M. Ivanov, Ph.D. n.
Corrosion of metals, especially iron and unalloyed steel, causes great harm to devices and pipelines operated in contact with water and air. This leads to a reduction in the service life of equipment and additionally creates conditions for water contamination with corrosion products.
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As is known, corrosion is an electrochemical process in which the oxidation of a metal occurs, that is, the release of electrons by its atoms. This process takes place in a microscopic part of the surface called the anodic region. It leads to a violation of the integrity of the metal, the atoms of which enter into chemical reactions, especially active in the presence of atmospheric oxygen and moisture.
Since metals are good conductors of electricity, the released electrons freely flow to another microscopic region, where, in the presence of water and oxygen, reduction reactions. This region is called the cathode.
The occurrence of electrochemical corrosion can be counteracted by applying a voltage from an external direct current source to shift the electrode potential of the metal to values at which the corrosion process does not occur.
On this basis, cathodic protection systems for underground pipelines, tanks and other metal structures have been built. If an electric potential is applied to the protected metal, such potential values are established on the entire surface of the metal structure at which only reduction cathode processes can occur: for example, metal cations will accept electrons and transform into ions of a lower oxidation state or neutral atoms.
Technically, the method of cathodic protection of metals is carried out as follows ( rice. 1). A wire is supplied to the metal structure to be protected, for example a steel pipeline, which is connected to the negative pole of the cathode station, as a result of which the pipeline becomes the cathode. At some distance from the metal structure, an electrode is located in the ground, which is connected with a wire to the positive pole and becomes the anode. The potential difference between the cathode and anode is created in such a way as to completely eliminate the occurrence of oxidative processes on the protected structure. In this case, weak currents will flow through the moist soil between the cathode and the anode in the soil thickness. Effective protection requires the placement of several anode electrodes along the entire length of the pipeline. If it is possible to reduce the potential difference between the protected structure and the soil to 0.85-1.2 V, then the rate of pipeline corrosion is reduced to significantly low values.
So, the cathodic protection system includes a source of constant electric current, control point and anodic grounding. Typically, a cathodic protection station consists of an AC transformer and a diode rectifier. As a rule, it is powered from a 220 V network; There are also stations powered by high (6-10 kV) voltage lines.
For the cathode station to operate effectively, the potential difference between the cathode and anode it creates must be at least 0.75 V. In some cases, about 0.3 V is sufficient for successful protection. At the same time, as technical parameters Cathodic protection stations use the nominal values of the output current and output voltage. Thus, usually the rated output voltage of stations is from 20 to 48 V. With a large distance between the anode and the protected object, the required output voltage of the station reaches 200 V.
Auxiliary inert electrodes are used as anodes. Anode grounding electrodes, for example, the AZM-3X model manufactured by JSC Katod (Razvilka village, Moscow region), are castings made of a corrosion-resistant alloy, equipped with a special wire with a copper core in reinforced insulation, as well as a sealed coupling for connection to the main cable of the cathodic protection station. It is most rational to use grounding conductors in environments with high and moderate corrosive activity when resistivity soil up to 100 Ohm.m. For optimal distribution of field strength and current density throughout the equipment body, special screens in the form of a backfill of coal or coke are placed around the anodes.
To assess the efficiency of a cathodic protection station, a system is required that consists of a measuring electrode and a reference electrode and is the main part of the control and measuring point. Based on the readings of these electrodes, the cathodic protection potential difference is regulated.
Measuring electrodes are made of high-alloy steel, silicon cast iron, platinized brass or bronze, and copper. Reference electrodes are silver chloride or copper sulfate. In my own way design reference electrodes can be submersible or remote. The composition of the solution used in them must be close to the composition of the medium, from harmful effects which the equipment needs to be protected.
One can note the long-acting bimetallic reference electrodes of the EDB type, developed by VNIIGAZ (Moscow). They are designed to measure the potential difference between an underground metal object (including a pipeline) and the ground to control a cathodic protection station in automatic mode under conditions of heavy load and at considerable depth, that is, where other electrodes cannot ensure constant maintenance of the given potential.
Equipment for cathodic protection is supplied mainly by domestic manufacturers. Thus, the mentioned CJSC “Kathod” offers the “Minerva-3000” station ( rice. 2), designed to protect main water supply networks. Its rated output power is 3.0 kW, output voltage is 96 V, protection current is 30 A. The accuracy of maintaining the protective potential and current value is 1 and 2%, respectively. The ripple value is no more than 1%.
