Effect Of Near Neutral Pre-corrosion On Mechanical Behavior Of Pipeline Steel With Banded Structure

Due to the high strength,good toughness,and low production cost,X80 pipeline steel is currently the main material for oil and natural gas pipeline transportation.The banded structure is a common defect organization in X80 pipeline steel.When the steel pipe outer wall anti-corrosion layer is broken in the working environment,the existence of the banded structure will affect the mechanical properties of the pipeline steel in the soil environment and consequently cause safety hazards.Therefore,this paper combines the three-dimensional digital image correlation method and the micro-area electrochemical scanning technology to study and analyze the tensile deformation behavior of pipeline steel before and after precorrosion.And the corrosion behavior of the surface band structure in a near-neutral corrosive environment is investigated.(1)Combining a 3D stereoscopic non-contact full-field strain measurement system,an in-situ stretching table and a controllable light source,a pipeline steel sample surface microzone structure deformation test system was built.The specimens were stretched with in-situ stretching table.The macroscopic tensile properties and the strain field of the ferrite/pearlite(F/P)band structure were tested and analyzed.The dual CCD cameras collected the grayscale images during the deformation of the specimens' surfaces.Using the natural texture of the F/P banded structure on the surfaces as the deformation identification marks,the deformation field and displacement field of the sample surfaces were calculated by the Vic-3D software in the computer.The banded structure was tested in three different directions(RD: Rolling Direction,45°,and TD: Transverse Direction)for the field measurement of strain.The results showed that the banded structure orientation could cause the macro-anisotropy of the pipeline steel,and the banded structure orientation along the rolling direction could withstand the maximum strength limit.The 45° orientation presented the least strength limit due to the distribution of banded structure parallel to the shear band during the tensile.The von Mises strain field of the F/P banded structure showed that the ferrite bands as the soft phase had obvious strain concentration during the stretching process,but the pearlite bands are less deformed as the hard phase.(2)The micro-area electrochemical scanning probe technology(Versa SCAN SVET)was used to detect the corrosion potential difference of the pipeline steel surface with banded structure in the NS4 near-neutral simulated corrosion solution.The pipeline steel specimens were wrapped in the epoxy resin and used as the working electrode,the platinum sheet was used as the auxiliary electrode to connect with the SVET probe,and the saturated calomel electrode was used as the reference electrode to form a three-electrode system.The results showed that the corrosion potential of pearlite bands is significantly higher than that of the ferrite bands.The Alicona three-dimensional surface measuring instrument was used to obtain the micro-area morphology images before and after the corrosion of the specimens' surface.It can be found that the boundary position of the ferrite and pearlite bands and the ferrite area in the pearlite bands emerged corrosion pits.This is mainly due to the dissolution behavior of ferrite as an anode.(3)In order to analyze the influence of the corrosion of the specimens' surfaces on the tensile strength of the pipeline steel,the specimens with the corroded surface were taken out from the epoxy resin to perform the in-situ tensile test.The results showed that compared with the tensile strength of the specimens before corrosion,the specimens in three orientation all had a decrease in the tensile strength after corrosion and the off-plane displacement at the fracture site of the specimens became lower.Therefore,the depression caused by the electrochemical corrosion at the banded boundary made the material prone to strain concentration behavior during the load-bearing process,which in turn caused the reduction of tensile properties.