Influence of electrolytic hydrogenation on stress-corrosion cracking of X70 steel and pipes of long-term exploited main gas pipeline
Keywords:electrolytic hydrogenation, potentiometry, scanning electron microscopy, slow strain rate test, stress-corrosion cracking.
Cathodic polarization, which is used in the complex anti-corrosion protection of main gas pipelines to reduce the corrosion rate to a technically acceptable level, causes the decomposition of the soil electrolyte with the recovery of hydrogen, and its penetration into the steel under the action of tensile stresses contributes to the change in the mechanical properties of the pipes and resistance to stress-corrosion cracking. Comprehensive studies were conducted, and the effect of electrolytic hydrogen penetration on stress-corrosion cracking of X70 steel specimens made from a steel sheet and pipe of a long-term main gas pipeline in a model soil electrolyte NS4 under cathodic polarization in the range of potentials from -0.75 to -1.05 V was determined. The following methods were used in the work: slow strain rate tests, potentiometry, voltammetry, electrolytic hydrogenation, the method of scanning electron microscopy, and optical microscopy. It was established that for both types of specimens, the concentration of hydrogen penetrating into the steel during cathodic polarization increases non-monotonically, with a potential shift from -0.75 to -1.05 V. It is noted that the hydrogenation of specimens of steel under investigation begins at a lower protective potential than steel sheet specimens: -0.95 and -1.05 V, respectively. The concentration of hydrogen diffusing into steel for specimens from a sheet changes in the range 0 0 0.057 mol/m3, for specimens from an operated pipe – 0 0.019 0.024 mol/m3. As a result of the course of the hydrogen diffusion process, there is an increase in the susceptibility to stress-corrosion cracking of specimens from the operated pipe compared to specimens from sheet steel. The coefficient of susceptibility to stress-corrosion cracking KS for the steel sheet changes less intensively than for the specimens from the operated pipe: 1.06 1.06 1.18 and 1.25 1.35 1.53, respectively. Therefore, it was established that specimens made from long-term operating under complex anti-corrosion protection of the main gas pipeline have an increased susceptibility to electrolytic hydrogenation compared to specimens made of steel sheet, and as a result, an increased susceptibility to stress-corrosion cracking.
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