PSI - Issue 16

Lubomyr Poberezhny et al. / Procedia Structural Integrity 16 (2019) 141–147 /XERP\U 3REHUH]KQ\ et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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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) t emperature 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 σ YS = 220 MPa, ultimate strength σ UTS = 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 Corrosion type 1 0.01 Soil corrosion 2 0.05 3 0.10 4 0.50

5 6 7 8

1.50 2.50 3.75 5.00

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