Another Russian manufacturer- JSC Energomera (Stavropol) - supplies modules of the MKZ-M12, PNKZ-PPCh-M10 and PN-OPE-M11 brands, providing effective cathodic protection of underground metal structures in areas of high corrosion hazard. The MKZ-M12 module has a rated current of 15 or 20 A; rated output voltage is 24 V. For models MKZ-M12-15-24-U2, the output voltage is 30 V. The accuracy of maintaining the protective potential reaches ±0.5%, the specified current is ±1%. The technical resource is 100 thousand hours, and the service life is at least 20 years.
LLC "Electronic Technologies" (Tver) offers cathodic protection stations "Tvertsa" ( rice. 3), equipped with a built-in microprocessor and a telemechanical remote control system. Control and measuring points are equipped with non-polarizing long-acting comparison electrodes with electrochemical potential sensors, which provide measurement of polarization potentials on the pipeline. These stations also include an adjustable source of cathode current and a block of sensors for electrical parameters of the circuit, which is connected through a controller to a remote access device. The transformer of this station is made on the basis of ferrite cores of the Epcos type. A voltage converter control system based on a UCC 2808A microcircuit is also used.
The Kurs-OP company (Moscow) produces Elkon cathodic protection stations, the output voltage of which varies in the range from 30 to 96 V, and the output current in the range from 20 to 60 A. Output voltage ripple - no more than 2% . These stations are designed to protect single-strand pipelines from soil corrosion, and with the use of a joint protection unit, multi-strand pipelines in areas without stray currents in moderate climate conditions (from -45 to +40 ° C). The stations include a single-phase power transformer, a converter with stepwise regulation of the output voltage, high-voltage equipment, a two-pole manual disconnector and surge suppressors.
One can also note cathodic protection installations of the NGK-IPKZ series manufactured by NPF Neftegazkompleks EKhZ LLC (Saratov), the maximum output current of which is 20 or 100 A, and the rated output voltage is 48 V.
One of the suppliers of cathodic protection stations from the CIS countries is Hoffmann Electric Technologies (Kharkov, Ukraine), which offers equipment for electrochemical protection against soil corrosion of main pipelines.
Electrochemical protection – effective method protection of finished products from electrochemical corrosion. In some cases it is impossible to renew the paint coating or protective wrapping material, then it is advisable to use electrochemical protection. Covering an underground pipeline or bottom sea vessel it is very labor-intensive 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 area of intense dissolution of the base metal or repassivation. Those. when there is intense destruction of metal structures.
The essence of electrochemical protection
A direct current (DC source or protector) is connected to the finished metal product from the outside. 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 due to the influence of a corrosive environment, it is not the metal of the structure that is destroyed, but the anode.
Depending on which direction (positive or negative) the metal potential shifts, electrochemical protection is divided into anodic and cathodic.
Cathodic corrosion protection
Cathodic electrochemical corrosion protection is used when the metal being protected 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 to the product from the negative pole, which polarizes the cathode sections of the corrosive elements, bringing the potential value closer to the anodic ones. The positive pole of the current source is connected to the anode. In this case, corrosion of the protected structure is almost reduced to zero. The anode gradually deteriorates and must be replaced periodically.
There are several options for cathodic protection: polarization from an external source of electric current; reducing the rate of the cathodic process (for example, deaeration of the electrolyte); contact with a metal whose free corrosion potential in a given environment is more electronegative (so-called sacrificial protection).
Polarization from an external source of electric current is used very often to protect structures located in soil, water (bottoms of ships, etc.). Besides this type 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.
The external current source is cathodic protection stations, which consist of a rectifier (converter), a current supply to the protected structure, anode grounding conductors, a reference electrode and an anode cable.
Cathodic protection Both independent and additional types of corrosion protection are used.
The main criterion by which one can judge the effectiveness of cathodic protection is protective potential. Protective potential is the potential at which the rate of metal corrosion in certain conditions environment takes the lowest (as far as possible) value.
There are disadvantages to using cathodic protection. One of them is danger redefense. Overprotection is observed with a large shift in the potential of the protected object in negative side. At the same time it stands out. The result is the destruction of protective coatings, hydrogen embrittlement of the metal, and corrosion cracking.
