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Influence of the gas hydrates on the corrosion rate of gas gathering pipelines
ScienceDirect ScienceDirect StructuralIntegrity Procedia 00 (2019) 000–000
Available online at www.sciencedirect.com
Available online at www.sciencedirect.com StructuralIntegrity Procedia 00 (2019) 000–000
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
ScienceDirect Procedia Structural Integrity 16 (2019) 141–147
6th International Conference “Fracture Mechanics of Materials and Structural Integrity” 6th International Conference “Fracture Mechanics of Materials and Structural Integrity”
Influence of the gas hydrates on the corrosion rate of gas gathering Influence of the gas hydratespipelines on the corrosion rate of gas gathering pipelines Lubomyr Poberezhny*, Andrii Hrysanchuk,Halyna Grytsuliak Lubomyr Poberezhny*, Andrii Hrysanchuk,Halyna Grytsuliak
Ivano-Frankivsk National Technical University of Oil and Gas,15 Karpatska St,. Ivano-Frankivsk 76019, Ukraine Ivano-Frankivsk National Technical University of Oil and Gas,15 Karpatska St,. Ivano-Frankivsk 76019, Ukraine
Abstract Abstract The regularities of corrosion degradation of the low carbon ferrite pearlite St 20 (0.2C) steel of gas gathering pipeline under the mutual effect of corrosion environment, mechanical stresses and hydrate formation were determined. A change in corrosion The regularities of corrosion degradation of the low carbon ferrite pearlite (0.2C)tosteel of gas gathering pipeline underThe the mechanism in chloride media at increasing chloride concentration from St 2.520mol/L higher concentration was found. mutual effect corrosion environment, mechanical and hydrate formation determined. A change inassociated corrosion revealed more of significant influence of mechanical stressstresses on localized corrosion rate thanwere on uniform corrosion one was mechanism in chloride media at increasing concentration 2.5 mol/L oftothe higher found. zone The with the formation of local galvanic elements chloride on the metal surface andfrom intensification metalconcentration dissolution inwas the tensile revealed more significant influence ofinteraction mechanicalcaused stress on ratebetween than on grains. uniformFatigue corrosion oneofwas due to the weakening of interatomic by localized increase corrosion the distance tests the associated pipe steel with the formation of local galvanic elementsshowed on the metal surface and intensification of the metal dissolution in the tensile specimens after exposure to gas hydrate a significant decrease in durability (by 15 – 25% depending on zone test due to the weakening of interatomic by increase the distance betweendamaging grains. Fatigue tests of thesurface pipe steel environment), the reason of which wasinteraction obviouslycaused associated with increasing of corrosion of the specimen as a specimens exposure result of theafter aggressive actiontoofgas gas hydrate hydrates.showed a significant decrease in durability (by 15 – 25% depending on test environment), the reason of which was obviously associated with increasing of corrosion damaging of the specimen surface as a result ofThe the aggressive action of gas © 2019 Author(s). Published by hydrates. Elsevier B.V. Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” © 2019 Published by Elsevier B.V. B.V. © 2019The TheAuthors. Author(s). Published by Elsevier organizers Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers. Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers Keywords: corrosion; mechanical stress; fatigue; hydrate formation, field pipeline Keywords: corrosion; mechanical stress; fatigue; hydrate formation, field pipeline
1. Introduction 1. Introduction Hydrates can easily influence various types of internal corrosion of gas pipelines. This corrosion refers to pitting corrosion, which is often observed both in neutral and acidic environments. This type of corrosion is very difficult to Hydrates can easily influence various types of internal corrosion of gas pipelines. This corrosion refers to pitting corrosion, which is often observed both in neutral and acidic environments. This type of corrosion is very difficult to * Corresponding author. Tel.: +380-342-72-7173; fax: +380-342-54-7139. E-mail address: [email protected] * Corresponding author. Tel.: +380-342-72-7173; fax: +380-342-54-7139. E-mail address: [email protected] 2452-3216© 2019 The Author(s). Published by Elsevier B.V.
Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” 2452-3216© organizers 2019 The Author(s). Published by Elsevier B.V. Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers
2452-3216 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers. 10.1016/j.prostr.2019.07.033
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detect, predict or prevent at the design stage of the pipeline or under operation due to corrosion products covering pits. However, pitting corrosion can cause a great deal of pipeline damage (Obanijesu et al. (2011)). The problem is complex due to physical and chemical processes that depend on the size of the formed hydrate, the stage and the period of contact with the pipeline, which results in breaching of protective films on the pipe surface. Acidic gases such as H2S and CO2, which are components in the formation of gas hydrates, dissolving in water can accelerate internal corrosion of gas pipelines (Poberezhny et al. (2017a)). Since the mechanism of corrosion in chloride environments is common for both internal and soil corrosion, for better description of the process and more accurate establishment of the general regularities of the influence of chloride ions on a metal, in addition to the results obtained in the study, the previously obtained data for soil corrosion were used (Poberezhny et al. (2017b)). The objective of the paper is to determine the regularities of the mutual effect of the corrosion environment, mechanical stresses and hydrate formation on corrosion of gathering pipelines. 2. Research technique Long-term exploitation of pipelines leads to significant changes of physical and mechanical properties of material, which results in formation of hardly predicted and controlled stress-strain condition in a pipeline. Therefore, developing methodological approaches, based on simulation of operation of structural elements and providing an effective control of static process of deformation and fracture by determining key parameters, is very important. The automated test system with computer, developed and described in detail by Kryzhanivskyy et al. (2001, 2004), was modified. The scheme of the test system is shown in Fig. 1. It includes the MV-1K and KN-1 test machines; it is used for a comprehensive study of kinetics of deformation, fracture and electrode potential of a pipe steel. The laboratory test system consists of computer, Mtech digital recorder, device for scanning fracture surfaces, further processing of received digital imprints in a graphic editor using a computer database and a Cole-Parmer A48405-25 metallographic microscope. Based on a reactor developed by specialists of Poltava National Technical University (Poberezhny et al. (2017a)), the reactor for synthesis of gas hydrates on the surface of pipeline steel specimens (Fig. 1b) was constructed and experimentally tested.
