Cathodic protection of pipelines against corrosion, scheme, principle of operation and video. Cathodic corrosion protection. Principle of operation, basic concepts

BUT. G. Semenov, general director, joint venture "Elkon", G. Chisinau; L. P. Sysa, leading engineer on ECP, NPC "Vector", G. Moscow

Introduction

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

The authors did not intend to express themselves strictly scientific language in defining concepts. In the process of communicating with the personnel of the 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 VCS itself.

A taskECP

cathodic protection is carried out when an electric current flows from the SKZ through a closed electrical circuit formed by three resistors connected in series:

· soil resistance between pipeline and 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 the consumable element, the dissolution of which provides the very possibility of ECP implementation. Its resistance during operation steadily increases due to dissolution, a decrease in the effective area of ​​the working surface and the formation of oxides.

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

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

A taskVHC

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

So, if the insulation is new, and it has not had time to get damaged, then its resistance to electric current is high and a small current is needed to maintain the desired potential. As the insulation ages, its resistance decreases. Consequently, the required compensating current from the RMS 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.

ModesworkVHC

There are four modes of operation of the ECP:

without stabilization of output values ​​of current or voltage;

I stabilize the output voltage;

stabilization of the output current;

· I stabilization of the protective potential.

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

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

In conditions where there are problems with the potential feedback mode, proceed as follows. When using the third mode, it is assumed that the state of the insulation changes little in the short term and its resistance remains practically stable. Therefore, it is enough to ensure the flow of a stable current through a stable insulation resistance, and we get a stable protective potential. In the medium and long term, the necessary adjustments can be made by a specially trained lineman. The first and second regimes do not impose high requirements on the SKZ. These stations are simple in execution and, as a result, cheap, both in manufacture and in operation. Apparently, this circumstance determines the use of such SCs in the ECP of objects located in conditions of low corrosive activity of the environment. If the external conditions (insulation state, temperature, humidity, stray currents) change to the limits when an unacceptable mode is formed on 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 performed.

CharacteristicsVHC

First of all, the VHC must be selected based on the requirements set out in normative 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 noise generated by RMS should not exceed the values ​​specified by GOST 16842, and the level of harmonics at the output should comply with GOST 9.602.

The SKZ passport usually indicates: I rated output power;

Efficiency at rated output power.

Rated output power - the power that the station can deliver at rated load. Typically this load is 1 ohm. The efficiency is defined as the ratio of the rated output power to the active power consumed by the station in the rated mode. And in this mode, the efficiency is the highest for any station. However, most VCSs operate far from the nominal mode. The power load factor ranges from 0.3 to 1.0. In this case, the real efficiency for most stations manufactured today will drop noticeably with a decrease in output power. This is especially noticeable for transformer SKZ using thyristors as a regulating element. For transformerless (high-frequency) RMS, the drop in efficiency with a decrease in output power is much less.

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

From fig. it can be seen that if you use the station, for example, with a nominal efficiency of 70%, then be prepared for the fact that you have spent 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 nominal, you should already be prepared for the fact that your energy losses will be equal to the useful energy spent. Where is so much energy being wasted?

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

· energy costs for the operation of the station control circuit;

Loss of energy in the form of radio emission; energy losses of the output current ripple of the station at 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 ripple coefficient, otherwise expensive energy is wasted. Not only that, at high levels of ripples and radio emission, power losses increase, but besides this, this uselessly dissipated energy interferes with normal operation. a large number electronic equipment located in the vicinity. The required total power is also indicated in the SKZ passport, let's try to deal with this parameter. The SKZ takes energy from the power grid and does it in every unit of time with such intensity as we have allowed it to do with the adjustment knob on the station control panel. Naturally, it is possible to take energy from the network with a power not exceeding the power of this network itself. 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 mains voltage passes through zero, no power can be taken from it. However, when the voltage sinusoid reaches its maximum, then at this moment our ability to take energy from the network is maximum. At any other time, this possibility is less. Thus, it turns out that at any time the power of the network differs from its power at a neighboring 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 the 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, we got 220 V - it was called the effective voltage. BUT maximum value the sinusoids of the voltage were called the amplitude voltage, and it is equal to 320 V. By analogy with the voltage, the concept of the effective value of the current 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 SKZ itself is not fully used, because. it has various reactive elements that do not waste energy, but use it, as it were, 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 energy returned back was called reactive energy. The energy that is transferred to the load is active energy. A parameter that indicates the relationship between the active energy to be transferred to the load and full energy, supplied to the RMS, is called the power factor and is indicated in the station's passport. And if we coordinate our capabilities with the capabilities of the supply network, i.e. synchronously with a sinusoidal change in the voltage of the network, we take power from it, then such a case is called ideal and the power factor of the RMS operating with the network in this way will be equal to one.

