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Special electrical device cathodic protection station. What is cathodic protection of pipelines and how does it work?

Corrosion has a detrimental effect on the technical condition of underground pipelines; under its influence, the integrity of the gas pipeline is compromised and cracks appear. To protect against such a process, electrochemical protection of the gas pipeline is used.

Corrosion of underground pipelines and means of protection against it

The condition of steel pipelines is influenced by soil moisture, its structure and chemical composition. The temperature of the gas conveyed through pipes, currents wandering in the ground caused by electrified transport and climatic conditions in general.

Types of corrosion:

Superficial. Spreads in a continuous layer over the surface of the product. Represents the least danger to the gas pipeline.
Local. Manifests itself in the form of ulcers, cracks, spots. The most dangerous type of corrosion.
Fatigue corrosion failure. The process of gradual accumulation of damage.

Methods of electrochemical protection against corrosion:

passive method;
active method.

The essence of the passive method of electrochemical protection is to apply a special protective layer to the surface of the gas pipeline that prevents harmful effects environment. Such coverage could be:

bitumen;
polymer tape;
coal tar pitch;
epoxy resins.

In practice, it is rarely possible to apply an electrochemical coating evenly to a gas pipeline. In places of gaps, the metal is still damaged over time.

The active method of electrochemical protection or cathodic polarization method is to create a negative potential on the surface of the pipeline, preventing the leakage of electricity, thereby preventing the occurrence of corrosion.

Operating principle of electrochemical protection

To protect a gas pipeline from corrosion, it is necessary to create a cathodic reaction and eliminate the anodic reaction. To do this, a negative potential is forcibly created on the protected pipeline.

Anode electrodes are placed in the ground, and the negative pole of an external current source is connected directly to the cathode - the protected object. To complete the electrical circuit, the positive pole of the current source is connected to the anode - an additional electrode installed in a common environment with the protected pipeline.

The anode in this electrical circuit performs the grounding function. Due to the fact that the anode has a more positive potential than the metal object, its anodic dissolution occurs.

The corrosion process is suppressed under the influence of the negatively charged field of the protected object. With cathodic protection against corrosion, the anode electrode will be directly subjected to deterioration.

To increase the service life of anodes, they are made of inert materials that are resistant to dissolution and other influences. external factors.

An electrochemical protection station is a device that serves as a source of external current in a cathodic protection system. This installation is connected to the network, 220 W and produces electricity with set output values.

The station is installed on the ground next to the gas pipeline. It must have a degree of protection IP34 or higher, since it works outdoors.

Cathodic protection stations can have various technical parameters and functional features.

Types of cathodic protection stations:

transformer;
inverter

Transformer stations for electrochemical protection are gradually becoming a thing of the past. They are a structure consisting of a transformer operating at a frequency of 50 Hz and a thyristor rectifier. The disadvantage of such devices is the non-sinusoidal shape of the generated energy. As a result, a strong current pulsation occurs at the output and its power decreases.

An inverter electrochemical protection station has an advantage over a transformer one. Its principle is based on the operation of high-frequency pulse converters. A feature of inverter devices is the dependence of the size of the transformer unit on the frequency of current conversion. With a higher signal frequency, less cable is required and heat loss is reduced. In inverter stations, thanks to smoothing filters, the ripple level of the produced current has a smaller amplitude.

The electrical circuit that powers the cathodic protection station looks like this: anodic grounding - soil - insulation of the protected object.

When installing a corrosion protection station, the following parameters are taken into account:

position of the anode grounding (anode-ground);
soil resistance;
electrical conductivity of the object's insulation.

Drainage protection installations for gas pipelines

With the drainage method of electrochemical protection, a current source is not required; the gas pipeline communicates with the traction rails using currents wandering in the ground railway transport. Electrical interconnection is achieved due to potential difference railway rails and gas pipeline.

By means of the drainage current, a displacement of the electric field of the gas pipeline located in the ground is created. The protective role in this design is played by fuses, as well as automatic maximum load switches with reset, which adjust the operation of the drainage circuit after a drop in the high voltage.

The polarized electric drainage system is carried out using valve block connections. Voltage regulation with this installation is carried out by switching active resistors. If the method fails, more powerful electrical drains are used in the form of electrochemical protection, where a railway rail serves as the anode grounding conductor.