Fig. 1. General scheme of the laboratory test system (a), scheme of the installation for synthesis of gas hydrates (b): 1 – experimental reactor, 2 – drain pipe, 3 – gas cylinder, 4 – inlet pipe, 5 – pressure gauge, 6 – refrigerating chamber, 7 – support, 8 – gear transmission, 9 – engine.
The developed methodology consists in the following procedure. The first stage is synthesis of gas hydrates (Fig. 1b). Several schemes for the formation of gas hydrates are singled out: scheme without mechanical vibration of the reactor, and scheme with mechanical vibration of the reactor. The second scheme is similar to the first one, but after the formation of hydrate on the sample surface the oscillator generator reactor is additionally activated. This method of testing makes it possible to better assess the
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effect of hydrate in the presence of a large amount of bottom water in conditions of significant turbulence of gas flow. The tested samples is placed into the rector for synthesis of gas hydrates in the following manner: (1) disconnection of the end part of the reactor 1 to provide access to the reactor; (2) close of drain valve 2; (3) water in volume of 1 L is flooded into the reactor through its end face with flange; (4) the studied specimen is fixed to the frame into the reactor, which prevents their distortion during research; (5) close of the reactor end with a metallic ring with an aperture, between which Plexiglas is placed in a thickness of 3 cm to observe the formation of crystals of gas hydrate; (6) gas methane is pumped through valve 4 by the gas compressor 3 until the pressure in the system is set at 45 atm, as indicated by the pressure gauge 5; (7) temperature of 2.5 °C is created in the refrigeration unit 6; (8) the reactor is mounted on the pillars 7 in the middle of the refrigeration unit; (9) at tests according to the second scheme with mechanical vibration it is necessary to activate the engine 9, which through the gearbox 8 will make the reactor vibrate; (10) the specimen of the pipeline material is kept in the reactor during a predetermined exposure time. At the second stage, the planning and implementation of the experiment are carried out, the main purpose of which is to reveal the interaction between deformation and fracture of pipelines based on fracture mechanics and tribology. At the third stage, a fractographic analysis of surface fractures of samples is carried out on metallographic and electron microscopes. The low carbon St 20 (0.2C) steel, with ferrite-pearlite microstructure (yield stress σYS = 220 MPa, ultimate strength σUTS = 450 MPa) was investigated. The fatigue tests at load frequency of 0.8 Hz were carried out. The studies were performed in three stages: at first stage the specimens were subjected to a gas hydrate medium in a constructed reactor at temperature of + 2.5 C and pressure of 45 atm for 170 hours (Poberezhny et al. (2017a, 2018); Hrabovskyy et al. (2017)), at the second one the specimens fatigue tests were performed in air, and at third one – fatigue tests were carried out in corrosion environment (ME5 solution, chemical composition of solutions for corrosion tests is presented in Table 1). Corrosion rate of steel samples was evaluated using weight loss method in test aqueous solutions with different concentration of NaCl, simulating soil media (Table 1). Mass loss rate was converted into penetration rate. Table 1. Chemical composition of solutions for corrosion tests. No МE
Concentration of NaCl, mol/L
1
0.01
2
0.05
3
0.10
4
0.50
5
1.50
6
2.50
7
3.75
8
5.00
Corrosion type Soil corrosion
Internal corrosion
3. Results and discussion Research results concerning influence of applied stresses on corrosion penetration rate of the St 20 steel at uniform and localized corrosion in the test soil solutions with different concentration of chloride, with and without gas hydrates on the surface, is presented in Fig. 2 – 4. A noticeable increase in general corrosion rate of the studied pipe steel with increasing concentration of chloride ions from ME1 to ME2 solution was revealed. The studied steel in ME7 and ME8 solutions was characterized by the highest corrosion rate. It should be noted that significant increasing of corrosion rate of the steel was observed at increasing chloride concentration from 2.50 to 3.75 mol/L (ME6 and ME7). Analysing data presented in Fig. 2, intensification of localized corrosion processes was noticed. The influence of the mechanical factor on rate of both general and localized corrosion increases significantly at
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increase in concentration of chloride ions in the test solution. For corrosion in highly mineralized produced waters (Fig. 3a), a sharp increase in the rate of corrosion was observed at the transition from ME6 to ME7. Such corrosion behavior ms associated, obviously, with accelerated destruction of passive films by chloride ions at reaching a certain critical concentration. An increase in the rate of general and localized corrosion for the samples with gas hydrates formed on the surface was observed (Fig. 3 and 4). Thus, the calculated corrosion rate of the steel with gas hydrates was 1.131 mm/year for general corrosion and 1.332 mm/year for localized corrosion.
Fig. 2. Dependence of corrosion penetration rate of the St 20 steel on applied stress at uniform (a) and localized (b) corrosion in the test soil solutions with different concentration of chloride.