The station must transmit 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 transfer and on the mode of operation. Without going into this vast field for discussion, we will only say that transformer and transformer-thyristor SKZs have reached their limit of improvement. They do not have the resources to improve the quality of their work. The future belongs to high-frequency VMS, which every year become more reliable and easier to maintain. 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, the weight and strength. Probably, it is not necessary to say that the smaller and lighter the station, the lower the cost of its transportation and installation, both during installation and repair.

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

3. Convenience in service. Ease of maintenance, in addition to ease of transportation and repair, is determined, in our opinion, as follows:

the presence of all the necessary indicators and measuring instruments, the possibility of remote control and monitoring of the operating mode of the SKZ.

conclusions

Based on the foregoing, several conclusions and recommendations can be drawn:

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 in all range of loadings;

power factor (cos I) not less than 0.75 in 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 principle of construction, 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 SKZ.

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

Electrochemical protection – effective method protection of finished products from electrochemical corrosion. In some cases, it is impossible to renew the paintwork or protective wrapping material, then it is advisable to use electrochemical protection. The coating of an underground pipeline or the bottom of a sea vessel is very laborious and expensive to renew, sometimes it is simply impossible. Electrochemical protection reliably protects the product from, preventing the destruction of underground pipelines, ship bottoms, various tanks, etc.

Electrochemical protection is used in cases where the potential for free corrosion is in the region of intensive dissolution of the base metal or overpassivation. Those. when there is an intensive destruction of the metal structure.

The essence of electrochemical protection

A direct current is connected to the finished metal product from the outside (source direct current or protector). Electric current on the surface of the protected product creates cathodic polarization of the electrodes of microgalvanic pairs. The result of this is that the anodic areas on the metal surface become cathodic. And as a result of exposure to a corrosive environment, not the metal of the structure is destroyed, but the anode.

Depending on which direction (positive or negative) the potential of the metal is shifted, electrochemical protection is divided into anode and cathode.

Cathodic corrosion protection

Cathodic electrochemical corrosion protection is used when the protected metal is not prone to passivation. This is one of the main types of protection of metals from corrosion. The essence of cathodic protection is the application of an external current from the negative pole to the product, which polarizes the cathodic sections of corrosion elements, bringing the potential value closer to the anode ones. The positive pole of the current source is connected to the anode. In this case, the corrosion of the protected structure is almost reduced to zero. The anode is gradually destroyed and must be replaced periodically.

There are several options for cathodic protection: polarization from an external source of electric current; decrease in the rate of the cathode process (for example, electrolyte deaeration); contact with a metal that has a more electronegative potential for free corrosion in a given environment (the so-called sacrificial protection).

Polarization from an external source of electric current is used very often to protect structures located in the soil, water (bottoms of ships, etc.). In addition, this type of corrosion protection is used for zinc, tin, aluminum and its alloys, titanium, copper and its alloys, lead, as well as high-chromium, carbon, alloy (both low and high alloy) steels.

An external current source is cathodic protection stations, which consist of a rectifier (converter), a current supply to the protected structure, anode ground electrodes, a reference electrode and an anode cable.

Cathodic protection is used as an independent and additional type of corrosion protection.

The main criterion by which one can judge the effectiveness of cathodic protection is protective potential. The protective potential is the potential at which the corrosion rate of the metal in certain conditions environment takes the lowest (as far as possible) value.

There are disadvantages to using cathodic protection. One of them is danger overprotection. Overprotection is observed with a large displacement of the potential of the protected object in negative side. At the same time, it stands out. As a result, the destruction of protective coatings, hydrogen embrittlement of the metal, corrosion cracking.

