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formation water with pH = 3.1 than the 32Г2 steel with higher strength and with

more dispersed microstructure, which is due to a larger fraction of structural-free

ferrite in the 50Г steel. The 50Г steel is also characterized by very low resistance to

2
brittle fracture in terms of impact toughness (22.3 J/cm ) and static fracture
1/2
toughness (30.5 MPa·m ) and by higher compared to the 32Г2 steel susceptibility
to hydrogen embrittlement under pre-electrolytic hydrogenation.

The limiting stages of electrochemical reactions on the ferritic-pearlite 32Г2

steel in 1% NaCl solution with pH = 3.1–6.3 and CO 2 bubbling into the solution,

modelling formation water, have been distinguished: cathodic control for

pH = 4.15–6.3 and cathodic-anodic control for pH = 3.1.

The degradation degree of the low-alloyed 20H2M and 35ХМ steels of sucker

rods after their long-term (5.5 years) operation has been estimated by decreasing

their corrosion resistance, resistance to stress corrosion cracking, fatigue and

corrosion-fatigue failure at the stages of crack initiation and crack propagation in an

acid chloride solution with pH = 3.1, modelling formation water. In particular, long-

term operation of the 20H2M and 35ХМ steels caused a decrease of their corrosion

resistance (by 10-15%), stress corrosion cracking resistance (by 2.7 and 5.4 times,

respectively) and corrosion-fatigue durability (in approximately 25%) in model

formation water with pH = 3.1.

It has been found that 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 stress corrosion cracking,

as indicated by the tendency to form a plateau on fatigue crack growth diagrams.

Environmentally friendly tannin has been proposed to protect the operated

steel of sucker rods against corrosion and corrosion-fatigue failure in formation

water. It has been shown that tannin provides effective inhibitory protection of the

exploited 20H2M steel in model formation water with pH = 3.1 against corrosion

and corrosion-fatigue failure at the stages of crack initiation and propagation. The
protection degree of the operated 20H2M steel in model formation water with

pH = 3.1 against corrosion is 85%, and against corrosion-fatigue failure – 80% at