Analysing corrosion behavior of the studied pipe steel in the investigated aggressive chloride environments it was revealed similar regularities of the joint effect of the corrosive environment and mechanical stress on corrosion resistance in all studied cases. Thus, for both studied cases of corrosion (uniform and localized one) intensification of the influence of the mechanical factor with an increase in the concentration of chlorides was observed. However, corrosion resistance of the steel at localized corrosion was more markedly influenced by mechanical stress than at general corrosion. In the ME1, ME4 and ME5 solutions slight changes in the steel corrosion resistance at general corrosion was revealed at transition from the elastic to the elastic-plastic deformations. In the ME2 solution under localized corrosion, a significant intensification of corrosion processes, especially in the area of elastic-plastic deformation (range within 1.45 σYS – 1.8 σYS for the St 20 steel) was found at increasing in the level of mechanical stress.
Fig. 3. Dependence of corrosion penetration rate of the St 20 steel on applied stress at uniform corrosion in the test soil solutions with different concentration of chloride: the steel sample without (a) and with gas hydrates (b) on the surface.
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Ii is suggested that more significant influence of mechanical stress on localized corrosion rate than on uniform corrosion one was associated with the formation of local galvanic elements on the metal surface (Saakiyan et al. (1989)) and intensification of the metal dissolution in the tensile zone due to the weakening of interatomic interaction caused by increase the distance between grains. In the process of development of localized corrosion damage, the strengthening of the role of the mechanical factor contributes to the concentration of stresses at the bottom of corrosion pits. This leads to creation of favourable conditions for their growth in depth. From other hand, chloride ions can destroy the integrity of the passive film and influence on crack initiation (Voloshyn et al. (2014)). As a result of these effects, a significant increase in the corrosion rate of both localized and uniform corrosion at the transition from ME6 to ME7 and ME8 solutions was observed. To assess operational risks properly, it is extremely important to know the corrosion penetration rate of the pipeline steel during operation in order to prevent any incidents, since it well known that corrosion degradation of the pipe steel is very often assisted with the degradation of its mechanical properties (Zvirko (2017), Zvirko et al. (2017)).
Fig. 4. Dependence of corrosion penetration rate of the St 20 steel on applied stress at localized corrosion in the test soil solutions with different concentration of chloride: the steel sample without (a) and with gas hydrates (b) on the surface.
In the case of soil uniform corrosion (ME1 – ME3 solutions) of pipe steel at damaging of insulation and improper cathodic protection, the annual corrosion depth of the steel amounts to 0.45 – 0.55 mm per year at operating loads, and at localization of corrosion process in the ME3 solution the maximum penetration of localized attack on the steel exceeds 6 mm/year, indicating a significant risk of overpressure of pipelines, especially for those that are operated for more than 15 – 25 years with outdated and short-lived bituminous insulation coating. It is especially important to perform groundwater analysis along pipelines regularly in order to timely assess the risks of corrosion degradation of pipeline steels and prevent pipeline failure (Chernov et al. (2002); Kryzhanivskyi et al. (2015); Yavorskyi et al. (2016)). A risk of pipeline failure due to action of corrosion and mechanical stresses is increased first of all for gathering and other industrial pipelines operated without active corrosion protection. Considering corrosion risk at metal corrosion in high salinity produced water, it should be emphasized that in such corrosion media corrosion rate of the St 20 steel is high and it increase at increasing salinity of solution as it was observed at corrosion tests in the ME6 – ME8 solutions.
Fig. 5. General view of corrosion damages of the specimens after exposure in gas hydrates for 7 days according to scheme 1 (upper specimen) and scheme 2 (lower specimen).
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The study of the influence of gas hydrates on corrosion resistance of the pipe steel was carried out according to two exposure schemes described above (Poberezhny et al. (2017a)). It was observed that at exposure according to the scheme 2, corrosion of the specimens was uniform, but at tests according to scheme 1 it was revealed localization of corrosion damages in the sites of crystallization and dissociation of hydrate (Fig. 5) indicated increasing risk of severe localized corrosion in this case. For investigation of the gas hydrates influence on the pipe steel durability fatigue tests of the St 20 steel were performed in air at a pure bending. Sets of specimens were tested: without any pre-treatment and after exposure to gas hydrate. The fatigue tests in air (Fig. 6a) showed that there is the three-stage kinetics of fatigue crack growth and a slight higher level of cyclic deformation on fatigue crack growth curve of the specimen after exposure to gas hydrate was observed, which may be associated with corrosion damage of the specimen surface.
Fig. 6. Fatigue crack growth curves for the St 20 steel specimens in air (a) and the ME5 solution (b) after exposure in hydrate (1) and without any pre-treatment (2).
The same three-stage kinetics of fatigue crack growth for the studied steels was also observed at tests in corrosion environment (Fig. 6b). Fatigue crack growth rate was higher by 5 – 7% for the specimen after exposure to hydrate compared with that without any pre-treatment, which can be associated with an increase of the surface damaging due to the action of the gas hydrates. A simultaneous growth of two cracks was observed on the fracture surface of the St 20 steel specimen after exposure in gas hydrate (Fig. 7). After fatigue tests in corrosion environment a smooth relief on the fracture surface of the steel specimen after exposure in gas hydrate was revealed, this indicates a higher rate of fatigue crack propagation in comparison with that in air. Corrosion defects act as stress concentrators and, consequently, as sites for cracks growth. This is confirmed by analysis of fracture surfaces.
Fig. 7. Fracture surfaces of the St 20 steel specimens after fatigue tests in air and in ME5: without any pre-treatment (a, c), after exposure in hydrate (b, d).