Tread protection (use of protector)
A type of cathodic protection is sacrificial. When using sacrificial protection, a metal with a more electronegative potential is connected to the protected object. In this case, it is not the structure that 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 a small transition resistance between the protector and the environment.
Each protector has its own radius of protective action, which is determined by the maximum possible distance, on which the protector can be removed without losing the protective effect. Protective protection is most often used when it is impossible or difficult and expensive to supply current to the structure.
Protectors are used to protect structures in neutral environments (sea or river water, air, soil, etc.).
The following metals are used to make protectors: magnesium, zinc, iron, aluminum. Pure metals do not fully fulfill their protective functions, therefore, when manufacturing protectors, they are additionally alloyed.
Iron protectors are made from carbon steel 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 sea corrosion (in salt water). If a zinc protector is used in slightly salted, fresh water or soils, it quickly becomes covered with a thick layer of oxides and hydroxides.
Magnesium protector
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 explained by the high rate of dissolution of magnesium 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 added to such protectors. Aluminum protectors are used in the coastal shelf and flowing sea water.
Anodic corrosion protection
Anodic electrochemical protection is used for structures made of titanium, low-alloy stainless steels, carbon steels, ferrous high-alloy alloys, and dissimilar passivating metals. Anodic protection is used in highly electrically conductive corrosive environments.
With anodic protection, the potential of the protected metal shifts in a more positive direction until a passive stable state of the system is achieved. 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 manufactured product and environment.
Anodic protection can be implemented in several ways: by shifting the potential in a positive direction 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.
Anodic protection using oxidizing agents defense mechanism similar to anodic polarization.
If passivating inhibitors with oxidizing properties are used, the protected surface becomes passive under the influence of the generated current. These include dichromates, nitrates, etc. But they pollute the surrounding technological environment quite heavily.
When additives are introduced into the alloy (mainly alloying with a noble metal), the depolarizer reduction reaction occurring at the cathode occurs with a lower overvoltage than at the protected metal.
If an electric current is passed through the protected structure, the potential shifts in a positive direction.
The installation for anodic electrochemical corrosion protection 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 one can 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, poorly soluble 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 passivable 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 anodic protection, the cathode arrangement is designed individually.
In order for anodic protection to be effective for a certain object, it is necessary that it meets certain requirements:
All welds must be made with high quality;
In a technological environment, the material from which the protected object is made must pass into a passive state;
The number of air pockets and cracks should be minimal;
There should be no rivet joints on the structure;
In the device being protected, the reference electrode and cathode must always be in solution.
To implement anodic protection in chemical industry Heat exchangers and installations that have a cylindrical shape are often used.
Electrochemical anodic protection of stainless steels is applicable for industrial storage of sulfuric acid, ammonia-based solutions, mineral fertilizers, as well as all kinds of collectors, tanks, and measuring tanks.
Anodic protection can also be used to prevent corrosive destruction of electroless nickel plating baths, heat exchange units in the production of artificial fibers and sulfuric acid.
Pipelines are by far the most common means of transporting energy carriers. Their obvious drawback is their susceptibility to rust. For this purpose, cathodic protection of main pipelines from corrosion is performed. What is its principle of operation?
Causes of corrosion
Networks of pipelines for life support systems are distributed throughout Russia. With their help, gas, water, petroleum products and oil are efficiently transported. Not long ago, a pipeline was laid to transport ammonia. Most types of pipelines are made of metal, and their main enemy is corrosion, of which there are many types.
The reasons for the formation of rust on metal surfaces are based on the properties of the environment, both external and internal corrosion of pipelines. Risk of corrosion for internal surfaces based on:
- Interaction with water.
- The presence of alkalis, salts or acids in the water.
Such circumstances may arise on main water supply systems, hot water supply (DHW), steam and heating systems. No less important factor is the method of laying the pipeline: above-ground or underground. The first one is easier to maintain and eliminate the causes of rust formation compared to the second one.
With the pipe-to-pipe installation method, the risk of corrosion is low. When directly installing a pipeline outdoors, rust may form due to interaction with the atmosphere, which also leads to a change in design.
Pipelines located underground, including steam and hot water, are most vulnerable to corrosion. The question arises about the susceptibility to corrosion of pipes located at the bottom of water sources, but only a small part of the pipelines are located in these places.
