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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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 σ YS – 1.8 σ YS 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.