The reason of soil corrosion of underground metal constructions is overflowing of electrons from the metal into the soil. It can be prevented by the establishment of direct current between an external anode (protector), made from more electronegative element as compared with the cathode (pipeline or reservoir). Although the total loss of metal at the same time increase, metal framework remains intact, and even local corrosion defects cease to grow.
During the operation of the objects protected only by anticorrosive coverings, rust-through damage of metal arises in 5-8 years (with stray currents. Installation of protectors can significantly increase the time limit and reduce operating costs for corrosion protection of pipelines and reservoirs.
"This kind of protection is used for relatively small structures (sea tankers, stationary and sinking platforms, jetties and piers in ports, gas and oil pipelines, storage facilities such as RVS-100000, -500000, -1000000 could hardly be called as a “small "- note . EnergyLand.info) or additionally coated with insulation metal constructions (eg, pipelines) with low power consumption, - says Valentin Grigoriev, professor, doctor of chemical sciences, Head of the Department of Electrochemistry, Chemical Faculty, Rostov State University. - the specified protection is effective. With the help of one magnesian anode we can protect up to 8 km of the pipeline with a covering, without it - only 30 m. Protector Defence is widespread - for example, in the U.S. for the production of anodes annually consumes about 11.5 tons of aluminum.
On the right you can see the Photo 4 - Magnesium protectors (unpacked) PM5U,-10U and-20U (photo ZAO PPMTS Permsnabsbyt ")
The given method is especially shown:- On the dangerous sections of pipelines, where is unprofitable or impossible to build a station of cathodic protection - CPS (underground passages, floodplains, ecologically fragile zone in the territory of industrial enterprises) as well as in zones of influence stray currents;
- On the pipeline in conjunction with the CPS to ensure the security potential on the section between the CPS;
- On main pipelines away from the sources of electricity;
- On Tanks for corrosive liquids - oil and petroleum products, commercial water, etc. (in which chemical decomposition of oil emulsions is allocated a mineralized water), causing extensive corrosion of the bottom and lower zones;
- On steel structures mounted in soils with high corrosion activity by State standards: 9.60289, R 5116498 (in this situation the protective coating is not enough);
- On offshore installations, ships, tankers, etc.
Material and design of a protector
The most wide spread for the production of protectors in the world are used alloys based on the three metals - zinc, magnesium, and aluminum (much less). In Russia traditionally for the mass production of anodes are used magnesium-based alloys of brands MP1 and MP2 State standard: 2625184. Many marks of magnesium, zinc and aluminum alloys are developed and are applied. In the production of some special protectors are also used niobium, silver, gold, platinum and other metals.
On the left the Photo 5 - Scheme of protection of the tank: 1 - Cathodic group, 2 - wire (rod), 3 – milestones for potential measurements, 4 - circuit on which are placed cathodic groups (Fig. Management Company SheshmaOyl)"For the protection of underground metal constructions, mainly - pipes, are used magnesium protectors, because only magnesium alloys are capable in these conditions to adequately protect metal, - said Alexander Redekop, gen. Director of "PPMTS Permsnabsbyt" (Perm). - Traditionally on the majority of the enterprises maintaining pipeline transport with magnesium protectors, try to protect metal casings in places of transitions under roads, ways, the rivers. Anodes are often used to protect product lines.
Most revealing is the experience of Tatneft, which indicates that the exploitation of magnesium anodes in the ground can be 10-15 years. 45-50% of all Tatneft piping is protected by protectors. Moreover, all new piping with two-sided polyethylene coating are protected from the adverse impacts only by magnesium anodes. They are widely used to protect tanks at the site of their contact with the ground. "
Magnesium anodes have different sizes: from a rectangular configuration with various sections to round, size from several millimeters to meters and weight from a few grams to tons.
The potential difference between anode and cathode determines the area of the protective effect of protector.
