Electrostatic cathodic protection. Options for cathodic protection of pipelines - advantages and disadvantages of methods

A. G. Semenov, general director, JV "Elkon", G. Chisinau; L. P. Sysa, leading engineer By ECP, NPK "Vector", G. Moscow

Introduction

Cathodic protection stations (CPS) are a necessary element of the electrochemical (or cathodic) protection system (ECP) of underground pipelines against corrosion. When choosing VCS, they most often proceed from the lowest cost, ease of service and the qualifications of their operating personnel. The quality of purchased equipment is usually difficult to assess. The authors propose to consider the technical parameters of the SCZ specified in the passports, which determine how well the main task of cathodic protection will be performed.

The authors did not pursue the goal of expressing themselves in strictly scientific language in defining concepts. In the process of communicating with the personnel of ECP services, we realized that it is necessary to help these people systematize the terms and, more importantly, give them an idea of ​​what is happening both in the power grid and in the VCP itself.

TaskECP

Cathodic protection is carried out when electric current flows from the SCZ through a closed electrical circuit formed by three resistances connected in series:

· soil resistance between the pipeline and the anode; I anode spreading resistance;

· pipeline insulation resistance.

The soil resistance between the pipe and the anode can vary widely depending on the composition and external conditions.

The anode is an important part of the ECP system, and serves as a consumable element, the dissolution of which ensures the very possibility of implementing ECP. Its resistance steadily increases during operation due to dissolution, a decrease in the effective working surface area and the formation of oxides.

Let's consider the metal pipeline itself, which is the protected element of the ECP. The outside of the metal pipe is covered with insulation, in which cracks form during operation due to the effects of mechanical vibrations, seasonal and daily temperature changes, etc. Moisture penetrates through the formed cracks in the hydro- and thermal insulation of the pipeline and contact of the pipe metal with the ground occurs, thus forming a galvanic couple that facilitates the removal of metal from the pipe. The more cracks and their sizes, the more metal is taken out. Thus, galvanic corrosion occurs in which a current of metal ions flows, i.e. electricity.

Since current is flowing, a great idea arose to take an external current source and turn it on to meet this very current, due to which metal is removed and corrosion occurs. But the question arises: what magnitude should this man-made current be given? It seems to be such that plus and minus give zero metal removal current. How to measure this current? The analysis showed that the voltage between the metal pipe and the ground, i.e. on both sides of the insulation, should be between -0.5 and -3.5 V (this voltage is called the protective potential).

TaskSKZ

The task of the SCP is not only to provide current in the ECP circuit, but also to maintain it so that the protective potential does not go beyond the accepted limits.

So, if the insulation is new and has not been damaged, then its resistance to electric current is high and a small current is needed to maintain the required potential. As insulation ages, its resistance decreases. Consequently, the required compensating current from the SCZ increases. It will increase even more if cracks appear in the insulation. The station must be able to measure the protective potential and change its output current accordingly. And nothing more, from the point of view of the ECP task, is required.

ModesworkSKZ

There can be four operating modes of the ECP:

· without stabilization of output current or voltage values;

· I output voltage stabilization;

· output current stabilization;

· I stabilization of protective potential.

Let us say right away that in the accepted range of changes in all influencing factors, the implementation of the ECP task is fully ensured only when using the fourth mode. Which is accepted as the standard for the VCS operating mode.

The potential sensor provides the station with information about the potential level. The station changes its current in the desired direction. Problems begin from the moment when it is necessary to install this potential sensor. You need to install it in a certain calculated location, you need to dig a trench for the connecting cable between the station and the sensor. Anyone who has laid any communications in the city knows what a hassle it is. Plus, the sensor requires periodic maintenance.

