Cathodic protection of pipelines against corrosion, diagram, principle of operation and video. Electrochemical protection

Corrosion is a chemical and electrochemical reaction of a metal with environment causing damage to it. It flows at different speeds, which can be reduced. From a practical point of view, the anticorrosive cathodic protection of metal structures in contact with the ground, with water and with the transported media is of interest. The outer surfaces of pipes are especially damaged from the influence of soil and stray currents.

Internally, corrosion depends on the properties of the medium. If it is a gas, it must be thoroughly cleaned of moisture and corrosive substances: hydrogen sulfide, oxygen, etc.

Principle of operation

Process objects electro chemical corrosion are the medium, the metal and the interfaces between them. The medium, which is usually wet soil or water, has good electrical conductivity. An electrochemical reaction takes place at the interface between it and the metal structure. If the current is positive (anode electrode), iron ions pass into the surrounding solution, which leads to a loss of metal mass. The reaction is corrosive. With a negative current (cathode electrode), these losses are absent, since electrons pass into the solution. The method is used in electroplating for the application of non-ferrous metal coatings to steel.

Cathodic corrosion protection is achieved when a negative potential is applied to the iron object.

To do this, an anode electrode is placed in the ground and a positive potential from a power source is connected to it. The minus is fed to the protected object. Cathodic-anodic protection leads to active destruction from corrosion only of the anode electrode. Therefore, it should be changed periodically.

Negative effect of electrochemical corrosion

Corrosion of structures can occur from the action of stray currents coming from other systems. They are useful for targets, but cause significant damage to nearby structures. Stray currents can spread from the rails of electrified vehicles. They go towards the substation and end up on the pipelines. When leaving them, anode areas are formed, causing intense corrosion. For protection, electrical drainage is used - a special drainage of currents from the pipeline to their source. Here it is also possible For this, it is necessary to know the magnitude of the stray currents, which is measured with special devices.

Based on the results of electrical measurements, the method of protecting the gas pipeline is selected. A universal remedy is the passive method of contact with the ground using insulating coatings. Cathodic protection of a gas pipeline is an active method.

Pipeline protection

Structures in the ground are protected from corrosion if you connect the minus of a direct current source to them, and plus to the anode electrodes buried nearby in the ground. The current will flow to the structure, protecting it from corrosion. Thus, cathodic protection of pipelines, tanks or pipelines located in the ground is carried out.

The anode electrode will deteriorate and should be replaced periodically. For a tank filled with water, the electrodes are placed inside. In this case, the liquid will be the electrolyte, through which the current will go from the anodes to the surface of the container. The electrodes are well controlled and easy to replace. It is more difficult to do this in the ground.

Power supply

Near oil and gas pipelines, in heating and water supply networks, for which cathodic protection is required, stations are installed from which voltage is supplied to objects. If they are located outdoors, their degree of protection must be at least IP34. Any is suitable for dry rooms.

Cathodic protection stations for gas pipelines and other large structures have a capacity of 1 to 10 kW.

Their energy parameters primarily depend on the following factors:

resistance between soil and anode;
electrical conductivity of the soil;
length of the protective zone;
insulating effect of the coating.

Traditionally, a cathodic protection converter is a transformer installation. Now it is being replaced by an inverter one, which has smaller dimensions, better current stability and greater efficiency. In important areas, controllers are installed that have the functions of regulating current and voltage, equalizing protective potentials, etc.

The equipment is on the market in different options... For specific needs, a providing Better conditions exploitation.

Power source parameters

To protect against corrosion for iron, the protective potential is 0.44 V. In practice, it should be higher due to the influence of inclusions and the state of the metal surface. The maximum value is 1 V. In the presence of coatings on the metal, the current between the electrodes is 0.05 mA / m 2. If the insulation is broken, it rises to 10 mA / m 2.

Cathodic protection is effective in combination with other methods, since less energy is consumed. If there is a paint-and-varnish coating on the surface of the structure, only the places where it is damaged are electrochemically protected.

Features of cathodic protection

Power sources are stations or mobile generators.
The location of the anode earthing switches depends on the specifics of the pipelines. The placement method can be distributed or concentrated, as well as located at different depths.
The anode material is selected with a low solubility so that it lasts 15 years.
The protective field potential for each pipeline is calculated. It is not regulated if the structures lack protective coatings.

Gazprom's standard requirements for cathodic protection

Action during the entire service life of protective equipment.
Protection against atmospheric overvoltage.
Placing the station in block-boxes or in a free-standing vandal-proof design.
Anode grounding is selected in areas with a minimum electrical resistance of the soil.
The characteristics of the transducer are selected taking into account the aging of the protective coating of the pipeline.

Protective protection

The method is a type of cathodic protection with the connection of electrodes from a more electronegative metal through an electrically conductive medium. The difference lies in the absence of an energy source. The protector takes on corrosion by dissolving in an electrically conductive environment.

After a few years, the anode should be replaced as it depletes.

The effect of the anode increases with a decrease in its contact resistance with the medium. Over time, it can become covered with a corrosive layer. This leads to a breakdown in electrical contact. If the anode is placed in a salt mixture that dissolves the corrosion products, the efficiency is increased.

The tread effect is limited. The radius of action is determined by the electrical resistance of the medium and the potential difference between

Protective protection is used in the absence of energy sources or when their use is economically impractical. It is also disadvantageous when used in acidic environments due to the high dissolution rate of the anodes. The protectors are installed in water, in the ground or in a neutral environment. Pure metal anodes are usually not made. The dissolution of zinc is uneven, magnesium corrodes too quickly, and a strong oxide film forms on the aluminum.

Protector materials

In order for the protectors to have the required performance properties, they are made from alloys with the following alloying additions.

