Corrosion of underground pipelines and protection against it

Corrosion of underground pipelines is one of the main reasons for their depressurization due to the formation of cavities, cracks and ruptures. Corrosion of metals, i.e. their oxidation is the transition of metal atoms from a free state to a chemically bound, ionic state. In this case, the metal atoms lose their electrons, and the oxidizing agents accept them. On an underground pipeline, due to the heterogeneity of the pipe metal and due to the heterogeneity of the soil (as in physical properties, however, by chemical composition) areas with different electrode potentials arise, which causes the formation of galvanic corrosion. The most important species corrosion are: superficial (solid over the entire surface), local in the form of shells, pitting, crevice and fatigue corrosion cracking. The last two types of corrosion pose the greatest danger to underground pipelines. Surface corrosion only in in rare cases leads to damage, whereas due to pitting corrosion most damage occurs. The corrosive situation in which a metal pipeline is located in the ground depends on large quantity factors related to soil and climatic conditions, route features, operating conditions. These factors include:

• soil moisture,
• chemical composition soil,
• acidity of the ground electrolyte,
• soil structure,
• temperature of transported gas

The most powerful negative manifestation stray currents in the ground, caused by electrified DC rail transport, is the electrocorrosive destruction of pipelines. The intensity of stray currents and their impact on underground pipelines depends on factors such as:

• rail-to-ground contact resistance;
• longitudinal resistance of running rails;
• distance between traction substations;
• current consumption by electric trains;
• number and cross-section of suction lines;
• electrical resistivity of soil;
• distance and location of the pipeline relative to the path;
• transition and longitudinal resistance of the pipeline.

It should be noted that stray currents in cathode zones have a protective effect on the structure, therefore, in such places, cathodic protection of the pipeline can be carried out without large capital costs.

Methods for protecting underground metal pipelines from corrosion are divided into passive and active.

The passive method of corrosion protection involves creating an impenetrable barrier between the metal of the pipeline and the surrounding soil. This is achieved by applying special protective coatings to the pipe (bitumen, coal tar pitch, polymer tapes, epoxy resins, etc.).

In practice, it is not possible to achieve complete continuity of the insulating coating. Various types coatings have different diffusion permeability and therefore provide different insulation of the pipe from environment. During construction and operation, cracks, scuffs, dents and other defects appear in the insulating coating. The most dangerous are through damage to the protective coating, where, in practice, ground corrosion occurs.

Since the passive method does not allow complete protection of the pipeline from corrosion, active protection is simultaneously applied, associated with the control of electrochemical processes occurring at the boundary of the pipe metal and the ground electrolyte. This type of protection is called comprehensive protection.

The active method of corrosion protection is carried out by cathodic polarization and is based on reducing the rate of dissolution of the metal as its corrosion potential shifts to an area of ​​more negative values ​​than the natural potential. Experienced way It was established that the cathodic protection potential of steel is minus 0.85 Volts relative to the copper sulfate reference electrode. Since the natural potential of steel in the ground is approximately -0.55...-0.6 Volts, to implement cathodic protection it is necessary to shift the corrosion potential by 0.25...0.30 Volts in the negative direction.

By applying an electric current between the metal surface of the pipe and the ground, it is necessary to achieve a reduction in the potential in defective areas of the pipe insulation to a value below the protective potential criterion of -0.9 V. As a result, the corrosion rate is significantly reduced.

2. Cathodic protection installations
Cathodic protection of pipelines can be carried out using two methods:

• the use of magnesium sacrificial protector anodes (galvanic method);
• using external direct current sources, the minus of which is connected to the pipe, and the plus to anode grounding (electrical method).

The galvanic method is based on the fact that different metals in the electrolyte have different electrode potentials. If you form a galvanic couple from two metals and place them in an electrolyte, the metal with a more negative potential will become the anode and will be destroyed, thereby protecting the metal with a less negative potential. In practice, protectors made of magnesium, aluminum and zinc alloys are used as sacrificial galvanic anodes.

The use of cathodic protection using protectors is effective only in low-resistivity soils (up to 50 Ohm-m). In high-resistivity soils, this method does not provide the necessary protection. Cathodic protection using external current sources is more complex and labor-intensive, but it depends little on the resistivity of the soil and has an unlimited energy resource.

As a rule, converters of various designs powered from an alternating current network are used as direct current sources. The converters allow you to regulate the protective current over a wide range, ensuring pipeline protection in any conditions.