In conditions where problems arise with the operating mode with potential feedback, proceed as follows. When using the third mode, it is assumed that the state of the insulation in the short term changes little and its resistance remains practically stable. Therefore, it is enough to ensure the flow of stable current through a stable insulation resistance, and we obtain a stable protective potential. In the medium to long term, the necessary adjustments can be made by a specially trained lineman. The first and second modes are not applied to VCS high requirements. These stations are simple in design and, as a result, cheap, both to manufacture and to operate. Apparently this circumstance determines the use of such SCZ in ECP of objects located in conditions of low corrosive activity of the environment. If external conditions (insulation state, temperature, humidity, stray currents) change to the extent that an unacceptable mode is formed at the protected object, these stations cannot perform their task. To adjust their mode, the frequent presence of maintenance personnel is necessary, otherwise the ECP task is partially completed.

CharacteristicsSKZ

First of all, VCS must be selected based on the requirements set out in regulatory documents. And, probably, the most important thing in this case will be GOST R 51164-98. Appendix “I” of this document states that the efficiency of the station must be at least 70%. The level of industrial interference created by the RMS must not exceed the values ​​specified by GOST 16842, and the level of harmonics at the output must comply with GOST 9.602.

The SPS passport usually indicates: I rated output power;

Efficiency at rated output power.

Rated output power is the power that a station can deliver at rated load. Typically this load is 1 ohm. Efficiency is defined as the ratio of the rated output power to the active power consumed by the station in rated mode. And in this mode, the efficiency is the highest for any station. However, most VCSs do not operate in nominal mode. The power load factor ranges from 0.3 to 1.0. In this case, the real efficiency for most stations produced today will drop noticeably as the output power decreases. This is especially noticeable for transformer SSCs using thyristors as a regulating element. For transformerless (high-frequency) RMS, the drop in efficiency with a decrease in output power is significantly less.

A general view of the change in efficiency for VMS of different designs can be seen in the figure.

From Fig. It can be seen that if you use a station, for example, with a nominal efficiency of 70%, then be prepared for the fact that you have wasted another 30% of the electricity received from the network uselessly. And this is in the best case of rated output power.

With an output power of 0.7 of the rated value, you should be prepared for the fact that your electricity losses will be equal to the useful energy expended. Where is so much energy lost?

· ohmic (thermal) losses in the windings of transformers, chokes and in active circuit elements;

· energy costs for operation of the station control circuit;

· energy losses in the form of radio emission; loss of pulsation energy of the station output current on the load.

This energy is radiated into the ground from the anode and does not produce useful work. Therefore, it is so necessary to use stations with a low pulsation coefficient, otherwise expensive energy is wasted. Not only do electricity losses increase at high levels of pulsation and radio emission, but in addition, this uselessly dissipated energy creates interference with normal operation large quantity electronic equipment located in the surrounding area. The SKZ passport also indicates the required total power, let's try to understand this parameter. The SKZ takes energy from the power grid and does this in each unit of time with the same intensity that we allowed it to do with the adjustment knob on the station control panel. Naturally, you can take energy from the network with a power not exceeding the power of this very network. And if the voltage in the network changes sinusoidally, then our ability to take energy from the network changes sinusoidally 50 times per second. For example, at the moment when the network voltage passes through zero, no power can be taken from it. However, when the voltage sinusoid reaches its maximum, then at that moment our ability to take energy from the network is maximum. At any other time this opportunity is less. Thus, it turns out that at any moment in time the power of the network differs from its power at the next moment in time. These power values ​​are called instantaneous power in this moment time and such a concept is difficult to operate. Therefore, we agreed on the concept of so-called effective power, which is determined from an imaginary process in which a network with a sinusoidal voltage change is replaced by a network with a constant voltage. When we calculated the value of this constant voltage for our electrical networks, it turned out to be 220 V - it was called the effective voltage. And the maximum value of the voltage sinusoid was called the amplitude voltage, and it is equal to 320 V. By analogy with voltage, the concept of effective current value was introduced. The product of the effective voltage value and the effective current value is called the total power consumption, and its value is indicated in the RMS passport.