Fatigue tests in corrosion environment showed an increase in deformation shifts for the steel specimen exposed to gas hydrate, which likely corresponds to increase in fatigue crack growth. The gas hydrate influence on duration of the low-frequency fatigue stages revealed itself in shortening of the stage III (Fig. 6), which corresponds to pipeline
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serviceability. Similar regularities were earlier noted for material of drill pipes and sea pipelines (Maruschak et al. (2013); Kryzhanivskyy et al. (2014)). 4. Conclusion Corrosion behaviour of the low carbon ferrite pearlite St 20 steel of gathering pipeline under the mutual effect of the corrosion environment, mechanical stresses and hydrate formation was studied. The regularities of corrosion degradation of the steel under investigated conditions were determined. A change in the mechanism of corrosion in chloride media was found at increasing chloride concentration from 2.5 mol/L to higher concentration. The revealed more significant influence of mechanical stress on localized corrosion rate than on uniform corrosion one was associated with the formation of local galvanic elements on the metal surface and intensification of the metal dissolution in the tensile zone due to the weakening of interatomic interaction caused by increase the distance between grains. A negative effect of the preliminary exposure of the steel to gas hydrates on the durability of the pipe steel in air and in corrosion environment was defined. The durability of the pipe steel in air was reduced by 25%, and in the ME5 solution – by 15%. The reason of a significant decrease in durability of the steel specimens after exposure in gas hydrate was obviously associated with increasing of corrosion damaging of the specimen surface as a result of the aggressive action of gas hydrates. References Chernov, V.Y., Makarenko, V.D., Kryzhanivs'kyi, E.I., Shlapak, L.S., 2002. Causes and mechanisms of local corrosion in oil-field pipelines. Materials Science 38, No. 5, 729–737. Hrabovskyy R., Mazur M., Hrytsanchuk A., Habinskyy V., 2017. Assessment of destruction conditions of the long-term operation gas pipeline. Scientific Journal of TNTU (Tern.) 87, No. 3, 38–47. Kryzhanivskyy Ye.I., Poberezhnyy L.Ya., 2001. Perspektyvy vykorystannya avtomatyzovanykh vyprobovuvalnykh system z EOM dlya otsinky koroziyno-mekhanichnykh vlastyvostey materialiv morskykh truboprovodiv. Problemy sozdanyya novykh mashyn y tekhnolohyy. Nauchnye trudy K·HPU. No. 1 (10), 21–23. (In Ukrainian) Kryzhanivskyy Ye.I., Poberezhnyy L.Ya., 2004. Metodolohiya doslidzhennya deformatsiyi ta ruynuvannya truboprovidnykh system. Materialy III mizhnarodnoyi konferentsiyi. Mekhanika ruynuvannya materialiv i mitsnist konstruktsiy. Lviv, pp. 419 – 424. (In Ukrainian) Kryzhanivs’kyi, E.I., Hoisan, I.M., Student, O.Z., 2014. Specific features of the growth of a tigue cracks in 36G2S steel of drill pipes after there covery heat treatment. Materials Science 50, No. 1, 92–97. Kryzhanivskyi, Y.I., Ivasiv, V.M., Rachkevych, R.V., Vasylyshyn, V.Y., 2015. Fatigue strength of oil well tubings screwed joints in wells curvelinear sections. Naukovyi Visnyk Natsionalnoho Hirnychoho Universytetu 5, 14–21. Maruschak, P., Poberezhny, L., Pyrig, T., 2013. Fatigue and brittle fracture of carbon steel of gas and oil pipelines. Transport 28, No. 3, 270–276. Maruschak, P., Panin, S., Danyliuk, I., Poberezhnyi, L., Pyrig, T., Bishchak, R., Vlasov, I., 2015. Structural and mechanical defects of materials of offshore and onshore main gas pipelines after long-term operation. Open Engineering 5(1), 365–372. Obanijesu, E.O., Pareek, V., Gubner, R., Tade, M.O., 2011. Hydrate formation and its influence on natural gas pipeline internal corrosion. Nafta 62(5–6), 164–173. Poberezhny, L., Maruschak, P., Hrytsanchuk, A., Poberezhna, L., Prentkovskis, O., Stanetsky, A., 2017a. Impact of gas hydrates and long-term operation on fatigue characteristics of pipeline steels. Procedia Engineering 187, 356–362. Poberezhnyi, L.Y., Marushchak, P.O., Sorochak, A.P., Draganovska, D., Hrytsanchuk, A.V., Mishchuk, B.V., 2017b. Corrosive and mechanical degradation of pipelines in acid soils. Strength of Materials 49(4), 539–549. Poberezhny, L., Hrytsanchuk, A., Hrytsuliak, G., Poberezhna, L., Kosmii, M., 2018. Influence of hydrate formation and wall shear stress on the corrosion rate of industrial pipeline materials. Koroze a ochrana materialu 62(4), 121–128. Saakiyan, L.S., Efremov, A.P., Ropyak, L.Ya., 1989. Effect of stress on the microelectrochemical heterogeneity of steel. Protection of Metals 25(2), 185–189. Voloshyn, V.A., Zvirko, O.I., Sydor, P.Y., 2014. Influence of the compositions of neutral soil media on the corrosion cracking of pipe steel. Materials Science 50, No 5, 44–47. Yavorskyi, A.V., Karpash, M.O., Zhovtulia, L.Y., Poberezhny, L.Y., Maruschak, P.O., Prentkovskis, O., 2016. Risk management of a safe operation of engineering structures in the oil and gas sector. Transport Means – Proceedings of the International Conference, Juodkrantė, Lithuania, 370–373. Zvirko, О.І., 2017. Electrochemical methods for the evaluation of the degradation of structural steels intended for long-term operation. Materials Science 52(4), 588–594. Zvirko, O.I., Mytsyk, A.B., Tsyrulnyk, O.T., Gabetta, G., Nykyforchyn, H.M., 2017. Corrosion degradation of steel of long-term operated gas pipeline elbow with large-scale delamination. Materials Science 52, No 6, 861–865.