Tread protection (tread application)

A type of cathodic protection is cathodic protection. When using sacrificial protection, a metal with a more electronegative potential is connected to the protected object. In this case, not the structure is destroyed, but the tread. Over time, the protector corrodes and must be replaced with a new one.

Tread protection is effective in cases where there is little transient resistance between the protector and the environment.

Each protector has its own radius of protective action, which is determined by the maximum possible distance at which the protector can be removed without losing the protective effect. Protective protection is used most often when it is impossible or difficult and expensive to bring current to the structure.

Protectors are used to protect structures in neutral environments (sea or river water, air, soil, etc.).

For the manufacture of protectors, the following metals are used: magnesium, zinc, iron, aluminum. Pure metals do not fulfill their protective functions to the full extent, so they are additionally alloyed during the manufacture of protectors.

Iron protectors are made of carbon steels or pure iron.

Zinc protectors

Zinc protectors contain about 0.001 - 0.005% lead, copper and iron, 0.1 - 0.5% aluminum and 0.025 - 0.15% cadmium. Zinc projectors are used to protect products from marine corrosion (in salt water). If the zinc protector is used in slightly saline, fresh water or soil, it is quickly covered with a thick layer of oxides and hydroxides.

Protector magnesium

Alloys for the manufacture of magnesium protectors are alloyed with 2–5% zinc and 5–7% aluminum. The amount of copper, lead, iron, silicon, nickel in the alloy should not exceed tenths and hundredths of a percent.

Magnesium protector is used in lightly salted, fresh waters, soils. The protector is used in environments where zinc and aluminum protectors are ineffective. An important aspect is that magnesium protectors must be used in an environment with a pH of 9.5 - 10.5. This is due to the high rate of magnesium dissolution and the formation of sparingly soluble compounds on its surface.

Magnesium protector is dangerous, because. is the cause of hydrogen embrittlement and corrosion cracking of structures.

Aluminum protectors

Aluminum protectors contain additives that prevent the formation of aluminum oxides. Up to 8% zinc, up to 5% magnesium and tenths to hundredths of silicon, cadmium, indium, and thallium are introduced into such protectors. Aluminum protectors are used in the coastal shelf and flowing sea water.

Anode corrosion protection

Anode electrochemical protection is used for structures made of titanium, low-alloy stainless, carbon steels, high-alloy ferrous alloys, dissimilar passivated metals. Anode protection is used in highly conductive corrosive environments.

With anodic protection, the potential of the protected metal is shifted to a more positive side until a passive stable state of the system is reached. The advantages of anodic electrochemical protection are not only a very significant slowdown in the corrosion rate, but also the fact that corrosion products do not enter the product and the medium.

Anode protection can be implemented in several ways: by shifting the potential to the positive side using an external electric current source or by introducing oxidizing agents (or elements into the alloy) into the corrosive environment, which increase the efficiency of the cathodic process on the metal surface.

Anode protection using oxidizers according to defense mechanism similar to anodic polarization.

If passivating inhibitors with oxidizing properties are used, then the protected surface passes into a passive state under the influence of the current that has arisen. These include dichromates, nitrates, etc. But they pollute the surrounding technological environment quite strongly.

With the introduction of additives into the alloy (mainly doping with a noble metal), the reduction reaction of depolarizers occurring at the cathode proceeds with a lower overvoltage than on the protected metal.

If an electric current is passed through the protected structure, the potential shifts in the positive direction.

An installation for anodic electrochemical protection against corrosion consists of an external current source, a reference electrode, a cathode, and the protected object itself.

In order to find out whether it is possible to apply anodic electrochemical protection for a certain object, anodic polarization curves are taken, with the help of which it is possible to determine the corrosion potential of the structure under study in a certain corrosive environment, the region of stable passivity and the current density in this region.

For the manufacture of cathodes, low-solubility metals are used, such as high-alloy stainless steels, tantalum, nickel, lead, and platinum.

In order for anodic electrochemical protection to be effective in a certain environment, it is necessary to use easily passivated metals and alloys, the reference electrode and cathode must always be in solution, and the connecting elements must be of high quality.

