Stress Corrosion Cracking Behavior And Mechanism Of X80 Pipeline Steel By Nitrate-reducing Bacterium,Bacillus Cereus

Buried pipeline steel can be corroded by a variety of microorganisms in soil,and stress corrosion cracking(SCC)occurs under the synergy of stress,which shortens the service life of pipeline and seriously threatens the safe operation of pipeline.In this paper,X80 steel was selected as the research material,and Bacillus cereus(B.cereus),which widely exists in the soil was taken as the factor.The effect and mechanism of B.cereus on the stress corrosion behavior of X80 steel were investigated by various means.In addition,the corrosion big data technology and machine learning method were used to analyze the dynamic mechanism of corrosion process.Finally,a series of Cucontaining X80 pipeline steel was designed and studied to resist B.cereus-SCC.The main results are as follows:In the neutral pH soil solution,B.cereus increased the SCC sensitivity of X80 steel from 3.21%-7.34%to 5.84%-18.10%in the sterile environment,and there was a synergistic effect between bacteria and stress.The uneven concentration of nitrite will lead to the formation of concentration difference corrosion cells.Within the concentration difference range of 10 mmol/L(base content is 0 mmol/L,the same below),the weight loss rate of X80 steel increases linearly with the increase of the concentration difference of nitrite.When the nitrite concentration difference was greater than 4.5 mmol/L,the main corrosion mechanism was concentration difference corrosion.Otherwise it was extracellular electron transfer.The effects of B.cereus on the SCC sensitivity of X80 steel were different at different cathode potentials.At-0.8 V and-0.9 V,the SCC increment is 8.14%and 17.92%,respectively.Then,with the negative shift of cathode potential,the SCC sensitivity increased,but the increment decreased.The reason for this phenomenon is that-0.9 V is the initial potential for hydrogen embrittlement and has little effect on bacterial activity.At the same time,B.cereus can secrete organic acids at the cathode potential to reduce the formation of calcium and magnesium deposits and increase the cathodic polarization current,thus promoting the hydrogen flux and apparent hydrogen concentration of X80 steel to increase the risk of hydrogen embrittlement.B.cereus has different effects on the SCC sensitivity of different microstructures in the heat affected zone of X80 steel.The SCC sensitivity increment of coarse-grained heat affected zone(CGHAZ)is the largest,which is 7.68%and 16.79%respectively at open circuit and-0.9V potential,followed by intercritical heat affected zone and finegrained heat affected zone.Due to the different characteristics of phase composition,grain boundary type,dislocation density,volta potential and other characteristics of different microstructure in the heat affected zone,there are differences in the initial adhesion number and initial adhesion position of B.cereus,resulting in different sensitivity to SCC.Using the corrosion big data technology and machine learning method,the dynamic mechanism and important factors of corrosion process of X80 steel with different mirostructures and stress levels in B.cereus environment were investigated.The total corrosion amount of CGHAZ+stress is the largest in the whole experimental period,and the early stage of corrosion is the core stage that affects the corrosion difference.The number of bacteria,the proportion of low angle grain boundary and temperature are the main factors affecting the corrosion of stress-free X80 steel,while the number of bacteria,the Kernel average misorientation and the prior austenite grain boundary are the main factors affecting the corrosion of stress-free X80 steel.Based on the above understanding and discovery of corrosion mechanism,a series of Cu alloying was designed and studied to improve the resistance of X80 steel to B.cereus-SCC.The sensitivity increment of B.cereus to SCC of X80 steel can be reduced from 8.61%to 3.89%at open circuit potential by 0.6 wt.%Cu,and from 7.40%to 6.07%at-0.9 V potential by 1.0 wt.%Cu.Nano Cu-rich phase distributed uniformly in the tissue can not only release copper ions to poison bacteria after corrosion,but also effectively act as a hydrogen trap to reduce the risk of hydrogen embrittance and reduce the sensitivity of SCC.The research results have reference value for stress corrosion protection of pipeline steel in microbial environment.