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Available online at www.sciencedirect.com StructuralIntegrity Procedia 00 (2019) 000–000
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
ScienceDirect Procedia Structural Integrity 16 (2019) 141–147
6th International Conference “Fracture Mechanics of Materials and Structural Integrity” 6th International Conference “Fracture Mechanics of Materials and Structural Integrity”
Influence of the gas hydrates on the corrosion rate of gas gathering Influence of the gas hydratespipelines on the corrosion rate of gas gathering pipelines Lubomyr Poberezhny*, Andrii Hrysanchuk,Halyna Grytsuliak Lubomyr Poberezhny*, Andrii Hrysanchuk,Halyna Grytsuliak
Ivano-Frankivsk National Technical University of Oil and Gas,15 Karpatska St,. Ivano-Frankivsk 76019, Ukraine Ivano-Frankivsk National Technical University of Oil and Gas,15 Karpatska St,. Ivano-Frankivsk 76019, Ukraine
Abstract Abstract The regularities of corrosion degradation of the low carbon ferrite pearlite St 20 (0.2C) steel of gas gathering pipeline under the mutual effect of corrosion environment, mechanical stresses and hydrate formation were determined. A change in corrosion The regularities of corrosion degradation of the low carbon ferrite pearlite (0.2C)tosteel of gas gathering pipeline underThe the mechanism in chloride media at increasing chloride concentration from St 2.520mol/L higher concentration was found. mutual effect corrosion environment, mechanical and hydrate formation determined. A change inassociated corrosion revealed more of significant influence of mechanical stressstresses on localized corrosion rate thanwere on uniform corrosion one was mechanism in chloride media at increasing concentration 2.5 mol/L oftothe higher found. zone The with the formation of local galvanic elements chloride on the metal surface andfrom intensification metalconcentration dissolution inwas the tensile revealed more significant influence ofinteraction mechanicalcaused stress on ratebetween than on grains. uniformFatigue corrosion oneofwas due to the weakening of interatomic by localized increase corrosion the distance tests the associated pipe steel with the formation of local galvanic elementsshowed on the metal surface and intensification of the metal dissolution in the tensile specimens after exposure to gas hydrate a significant decrease in durability (by 15 – 25% depending on zone test due to the weakening of interatomic by increase the distance betweendamaging grains. Fatigue tests of thesurface pipe steel environment), the reason of which wasinteraction obviouslycaused associated with increasing of corrosion of the specimen as a specimens exposure result of theafter aggressive actiontoofgas gas hydrate hydrates.showed a significant decrease in durability (by 15 – 25% depending on test environment), the reason of which was obviously associated with increasing of corrosion damaging of the specimen surface as a result ofThe the aggressive action of gas © 2019 Author(s). Published by hydrates. Elsevier B.V. Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” © 2019 Published by Elsevier B.V. B.V. © 2019The TheAuthors. Author(s). Published by Elsevier organizers Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers. Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers Keywords: corrosion; mechanical stress; fatigue; hydrate formation, field pipeline Keywords: corrosion; mechanical stress; fatigue; hydrate formation, field pipeline
1. Introduction 1. Introduction Hydrates can easily influence various types of internal corrosion of gas pipelines. This corrosion refers to pitting corrosion, which is often observed both in neutral and acidic environments. This type of corrosion is very difficult to Hydrates can easily influence various types of internal corrosion of gas pipelines. This corrosion refers to pitting corrosion, which is often observed both in neutral and acidic environments. This type of corrosion is very difficult to * Corresponding author. Tel.: +380-342-72-7173; fax: +380-342-54-7139. E-mail address: [email protected] * Corresponding author. Tel.: +380-342-72-7173; fax: +380-342-54-7139. E-mail address: [email protected] 2452-3216© 2019 The Author(s). Published by Elsevier B.V.
Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” 2452-3216© organizers 2019 The Author(s). Published by Elsevier B.V. Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers
2452-3216 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers. 10.1016/j.prostr.2019.07.033
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detect, predict or prevent at the design stage of the pipeline or under operation due to corrosion products covering pits. However, pitting corrosion can cause a great deal of pipeline damage (Obanijesu et al. (2011)). The problem is complex due to physical and chemical processes that depend on the size of the formed hydrate, the stage and the period of contact with the pipeline, which results in breaching of protective films on the pipe surface. Acidic gases such as H2S and CO2, which are components in the formation of gas hydrates, dissolving in water can accelerate internal corrosion of gas pipelines (Poberezhny et al. (2017a)). Since the mechanism of corrosion in chloride environments is common for both internal and soil corrosion, for better description of the process and more accurate establishment of the general regularities of the influence of chloride ions on a metal, in addition to the results obtained in the study, the previously obtained data for soil corrosion were used (Poberezhny et al. (2017b)). The objective of the paper is to determine the regularities of the mutual effect of the corrosion environment, mechanical stresses and hydrate formation on corrosion of gathering pipelines. 2. Research technique Long-term exploitation of pipelines leads to significant changes of physical and mechanical properties of material, which results in formation of hardly predicted and controlled stress-strain condition in a pipeline. Therefore, developing methodological approaches, based on simulation of operation of structural elements and providing an effective control of static process of deformation and fracture by determining key parameters, is very important. The automated test system with computer, developed and described in detail by Kryzhanivskyy et al. (2001, 2004), was modified. The scheme of the test system is shown in Fig. 1. It includes the MV-1K and KN-1 test machines; it is used for a comprehensive study of kinetics of deformation, fracture and electrode potential of a pipe steel. The laboratory test system consists of computer, Mtech digital recorder, device for scanning fracture surfaces, further processing of received digital imprints in a graphic editor using a computer database and a Cole-Parmer A48405-25 metallographic microscope. Based on a reactor developed by specialists of Poltava National Technical University (Poberezhny et al. (2017a)), the reactor for synthesis of gas hydrates on the surface of pipeline steel specimens (Fig. 1b) was constructed and experimentally tested.
