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steel at a cathodic polarization at a potential of -1.0 V, which is characterized by

the length of falling regions in the fracture curves, is lower than for X70 steel,

indicating its higher susceptibility to brittle fracture.

4. It was for the first time revealed, that at a cathodic protection at a

o
temperature of 20 C, degradation of polyurethane and epoxy coatings on X70
and X80 steels in a solution of 3% NaCl is determined only by the properties of

the coatings and by the polarization potential. At a cyclic change in temperature

о
о
(from 20 С to 50 С), cathodic disbondment of coatings from X80 steel runs
more intensely, which is predetermined by the structure of a near-surface layer

and by the reduction (in absolute value) of the hydrogen evolution potential on

the mentioned steel.

5. It was for the first time revealed (using a set of corrosion-mechanical,

fractographic and electrochemical studies), that the mechanism of corrosion

cracking of pipe X70 steel in the model soil environment varies depending on

the value of the protective potential. Three areas of potentials were revealed,

where the corrosion cracking of steel occurs by different mechanisms: in the

region of potentials, which are more positive than -0.75 V, it proceeds mainly by

the mechanism of local anodic dissolution; in the range of potentials from

-0.75 V to -1.05 V, local anodic dissolution and hydrogen embrittlement occur

simultaneously, at the potentials more negative than -1.05 V, the predominant

mechanism of corrosion cracking is hydrogen embrittlement.

Practical importance of the obtained results.
1. A new methodology for evaluating the susceptibility of pipe X70 steel

to corrosion cracking in the conditions of modeling a complex of external and
internal factors (Ukrainian patent for invention 107381) was proposed, the

coefficient K was introduced, which takes into account the change in the
S
ductile properties of the metal in a corrosive environment as compared to its

properties in air. The criterion of susceptibility of pipe steel to corrosion

cracking, confirmed by the results of laboratory and full-scale tests, was