Galvanic electrochemical protection installations

The use of protective installations for galvanic pipeline protection is justified if there is no voltage source near the facility - a power line, or the gas pipeline section is not large enough in size.

Galvanic equipment serves to protect against corrosion:

underground metal structures not connected by an electrical circuit to external current sources;
individual unprotected parts of gas pipelines;
parts of gas pipelines that are isolated from the current source;
pipelines under construction that are temporarily not connected to corrosion protection stations;
other underground metal structures (piles, cartridges, tanks, supports, etc.).

Galvanic protection will work best in soils with electrical resistivity within 50 ohms.

Installations with extended or distributed anodes

When using a corrosion protection transformer station, the current is distributed along a sinusoid. This has an adverse effect on the protective electric field. Either excess voltage occurs at the protection point, which entails high energy consumption, or uncontrolled current leakage, which makes the electrochemical protection of the gas pipeline ineffective.

The practice of using extended or distributed anodes helps to circumvent the problem of uneven distribution of electricity. The inclusion of distributed anodes in the gas pipeline electrochemical protection scheme helps to increase the corrosion protection zone and smooth out the voltage line. With this scheme, anodes are placed in the ground along the entire gas pipeline.

An adjusting resistance or special equipment ensures that the current changes within the required limits, the anodic grounding voltage changes, and with this the protective potential of the object is regulated.

If several ground electrodes are used at once, the voltage of the protective object can be changed by changing the number of active anodes.

The ECP of a pipeline using protectors is based on the potential difference between the protector and the gas pipeline located in the ground. The soil in this case is an electrolyte; the metal is restored, and the protector body is destroyed.

Video: Protection against stray currents

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Cathodic protection of the gas pipeline must operate uninterruptedly. For each VCS a certain mode is established depending on its operating conditions. When operating a cathode station, a log of its electrical parameters and the operation of the current source is kept. Constant monitoring of the anodic grounding is also necessary, the state of which is determined by the value of the RMS current.

Characteristics of the state of the protective coating and its conductivity.

Cathodic protection of the gas pipeline must operate uninterruptedly. In sections of the route where power supply is interrupted for several hours a day, batteries are used to provide protection during power outages. The capacity of the battery is determined by the value of the RMS protective current.

Cathodic protection of gas pipelines from the effects of stray currents or ground corrosion is carried out using direct electric current from an external source. The negative pole of the current source is connected to the protected gas pipeline, and the positive pole is connected to a special ground - the anode.

Cathodic protection of gas pipelines from corrosion is carried out due to their cathodic polarization using an external current source.

The influence of cathodic protection of gas pipelines on railway rail circuits.

For cathodic protection of a gas pipeline, standard instruments of electrical installations and special corrosion-measuring and auxiliary instruments are used. To measure the potential difference between an underground structure and the ground, which is one of the criteria for assessing the danger of corrosion and the presence of protection, voltmeters with a large internal resistance value of 1 on the scale are used so that their inclusion in the measuring circuit does not violate the potential distribution in the latter. This requirement is determined by both the high internal resistance of the underground structure and the ground system, and the difficulty of creating a low grounding resistance at the point of contact of the measuring electrode with the ground, especially when using non-polarizing electrodes. To obtain a measuring circuit with a high input resistance, potentiometers and high-resistance voltmeters are used.

For gas pipeline cathodic protection stations as a source of electricity, it is recommended to use high-temperature fuel cells with a ceramic electrode. Such fuel cells can operate for a long time along the gas pipeline route, supplying electricity to cathodic protection stations, as well as the houses of line repairmen, signaling systems and automatic control valves. This power supply method linear structures and gas pipeline installations that do not require high power, greatly simplifies operational maintenance.

Very often, the cathodic protection parameters of gas pipelines obtained by calculation differ significantly from the SPS parameters obtained in practice by measurements. This is due to the impossibility of taking into account the whole variety of factors influencing natural conditions on protection parameters.

One of the frequently used methods of electrochemical protection of various metal structures from rust is cathodic protection. In most cases, it is used in conjunction with the application of special coatings to metal surfaces.

