The principle of operation of cathodic protection of pipelines. Cathodic Protection: Applications and Standards

cathodic protection that - the most widespread type of electrochemical protection. It is used in cases where the metal is not prone to passivation, that is, it has an extended region of active dissolution, a narrow passive region, high passivation current (i p) and passivation potential (c p).

Cathodic polarization can be carried out by connecting the protected structure to the negative pole of an external current source. Cathodic protection is carried out by an external current. .

The scheme of cathodic protection is shown in fig. 4. The negative pole of the external current source 4 is connected to the protected metal structure 1, and the positive pole is connected to the auxiliary electrode 2, which works as an anode. In the process of protection, the anode is actively destroyed and is subject to periodic restoration.

Cast iron, steel, coal, graphite, scrap metal (old pipes, rails, etc.) are used as the anode material. The sources of external current in cathodic protection are cathodic protection stations, the mandatory elements of which are: a converter (rectifier) ​​that generates current; current supply to the structure to be protected, reference electrode, anode earth electrodes, anode cable.

Cathodic protection of factory equipment (refrigerators, heat exchangers, condensers, etc.) exposed to an aggressive environment is carried out by connecting an external current source to the negative pole and immersing the anode in this environment.

Cathodic protection by external current is impractical under conditions of atmospheric corrosion, in a vaporous medium, in organic solvents, since in this case the corrosive medium does not have sufficient electrical conductivity.

Protective protection. Protective protection is a type of cathodic protection. The scheme of the protective protection of the pipeline is shown in fig. 5. A more electronegative metal is attached to the protected structure 2 - protector 3, which, dissolving in the environment, protects the main structure from destruction.

After complete dissolution of the protector or loss of contact with the protected structure, it is extremely important to replace the protector.

Figure 5 Scheme of the protective protection of the pipeline

The protector works effectively if the contact resistance between it and the environment is small. During operation, the protector, for example, zinc, can be covered with a layer of insoluble corrosion products, which isolate it from the environment and dramatically increase the transition resistance. To combat this, the protector is placed in filler 4 - a mixture of salts, which creates a certain environment around it, which facilitates the dissolution of corrosion products and increases the efficiency and stability of the protector in the ground.

Protective protection, compared with cathodic protection by external current, is advisable to use in cases where obtaining energy from the outside is difficult or if the construction of special power lines is not economically viable.

Today, tread protection is used to combat corrosion of metal structures in sea and river water, soil and other neutral media. The use of protective protection in acidic environments is limited by the high rate of self-dissolution of the protector.

Metals can be used as protectors: Al, Fe, Mg, Zn. At the same time, it is not always advisable to use pure metals as protectors. To give the protectors the required performance properties, alloying elements are introduced into their composition.

Cathodic protection - concept and types. Classification and features of the cathodic protection category 2017, 2018.

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The cathodic protection of the gas pipeline must operate uninterruptedly. For each SKZ, a certain mode is set depending on the conditions of its operation. During the operation of the cathode station, a log of its electrical parameters and the operation of the current source is kept. It is also necessary to constantly monitor the anode grounding, the state of which is determined by the magnitude of the RMS current.

The cathodic protection of the gas pipeline must operate uninterruptedly. On sections of the route with interruptions in the supply of electricity for several hours a day, batteries are used that provide protection during a power outage. The capacity of the battery is determined by the value of the protective current RMS.

Cathodic protection of gas pipelines from the effects of stray currents or soil corrosion is carried out using a direct electric current from an external source. The negative pole of the current source is connected to the protected gas pipeline, and the positive pole to a special ground - the anode.

Cathodic protection of gas pipelines against corrosion is carried out due to their cathodic polarization using an external current source.

Influence of cathodic protection of gas pipelines on rail chains of railways.

For cathodic protection of a gas pipeline, standard instruments of electrical installations and special corrosion-measuring and auxiliary instruments are used. To measure the potential difference between an underground structure and the earth, which is one of the criteria for assessing the risk of corrosion and the presence of protection, voltmeters with a large internal resistance by 1 on the scale are used so that their inclusion in the measuring circuit does not violate the potential distribution in the latter. This requirement is due to both the high internal resistance of the underground structure - ground system, and the difficulty of creating a low ground resistance at the point of contact of the measuring electrode with the ground, especially when using non-polarizable electrodes. To obtain a measuring circuit with a high input resistance, potentiometers and high-resistance voltmeters are used.