Zn + 0.025-0.15% Cd + 0.1-0.5% Al - protection of equipment in sea water.
Al + 8% Zn + 5% Mg + Cd, In, Gl, Hg, Tl, Mn, Si (fractions of a percent) - operation of structures in running seawater.
Mg + 5-7% Al + 2-5% Zn - protection of small structures in soil or water with a low concentration of salts.

Improper use of some types of protectors leads to negative consequences. Magnesium anodes can cause equipment cracking due to the development of hydrogen embrittlement.

Joint protective cathodic protection with anticorrosive coatings increases its effectiveness.

The distribution of the protective current is improved and significantly fewer anodes are required. A single magnesium anode protects a bitumen-coated pipeline over a length of 8 km, and an uncoated one only 30 m.

Corrosion protection of car bodies

If the coating is broken, the thickness of the car body can decrease in 5 years to 1 mm, i.e., it can corrode through and through. The restoration of the protective layer is important, but besides it there is a way to completely stop the corrosion process using cathodic protection. If you turn the body into a cathode, metal corrosion stops. Anodes can be any conductive surfaces located nearby: metal plates, ground loop, garage body, wet road surface. In this case, the effectiveness of protection increases with an increase in the area of the anodes. If the anode is a road surface, a metallized rubber "tail" is used for contact with it. It is placed against the wheels for better splashing. The "tail" is insulated from the body.

The plus of the storage battery is connected to the anode through a 1 kOhm resistor and an LED connected in series with it. When the circuit is closed through the anode, when the minus is connected to the body, in normal mode the LED barely shines. If it burns brightly, then a short circuit has occurred in the circuit. The cause must be found and eliminated.

For protection, a fuse must be installed in series in the circuit.

When the car is in the garage, it is connected to the ground anode. During the movement, the connection occurs through the "tail".

Conclusion

Cathodic protection is a way to increase the operational reliability of underground pipelines and other structures. In this case, one should take into account its negative impact on neighboring pipelines from the influence of stray currents.

A.I. Heifetz, Head of Electrochemical Protection Service,
JSC "Heating network of St. Petersburg", St. Petersburg

Introduction

Protection of pipelines of heating networks from corrosion is a very important task, the solution of which largely depends on the reliability of the entire system. district heating... In St. Petersburg, underground heating networks prevail, which operate in corrosive conditions caused by both a dense network of long underground utilities and developed electrified transport, as well as saturation of soils and grounds with moisture and chemical reagents. There are two main methods of protecting metals from corrosion: passive is the application of insulating coatings to their surface and active is the use of electrochemical protection means.

A bit of theory

Metal structures operating in various environments (atmosphere, water, soil) are exposed to the destructive effects of this environment. The destruction of a metal due to its interaction with the external environment is called corrosion. The essence of the corrosion process is the removal of atoms from the metal lattice, which can occur in two ways, and therefore distinguish between just chemical and electrochemical corrosion.

Corrosion is chemical if, after breaking a metal bond, the metal atoms are directly bonded by a chemical bond with those atoms or groups of atoms that are part of the oxidizing agents that take away the valence electrons of the metal. The process takes place without the participation of free electrons and is not accompanied by the appearance electric current... An example is the formation of scale during the interaction of iron-based materials with high temperature with oxygen.

Corrosion is electrochemical if a positively charged metal ion exits the metal lattice, i.e. the cation enters into a bond not with the oxidizing agent, but with other components of the corrosive medium, while the electrons released during the formation of the cation are transferred to the oxidizing agent. In electrochemical corrosion, the removal of atoms from the metal lattice is carried out as a result of not one, as in chemical corrosion, but two independent, but coupled with each other, electrochemical processes: anodic (the transition of "trapped" metal cations into solution) and cathodic (binding of released electrons by an oxidizer). Oxidizing agents are hydrogen ions, which are found wherever water is present, and oxygen molecules. Electrochemical corrosion is accompanied by the appearance of an electric current.

Heat pipelines are extended objects and their various sections are not in equal conditions from the point of view of the development of corrosion processes. Soils and grounds absorb atmospheric precipitation and melt water in different ways, and have different air permeability. The electrical resistivity of soils is also different; it is its value (the lower, the more dangerous) characterizes the corrosive aggressiveness of the environment. As a result, sections are formed along the surface of the pipelines where either anodic or cathodic reactions are predominantly carried out. The electrical conductivity of the metal is very high; electrons are almost instantly redistributed from the places where the anodic reaction takes place to the places where the cathodic one proceeds (Fig. 1). In fact, there are similarities of galvanic cells, batteries, in which the role of the electrolyte is played by the soil, and the external circuit is an underground metal structure. The anode zones are the positive electrode ("+") and the cathode zones are the negative electrode ("-"). When an electric current flows in the anode zones, atoms are continuously released from the metal lattice into external environment, i.e. dissolution of metal.

A particular danger to pipelines of heating networks is posed by stray currents that arise as a result of leakage from transport electrical circuits of a part of the current into the soil or aqueous solutions where they end up on metal structures. Anodic dissolution of the metal occurs in the places where the current leaves these structures again into the soil or water. Such zones are especially common in areas of ground electric transport. Stray current corrosion is sometimes referred to as electrical corrosion. Such currents can be as high as several amperes. To represent: a current with a force of 1 A, in accordance with the first Faraday's law, causes the dissolution of iron in an amount of 9.1 kg during the year. If the current is concentrated on a section of 1 m 2, then this corresponds to a decrease in the pipe wall thickness by 1.17 mm per year, i.e. in 6 years it would have decreased by 7 mm.

The principle of operation of electrochemical protection (ECP) of the outer surface of a metal against corrosion is based on the fact that by shifting the potential of a metal by passing an external electric current, it is possible to change the rate of its corrosion. The relationship between potential and corrosion rate is nonlinear and ambiguous.

