Study Of Corrosion And Stress Corrosion Cracking Of X70 Pipeline Steel In Simulated Deep-sea Environment

Pipeline steel had been widely used to transport oil and natural transmission,with a continuously growing demand due to the exploitation of the resources in deep sea.Pitting and stress corrosion crack are the main cause failure for the pipeline steel.The hydrostatic pressure in deep sea environment is the main different compared with shallow sea.Nevertheless,there is few reports about hydrostatic pressure effects on pitting and stress corrosion crack.This can provide the reference and theoretical support for the pipeline steel in deep-sea environment serve.Base on the above,hydrostatic pressure effects on pitting and stress corrosion crack behavior of X70 pipeline steel were studied in this paper.In the first place,hydrostatic pressure effects on the stress corrosion crack,corrosion kinetics,and pitting initiation and extension behavior of X70 steel in 3.5%NaCl solution were investigated by a home-made autoclave system with loading device.In the second place,as is known to all,local corrosion leads to local acidification in the crack tip due to the effect of the occluded cell.Therefore,the second aim of this study was to examine the threshold stress of X70 steel in an acidified solution.In addition,the inclusion type in the X70 steel is described detailedly and the mechanism of inclusion induce pitting initiation is discussed in this paper.The main results were follows:(1)The constant load test results indicate that,at atmospheric pressure(0.1MPa),the threshold stress,?scc,for SCC is 0.89 ?b,in 3.5%NaCl solution(pH=7),the SCC susceptibility is not obvious.When the hydrostatic pressure increases to 5MPa and lOMPa,the threshold stress decreases to 0.78 ?b and 0.68?b,respectively.These results indicate that the hydrostatic pressure(HP)promotes SCC of X70 steel.When the 3.5%NaCl solution pH value decreased to 3,?scc decreased further to 0.83 ?b,0.68 ?b and 0.53 ?b,corresponding to 0.1 MPa,5 MPa and 10 MPa,respectively.These results indicate that an acidified sodium chloride solution promotes SCC of X70 steel.On the one hand,the hydrogen concentration was 0.10 ppm,0.17 ppm and 0.23 ppm after a 200-h immersion in a 3.5%NaCl solution(pH=7)of 0.1 MPa,5 MPa and 10 MPa,respectively,revealing that the hydrogen concentration increased approximately linearly with the HP.On the other hand,scanning electron microscopy(SEM)and confocal scanning laser microscopy showed that on the sample surface,the HP promotes pitting propagation,HP promotes anodic dissolution of X70 steel in 3.5%NaCl solution.(2)HP affects the corrosion behavior of the X70 steel during the whole immersion period.At the initial immersion,HP promote the electrochemical reaction of corrosion process.Pitting initiation rate and propagation are both promoted by HP,corrosion rate increases with HP at the initial immersion period.At the stable immersion period,inner layer corrosion product is complete at 0.1 MPa,obvious cracks can be observed at 10 MPa.HP promotes Cl-transport and adsorption on the rust layer,corrosion rate slightly increases with HP.(3)Hydrostatic pressure effects on pitting initiation and propagation in X70 steel are investigated by evaluating metastable pitting probability using electrochemical methods and immersion corrosion tests in containing chlorine ion solution.Metastable test results revealed that hydrostatic pressure can increase metastable pitting formation frequency and promote stabilization of metastable pitting growth.Electrochemical impedance spectroscopy(EIS)results indicate that Hydrostatic pressure decreases the charge transfer resistance and increases the dissolution rate within the cavities.(4)There are three inclusion types in the X70 steel.Corrosion test results indicated that pitting corrosion resistance are:typeA inclusion<typeC inclusion<type B inclusion;rich(Mn,Ca)S<matrix<rich(Al,Ca)O.SKPFM test results show that type A inclusion exhibited a lower potential and a higher potential than the matrix,type B inclusion exhibited a higher potential than the matrix.The corrosion test and the SKPFM potential test results are consistent.Potentiodynamic polarization results indicate that the type A and C are active inclusions and type B is inactive inclusion.Three kinds of possible mechanisms of inclusion-induced pitting corrosion are established for the X70 steel.