The actual scientific and practical problem is performed in the dissertation work - the resistance of ferrite-pearlite class steels, which are used in the main gas pipelines of Ukraine, 09G2S, 17G1S, 10G2FB, and welded joints, against stress-corrosion cracking in a near-neutral pH soil environment (model soil electrolyte NS4) under cathodic polarization in the standardized DSTU 4219 range of protective potentials from -0.750 to -1.050 V was determined.
The gas transportation system of Ukraine includes a network of main gas pipelines approximately 40.000 km long, mostly underground. To protect them from soil corrosion, complex anti-corrosion protection is used (protective polymer coatings and electrochemical protection – applying the cathodic polarization). This makes it possible to reduce the corrosion rate of steel to a technically acceptable level (less than 0,01 mm/year).
In the fundamental document on the protection of main pipelines from corrosion, DSTU 4219, the range of protective potentials from -0,850 V to -1,150 V (relative to the copper sulfate reference electrode, which corresponds to potentials from -0,750 to -1,050 V relative to the silver chloride electrode) is established. But operational experience shows that even when the potential is maintained in this range, the possibility of the development of various types of corrosion is not excluded, the most dangerous of which is stress-corrosion cracking. It is likely that the level of cathodic protection potential will also have a different effect on the development of this phenomenon.
The problem of stress-corrosion cracking has been acute in the world since the 60s of the last century, and the consequences of man-made accidents on gas pipelines caused by stress-corrosion cracking affect the safety of people and the environment in large areas. The part of accidents caused by stress-corrosion cracking reaches 40 %. Considering the danger and scale of stress-corrosion cracking, the importance of a more detailed study of the stress-corrosion cracking of main gas pipelines made of ferrite-pearlite-class steels under cathodic protection conditions is relevant.
The purpose of the work is to determine the resistance of the ferrite-pearlite classes of steels, which are used in the main gas pipelines of Ukraine, 09G2S, 17G1S, 10G2FB, and welded joints, against corrosion cracking in a soil environment with a pH close to neutral (model soil electrolyte NS4), under cathodic polarization conditions in the normalized DSTU 4219 range of protective potentials from
0,750 to -1,050 V relative to the silver chloride reference electrode.
Scientific novelty of the obtained results.
1. The concept of the resistance of ferrite-pearlitic steels of the 09G2S, 17G1S, and 10G2FB classes against stress-corrosion cracking in a near-neutral pH soil environment (model soil electrolyte NS4) under cathodic polarization has been expanded, which consists in the fact that depending on the level of cathodic polarization in the normalized DSTU 4219 range of protective potentials from
-0,750 to -1,05 V, stress-corrosion cracking occurs by the mechanisms of local anodic dissolution, hydrogen cracking, or mixed, and the potentials ranges of these mechanisms differ for steels of different chemical composition.
2. For the first time, the limits of the potential ranges at which the stress-corrosion cracking mechanism of ferrite-pearlite class steels in the model soil electrolyte NS4 in the normalized DSTU 4219 range of protective potentials changes. For steels 09G2S, 17G1S and 10G2FB, the mixed mechanism is implemented in the ranges: from -0,850 V to -1,000 V, from -0,800 V to -0,980 V, from -0,750 to
-1,050 V, respectively. At potentials more positive than the lower limit of that ranges, the mechanism of local anodic dissolution operates, and at potentials more negative than the upper limit of that ranges – hydrogen cracking occurs.
3. It was established for the first time that welded joints of 10G2FB steel are not susceptible to stress-corrosion cracking by the hydrogen mechanism in the range of protective potentials from -0,750 to -1,05 V normalized by DSTU 4219 in the NS4 solution. The breaking of the welded joint occurs on the base metal, which is proven by metallographic studies and is consistent with cases of stress-corrosion cracking on main gas pipelines.
4. For the first time, the legality of using the coefficient of susceptibility to stress-corrosion cracking KS, introduced for the base metal, for the comparative assessment of the stability of defect-free welded joints has been established.