For each case of anode protection, the layout of the cathodes is designed individually.

In order for the anode protection to be effective for a certain object, it is necessary that it meets certain requirements:

All welds must be of high quality;

In the technological environment, the material from which the protected object is made must pass into a passive state;

The number of air pockets and slots should be kept to a minimum;

There should be no riveted joints on the structure;

In the device to be protected, the reference electrode and cathode must always be in solution.

To implement anode protection in the chemical industry, heat exchangers and cylindrical units are often used.

Electrochemical anodic protection of stainless steels is applicable for industrial storages of sulfuric acid, ammonia-based solutions, mineral fertilizers, as well as all kinds of collections, tanks, mernikov.

Anode protection can also be used to prevent corrosion damage in chemical nickel plating baths, heat exchangers in production artificial fiber and sulfuric acid.

They allow to extend the service life of the metal structure, as well as to preserve its technical and physical properties during operation. Despite the variety of methods for providing anti-corrosion action, it is possible to completely protect objects from rust damage only in rare cases.

The effectiveness of such protection depends not only on the quality of the tread technology, but also on the conditions of its application. In particular, in order to preserve the metal structure of pipelines, their best properties demonstrates electrochemical corrosion protection based on the operation of cathodes. The prevention of rust formation on such communications is, of course, not the only area of ​​application of this technology, but in terms of the combination of characteristics, this direction can be considered as the most relevant for electrochemical protection.

General information about electrochemical protection

The protection of metals from rust by electrochemical action is based on the dependence of the size of the material on the rate of the corrosion process. Metal structures must be operated in the range of potentials where their anodic dissolution will be below the permissible limit. The latter, by the way, is determined by the technical documentation for the operation of the structure.

In practice, electrochemical corrosion protection involves connecting a source with direct current to the finished product. The electric field on the surface and in the structure of the protected object forms the polarization of the electrodes, which controls the process of corrosion damage. In essence, the anode zones on the metal structure become cathodic, which allows negative processes to be displaced, ensuring the preservation of the structure of the target object.

How Cathodic Protection Works

There is cathodic and anode protection electrochemical type. However, the first concept, which is used to protect pipelines, has gained the greatest popularity. By general principle, when implementing this method, a current with a negative pole is supplied to the object from an external source. In particular, a steel or copper pipe can be protected in this way, as a result of which the polarization of the cathode sections will occur with the transition of their potentials to the anode state. As a result, the corrosive activity of the protected structure will be reduced to almost zero.

At the same time, cathodic protection can have different variants execution. The above-described technique of polarization from an external source is widely practiced, but the electrolyte deaeration method with a decrease in the rate of cathodic processes, as well as the creation of a protective barrier, also works effectively.

It has been noted more than once that the principle of cathodic protection is implemented by means of an external current source. In fact, his work is main function These tasks are performed by special stations, which, as a rule, are part of the general pipeline maintenance infrastructure.

Stations against corrosion

The main function of the cathode station is to provide stable current to the target metal object in accordance with the cathodic polarization method. Such equipment is used in the infrastructure of underground gas and oil pipelines, in water supply pipes, heating networks, etc.

There are many varieties of such sources, while the most common cathodic protection device provides for the presence of:

• current converter equipment;
• wires for connecting to the protected object;
• anode grounding.

At the same time, there is a division of stations into inverter and transformer ones. There are other classifications, but they are focused on the segmentation of installations, either by application, or by technical characteristics and input data parameters. Basic principles The works most clearly illustrate the designated two types of cathode stations.

Transformer plants for cathodic protection

It should immediately be noted that this type of station is obsolete. It is being replaced by inverter analogues, which have both pluses and minuses. One way or another, transformer models are used even at new points for providing electrochemical protection.

A 50 Hz low-frequency transformer is used as the basis for such objects, and the simplest devices are used for the thyristor control system, including phase-pulse power controllers. A more responsible approach to solving control problems involves the use of controllers with wide functionality.

