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THE PROBLEM OF PIPELINES CORROSION AND DIFFERENT WAYS OF THEIR PROTECTION
Bychenkov Anton Dmitrievich, Astrakhan State Technical University, Astrakhan
E-mail: anton.xm8@mail.ru
Dzhantemirov Malik Ramazanovich, Astrakhan State Technical University, Astrakhan
E-mail: malik.dzhantemirov@mail.ru
Kochegarova Natalya Aleksandrovna, Astrakhan State Technical University, Astrakhan
E-mail: na-rova@yandex.ru
Abstract. This paper addresses the problems of pipelines protection from corrosion. There were considered types of corrosion and methods of pipelines protections in the oil and gas industry.
Key words: protection, corrosion, pipelines, gas and oil industry.
Corrosion is a naturally occurring phenomenon, which happens when metal reacts with the environment, such as water or soil. Corrosion of pipelines has always been a hot topic in the oil and gas industry and will continue to be. Corrosion of the oil and gas pipelines not only reduces the service life of the pipelines, serious still can cause major catastrophic incidents.
There are many types and causes of corrosion. In the oil and gas production industries, the major forms of corrosion include sweet corrosion, sour corrosion, and erosion corrosion.
1. Sweet corrosion (CO2 corrosion). CO2 corrosion has been a recognized problem in oil and gas production and transportation facilities for many years. CO2 is one of the main corroding agents in the oil and gas production systems. Dry CO2 gas is not itself corrosive at the temperatures encountered within oil and gas production
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systems but is so when dissolved in an aqueous phase through which it can promote an electrochemical reaction between steel and the contacting aqueous phase. CO2 will mix with the water, forming carbonic acid making the fluid acidic. CO2 corrosion is influenced by temperature, increase in pH value, composition of the aqueous stream, presence of non-aqueous phases, flow condition, and metal characteristics and is by far the most prevalent form of attack encountered in oil and gas production.
2. Sour corrosion (H2S corrosion). The deterioration of metal due to contact with hydrogen sulfide (H2S) and moisture is called sour corrosion. Although H2S is not corrosive by itself, it becomes a severely corrosive agent in the presence of water, leading to pipeline embrittlement. Hydrogen sulfide when dissolved in water is a weak acid, and therefore, it is a source of hydrogen ions and is corrosive. The corrosion products are iron sulfides (FeSx) and hydrogen. The general equation of sour corrosion can be expressed as follows:
H2S + Fe + H2O ^FeSx + 2H + H2O
3. Erosion corrosion. The erosion corrosion mechanism increases corrosion reaction rate by continuously removing the passive layer of corrosion products from the wall of the pipe. The passive layer is a thin film of corrosion product that actually _ serves to stabilize the corrosion reaction and slow it down. As a result of the turbulence and high shear stress in the line, this passive layer can be removed, causing the corrosion rate to increase. The erosion corrosion is always experienced where there
is high turbulence flow regime with significantly higher rate of corrosion and is dependent on fluid flow rate and the density and morphology of solids present in the fluid. This form of corrosion is often overlooked or recognized as being caused by wear.
But we need in pipelines protection from these different types of corrosion. That’s why we’d like to suggest some methods of corrosion protection.
One of them is pipelines coating. Coatings normally are intended to form a continuous film of an electrically insulating material over the metallic surface to be protected. The function of such a coating is to isolate the metal from direct contact with the surrounding electrolyte (preventing the electrolyte from contacting the metal) and to interpose such a high electrical resistance that the electrochemical reactions cannot readily occur. In reality, all coatings, regardless of overall quality, contain holes, referred to as holidays that are formed during application, or during transport. Holidays in coatings also develop in service as a result of degradation of the coating, soil stresses, or movement of the pipe in the ground. Degradation of the coating in service also can lead to disbonding from the pipe surface, further exposing metal to the underground environment. A high corrosion rate at a holiday can result in a leak or rupture, even where the coating effectively protects a high percentage of the pipe surface.
Another method of corrosion protection is cathodic protection (CP). It is a
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technique to reduce the corrosion rate of a metal surface by making it the cathode of an electrochemical cell. This is accomplished by shifting the potential of the metal in the negative direction by the use of an external power source (referred to as impressed current CP) or by utilizing a sacrificial anode. In the case of an impressed current system, a current is impressed on the structure by means of a power supply, referred to as a rectifier, and an anode buried in the ground. In the case of a sacrificial anode system, the galvanic relationship between a sacrificial anode material, such as zinc or magnesium, and the pipe steel is used to supply the required CP current.
