrial air. In its mechanism, this case is somewhat reminiscent of corrosion in the zone of variable wetting - also a large number of locally moist areas, only even smaller ones. In this case, the only way to ensure electrochemical protection of the entire surface of the protected product is to provide your own local ECP system at each wetted area. This goal is achieved by applying a special coating to the surface of the product containing metal particles that have protective properties in relation to steel. Typically this metal is zinc. Thus, each surface area is provided with its own small installation of tread protection, which is activated when moistened.

In this article, we talked about only a few main cases of application of electrochemical protection of various metal structures. In fact, many more such examples can be given - ECP can be used everywhere: car bodies, marine hulls, household water heaters, offshore pipelines, etc. Sometimes you even have to provide electrochemical protection reinforced concrete structures, but this is such a voluminous topic that it requires a separate review. Therefore, we can safely say that while our age of metal has not been replaced by the age of composite materials, it is electrochemical protection will be one of the most important and sought after technologies by humanity.

Protection of pipelines from corrosion can be carried out using a variety of technologies, the most effective of which is the electrochemical method, which includes cathodic protection. Often, anti-corrosion cathodic protection is used comprehensively, together with treatment steel structure insulating compounds.

This article examines the electrochemical protection of pipelines and studies its cathodic subtype in particular detail. You will learn what the essence of this method is, when it can be used and what equipment is used for cathodic protection of metals.

Contents of the article

Types of cathodic protection

Cathodic corrosion protection of steel structures was invented in the 1820s. For the first time, the method was used in shipbuilding - the copper hull of the ship was sheathed with protective anode protectors, which significantly reduced the rate of copper corrosion. The technique was adopted and began to actively develop, making it one of the most effective methods of anti-corrosion protection today.

Cathodic protection of metals, according to technology, is classified into two types:

• method No. 1 - an external current source is connected to the protected structure, in the presence of which the metal product itself acts as a cathode, while third-party inert electrodes act as anodes.
• method No. 2 – “ galvanic technology“: the protected structure is in contact with a tread plate made of a metal having a higher electronegative potential (such metals include zinc, aluminum, magnesium and their alloys). The function of the anode in this method both metals perform, while the electrochemical dissolution of the metal of the tread plate ensures flow through the protected structure the required minimum cathode current. Over time, the tread plate is completely destroyed.

Method No. 1 is the most common. This is an easy-to-implement anti-corrosion technology that effectively copes with many types of metal corrosion:

• intercrystalline corrosion of stainless steel;
• pitting corrosion;
• cracking of brass from increased stress;
• corrosion under the influence of stray currents.

Unlike the first method, suitable for protecting large structures (used for underground and above-ground pipelines), galvanic electrochemical protection is intended for use with small-sized products.

The galvanic method is widespread in the USA; in Russia it is practically not used, since the technology for constructing pipelines in our country does not provide for the treatment of pipelines with a special insulating coating, which is a prerequisite for galvanic electrical protection.

Note that without the corrosion of steel increases significantly under the influence of groundwater, which is especially typical for spring period and autumn. In winter, after water freezes, corrosion from moisture slows down significantly.

The essence of technology

Cathodic anti-corrosion protection is carried out through the use of direct current, which is supplied to the protected structure from an external source (most often, rectifiers that convert alternating current to constant) and makes its potential negative.

The object itself, connected to direct current, is a “minus” - a cathode, while the anode grounding connected to it is a “plus”. Key condition The effectiveness of cathodic protection is the presence of a well-conducting electrolytic medium, which, when protecting underground pipelines the ground protrudes while electronic contact is achieved through the use of metal materials with high conductivity.

In the process of implementing the technology, the required current potential difference is constantly maintained between the electrolytic medium (soil) and the object, the value of which is determined using a high-resistance voltmeter.

Features of cathodic protection of pipelines

Corrosion is the main cause of depressurization of all types of pipelines. Due to damage to the metal by rust, ruptures, cavities and cracks form on it, leading to the destruction of the steel structure. This problem is especially critical for underground pipelines that are constantly in constant contact with groundwater.

Cathodic protection of gas pipelines against corrosion is carried out using one of the above methods (using an external rectifier or galvanic method). The technology, in this case, makes it possible to reduce the rate of oxidation and dissolution of the metal from which the pipeline is made, which is achieved by shifting its natural corrosion potential to the negative side.

Through practical tests, it was found that the potential of cathodic polarization of metals, at which all corrosion processes slow down, is equal to -0.85 V, whereas for underground pipelines in natural mode it is -0.55 V.

For anti-corrosion protection to be effective, it is necessary to reduce the cathodic potential of the metal from which the pipeline is made by -0.3 V using direct current. In this case, the rate of corrosion of steel does not exceed 10 micrometers over the course of a year.

Cathodic protection is the most effective method protection of underground pipelines from stray currents. The concept of stray currents means electric charge, which enters the ground as a result of the operation of grounding points of power lines, lightning rods, or the movement of trains along railway lines. Exact time and the location of the appearance of stray currents is impossible to find out.

The corrosive effect of stray currents on metal occurs if metal structure has a positive potential relative to the electrolyte (for underground pipelines the electrolyte is the soil). Cathodic protection makes the metal potential of underground pipelines negative, which eliminates the risk of their oxidation under the influence of stray currents.

The technology of using an external current source for cathodic protection of underground pipelines is preferable. Its advantages are unlimited energy resources that can overcome resistivity soil.

Anti-corrosion protection is used as a current source air lines power transmission lines with a capacity of 6 and 10 kW, but if there are no power lines on the territory, mobile generators running on gas and diesel fuel can be used.

Detailed overview of cathodic corrosion protection technology (video)

Cathodic protection equipment

For anti-corrosion protection of underground pipelines it is used special equipment – cathodic protection stations(SKZ), consisting of the following units:

• grounding (anode);
• DC source;
• control, monitoring and measurement point;
• connecting cables and wires.

One SCP connected to the power grid or to an autonomous generator can perform cathodic protection of several nearby underground pipelines. Current adjustment can be done manually (by replacing the winding on the transformer) or automatically (if the system is equipped with thyristors).

Among cathodic protection stations used in domestic industry, the most technologically advanced installation is considered to be Minerva-3000 (designed by engineers from France at the request of Gazprom). The power of this VS is sufficient for effective protection 30 km of underground pipeline.

The advantages of the installation include:

• increased power;
• overload recovery function (update occurs in 15 seconds);
• availability of digital control systems to control operating conditions;
• complete sealing of critical components;
• possibility of connecting equipment for remote control.

ASKG-TM units are also widely in demand in domestic construction; in comparison with Minerva-3000, they have a reduced power (1-5 kW), however, in the stock configuration, the system is equipped with a telemetry complex, which automatically controls the operation of the SCP and has the ability to be remotely controlled .

Cathodic protection stations Minerva-3000 and ASKG-TM require power from a 220 V power supply. Remote control equipment is performed using built-in GPRS modules. SKZ have quite larger dimensions - 50*40*90 cm and weight - 50 kg. The minimum service life of the devices is 20 years.