Modern cathodic corrosion protection of pipelines with such equipment allows you to adjust the parameters of the output current, voltage indicators, as well as equalize the protective potentials. As for the disadvantages of transformer equipment, they come down to a high degree of current ripple at the output at a low power factor. This defect is explained not by the sinusoidal form of the current.

To a certain extent, the introduction of a low-frequency choke into the system allows solving the problem with ripple, but its dimensions correspond to the dimensions of the transformer itself, which does not always make such an addition possible.

Cathodic protection inverter station

Settings inverter type are based on pulsed high-frequency converters. One of the main advantages of using stations of this type is high efficiency, reaching 95%. For comparison, for transformer installations, this figure reaches an average of 80%.

Sometimes other advantages come to the fore. For example, the small dimensions of inverter stations expand the possibilities for their use in difficult areas. There are also financial advantages, which are confirmed by the practice of using such equipment. So, inverter cathodic protection against corrosion of pipelines quickly pays off and requires minimum investment into technical content. However, these qualities are clearly visible only when compared with transformer installations, but even today there are more effective new means of providing current for pipelines.

Structures of cathode stations

Such equipment is presented on the market in different cases, shapes and dimensions. Of course, the practice of individual design of such systems is also widespread, which makes it possible not only to obtain an optimal design for specific needs, but also to provide the necessary operational parameters.

A rigorous calculation of the characteristics of the station allows further optimization of the costs of its installation, transportation and storage. For example, cathodic protection against corrosion of pipelines based on an inverter with a mass of 10-15 kg and a power of 1.2 kW is quite suitable for small objects. Equipment with such characteristics can be serviced by a car, however, for large-scale projects, more massive and heavy stations can be used, requiring the connection of trucks, a crane and installation teams.

Protective functionality

Particular attention in the development of cathode stations is paid to the protection of the equipment itself. For this, systems are integrated that allow protecting stations from short circuits and load interruptions. In the first case, special fuses are used to handle emergency operation of the installations.

As for power surges and breaks, the cathodic protection station is unlikely to be seriously affected by them, but there may be a risk of electric shock. For example, if in normal mode the equipment is operated with a low voltage, then after a break, the jump in indicators can be brought up to 120 V.

Other types of electrochemical protection

In addition to cathodic protection, electrical drainage technologies are also practiced, as well as tread methods for preventing corrosion. Most promising direction It is considered to be a special protection against the formation of corrosion. AT this case active elements are also connected to the target object, providing the transformation of the surface with cathodes by means of current. For example, a steel pipe as part of a gas pipeline can be protected by zinc or aluminum cylinders.

Conclusion

Methods of electrochemical protection cannot be attributed to new and, moreover, innovative. The effectiveness of the use of such techniques in the fight against rusting processes has been mastered for a long time. However, one serious drawback prevents the wide distribution of this method. The fact is that cathodic corrosion protection of pipelines inevitably produces the so-called They are not dangerous for the target structure, but can have negative impact to nearby objects. In particular, the stray current contributes to the development of the same corrosion on the metal surface of adjacent pipes.

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

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

Principle of operation

The objects of the electrochemical corrosion process are the medium, the metal and the interface between them. The medium, which is usually moist soil or water, has good electrical conductivity. An electrochemical reaction takes place at the interface between it and the metal structure. If the current is positive (anode electrode), the iron ions pass into the surrounding solution, resulting in a mass loss of the metal. The reaction causes corrosion. With a negative current (cathode electrode), these losses are absent, since electrons pass into the solution. The method is used in electroplating for coating steel with non-ferrous metals.

Cathodic corrosion protection is achieved when a negative potential is applied to an iron object.

To do this, an anode electrode is placed in the ground and a positive potential is connected to it from a power source. The minus is applied to the protected object. Cathodic-anodic protection leads to active corrosion destruction of only the anode electrode. Therefore, it should be changed periodically.

Negative effect of electrochemical corrosion

Corrosion of structures can occur from the action of stray currents from other systems. They are useful for target objects, but cause significant damage to nearby structures. Stray currents can spread from the rails of electrified vehicles. They pass towards the substation and enter the pipelines. When leaving them, anode sections are formed, causing intense corrosion. For protection, electrical drainage is used - a special removal of currents from the pipeline to their source. It is also possible here. For this, it is necessary to know the magnitude of the stray currents, which is measured by special devices.

