Study On The Structure And Electrochemical Performance Of The CO₂ Corrosion Product Film Of P110SS Steel Under Tensile Stress

CO2 corrosion is a common problem in oil and gas industry.Although CO2 corrosion can be alleviated to some extent with the development of anti-corrosion technology,it can not be completely avoided.Along with the reconnoitering and development of petroleum and natural gas to exploit oil became a difficult and challenging issue.There are many complex corrosion factors,which have influence on casing steels seriously and cause severely corrosion.As a kind of common casing steels,P110SS mainly used in oil and gas fields of high-sour and high sulfur content,and also widely used in fields of high CO2 and slight sulfur content.In this paper,with loaded tensile stress,the corrosion behaviors of P110SS steels in high-temperature and high-pressure environment CO2 were studied.The experimental results have a certain practical significance for the safe exploitation and operation of oil and gas industry.In this paper,the four-point bending stress loading method was investigated and designed corresponding four-point bending jigs.The basic physical and chemical properties of P110SS steel were tested by metallographic microscope,elemental analyzer and tensile testing machine.Electrochemical method was used to study the influence of tensile stress on the surface activity of P110SS steels.The experimental results showed that tensile stress could increase the surface activity and corrosion tendency of P110SS steels,and accelerate the corrosion rate.The PI 10SS steels were corroded for 6 h,18 h,36 h and 72 h respectively under the high-temperature and high-pressure CO2 environment induced by tensile stress in the autoclave.The microstructure,phase composition and electrochemical properties of different corrosion product films formed on P110SS steels have been measured.The experimental results show that the structure of corrosion products films become worse,and the charge transfer resistance and polarization resistance are reduced,then the corrosion current density increased due to the influence of tensile stress.However the phase composition of corrosion product films is FeCO3 and not changed due to tensile stress.According to the experimental rules,combining with the supersaturation theory and the stress corrosion mechanism,the physical and chemical models corroded for particular time period were proposed.It is believed that tensile stress can cause more defects on the surface of the matrix,which lead to the enhancement of nucleation.Since then,due to the concentration of solution in defects and the competing growth of grains,the structure of corrosion product films become worse and the protective effect decreased.Integrated FeCO3 films was gained by high temperature autoclave for 72 hours induced by different tensile stress.Then the samples with corrosion product films were soaked in 0.5 mol/L CaCl2,MgCl2,Na2SO4 and NaHCO3 solutions respectively,and the microstructure and electrochemical properties of films were tested.Corrosion product films are damaged and their protective effect reduced under the action of tensile stress and CaCl2,MgCl2,Na2SO4 solutions.Besides CaCl2 solution has the most obvious destructive effect,then followed by MgC12 and Na2SO4 solution.While NaHCO3 can promote the formation of corrosion product films and make it more compact and dense,so films have better protection.Even though tensile stress can slightly influence the structure of films in NaHCO3 solution,the overall protective effect of films is still relatively high.