For gas pipeline cathodic protection stations as a source of electricity, it is recommended to use high-temperature fuel cells with a ceramic electrode. Such fuel cells can operate for a long time on the gas pipeline route, supplying cathodic protection stations with electricity, as well as the houses of line repairers, signaling systems and automatic control of the yards. This method of supplying linear structures and installations on a gas pipeline, which do not require high power, greatly simplifies operational maintenance.

Very often, the parameters of cathodic protection of gas pipelines, obtained by calculation, differ significantly from the RMS parameters obtained in practice by measurements. This is due to the impossibility of taking into account the whole variety of factors that influence protection parameters in natural conditions.

There are various methods for treating metal pipes, but the most effective of them is the cathodic protection of pipelines from corrosion. It is necessary to prevent their premature depressurization, which will lead to the formation of cracks, cavities and ruptures.

Metal corrosion is a natural process in which metal atoms change. As a result, their electrons go to oxidizing agents, which leads to the destruction of the material structure.

For underground pipelines, an additional factor in the corrosive effect is the composition of the soil. It contains areas of different electrode potential, which is the reason for the formation of corrosive galvanic cells.

There are several types of corrosion, including:

• Solid. Differs in the big continuous area of ​​distribution. In rare cases, it causes damage to the pipeline, as it often does not penetrate deep into the metal structure;


• Local corrosion - becomes the most common cause of ruptures, as it does not cover a large area, but penetrates deeply. It is subdivided into ulcerative, filiform, through, subsurface, spotty, knife, intergranular, corrosion brittleness and cracking.

Underground pipeline protection methods

Metal corrosion protection can be either active or passive. Passive methods involve creating conditions for the pipeline in which it will not be affected by the surrounding soil. To do this, special protective compounds are applied to it, which become a barrier. The most commonly used coatings are bitumen, epoxy resins, polymer tapes or coal tar pitch.

For the active method, cathodic protection of pipelines against corrosion is most often used. It is based on the creation of polarization, which reduces the rate of dissolution of the metal. This effect is realized due to the shift of the corrosion potential to a more negative area. To do this, an electric current is conducted between the metal surface and the soil, which significantly reduces the corrosion rate.

Ways to implement cathodic protection:

• With the use of external current sources that are connected to the protected pipe and to the anode ground;


• Using the galvanic method (magnesium sacrificial anode protectors).

Cathodic corrosion protection of pipelines using external sources is more complex. Since it requires the use of special designs that provide direct current. The galvanic method, in turn, is implemented through protectors, which allow effective protection only in soils with low electrical resistance.

Can be used for pipeline protection and anode method. It is used in contact with aggressive chemical environment. The anode method is based on the transfer of the active state of the metal into a passive state and its maintenance due to the influence of an external anode.

Despite certain difficulties in implementation, this method is actively used where cathodic corrosion protection of pipelines cannot be implemented.

Examples of cathodic corrosion protection of pipelines at the exhibition

The experience of use and new developments in this area are covered at the annual industry exhibition "Naftogaz", which takes place at the Expocentre Fairgrounds.

The exhibition is a major industry event and an excellent platform for familiarizing specialists with new developments, as well as launching new projects. The Naftogaz exhibition will be held at the Expocentre Fairgrounds in Moscow on Krasnaya Presnya.

Read our other articles.

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 laying methods, except for above-ground, pipelines are subject to comprehensive protection against corrosion by protective coatings and electrochemical protection (ECP), regardless of the corrosive activity of the soil.

ECP facilities include cathodic, sacrificial and electrical drainage protection.

Protection against soil corrosion is carried out by cathodic polarization of pipelines. If cathodic polarization is performed 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 having a more negative potential, then such protection is called tread protection.

cathodic protection

A 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, coming through the transformer point 2, is converted into direct current.