Fig. 1. General scheme of the laboratory test system (a), scheme of the installation for synthesis of gas hydrates (b): 1 – experimental reactor, 2 – drain pipe, 3 – gas cylinder, 4 – inlet pipe, 5 – pressure gauge, 6 – refrigerating chamber, 7 – support, 8 – gear transmission, 9 – engine.
The developed methodology consists in the following procedure. The first stage is synthesis of gas hydrates (Fig. 1b). Several schemes for the formation of gas hydrates are singled out: scheme without mechanical vibration of the reactor, and scheme with mechanical vibration of the reactor. The second scheme is similar to the first one, but after the formation of hydrate on the sample surface the oscillator generator reactor is additionally activated. This method of testing makes it possible to better assess the
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effect of hydrate in the presence of a large amount of bottom water in conditions of significant turbulence of gas flow. The tested samples is placed into the rector for synthesis of gas hydrates in the following manner: (1) disconnection of the end part of the reactor 1 to provide access to the reactor; (2) close of drain valve 2; (3) water in volume of 1 L is flooded into the reactor through its end face with flange; (4) the studied specimen is fixed to the frame into the reactor, which prevents their distortion during research; (5) close of the reactor end with a metallic ring with an aperture, between which Plexiglas is placed in a thickness of 3 cm to observe the formation of crystals of gas hydrate; (6) gas methane is pumped through valve 4 by the gas compressor 3 until the pressure in the system is set at 45 atm, as indicated by the pressure gauge 5; (7) temperature of 2.5 °C is created in the refrigeration unit 6; (8) the reactor is mounted on the pillars 7 in the middle of the refrigeration unit; (9) at tests according to the second scheme with mechanical vibration it is necessary to activate the engine 9, which through the gearbox 8 will make the reactor vibrate; (10) the specimen of the pipeline material is kept in the reactor during a predetermined exposure time. At the second stage, the planning and implementation of the experiment are carried out, the main purpose of which is to reveal the interaction between deformation and fracture of pipelines based on fracture mechanics and tribology. At the third stage, a fractographic analysis of surface fractures of samples is carried out on metallographic and electron microscopes. The low carbon St 20 (0.2C) steel, with ferrite-pearlite microstructure (yield stress σYS = 220 MPa, ultimate strength σUTS = 450 MPa) was investigated. The fatigue tests at load frequency of 0.8 Hz were carried out. The studies were performed in three stages: at first stage the specimens were subjected to a gas hydrate medium in a constructed reactor at temperature of + 2.5 C and pressure of 45 atm for 170 hours (Poberezhny et al. (2017a, 2018); Hrabovskyy et al. (2017)), at the second one the specimens fatigue tests were performed in air, and at third one – fatigue tests were carried out in corrosion environment (ME5 solution, chemical composition of solutions for corrosion tests is presented in Table 1). Corrosion rate of steel samples was evaluated using weight loss method in test aqueous solutions with different concentration of NaCl, simulating soil media (Table 1). Mass loss rate was converted into penetration rate. Table 1. Chemical composition of solutions for corrosion tests. No МE
Concentration of NaCl, mol/L
1
0.01
2
0.05
3
0.10
4
0.50
5
1.50
6
2.50
7
3.75
8
5.00
Corrosion type Soil corrosion
Internal corrosion
3. Results and discussion Research results concerning influence of applied stresses on corrosion penetration rate of the St 20 steel at uniform and localized corrosion in the test soil solutions with different concentration of chloride, with and without gas hydrates on the surface, is presented in Fig. 2 – 4. A noticeable increase in general corrosion rate of the studied pipe steel with increasing concentration of chloride ions from ME1 to ME2 solution was revealed. The studied steel in ME7 and ME8 solutions was characterized by the highest corrosion rate. It should be noted that significant increasing of corrosion rate of the steel was observed at increasing chloride concentration from 2.50 to 3.75 mol/L (ME6 and ME7). Analysing data presented in Fig. 2, intensification of localized corrosion processes was noticed. The influence of the mechanical factor on rate of both general and localized corrosion increases significantly at
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increase in concentration of chloride ions in the test solution. For corrosion in highly mineralized produced waters (Fig. 3a), a sharp increase in the rate of corrosion was observed at the transition from ME6 to ME7. Such corrosion behavior ms associated, obviously, with accelerated destruction of passive films by chloride ions at reaching a certain critical concentration. An increase in the rate of general and localized corrosion for the samples with gas hydrates formed on the surface was observed (Fig. 3 and 4). Thus, the calculated corrosion rate of the steel with gas hydrates was 1.131 mm/year for general corrosion and 1.332 mm/year for localized corrosion.
Fig. 2. Dependence of corrosion penetration rate of the St 20 steel on applied stress at uniform (a) and localized (b) corrosion in the test soil solutions with different concentration of chloride.
