Cathodic corrosion protection of pipelines: equipment, operating principle
Corrosion protection means can extendthe service life of the metal structure, as well as maintain 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 thequality protector technology, but also on the conditions of 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 on the totality of characteristics this direction can be considered as the most relevant for electrochemical protection.
General information on electrochemical protection
Protection of metals against rust throughelectrochemical influence 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 protectioninvolves the connection to the finished product of a source with a direct 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 protectionelectrochemical 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, a steel or copper pipe can be protected in this way, resulting in 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.
Moreover, cathodic protection can have differentvariants of execution. 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 been noted more than once that the principle of cathodeprotection is realized due to an external source of current. Actually, the main function of corrosion protection is in its work. These tasks are performed by special stations, which, as a rule, are part of the overall pipeline maintenance infrastructure.
Stations of cathodic corrosion protection
The main function of the cathode station isstable 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 intoinverter and transformer. There are other classifications, but they are oriented toward segmentation of plants or by application, or by technical characteristics and parameters of input data. The basic principles of operation most clearly illustrate the two types of cathode stations.
Transformer installations for cathodic protection
It should immediately be noted that this type of stationis obsolete. It is replaced by inverter analogues, which have both pluses and minuses. Either way, transformer models are used even at new points of electrochemical protection.
As the basis of such objects is usedlow-frequency transformer at 50 Hz and 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 corrosion protectionpipelines with such equipment allows you to adjust the parameters of the output current, voltage indicators, 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 sinusoidal form of current.
Solve the problem with pulsation to a certain extentallows the introduction of a low-frequency throttle 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 of cathodic protection
Inverter-type installations are based onpulse high-frequency converters. One of the main advantages of using this type of station is a high efficiency, reaching 95%. For comparison, this indicator on average reaches 80% for transformer plants.
Sometimes others come to the forefrontdignity. 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 differentcases, shapes and dimensions. Of course, the practice of individual design 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 allowsfurther optimize the costs of its installation, transportation and storage. For example, for small objects, the cathodic protection against corrosion of pipelines on an inverter base weighing 10-15 kg and a power of 1.2 kW is quite suitable. Equipment with such characteristics can be serviced by a car, however for large-scale projects, more massive and heavy stations can be used that require the connection of trucks, cranes and assembling teams.
Particular attention in the development of cathode stationsprotection 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 modes of the units.
As regards jumps and breaks in 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, technologieselectric drainage, as well as protector methods of corrosion prevention. 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 may be protected by zinc or aluminum cylinders.
Methods of electrochemical protection can not be attributed tonew and, especially, innovative. The effectiveness of the application of such techniques in the fight against rusting processes has been mastered for a long time. However, one serious shortcoming hampers the wide spread of this method. 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 stray current promotes the development of the same corrosion on the metal surface of adjacent pipes.