The negative pole of the source is connected to the protected pipeline 6 by means of a connecting wire 4, and the positive pole is connected to the anode ground 5. When the current source is turned on, the electrical circuit is closed through the soil electrolyte.

Schematic diagram of cathodic protection

1 - power line; 2 - transformer point; 3 - cathodic protection station; 4 - connecting wire; 5 - anode grounding; 6 - pipeline

The principle of operation of cathodic protection is as follows. Under the influence of the applied electric field of the source, the movement of semi-free valence electrons begins in the direction "anode grounding - current source - protected structure". Losing electrons, the metal atoms of the anode grounding pass in the form of ion-atoms into the electrolyte solution, i.e. the anode ground is destroyed. Ion-atoms undergo hydration and are discharged into the depth of the solution. In 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 reactions of oxygen and hydrogen depolarization, which are characteristic of the cathode.

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

J dr \u003d j 3 F 3 K 0

where j 3 is the required value of the protective current density; F 3 - the total surface of contact of underground structures with the soil; K 0 - coefficient of bare 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 p g according to the graph shown in the figure below.

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

Protective protection

The principle of operation of the sacrificial 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 immersed in soil electrolyte and connected by wire 3. Since the protector material is more electronegative, under the action of a potential difference, a directed movement of electrons from the protector to the pipeline occurs along the conductor 3. At the same time, the ion-atoms of the protector material go into solution, which leads to its destruction. In this case, the current strength is controlled using a control column 4.

Dependence of the bareness coefficients of underground pipelines on the transition resistance of the insulating coating for soils with specific resistance, 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 column

Thus, the destruction of the metal still takes place. But not a pipeline, but a protector.

Theoretically, to protect steel structures from corrosion, all metals located in the electrochemical series of voltages 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 mass of the protector (current output) should be maximum;
• the ratio of the protector mass spent on the creation of the protective current to the total loss of the protector mass (utilization factor) should be the largest.

Alloys based on magnesium, zinc and aluminum satisfy these requirements to the greatest extent.

Protective protection is carried out by 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

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

Apply direct, polarized and reinforced drainage.

Schematic diagrams of electrical drainage protection

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

Direct electrical drainage is a two-way conductive drainage device. The direct electrical drainage circuit includes: a rheostat K, a knife switch K, a fuse Pr and an alarm relay C. The current strength in the “pipeline - rail *” circuit is regulated by a rheostat. If the current exceeds the allowable value, then the fuse will burn out, the current will flow through the relay winding, which, when turned on, turns on an audible 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 diverted. Otherwise, the drainage will turn into a channel for the leakage of stray currents into the pipeline.

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

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

Due to such a connection scheme, it is provided: firstly, polarized drainage (due to the operation of valve elements in the SKZ circuit), and secondly, the cathode station maintains the necessary protective potential of the pipeline.

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

Cathodic protection stations (CPS) are a necessary element of the system of electrochemical (or cathodic) protection (ECP) of underground pipelines against corrosion. When choosing a VCS, they most often proceed from the lowest cost, ease of maintenance and qualifications of their 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 pursue the goal of expressing themselves in a strictly scientific language in the definition of 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.

ECP task

Cathodic protection is carried out when an electric current flows from the RMS 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 implementing the ECP. 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 sizes, the more metal is carried 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 voltage between the metal pipe and the ground, i.e. on both sides of the insulation, must be between -0.5 and -3.5 V (this voltage is called the protective potential).

The task of the VHC

The task of the CPS 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.

SKZ operating modes

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.

Characteristics of SKZ

First of all, the SKZ must be selected based on the requirements set forth in the 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 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, the loss of electricity increases, but besides this, this uselessly dissipated energy interferes with the normal operation of a large number of 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 at a given time and it is difficult to operate with such a concept. 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. And the maximum value of the sinusoid of the voltage was 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. The parameter that indicates the ratio between the active energy that must be transferred to the load and the total energy supplied to the RMS 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 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 evaluated by the efficiency factor. 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.

Consumer properties

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

1. Dimensions, 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. Ease of maintenance. 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.

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 gas pipeline 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.