ECP, based on the imposition of a cathodic current, is called cathodic protection. In a production environment, it is implemented in two versions.

1. In the first variant, the required potential shift is provided by connecting the protected structure to an external voltage source as a cathode, and auxiliary electrodes are used as an anode (Fig. 2).

The source is an adjustable rectifier, which converts the power frequency voltage into a constant one, and the anode ground electrodes are combined into a circuit, the composition and location of the electrodes of which are determined by calculation. During operation, the mass of the electrodes of the anode ground loop decreases monotonically.

Cathodic polarization uninsulated metal structure to the value of the minimum protective potential requires significant currents, therefore, usually cathodic protection is used in conjunction with insulating coatings applied to the outer surface of the protected structure. The surface coating reduces the required current by several orders of magnitude. With cathodic protection, it is necessary to control the value of the maximum potential, since too high a value can lead to peeling of the insulating coating from the pipeline wall. Regulatory documents (Typical instructions for the protection of pipelines of heating networks from external corrosion RD 153-34.0-20.518-2003) establish that the minimum protective potential for heating networks is 1.1 V, and the maximum 2.5 V is in the negative direction with respect to non-polarizing copper sulfate reference electrode. Such values must be ensured throughout the entire length of the protected area, and this is achieved all the more true, the better the metal is insulated from the ground.

2. The second option for cathodic protection is galvanic (or protective) protection (Fig. 3). Its principle of operation is based on the fact that different metals are characterized by different values of standard electrode potentials. Cathodic polarization of the protected structure is achieved through its contact with a more electronegative metal. The latter acts as an anode, and its electrochemical dissolution ensures the flow of the cathodic current through the protected metal. The anode itself, made of magnesium, zinc, aluminum and their alloys, is gradually destroyed. The advantage of protective protection is that it does not require an external voltage source, but this type of protection can only be used on relatively short pipelines (up to 60 m), as well as on steel cases.

3. To protect pipelines of heating networks from external corrosion under the influence of stray currents, electrical drainage (drainage) is used - a connection by a metal conductor of the area from which these currents flow down to the rail of tram or railway tracks. With a large distance to the rail, when such drainage is difficult to implement, an additional cast iron anode is used, which is buried in the ground and connected to the protected area.

In places where the electrolytic effect of stray currents is added to the currents of galvanic couples, sharp increase the rate of corrosion processes. In such cases, reinforced drainage installations are used (Fig. 4), which allow not only to divert stray currents from pipelines, but also to provide them with the necessary amount of protective potential. Reinforced drainage is a conventional cathode station connected with the negative pole to the protected structure, and the positive pole not to the anode grounding, but to the rails of the electrified transport.

4. A strong corrosive effect on pipelines of heating networks can be exerted by ECP installations of owners of adjacent underground utilities, for example, gas pipelines (Fig. 5a). If the pipelines are in the zone of action of the cathode current of the "foreign" installation, then the destruction in the places where this current exits from the steel pipe into the ground will be the same as under the action of stray currents. For protection, it is necessary to connect the pipelines of heating networks with the negative pole of the voltage source (Fig. 5b).

It is possible to shift the potential of a metal to protect it from corrosion not only towards negative, but also positive values. In this case, some metals pass into a passive state, and the dissolution current of the metal drops tenfold. Such protection is called anodic protection, its advantage is that low currents are required to maintain the passive state of the metal. However, if the electrolyte contains chlorine and sulfur ions, metal corrosion can sharply increase and the anode-polarized equipment itself can fail. Anode protection for heating networks is not applied.

ECP in JSC "Heating Grid of St. Petersburg" is operated and developed as a system, i.e. a set of interrelated components: stationary technical means, instrumental control and information database.

In accordance with the schedules, the specialists of the ECP service routinely carry out corrosion measurements according to the established methodology at all sections of the main and distribution networks at the points of access to underground pipelines(thermal cameras). After processing the measurement results, anode and cathodic zones on pipelines, protection zones, and areas of hazardous effects of stray currents are determined. In addition, corrosion measurements are carried out during planned pits and during the elimination of defects in heating networks, where they are supplemented by the result of chemical analysis of the soil. The measurement results are systematized and archived, they are valuable information for both correct organization operation of thermal mechanical equipment, and for planning the construction of additional ECP facilities.

More detailed and thorough corrosion surveys of the heating mains are carried out by a specialized contractor. These surveys are carried out in corrosive areas, usually after reconstruction (re-laying) of heating networks, because the use of modern types of insulation, structures and technologies provides a better galvanic isolation of metal from concrete and from the ground than before. This means, among other things, a possible change in the boundaries of the anodic and cathodic zones, areas of influence of stray currents. The survey results are presented in the form of reports containing information on changes in the values of electrode potentials in different sections of the surface of pipelines under various operating modes (Fig. 6), not only for our own, but also for third-party ECP facilities. Mathematical modeling methods (Fig. 7) are used to calculate the type, quantity and location of the necessary additional ECP tools for further design.

At present, OJSC “Heating Grid of St. Petersburg” owns 432 ECP units, of which: cathodic protection units - 204 pcs. (including cathodic protection installations belonging to the category of joint protection against external corrosion of pipelines of heating networks and gas pipelines laid alongside - 20 pcs.); reinforced drainage installations - 8 pcs.; tread protection installations - 220 pcs. OJSC "Antikor" is engaged in technical maintenance of installations of cathodic joint protection.

In accordance with the requirements of regulatory documents (Protection against corrosion. Design of electrochemical protection of underground structures. STO Gazprom 2-3.5-047-2006) ECP installations should not have a negative impact on neighboring communications. OJSC "Anticor", which is engaged in electrochemical protection of gas pipelines in St. Petersburg, during the reconstruction and new construction of its installations, promptly notifies OJSC "Heating Grid of St. Petersburg" about the technical feasibility of connecting sections of heating networks to the ECP of gas pipelines, if it is provided for by the project.

