Principle and scheme of cathodic protection. Cathodic corrosion protection

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cathodic protection 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.

Characteristics of the state of the protective coating and its conductivity.

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. Capacity battery determined by the magnitude 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 of an underground structure - the earth, which is one of the criteria for assessing the risk of corrosion and the presence of protection, voltmeters are used with a large value of internal resistance by 1 on the scale 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 long time work 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 affecting natural conditions to the security settings.

Passive protection of underground gas pipelines with insulating coatings is complemented by electrical protection. The tasks of electrical protection are as follows.

1. Removal of stray electric currents from the protected gas pipeline and their organized return to electrical installations and DC networks, which are the source of these currents.
2. Suppression of currents flowing through the gas pipeline at the points of their exit to the ground (anodic zones) by currents from an external source, as well as currents arising due to soil electrochemical corrosion, by creating a galvanic circuit and a protective electrical potential on the gas pipeline pipes.
3. Prevention of the spread of electric currents through gas pipelines by sectioning the latter with insulating flanges.

The problem of diversion of stray currents can be solved by creating:

1. additional grounding to drain currents to the ground. Disadvantage - Opportunity harmful influence to adjacent pipelines of currents flowing from the protected gas pipeline;
2. simple or direct drainage protection, i.e. electrical connection of the protected gas pipeline with tram rails or electrical railway in order to return currents through them to their source. Simple drainage has two-sided conductivity, i.e. can pass current back and forth and is therefore used in stable anode zones. The disadvantage of this protection is the need to turn off the drainage if the polarity of the current has changed or if the potential on the gas pipeline has become lower than on the rails;
3. polarized drainage protection, i.e. drainage with one-sided conductivity, which excludes the reverse flow of current from the rails to the protected gas pipeline;
4. enhanced drainage protection, i.e. such protection, in the circuit of which an external current source is included to increase efficiency. Thus, enhanced drainage is a combination of polarized drainage with cathodic protection.

The task of suppressing currents flowing through the protected gas pipeline can be solved using:

1. Cathodic protection by external current (electrical protection), i.e. by connecting the protected gas pipeline to an external current source - to its negative pole as a cathode. The positive pole of the current source is connected to ground - the anode. A closed circuit is created in which the current flows from the anode through the ground to the protected gas pipeline and further to the negative pole of the external current source. In this case, the anode grounding is gradually destroyed, but the protection of the gas pipeline is ensured due to its cathodic polarization and the prevention of current flow from the pipes into the ground. As an external source, cathodic protection stations (CPS) can be used;
2. Protective protection, i.e. protection by using in the electrical circuit protectors made of metals that have a more negative potential in a corrosive environment than the metal of the pipeline. Electricity occurs in the tread protection system, as well as in a galvanic cell, and the electrolyte is the soil containing moisture, and the electrodes are the gas pipeline and the metal of the protector. The resulting protective current suppresses electrochemical corrosion currents and ensures the creation of a protective electrical potential on the pipeline.

Schematic diagram of cathodic protection of an underground gas pipeline

1 - anode grounding; 2.4 - drainage cables; 3 - external source of electric current; 5 - point of attachment of the drainage cable; 6 - protected gas pipeline

Schematic diagram of the tread protection of an underground gas pipeline

1 - protected gas pipeline; 2 - insulated cables; 3 - control output; 4 - protector; 5 - tread filler

The problem of electrical sectioning of pipelines is solved by installing insulating flanges with paronite or textolite gaskets, textolite bushings and washers. An example of the design of insulating flanges is shown in the figure below.

Installation of insulating flanges

1 - insulating textolite or paronite bushing; 2 - insulating washer made of textolite, rubber or vinyl chloride; 3 - steel washer; 4 - lead washers; 5 - textolite ring-gasket

The main factors characterizing the degree of corrosion impact on underground steel gas pipelines are:

• magnitude and direction of stray currents in the soil;
• the magnitude and polarity of the potential of the gas pipeline relative to other metal underground utilities and rails of electrified transport;
• direction and strength of currents flowing through the gas pipeline;
• state of anti-corrosion protection of gas pipelines;
• the value of the electrical resistivity of the pound.

All these factors are subject to periodic monitoring.

The frequency of electrical measurements is as follows:

• in the areas of electrical protection installations for gas pipelines and other protected structures, as well as near traction substations and depots of electric transport, near famway rails and electrified railways and at the places where gas pipelines intersect with them - at least once every 3 months, as well as when changing modes of installation - innovations in electrical protection, protected structures or sources of stray currents;
• in non-hazardous areas from the point of view of electrical protection - at least once a year in summer time, as well as with any changes in conditions that can cause electrocorrosion.

For tread protection, non-ferrous metal protectors are used - usually magnesium, zinc, aluminum and their alloys.

The control of the operation of electrical protective installations and the measurement of potentials at the contacts are carried out (at least): at drainage installations - 4 times a month; on cathodic installations - 2 times a month; on tread installations - 1 time per month.