According to their purpose, pipelines with a risk of corrosion are divided into:
- main lines;
- fishing;
- for heating and life support systems;
- for waste water from industrial enterprises.
Susceptibility to corrosion of main pipeline networks
Corrosion of pipelines of this type is most well studied, and their protection from exposure external factors defined by standard requirements. Regulatory documents discuss methods of protection, and not the reasons for the formation of rust.
It is equally important to take into account that in this case only external corrosion is considered, to which the outer section of the pipeline is susceptible, since inert gases pass inside the pipeline. In this case, contact of the metal with the atmosphere is not so dangerous.
For protection against corrosion, according to GOST, several sections of the pipeline are considered: increased and high danger, as well as corrosion-hazardous.
Impact of negative factors from the atmosphere for areas increased danger or types of corrosion:
- Stray currents arise from direct current sources.
- Exposure to microorganisms.
- The created stress provokes cracking of the metal.
- Waste storage.
- Salty soils.
- The temperature of the transported substance is above 300 °C.
- Carbon dioxide corrosion of an oil pipeline.
An installer for protecting underground pipelines from corrosion must know the design of the pipeline and the requirements of SNiP.
Electrochemical corrosion from soil
Due to the difference in voltages formed in individual sections of pipelines, an electron flow occurs. The process of rust formation occurs according to the electrochemical principle. Based on this effect, part of the metal in the anodic zones cracks and flows into the base of the soil. After interaction with the electrolyte, corrosion forms.
One of the significant criteria for ensuring protection against negative manifestations is the length of the line. On the way there are soils with different composition and characteristics. All this contributes to the emergence of a voltage difference between parts of the laid pipelines. The mains have good conductivity, so the formation of galvanic couples with a fairly large extent occurs.
An increase in the rate of pipeline corrosion provokes high density electron flow. The depth of the lines is no less important, since it retains a significant percentage of humidity and the temperature is not allowed to fall below the “0” mark. Mill scale also remains on the surface of the pipes after processing, and this affects the appearance of rust.
By conducting research work A direct relationship has been established between the depth and area of rust formed on the metal. This is based on the fact that metal with a larger surface area is most vulnerable to external negative manifestations. Special cases include the occurrence of significantly smaller amounts of destruction on steel structures under the influence of the electrochemical process.
The aggressiveness of soils to metal is, first of all, determined by their own structural component, humidity, resistance, alkali saturation, air permeability and other factors. The installer for the protection of underground pipelines from corrosion must be familiar with the pipeline construction project.
Corrosion under the influence of stray currents
Rust can arise from an alternating and constant flow of electrons:
- Rust formation under the influence of constant current. Stray currents are currents found in the soil and in structural elements located underground. Their origin is anthropogenic. They arise as a result of the operation of technical devices of direct current, spreading from buildings or structures. They can be welding inverters, cathode protection systems and other devices. The current tends to follow the path of least resistance, as a result, with existing pipelines in the ground, it will be much easier for the current to pass through the metal. The anode is the section of the pipeline from which the stray current exits to the soil surface. The part of the pipeline into which the current enters acts as a cathode. On the described anodic surfaces, currents have an increased density, so it is in these places that significant corrosion spots form. The corrosion rate is not limited and can be up to 20 mm per year.
- Rust formation under the influence of alternating current. When located near power lines with a network voltage of over 110 kV, as well as in a parallel arrangement of pipelines, corrosion occurs under the influence of alternating currents, including corrosion under the insulation of pipelines.
Stress Corrosion Cracking
If a metal surface is simultaneously exposed to external negative factors And high voltage from power lines that create tensile forces, rust formation occurs. According to the research carried out, the new hydrogen-corrosion theory gained its place.
Small cracks are formed when the pipe is saturated with hydrogen, which then ensures an increase in pressure from the inside to levels higher than the required equivalent of the bond of atoms and crystals.
Under the influence of proton diffusion, hydrogenation of the surface layer occurs under the influence of hydrolysis at elevated levels cathodic protection and simultaneous exposure to inorganic compounds.
After the crack opens, the rusting process of the metal accelerates, which is provided by the ground electrolyte. As a result, under the influence of mechanical influences, the metal undergoes slow destruction.
Corrosion due to microorganisms
Microbiological corrosion is the process of rust formation on a pipeline under the influence of living microorganisms. These can be algae, fungi, bacteria, including protozoa. It has been established that the proliferation of bacteria most significantly influences this process. To maintain the vital activity of microorganisms, it is necessary to create conditions, namely nitrogen, humidity, water and salts. Also the conditions are:
- Temperature and humidity indicators.