During the operation of all, except for drainage, ECP installations, the mass of their grounded electrodes is continuously lost, because this constitutes the physical essence of electrochemical protection. Inevitably, the moment of "death" of the anode ground loop or protector comes. It is possible and necessary to ensure the specified period of operation between major overhauls of ECP units with the correct calculation.

the required number and location of elements, the choice of quality materials, strict adherence to the installation technology. Cases of electrode failure due to local point damage are possible. Since 2010, during reconstruction and new construction, we have been using ElZhK-1500 ferrosilide anode earthing switches with contact unit protection instead of the previous EGT-1450. Over the course of a series recent years in EKhZ installations, only automatic converters of the UKZTA and PKZ-AR type are used (Fig. 8), which allow to continuously maintain the set values of the anode current or protective potential on the pipeline.

The practice of equipping ECP installations with telemetric recorders has acquired particular importance (Fig. 9). These devices, made in the form of built-in units, continuously remotely transmit information about the values of electrical quantities changing over time to a dedicated computer (Fig. 10). Archives are created to analyze the operation of ECP installations. In addition, the telemetry system has a function of signaling about unauthorized access of unauthorized persons to the installations.

It is worth noting that before the start of construction and installation work, the contractor notifies the customer, the design organization, the organization that supervises the construction, and the organization to service which the protective installations under construction will be transferred about the start date of the work.

Our company has been engaged in electrochemical protection of heating networks from external corrosion since 1960, i.e. over 50 years. Over the years, ECP specialists were part of various production divisions, and after the formation of St. Petersburg Heating Grid in 2010, a separate ECP service was created. Today it includes 13 people who solve technical and organizational problems.

The technical tasks include: daily detours of two teams of electricians along the specified routes of the ECP units with maintenance. At the same time, it is monitored whether they are not conducted by third-party organizations without proper registration excavation in the area of our installations.

Maintenance of ECP units includes:

■ inspection of all elements of the installation in order to identify external defects, check the density of contacts, serviceability of installation, absence mechanical damage individual elements, the absence of burns and traces of overheating, the absence of excavations along the route of drainage cables and anode groundings;

■ checking the functionality of the fuses (if any);

■ cleaning the housing of the drainage and cathode converter, the joint protection unit outside and inside;

■ measurement of current and voltage at the output of the converter or between galvanic anodes (protectors) and pipes;

■ measuring the potential of the pipeline at the point of connection of the installation;

■ making an entry in the installation log about the results of the work performed;

■ measuring potentials at permanently fixed measuring points.

Periodically held Maintenance and monitoring the efficiency of ECP equipment. ECP service specialists conduct technical supervision of production overhaul, reconstruction and capital construction of ECP units by contractors. The compliance of the construction and installation works with the project is monitored.

Current repairs include:

■ measurement of insulation resistance of supply cables;

■ repair of power lines;

■ repair of the rectifier unit;

■ repair of a drainage cable.

Monitoring the efficiency of the ECP installation consists in measuring the protective potentials at measuring points throughout the entire protection zone of this ECP installation. Control of the efficiency of ECP of pipelines of heating networks is carried out at least 2 times a year, as well as when the operating parameters of ECP plants change and when corrosive conditions change, associated with:

■ laying of new underground structures;

■ in connection with the repair work on heating networks;

■ installation of ECP on adjacent underground utilities.

ECP service specialists conduct technical supervision over the overhaul, reconstruction and capital construction of ECP units by contractors. The compliance of the construction and installation works with the project is monitored.

The organizational tasks include, first of all, obtaining a permit for power supply of ECP stations from the networks of JSC Lenenergo. This is a multi-pass algorithm, accompanied by a large amount of documentation. In addition to power supply, the ECP service is engaged in the preparation of targeted programs for new construction and repair, verification and approval of projects, preparation of technical specifications.

ECP installations against external corrosion of metal structures have been used for 100 years. The physicochemical principle of their work remains unchanged, but in order to increase the resource of their work, reduce capital and operating costs, it is necessary to look for and find new technical solutions. The use of extended electrodes for anode grounding seems to be promising. Elastomeric electrodes are laid horizontally in a trench along the pipelines of the heating network at a depth

1.5 m and are divided into several sections to improve maintainability. The cost of such installations is less than when using traditional anode ground loops. In 2011, two installations with horizontal electrodes have already been built.

The equipping of ECP units with telemetry units will continue, and in the future information on the operation of all units will be remotely transmitted and archived.

In 2011, a project for automated electricity metering was completed for 59 ECP units, and its implementation is scheduled for 2012.

Work has already begun on entering the database on ECP units into the unified information and analytical system of St. Petersburg Heating Grid OJSC. In the future, this will make it possible to quickly and reliably determine priorities when drawing up a program for the reconstruction of sections of heating networks, correctly organize earthworks when eliminating defects.

The main purpose of the ECP of heating networks is to ensure the operation of pipelines without damage during the entire standard period (25 years). To achieve this goal, it is necessary to treat ECP exactly as a system, without neglecting any of its components indicated in this article. Several general considerations may be helpful.

1. In corrosive-hazardous zones, it is necessary to put into operation the ECP as soon as possible after the construction or reconstruction of the heating network section, i.e. protect metal from scratch.

2. On the section of pipelines that are electrically poorly insulated from the ground (destruction of thermal insulation, metal contact with concrete structures, etc.), the ECP installation will be ineffective, since the protective current created by it will not be distributed for hundreds of meters along the pipes, but will drain into the ground at the “short circuit”.

3. With the identified low efficiency existing installation ECP (small difference in the value of the metal potential when the installation is turned on and off), it is necessary to reconstruct it with a change in the location of the anode grounding loop (KAZ) in relation to the protected pipelines.