Corrosion has a detrimental effect on the technical condition of underground pipelines, under its influence the integrity of the gas pipeline is violated, cracks appear. To protect against such a process, electrochemical protection of the gas pipeline is used.

Corrosion of underground pipelines and means of protection against it

On condition steel pipelines influenced by soil moisture, its structure and chemical composition. The temperature of the gas conveyed through the pipes, currents straying in the ground caused by electrified transport and climatic conditions in general.

Types of corrosion:

• Surface. It spreads in a continuous layer over the surface of the product. Represents the least danger to the gas pipeline.
• Local. It manifests itself in the form of ulcers, cracks, spots. Most dangerous view corrosion.
• Fatigue corrosion failure. The process of gradual accumulation of damage.

Methods of electrochemical protection against corrosion:

• passive method;
• active method.

The essence of the passive method of electrochemical protection is to apply a special protective layer to the surface of the gas pipeline, which prevents harmful effects environment. This coverage could be:

• bitumen;
• polymer tape;
• coal tar pitch;
• epoxy resins.

In practice, it is rarely possible to apply an electrochemical coating evenly on a gas pipeline. In places of gaps, over time, the metal is still damaged.

The active method of electrochemical protection or the method of cathodic polarization is to create a negative potential on the surface of the pipeline, which prevents the leakage of electricity, thereby preventing the occurrence of corrosion.

The principle of operation of electrochemical protection

To protect the gas pipeline from corrosion, it is necessary to create a cathodic reaction and eliminate the anodic one. To do this, a negative potential is forcibly created on the protected pipeline.

Anode electrodes are placed in the ground, the negative pole of the external current source is connected directly to the cathode - the protected object. To close the electrical circuit, the positive pole of the current source is connected to the anode - an additional electrode installed in common environment with protected pipeline.

The anode in this electrical circuit performs the function of grounding. Due to the fact that the anode has a more positive potential than the metal object, its anodic dissolution occurs.

The corrosion process is suppressed under the influence of the negatively charged field of the protected object. With cathodic corrosion protection, the anode electrode will be subjected to the deterioration process directly.

To increase the service life of anodes, they are made of inert materials that are resistant to dissolution and other external influences.

An electrochemical protection station is a device that serves as a source of external current in a cathodic protection system. This installation connects to the mains, 220 W and produces electricity with set output values.

The station is installed on the ground next to the gas pipeline. It must have a degree of protection of IP34 and above, as it works outdoors.

Cathodic protection stations may have different technical specifications and functional features.

Types of cathodic protection stations:

• transformer;
• inverter.

Transformer stations of electrochemical protection are gradually becoming a thing of the past. They are a construction of a transformer operating at a frequency of 50 Hz and a thyristor rectifier. The disadvantage of such devices is the non-sinusoidal form of the generated energy. As a result, a strong current ripple occurs at the output and its power decreases.

The inverter station of electrochemical protection has an advantage over the transformer one. Its principle is based on the operation of high-frequency pulse converters. A feature of inverter devices is the dependence of the size of the transformer unit on the frequency of current conversion. With a higher signal frequency, less cable is required, and heat losses are reduced. In inverter stations, thanks to smoothing filters, the ripple level of the produced current has a lower amplitude.

The electrical circuit that puts the cathodic protection station into operation looks like this: anode grounding - soil - insulation of the protected object.

When installing a corrosion protection station, the following parameters are taken into account:

• position of anode grounding (anode-ground);
• soil resistance;
• electrical conductivity of the insulation of the object.

Drainage protection installations for a gas pipeline

With the drainage method of electrochemical protection, a current source is not required, the gas pipeline communicates with traction rails using currents wandering in the ground railway transport. An electrical interconnection is carried out due to the potential difference between the railway rails and the gas pipeline.

By means of the drainage current, a displacement of the electric field of the gas pipeline located in the ground is created. The protective role in this design is played by fuses, as well as automatic overload switches with a return, which adjust the operation of the drainage circuit after a high voltage drop.

The system of polarized electrical drainage is carried out with the help of valve block connections. Voltage regulation with this installation is carried out by switching active resistors. If the method fails, more powerful electrical drains are used in the form of electrochemical protection, where a railway rail serves as an anode ground electrode.

Installations of galvanic electrochemical protection

The use of protective installations for galvanic protection of the pipeline is justified if there is no voltage source near the object - power lines, or the gas pipeline section is not impressive enough in size.

Galvanic equipment serves to protect against corrosion:

• underground metal structures not connected by an electrical circuit to external current sources;
• individual unprotected parts of gas pipelines;
• parts of gas pipelines that are isolated from the current source;
• pipelines under construction, temporarily not connected to corrosion protection stations;
• other underground metal structures (piles, cartridges, tanks, supports, etc.).

Galvanic protection will work the best way in soils with specific electrical resistance within 50 ohms.

Plants with extended or distributed anodes

When using a corrosion protection transformer station, the current is distributed along a sinusoid. This adversely affects the protective electric field. There is either excessive voltage at the place of protection, which entails a high consumption of electricity, or an uncontrolled leakage of current, which makes the electrochemical protection of the gas pipeline ineffective.

