Gas pipelines are protected against corrosion in several ways. This is due to the different nature of the origin of the deformation itself, which depends on the type of location of the highway and environmental conditions. Corrosion of metal wires means spontaneous deformation of these elements due to chemical or electrochemical processes. The main types of deformations are liquid, atmospheric, and underground.
Causes
Below are brief definitions of the damage that are leveled by the corrosion protection of the gas pipelines:
- Chemical effect - spontaneous oxidation of metal parts due to its transformation into a stable ionic region, under the influence of non-conductive compositions.
- Electrochemical corrosion - the metal is destroyed at a speed depending on the electrode implants. This is due to the fact that atoms are ionized separately, with the renewal of the oxidizing agent in the electrolyte.
- The most dangerous corrosion is an attack by stray type currents. This problem is observed near electrically conductive systems, for example, in the area of railway lines with a contact network.
general information
The main types of protection of gas pipelines against corrosion include three types: tread, cathodic and drainage methods. In order to maximize the safety of serviced objects, apply comprehensive measures, including cathodic, tread, drainage protection. Cathodic stations are being erected with several drainage compartments and scattered anodes to avoid the shielding effect of underground utilities.
Cathodic protection of gas pipelines against corrosion
This method consists in connecting the positive pole of the DC generator with the conductor of the anode-ground electrode. From it, currents enter the soil, flowing through damaged sections of insulation into the pipeline. Through the pipe they are directed to the place of connection of the conductor, then to the negative line of the source.
If there is a sufficient voltage level, the entire working part of the gas pipeline becomes negative-cathode. This makes it possible to prevent the formation of active corrosion. In this case, grounding (waste metal) becomes the anode section. As a result, the pipe is negatively potentiated with respect to the soil.
Protective resistance
The tread protection of the gas pipeline against corrosion provides for the creation of a blocking potential by connecting metal protectors with pipes with a more negative indicator than the parameter of the pipeline itself. When using this method, an external current source is not provided, the required characteristics are created by means of a galvanic anode element. Under the influence of the tread on the gas pipeline, cathodic polarization acts, which contributes to the cessation of corrosion processes.
The working material can be zinc, aluminum, magnesium in the form of special alloys (ML, CO, Ts1, and the like). The specified type of protection is as simple as possible, does not require additional maintenance. This method in combination with other methods is relevant to apply to protect individual compartments that are not crossed by adjacent sections of cathodic safety. The tread protection of the gas pipeline against corrosion is appropriate for special casings at the crossings of railways and highways, at facilities with developed underground structures.
Protectors are mounted by rafts of several elements, connected directly to the pipe or to the cathode exit. They are interconnected using a special cable, wire made of steel or copper. To increase the effectiveness of protection, protectors are located in the filler, which reduces the transition resistance. The composition is magnesium sulfate or sodium with clay. The distance of installation of protectors from the pipeline is about 3-6 meters.
Drainage
Very often, tram and railway rails on electrified tracks do not have proper conductivity, which causes part of the electric current to get into the ground. It is from this that it is necessary to protect pipelines running near the railways. At the point of entry of stray currents into the tube, a cathode potential is formed, and at the output, an anode zone. It is in the last places that active metal damage occurs.
Drainage protection of steel gas pipelines against corrosion is an effective way to deal with stray currents. This is very important, because under the influence of this effect the pipes are deformed through in a very short period. The specified type of protection involves the removal of currents from the pipeline to the primary source using a conductor. At the same time, the potential of the pipes with respect to the ground decreases, which helps to eliminate alternating and anode sections while stopping the leakage of currents into the soil.
Drainage features
The location of the drainage lines depends on the location of the potential threat object. Corrosion protection of the main gas pipeline is erected on the negative rail of the traction substation or on railway rails. In the first case, the connection can be direct or polarized.
Direct drainage is appropriate if the pipeline potential is higher than a similar parameter of the stray current drainage system. When arranging electric drainage on rails, the connection must be exclusively polarized. It differs from the direct version in that the circuit provides special settings to prevent the return of electric currents to pipes. The drainage line is in cable or atmospheric design; instrumentation is mounted on it.
Underground Pipeline Corrosion
The indicated type of damage to the pipes refers to one of the main factors of their destruction due to the formation of cracks and tears. Corrosion as a result of the reaction of the metal with the environment causes changes in its structure, which leads to corresponding deformations. To prevent such malfunctions, the electrochemical protection of the pipeline from corrosion allows, since most reactions are caused in a similar way. That is, cathodic and anodic zones are formed in different sections of the pipe.
Under the influence of an electromotive flow of a galvanic pair, electrons through metal elements enter the cathode compartment, flowing into the ground and creating a reaction with an oxidizing electrolyte, provoking the formation of oxygen and hydrogen ions. The electrolytic balance is disturbed, at the anode site, positive iron particles go into the soil, which causes galvanic damage due to the loss of metal mass.
Protection of underground gas pipelines against corrosion
In this direction, there are two methods of protection: active and passive. In the second case, it is supposed to create a tight barrier between the pipe metal and the surrounding soil. To do this, use various coatings such as polymer tapes, bitumen, resins.
All insulation coatings for passive corrosion protection of gas pipelines must comply with certain standards and requirements. Among them:
- resistance to chemical attack;
- high electrical resistance;
- acceptable rate of adhesion to a metal surface;
- possession of high mechanical strength;
- non-susceptibility to climatic factors;
- preservation of its properties when exposed to high and low temperatures;
- lack of mechanical and factory defects;
- the composition should not contain components that have a corrosive effect on the metal;
- resistance to attack of various bacteria.
Efficiency
As practice shows, it is almost impossible to achieve an optimal continuous layer by applying an insulating coating. Different types of materials have unequal diffuse permeability, which leads to a different quality of processing pipelines from the environment. In addition, dents, cracks and other defects form on the coating during construction and installation. End-to-end damage to the passive protection is the most dangerous, since in these places the process of soil corrosion is actively ongoing.
Since this method is ineffective for complete pipe safety, the pipeline is actively protected against corrosion. It is based on the control of electrochemical processes taking place at the boundary of pipe metal and ground electrolyte. This approach is called comprehensive protection. In the active phase, cathodic polarization is provided, which contributes to a decrease in the dissolution rate of the metal as the corrosion potential shifts to a negative value, to a greater side from the natural parameter.
Cathodic polarization principle
The cathodic protection of underground pipelines is carried out using sacrificial anodes or through polarization from a direct current source. In the first case, the calculation is made on the fact that different metals in the electrolyte have different potentials. Therefore, when creating a galvanic pair of two materials and immersing them in an electrolyte, the metal, whose potential has a large negative value, will be the anode. As a result, the opposite material undergoes less destruction.
In practical terms, sacrificial galvanic cells consist of magnesium, aluminum or zinc protectors. Such protection is effective in soils with low ohms (up to 50 Ohm · m).
External sources
The cathodic protection of gas pipelines from corrosive processes using external sources is more complex. Despite the complexity of organizing the process, such a system does not depend on the specific soil resistance and has an unlimited energy resource. The role of DC sources is played by converters of various configurations and designs, which are powered by a variable electrical network.
Converting elements make it possible to adjust the current of the protective direction in wide ranges. At the same time, gas pipeline protection is guaranteed, regardless of environmental conditions. Main power sources:
- 0.4 / 6.0 / 10.0 kW overhead power lines;
- diesel generators;
- thermal, gas and other analogues.
Protective current flows acting on the pipes create a potential difference from metal to soil and are distributed unevenly along the length of the pipeline.