According to the results electrical measurements the method of protection of the gas pipeline is selected. A universal remedy is a passive method of contact with the ground using insulating coatings. Cathodic protection of the gas pipeline refers to the active method.

Pipeline protection

Structures in the ground are protected from corrosion if the minus of a direct current source is connected to them, and the plus is connected to anode electrodes buried nearby in the ground. The current will go to the structure, protecting it from corrosion. In this way, cathodic protection of pipelines, tanks or pipelines located in the ground is carried out.

The anode electrode will degrade and should be replaced periodically. For a tank filled with water, the electrodes are placed inside. In this case, the liquid will be the electrolyte through which the current will flow from the anodes to the surface of the container. The electrodes are well controlled and easy to change. It is more difficult to do this in the ground.

Source of power

Near oil and gas pipelines, in heating and water supply networks that require cathodic protection, stations are installed from which voltage is supplied to objects. If they are placed on outdoors, their degree of protection must be at least IP34. For dry rooms, any is suitable.

Cathodic protection stations for gas pipelines and other large structures have a capacity of 1 to 10 kW.

Their energy parameters primarily depend on the following factors:

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

Traditionally, a cathodic protection converter is a transformer installation. Now it is being replaced by an inverter one, which has smaller dimensions, better current stability and greater efficiency. In important areas, controllers are installed that have the functions of regulating current and voltage, equalizing protective potentials, etc.

The equipment is on the market in various options. For specific needs, the one providing the best operating conditions is used.

Current source parameters

For corrosion protection for iron, the protective potential is 0.44 V. In practice, it should be higher due to the influence of inclusions and the condition of the metal surface. The maximum value is 1 V. In the presence of coatings on the metal, the current between the electrodes is 0.05 mA/m 2 . If the insulation is broken, it rises to 10 mA/m 2 .

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

Features of cathodic protection

1. Stations or mobile generators serve as power sources.
2. The location of the anode ground electrodes depends on the specifics of the pipelines. The placement method can be distributed or concentrated, as well as located at different depths.
3. The anode material is chosen with low solubility to last for 15 years.
4. The protective field potential for each pipeline is calculated. It is not regulated if there are no protective coatings on the structures.

Gazprom standard requirements for cathodic protection

• Action during the entire period of operation of protective equipment.
• Protection against atmospheric surges.
• Placement of the station in block-boxes or separately standing in anti-vandal design.
• Anode grounding is selected in areas with a minimum electrical resistance soil.
• The characteristics of the transducer are selected taking into account the aging of the protective coating of the pipeline.

Protective protection

The method is a type of cathodic protection with the connection of electrodes made of a more electronegative metal through an electrically conductive medium. The difference lies in the absence of an energy source. The tread absorbs corrosion by dissolving into the electrically conductive environment.

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

The effect of the anode increases with a decrease in its contact resistance with the medium. Over time, it can become covered with a corrosive layer. This leads to a breakdown in electrical contact. If the anode is placed in a mixture of salts, which ensures the dissolution of corrosion products, the efficiency increases.

Protector influence is limited. The radius of action is determined by the electrical resistance of the medium and the potential difference between

Protective protection is used in the absence of energy sources or when their use is not economically feasible. It is also disadvantageous in acidic applications due to the high dissolution rate of the anodes. Protectors are installed in water, in soil or in a neutral environment. Anodes are usually not made of pure metals. The dissolution of zinc occurs unevenly, magnesium corrodes too quickly, and a strong film of oxides forms on aluminum.

Tread materials

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

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

Incorrect use of some types of protectors leads to negative consequences. Magnesium anodes can cause equipment cracking due to the development of hydrogen embrittlement.

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

The distribution of the protective current is improved, and significantly fewer anodes are required. One magnesium anode protects a bitumen-coated pipeline for a length of 8 km, and without a coating - only 30 m.