1 General information about cathodic protection

Such protection of metals was first described in the 1820s by Humphry Davy. Based on his reports, in 1824, on the ship HMS Samarang, the theory provided was tested. 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 metal structures (pipelines, car elements, etc.) is recognized as the most effective and widely used.

IN production conditions Such protection of metals (often called cathodic polarization) is carried out using two main methods.

The structure, which is protected from destruction, is connected to an external current source. In this case, the metal product acts as a cathode. And anodes are inert additional electrodes. This technique is usually used to protect pipelines, welded metal foundations, and drilling platforms.
Cathodic polarization of galvanic type. 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 refers to both metals (main and protective). The dissolution (meaning a purely electrochemical process) of an electronegative material leads to the flow of the necessary cathode current through the protected product. Over time, the “protector” metal is completely destroyed. Galvanic polarization is effective for structures that have an insulating layer, as well as for relatively small metal products.

The first technique found wide application all over the world. It is quite simple and economically feasible, making 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 caused by the stresses under which they operate.

The galvanic circuit has found greater use in the USA. In our country it is used less frequently, although its effectiveness is high. The limited use of sacrificial protection for metals in Russia is due to the fact that many pipelines in our country do not have a special coating applied, and this is prerequisite for the implementation of anti-corrosion galvanic techniques.

2 How does standard cathodic polarization of metals work?

Cathodic corrosion protection is achieved through the use of superimposed current. It is supplied to the structure from a rectifier or other source of (external) current, where industrial-frequency alternating current is modified into the required direct current. The object being protected is connected to rectified current (to the “minus” pole). The structure is thus a cathode. The anodic grounding (second electrode) is connected to the “plus”.

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

Regulation of cathodic anti-corrosion protection is carried out by maintaining the protective potential between the electrolytic medium and the polarization potential indicator (or the structure itself) 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 another component - a drop in (ohmic) voltage. This drop occurs due to the flow of cathode current through the effective resistance. 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.

Effective regulation of the polarization of metals, taking into account all of the above, becomes possible in the case when the indicator of the ohmic component is excluded from the value of the resulting potential difference. This can be achieved using a special circuit 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, cathode technology is used in conjunction with the application of special protective materials to the external surface of products protected from corrosion.

To protect uninsulated 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 very aggressive process. High-quality protection of car components from rust causes many problems for car enthusiasts. All vehicles without exception are subject to corrosive destruction, because rusting begins even when a small scratch appears on the paintwork of the car.

Cathodic technology for protecting a car from corrosion is quite common these days. It is used along with the use of all kinds of mastics. This technique refers to the application of electrical potential to the surface of a particular car part, which leads to an effective and long-term inhibition of rusting.

With the described protection vehicle The cathode is made up of special plates that are placed on its most vulnerable components. And the role of the anode is played by the car body. Such a distribution of potentials ensures the integrity of the machine body, since only the cathode plates are destroyed, and the base metal does not corrode.

The vulnerabilities of a vehicle that can be protected using the cathodic method are understood as:

rear and front parts of the bottom;
rear wheel arch;
areas for fixing sidelights and headlights themselves;
wing-wheel joints;
internal areas of doors and thresholds;
space behind the wheel guards (front).

To protect the car, you need to purchase a special electronic module (some craftsmen make it themselves) and protector plates. The module is mounted in the car interior and connected to the on-board network (it must be powered when the car engine is turned off). Installing 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 of different sizes. Their number also differs depending on where in the car they are mounted, as well as on what geometric parameters has an electrode. In practice, the smaller the electrode size, the fewer plates you need.

Car corrosion protection using the cathodic method is also carried out by other comparative in simple ways. The most basic one is to connect the plus wire of the car battery to a regular one. metal garage. Please note that you must use a resistor for connection.

4 Protection of pipelines using cathodic polarization method

Depressurization of pipelines of various purposes occurs in many cases due to their corrosion destruction caused by the appearance of ruptures, cracks and cavities. Particularly susceptible to rust underground communications. Zones with different potentials (electrodes) are formed on them, which is caused by the heterogeneity of the soil and the heterogeneous 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 reducing the rate of dissolution of the pipe material during their operation. Such a reduction is achieved by shifting the corrosion potential to a zone that has more negative indicators in relation 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 -0.85 volts. In most soils the natural potential metal structures is in the range from –0.55 to –0.6 volts.