And the full power in the VCS itself is not fully used, because it contains various reactive elements that do not waste energy, but use it as if to create conditions for the rest of the energy to pass into the load, and then return this tuning energy back to the network. This returned energy is called reactive energy. The energy that is transferred to the load is active energy. The parameter that indicates the relationship between the active energy that must be transferred to the load and the total energy supplied to the VMS is called the power factor and is indicated in the station passport. And if we coordinate our capabilities with the capabilities of the supply network, i.e. synchronously with the sinusoidal change in the network voltage, we take power from it, then this case is called ideal and the power factor of the VMS operating with the network in this way will be equal to unity.

The station must transfer active energy as efficiently as possible to create a protective potential. The efficiency with which the VHC does this is assessed by the coefficient useful action. How much energy it spends depends on the method of energy transmission and the operating mode. Without going into this extensive field for discussion, we will only say that transformer and transformer-thyristor SSCs have reached their limit of improvement. They don't have the resources to improve the quality of their work. The future belongs to high-frequency VMS, which are becoming more reliable and easier to maintain every year. In terms of efficiency and quality of their work, they already surpass their predecessors and have a large reserve for improvement.

Consumerproperties

TO consumer properties Such a device as an SCS can include the following:

1. Dimensions, weight And strength. There is probably no need to say that the smaller and lighter the station, the lower the costs for its transportation and installation, both during installation and repair.

2. Maintainability. The ability to quickly replace a station or assembly on site is very important. With subsequent repairs in the laboratory, i.e. modular principle of construction of VCS.

3. Convenience V service. Ease of maintenance, in addition to ease of transportation and repair, is determined, in our opinion, by the following:

the presence of all necessary indicators and measuring instruments, the ability to remotely control and monitor the operating mode of the VCS.

conclusions

Based on the above, several conclusions and recommendations can be made:

1. Transformer and thyristor-transformer stations are hopelessly outdated in all respects and do not meet modern requirements, especially in the field of energy saving.

2. A modern station must have:

· high efficiency over the entire load range;

· power factor (cos I) not lower than 0.75 over the entire load range;

· output voltage ripple factor no more than 2%;

· current and voltage regulation range from 0 to 100%;

· lightweight, durable and small-sized body;

· modular construction principle, i.e. have high maintainability;

· I energy efficiency.

Other requirements for cathodic protection stations, such as protection against overloads and short circuits; automatic maintenance of a given load current - and other requirements are generally accepted and mandatory for all VCS.

In conclusion, we offer consumers a table comparing the parameters of the main cathodic protection stations produced and currently in use. For convenience, the table shows stations of the same power, although many manufacturers can offer a whole range of produced stations.

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

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

Contents of the article

Types of cathodic protection

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

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

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

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

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

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

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

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

The essence of technology

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

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

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

Features of cathodic protection of pipelines

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

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

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

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

Cathodic protection is the most effective method of protecting underground pipelines from stray currents. The concept of stray currents refers to an electric charge that enters the ground as a result of the operation of grounding points of power lines, lightning rods, or the movement of trains along railway lines. It is impossible to find out the exact time and place of the appearance of stray currents.

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

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

Overhead power lines with a power of 6 and 10 kW are used as a current source for anti-corrosion protection; if there are no power lines on the territory, mobile generators running on gas and diesel fuel can be used.

Detailed overview of cathodic corrosion protection technology (video)

Cathodic protection equipment

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

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

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

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

The advantages of the installation include:

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

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

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

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 (both in physical properties and chemical composition), areas with different electrode potentials appear, which causes the formation of galvanic corrosion. The most important types of 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 rarely causes damage, while pitting corrosion causes the greatest number of damage. The corrosive situation in which a metal pipeline is located in the ground depends on a large number of factors related to soil and climatic conditions, route features, operating conditions. These factors include:

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

The most powerful negative manifestation of stray currents in the ground, caused by electrified DC rail transport, is 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;
• specific electrical resistance 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. Different kinds coatings have different diffusion permeability and therefore provide different insulation of the pipe from the 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 cannot be implemented full protection pipeline against corrosion, at the same time active protection associated with control is applied electrochemical processes, flowing at the boundary of the pipe metal and the ground electrolyte. This type of protection is called comprehensive protection.