- Pressure.
- Availability of lighting.
- Oxygen.
Organisms that produce acidic conditions can also cause corrosion. Under their influence, cavities appear on the surface, which are black in color and bad smell hydrogen sulfide. Sulfate-containing bacteria are present in virtually all soils, but the rate of corrosion increases as their numbers increase.
What is electrochemical protection
Electrochemical protection of pipelines against corrosion is a set of measures aimed at preventing the development of corrosion under the influence of an electric field. Specialized rectifiers are used to convert direct current.
Corrosion protection is achieved by creating electromagnetic field, as a result of which a negative potential is acquired or the area acts as a cathode. That is, a section of steel pipelines, protected from rust formation, acquires a negative charge, and the grounding becomes positive.
Cathodic protection of pipelines against corrosion is accompanied by electrolytic protection with sufficient conductivity of the medium. This function is performed by soil when laying metal underground highways. Contacting of the electrodes is carried out through conductive elements.
The indicator for determining corrosion indicators is a high-voltage voltmeter or corrosion gauge. Using this device, the indicator between the electrolyte and the soil is monitored, specifically for this case.
How is electrochemical protection classified?
Corrosion and protection of main pipelines and tanks from it are controlled in two ways:
- A current source is connected to the metal surface. This area acquires a negative charge, that is, it acts as a cathode. Anodes are inert electrodes that have nothing to do with design. This method is considered the most common, and electrochemical corrosion does not occur. This technique is aimed at preventing the following types of corrosion: pitting, due to the presence of stray currents, crystalline type of stainless steel, as well as cracking of brass elements.
- Galvanic method. Protection of main pipelines or sacrificial protection is carried out by metal plates with high levels of negative charges, made of aluminum, zinc, magnesium or their alloys. Anodes are two elements, so-called inhibitors, while the slow destruction of the protector helps maintain the cathode current in the product. Protective protection is used extremely rarely. ECP is performed on the insulating coating of pipelines.
About the features of electrochemical protection
The main cause of pipeline destruction is the result of corrosion of metal surfaces. After the formation of rust, cracks, ruptures, and cavities form, which gradually increase in size and contribute to the rupture of the pipeline. This phenomenon occurs more often near highways laid underground or in contact with groundwater.
The principle of cathodic protection is the creation of a voltage difference and the action of the two methods described above. After carrying out measuring operations directly at the location of the pipeline, it was found that the required potential to help slow down the destruction process should be 0.85V, and for underground elements this value is 0.55V.
To slow down the corrosion rate, the cathode voltage should be reduced by 0.3V. In this situation, the corrosion rate will not exceed 10 microns/year, and this will significantly extend the service life of technical devices.
One of the significant problems is the presence of stray currents in the soil. Such currents arise from the grounding of buildings, structures, rail tracks and other devices. Moreover, it is impossible to make an accurate assessment of where they may appear.
To create a destructive effect, it is enough to charge steel pipelines with a positive potential in relation to the electrolytic environment, these include pipelines laid in the ground.
In order to provide the circuit with current, it is necessary to supply an external voltage, the parameters of which will be sufficient to break through the resistance of the soil foundation.
As a rule, such sources are power lines with power ratings from 6 to 10 kW. If electric current cannot be supplied, then diesel or gas generators can be used. The installer for the protection of underground pipelines from corrosion must be familiar with the design solutions before performing work.
Cathodic protection
To reduce the percentage of rust on the surface of pipes, electrode protection stations are used:
- Anode, made in the form of grounding conductors.
- Converters of constant electron flows.
- Equipment for process control and monitoring of this process.
- Cable and wire connections.
Cathodic protection stations are quite effective; when directly connected to a power line or generator, they provide an inhibitory effect of currents. This ensures protection of several sections of the pipeline simultaneously. Parameters can be adjusted manually or automatically. In the first case, transformer windings are used, and in the second, thyristors are used.
The most common in Russia is the high-tech installation - Minevra -3000. Its power is sufficient to protect 30,000 m of highways.
Advantages of the technical device:
- high power characteristics;
- updating the operating mode after overloads in a quarter of a minute;
- using digital regulation, operating parameters are monitored;
- tightness of highly critical connections;
- connecting the device to remote process control.