This means that to effectively protect pipelines, it is necessary to “move” the corrosion potential to the negative side by 0.25-0.3 volts. With such a magnitude, the practical effect of rusting on the condition 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 labor-intensive and quite complex. But it provides a high level of protection for pipelines, its energy resource is not limited by anything, and the resistance (specific) of the soil has minimal impact on the quality of protective measures.

Power sources for cathodic polarization are usually overhead power lines at 0.4; 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 - at the place where the source is connected. The greater the distance from this point, the less protected the pipes are. At the same time, excessive current directly in the connection area 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 low resistivity (up to 50 ohm*m). It is not used in soils of the high-resistivity group, since it does not give any special results. It is worth adding here that anodes are made from alloys based on aluminum, magnesium and zinc.

5 Briefly about cathodic protection stations (CPS)

For anti-corrosion protection of pipelines laid underground, SCPs are installed along their route, including:

anodic grounding;
current source;
control and measurement point;
cables and wires performing connecting functions.

Stations are connected to electrical networks or to autonomous devices. It is allowed to install several grounding connections and energy sources at the VCS when two or more pipeline lines are laid in one underground corridor. This, however, entails an increase in costs for 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 couples that occur when installing blind jumpers on pipe products. These blocks isolate pipes from each other, and also make it possible to select the required potential on each pipeline element, guaranteeing maximum protection of the structure from rust.

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

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

Let us add that regulation in automatic mode is carried out according to one of two indicators:

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

6 Information on known cathodic protection stations

Among the popular domestic VCSs, several installations can be distinguished. The station is in great demand Minerva–3000– 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 rust. The station has the following main advantages:

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

In addition, to Minerva–3000 you can connect installations for remote control over the operation 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 output power ratings (from 1 to 5 kilowatts). They have a multifunctional telemetry complex that allows you to select a specific VCS operating mode, monitor and change station parameters, as well as process incoming information and send it to the operator.

Benefits of use ASKG-TM:

possibility of integration into SCADA complexes due to support of OPC technology;
backup and main communication channel;
selection of power value (output);
increased fault tolerance;
wide operating temperature range;
unique accuracy of setting output parameters;
voltage protection of system power outputs.

There are SKZ and 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 cars and pipelines, the polarization techniques under consideration are actively used to protect reinforcement included in reinforced concrete structures (buildings, road facilities, foundations, etc.) from corrosion. Typically, the fittings are a single electrical system, which actively corrodes when chlorides and water enter it.

Cathodic polarization in combination with concrete sanitation stops corrosion 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 silicon 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 the protection of permanent structures located on the continental shelf, in the gas and oil fields. Initial protective coatings It is impossible to restore such objects (they require dismantling and transportation to dry hangars), which means that there is only one option left - cathodic protection of metals.

To protect against sea corrosion, galvanic polarization of civilian ships is used using anodes made of zinc, magnesium, and aluminum alloys. On shore (during repairs and moorings), ships are connected to SCZ, the anodes for which are made of platinized titanium.

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

There are various methods for treating metal pipes, but the most effective of them is cathodic protection of pipelines against corrosion. It is necessary to prevent their premature depressurization, which will lead to the formation of cracks, cavities and ruptures.

Metal corrosion is a natural process in which the atoms of a metal change. As a result, their electrons pass to oxidizing agents, which leads to destruction of the structure of the material.

For underground pipelines, an additional factor of corrosion influence is the composition of the soil. It contains areas of different electrode potential, which causes the formation of corrosive galvanic cells.

There are several types of corrosion, including:

Solid. It is distinguished by a large continuous area of distribution. In rare cases, it causes damage to the pipeline, since it often does not penetrate deep into the metal structure;

Local corrosion – becomes the most common cause ruptures, since it does not cover a large area, but penetrates deeply. It is divided into pitting, filamentous, through, subsurface, spotty, knife, intergranular, corrosion brittleness and cracking.

Methods for protecting underground pipelines

Protection against metal corrosion can be either active or passive. Passive methods involve creating conditions for the pipeline in which it will not be affected by the surrounding soil. To do this, special protective compounds are applied to it, which become a barrier. The most commonly used coatings are bitumen, epoxy resins, polymer tapes or coal tar pitch.

