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 maintenance and qualifications of their service 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 from the RMS via 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 it ourselves metal pipeline, 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 removed. 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 do not impose high demands on VCS. 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 interferes with the normal operation of a large number of 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 SKZ does this is assessed by the efficiency factor. 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

The consumer properties of such a device as SKZ include the following:

1. Dimensions, weight And strength. There is probably no need to say that the smaller and lighter the station, the less 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:

availability of all necessary indicators and measuring instruments, availability of remote control and monitoring 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 specified 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 gas pipelines from corrosion is divided into passive and active.

Passive protection. This type of protection involves insulating the gas pipeline. In this case, a coating based on bitumen-polymer, bitumen-mineral, polymer, ethylene and bitumen-rubber mastics is used. The anti-corrosion coating must have sufficient mechanical strength, plasticity, good adhesion to pipe metal, have dielectric properties, and it should not be destroyed by biological influences and contain components that cause corrosion of pipe metal.

One of the widely used methods of passive protection is insulation with adhesive polymer tapes with a width of 400, 450, 500 mm or upon request. According to GOST 20477-86, depending on the thickness of the tape, its base can be grade A or B.

Active protection. Active protection methods (cathodic, sacrificial, electrical drainage) mainly come down to creating an electrical regime for a gas pipeline in which corrosion of the pipeline stops.

Rice. 1. Cathodic protection scheme:

/ - drainage cable; 2 - source direct current; 3 - connection cable; 4 — ground electrode (anode); 5 - gas pipeline; b — drainage point

Cathodic protection. With cathodic protection (Fig. 1), an external power source is used to create a galvanic pair 2. In this case, the cathode is gas pipeline 5, connected at the drainage point 6 via a drain cable to the negative electrode of the power source; the anode is a metal rod 4, buried in the ground below its freezing zone.

One cathode station provides protection for gas pipelines up to 1,000 m long.

Protective (electrode) protection. With sacrificial protection, a section of the gas pipeline is converted into a cathode not due to the power source, but through the use of a protector. The latter is connected by a conductor to the gas pipeline and forms a galvanic couple with it, in which the gas pipeline is the cathode and the protector is the anode. A metal with a more negative potential than iron is used as a protector.

The operating principle of the tread protection is shown in Fig. 2. Current from the protector 3 through the ground it enters the gas pipeline 6, and then along an insulated connecting cable to the protector. The protector will collapse when current drains from it, protecting the gas pipeline.

The coverage area of ​​the protector installation is approximately 70 m. The main purpose of the protector installations is to complement drainage or cathodic protection on remote gas pipelines for complete removal of positive potentials.

Rice. 2. Scheme of tread (electrode) protection:

/ - check Point; 2 — connecting cables; 3 — protector (electrode);

4 — filler (salt + clay + water); 5 — paths of protective current movement in the ground; 6 — gas pipeline

Electrical drainage protection. With electrical drainage protection, the current is diverted from the anode zone of the gas pipeline to the source (rail or negative bus of the traction substation). The protection zone is about 5 km.

Three types of drainage are used: direct (simple), polarized and reinforced.

Direct drainage is characterized by bilateral conductivity (Fig. 3). The drain cable is connected only to the minus bus. The main disadvantage is the emergence of a positive potential on the gas pipeline when the butt joints of the rails are broken, therefore, despite their simplicity, these installations are not used in urban gas pipelines.

Polarized drainage has one-way conductivity from the gas pipeline to the source. When a positive potential appears on the rails, the drainage cable is automatically switched off, so it can be connected to the rails.

Rice. 3. Scheme of direct (simple) drainage:

/ - protected gas pipeline; 2 — adjusting rheostat; 3 - ammeter; 4 — fuse; 5 — negative busbar (suction cable)

Enhanced drainage is used when the gas pipeline remains at a positive or alternating potential with respect to the ground, and the rail potential at the current drainage point is higher than the gas pipeline potential. In enhanced drainage, an EMF source is additionally included in the circuit, which makes it possible to increase the drainage current. The grounding in this case is the rails.

Insulating flange connections and inserts. They are used in addition to electrochemical protection devices and allow the gas pipeline to be divided into separate sections, reducing the conductivity and strength of the current flowing through the gas pipeline. Electrical insulating connections (EIS) are gaskets between rubber or hard rubber flanges. Inserts from polyethylene pipes used to isolate various underground structures from each other. Installing an EIS leads to a reduction in energy costs by eliminating current flow losses to adjacent communications. EIS is installed at inputs to consumers, underground and overwater passages of gas pipelines through obstacles, as well as at inputs of gas pipelines to gas distribution stations, hydraulic fracturing and GRU.