Analysing corrosion behavior of the studied pipe steel in the investigated aggressive chloride environments it was revealed similar regularities of the joint effect of the corrosive environment and mechanical stress on corrosion resistance in all studied cases. Thus, for both studied cases of corrosion (uniform and localized one) intensification of the influence of the mechanical factor with an increase in the concentration of chlorides was observed. However, corrosion resistance of the steel at localized corrosion was more markedly influenced by mechanical stress than at general corrosion. In the ME1, ME4 and ME5 solutions slight changes in the steel corrosion resistance at general corrosion was revealed at transition from the elastic to the elastic-plastic deformations. In the ME2 solution under localized corrosion, a significant intensification of corrosion processes, especially in the area of elastic-plastic deformation (range within 1.45 σYS – 1.8 σYS for the St 20 steel) was found at increasing in the level of mechanical stress.
Fig. 3. Dependence of corrosion penetration rate of the St 20 steel on applied stress at uniform corrosion in the test soil solutions with different concentration of chloride: the steel sample without (a) and with gas hydrates (b) on the surface.
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Ii is suggested that more significant influence of mechanical stress on localized corrosion rate than on uniform corrosion one was associated with the formation of local galvanic elements on the metal surface (Saakiyan et al. (1989)) and intensification of the metal dissolution in the tensile zone due to the weakening of interatomic interaction caused by increase the distance between grains. In the process of development of localized corrosion damage, the strengthening of the role of the mechanical factor contributes to the concentration of stresses at the bottom of corrosion pits. This leads to creation of favourable conditions for their growth in depth. From other hand, chloride ions can destroy the integrity of the passive film and influence on crack initiation (Voloshyn et al. (2014)). As a result of these effects, a significant increase in the corrosion rate of both localized and uniform corrosion at the transition from ME6 to ME7 and ME8 solutions was observed. To assess operational risks properly, it is extremely important to know the corrosion penetration rate of the pipeline steel during operation in order to prevent any incidents, since it well known that corrosion degradation of the pipe steel is very often assisted with the degradation of its mechanical properties (Zvirko (2017), Zvirko et al. (2017)).
Fig. 4. Dependence of corrosion penetration rate of the St 20 steel on applied stress at localized corrosion in the test soil solutions with different concentration of chloride: the steel sample without (a) and with gas hydrates (b) on the surface.
In the case of soil uniform corrosion (ME1 – ME3 solutions) of pipe steel at damaging of insulation and improper cathodic protection, the annual corrosion depth of the steel amounts to 0.45 – 0.55 mm per year at operating loads, and at localization of corrosion process in the ME3 solution the maximum penetration of localized attack on the steel exceeds 6 mm/year, indicating a significant risk of overpressure of pipelines, especially for those that are operated for more than 15 – 25 years with outdated and short-lived bituminous insulation coating. It is especially important to perform groundwater analysis along pipelines regularly in order to timely assess the risks of corrosion degradation of pipeline steels and prevent pipeline failure (Chernov et al. (2002); Kryzhanivskyi et al. (2015); Yavorskyi et al. (2016)). A risk of pipeline failure due to action of corrosion and mechanical stresses is increased first of all for gathering and other industrial pipelines operated without active corrosion protection. Considering corrosion risk at metal corrosion in high salinity produced water, it should be emphasized that in such corrosion media corrosion rate of the St 20 steel is high and it increase at increasing salinity of solution as it was observed at corrosion tests in the ME6 – ME8 solutions.
Fig. 5. General view of corrosion damages of the specimens after exposure in gas hydrates for 7 days according to scheme 1 (upper specimen) and scheme 2 (lower specimen).
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The study of the influence of gas hydrates on corrosion resistance of the pipe steel was carried out according to two exposure schemes described above (Poberezhny et al. (2017a)). It was observed that at exposure according to the scheme 2, corrosion of the specimens was uniform, but at tests according to scheme 1 it was revealed localization of corrosion damages in the sites of crystallization and dissociation of hydrate (Fig. 5) indicated increasing risk of severe localized corrosion in this case. For investigation of the gas hydrates influence on the pipe steel durability fatigue tests of the St 20 steel were performed in air at a pure bending. Sets of specimens were tested: without any pre-treatment and after exposure to gas hydrate. The fatigue tests in air (Fig. 6a) showed that there is the three-stage kinetics of fatigue crack growth and a slight higher level of cyclic deformation on fatigue crack growth curve of the specimen after exposure to gas hydrate was observed, which may be associated with corrosion damage of the specimen surface.
Fig. 6. Fatigue crack growth curves for the St 20 steel specimens in air (a) and the ME5 solution (b) after exposure in hydrate (1) and without any pre-treatment (2).
The same three-stage kinetics of fatigue crack growth for the studied steels was also observed at tests in corrosion environment (Fig. 6b). Fatigue crack growth rate was higher by 5 – 7% for the specimen after exposure to hydrate compared with that without any pre-treatment, which can be associated with an increase of the surface damaging due to the action of the gas hydrates. A simultaneous growth of two cracks was observed on the fracture surface of the St 20 steel specimen after exposure in gas hydrate (Fig. 7). After fatigue tests in corrosion environment a smooth relief on the fracture surface of the steel specimen after exposure in gas hydrate was revealed, this indicates a higher rate of fatigue crack propagation in comparison with that in air. Corrosion defects act as stress concentrators and, consequently, as sites for cracks growth. This is confirmed by analysis of fracture surfaces.
Fig. 7. Fracture surfaces of the St 20 steel specimens after fatigue tests in air and in ME5: without any pre-treatment (a, c), after exposure in hydrate (b, d).