Protection of car bodies from corrosion

In case of violation of the coating, the thickness of the car body can decrease in 5 years to 1 mm, i.e., rust through. Restoration of the protective layer is important, but in addition to it, there is a way to completely stop the corrosion process using cathodic-protective protection. If you turn the body into a cathode, the corrosion of the metal stops. Anodes can be any conductive surfaces located nearby: metal plates, ground loop, garage body, wet road surface. In this case, the protection efficiency increases with an increase in the area of ​​the anodes. If the anode is a road surface, a "tail" of metallized rubber is used to contact it. It is placed opposite the wheels so that splashes get better. "Tail" is isolated from the body.

Plus is connected to the anode battery through a 1 kΩ resistor and an LED connected in series with it. When the circuit is closed through the anode, when the minus is connected to the body, in normal mode the LED barely noticeably glows. If it burns brightly, then a short circuit has occurred in the circuit. The cause must be found and eliminated.

For protection, a fuse must be installed in series in the circuit.

When the car is in the garage, it is connected to a grounding anode. During the movement, the connection occurs through the "tail".

Conclusion

Cathodic protection is a way to improve the operational reliability of underground pipelines and other structures. At the same time, its negative impact on adjacent pipelines from the influence of stray currents should be taken into account.

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

Causes of corrosion

Networks of pipelines of life support systems are distributed throughout Russia. With their help, gas, water, oil products and oil are efficiently transported. Not so long ago, pipelines were laid for the transportation of 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 danger of corrosion formation for internal surfaces is based on:

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

Such circumstances can develop on main water pipelines, hot water supply systems (DHW), steam and heating. Not less than an important factor is the method of laying the pipeline: ground or underground. The first is easier to maintain and eliminate the causes of rust formation, compared to the second.

With the pipe-in-pipe laying method, the risk of corrosion is at a low level. With direct installation of the pipeline in the open air, rust may form from interaction with the atmosphere, which also leads to a change in design.

Pipelines located underground, including steam and hot water, are the 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 mains is located in these places.

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

• trunk;
• commercial;
• for heating systems and life support of the population;
• 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 against external factors is determined by standard requirements. Regulatory documents deal with methods of protection, and not the reasons for the formation of rust.

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

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

The impact of negative factors from the atmosphere for high-risk areas or types of corrosion:

1. From direct current sources, the occurrence of stray currents.
2. The impact of microorganisms.
3. The stress created causes the metal to crack.
4. Waste storage.
5. Salty soils.
6. The temperature of the transported substance is above 300 °C.
7. Carbon dioxide corrosion of the oil pipeline.

The installer for the protection of 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 separate 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 anode zones cracks and flows into the base of the soil. After interaction with the electrolyte, corrosion is formed.

One of the significant criteria for ensuring protection against negative manifestations is the length of the main. On the way there are soils with different composition and characteristic. All this contributes to the occurrence of a voltage difference between the parts of the laid pipelines. The mains have good conductivity, therefore, the formation of galvanic couples with a sufficiently large length occurs.

An increase in the pipeline corrosion rate provokes a high electron flux density. No less important is the depth of the pipelines, since it retains a significant percentage of humidity, and the temperature below the “0” mark is not released. 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 ​​the formed rust on the metal. This is based on the fact that the metal larger area surface is most vulnerable to external negative manifestations. Particular cases include the manifestation on steel structures of much smaller amounts of destruction under the action of an electrochemical process.

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

Corrosion under the influence of stray currents

Rust can occur from variable and constant electron flow:

• Rust formation under the influence of constant current. Stray currents are called currents in the soil and in structural elements located underground. Their origin is anthropogenic. They result from the use technical devices direct current propagating 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 the 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 comes out to the soil surface. The part of the pipeline into which the current enters plays the role of the cathode. On the described anode surfaces, the 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 the mains of power lines with a network voltage of over 110 kV, as well as parallel arrangement pipelines under the influence of alternating currents, corrosion is formed, including corrosion under the insulation of pipelines.

Stress corrosion cracking

If the metal surface is simultaneously affected by external negative factors and high voltage from power lines, creating tensile forces, then rust is formed. According to the conducted research, a new hydrogen-corrosion theory has taken its place.