An 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 a region of higher negative values than natural potential. It was experimentally established that the value of 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 equal to - 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 by 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 of this difference is located 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 source of stray currents. 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 connections are allowed if a special protective device is included in the drainage circuit. 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.

Enough effective protection Protector 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 non-productive energy consumption, and to a reduction protective zone installations. 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 the ZHZ technological scheme with distributed anodes, various layouts of anode grounding can be used. 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, the ground electrodes closest to the converter should 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 of metal structures in the world are 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

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, 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.

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 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 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 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 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.

Pipelines are by far the most common means of transporting energy carriers. Their obvious drawback is their susceptibility to rust. For this purpose, cathodic protection of main pipelines from corrosion is performed. What is its principle of operation?

Causes of corrosion

Networks of pipelines for life support systems are distributed throughout Russia. With their help, gas, water, petroleum products and oil are efficiently transported. Not long ago, a pipeline was laid to transport ammonia. Most types of pipelines are made of metal, and their main enemy is corrosion, of which there are many types.

The reasons for the formation of rust on metal surfaces are based on the properties of the environment, both external and internal corrosion of pipelines. The risk of corrosion for internal surfaces is based on:

1. Interaction with water.
2. The presence of alkalis, salts or acids in the water.

Such circumstances may arise on main water supply systems, hot water supply (DHW), steam and heating systems. An equally important factor is the method of laying the pipeline: above ground or underground. The first one is easier to maintain and eliminate the causes of rust formation compared to the second one.

With the pipe-to-pipe installation method, the risk of corrosion is low. When directly installing a pipeline outdoors, rust may form due to interaction with the atmosphere, which also leads to a change in design.

Pipelines located underground, including steam and hot water, are most vulnerable to corrosion. The question arises about the susceptibility to corrosion of pipes located at the bottom of water sources, but only a small part of the pipelines are located in these places.

According to their purpose, pipelines with a risk of corrosion are divided into:

• main lines;
• fishing;
• for heating and life support systems;
• For waste water from industrial enterprises.

Susceptibility to corrosion of main pipeline networks

Corrosion of pipelines of this type is the most well studied, and their protection from external factors is determined by standard requirements. Regulatory documents discuss methods of protection, and not the reasons for the formation of rust.

It is equally important to take into account that in this case only external corrosion is considered, to which the outer section of the pipeline is susceptible, since inert gases pass inside the pipeline. In this case, contact of the metal with the atmosphere is not so dangerous.

For protection against corrosion, according to GOST, several sections of the pipeline are considered: increased and high danger, as well as corrosion-hazardous.

Impact of negative factors from the atmosphere for areas increased danger or types of corrosion:

1. Stray currents arise from direct current sources.
2. Exposure to microorganisms.
3. The created stress provokes cracking of the metal.
4. Waste storage.
5. Salty soils.
6. The temperature of the transported substance is above 300 °C.
7. Carbon dioxide corrosion of an oil pipeline.

An installer for protecting underground pipelines from corrosion must know the design of the pipeline and the requirements of SNiP.

Electrochemical corrosion from soil

Due to the difference in voltages formed in individual sections of pipelines, an electron flow occurs. The process of rust formation occurs according to the electrochemical principle. Based on this effect, part of the metal in the anodic zones cracks and flows into the base of the soil. After interaction with the electrolyte, corrosion forms.

One of the significant criteria for ensuring protection against negative manifestations is the length of the line. Along the way you come across soils with different compositions and characteristics. All this contributes to the emergence of a voltage difference between parts of the laid pipelines. The mains have good conductivity, so the formation of galvanic couples with a fairly large extent occurs.

An increase in the rate of pipeline corrosion is provoked by a high electron flux density. The depth of the lines is no less important, since it retains a significant percentage of humidity and the temperature is not allowed to fall below the “0” mark. Mill scale also remains on the surface of the pipes after processing, and this affects the appearance of rust.

Through research, a direct relationship has been established between the depth and area of ​​rust formed on the metal. This is based on the fact that metal with a larger surface area is most vulnerable to external negative manifestations. Special cases include the occurrence of significantly smaller amounts of destruction on steel structures under the influence of the electrochemical process.

