Effect Of Mg Treatment On Corrosion Resistance Of X70 Submarine Pipeline Steel

Offshore oil and gas development is not only an important part of China’s strategy of becoming a strong country,but also an important part of the construction of the Maritime Silk Road Economic Belt in the"One Belt And One Road"strategy.Submarine pipeline steel is the key material for the development of offshore oil and gas resources.Its service environment is severe,facing high pressure and low temperature seawater,high concentration of H₂S,salinity transport medium and stress/strain load,etc.These factors will lead to two essential risks of the integrity and reliability of deep-sea pipeline steel in service:One is the hydrogen induced damage caused by wet hydrogen sulfide inside the pipe,especially the strongest and most concerned destructive forces of hydrogen induced cracking（HIC）and sulfide stress corrosion cracking（SSCC）,and the risk increases with the increase of steel grade;The second is Cl^-corrosion in the seawater outside the pipe,especially in the initial stages.Numerous studies have shown that large-sized inclusions are the main cause of HIC and SSCC crack nucleation,and are also the"fatal crux"of Cl^-environment induced initial corrosion of steel.Numerous studies have shown that large-sized inclusions are the main cause of HIC and SSCC crack nucleation,and are also the"fatal crux"of Cl^-environment induced initial corrosion of steel.Magnesium treated oxide metallurgy technology（referred to as"Mg treatment"）can generate a large number of micro and nano sized and dispersed inclusions in steel,which is a feasible approach to achieve the integrated design of high strength steel’s resistance to H and Cl^-medium corrosion.However,there is a lack of relevant research data on the effect of Mg treatment on the corrosion resistance of X70 submarine pipeline steel.This paper focuses on the problems of wet hydrogen sulfide corrosion inside and seawater corrosion outside of submarine pipeline steel.X70 submarine pipeline tested steels with different Mg additions were smelted and rolled in a pilot plant.The effect and mechanism of Mg treatment on the HIC and SSCC susceptibility and seawater corrosion of tested steel were studied,providing theoretical basis and data support for the industrial application of Mg treatment and the development of high-grade submarine pipeline steel.The main research findings and progress are as follows:（1）The effect of Mg treatment on the modification of inclusions in tested steel had been explored.The addition of 0.003%Mg treatment refined the inclusions in the steel,improved the morphology of the inclusions,and formed composite inclusions with Al-Ti-Mg-O as the core and Mn S as the shell in a"core-shell"and similar"core-shell"structure.However,when the amount of Mg added increased to 0.005%,the modified Al-Ti-Mg-O-Mn-S inclusions were too numerous and difficult to disperse in the molten steel.Some of them aggregated and merged into larger sized inclusions,and even chain like inclusions appeared.（2）The effect and mechanism of Mg treatment on hydrogen trapping efficiency and HIC susceptibility of tested steel were studied.It was found that the small and dispersed"core-shell"and similar"core-shell"structure composite inclusions in tested steel with 0.003%Mg treatment was not only hydrogen trapping site at the interface with the matrix,but also hydrogen trapping site at the"core-shell"interface composed of different inclusions.This provided more hydrogen trapping sites,hindered diffusion of hydrogen atoms,improved the hydrogen trapping efficiency of tested steel,dispersed local hydrogen pressure,and reduced HIC susceptibility.However,partially polymerized composite inclusions or chain like inclusions in tested steel with0.005%Mg treatment reduced the hydrogen trapping efficiency,causing local hydrogen aggregation and pressure increase,thereby increasing the HIC susceptibility of tested steel.（3）The effect and mechanism of Mg treatment on SSCC susceptibility of tested steel were studied under external tensile stress.It was found that adding 0.003%Mg treatment not only resulted in the formation of hard Al-O and Ti-O inclusions with Mg-O to form Al-Ti-Mg-O and wrap it around a soft Mn S shell,but also caused elongated Mn S inclusions to precipitate along the surface of small spherical Al-Ti-Mg-O,achieving spheroidization,refinement,and softening of inclusions in tested steel.The modified inclusions not only reduced the initiation of hydrogen induced cracks in the steel,but also hindered the propagation of stress cracks,thereby improving the SSCC sensitivity of tested steel under external tensile stress.However,partially polymerized composite inclusions or chain like inclusions in tested steel with0.005%Mg treatment not only cleaved the matrix,but also induced hydrogen induced cracking initiation and promoted crack propagation,resulting in an increase in SSCC susceptibility of tested steel.（4）The effect and mechanism of Mg treatment on seawater corrosion of tested steel were studied.It was found that long strip shaped Mn S inclusions preferentially dissolve,large-sized spherical composite inclusions dissolve at the interface with the matrix,and chain shaped inclusions and their nearby matrix corrode and dissolve more severely in tested steel.However,the small and dispersed inclusions and their interface with matrix in tested steel with 0.003%Mg treatment,were difficult to undergo corrosion and dissolution,resulting in high stability.Furthermore,the integrity of the oxide film on the surface of tested steel was improved,the corrosion dissolution damage caused by inclusions was reduced,and the seawater corrosion resistance of tested steel was improved.（5）Mg treatment improved the wet hydrogen sulfide corrosion resistance inside and seawater corrosion resistance outside of X70 submarine pipeline steel by changing the number,composition,and size distribution of inclusions.The optimal addition amount of Mg treatment was 0.003%（mass fraction）in the study,providing a theoretical basis for the development of high-grade submarine pipeline steel with good corrosion resistance using Mg treatment.