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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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