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

145

5

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significant risk of overpressure of pipelines, especially for those that are operated for more than 15 – 25 years with outdated and short-lived bituminous insulation coating.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).