0.4 overhead lines are used as power sources for cathodic protection installations; 6; 10 kV. The protective current applied to the pipeline from the converter and creating a “pipe-ground” potential difference is distributed unevenly along the length of the pipeline. Therefore, the maximum absolute value the value of this difference is at the point of connection of the current source (drainage point). As you move away from this point, the pipe-ground potential difference decreases. Excessively increasing the potential difference negatively affects the adhesion of the coating and can cause hydrogenation of the pipe metal, which can cause hydrogen cracking. Cathodic protection is one of the methods of combating metal corrosion in aggressive chemical environments. It is based on transferring a metal from an active state to a passive state and maintaining this state using an external cathode current. To protect underground pipelines from corrosion, cathodic protection stations (CPS) are built along their route. The VCS includes a direct current source (protective installation), anode grounding, a control and measuring point, connecting wires and cables. Depending on the conditions, protective installations can be powered from an alternating current network 0.4; 6 or 10 kV or from autonomous sources. When protecting multi-line pipelines laid in one corridor, several installations can be installed and several anode groundings can be constructed. However, taking into account that during interruptions in the operation of the protection system, due to the difference in natural potentials of the pipes connected by a blind jumper, powerful galvanic couples are formed, leading to intense corrosion, the connection of the pipes to the installation must be carried out through special joint protection units. These blocks not only disconnect the pipes from each other, but also allow you to set the optimal potential on each pipe. Converters powered by a 220 V industrial frequency network are mainly used as direct current sources for cathodic protection in VSCs. The output voltage of the converter is adjusted manually, by switching the taps of the transformer winding, or automatically, using controlled valves (thyristors). If cathodic protection installations operate under time-varying conditions, which may be caused by the influence of stray currents, changes in soil resistivity or other factors, then it is advisable to provide converters with automatic control of the output voltage. Automatic regulation can be carried out according to the potential of the protected structure (potentiostat converters) or according to the protection current (galvanostat converters).

3. Drainage protection installations

Electric drainage is the simplest type of active protection that does not require a current source, since the pipeline is electrically connected to the traction rails of the stray current source. The source of the protective current is the pipeline-rail potential difference, which arises as a result of the operation of electrified railway transport and the presence of a field of stray currents. The flow of drainage current creates the required potential shift in the underground pipeline. As a rule, fuses are used as a protective device, but maximum load circuit breakers with reset are also used, that is, they restore the drainage circuit after the current dangerous for the installation elements subsides. As a polarized element, valve blocks assembled from several avalanche silicon diodes connected in parallel are used. The current in the drainage circuit is regulated by changing the resistance in this circuit by switching active resistors. If the use of polarized electric drains is ineffective, then reinforced (forced) electric drains are used, which are a cathodic protection installation, the rails of an electrified railway are used as an anode grounding electrode. The current of forced drainage operating in cathodic protection mode should not exceed 100A, and its use should not lead to the appearance of positive rail potentials relative to the ground in order to prevent corrosion of rails and rail fastenings, as well as structures attached to them.

Electrical drainage protection can be connected to the rail network directly only to the middle points of track choke transformers through two to a third choke points. More frequent connection is allowed if a special connection is included in the drainage circuit protective device. A choke can be used as such a device, the total input resistance of which to the signal current of the trunk signaling system railways frequency 50 Hz is at least 5 ohms.

4. Galvanic protection installations

Galvanic protection installations (protector installations) are used for cathodic protection of underground metal structures in cases where the use of installations powered by external current sources is not economically feasible: the absence of power lines, the short length of the facility, etc.

Typically, protector installations are used for cathodic protection of the following underground structures:

• tanks and pipelines that do not have electrical contacts with adjacent extended communications;
• individual sections of pipelines that are not provided with a sufficient level of protection from converters;
• sections of pipelines electrically isolated from the main line by insulating connections;
• steel protective casings (cartridges), underground tanks and containers, steel supports and piles and other concentrated objects;
• the linear part of the main pipelines under construction before the commissioning of permanent cathodic protection installations.

Sufficiently effective protection with protective installations can be carried out in soils with a specific electrical resistivity of no more than 50 Ohms.

