Page 22 - НАЦІОНАЛЬНА АКАДЕМІЯ НАУК УКРАЇНИ

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The influence of microstructure features of steels of casing pipes with different
strength on their corrosion resistance, brittle fracture resistance, susceptibility to hydrogen
embrittlement has been analysed. It has been established that the initial stage of corrosion
of the ferritic-pearlitic 50Г and 32Г2 steels in an acid chloride solution modelling
formation water consists in the predominant dissolution of structural-free ferrite, the
fraction of which in the steel microstructure is the dominant factor of corrosion
intensification. The limiting stages of electrochemical reactions on the ferritic-perlite 32Г2
steel in chloride solution depending on pH have been distinguished: cathodic control for
pH = 4.15–6.3 and cathodic-anodic control for pH = 3.1. The influence of microstructure
of ferritic-pearlitic steels of casing pipes with different strength on resistance to hydrogen
embrittlement has been shown: the 32Г2 steel with higher strength and with more
dispersed microstructure is more susceptible to it under pre-electrolytic hydrogenation than
the 50Г steel with lower strength and with coarse-grained microstructure (coarse-grained
pearlite edged by ferrite).
For the first time, degradation degree of the low-alloyed 20H2M and 35ХМ steels of
pump rods after their long-term (5.5 years) operation has been estimated by reducing their
corrosion resistance, resistance to stress corrosion cracking, fatigue and corrosion-fatigue
at the stages of crack initiation and crack propagation in a model solution of formation
water with pH = 3.1. It has been found that a model formation water with pH = 3.1
intensifies the fatigue crack growth in the 20H2M steel in the medium-amplitude region of
loading due to a combination of mechanisms of fatigue failure and syress corrosion
cracking, as indicated by the tendency to form a plateau on fatigue crack growth diagrams.
For the first time, the effectiveness of protection of the operated low-alloyed 20H2M
steel in a model solution of formation water with pH = 3.1 against corrosion and corrosion-
fatigue at the stages of crack initiation and propagation with environmentally friendly tannin
(protection degree of 80-85%) has been revealed. The correlation between charge for
oxidation of freshly formed surface of the 20H2M steel and resistance to corrosion-fatigue
crack initiation in the steel in a model solution of formation water at different tannin
concentration has been established. On this basis, a method for predicting corrosion-fatigue
durability of ferritic-pearlite steels of pump rods in formation water with addition of an
inhibitor has been developed.
The influence of operation conditions and hydrogenation on degree of anisotropy of
characteristics of strength, plasticity, impact toughness, resistance to hydrogen
embrittlement and fracture toughness of rolled pipeline steels depending on the orientation
of the plane of fracture propagation in axial, tangential and radial directions have been
assessed. It has been established that the tensile deformation energy of the hydrogenated
specimen with a crack before its start in the direction between the fibers is 60 times less
than across the fibers.
Keywords: steel, corrosion, electrochemical properties, inhibitor, hydrogenation,
corrosion-mechanical fracture, corrosion fatigue, operation.