Corrosion is a chemical and electrochemical reaction of a metal with its environment, causing its damage. It proceeds at different speeds, which can be reduced. From a practical point of view, the anti-corrosion cathodic protection of metal structures in contact with the ground, with water and with transported media is of interest. The outer surfaces of pipes are especially damaged by the influence of soil and stray currents.
Corrosion inside depends on the properties of the medium. If it is a gas, it must be thoroughly cleaned of moisture and aggressive substances: hydrogen sulfide, oxygen, etc.
Principle of operation
The objects of the process of electrochemical corrosion are the medium, metal and the interface between them. The environment, which is usually wet soil or water, has good electrical conductivity. An electrochemical reaction occurs at the interface between it and the metal structure. If the current is positive (anode electrode), iron ions pass into the surrounding solution, which leads to a loss of metal mass. The reaction causes corrosion. At a negative current (cathode electrode), these losses are absent, since electrons pass into the solution. The method is used in electroplating for applying non-ferrous metal coatings to steel.
Cathodic corrosion protection is carried out when a negative potential is applied to an object made of iron.
For this, an anode electrode is placed in the ground and a positive potential from a power source is connected to it. Minus is fed to the protected object. Cathode-anode protection leads to the active destruction of only the anode electrode from corrosion. Therefore, it should be changed periodically.
Negative effects of electrochemical corrosion
Corrosion of structures can occur from the action of stray currents from other systems. They are useful for target objects, but cause significant harm to nearby structures. Stray currents can spread from the rails of electrified vehicles. They pass towards the substation and enter pipelines. When they exit from them, anode sections are formed that cause intense corrosion. For protection, electric drainage is used - a special tap of currents from the pipeline to their source. Here, cathodic protection of pipelines against corrosion is also possible . For this, it is necessary to know the value of stray currents, which is measured by special devices.
Based on the results of electrical measurements, a method of protecting the gas pipeline is selected. A universal tool is a passive method of isolating pipes from contact with the ground using insulating coatings. The cathodic protection of a gas pipeline refers to the active method.
Pipeline protection
Structures in the ground protect against corrosion if you connect a minus source of direct current to them, and a plus to anode electrodes buried nearby in the ground. Current will flow to the structure, protecting it from corrosion. Thus, cathodic protection of pipelines, reservoirs or pipelines located in the ground is performed.
The anode electrode will be destroyed and should be replaced periodically. For a tank filled with water, electrodes are placed inside. In this case, the liquid will be an electrolyte through which current will flow from the anodes to the surface of the vessel. The electrodes are well controlled and easy to replace. In the ground it is more difficult to do.
Source of power
Near oil and gas pipelines, in heating and water supply networks, for which cathodic protection is required, stations are installed from which voltage is supplied to the objects. If they are placed outdoors, their degree of protection should not be lower than IP34. Any room is suitable for dry rooms.
Stations of cathodic protection of gas pipelines and other large structures have a capacity of 1 to 10 kW.
Their energy parameters primarily depend on the following factors:
- resistance between soil and anode;
- soil conductivity;
- length of the protective zone;
- insulating effect of the coating.
Traditionally, the cathodic protection converter is a transformer installation. Now it is being replaced by an inverter one, which has smaller dimensions, better current stability and greater efficiency. In important areas, controllers are installed that have the functions of regulating current and voltage, equalizing protective potentials, etc.
The equipment is on the market in various versions. For specific needs, individual design is used to ensure the best operating conditions.
Current Source Parameters
To protect against corrosion for iron, the protective potential is 0.44 V. In practice, it should be greater due to the influence of inclusions and the state of the metal surface. The maximum value is 1 V. In the presence of coatings on the metal, the current between the electrodes is 0.05 mA / m 2 . If the insulation is broken, it rises to 10 mA / m 2 .
Cathodic protection is effective in combination with other methods, since less energy is consumed. If there is a paint and varnish coating on the surface of the structure, only the places where it is broken are protected by the electrochemical method.