Fatigue tests in corrosion environment showed an increase in deformation shifts for the steel specimen exposed to gas hydrate, which likely corresponds to increase in fatigue crack growth. The gas hydrate influence on duration of the low-frequency fatigue stages revealed itself in shortening of the stage III (Fig. 6), which corresponds to pipeline
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serviceability. Similar regularities were earlier noted for material of drill pipes and sea pipelines (Maruschak et al. (2013); Kryzhanivskyy et al. (2014)). 4. Conclusion Corrosion behaviour of the low carbon ferrite pearlite St 20 steel of gathering pipeline under the mutual effect of the corrosion environment, mechanical stresses and hydrate formation was studied. The regularities of corrosion degradation of the steel under investigated conditions were determined. A change in the mechanism of corrosion in chloride media was found at increasing chloride concentration from 2.5 mol/L to higher concentration. The revealed more significant influence of mechanical stress on localized corrosion rate than on uniform corrosion one was associated with the formation of local galvanic elements on the metal surface and intensification of the metal dissolution in the tensile zone due to the weakening of interatomic interaction caused by increase the distance between grains. A negative effect of the preliminary exposure of the steel to gas hydrates on the durability of the pipe steel in air and in corrosion environment was defined. The durability of the pipe steel in air was reduced by 25%, and in the ME5 solution – by 15%. The reason of a significant decrease in durability of the steel specimens after exposure in gas hydrate was obviously associated with increasing of corrosion damaging of the specimen surface as a result of the aggressive action of gas hydrates. References Chernov, V.Y., Makarenko, V.D., Kryzhanivs'kyi, E.I., Shlapak, L.S., 2002. Causes and mechanisms of local corrosion in oil-field pipelines. Materials Science 38, No. 5, 729–737. Hrabovskyy R., Mazur M., Hrytsanchuk A., Habinskyy V., 2017. Assessment of destruction conditions of the long-term operation gas pipeline. Scientific Journal of TNTU (Tern.) 87, No. 3, 38–47. Kryzhanivskyy Ye.I., Poberezhnyy L.Ya., 2001. Perspektyvy vykorystannya avtomatyzovanykh vyprobovuvalnykh system z EOM dlya otsinky koroziyno-mekhanichnykh vlastyvostey materialiv morskykh truboprovodiv. Problemy sozdanyya novykh mashyn y tekhnolohyy. Nauchnye trudy K·HPU. No. 1 (10), 21–23. (In Ukrainian) Kryzhanivskyy Ye.I., Poberezhnyy L.Ya., 2004. Metodolohiya doslidzhennya deformatsiyi ta ruynuvannya truboprovidnykh system. Materialy III mizhnarodnoyi konferentsiyi. Mekhanika ruynuvannya materialiv i mitsnist konstruktsiy. Lviv, pp. 419 – 424. (In Ukrainian) Kryzhanivs’kyi, E.I., Hoisan, I.M., Student, O.Z., 2014. Specific features of the growth of a tigue cracks in 36G2S steel of drill pipes after there covery heat treatment. Materials Science 50, No. 1, 92–97. Kryzhanivskyi, Y.I., Ivasiv, V.M., Rachkevych, R.V., Vasylyshyn, V.Y., 2015. Fatigue strength of oil well tubings screwed joints in wells curvelinear sections. Naukovyi Visnyk Natsionalnoho Hirnychoho Universytetu 5, 14–21. Maruschak, P., Poberezhny, L., Pyrig, T., 2013. Fatigue and brittle fracture of carbon steel of gas and oil pipelines. Transport 28, No. 3, 270–276. Maruschak, P., Panin, S., Danyliuk, I., Poberezhnyi, L., Pyrig, T., Bishchak, R., Vlasov, I., 2015. Structural and mechanical defects of materials of offshore and onshore main gas pipelines after long-term operation. Open Engineering 5(1), 365–372. Obanijesu, E.O., Pareek, V., Gubner, R., Tade, M.O., 2011. Hydrate formation and its influence on natural gas pipeline internal corrosion. Nafta 62(5–6), 164–173. Poberezhny, L., Maruschak, P., Hrytsanchuk, A., Poberezhna, L., Prentkovskis, O., Stanetsky, A., 2017a. Impact of gas hydrates and long-term operation on fatigue characteristics of pipeline steels. Procedia Engineering 187, 356–362. Poberezhnyi, L.Y., Marushchak, P.O., Sorochak, A.P., Draganovska, D., Hrytsanchuk, A.V., Mishchuk, B.V., 2017b. Corrosive and mechanical degradation of pipelines in acid soils. Strength of Materials 49(4), 539–549. Poberezhny, L., Hrytsanchuk, A., Hrytsuliak, G., Poberezhna, L., Kosmii, M., 2018. Influence of hydrate formation and wall shear stress on the corrosion rate of industrial pipeline materials. Koroze a ochrana materialu 62(4), 121–128. Saakiyan, L.S., Efremov, A.P., Ropyak, L.Ya., 1989. Effect of stress on the microelectrochemical heterogeneity of steel. Protection of Metals 25(2), 185–189. Voloshyn, V.A., Zvirko, O.I., Sydor, P.Y., 2014. Influence of the compositions of neutral soil media on the corrosion cracking of pipe steel. Materials Science 50, No 5, 44–47. Yavorskyi, A.V., Karpash, M.O., Zhovtulia, L.Y., Poberezhny, L.Y., Maruschak, P.O., Prentkovskis, O., 2016. Risk management of a safe operation of engineering structures in the oil and gas sector. Transport Means – Proceedings of the International Conference, Juodkrantė, Lithuania, 370–373. Zvirko, О.І., 2017. Electrochemical methods for the evaluation of the degradation of structural steels intended for long-term operation. Materials Science 52(4), 588–594. Zvirko, O.I., Mytsyk, A.B., Tsyrulnyk, O.T., Gabetta, G., Nykyforchyn, H.M., 2017. Corrosion degradation of steel of long-term operated gas pipeline elbow with large-scale delamination. Materials Science 52, No 6, 861–865.