Electrical jumpers. Electrical jumpers are installed on adjacent metal structures in the case when one structure has positive potentials (anode zone), and the other has negative potentials (cathode zone), while negative potentials are established at both structures. Jumpers are used when laying gas pipelines of various pressures along one street.

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

1 General information about cathodic protection

Such protection of metals was first described in the 1820s by Humphry Davy. Based on his reports, in 1824, on the ship HMS Samarang, the theory provided was tested. Iron anode protectors were installed on the copper plating of the ship, which significantly reduced the rate of rusting of copper. The technique began to be developed, and today the cathode of all kinds of metal structures (pipelines, car parts, 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 metal structure contacts 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. Happens over time complete destruction"protector" metal. Galvanic polarization is effective for structures that have an insulating layer, as well as for relatively small metal products.

The first technique found wide application Worldwide. It is quite simple and economically feasible, making it possible to protect metal from general corrosion and from many of its varieties - intergranular corrosion of “stainless steel”, pitting, cracking of brass products due to the stresses under which they operate.

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

2 How does standard cathodic polarization of metals work?

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

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

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

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

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

Usually, cathode technology It is used in conjunction with the application of special protective materials to the external surface of products protected from corrosion.

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

3 Cathodic protection of vehicle elements

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

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

In the described vehicle protection, the cathode is 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.

Vulnerable spots of a vehicle that can be protected using the cathodic method are understood as:

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

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

Protective plates may have different size. Their number also differs depending on where in the car they are mounted, as well as on what geometric parameters has an electrode. In practice, the fewer plates you need, the more larger size has an electrode.

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

4 Protection of pipelines using cathodic polarization method

Depressurization of pipelines of various purposes occurs in many cases due to their corrosion destruction caused by the appearance of ruptures, cracks and cavities. 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 of active formation of corrosive galvanic components begins.

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

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

This means that to effectively protect pipelines, it is necessary to “move” the corrosion potential into negative side at 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 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 VCSs operate under time-varying conditions, it is recommended to operate stations with automatic adjustment voltage.

They themselves monitor the resistance indicators of (specific) soil, the appearance of stray currents and other factors that have an impact. 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– a powerful system developed by French and Russian engineers for Gazprom facilities. One Minerva is enough to reliably protect up to 30 kilometers of pipelines from rust. The station has the following main advantages:

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

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

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

Benefits of use ASKG-TM:

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

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

7 What objects can be protected using cathodic polarization?

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

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

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

By 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 wastewater. industrial waters and other aggressive electrolytes. Polarization in this case increases the time of maintenance-free use of these structures by 2–3 times.

When laying an insulated pipeline in a trench and then backfilling it, the insulating coating may be damaged, and during the operation of the pipeline it gradually ages (loses its dielectric properties, water resistance, adhesion). Therefore, for all installation methods, except above-ground, pipelines are subject to comprehensive corrosion protection protective coatings and means of electrochemical protection (ECP) regardless of the corrosive activity of the soil.

ECP means include cathodic, sacrificial and electrical drainage protection.

Protection against soil corrosion is carried out by cathodic polarization of pipelines. If cathodic polarization is carried out using an external direct current source, then such protection is called cathodic, but if polarization is carried out by connecting the protected pipeline to a metal that has a more negative potential, then such protection is called sacrificial.

Cathodic protection

The schematic diagram of cathodic protection is shown in the figure.

The source of direct current is the cathodic protection station 3, where, with the help of rectifiers, the alternating current from the along-route power line 1, entering through the transformer point 2, is converted into direct current.

The negative pole of the source is connected to the protected pipeline 6 using connecting wire 4, and the positive pole is connected to the anode grounding 5. When the current source is turned on electrical circuit closes through the soil electrolyte.

Schematic diagram of cathodic protection

1 - power lines; 2 - transformer point; 3 — cathodic protection station; 4 - connecting wire; 5 - anodic grounding; 6 - pipeline

The operating principle of cathodic protection is as follows. Under the influence of the applied electric field source, the movement of half-free valence electrons begins in the direction “anode grounding - current source - protected structure”. Losing electrons, the anodic grounding metal atoms pass in the form of ion atoms into the electrolyte solution, i.e. the anodic grounding is destroyed. Ion atoms undergo hydration and are removed into the depth of the solution. At the protected structure, due to the operation of the direct current source, an excess of free electrons is observed, i.e. conditions are created for the occurrence of oxygen and hydrogen depolarization reactions characteristic of the cathode.