5. Installations with extended or distributed anodes.

As already noted, when using a traditional cathodic protection scheme, the distribution of the protective potential along the pipeline is uneven. The uneven distribution of the protective potential leads to both excessive protection near the drainage point, i.e. to unproductive energy consumption and to a reduction in the protective zone of the installation. This disadvantage can be avoided by using a circuit with extended or distributed anodes. The ECP technological scheme with distributed anodes makes it possible to increase the length of the protective zone compared to the cathodic protection scheme with concentrated anodes, and also ensures a more uniform distribution of the protective potential. When using technological scheme ZHZ with distributed anodes can use different layouts of anode grounding. The simplest is the scheme with anode groundings evenly installed along the gas pipeline. Adjustment of the protective potential is carried out by changing the anodic grounding current using an adjusting resistance or any other device that ensures a change in the current within the required limits. In the case of grounding from several grounding electrodes, the protective current can be adjusted by changing the number of connected grounding electrodes. IN general case the ground electrodes closest to the converter must have a higher contact resistance. Protective protection Electrochemical protection using protectors is based on the fact that due to the potential difference between the protector and the protected metal in an electrolyte environment, the metal is restored and the protector body dissolves. Since the bulk metal structures In the world it is made of iron; metals with a more negative electrode potential than iron can be used as a protector. There are three of them - zinc, aluminum and magnesium. The main difference between magnesium protectors is the largest potential difference between magnesium and steel, which has a beneficial effect on the radius of protective action, which ranges from 10 to 200 m, which allows the use of fewer magnesium protectors than zinc and aluminum. In addition, magnesium and magnesium alloys, unlike zinc and aluminum, do not have polarization, accompanied by a decrease in current output. This feature determines the main use of magnesium protectors for the protection of underground pipelines in soils with high resistivity

Pipelines running underground are subject to the destructive effects of corrosion. Pipeline corrosion affects metal pipes when conditions arise where metal atoms can become ionic.

In order for a neutral atom to become an ion, it is necessary to give up an electron, and this is possible if there is an anode that will accept it. This situation is possible when a potential difference occurs between individual sections of the pipe: one section is the anode, the other is the cathode.

Reasons for electrolytic reactions

There are several reasons for the formation of a potential difference (the magnitude of its value) in individual sections of the pipe:

• different soil compositions according to physical and chemical properties;
• metal heterogeneity;
• soil moisture;
• the value of the operating temperature of the transported substance;
• indicator of soil electrolyte acidity;
• the passage of an electric transport line that creates stray currents.

Important! Areas that require protection are determined at the design stage of the facility. All necessary structures are built in parallel with the laying of pipes.

As a result, two types of corrosion damage can occur:

• superficial, which does not lead to destruction of the pipeline;
• local, which results in the formation of shells, cracks, and cracking.

Types of corrosion protection

To protect pipes from destruction, pipeline corrosion protection is used.

There are two main methods of protection:

• passive, in which a protective shell is created around the pipes, completely separating them from the ground. Typically this is a coating made of bitumen, epoxy resin, or polymer tape;
• active, allowing you to control the electrochemical processes that occur at the points of contact between the pipe and the ground electrolyte.

The active method is divided into three types of protection:

• cathode;
• tread;
• drainage

Drainage protects pipelines from corrosion caused by stray currents. Such currents are diverted in the direction of the source that creates them or directly into the soil layer. Drainage can be earthen (grounding the anode zones of the pipeline), direct (disconnected from negative pole stray current source). Polarized and enhanced drainage is used less frequently.

Methods for organizing cathodic protection

Cathodic protection of a pipeline against corrosion is achieved if external electric field to organize cathodic polarization of the pipeline, and transfer the damage to the external anode, which will undergo destruction.

Cathode is divided into two types:

• galvanic with the use of protective anodes, for the manufacture of which alloys of magnesium, aluminum, and zinc are used;
• electric, which uses an external direct current source with a connection diagram: minus to the pipe, plus to the grounded anode.

The basis of the galvanic method of cathodic protection: the use of the property of metals to have different potentials when they are used in the form of an electrode. If the electrolyte contains two metals with different meaning potential, then the one with the least value will be destroyed.

The tread material is selected so that certain requirements are met:

• negative potential with a large value compared to the potential of the pipeline;
• significant efficiency;
• high specific current output;
• low anodic polarizability, so that oxide films do not form.

Pay attention! The highest efficiency is for anodes made of zinc and aluminum alloy, the lowest for magnesium.

To increase the efficiency and effectiveness of protection, protectors are immersed in an activator, which reduces the protector’s own corrosion and the amount of resistance to current spreading from the protector, and reduces anodic polarizability.

The protector protective installation consists of a protector, an activator, a conductor connecting the protector and the pipeline, and a point for monitoring and measuring electrical parameters.

Efficiency tread protection from corrosion of pipelines depends on the magnitude of soil resistivity. It works well if this indicator does not exceed 50 Ohm*m; with a higher value, the protection will be partial. To increase efficiency, tape protectors are used.

