In the dissertation work, a scientific and applied problem is solved – the regularities of corrosion cracking of steel of main gas pipelines under their cathodic protection are established, the solution of which expands the understanding of the mechanism of stress corrosion cracking of main gas pipelines, which makes it possible to scientifically substantiate approaches to the choice of methods for its prevention.
The methodology for assessment the susceptibility of steel X70 to CC under cathodic protection conditions has been scientifically substantiated, a coefficient of susceptibility to CC Ks in which the changing of plastic properties of metal in the corrosive environment in comparison with its properties in air was takes into account been offered. An assessment criteria of susceptibility of pipe steel to CC was introduced: if Ks is equal or more than 1,6 steel is susceptible to CC, which confirmed by the results of laboratory and field studies. A complex of factors which are causing the degradation of protective polymer coatings (in particular, polymer tape coating), namely the presence of a defect in the coating, its contact with a corrosive medium and cathodic polarization was revealed. By the method infrared spectroscopy the degradation of the primer layer of the polymer tape coating was confirmed and it was proved that the presence of degradation products of polymer tape coating in the solution increases the susceptibility of pipe steel to CC: increases from ~1,7 in the solution in which polymer coating was not in contact to ~2,63 in the solution after contact with the cover. Under conditions of long-term operation at constant deformation in the model soil electrolyte at a potential close to the maximum protective -1,0 V, for X70 steel is more susceptible to subsurface corrosion, and for X80 – intergranular fracture caused by hydrogen.
For steels of different strength level, a new method for assessment of their tendency to CC was proposed, based on the analysis of the length of the descending sections of the fracture curves, and the corresponding coefficient Kt is introduced. It was established that at the temperature 50 оC the properties of the steel substrate make the effect on the cathodic disbondment of polymer coatings: on steel X80, the disbondment process of the coating proceeds more intensively than on X70, which is due to decreasing in the hydrogen evolution potential on steel X80 and the structure of the near-surface layer.
It has been experimentally proven that three potential regions exists in which the CC of X70 steel proceeds according to different mechanisms: at potentials more positive than -0,75 V – by the mechanism of local anodic dissolution, in the potential range from -0,75 V to -1,05 V, a mixed mechanism of CC acts (local anodic dissolution and hydrogen embrittlement occur simultaneously), at potentials more negative than -1,05 V – by the mechanism of hydrogen embrittlement. The regularities of CC are confirmed by the changing in the corrosion-mechanical properties of steel, estimated by the coefficient Ks, and by fractographic signs of destruction. Decreasing of the cathodic potential to the minimum protective potential -0,75 V (c.s.e.) provides the preservation of the protective properties of polymer coatings: new and artificially aged tape coating – by ~9,4 and ~26,9 times, respectively; new hybrid-epoxy coating – by ~3,3 times, artificially aged – by ~1,7 times; new and artificially aged polyurethane coating – by ~20 times.
A methodology for identifying of potentially corrosive dangerous areas of gas pipelines under cathodic protection was developed and implemented which is based on the calculation of the probability of corrosion cracking using on the data of design, executive, operational documentation and the results of ground technical diagnostics and laboratory studies. The probability of corrosion cracking on sections of three main gas pipelines of Ukraine is estimated. It is shown that in regions that are identified as potentially corrosive dangerous, failures due to corrosion cracking were detected. It is confirmed that the developed methodology allows to determine the regions of gas pipelines of increased corrosion danger quite accurately.
The results of the work were used in the development of SOU 60.3-30019801-070, DSTU N B A.3.1-29, changes No. 1 to DSTU 4219.