Underground communications of oil depots are protected by cathode installations with various types anode grounding. The required protective current strength of the cathode installation is determined by the formula

J dr =j 3 ·F 3 ·K 0

where j 3 is the required value of the protective current density; F 3 - total contact surface of underground structures with the ground; K 0 is the coefficient of exposure of communications, the value of which is determined depending on the transition resistance of the insulating coating R nep and the electrical resistivity of the soil r g according to the graph shown in the figure below.

The required value of the protective current density is selected depending on the characteristics of the soil at the oil depot site in accordance with the table below.

Tread protection

The principle of operation of the tread protection is similar to the operation of a galvanic cell.

Two electrodes: pipeline 1 and protector 2, made of a more electronegative metal than steel, are lowered into the soil electrolyte and connected by wire 3. Since the protector material is more electronegative, under the influence of a potential difference, a directed movement of electrons occurs from the protector to the pipeline along the conductor 3. At the same time, the ion atoms of the protector material go into solution, which leads to its destruction. The current strength is controlled using control and measuring column 4.

Dependence of the coefficients of exposure of underground pipelines on the transition resistance of the insulating coating for soil resistivity, Ohm-m

1 — 100; 2 — 50; 3 — 30; 4 — 10; 5 — 5

Dependence of protective current density on soil characteristics

Schematic diagram of tread protection

1 - pipeline; 2 — protector; 3 - connecting wire; 4 - control and measuring column

Thus, metal destruction still occurs. But not the pipeline, but the protector.

Theoretically, to protect steel structures from corrosion, all metals located in the electrochemical voltage series to the left of iron can be used, since they are more electronegative. In practice, protectors are made only from materials that meet the following requirements:

• the potential difference between the tread material and iron (steel) should be as large as possible;
• the current obtained by electrochemical dissolution of a unit of mass of the protector (current output) must be maximum;
• the ratio of the tread mass used to create protective current to the total loss of tread mass (utilization factor) should be the greatest.

These requirements are best met by alloys based on magnesium, zinc and aluminum.

Tread protection is carried out with concentrated and extended protectors. In the first case, the electrical resistivity of the soil should be no more than 50 Ohm-m, in the second - no more than 500 Ohm-m.

Electrical drainage protection of pipelines

A method of protecting pipelines from destruction by stray currents, providing for their removal (drainage) from the protected structure to a structure that is a source of stray currents or special grounding, is called electrical drainage protection.

Direct, polarized and reinforced drainage are used.

Schematic diagrams of electrical drainage protection

a - direct drainage; b — polarized drainage; c - enhanced drainage

Direct electric drainage is drainage device bilateral conductivity. The direct electrical drainage circuit includes: rheostat K, switch K, fuse Pr and signal relay C. The current strength in the pipeline-rail circuit* is regulated by the rheostat. If the current value exceeds the permissible value, the fuse will burn out and current will flow through the relay winding, which, when turned on, turns on a sound or light signal.

Direct electrical drainage is used in cases where the potential of the pipeline is constantly higher than the potential of the rail network, where stray currents are discharged. Otherwise, the drainage will turn into a channel for stray currents to flow into the pipeline.

Polarized electrical drainage is a drainage device that has one-way conductivity. Polarized drainage differs from direct drainage by the presence of a one-way conductivity element (valve element) VE. With polarized drainage, current flows only from the pipeline to the rail, which eliminates the flow of stray currents onto the pipeline through the drainage wire.

Enhanced drainage is used in cases where it is necessary not only to remove stray currents from the pipeline, but also to provide the required protective potential on it. Enhanced drainage is a conventional cathode station, connected with the negative pole to the protected structure, and the positive pole not to the anode grounding, but to the rails of electrified transport.

Due to this connection scheme, the following is ensured: firstly, polarized drainage (due to the operation of valve elements in the SCP circuit), and secondly, the cathode station maintains the necessary protective potential of the pipeline.

After the pipeline is put into operation, the operating parameters of the corrosion protection system are adjusted. If necessary, taking into account the actual state of affairs, additional cathodic and drainage protection stations, as well as protector installations, can be put into operation.

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 comprehensively, together with treatment steel structure insulating compounds.

This article examines the electrochemical protection of pipelines and studies its cathodic subtype 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). In this method, both metals perform the anode function, while the electrochemical dissolution of the metal of the tread plate ensures flow through the protected structure the required minimum cathode current. 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 the treatment of pipelines with a special insulating coating, which is a prerequisite for galvanic electrical protection.

Note that without the corrosion of steel increases significantly under the influence of groundwater, which is especially typical for spring period 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, which 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). The technology, in this case, makes it possible 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 cathodic 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 resistivity soil.

Anti-corrosion protection is used as a current source air lines power transmission lines with a capacity of 6 and 10 kW, but 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.