Cathodic protection: application and standards

Corrosion is the chemical and electrochemical reaction of metal with the environment, causing its damage. It flows at different speeds, which can be reduced. From the practical point of view of interest is the anticorrosive cathodic protection of metal structures in contact with the ground, with water and with transported media. Particularly damaged are the outer surfaces of the pipes from the influence of soil and wandering currents.

Inside, corrosion depends on the properties of the medium. If it is a gas, it must be thoroughly cleaned of moisture and aggressive substances: hydrogen sulphide, oxygen, etc.

Principle of operation

The objects of the process of electrochemical corrosion are the medium, the metal and the interfaces between them. The medium, which is usually wet ground 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), the iron ions pass into the surrounding solution, which leads to a loss of the mass of the metal. The reaction causes corrosion. With a negative current (cathode electrode), these losses are not present, since electrons pass into the solution. The method is used in electroplating for coating non-ferrous metals on steel.

Cathodic corrosion protection is carried out when a negative potential is applied to the object from the iron.

For this purpose, an anode electrode is placed in the soil and a positive potential is connected to it from the power source. A minus is applied to the protected object. Cathodic-anodic protection leads to active destruction of the anode electrode from corrosion only. Therefore, it should be periodically changed.

Negative effect of electrochemical corrosion

Corrosion of structures can occur from the action of stray currents falling from other systems. They are useful for target objects, but cause significant damage to nearby structures. Wandering currents can spread from rails of electrified transport. They pass in the direction of the substation and fall onto the pipelines. At the exit from them, anodic areas are formed, causing intense corrosion. For protection, electric drainage is used - a special tapping of the currents from the pipeline to their source. Here, the cathodic protection of pipelines against corrosion is also possible . To do this, it is necessary to know the magnitude of stray currents, which are measured by special devices.

Based on the results of electrical measurements, the method of protecting the gas pipeline is chosen. Universal means is a passive way of isolating pipes from contact with the ground with the help of insulating coatings. Cathodic protection of the gas pipeline is an active method.

Pipeline Protection

The structures in the ground are protected from corrosion if they connect a minus DC source to them, and plus - to the anode electrodes buried in the ground. The current will go to the design, protecting it from corrosion. Thus, cathodic protection of pipelines, reservoirs or pipelines in the ground is made.

The anode electrode will be destroyed, and it should be periodically changed. For a tank filled with water, the electrodes are placed inside. In this case, the liquid will be an electrolyte through which the current will flow from the anodes to the surface of the container. Electrodes are well controlled and easy to replace. In the ground, this is harder to do.

Source of power

Near the oil and gas pipelines, in heating and water supply networks, for which cathodic protection is required, establish stations, from which voltage is supplied to the facilities. If they are placed in the open air, the degree of their protection should not be lower than IP34. For dry rooms, any one is suitable.

Stations of cathodic protection of gas pipelines and other large structures have a power from 1 to 10 kW.

Their energy parameters primarily depend on the following factors:

• Resistance between soil and anode;
• Soil electrical conductivity;
• Length of the protective zone;
• Insulating action of the coating.

Traditionally, the cathodic protection converter is a transformer installation. Now it is replaced by an inverter, which has smaller dimensions, better current stability and greater economy. In important areas, install controllers that have functions for regulating current and voltage, equalizing protective potentials, etc.

The equipment is presented in the market in various variants. 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 more because of 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 insulation breaks, it increases to 10 mA / m ² .

Cathodic protection is effective in combination with other methods, since less energy is consumed. If there is a paint coating on the surface of the structure, only the places where it is disturbed are electrochemically protected.

Features of cathodic protection

1. Power sources are stations or mobile generators.
2. The location of the anode earthing switches depends on the specific pipelines. The arrangement method can be distributed or concentrated, and also be located at different depths.
3. The anode material is chosen with a low solubility to be sufficient for 15 years.
4. The potential of the protective field for each pipeline is calculated. It is not regulated if there are no protective coatings on the structures.

Gazprom's standard requirements for cathodic protection

• Action throughout the life of the protective equipment.
• Protection against atmospheric overvoltage.
• Placement of the station in block-boxes or in stand-alone vandalism.
• Anode grounding is selected in areas with a minimum electrical resistance of the ground.
• The characteristics of the converter are chosen taking into account the aging of the protective coating of the pipeline.

Protector protection

The method is a kind of cathodic protection with the connection of electrodes from a more electronegative metal through an electrically conductive medium. The difference is in the absence of an energy source. Protector takes corrosion on itself, dissolving in an electrically conductive environment.

A few years later, the anode should be replaced, since it is produced.

The effect of the anode increases with a decrease in its transient resistance with the medium. Over time, it can be covered with a corrosive layer. This leads to a breakdown of the electrical contact. If the anode is placed in a mixture of salts, which dissolves the corrosion products, the efficiency rises.

The influence of the tread is limited. The range of action is determined by the electrical resistance of the medium and the potential difference between the anode and the cathode.

Protector protection is used in the absence of energy sources or when their use is economically impractical. It is also disadvantageous when used in acid media because of the high dissolution rate of the anodes. Protectors are installed in water, in the ground or in a neutral environment. Anodes from pure metals usually do not. Dissolution of zinc occurs unevenly, magnesium corrodes too quickly, and a strong film of oxides forms on aluminum.

Protector materials

To protectors have the necessary performance 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 in seawater.
• Al + 8% Zn +5% Mg + Cd, In, Gl, Hg, Tl, Mn, Si (percentage share) - operation of structures in running sea water.
• Mg + 5-7% Al + 2-5% Zn - protection of small structures in the ground or in water with a low concentration of salts.

Incorrect application of some types of protectors leads to negative consequences. Anodes from magnesium can cause cracking of equipment due to the development of hydrogen embrittlement.

Joint tread protection with anticorrosion coatings increases its efficiency.

The distribution of the protective current improves, and the anodes are required significantly less. One magnesium anode protects the bitumen-covered pipeline at a length of 8 km, and without a coating, only 30 m.

Protection of car bodies from corrosion

If there is a violation of the coating, the thickness of the car body may decrease for 5 years to 1 mm, i.e. rust through. Restoration of the protective layer is important, but apart from it there is a way to completely stop the corrosion process with the help of cathodic protection. If you turn the body into a cathode, metal corrosion stops. Anodes can be any conductive surfaces, located next: metal plates, ground loop, garage housing, wet road surface. In this case, the efficiency of protection increases with the growth of the anode area. If the anode is a road surface, a "tail" of metallized rubber is used to contact it. It is placed in front of the wheels to get a better spray. "Tail" is isolated from the body.

To the anode, a plus battery is connected via a 1 kΩ resistor and a LED in series connected to it. When the circuit is closed through the anode, when the minus is connected to the body, in normal operation, the LED is hardly visible. If it is brightly lit, it means that a short circuit has occurred in the circuit. The reason must be found and eliminated.

To protect the circuit in series, a fuse must be installed.

When the car is in the garage, it is connected to the grounding anode. During the movement, the connection is via the "tail".

Conclusion

Cathodic protection is a way to improve the operational reliability of underground pipelines and other structures. In this case, it should take into account its negative impact on neighboring pipelines from the influence of wandering currents.