In magnesium protectors anodic potential is almost two times higher than that of zinc, therefore they are required approximately twice less, than zinc or aluminum. Also magnesium and its alloys have no polarization, decreasing current-less. Aluminum is prone to the formation on the surface of a dense an oxide layer - these factors determine the preferential use of magnesium anodes for protection of underground facilities.
Normal ratio of the tread surface and protected metal - from 1:200 to 1:1000. In the situation of underground corrosion efficiency of the use of magnesium tread reaches 60%, aluminum - 20-50%, zinc - 80%. Useful life (until a protector of the body output) depends on the nature of the soil, insulation quality and weight of the tread and is 5-15 years.
On the right - Photo 7 - Aluminum treads P-PLA-5 alloy AP2 (photo NPO "RosAntikor")With the inner surface of tanks containing flammable liquids, the situation is different. Aluminum and zinc anodes, in contrast to magnesium, have low capacity to sparking. For example, the protectors from aluminum alloy AP-3 for over 10 years ensured the integrity of the inner surface of the reservoir-tanks type as FAR for some OGPD Nizhnevartovsk. Especially for tanks subjected to sand-paraffin residues (with a low electrical conductivity) on the bottoms, Federal State Unitary Enterprise "Prometheus" has developed alloy AP4N with higher anodic activity.
"Compared to aluminum, Protectors from the zinc alloy CP-1 have a moderate working capacity, - said Victor Vasilyev, commercial director of NPO" RosAntikor "(Chelyabinsk). - In addition, they completely fulfills the terms of fire safety, with their anodic dissolution not generating products that may pollute the environment and affect the quality of petroleum products. Therefore, by the current standards for cathodic protection of internal surfaces of oil tanks (especially the bottoms and lower zones), as well as trucks, and fuel tanks of oil tankers shall be apply only protectors from zinc alloy.
The use of magnesium anodes in such situations is unacceptable. First, magnesium is extremely fire-unsafe (its collision with the steel produces a spark), and at its dissolution is released hydrogen that also creates a highly explosive environment. Secondly, because of the high working capacity of the protective magnesium alloy (-1.45 W on silver chloride reference electrode) comes its rapid deterioration, unacceptable for practical use. Thirdly, in reservoirs with colored inner surface it increases the probability of failure of refinishing because of intensive cathodic polarization of painted steel near the protectors, in which segregating on the steel surface hydrogen swells paint.
On the Left - photo 2 - special water type protectors from alloy CP-1 (photo NPO "RosAntikor")
In Russia, is adjusted industrial production of standardized protectors from zinc alloy CP-1 with mass 5-168 kg, with the chemical composition of State Standard 26251-84, developed by the Federal State Unitary Enterprise "CRISM" Prometheus ".
How to install a protector in the groundFor efficient operation of the tread is usually used activator (filler, filling) - a mixture of gypsum, clay, sodium sulphate and other salts, having an increased electrical conductivity. Activator stabilizes the resistivity of the soil around the tread and helps to reduce the spreading resistance of the tread. Gypsum, which is a part of activators, prevents the formation on the surface of the tread some layers with poor conductivity. Sodium sulfate gives soluble compounds with corrosion products, which contributes to maintaining a constant steady-state capacity and reduce the resistance of the tread. Other components (eg, bentonite and tripoli) retain moisture and slow down the dissolution of salt groundwater. A layer of activator can be a thickness of 10 cm. Protectors with soldered to it drain conductor are placed together with the activator in a cotton bag, which is set in the ground.
On the Right - Photo 3 - Tread in the bag with the activator (Fig. LLC Elmet ")
There are various schemes how to install protectors - both single and in groups, depending on the structures that must be protected, and the environments in which these buildings are located.
Rate of protector’s charge on 1 km of the pipeline depends on the soil properties and the mass of the tread. For example, 10kilogrammovy magnesium protector protects a segment of the pipeline length of 1-2 km, with protectors can be installed singly or in groups of 5-15 units at a distance of 5 meters from the pipeline. There can be other variants of the pipe: with intermittent extended tread with one continuous, with two continuous - in these cases Protector fits in one trench with piping. The cost of installation of magnesium anodes is less than 5% of the total cost of the pipeline, and today is one of the most tech ways to protect against corrosion.