The limitation for the use of sacrificial protection is the electrical contact of the pipeline and adjacent extended communications.

Cathodic protection stations

More complex to organize, but the most effective is the electric one. To organize it, an external direct current source is constructed - a cathodic protection station. IN power station alternating current is converted to direct current.

Cathodic protection elements:

• anodic grounding;
• DC connection line;
• protective grounding;
• DC source;
• cathode terminal.

The electrical method is an analogue of the electrolysis process.

Under the influence of the external field of the current source, valence electrons move away from the anode grounding towards the current source and the pipe. The grounded anode is gradually destroyed. And near a pipeline from a direct current source, an incoming excess of free electrons leads to depolarization (like a cathode during electrolysis).

To prevent corrosive destruction of several pipes, several stations are constructed and an appropriate number of anodes are installed.

With cathodic protection of a pipeline, the positive pole of the direct current source (anode) is connected to a special anode grounding conductor, 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 electric field the movement of electrons from the anode grounding conductor to the protected structure begins. 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 after 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).

Rice. 2.26. Electrical drainage protection diagram

1 - pipeline; 2 - drainage cable; 3 - ammeter; 4 - rheostat; 5 - switch; 6 - valve element; 7 - fuse; 8 – signal relay; 9 – rail

Until now, when constructing long industrial pipelines, the most popular pipe material is steel. Possessing many remarkable properties, such as mechanical strength, the ability to function at high values ​​of internal pressure and temperature and resistance to seasonal weather changes, steel also has a serious drawback: a tendency to corrosion, leading to the destruction of the product and, accordingly, the inoperability of the entire system.

One of the methods of protection against this threat is electrochemical, including cathodic and anodic protection of pipelines; The features and types of cathodic protection will be discussed below.

Definition of electrochemical protection

Electrochemical protection of pipelines from corrosion is a process carried out under the influence of a constant electric field on a protected object made of metals or alloys. Since alternating current is usually available for operation, special rectifiers are used to convert it to direct current.

In the case of cathodic protection of pipelines, the protected object acquires a negative potential by applying an electromagnetic field to it, that is, it becomes a cathode.

Accordingly, if a section of pipe protected from corrosion becomes a “minus”, then the grounding connected to it becomes a “plus” (i.e. an anode).

Anti-corrosion protection using this method is impossible without the presence of an electrolytic medium with good conductivity. In the case of underground pipelines, its function is performed by the soil. The contact of the electrodes is ensured by the use of elements made of metals and alloys that conduct electric current well.

During the process, a constant potential difference arises between the electrolyte medium (in this case, soil) and the element protected from corrosion, the value of which is controlled using high-voltage voltmeters.

Classification of electrochemical cathodic protection techniques

This method of preventing corrosion was proposed in the 20s of the 19th century and was initially used in shipbuilding: the copper hulls of ships were sheathed with anode protectors, which significantly reduced the rate of metal corrosion.

After the effectiveness of the new technology was established, the invention began to be actively used in other areas of industry. After some time it was recognized as one of the most effective ways protection of metals.

There are currently two main types of cathodic protection of pipelines against corrosion:

1. The easiest way: an external source is supplied to a metal product that requires protection from corrosion electric current. In this design, the part itself acquires a negative charge and becomes the cathode, while the role of the anode is performed by inert, design-independent electrodes.
2. Galvanic method. The part in need of protection comes into contact with a protective (tread) plate made of metals with high values ​​of negative electric potential: aluminum, magnesium, zinc and their alloys. In this case, both become anodes metal element, and the slow electrochemical destruction of the protector plate guarantees the maintenance of the required cathode current in the steel product. Through more or less for a long time, depending on the parameters of the plate, it dissolves completely.

Characteristics of the first method

This method of ECP of pipelines, due to its simplicity, is the most common. It is used to protect large structures and elements, in particular, underground and above-ground pipelines.

The technique helps to resist:

• pitting corrosion;
• corrosion due to the presence of stray currents in the area where the element is located;
• corrosion of intercrystal type stainless steel;
• cracking of brass elements due to increased stress.

Characteristics of the second method

This technology, unlike the first one, is intended, among other things, to protect small-sized products. The technique is most popular in the USA, while in Russian Federation rarely used. The reason is that to carry out galvanic electrochemical protection of pipelines, it is necessary to have an insulating coating on the product, and in Russia main pipelines are not treated in this way.