4. During the reconstruction and new construction of ECP units, it is advisable to use the best brands of electrodes for KAZ, since a circuit failure is a failure of the entire installation, and to restore the KAZ, expensive earthworks will have to be carried out.

5. Coordination of activities in terms of ECP with other owners of underground utilities will allow taking measures to protect pipelines of heating networks from the harmful influence of "foreign" ECP installations, as well as, in some cases, organize joint protection.

The experience of operating the heating networks of St. Petersburg Heating Grid convincingly proves that ECP was and remains an important component in the complex of measures to improve the reliability of heat supply in St. Petersburg.

Passive protection of underground gas pipelines with insulating coatings is complemented by electrical protection. The tasks of electrical protection are as follows.

Removal of stray electric currents from the protected gas-wire and their organized return to electrical installations and DC networks, which are the source of these currents.
Suppression of currents flowing through the gas pipeline at the points of their exit to the ground (anode zones) by currents from an external source, as well as currents arising from soil electrochemical corrosion, by creating a galvanic circuit and protective electrical potential on the gas pipeline pipes.
Prevention of the spread of electric currents through gas pipelines by sectioning the latter with insulating flanges.

The problem of diverting stray currents can be solved by creating:

additional grounding to drain currents into the ground. Shortage - the possibility of a harmful effect on neighboring pipelines of currents flowing from the protected gas pipeline;
simple or direct drainage protection, i.e. electrical connection of the protected gas pipeline with the rails of a tram or electric railway in order to return currents through them to their source. Simple drainage is bilaterally conductive, i.e. can pass current back and forth, and therefore is used in stable anode zones. The disadvantage of this protection is the need to turn off the drainage if the polarity of the current has changed or if the potential on the gas pipeline has become lower than on the rails;
polarized drainage protection, i.e. drainage with one-sided conductivity, excluding the reverse flow of current from the rails to the protected gas pipeline;
enhanced drainage protection, i.e. such protection, in the circuit of which an external current source is included in order to increase efficiency. Thus, enhanced drainage is a combination of polarized drainage with cathodic protection.

The problem of suppressing currents flowing through the protected gas line can be solved using:

Cathodic protection by external current (electrical protection), i.e. when the protected gas pipeline is connected to an external current source - to its negative pole as a cathode. The positive pole of the current source is connected to the ground - the anode. A closed circuit is created in which the current flows from the anode through the ground to the protected gas pipeline and then to the negative pole of the external current source. In this case, a gradual destruction of the anode grounding occurs, but the protection of the gas pipeline is ensured due to its cathodic polarization and the prevention of currents flowing from the pipes into the ground. Cathodic protection stations (SKZ) can be used as an external source;
Protective protection, i.e. protection by using protectors made of metals in the electrical circuit, which have a more negative potential in a corrosive environment than the metal of the pipeline. Electric current arises in the protector protection system, as well as in a galvanic cell, and the electrolyte is the soil containing moisture, and the electrodes are the gas pipeline and the protector metal. The arising protective current suppresses the currents of electrochemical corrosion and ensures the creation of a protective electrical potential on the gas pipeline.

Schematic diagram of cathodic protection of an underground gas pipeline

1 - anode grounding; 2.4 - drainage cables; 3 - external source of electric current; 5 - point of connection of the drainage cable; 6 - protected gas pipeline

Schematic diagram of the protective protection of an underground gas pipeline

1 - protected gas pipeline; 2 - insulated cables; 3 - control pin; 4 - protector; 5 - filler for a protector

The task of electrical sectioning of pipelines is solved by installing insulating flanges with paronite or textolite gaskets, textolite bushings and washers. An example of the design of insulating flanges is shown in the figure below.

Insulating flange arrangement

1 - insulating textolite or paronite bushing; 2 - insulating washer made of PCB, rubber or PVC; 3 - steel washer; 4 - lead washers; 5 - textolite ring-gasket

The main factors characterizing the degree of corrosive attack on underground steel gas pipelines are:

the magnitude and direction of stray currents in the ground;
the magnitude and polarity of the potential of the gas pipeline relative to other metal underground communications and rails of electrified transport;
direction and strength of currents flowing through the gas pipeline;
the state of anticorrosive protection of gas pipelines;
resistivity value lb.

All of these factors are subject to periodic monitoring.

The frequency of electrical measurements is as follows:

in the areas of electrical protection installations for gas pipelines and other protected structures, as well as near traction substations and electric transport depots, near the rails of the famway and electrified railways and at the intersections of gas pipelines with them - at least once every 3 months, as well as when changing the modes of the mouth - new electrical protection, protected structures or sources of stray currents;
in areas that are not dangerous from the point of view of electrical protection - at least once a year in the summer, as well as in case of any changes in conditions that may cause electrical corrosion.

For tread protection, protectors made of non-ferrous metals are used - usually magnesium, zinc, aluminum and their alloys.

Control of the operation of electrical protection installations and measurement of potentials at the contacts are carried out (at least): at drainage installations - 4 times a month; on cathode installations - 2 times a month; on tread installations - once a month.

One of the most commonly used methods of electrochemical protection various designs of metals from rusting is cathodic protection. In most cases, it is used in conjunction with the application of special coatings to metal surfaces.

1 General information on cathodic protection

For the first time such protection of metals was described in the 1820s by Humphrey Davy. On the basis of his reports in 1824, the theory provided was tested on the HMS Samarang. Iron anode protectors were installed on the copper plating of the ship, which significantly reduced the rate of rusting of copper. The technique began to be developed, and today the cathode of all kinds of structures made of metals (pipelines, automobile elements, etc.) is recognized as the most effective and widely used.

Under industrial conditions, such protection of metals (it is often called cathodic polarization) is carried out according to two main methods.

