Stress-corrosion cracking of the steels of main gas pipeline: assessment and prevention

Abstract

In the monography the methodology for assessing the susceptibility of steel X70 to stress-corrosion cracking under cathodic protection conditions has been scientifically substantiated, a coefficient of susceptibility to stress-corrosion cracking (SCC) KS in which the changing of plastic properties of metal in the corrosive environment in comparison with its properties in the air was taken into account been offered. An assessment criterion of susceptibility of pipe steel to SCC was introduced: if KS ³ 1,6 steel is susceptible to SCC, which was confirmed by the results of laboratory and field studies. A complex of factors that 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 of 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 SCC: KS increases from ~1,7 in the solution in which polymer coating was not in contact to ~2,63 in the solution after contacting 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, X70 steel is more susceptible to subsurface corrosion, and for X80 – intergranular fracture caused by hydrogen. For steels of different strength levels, 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 is introduced. It was established that at the temperature of 50 оC the properties of the steel substrate make an effect on the cathodic disbandment of polymer coatings: on steel X80, the disbandment process of the coating proceeds more intensively than on X70, which is due to a decrease 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 exist 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, and by fractographic signs of destruction. Decreasing 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 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 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 in 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 us to determine the regions of gas pipelines of increased corrosion danger quite accurately.