Protective current. Cathodic protection against corrosion of pipelines: equipment, operating principle

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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. During operation 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.

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-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 long time work on 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 method of power supply to 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.

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

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

Principle of operation

The objects of the electrochemical corrosion process are the environment, the metal and the interfaces between them. The medium, which is usually moist soil or water, has good electrical conductivity. An electrochemical reaction occurs at the interface between it and the metal structure. If the current is positive (anode electrode), iron ions move into the surrounding solution, which leads to a loss of metal mass. The reaction causes corrosion. With a negative current (cathode electrode), these losses do not exist, since electrons pass into the solution. The method is used in electroplating for applying non-ferrous metal coatings to steel.

Cathodic corrosion protection occurs when a negative potential is applied to an 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 signal is applied to the protected object. Cathodic-anodic protection leads to active destruction of only the anode electrode from corrosion. Therefore, it should be changed periodically.

Negative effects of electrochemical corrosion

Corrosion of structures can occur from the action of stray currents coming from other systems. They are useful for target objects, but cause significant damage to nearby structures. Stray currents can spread from the rails of electrified transport. They pass towards the substation and end up on the pipelines. When leaving them, anodic areas are formed, causing intense corrosion. For protection, electrical drainage is used - a special drainage of currents from the pipeline to their source. It is also possible here. To do this, you need to know the magnitude of stray currents, which is measured with special instruments.

Based on the results of electrical measurements, a method of protecting the gas pipeline is selected. A universal remedy is a passive method of preventing 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 DC source to them, and the plus to the anode electrodes buried nearby in the ground. The current will flow to the structure, protecting it from corrosion. In this way, 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 an electrolyte through which the current will flow 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 that require cathodic protection, stations are installed from which voltage is supplied to the objects. If they are placed 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 power of 1 to 10 kW.

Their energy parameters primarily depend on the following factors:

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

Traditionally, the cathodic protection converter is a transformer unit. Now it is being replaced by an inverter, 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 presented on the market in various versions. For specific needs, supporting Better conditions operation.

Current source parameters

For corrosion protection for iron, the protective potential is 0.44 V. In practice, it should be higher due to the influence of inclusions and the condition 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 increases to 10 mA/m2.

Cathodic protection is effective in combination with other methods, since less electricity is consumed. If there is a paint coating on the surface of the structure, only the places where it is damaged are protected by electrochemical means.

Features of cathodic protection

1. Power sources are stations or mobile generators.
2. The location of the anode grounding electrodes depends on the specifics of the pipelines. The placement method can be distributed or concentrated, and also located at different depths.
3. The anode material is selected with low solubility so that it lasts for 15 years.
4. The protective field potential for each pipeline is calculated. It is not regulated if there are no protective coatings on the structures.

Gazprom standard requirements for cathodic protection

• Valid throughout the entire service life of the protective equipment.
• Protection against atmospheric surges.
• Placement of the station in block boxes or in a stand-alone, vandal-proof design.
• Anodic grounding is selected in areas with minimal electrical resistance soil.
• The characteristics of the converter are selected taking into account the aging of the protective coating of the pipeline.

Tread 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 is the absence of an energy source. The protector takes on corrosion by dissolving in the electrically conductive environment.

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

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 corrosion products, efficiency increases.

The influence of the tread is limited. The range 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 not economically feasible. It is also unfavorable when used in acidic environments due to the high rate of dissolution of the anodes. Protectors are installed in water, in soil or in a neutral environment. Anodes are usually not made from pure metals. The dissolution of zinc occurs unevenly, magnesium corrodes too quickly, and a strong film of oxides forms on aluminum.

Protector materials

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

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

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

Joint sacrificial cathodic protection with anti-corrosion coatings increases its effectiveness.

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

Protection of car bodies from corrosion

If the coating is damaged, the thickness of the car body can decrease to 1 mm over 5 years, i.e., rust through. Restoring 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. Moreover, the effectiveness of protection increases with increasing area of ​​the anodes. If the anode is a road surface, a “tail” made of metallized rubber is used for contact with it. It is placed opposite the wheels to allow splashes to fall better. The "tail" is isolated from the body.

The plus of the 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 negative is connected to the body, in normal mode the LED glows barely noticeably. If it lights up brightly, there is a short circuit 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 grounding anode. During movement, the connection occurs through the “tail”.

Conclusion

Cathodic protection is a way to improve the operational reliability of underground pipelines and other structures. It should be taken into account negative impact on neighboring pipelines from the influence of stray currents.

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 gaps, as it does not cover 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. For this purpose, special protective compounds 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 current sources that are connected to the protected pipe and to 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 using protectors that 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.

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 internal pressures and temperatures, and resistance to seasonal changes weather, 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 against corrosion is a process carried out under the influence of 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 by applying electromagnetic field acquires a negative potential, 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 between the medium and the electrolyte (in in this case soil) and the element protected from corrosion, a constant potential difference arises, 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.

Once effectiveness has been established new technology, 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 of electric current is supplied to a metal product that requires protection from corrosion. 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 electrical potential: aluminum, magnesium, zinc and their alloys. In this case, both metal elements become anodes, and the slow electrochemical destruction of the protector plate ensures that the required cathode current is maintained 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 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 of 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 operating principle 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 transmission 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” resistivity soil.

Typically, such a source is air lines energy transmission with powers of 6 and 10 kW. 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 among the cathodic protection stations 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 power level;
• 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.

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 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 after 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).

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