The aggressiveness of soils to metal is, first of all, determined by their own structural component, humidity, resistance, alkali saturation, air permeability and other factors. The installer for the protection of underground pipelines from corrosion must be familiar with the pipeline construction project.

Corrosion under the influence of stray currents

Rust can arise from an alternating and constant flow of electrons:

• Rust formation under the influence of constant current. Stray currents are currents found in the soil and in structural elements located underground. Their origin is anthropogenic. They arise as a result of the operation of technical devices of direct current, spreading from buildings or structures. They can be welding inverters, cathode protection systems and other devices. The current tends to follow the path of least resistance, as a result, with existing pipelines in the ground, it will be much easier for the current to pass through the metal. The anode is the section of the pipeline from which the stray current exits to the soil surface. The part of the pipeline into which the current enters acts as a cathode. On the described anodic surfaces, currents have an increased density, so it is in these places that significant corrosion spots form. The corrosion rate is not limited and can be up to 20 mm per year.
• Rust formation under the influence of alternating current. When located near power lines with network voltages above 110 kV, as well as in parallel arrangement of pipelines, corrosion occurs under the influence of alternating currents, including corrosion under the insulation of pipelines.

Stress Corrosion Cracking

If a metal surface is simultaneously exposed to external negative factors and high voltage from power lines, which creates tensile forces, then rust formation occurs. According to the research carried out, the new hydrogen-corrosion theory gained its place.

Small cracks are formed when the pipe is saturated with hydrogen, which then ensures an increase in pressure from the inside to levels higher than the required equivalent of the bond of atoms and crystals.

Under the influence of proton diffusion, hydrogenation of the surface layer occurs under the influence of hydrolysis at elevated levels cathodic protection and simultaneous exposure to inorganic compounds.

After the crack opens, the rusting process of the metal accelerates, which is provided by the ground electrolyte. As a result, under the influence of mechanical influences, the metal undergoes slow destruction.

Corrosion due to microorganisms

Microbiological corrosion is the process of rust formation on a pipeline under the influence of living microorganisms. These can be algae, fungi, bacteria, including protozoa. It has been established that the proliferation of bacteria most significantly influences this process. To maintain the vital activity of microorganisms, it is necessary to create conditions, namely nitrogen, humidity, water and salts. Also the conditions are:

1. Temperature and humidity indicators.
2. Pressure.
3. Availability of lighting.
4. Oxygen.

Organisms that produce acidic conditions can also cause corrosion. Under their influence, cavities appear on the surface, which are black in color and have an unpleasant odor of hydrogen sulfide. Sulfate-containing bacteria are present in virtually all soils, but the rate of corrosion increases as their numbers increase.

What is electrochemical protection

Electrochemical protection of pipelines against corrosion is a set of measures aimed at preventing the development of corrosion under the influence of an electric field. Specialized rectifiers are used to convert direct current.

Protection against corrosion is carried out by creating an electromagnetic field, as a result of which a negative potential is acquired or the area acts as a cathode. That is, the segment steel pipelines, protected from rust formation, acquires a negative charge, and grounding acquires a positive charge.

Cathodic protection of pipelines against corrosion is accompanied by electrolytic protection with sufficient conductivity of the medium. This function is performed by soil when laying metal underground highways. Contacting of the electrodes is carried out through conductive elements.

The indicator for determining corrosion indicators is a high-voltage voltmeter or corrosion gauge. Using this device, the indicator between the electrolyte and the soil is monitored, specifically for this case.

How is electrochemical protection classified?

Corrosion and protection of main pipelines and tanks from it are controlled in two ways:

• A current source is connected to the metal surface. This area acquires a negative charge, that is, it acts as a cathode. Anodes are inert electrodes that have nothing to do with design. This method is considered the most common, and electrochemical corrosion does not occur. This technique is aimed at preventing the following types of corrosion: pitting, due to the presence of stray currents, crystalline type of stainless steel, as well as cracking of brass elements.
• Galvanic method. Protection of main pipelines or sacrificial protection is carried out by metal plates with high levels of negative charges, made of aluminum, zinc, magnesium or their alloys. Anodes are two elements, so-called inhibitors, while the slow destruction of the protector helps maintain the cathode current in the product. Protective protection is used extremely rarely. ECP is performed on the insulating coating of pipelines.