Let’s speak about two types of cathodic protection. The First one is Cathodic Protection with Galvanic Anodes. In corrosion cell, one metal is active (negative) with respect to the other and corrodes. In CP with galvanic anodes, this effect is taken advantage of by purposely establishing a dissimilar metal cell strong enough to counteract corrosion cells normally existing on pipelines. This is accomplished by connecting a very active metal to the pipeline. This metal will corrode and, in so doing, will discharge current to the pipeline. In the case of CP with galvanic anodes, CP does not eliminate corrosion; rather, it displaces corrosion from the structure being protected to the galvanic anodes. Under normal circumstances, the current available 0 from galvanic anodes is limited. For this reason, CP by galvanic anodes normally is used where the current required for protection is small. Similarly, the driving voltage existing between pipe steel and galvanic anode metals is limited. Therefore, the contact resistance between the anodes and the earth must be low for the anodes to discharge a useful amount of current. This means that, for normal installations, galvanic anodes are used in low-resistivity soils. A normal installation, as considered here, is one in which the current from a galvanic anode installation is expected to protect a substantial length of pipeline. There are also instances where galvanic anodes are placed at specific points on a pipeline (often termed hot spots) and may be expected to protect only a few feet of pipe, especially where the line is bare.
The second one is Cathodic Protection with Impressed Current. To be free of the limited driving voltage associated with galvanic anodes, current from some outside power source may be impressed on the pipeline by using a ground bed and a power source. The most common power source is the rectifier. This device converts alternating current (AC) electric power to low-voltage direct current (DC) power. Rectifiers usually are provided with the means for varying the DC output voltage, in small "increments", over a reasonably wide range. Although the maximum output voltage may be less than 10 V or close to 100 V, most pipeline rectifiers operate in the range between 10 and 50 V and can be obtained with maximum current outputs ranging from less than 10 A to several hundred amperes. This serves to illustrate the flexibility in choice of power source capacity available to the corrosion engineer when planning an impressed current CP system. Any other reliable source of DC electric power can be used for impressed current CP systems.
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Fig. 1 Cathodic protection by impressed current
Now, let’s return to the third method of corrosion protection. It’s named as corrosion inhibitors. This technology of corrosion inhibitor is mainly applied to the inner side of pipelines. By using corrosion inhibitor, it changes the surface of corroded metal the reactions between anode and cathode, which decreases the reaction rate and slows the corrosion. With a small amount of the inhibitor, the pipelines’ life span would be expanded.
Liquid phase inhibitors (LPIs): LPIs are in wide scale use to combat CO2 corrosion in pipelines. The inhibitor is that is injected into the pipeline is transported in low concentrations and adsorbs onto the surface of the steel. The inhibitor is in the liquid phase only the interior of the pipeline needs to be in contact with the liquid either continuously. One of the main benefits in using inhibitors to protect from CO2 corrosion is that it can continuously be added to the flow to provide continuous protection.
Volatile corrosion inhibitors (VCIs): VCIs are similar to liquid phase inhibitors in that they are present in low concentrations in the pipeline. They can easily enter the vapour phase if one is present. VCIs can adsorb onto the steel directly from the vapour phase and can penetrate into complex shapes and imperfections better than LPIs. Any condensate that may form on the inside of the pipe may contain water and CO2 which will contribute to corrosion in the pipe. If VCIs are present in the vapour phase they will also condense and provide a protective film.
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Metal Surface
VCI Molecules
VCI carrier -paper or film
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Fig. 2 VCIs Molecules Adsorbing to a Metal Surface
The volatility of the inhibitor is dependent on the vapour pressure of the compound. For the prevention of CO2 corrosion on steel, amines are used as the VCIs. The vapour pressure of amines is ideal so that the VCIs will be present in both phases and provide ideal coverage for the pipeline.
And the last method of corrosion protection is anodic protection. Anodic protection (AP) is a technique to control the corrosion of a metal surface by making it the anode of an electrochemical cell and controlling the potential in a range where the metal is passive.
AP is used to protect metals that exhibit passivation in environments where the current density in the freely corroding metals.
Anodic protection is used for carbon steel storage tanks where cathodic protection is not suitable due to very high current requirements. Examples include extreme pH environments such as concentrated sulfuric acid and 50 percent caustic soda. It is also used on a variety of stainless steels, titanium, and similar alloys in very acidic environments. The mining and the pulp and paper industries use these systems.
Conclusion
Corrosion is a world problem and we must find a solution for this problem. According to the world-wide organization, corrosion damages to the world's economy globally, for example, 2 trillion dollars annually. Using different methods of pipelines protection we have obtained good results in the reduction of leaks and extension of the useful life of pipelines.
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Literature:
1. Brondel D, Edwards R, Hayman A, Hill D, Mehta S, Semerad T (1994) Corrosion in the oil industry. Oilfield Rev
2. Nalli K (2010) Corrosion and its mitigation in the oil and gas industry PM-Pipeliner Report
3. Kermani MB, Harrop D (1996) The impact of corrosion on the oil and gas industry 11: SPE Production Facilities pp 186-190
5. Schmitt G (1984) Fundamental aspects of C02 corrosion Houston: NACE p 10
6. de Waard C, Lotz U (1994) Prediction of CO2 corrosion of carbon steel London: The Institute of Materials
7. Chilingar GV, Beeson CM (1969) Surface operations in petroleum production New York: American Elsevier p 397
8. A.W. Peabody, Control of Pipeline Corrosion (Houston, TX: NACE, 1967).
9. R.L.Bianchetti, Impressed Current Cathodic Protection(2001)
10. R.L.Bianchetti, Cathodic Protection with Galvanic Anodes (2001).
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