Corrosion Behavior And Protection Methods Of 9Ni Steel In Seawater

In recent years,the demand for green development has continuously increased.With the rapid increase of the application of green energy and natural gas,the construction of oil and gas industry facilities is also developing.LNG?Liquefied natural gas?storage tanks are important for the LNG storage and transportation process,and 9Ni steel is the main material of LNG storage tanks.Hydraulic pressure test is an indispensable process in tank construction,and due to the demand for environmental protection and resource conservation,seawater is used instead of freshwater.However,seawater will cause severe corrosion of 9Ni steel tank.Therefore,it is very important to study the corrosion behavior and protection methods of 9Ni steel in seawater.In this experiment,the corrosion behavior of 9Ni steel in the natural seawater from the China's four sea areas,the South China Sea,the Yellow Sea,the Bohai Sea and the East China Sea,was studied via immersion weightlessness test,metallographic microscope,X ray diffraction,Raman spectroscopy,Infrared spectroscopy,scanning electron microscope,EDS spectrum analysis,and open circuit potential measurement,electrochemical impedance spectrum,potentiodynamic polarization and so on.The effect of galvanic corrosion between 9Ni steel and 304L stainless steel as holders within tanks had also been studied in Yellow seawater.Meawhile,the influence of temperature and Cl^-concentration on the corrosion of 9Ni steel in simulated seawater and the corrosion mechanism were clarified.The three corrosion protection methods including corrosion inhibitor,sacrificial anodes and impressed current had also been compared.The experimental results showed that the corrosion characteristics of 9Ni steel were similar in the natural seawater from the China's four sea areas.Selective corrosion of 9Ni steel mainly occured,and the product was loose and easy to fall off.The difference was mainly related to the water conductivity and salinity of each sea area.It was found that the salinity and conductivity decreases in the order of the South China Sea,the Yellow Sea,the Bohai Sea and the East China Sea,and the corrosion rates were 0.0761,0.0742,0.0712,and 0.0627 g/m²·h,respectively.The 9Ni steel surface did not produce a protective product layer within 30 days,and the corrosion rate accelerated as the immersing time increased.The 304L stainless steel holder and weld in storage tank were relatively stable compared to 9Ni steel,and there was a galvanic potential of approximately±0.56 V between 304L stainless steel and the 9Ni steel,which may cause galvanic corrosion and accelerate the corrosion of the 9Ni steel.In 3.5%NaCl simulated seawater,the corrosion of 9Ni steel became more severe as the temperature increases from 5-35°C.Corrosion rate of 9Ni steel slightly increased with the increase of Cl^-concentration in the range of 0.06-0.6 M.The morphology and composition analysis revealed that the martensite and reversed austenite microstructures of 9Ni steel mainly caused selective corrosion,and the outer layers of corrosion products were?-FeOOH,?-FeOOH and Fe₃O₄.The presence of a Ni-rich layer in the middle layer did not protect 9Ni steel.Instead,it accelerated the corrosion of the substrate.According to the comparative study of three corrosion protection methods,the best protection technology was sacrificial anode cathodic protection and the degree of protection can reach more than 85%.