The structure, protected from destruction, is connected to an external current source. In this case, the metal product acts as a cathode. And the anodes are inert additional electrodes. This technique is commonly used to protect pipelines, welded metal substrates, and drilling platforms.
Galvanic cathodic polarization. With this scheme, the metal structure is in contact with a metal that has a higher electronegative potential (aluminum, magnesium, aluminum alloys, zinc). In this case, the anode is understood to mean both metals (basic and protective). Dissolution (meaning a purely electrochemical process) of an electronegative material leads to the flow of the required cathodic current through the protected product. Over time, there is a complete destruction of the metal "protector". Galvanic polarization is effective for structures with an insulating layer, as well as for relatively small metal products.

The first technique has found widespread use throughout the world. It is quite simple and economically feasible, it makes it possible to protect the metal from general corrosion and from many of its varieties - intergranular corrosion of "stainless steel", pitting, cracking of brass products due to the stresses at which they work.

The galvanic circuit has found widespread use in the United States. In our country, it is used less often, although its effectiveness is high. The limited use of protective metal protection in Russia is due to the fact that many pipelines in our country do not have a special coating, and this is a prerequisite for the implementation of an anti-corrosion galvanic technique.

2 How does standard cathodic metal polarization work?

Cathodic corrosion protection is achieved by using an injected current. It enters the structure from a rectifier or other source (external) current, where industrial frequency alternating current is modified to the required constant. The object to be protected is connected to rectified current (to the "minus" pole). The structure is thus a cathode. The anode ground (second electrode) is connected to "plus".

It is important that there is good electrolytic and electronic contact between the secondary electrode and the structure. The first is provided by soil, into which the anode and the object of protection are immersed. In this case, the soil acts as an electrolytic medium. And electronic contact is achieved using conductors made of metallic materials.

The regulation of cathodic corrosion protection is carried out by maintaining the protective potential between the electrolytic medium and the polarization potential indicator (or directly by the structure) at a strictly defined value. The indicator is measured with a voltmeter with a high-resistance scale.

Here it is necessary to understand that the potential has not only a polarization component, but also one more component - the (ohmic) voltage drop. This drop occurs due to the flow through the effective resistance of the cathodic current. Moreover, the quality of cathodic protection depends solely on the polarization on the surface of the product, which is protected from rusting. For this reason, two characteristics of the security of a metal structure are distinguished - the highest and lowest polarization potentials.

Taking into account all of the above, effective regulation of the polarization of metals becomes possible when the ohmic component is excluded from the value of the resulting potential difference. This can be achieved using a special scheme for measuring the polarization potential. We will not describe it within the framework of this article, since it is replete with many specialized terms and concepts.

As a rule, the cathodic technology is used in conjunction with the application of special protective materials to the outer surface of products protected from corrosion.

To protect bare pipelines and other structures, it is necessary to use significant currents, which is economically unprofitable and technically difficult.

3 Cathodic protection of vehicle elements

Corrosion is an active and highly aggressive process. High-quality protection of vehicle components from rusting causes many problems for motorists. All vehicles, without exception, are subject to corrosion, because rusting begins even when paintwork a small scratch appears on the machine.

Cathodic technology to protect cars from corrosion is quite common these days. It is used along with the use of all kinds of mastics. This technique is understood as the supply of an electric potential to the surface of a particular car part, which leads to an effective and long-term deceleration of rust.

In the described protection of the vehicle, the cathode is special plates that are applied to the most vulnerable parts of it. And the role of the anode is played by the car body. This potential distribution ensures the integrity of the machine body, since only the cathode plates are destroyed, and the base metal does not corrode.

Vulnerabilities in a vehicle that can be protected by the cathodic technique are understood as:

back and front parts of the bottom;
rear wheel arch;
areas for fixing sidelights and headlights directly;
wing-to-wheel joints;
inner zones of doors and thresholds;
the space behind the wheel flaps (front).

To protect the car, you need to purchase a special electronic module(some craftsmen make it themselves) and protectors-plates. The module is mounted in the passenger compartment of the car, connected to the on-board network (it must be powered when the engine is turned off). Installation of the device takes literally 10-15 minutes. Moreover, it takes a minimum of energy, and guarantees very high quality anti-corrosion protection.

Protective plates can be different size... Their number also differs depending on where in the car they are mounted, as well as on what geometric parameters the electrode has. In practice, the larger the size of the electrode, the smaller the plates are needed.

Corrosion protection of a car using the cathodic method is also performed in other relatively simple ways. The most elementary one is to connect the "plus" wire of the car battery to an ordinary metal garage. Please note - it is imperative to use a resistor for connection.

4 Protection of pipelines by cathodic polarization

Depressurization of pipelines of various purposes occurs in many cases due to their corrosion destruction caused by the appearance of ruptures, cracks and caverns. Underground utilities are especially susceptible to rusting. Zones with different potential (electrode) are formed on them, which is caused by the heterogeneity of the soil and the inhomogeneous composition of the metals from which the pipes are made. Due to the appearance of these zones, the process of active formation of corrosive galvanic components begins.

Cathodic polarization of pipelines, carried out according to the schemes described at the beginning of the article (galvanization or an external energy source), is based on a decrease in the rate of dissolution of the pipe material during their operation. A similar reduction is achieved by shifting the corrosion potential to a zone that has more negative indicators with respect to the natural potential.

Back in the first third of the 20th century, the potential for cathodic polarization of metals was determined. Its indicator is equal to –0.85 volts. In most soils, the natural potential of metal structures is in the range from –0.55 to –0.6 volts.

This means that for effective protection of pipelines it is required to "move" the corrosion potential in the negative direction by 0.25-0.3 volts. With such a value, the practical effect of rusting on the state of communications is almost completely leveled out (corrosion per year has a rate of no more than 10 micrometers).

