Study On Stress Corrosion Cracking Mechanism Of Pipeline Steel Under Peeling Coating

The high strength X90 pipeline steel is used as the research object.Electrochemical test,slow strain rate tensile test,corrosion fatigue test and other test methods are used,in this paper.The X90 pipeline steel under the peeling coating is in the near neutral pH solution(Urumqi The stress corrosion cracking mechanism of the regional soil simulation solution was analyzed and studied.The electrochemical behavior of X90 pipeline steel in near-neutral pH solution,the sensitivity of specimen in stress corrosion cracking?SCC?and the crack propagation behavior of the specimen under test conditions were tested,in this paper.The analysis was carried out to discuss the stress corrosion cracking mechanism under the test conditions.At the same time,the crack growth rate was also analyzed.The conclusions are as follows:Electrochemical test results of X90 pipeline steel under near-neutral pH solution under peeling coating showed that:the self-corrosion potentials at different positions were-718.6 mV,-733.8 mV,-722.4 mV,-686.3 mV,-763.7 mV,.The measured polarization curves were characteristic of typical active dissolution,and no activation-blunt phenomenon was found.The corrosion current density Icorr of the sample in the stripping zone decreases first and then increases with the increase of the distance between the sample and the damaged port.The corrosion rate is faster at the point of d=5cm at the leak point and at the farthest point from the position of the leak point,that is,d=20cm,and the corrosion rate is slower at the central position d=10cm and d=15cm.The results of the slow strain rate tensile test show that:there is a certain degree of stress corrosion sensitivity in different retentate,and the stress corrosion sensitivity is high at the near leak point and the bottom of the stripping area,and the stress corrosion sensitivity in the middle of the stripping area is small.The tensile fracture of X90 pipeline steel in the near-neutral pH retentate environment under the stripping coating is transgranular stress corrosion cracking;Under different applied potential conditions,there is obvious stress corrosion sensitivity.With the negative shift of the applied potential,the SCC sensitivity coefficient of X90 pipeline steel shows a trend of decreasing first and then increasing,and having SCC characteristics with obvious hydrogen embrittlement mechanism.When the potential is Eocp,the SCC mechanism is the anodic dissolution mechanism;When the potential is-850mV,the SCC mechanism is a mixed mechanism of the two mechanisms of anodic dissolution+hydrogen embrittlement mechanism;when the potential is lower than-850mV,the SCC mechanism is a hydrogen embrittlement mechanism.The corrosion fatigue test results show that:Stress corrosion cracking of X90 pipeline steel under peeling coating has high fracture sensitivity in near-neutral pH solution environment?Urumqi soil simulation solution?,X90 pipeline steel near-threshold in near-neutral pH solution environment The stress corrosion crack growth rate of the zone can be approximated by da/dN=4.41×10^-9??K-6.48?^1.46,and can be expressed by da/dN=5.81×10^-10??K-8.63?² in the crack stable extension zone;The fractures of the samples in the air or in the soil simulated solution environment in Urumqi show brittle fracture characteristics,and there are secondary cracks in the sections.The cracks are mainly caused by transgranular cracking,and there are also a few intergranular cracks.The crack cracking mechanism is generally characterized by a mixed cracking pattern.