ASKG-TM are also used, although their power is low, they are equipped with a telemetry complex or remote control allows them to be no less popular.
A diagram of the insulation main of the water supply or gas pipeline must be available at the work site.
Video: cathodic protection against corrosion - what is it and how is it performed?
Corrosion protection by installing drainage
The corrosion protection installer for underground pipelines must be familiar with the drainage system. Such protection against rust formation of pipelines from stray currents is carried out by a drainage device necessary to divert these currents to another section of the ground. There are several drainage options.
Types of execution:
- Executed underground.
- Straight.
- With polarities.
- Reinforced.
When carrying out earthen drainage, electrodes are installed in the anode zones. To ensure a straight drainage line, an electrical jumper is made connecting the pipeline to the negative pole of current sources, for example, grounding from a residential building.
Polarized drainage has one-way conductivity, that is, when a positive charge appears on the ground loop, it automatically turns off. Enhanced drainage operates from a current converter additionally connected to electrical diagram, and this improves the removal of stray currents from the main line.
The increase for pipeline corrosion is carried out by calculation, according to the RD.
In addition, inhibitor protection is used, that is, it is used on pipes special composition for protection against aggressive environments. Standstill corrosion occurs when boiler equipment is idle for a long time; to prevent this from happening, equipment maintenance is necessary.
An installer for the protection of underground pipelines from corrosion must have knowledge and skills, be trained in the Rules and periodically undergo a medical examination and pass exams in the presence of an inspector from Rostechnadzor.
Protecting metal from corrosion by applying an external direct electric current, which radically changes the electrode potential of the material and changes the rate of its corrosion, is called electrochemical protection. It reliably protects surfaces from corrosion, preventing the destruction of underground tanks, pipelines, ship bottoms, gas tanks, hydraulic structures, gas pipelines, etc. This method is used in cases where the corrosion potential is in a zone of intense decay or during passivation, that is, when active destruction of metal structures occurs.
Operating principle of electrochemical protection
A source of direct electric current is connected to the metal structure from the outside. On the surface of the product, an electric current forms a cathodic polarization of the electrodes, as a result of which an exchange occurs and the anodic areas are transformed into cathodic ones. As a result, under the influence of a corrosive environment, the anode is destroyed, and not the source material. This kind of protection is divided into cathodic and anodic, it depends on which direction (negative or positive) the potential of the metal shifts.
Cathodic corrosion protection
Example: (+0.8)Au/Fe(-0.44)
To increase the stability of metal parts in contact with any aggressive environment or during operation exposed to sea water or soil, cathodic corrosion protection is used. In this case, cathodic polarization of the stored metal is achieved by forming a microgalvanic couple with another metal (aluminum, zinc, magnesium), reducing the speed of the cathodic process (deaeration of the electrolyte) or applying an electric current from an external source.
This technique is usually used to preserve ferrous metals, because most of the objects located in soil and water are made from them - for example, piers, pile structures, pipelines. This method has also found wide application in mechanical engineering, in the prevention of corrosion processes in new and in-use machines, in the treatment of car bodies, cavities of side members, chassis components, etc. It should be noted that the same method is used to produce effective protection the underbody of the car, which is most often exposed to aggressive environments.
Cathodic protection, with many advantages, still has disadvantages. One of them is an excess of protection; this phenomenon is observed when the potential of the stored product is strongly shifted in the negative direction. The result is metal fragility, corrosion cracking of the material and destruction of all protective coatings. Its type is tread protection. When using it, a metal with a negative potential (protector) is attached to the stored product, which is subsequently destroyed, preserving the object.
Anodic protection
Example: (-0.77)Cd/Fe(-0.44) 
Anodic protection against metal corrosion is used for products made of high-alloy ferrous alloys, carbon and acid-resistant steel, located in corrosive environments with good electrical conductivity. With this method, the potential of the metal shifts in a positive direction until it reaches a stable (passive) state.
The anode electrochemical installation includes: a current source, a cathode, a reference electrode and a stored object.