About the features of electrochemical protection

The main cause of pipeline destruction is the result of corrosion of metal surfaces. After the formation of rust, cracks, ruptures, and cavities form, which gradually increase in size and contribute to the rupture of the pipeline. This phenomenon occurs more often near highways laid underground or in contact with groundwater.

The principle of cathodic protection is the creation of a voltage difference and the action of the two methods described above. After carrying out measuring operations directly at the location of the pipeline, it was found that the required potential to help slow down the destruction process should be 0.85V, and for underground elements this value is 0.55V.

To slow down the corrosion rate, the cathode voltage should be reduced by 0.3V. In this situation, the corrosion rate will not exceed 10 microns/year, and this will significantly extend the service life of technical devices.

One of significant problems– this is the presence of stray currents in the soil. Such currents arise from the grounding of buildings, structures, rail tracks and other devices. Moreover, it is impossible to make an accurate assessment of where they may appear.

To create a destructive effect, it is enough to charge steel pipelines with a positive potential in relation to the electrolytic environment, these include pipelines laid in the ground.

In order to provide the circuit with current, it is necessary to supply an external voltage, the parameters of which will be sufficient to break through the resistance of the soil foundation.

As a rule, such sources are power lines with power ratings from 6 to 10 kW. If electric current cannot be supplied, then diesel or gas generators. The installer for the protection of underground pipelines from corrosion must be familiar with the design solutions before performing work.

Cathodic protection

To reduce the percentage of rust on the surface of pipes, electrode protection stations are used:

1. Anode, made in the form of grounding conductors.
2. Converters of constant electron flows.
3. Equipment for process control and monitoring of this process.
4. Cable and wire connections.

Cathodic protection stations are quite effective; when directly connected to a power line or generator, they provide an inhibitory effect of currents. This ensures protection of several sections of the pipeline simultaneously. Parameters can be adjusted manually or automatically. In the first case, transformer windings are used, and in the second, thyristors are used.

The most common in Russia is the high-tech installation - Minevra -3000. Its power is sufficient to protect 30,000 m of highways.

Advantages of the technical device:

• high power characteristics;
• updating the operating mode after overloads in a quarter of a minute;
• using digital regulation, operating parameters are monitored;
• tightness of highly critical connections;
• connecting the device to remote process control.

ASKG-TM are also used, although their power is low, their equipment with a telemetry complex or remote control allows them to be no less popular.

A diagram of the insulation main of the water supply or gas pipeline must be available at the work site.

Video: cathodic protection against corrosion - what is it and how is it performed?

Corrosion protection by installing drainage

The corrosion protection installer for underground pipelines must be familiar with the drainage system. Such protection against the formation of rust of pipelines from stray currents is carried out by a drainage device necessary to divert these currents to another section of the earth. There are several drainage options.

Types of execution:

1. Executed underground.
2. Straight.
3. With polarities.
4. Reinforced.

When carrying out earthen drainage, electrodes are installed in the anode zones. To ensure a straight drainage line, an electrical jumper is made connecting the pipeline to the negative pole of current sources, for example, grounding from a residential building.

Polarized drainage has one-way conductivity, that is, when a positive charge appears on the ground loop, it automatically turns off. Enhanced drainage operates from a current converter, additionally connected to the electrical circuit, and this improves the removal of stray currents from the main line.

The increase for pipeline corrosion is carried out by calculation, according to the RD.

In addition, inhibitor protection is used, that is, a special composition is used on the pipes to protect against aggressive environments. Standstill corrosion occurs when boiler equipment is idle for a long time; to prevent this from happening, equipment maintenance is necessary.

An installer for the protection of underground pipelines from corrosion must have knowledge and skills, be trained in the Rules and periodically undergo a medical examination and pass exams in the presence of an inspector from Rostechnadzor.