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

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

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

Corrosion under the influence of microorganisms

Microbiological corrosion is the process of rust formation on the pipeline under the influence of living microorganisms. It can be algae, fungi, bacteria, including the simplest organisms. It has been established that the reproduction of bacteria most significantly affects this process. To maintain the vital activity of microorganisms, it is necessary to create conditions, namely, nitrogen, humidity, water and salt are needed. Also conditions such as:

1. Temperature and humidity indicators.
2. Pressure.
3. The presence of illumination.
4. Oxygen.

When emitting an acidic environment, organisms can also cause corrosion. Under their influence, cavities appear on the surface, which have a black color and bad smell hydrogen sulfide. Sulfate-containing bacteria are present in almost 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.

Corrosion protection is produced by creating electromagnetic field, as a result of which a negative potential is acquired or the site plays the role of a cathode. That is, the segment steel pipelines, protected from rust, acquires a negative charge, and ground - a positive one.

Cathodic protection of pipelines against corrosion accompanies electrolytic protection with sufficient conductivity of the medium. This function is performed by the soil, when laying metal underground highways. The electrodes are contacted through conductive elements.

The corrosion indicator is a high voltage voltmeter or corrosion gauge. With the help of this device, the indicator between the electrolyte and the soil is controlled, specifically for this case.

How electrochemical protection is classified

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

• A current source is supplied to the metal surface. This area acquires a negative charge, that is, it plays the role of a cathode. Anodes are inert electrodes that have nothing to do with the 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, crystal type of stainless steel, as well as cracking of brass elements.
• galvanic way. Protection of main pipelines or tread protection is carried out by metal plates with great performance negative charges made of aluminum, zinc, magnesium or their alloys. Anodes are two elements, the so-called inhibitors, while the slow destruction of the protector helps to maintain the cathode current in the product. Protective protection is used extremely rarely. ECP is carried out on the insulating coating of pipelines.

On the features of electrochemical protection

The main reason for the destruction of pipelines is a consequence of corrosion of metal surfaces. After the formation of rust, cracks, ruptures, cavities are formed, which gradually increase in size and contribute to the rupture of the pipeline. This phenomenon occurs more often at highways laid underground, or in contact with groundwater.

The principle of cathodic protection is based on the creation of a voltage difference and action by the two methods described above. After the measurement operations carried out directly at the location of the pipeline, it was found that the required potential to 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 scenario, the corrosion rate will not exceed 10 microns/year, and this will significantly extend the service life of technical devices.

One of the significant problems is the presence of stray currents in the ground. 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 can 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 lines 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 base.

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 can be used. The installer for the protection of underground pipelines from corrosion before performing work must be familiar with the design solutions.

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 flows of electrons.
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 the inhibitory effect of currents. At the same time, several sections of the pipeline are protected at the same time. The parameters can be adjusted manually or automatically. In the first case, transformer windings are used, and in the second case, thyristors are used.

The most common in Russia is a high-tech installation - Minevra -3000. Its power is enough 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;
• with the help of digital regulation, control over the operating parameters is carried out;
• tightness of highly responsible connections;
• connecting the device to remote process control.

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

The scheme of the insulating line of the water or gas pipeline must be at the place of work.

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

Corrosion protection by arranging drainage

The installer for the protection of underground pipelines from corrosion must be familiar with the drainage device. Such protection against the formation of rust of pipelines from stray currents is provided by a drainage device necessary to divert these currents to another piece of land. In total there are several options for drainage.

Varieties of performance:

1. Made underground.
2. Straight.
3. with polarities.
4. Reinforced.

When carrying out earth drainage, electrodes are installed to the anode zones. To provide a direct drainage line, an electrical jumper is made connecting the pipeline to the negative pole from current sources, for example, to ground from a residential building.

Polarized drainage has one-sided conductivity, that is, when a positive charge appears on the ground loop, it automatically turns off. Reinforced drainage functions from a current converter, additionally connected to wiring diagram, and this improves the removal of stray currents from the main.

The allowance for corrosion of pipelines is carried out by calculation, in accordance with the RD.

In addition, inhibitor protection is used, that is, a special composition is used on the pipes to protect against aggressive environments. Parking corrosion occurs when boiler equipment is idle for a long time, so that this does not happen, it is necessary Maintenance equipment.

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 Rostekhnadzor.