Study Of The Pitting Corrosion Behavior Of 2205 Duplex Stainless Steel In CO₂-contained Solution

With the increasing demand for Marine engineering equipment such as offshore oil and gas drilling and acquisition,the application of stainless steel pipe in Marine engineering is gaining popularity due to its excellent corrosion resistance.The offshore oil and gas production environment is characterized by high temperature,pressure,rich CO₂ and Cl-content,which makes stainless steel prone to pitting corrosion.In terms of the process of offshore oil and gas exploitation,many scholars have obtained a lot of conclusions on the pitting issues concerning austenitic stainless steel,but the research on the topic of pitting about duplex stainless steel under the environment of CO₂-contained solution is little.What's more,the pitting mechanism and its development process are still unclear.Therefore,in this paper,the corrosion behavior of2205 duplex stainless steel in different CO₂-contained solutions was studied,and the three main tasks are as following.1)The pitting behavior of 2205 duplex stainless steel in CO₂-contained solutions with different oxygen content was studied through critical pitting temperature measurement,polarization curve,electrochemical impedance spectrum and morphological characterization analysis.The experimental results showed that when the solution temperature was higher than the critical pitting temperature,the existence of dissolved oxygen enhanced the active dissolution process in the steady pitting pit,which increased the steel's pitting corrosion sensitivity and pitting depth.In addition,the study also found that the co-existence of CO₂and O₂ would greatly increase the depth of pitting pits and change the morphology of steady-state pitting pits,which implied some certain synergistic effect between CO₂ and O₂promoting the development of pitting pits.2)The pitting behavior of 2205 duplex stainless steel at different temperature gradients in CO2-contained solutions with various concentrations of chloride ions and sulfate ions was studied using polarization curves and electrochemical impedance spectroscopy.It is found that the relationship between logarithm of chloride ion concentration and pitting potential are linear above the critical pitting temperature.Sulfate ions in a certain concentration range will improve the pitting resistance of 2205 duplex stainless steel,but excessive sulfate ions will significantly increase the passivation current density.3)The pitting behavior of 2205 duplex stainless steel in 3.5%NaCl solution containing no CO₂ and saturated CO₂ at 70°C was studied using electrochemical impedance spectroscopy,electrochemical noise and morphological characterization analysis.The impedance characteristics and electrochemical noise parameters of 2205 duplex stainless steel in different pitting development stages in two solution environments were discussed.It is found that the pitting corrosion resistance of 2205 duplex stainless steel in CO₂-containing NaCl solution is worse than that without CO₂.When the solution temperature is above the critical pitting temperature,2205 duplex stainless steel takes a short time from pitting initiation stage through metastable pitting stage to steady-state pitting stage in the CO₂-containing NaCl solution.During the development of pitting corrosion of 2205 duplex stainless steel,a non-uniform FeCO3 corrosion product film was formed on the surface.