The technique using a current source (external) is considered laborious and rather complicated. But it provides high level protection of pipelines, its energy resource is not limited by anything, while the resistance (specific) of the soil has a minimal effect on the quality of protective measures.

Power sources for cathodic polarization are usually 0.4 overhead power lines; 6 and 10 kV. In areas where there are none, it is allowed to use gas, thermal and diesel generators as energy sources.

The "protector" current is distributed unevenly along the length of the pipelines. Its greatest value is noted at the so-called drainage point - in the place where the source is connected. The further the distance from this point, the less protected the pipes. At the same time, an excessive current directly in the connection zone has a negative effect on the pipeline - there is a high probability of hydrogen cracking of metals.

The method using galvanic anodes demonstrates good efficiency in soils with a low ohmic index (up to 50 ohm * m). In soils of a high-resistance group, it is not used, since it does not give special results. It should be added here that anodes are made from alloys based on aluminum, magnesium and zinc.

5 Briefly about cathodic protection stations (RMS)

For anticorrosive protection of pipelines laid underground, along the route of their occurrence, SCZ is installed, including:

anode grounding;
current source;
control and measurement point;
cables and wires that perform connecting functions.

The stations are connected to electric networks or to autonomous devices. It is allowed to install several groundings and energy sources at the RCC when two or more pipeline lines are laid in one underground corridor. This, however, entails an increase in the cost of carrying out anti-corrosion measures.

If only one installation is installed on multi-line communications, its connection to the pipes is carried out using special blocks. They do not allow the formation of strong galvanic vapors arising from the installation of blind bridges on pipe products. These blocks isolate the pipes from each other, and also make it possible to select the required potential on each element of the pipelines, which guarantees maximum protection of the structure from rusting.

The output voltage at the cathode stations can be regulated automatically (in this case, the installation is equipped with thyristors) or manually (the operator switches the transformer windings, if necessary). In situations where RMSs operate under time-varying conditions, it is recommended to operate stations with automatic voltage regulation.

They themselves monitor the indicators of resistance (specific) soil, the appearance of stray currents and other factors that have a negative impact on the quality of protection, and automatically adjust the operation of the SCZ. But in systems where the protective current and the resistance indicator in its circuit remain unchanged, it is better to use installations with manual setting output voltage.

We add that the regulation in automatic mode is produced according to one of two indicators:

over current protection (galvanostatic converters);
according to the potential of the object being protected (potentiostatic converters).

6 Information on known cathodic protection stations

Several installations can be distinguished among the popular domestic VCRs. The station is in great demand. Minerva – 3000 Is a powerful system developed by French and Russian engineers for Gazprom facilities. One Minerva is enough to reliably protect up to 30 kilometers of pipelines from rusting. The station has the following main advantages:

unique manufacturability of production of all its components;
increased power of the SCZ (it is possible to protect communications with a very poor protective coating);
self-recovery (after emergency overloads) of the station operating modes for 15 seconds;
availability of high-precision digital equipment for monitoring operating modes and a thermal control system;
the presence of protective circuits against overvoltage of the measuring and input circuits;
the absence of moving parts and the tightness of the electrical cabinet.

In addition, to Minerva – 3000 you can connect installations for remote control over the work of the station and remote control her equipment.

The systems also have excellent technical performance. ASKG-TM- modern telemechanized adaptive stations for the protection of electrical cables, city and main pipelines, as well as tanks in which gas and oil products are stored. Such devices are available with different indicators (from 1 to 5 kilowatts) of output power. They have a multifunctional telemetry complex that allows you to select a specific operating mode of the RMS, monitor and change the parameters of the station, as well as process the incoming information and send it to the operator.

Benefits of using ASKG-TM:

the possibility of embedding in SCADA-complexes due to the support of OPC-technology;
backup and main communication channel;
choice of power value (output);
increased fault tolerance;
wide range of operating temperatures;
unique accuracy of setting the output parameters;
voltage protection of power outputs of the system.

There are VHCs of other types, information about which is easy to find on specialized sites on the Internet.

7 What objects can be protected using cathodic polarization?

In addition to protecting vehicles and pipelines, the considered polarization techniques are actively used to prevent corrosion of reinforcement included in reinforced concrete structures (buildings, road facilities, foundations, and so on). Typically, fittings are a single electrical system, which actively corrodes when chlorides and water enter it.

Cathodic polarization in combination with concrete sanitation stops corrosive processes. In this case, it is necessary to use two types of anodes:

the main ones are made of titanium, graphite or their combination with a metal-oxide coating, as well as siliceous cast iron;
distribution rods - rods made of titanium alloys with an additional layer of metal protection or with a non-metallic electrically conductive coating.

By regulating the external current supplied to the reinforced concrete structure, the potential of the reinforcement is selected.

Polarization is considered an indispensable technique for protecting permanent structures located on the continental shelf, in the oil and gas fields. The initial protective coatings at such facilities cannot be restored (they must be dismantled and transported to dry hangars), which means that there is only one way out - cathodic protection of metals.

Galvanic polarization of civil ships by means of zinc, magnesium, and aluminum alloy anodes is used to protect against marine corrosion. Onshore (during repairs and mooring), the vessel is connected to the SCZ, the anodes for which are made of platinized titanium.

Also, cathodic protection is used to protect against destruction of the inner parts of vessels and containers, as well as pipes that come into contact with industrial waste water and other aggressive electrolytes. Polarization in this case increases the time of maintenance-free use of these structures by 2–3 times.

They allow extending the service life of the metal structure, as well as preserving its technical and physical properties during operation. Despite the variety of methods for ensuring anti-corrosion action, it is possible to completely protect objects from rust damage only in rare cases.

