Peculiarities of corrosion-mechanical fracture of 09G2S pipe steel samples in the conditions of cathodic protection were investigated. It was established that depending on the level of protective potential, stress-corrosion cracking of pipe steel of a ferrite-pearlite class 09G2S can occur by different mechanisms. The range of protective potentials was determined, at which the anodic dissolution and hydrogen embrittlement occur simultaneously during the fracture of steel, namely from -0.85 V to -1.0 V. The existence of the above mechanisms is confirmed by the change in the strength and viscosity properties of the steel and the morphology of the fractures. For steels of other manufacturing technology and grades, these potential areas may differ.
Considered are some issues of stress corrosion failure of metal of main gas pipeline under operation. It is shown that, except for conventional orientation of stress corrosion cracks in longitudinal direction, the defects of such a type can propagate in transverse direction relatively to axis of the gas pipeline. Found transverse stress corrosion cracks, including ones passing through the whole thickness of the pipe wall, were formed in a zone of girth weld, joining 1420 mm diameter pipes of K60 strength class. Defective zone of 800 mm length was located from both sides of the weld and propagated at approximately 60 mm width from fusion line. Failure surface in zone of defects has specific step nature as a result of coalescence of finer semi-elliptical cracks, nucleated in different planes being displaced relatively to each other. The cracks are mainly characterized by rounded tips, that is typical for stress corrosion defects, formed as a result of anode metal dissolution.
The paper presents the results of stress corrosion resistance studies of welded joints of low-alloy steel 17G1SU, obtained by high-frequency welding (HFW). The potentiometry method has established that the welded joint in the state after welding and after linear heat treatment is resistant to corrosion, because the potential difference between the weld and the base metal does not exceed (30-50) mV. According to the results of accelerated corrosion-mechanical tests in 3% NaCl under conditions of constant load under different stress, it was found that the rate of uniform corrosion of both types of welded joints is almost the same as the base metal. Slightly higher corrosion rate of the welded junction after linear heat treatment correlates with the electrochemical data. In general, the welded joint, made according to the factory technology, has resistance to corrosion and mechanical destruction in a solution of 3% NaCl at the level of the base metal, in the absence of weld defects. In the range of protective polarization potentials normalized by the standard of Ukraine, the ratio of the cathodic protection current to the diffusion current limit for the base metal and for the weld metal practically does not differ. It can be expected that under the conditions of cathodic protection, the predominant local flooding of the weld metal or the parent metal is not expected.
According to the results of complex electrochemical, corrosion-mechanical and fractographic studies, the existence of three potential regions, in which the stress-corrosion cracking (SCC) of 17G1S (17G1S-U) steel in the NS4 model soil electrolyte occurs by different mechanisms was established and experimentally confirmed: at potentials positively than -0.8 V – by the mechanism of local anodic dissolution, at potentials region from -0.8 V to -0.98 V – by the mixed mechanism, at potentials less than -0.98 V by hydrogen breaking mechanism. The susceptibility to SCC of high-frequency weld joints, estimated by the coefficient of KS, in the potential range from the corrosion potential to -1.2 V increases (KS increases from 1.1 to 1.8), which is less intense than for steel 17G1S/17G1S-U (KS increases from 1.1 to 2.8), for arc weld joints – does not change much enough (KS increases from 1.1 to 1.3). The validity of KS coefficient introduced for the base metal for comparative assessment of the susceptibility to SCC of welded joints, is provided in case that there are no defects in the welds and SCC occurs on base metal.
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