Research On Corrosion Mechanism Of HP-13Cr Stainless Steel For Nature Gas Well In The Extremely Aggressive Environment

HP-13Cr stainless steel has been widely used in the oil and gas exploitation characteristics with super depth,superhigh temperature,and superhigh salinity due to its high strength,hardness and reasonable CO₂ corrosion resistance.Unfortunately,more than 30%of the pipelines suffered from either serious corrosion or even fractured in the early stage of its working-life.This thesis elaborated the corrosion mechanism of stainless steel turbings from the angle of thermodynamics firstly,and then focused on the basic theory research of HP-13Cr stainless steel in the extremely aggressive oilfield environment from the aspects of acidizing and wall shear stress,finally,a fast and efficient experimental method is established.The Pourbaix diagram for HP-13Cr stainless steel in the extremely aggressive oilfield environment has been revised.The area of the corrosion of HP-13Cr SS and the pH value of environment have been calibrated on the Pourbaix diagram using electrochemical measurements and surface analysis.The results indicated that the quinary Pourbaix diagram?Fe-Cr-H₂O-Cl^--CO₂?can be obtained by the overlap of three ternary Pourbaix diagrams,i.e.,Fe-Cr-H₂O,Fe-Cr-Cl^-and Fe-Cr-CO_2,and the Pourbaix diagrams indentify the regions of immunity,corrosion,passivity and passivity/possible regions,respectively.The domain of the thermodynamic immunity and passivity region decrease with the temperature and pressure increasing,while the domain of the corrosion and passivity/possible region increase.The operational area of HP-13Cr SS in the extremely aggressive oilfield environment located in the passivity region.Cr₂O₃,and Cr?OH?₃?s?were detected as the composition of the passive film.The content Cr₂O₃ increases with the increase of temperature and pressure.On the contrary,it decreases for the content of Cr?OH?₃?s?.The effect of acidizing on the corrosion mechanism of HP-13Cr SS were investigated using surface analysis and electrochemical measurements.The synergistic effect was found to exist among the process of live acid,spent acid,and formation water for the HP-13Cr SS.The synergistic effect between the processes of live acid and spend acid was due to Cu film,which acted as the barrier cover to prevent the diffusion of Cr³⁺outward to the solution.The accumulated Cr³⁺underwent hydrolysis,and a local acidity developed beneath the Cu film,which significantly accelerated the corrosion rate.The synergistic effect between the processes of acidizing and formation water was due to the variations in toughness of the Cu film and the surface of HP-13Cr SS after acidizing,due to which,the fluid state changed and the increases of the wall shear stress.The Cu film sheared out to form the incomplete cover,and then,the highly aggressive solution diffused to the matrix through loose film.As a consequence,the corrosion rate of the matrix noticeably increased.Based on high temperature and pressure autoclave and rotary cage,the bump-shape electrode was designedand arranged with variety distances from the center for each rotate cage layer to obtain the difference of wall shear stress for different bump-shape electrode's surfaces so as to improve the efficiency of the experiment.The computational fluid dynamics model and weight loss were used test to certify the feasibility of this approach.The results indicated that the flow field around each electrode does not affect each other when the electrode is arranged evenly,and there existed significant differences of wall shear stress for different bump-shape electrode's surfaces,each surface was regarded as an independent individual to analysis.Thus,the efficiency of the experiment would be improved further by the bump-shape electrode and it meeting the project of"high-throughput experimental method and design"of the Materials Genome Initiative.The influence of flow-induced shear stress on corrosion mechanism of HP-13Cr SS was investigated combined with electrochemical techniques and mechanical properties of the passive film.The results showed that the passive film of HP-13Cr SS in highly aggressive formation water was composed of Cr?OH?₃?s?and Cr₂O₃,of which Cr?OH?₃?s?was controlled by the dissolution-precipitation process,while Cr₂O₃ was controlled by the electro-migration process.Flow-inducedshearstressexhibitedasignificantinfluenceonthe dissolution-precipitation process,resulting in decrease in thickness of passive film and volume fraction of amorphous Cr?OH?₃?s?.Furthermore,the density of Cr₂O₃/Cr?OH?₃?s?phase boundary,and the donor densities increased such that the pitting corrosion was easier to occur.Furthermore,FISS altered the flow state within the pitting,and promoted the change in pitting geometry from bullet shape to shallow-disk shape.Meanwhile,the uniform corrosion accelerated under FISS condition,and exhibited deeper pitting.Therefore,FISS promoted the pitting growth rate both in horizontal and vertical directions.