Page 10 - DISS_NYRKOVA

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evaluation of steel susceptibility to corrosion cracking. For this purpose, the

coefficient of susceptibility to corrosion cracking K was introduced, which
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takes into account the variation of reduction in area of the sample in air as

compared to the solution and the criterion of susceptibility to corrosion cracking,
confirmed by the results of laboratory and full-scale studies: if K  1,60, the
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steel is susceptible to CC.

A set of the following factors was revealed, that cause degradation of

protective polymeric coatings (in particular, of a tape one) at a cathodic

protection: presence of defect in the coating, its contact with a corrosive

environment and cathodic polarization. It was shown that the process of

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degradation is accelerated with an increase in temperature from 20 С to 50 С
and continues its development over time. With a help of infrared spectroscopy, it

was shown that over that period destruction of polyethylene does not occur. The

primer layer is subjected to degradation after holding the coating in the

environment during 2.5 months, which is confirmed by the appearance of bands

of inorganic fillers (calcium carbonate and talc) in the infrared spectra. Presence

of degradation products of the primer layer in the solution improves the

susceptibility of X70 steel to corrosion cracking: K grows from 1.70 in the
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solution that did not contact with the coating to 2.63 in the solution after being

in contact with the coating.

In the conditions of a long-term action of constant deformation in the

model soil electrolyte at a potential close to the maximum protective one being

-1.0 V, it was found that for Х70 steel, subsurface corrosion is typical, and for

X80 steel, intergranular destruction caused by hydrogen is characteristic. A new

method of comparative evaluation of the susceptibility of steel with a different

strength to CC was substantiated and proposed. It is based on analysing the

length of falling regions in the fracture curves. The coefficient K was
τ
introduced. This allowed comparing the susceptibility of pipe steel with

different values of strength to CC at the same combination of external and