In order for protection to be as effective as possible for any specific item, certain rules must be followed:
minimize the number of cracks, crevices and air pockets;
the quality of welds and connections of metal structures must be maximum;
the cathode and reference electrode must be placed in the solution and remain there permanently
METAL STRUCTURES"
Cathodic protection of underground metal structures
Operating principle of cathodic protection
When metal comes into contact with soils related to electrolytic environments, a corrosion process occurs, accompanied by the formation of an electric current, and a certain electrode potential is established. The magnitude of the electrode potential of the pipeline can be determined by the potential difference between two electrodes: the pipeline and the non-polarizing copper sulfate element. Thus, the value of the pipeline potential is the difference between its electrode potential and the potential of the reference electrode with respect to the ground. On the surface of the pipeline, electrode processes occur in a certain direction and changes in time are stationary in nature.
Stationary potential is usually called natural potential, implying the absence of stray and other induced currents on the pipeline.
The interaction of a corroding metal with an electrolyte is divided into two processes: anodic and cathodic, which take place simultaneously at various areas interface between metal and electrolyte.
When protecting against corrosion, territorial separation of the anodic and cathodic processes is used. A current source with an additional grounding electrode is connected to the pipeline, with the help of which an external direct current is applied to the pipeline. In this case, the anodic process occurs on an additional grounding electrode.
Cathodic polarization of underground pipelines is carried out by applying an electric field from an external direct current source. The negative pole of the direct current source is connected to the structure being protected, while the pipeline is the cathode in relation to the ground, and the artificially created grounding anode is the positive pole.
Schematic diagram cathodic protection is shown in Fig. 14.1. With cathodic protection, the negative pole of the current source 2 is connected to the pipeline 1, and the positive pole is connected to the artificially created anode-grounding device 3. When the current source is turned on, the current source from its pole through the anodic grounding enters the ground and through damaged areas of insulation 6 to the pipe. Then, through the drainage point 4 along the connecting wire 5, the current returns again to the minus of the power source. In this case, the process of cathodic polarization begins in the exposed sections of the pipeline.
Rice. 14.1. Schematic diagram of pipeline cathodic protection:
1 - pipeline; 2 - external source direct current; 3 - anodic grounding;
4 - drainage point; 5 - drainage cable; 6 - cathode terminal contact;
7 - cathode terminal; 8 - damage to pipeline insulation
Since the voltage of the external current applied between the grounding electrode and the pipeline significantly exceeds the potential difference between the electrodes of the corrosion macropairs of the pipeline, the stationary potential of the anodic grounding does not play a decisive role.
With the inclusion of electrochemical protection ( j 0a.add) the distribution of currents of corrosion macropairs is disrupted, the values of the potential difference “pipe - ground” of the cathode sections ( j 0k) with the potential difference of the anode sections ( j 0a), conditions for polarization are provided.
Cathodic protection is regulated by maintaining the required protective potential. If, by applying an external current, the pipeline is polarized to the equilibrium potential ( j 0k = j 0a) dissolution of the metal (Fig. 14.2 a), then the anodic current stops and corrosion stops. A further increase in the protective current is impractical. At more positive potential values, the phenomenon of incomplete protection occurs (Fig. 14.2 b). It can occur during cathodic protection of a pipeline located in an area of strong influence of stray currents or when using protectors that do not have a sufficiently negative electrode potential (zinc protectors).
The criteria for protecting metal from corrosion are protective current density and protective potential.
Cathodic polarization of a bare metal structure to the protective potential requires significant currents. The most probable values of current densities required for the polarization of steel in various environments to the minimum protective potential (-0.85 V) in relation to the copper-sulfate reference electrode are given in Table. 14.1
Rice. 14.2. Corrosion diagram for the case of full polarization (a) and
incomplete polarization (b)
Typically, cathodic protection is used in conjunction with insulating coatings applied to the outer surface of the pipeline. Surface coating reduces the required current by several orders of magnitude. Thus, for cathodic protection of steel with a good coating in soil, only 0.01 ... 0.2 mA/m 2 is required.
Table 14.1
Current density required for cathodic protection
bare steel surface in various environments
The protective current density for insulated main pipelines cannot become a reliable protection criterion due to the unknown distribution of damaged pipeline insulation, which determines the actual contact area of the metal with the ground. Even for an uninsulated pipe (cartridge at an underground passage through railways and highways), the protective current density is determined by the geometric dimensions of the structure and is fictitious, since the proportion of the surface of the cartridge remains unknown, covered with constantly present passive protective layers (scale, etc.) and not participating during the process of depolarization. Therefore, the protective current density as a protection criterion is used for some laboratory research performed on metal samples.
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