The effectiveness of such protection depends not only on the quality of the tread technology, but also on the conditions of its use. In particular, in order to preserve the metal structure of pipelines, electrochemical corrosion protection based on the operation of cathodes demonstrates its best properties. Prevention of rust formation on such communications, of course, is not the only area of application of this technology, but in terms of the totality of its characteristics, this direction can be considered as the most relevant for electrochemical protection.

General information on electrochemical protection

The protection of metals from rust by means of electrochemical action is based on the dependence of the size of the material on the rate of the corrosion process. Metal structures should be operated in the potential range where their anodic dissolution will be lower acceptable limit... The latter, by the way, is determined by the technical documentation for the operation of the structure.

In practice, electrochemical corrosion protection involves connecting a source with direct current. Electric field on the surface and in the structure of the protected object forms the polarization of the electrodes, due to which the process of corrosion damage is also controlled. In essence, the anodic zones on the metal structure become cathodic, which allows the negative processes to be displaced, ensuring the preservation of the structure of the target object.

How cathodic protection works

There are cathodic and anodic protection of the electrochemical type. The most popular is the first concept, which is used to protect pipelines. According to the general principle, when implementing this method, a current with a negative pole from an external source is supplied to the object. In particular, in this way a steel or copper pipe can be protected, as a result of which the polarization of the cathode sections will occur with the transition of their potentials to the anodic state. As a result, the corrosive activity of the protected structure will be reduced to almost zero.

Moreover, the cathodic protection can have different versions. The above-described technique of polarization from an external source is widely practiced, but the method of deaeration of the electrolyte is also effective, with a decrease in the rate of cathodic processes, as well as the creation of a protective barrier.

It has already been noted more than once that the principle of cathodic protection is implemented by means of an external current source. Actually, its main function lies in its work. These tasks are performed by special stations, which, as a rule, are part of the general infrastructure of pipeline maintenance.

Anti-corrosion stations

The main function of the cathode station is to provide stable current to the target metal object in accordance with the cathodic polarization method. Such equipment is used in the infrastructure of underground gas and oil pipelines, in water supply pipes, heating networks, etc.

There are many varieties of such sources, while the most common cathodic protection device provides for the presence of:

current converter equipment;
wires for connecting to the protected object;
anode earthing switch.

At the same time, there is a division of stations into inverter and transformer. There are other classifications, but they are focused on the segmentation of attitudes or by areas of application, or by technical specifications and parameters of the input data. The basic principles of operation are most clearly illustrated by the indicated two types of cathode stations.

Cathodic protection transformer installations

It should be noted right away that given view stations is obsolete. It is replaced by inverter counterparts, which have both pros and cons. One way or another, transformer models are used even at new points for providing electrochemical protection.

As the basis of such objects, a low-frequency transformer at 50 Hz is used and the simplest devices are used for the thyristor control system, among which are phase-pulse power regulators. A more responsible approach to solving control problems involves the use of controllers with broad functionality.

Modern cathodic protection against corrosion of pipelines with such equipment allows you to adjust the parameters of the output current, voltage indicators, as well as equalize protective potentials. As for the disadvantages of transformer equipment, they boil down to high degree output ripple current at low power factor. This flaw is explained by a non-sinusoidal current waveform.

To a certain extent, the introduction of a low-frequency choke into the system allows solving the problem with ripple, but its dimensions correspond to the dimensions of the transformer itself, which does not always make such an addition possible.

Inverter cathodic protection station

Inverter-type installations are based on pulse high-frequency converters. One of the main advantages of using stations of this type is high efficiency, reaching 95%. For comparison, for transformer installations this figure reaches 80% on average.

Sometimes other advantages come to the fore. For example, the small size of inverter stations expands the possibilities for their use in difficult areas. There are also financial advantages, which are confirmed by the practice of using such equipment. So, inverter cathodic protection against pipeline corrosion quickly pays off and requires minimum investment into technical content. However, these qualities are clearly visible only when compared with transformer installations, but today more effective new means of supplying current for pipelines are emerging.

Cathode station designs

Such equipment is available on the market in different cases, shapes and sizes. Of course, the practice of individual design of such systems is also widespread, which makes it possible not only to obtain an optimal design for specific needs, but also to provide the necessary operational parameters.

A rigorous calculation of the characteristics of the station allows you to further optimize the costs of its installation, transportation and storage. For example, for small objects, cathodic protection against corrosion of pipelines on an inverter basis with a mass of 10-15 kg and a power of 1.2 kW is quite suitable. Equipment with such characteristics can be serviced and by car, however, for large-scale projects, more massive and heavy stations can also be used, requiring the connection of trucks, a crane and assembly teams.

Protective functionality

Particular attention is paid to the protection of the equipment itself when developing cathode stations. For this, systems are integrated to protect stations from short circuit and breakage of loads. In the first case, special fuses are used to handle emergency operating modes of installations.

As for voltage surges and interruptions, the cathodic protection station is unlikely to be seriously damaged by them, but there may be a danger of electric shock. For example, if in normal mode the equipment is operated with a low voltage, then after a break, a jump in indicators can bring up to 120 V.

Other types of electrochemical protection

In addition to cathodic protection, electrical drainage technologies are also practiced, as well as protective methods to prevent corrosion. Most promising direction it is the special protection against corrosion that is considered. In this case, active elements are also connected to the target object, ensuring the conversion of the surface with cathodes by means of current. For example, a steel pipe as part of a gas pipeline can be protected by zinc or aluminum cylinders.

Conclusion

Electrochemical protection methods cannot be classified as new and, moreover, innovative. The effectiveness of using such techniques in the fight against rusting processes has been mastered for a long time. However, there is one major drawback that hinders widespread adoption of this method. The fact is that cathodic protection against pipeline corrosion inevitably develops the so-called They are not dangerous for the target structure, but can have a negative effect on nearby objects. In particular, the stray current contributes to the development of the same corrosion on metal surface adjacent pipes.