The bottoms of the RVS are protected by protector groups uniformly distributed along the circle with a radius of 3-5 meters longer than the radius of the tank. Anodes are placed in vertical trenches. In all cases, in the location is organized control equipment, with which we track the status of each tread.
Location and number of protectors is defined by the project. Initial data for calculation is: the resistance of coatings, the diameter of the pipeline (tank), the electrochemical characteristics of the tread and the resistivity of the soil. The main design parameters: the current in the circuit of "protector -installation", the length of the protected area and tread life. The project will determine which parts of the building have to be equipped with CPS, which -with protectors.
|
Resistivity. Ohm * m |
Soil Corrosivity |
Applied Protectors |
|
Up to 5 |
very high |
Magnesium and zinc
protectors weighing 20kg (with PH <4 magnesium protectors are not applied) |
|
5-10 |
high |
Magnesium and zinc
protectors weighing 10 - 20kg (with PH <4 magnesium protectors are not
applied) |
|
10-20 |
increased |
Magnesium protectors weighing 10kg |
|
20-50 |
average |
Magnesium protectors weighing 5kg |
Areas
of application of protectors, depending on the corrosiveness of the soil
Coatings has not been canceled
Although the Protector is able to protect steel structures from corrosion and without dyeing (it is sufficient to ensure a higher density of protective current, which would require increasing the number of protectors. It is more appropriate to use the protectors at the same time with covering.) The effect from the combination of protections is very high and is synergistic in nature: the "bare" steel pipe in the ground requires the installation of magnesium anodes every 30 m, and isolated is protected by the same tread pattern for 8 km. As for tanks, the integrated protection often provides a threefold increase of their service life in hostile environments.
On the Left - Photo 6 - Thus zinc anodes P-PBC (A)are mounted on the pipe (photo NPO "RosAntikor")"Complete protection provides a more uniform current distribution over the surface of structures, compensate the defects of cover related to its inevitable destruction during the installation, transportation and operation, including as a result of natural aging (swelling, blistering, cracking, delamination) - Viktor Vasiliev
- The protective current goes on those sites of a surface of metal where the density of a covering is broken, and prevents corrosion of metal.
Thus on the bared surface of metal at its cathodic polarization in sea, tabular and commercial waters drops out the cathodic salt deposit consisting of insoluble salts of calcium and magnesium which plays a role of an additional covering ».
When choosing a method for underground structures it is necessary to know the corrosivity of the soil, divided into five categories: very high (resistivity of 5 ohm • m), high (5-10 ohm • m), high (10-20 ohm • m), average (20-100 ohm • m), low (more than 100 Ohm • m). The thickness and material for the isolation of certain areas are determined by the local soil category. Standard requirements for coating: water resistance, adhesion strength to metal, good insulation from electric current; sufficient strength, low cost.
To protect pipelines from internal corrosion (under the influence of technological environments) use paints, epoxy resins, coatings and inhibitors. On the outside the pipelines and reservoirs are protected by bituminous-rubber coats and polymer films.
This Material was prepared by: Oleg Nikitin, EnergyLand.info
Translation : Mukhametyanov Timur
On the first photo: Protector of P-PAP-1200 from alloy AP4N for the underwater pipeline in the Barents Sea (photo SPA "RosAntikor")
Links:
Construction of main and field pipelines. Means and installation of electrochemical protection
SRT 17330282.27.060.002-2008 heat pipelines. Protection against corrosion. Organization of operation and maintenance. Standards and requirements
State Standard 9.602-2005. Unified protection from corrosion and aging. Installations underground. General requirements for protection against corrosion
Manual for operation with corrosion protection of underground pipelines
Bonus: sandblasting works with the tank before its painting