By now about 60% of large diameter gas pipelines in Russia are subject to undercoating corrosion processes, the most hazardous of which is stress corrosion cracking (SCC).
The report considers SCC-specific prediction models for pipes steels describing main stages of the process, rate and implementation conditions. It was carried out electrochemical, corrosion-mechanical, mechanical, operational-life tests, X-ray tests of layer-by-layer texture and residual stresses, investigations of micro and dislocational structure for ?70 pipes steels manufactured at Germany and USSR after prolonged operation in pipelines.
Growth of shallow cracks is determined by the level of residual process stresses on the steel surface and by the aspect ratio. The texture non-uniformity of steels and residual process stresses in the pipe are the parameters featuring potential for growth or slowdown of cracks at initial stages. The plastic deformation in the tip of the cracks is slows the growth of the SCC-defects. After cyclic tests, no indicators of fatigue increment and significant changes of dislocational structure was identified nearby of cracks with up to 20% deep of pipe wall thickness (DPWT). By fractographic investigation of fractures after cyclic tests, violation of the oxidic film integrity is observed, which may stimulate growth of cracks according to anodic dissolution mechanism during access of the electrolyte. During static and low-amplitude cyclic loads the crack growth rate in the test media with ?? 5.5-7.0 accelerates with available component (sulfide, carbonate and phosphate) stimulating the anodic dissolution.
It was found that SCC-cracks up to 20% DPWT do not present immediate hazard in relation to pipelines reliability, and, under exposure to corrosion environment, the predicted development rates of such damages do not exceed 0.3-0.4 mm/year, with the exception of cracks located in the welded joints and along the weld-fusion line. SCC-cracks growth with up to 20% DPWT will stop without electrolyte.