Features of cathodic protection
- Power sources are stations or mobile generators.
- The location of the anode earthing switches depends on the specifics of the pipelines. The placement method can be distributed or concentrated, as well as located at different depths.
- The anode material is selected with low solubility, so that it is enough for 15 years.
- The protective field potential for each pipeline is calculated. It is not regulated if there are no protective coatings on the structures.
Gazprom standard requirements for cathodic protection
- Effects throughout the life of protective equipment.
- Surge protection.
- Placing the station in block boxes or in a stand-alone anti-vandal version.
- Anode grounding is selected in areas with minimal electrical resistance of the soil.
- The characteristics of the converter are selected taking into account the aging of the protective coating of the pipeline.
Tread protection
The method is a type of cathodic protection with the connection of electrodes of a more electronegative metal through an electrically conductive medium. The difference is the lack of an energy source. The tread takes corrosion by dissolving in an electrically conductive environment.
After a few years, the anode should be replaced as it is being developed.
The effect of the anode increases with a decrease in its transition resistance with the medium. Over time, it may become coated with a corrosive layer. This leads to a violation of electrical contact. If you place the anode in a mixture of salts, which ensures the dissolution of corrosion products, the efficiency increases.
The effect of the tread is limited. The radius of action is determined by the electrical resistance of the medium and the potential difference between the anode and cathode.
Protective protection is applied in the absence of energy sources or when their use is not economically feasible. It is also disadvantageous when used in acidic environments due to the high dissolution rate of the anodes. Protectors are installed in water, in soil or in a neutral environment. Pure metal anodes usually do not. The dissolution of zinc does not occur evenly, magnesium corrodes too quickly, and a strong oxide film forms on aluminum.
Tread Materials
So that the treads possess the necessary operational properties, they are made of alloys with the following alloying additives.
- Zn + 0.025-0.15% Cd + 0.1-0.5% Al - protection of equipment located in sea water.
- Al + 8% Zn +5% Mg + Cd, In, Gl, Hg, Tl, Mn, Si (fractions of a percent) - the operation of structures in running sea water.
- Mg + 5-7% Al + 2-5% Zn - protection of small structures in soil or in water with a low concentration of salts.
Incorrect use of certain types of protectors leads to negative consequences. Magnesium anodes can cause equipment cracking due to the development of hydrogen embrittlement.
Joint tread cathodic protection with anti-corrosion coatings increases its effectiveness.
The distribution of the protective current is improved, and the anodes require much less. One magnesium anode protects the pipeline covered with bitumen for a length of 8 km, and without coating - only 30 m.
Protection of car bodies against corrosion
In case of violation of the coating, the thickness of the car body may decrease in 5 years to 1 mm, i.e., rust through. The restoration of the protective layer is important, but besides it there is a way to completely stop the corrosion process using cathodic-protective protection. If you turn the body into a cathode, metal corrosion stops. Anodes can be any conductive surface located nearby: metal plates, ground loop, garage case, wet road surface. Moreover, the effectiveness of the protection increases with the increase in the area of ββthe anodes. If the anode is a road surface, a βtailβ of metallized rubber is used to contact it. It is placed opposite the wheels to get better splashes. The tail is isolated from the body.
A plus battery is connected to the anode through a 1 kΞ© resistor and a LED connected in series with it. When the circuit is closed through the anode, when the minus is connected to the body, in normal mode, the LED hardly glows noticeably. If it burns brightly, it means that a short circuit has occurred in the circuit. The reason must be found and eliminated.
For protection, a fuse must be installed in series with the circuit.
When the car is in the garage, it is connected to the grounding anode. During movement, the connection occurs through the "tail".
Conclusion
Cathodic protection is a way to increase the operational reliability of underground pipelines and other structures. In this case, one should take into account its negative impact on neighboring pipelines from the influence of stray currents.