Study On Corrosion Behavior Of X100 Steel In Marine Dry-Wet Cycle Environment

With the exploitation of oil and gas resources in the South China Sea,X100 steel can meet the growing demand for pipeline transportation of oil and gas due to its economic and performance advantages such as low construction cost,high pressure resistance,high strength and corrosion resistance,and which has broad application prospects.However,the corrosion of high-strength pipeline steel caused by the harsh marine environment has become one of the challenges in the safe operation and maintenance of marine oil and gas pipelines,especially the risers in the marine wet-dry cycle environment.Under such a circumstance,the aim of this paper is to investigate the corrosion behavior of X100 steel in the dry-wet cycle of the South China Sea.In general,the steel corrosion in the wet-dry cycle condition can be considered as corrosion under the thin electrolyte layers(TELs)formed on the surface of the steel.Therefore,we proposed the accelerated wet-dry cycle corrosion test(CCT)methods for simulating the South China Sea,electrochemical tests and slow strain rate tensile test(SSRT)platforms under TELs.This manuscript systematically studied the effects of four key factors on the electrochemical corrosion and stress corrosion behavior of X100 pipeline steel base metal and welded joints,such as wet-dry cycle time,dissolved oxygen,chloride ion and bicarbonate ion concentrations.We revealed the corrosion mechanism of X100 steel under different key factors in wet-dry cycle environment based on the surface corrosion product of X100 steel characterization results.The research results can provide a scientific basis for the application of X100 steel in the future construction of marine oil and gas pipelines.The research results are as follows:(1)The marine wet-dry cycle environment significantly increases the corrosion rate and stress corrosion cracking(SCC)sensitivity of X100 steel base metal and welded joints.The TELs form on the surface of the X100 steel sample accelerates the electrolyte transfer.The marine wet-dry corrosion process starts with the formation of?-Fe OOH.The corrosion rate of X100 steel increases with the CCT times,and the surface corrosion products are transformed from?-Fe OOH to?-Fe OOH with loose and porous structure.In addition,the corrosion products on the surface of the welded joint sample show a shedding phenomenon.The erosive ions in TEL penetrate the loose corrosion product layer,and under the action of local hydrogen evolution(HE)and anodic dissolution(AD)mixing mechanism,the SCC sensitivity of X100steel is increased.Furthermore,the SCC sensitivity of the welded joint is greater than that of the base metal under the same conditions.(2)The corrosion rate of X100 steel base metal and welded joint sample increases first and decreases then with the increase of dissolved oxygen concentration in the wet-dry cycle environment.For the dissolved oxygen concentration below 6 ppm,the electrochemical corrosion cathode reaction rate is limited by the amount of dissolved oxygen in TEL,and the increase of the dissolved oxygen concentration accelerates the diffusion rate.The oxygen reduction reaction and AD mechanism control the SCC sensitivity of X100 steel.For the dissolved oxygen concentration above this value,the relative proportion of protective?-Fe OOH and?-Fe₂O₃in the corrosion product increases,forming a protective layer with a barrier between the X100 steel matrix and the electrolyte,which alleviates the anode dissolution process.(3)The electrochemical corrosion rate of X100 steel base metal and welded joint samples show a logarithmic growth trend with the change of Cl^–concentration in the wet-dry cycle environment of the South China Sea.In the range of Cl^–concentration 0.5?2 mol/L,the increase of Cl^–concentration increases the conductivity of the thin liquid layer and causes the corrosion rate of the sample to rise rapidly.Meanwhile,the surface corrosion products of the X100 steel sample gradually fall off,and the high concentration of Cl^–directly contacts the substrate or penetrates the corrosion product layer to acidify TEL.The relative content of?-Fe OOH in the corrosion product layer increases,and which dominates the SCC sensitivity of X100 steel sample.For the Cl^–concentration above 2 mol/L,the rate of oxygen reduction and the rate of corrosion decrease under the restriction of low concentration of dissolved oxygen in TEL.The SCC mechanism is controlled by both AD and cathode oxygen reduction.(4)Under the simulated marine wet-dry cycle,the corrosion resistance of X100 steel obey an exponential decay function with the HCO₃^–concentration.High concentration of HCO₃^–has an inhibitory effect on the cathodic reaction,mainly due to the increase in p H.However,the HCO₃^–dissociation reaction provides an additional source of hydrogen for the electrochemical cathodic reaction,which increases the electrochemical corrosion rate and stress corrosion sensitivity of the sample under the action of the Bockris mechanism.At the same time,Fe₂(OH)₂CO₃ is converted into?-Fe OOH and Fe CO₃ in the corrosion products and transitional passivation occurs on the surface of the sample as the concentration of HCO₃^–increases,which slows down the anodic dissolution process of electrochemical corrosion.SCC is a combination of HE and AD mechanisms.