Features of ECP of pipelines

The main reason for pipeline failure (partial depressurization or complete destruction) individual elements) is metal corrosion. As a result of the formation of rust on the surface of the product, micro-tears, cavities and cracks appear on its surface, gradually leading to system failure. This problem is especially relevant for pipes that run underground and are constantly in contact with groundwater.

The principle of operation of cathodic protection of pipelines against corrosion involves the creation of an electrical potential difference and is implemented in the two ways described above.

After carrying out measurements on the ground, it was found that the required potential at which any corrosion process slows down is –0.85 V; for pipeline elements located under the earth layer, its natural value is –0.55 V.

In order to significantly slow down the processes of destruction of materials, it is necessary to reduce the cathode potential of the protected part by 0.3 V. If this is achieved, the corrosion rate of steel elements will not exceed 10 μm/year.

One of the most serious threats to metal products is stray currents, that is, electrical discharges penetrating into the ground due to the operation of grounding power lines (power lines), lightning rods, or movement on train rails. It is impossible to determine at what time and where they will appear.

The destructive effect of stray currents on steel structural elements appears when these parts have a positive electrical potential relative to the electrolytic medium (in the case of pipelines, soil). The cathodic technique imparts a negative potential to the protected product, as a result of which the risk of corrosion due to this factor is eliminated.

The optimal way to provide the circuit with electric current is to use an external energy source: it guarantees the supply of voltage sufficient to “break through” the soil resistivity.

Typically, overhead power transmission lines with powers of 6 and 10 kW act as such a source. If there are no power lines in the pipeline area, mobile generators operating on gas and diesel fuel should be used.

What is needed for cathodic electrochemical protection

To ensure a reduction in corrosion in pipeline areas, special devices called cathodic protection stations (CPS) are used.

These stations include the following elements:

• grounding acting as an anode;
• DC generator;
• control, measurement and process control point;
• connecting devices (wires and cables).

Cathodic protection stations quite effectively perform their main function, when connected to an independent generator or power line, simultaneously protecting several nearby sections of pipelines.

You can adjust the current parameters either manually (by replacing transformer windings) or in an automated mode (in the case where there are thyristors in the circuit).

Minerva-3000 is recognized as the most advanced cathodic protection station used in the Russian Federation (the SKZ project commissioned by Gazprom was created by French engineers). One such station makes it possible to ensure the safety of about 30 km of underground pipeline.

Pros of "Minerva-3000":

• high level power;
• the ability to quickly recover after overloads occur (no more than 15 seconds);
• equipped with the digital control units of the system necessary for monitoring operating modes;
• absolutely sealed critical components;
• the ability to control the operation of the installation remotely when connecting special equipment.

The second most popular SKZ in Russia is “ASKG-TM” (adaptive telemechanized cathodic protection station). The power of such stations is less than those mentioned above (from 1 to 5 kW), but their automatic control capabilities are improved due to the presence of a telemetry complex with remote control in the original configuration.

Both stations require a 220 V voltage source, are controlled using GPRS modules and are characterized by fairly modest dimensions - 500x400x900 mm and a weight of 50 kg. The service life of the SCP is from 20 years.

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, the theory provided was tested on the ship 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 metal structures (pipelines, car elements, etc.) is recognized as the most effective and widely used.

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

1. 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.
2. 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 has found widespread use around 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 of metals in Russia is due to the fact that many pipelines in our country do not have a special coating applied, and this is a 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 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 rusting of car components 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.

Under vulnerabilities vehicles that can be protected using the cathodic method understand:

• 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 may have different size. Their number also differs depending on where in the car they are mounted, as well as on the geometric parameters of the 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 comparatively in simple ways. The most basic one is to connect the positive wire of the car battery to a regular 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. Underground communications are especially susceptible to rust. 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 begins active formation corrosive galvanic components.

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 of metal structures is in the range of -0.55 to -0.6 volts.

This means that for effective protection pipelines, it is required 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 negative influence 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 the 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 of (specific) soil, the appearance of stray currents and other factors that affect negative impact on the quality of protection, and automatically adjust the operation of the VCS. But in systems where the protective current and the resistance value in its circuit remain unchanged, it is better to use settings with manual adjustment of the 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 – 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 of its 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 made of titanium alloys with an additional layer metal protection or with a non-metallic electrically conductive coating.

By adjusting the external current supplied to reinforced concrete structure, select the potential of the reinforcement.

Polarization is considered an indispensable technique for the protection of permanent structures located on the continental shelf, in the gas and oil fields. The original protective coatings on such objects cannot be restored (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 industrial wastewater and other aggressive electrolytes. Polarization in this case increases the time of maintenance-free use of these structures by 2–3 times.