For the active method, cathodic protection of pipelines against corrosion is most often used. It is based on the creation of polarization, which makes it possible to reduce the rate of metal dissolution. This effect is realized by shifting the corrosion potential to a more negative area. To do this, an electric current is passed between the metal surface and the soil, which significantly reduces the corrosion rate.

Methods for implementing cathodic protection:

Using external sources currents that are connected to the protected pipe and to the anode grounding;

Using the galvanic method (magnesium sacrificial anode protectors).

Cathodic protection of pipelines against corrosion using external sources is more complex. Since it requires the use of special designs that provide direct current. The galvanic method, in turn, is implemented through protectors, which make it possible to provide effective protection only in soils with low electrical resistance.

Can be used to protect the pipeline and the anodic method. It is used in conditions of contact with an aggressive chemical environment. The anodic method is based on converting the active state of the metal into a passive one and maintaining it due to the influence of an external anode.

Despite certain difficulties in implementation, this method is actively used where cathodic protection of pipelines against corrosion cannot be implemented.

Examples of cathodic protection of pipelines against corrosion at the exhibition

Experience of use and new developments in this area are highlighted at the annual industry exhibition “Neftegaz”, which takes place at the Expocentre Fairgrounds.

The exhibition is a major industry event and an excellent platform for introducing specialists to new developments, as well as launching new projects. The Neftegaz exhibition will be held at the Expocentre Fairgrounds in Moscow on Krasnaya Presnya.

Read our other articles.

With cathodic protection of a pipeline, the positive pole of the direct current source (anode) is connected to a special anode grounding electrode, and the negative pole (cathode) is connected to the protected structure (Fig. 2.24).

Rice. 2.24. Pipeline cathodic protection scheme

1- power line;

2 - transformer point;

3 - cathodic protection station;

4 - pipeline;

5 - anodic grounding;

6 - cable

The operating principle of cathodic protection is similar to electrolysis. Under the influence of an electric field, electrons begin to move from the anode grounding conductor to the protected structure. Losing electrons, the metal atoms of the anode ground electrode pass in the form of ions into the soil electrolyte solution, that is, the anode ground electrode is destroyed. An excess of free electrons is observed at the cathode (pipeline) (reduction of the metal of the protected structure).

49. Tread protection

When laying pipelines in hard-to-reach areas remote from power sources, sacrificial protection is used (Fig. 2.25).

1 - pipeline;

2 - protector;

3 - conductor;

4 - control and measuring column

Rice. 2.25. Tread protection scheme

The operating principle of tread protection is similar to galvanic couple. The two electrodes, the conduit and the protector (made of a more electronegative metal than steel), are connected by a conductor. In this case, a potential difference arises, under the influence of which a directed movement of electrons occurs from the anode protector to the cathode pipeline. Thus, the protector is destroyed, not the pipeline.

The tread material must meet the following requirements:

Provide the greatest potential difference between the protector metal and steel;

The current when dissolving a unit of tread mass should be maximum;

The ratio of the tread mass used to create the protective potential to the total tread mass should be the greatest.

The requirements are best met magnesium, zinc and aluminum. These metals provide almost equal protection effectiveness. Therefore, in practice, their alloys are used with the use of improving additives ( manganese, increasing current output and India– increasing the activity of the protector).

50. Electrical drainage protection

Electrical drainage protection is designed to protect the pipeline from stray currents. The source of stray currents is electric vehicles operating according to the “wire-to-ground” circuit. The current from the positive bus of the traction substation (contact wire) moves to the engine, and then through the wheels to the rails. The rails are connected to the negative bus of the traction substation. Due to the low transition resistance “rails-to-ground” and the violation of the jumpers between the rails, part of the current flows into the ground.

If there is a pipeline with damaged insulation nearby, current flows through the pipeline until there are favorable conditions for returning to the negative bus of the traction substation. Where the current exits, the pipeline is destroyed. Destruction occurs in a short time, since the stray current flows from a small surface.

Electrical drainage protection is the removal of stray currents from the pipeline to a source of stray currents or special grounding (Fig. 2.26).