Cathodic corrosion protection of pipelines: equipment, operating principle

Corrosion protection means can prolong the service life of the metal structure, as well as preserve its technical and physical properties during operation. Despite the variety of methods for providing anti-corrosive action, it is possible to completely protect objects from rust damage only in rare cases.

The effectiveness of such protection depends not only on the quality of the protective technology, but also on the conditions for its application. In particular, for the preservation of the metallic structure of pipelines, their best properties are demonstrated by electrochemical corrosion protection based on the operation of cathodes. Preventing the formation of rust on such communications, of course, is not the only sphere of application of this technology, but in terms of the combination of characteristics this direction can be considered as the most relevant for electrochemical protection.

General information on electrochemical protection

The protection of metals from rust by means of electrochemical action is based on the dependence of the electrode potential of the material on the speed of the corrosion process. Metal structures should be operated in the range of potentials where their anodic dissolution will be below the permissible limit. The latter, by the way, is determined by the technical documentation for the operation of the structure.

In practice, electrochemical corrosion protection involves the connection to a finished product of a source with a constant current. The electric field on the surface and in the structure of the protected object forms the polarization of the electrodes, which also controls the process of corrosive damage. In essence, the anode zones on the metal structure become cathodic, which allows shifting the negative processes, ensuring the safety of the structure of the target object.

Principle of operation of cathodic protection

There is cathodic and anodic protection of the electrochemical type. The first concept, which is used to protect pipelines, still gained the greatest popularity. By the general principle, when this method is implemented, a current is applied to the object with a negative pole from an external source. In particular, thus, a steel or copper pipe can be protected, resulting in the polarization of the cathode sections with the transition of their potentials to the anode state. As a result, the corrosive activity of the protected structure will be reduced to almost zero.

At the same time, cathodic protection can have different versions. The above described technique of polarization from an external source is widely practiced, but the method of deaeration of an electrolyte with a decrease in the rate of cathode processes, as well as the creation of a protective barrier, is effective.

It has already been noted more than once that the principle of cathodic protection is realized due to an external current source. Actually, in its work and is the main function of anti-corrosion protection. These tasks are fulfilled by special stations, which, as a rule, are included in the general infrastructure of maintenance of pipelines.

Stations of cathodic corrosion protection

The main function of the cathode station is a stable current supply of the target metal object in accordance with the cathodic polarization method. They use such equipment in the infrastructure of underground gas and oil pipelines, in water supply pipes, heat networks, etc.

There are many varieties of such sources, and the most common device for cathodic protection involves the presence of:

• Current converter equipment;
• Wires for connection to the object to be protected;
• Anodic earthing.

In this case, there is a separation of stations into inverter and transformer ones. There are other classifications, but they are oriented toward segmentation of plants or by application, or by technical characteristics and parameters of input data. Basic principles of operation most clearly illustrate the two types of cathode stations.

Transformer installations for cathodic protection

It should be noted at once that this type of station is obsolete. It is replaced by inverter analogs, which have both pluses and minuses. Anyway, transformer models are used even at new points of electrochemical protection.

The basis of such objects is a low-frequency transformer at 50 Hz and a thyristor converter. For the thyristor control system, the simplest devices are used, among which the phase-impulse power controllers. A more responsible approach to solving management tasks involves the use of controllers with a wide range of functions.

Modern cathodic protection against corrosion of pipelines with such equipment allows you to adjust the parameters of the output current, voltage values, and even out the protective potentials. As for the disadvantages of transformer equipment, they are reduced to a high degree of current ripple at the output with a low power factor. This defect is explained not by a sinusoid form of current.

Solving the problem with pulsation to a certain extent allows the introduction of a low-frequency choke into the system, but its dimensions correspond to the dimensions of the transformer itself, which does not always make such an addition possible.

Inverter station for cathodic protection

Inverter-type installations are based on pulsed high-frequency converters. One of the main advantages of using this type of station is a high efficiency, reaching 95%. For comparison, in transformer plants this figure on average reaches 80%.

Sometimes other dignities come to the fore. For example, small dimensions of inverter stations expand the possibilities for their application in complex areas. There are also financial advantages, which confirm the practice of using such equipment. Thus, inverter cathodic corrosion protection of pipelines quickly pays off and requires minimal investment in technical content. However, these qualities are clearly visible only when compared with transformer installations, but today there are more effective new means of providing current for pipelines.

Constructions of cathode stations

Such equipment is presented on the market in different buildings, shapes and dimensions. Of course, the practice of individual designing of such systems is widespread, which makes it possible not only to obtain an optimal design for specific needs, but also to provide the necessary operational parameters.

Strict calculation of the characteristics of the station allows to further optimize the costs of its installation, transportation and storage. For example, for small objects, the cathodic protection against corrosion of pipelines on an inverter-based basis with a mass of 10-15 kg and a power of 1.2 kW is quite suitable. Equipment with these characteristics can be serviced by a passenger car, however, for large-scale projects, more massive and heavy stations can be used, requiring the connection of trucks, cranes and assembling teams.

Protective functionality

Particular attention in the development of cathode stations is given to the protection of the equipment itself. To this end, systems are integrated to prevent stations from short-circuiting and breaking loads. In the first case, special fuses are used, which allow to handle emergency operation of the units.

As for the jumps and breaks of the voltage, the cathodic protection station is unlikely to be seriously affected by them, but there may be a risk of electric shock. For example, if in normal mode the equipment is operated with a low voltage, then after a break the jump in the indicators can bring to 120 V.

Other types of electrochemical protection

In addition to cathodic protection, both electric drainage technologies and protector methods for preventing corrosion are practiced. The most promising direction is considered to be special protection from corrosion. In this case, active elements are also connected to the target object, providing the transformation of the surface with cathodes by means of a current. For example, a steel pipe in a gas pipeline can be protected by zinc or aluminum cylinders.

Conclusion

Methods of electrochemical protection can not be attributed to new and, especially, innovative. The effectiveness of the use of such techniques in the fight against rusting processes has long been mastered. However, the wide spread of this method is hindered by one serious flaw. The fact is that the cathodic protection against corrosion of pipelines inevitably produces so-called wandering currents. They are not dangerous to the target design, but can have a negative impact on nearby objects. In particular, the wandering current contributes to the development of the same corrosion on the metal surface of adjacent pipes.