The practice of using extended or distributed anodes helps to circumvent the problem of uneven distribution of electricity. The inclusion of distributed anodes in the gas pipeline electrochemical protection scheme helps to increase the corrosion protection zone and smooth the voltage line. Anodes with this scheme are placed in the ground, throughout the entire gas pipeline.

Adjusting resistance or special equipment provides a change in current within the required limits, the voltage of the anode ground changes, with the help of this the protective potential of the object is regulated.

If several grounding conductors are used at once, the voltage of the protective object can be changed by changing the number of active anodes.

The ECP of a pipeline by means of protectors is based on the potential difference between the protector and the gas pipeline located in the ground. soil in this case is an electrolyte; the metal is restored, and the body of the protector is destroyed.

Video: Protection against stray currents

Electrochemical corrosion protection consists of cathodic and drainage protection. Cathodic protection of pipelines is carried out by two main methods: the use of metal anode protectors (galvanic cathodic method) and the use of external direct current sources, the minus of which is connected to the pipe, and the plus to anode grounding (electrical method).

Rice. 1. Working principle of cathodic protection

Galvanic sacrificial corrosion protection

The most obvious way to implement electrochemical protection of a metal structure in direct contact with the electrolytic environment is the galvanic protection method, which is based on the fact that different metals in the electrolyte have different electrode potentials. Thus, if you form a galvanic pair of two metals and place them in an electrolyte, then the metal with a more negative potential will become a protective anode and will be destroyed, protecting the metal with a less negative potential. Protectors essentially serve as portable sources of electricity.

Magnesium, aluminum and zinc are used as the main materials for the manufacture of protectors. From a comparison of the properties of magnesium, aluminum and zinc, it can be seen that of the considered elements, magnesium has the highest electromotive force. At the same time, one of the most important practical characteristics of protectors is the coefficient useful action, showing the proportion of the tread mass used to obtain useful electrical energy in the chain. K.P.D. protectors made of magnesium and magnesium alloys, rarely exceed 50% c, in contrast to Zn and Al-based protectors with efficiency. 90% or more.

Rice. 2. Examples of magnesium protectors

Typically, protective installations are used for cathodic protection of pipelines that do not have electrical contacts with adjacent extended communications, individual sections of pipelines, as well as tanks, steel protective casings (cartridges), underground tanks and tanks, steel supports and piles, and other concentrated objects.

At the same time, tread units are very sensitive to errors in their placement and configuration. Incorrect selection or placement of tread settings results in sharp decline their effectiveness.

Cathodic corrosion protection

The most common method of electrochemical protection against corrosion of underground metal structures is cathodic protection, carried out by cathodic polarization of the protected metal surface. In practice, this is implemented by connecting the protected pipeline to the negative pole of an external DC source, called a cathodic protection station. The positive pole of the source is connected by a cable to an external additional electrode made of metal, graphite or conductive rubber. This external electrode is placed in the same corrosive environment as the protected object, in the case of underground field pipelines, in the soil. Thus, a closed electrical circuit: additional external electrode - soil electrolyte - pipeline - cathode cable - direct current source - anode cable. As part of this electrical circuit, the pipeline is the cathode, and an additional external electrode connected to the positive pole of the DC source becomes the anode. This electrode is called an anode ground. The negatively charged pole of the current source connected to the pipeline, in the presence of external anode grounding, cathodically polarizes the pipeline, while the potential of the anode and cathode sections is practically equalized.

Thus, the cathodic protection system consists of a protected structure, a direct current source (cathodic protection station), anode grounding, connecting anode and cathode lines, an electrically conductive medium (soil) surrounding them, as well as monitoring system elements - control and measuring points.

Drainage corrosion protection

Drainage protection of pipelines against corrosion by stray currents is carried out by directed removal of these currents to the source or to the ground. Installation of drainage protection can be of several types: earthen, straight, polarized and reinforced drainage.

Rice. 3. Drainage protection station

Earth drainage is carried out by grounding the pipelines with additional electrodes in the places of their anode zones, direct drainage - by creating an electrical jumper between the pipeline and the negative pole of the stray current source, for example, the rail network of an electrified railway. Polarized drainage, unlike direct drainage, has only one-sided conductivity, therefore, when a positive potential appears on the rails, the drainage is automatically turned off. In reinforced drainage, a current converter is additionally included in the circuit, which allows increasing the drainage current.

When laying an insulated pipeline in a trench and its subsequent backfilling, 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 aboveground, 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 carried out using an external direct current source, then such protection is called cathodic, 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 using 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 closes 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 tank farms are protected by cathode installations with various types anode grounds. Required Force protective current of the cathode installation is determined by the formula

J dr \u003d j 3 F 3 K 0

where j 3 - the required value of the protective current density; F 3 - total contact surface underground structures with soil; K 0 - the coefficient of bare communications, the value of which is determined depending on the transition resistance of the insulating coating Rnep and the electrical resistivity of the soil pg 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 along the conductor occurs 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, 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.