Research On Microstructure And Corrosion Behavior Of Corrosion Resistance Steel Of Cargo Oil Tanks

With the rapid growth of oil demand and development of shipbuilding technology,the ocean transportation of oil tanker has become an important form of the world's oil trade.Crude oil contains a variety of acidic mediums,which will cause the material corrosion failure,and the most important thing is that greatly shorten the service life of oil tanker.In this paper,a new type of low alloy steel is developed for cargo oil tanks(COT)by adjusting the process parameters.The main research contents and results are involved as follows:(1)The continuous cooling transformation(CCT)curves of undeformed austenite and deformed austenite for corrosion resistant steel are obtained,which is tested on MMS-300 thermal mechanical simulation under different cooling rates.The results show that the dynamic CCT curve compared to static CCT curve obviously move to the left and up direction,the cooling transformation initiating temperature of austenite increase,the microstructure of the studied steel gradually change from polygonal ferrite and pearlite to a small amount of acicular ferrite,granular bainite,finally transform into bainite ferrite and martensite with the cooling rate increasing.(2)The main rolling parameters of corrosion resistance steel are obtained through multi-pass thermal simulation test.The results indicate that the final rolling temperature is about 820? and the final cooling temperature is about 600? and the cooling rate should be controlled between 10?/s and 20?/s.The mechanical properties meet the DH36 steel plate requirement.The obtained thermal simulation experiment results are feasible and can instruct the actual rolling production.(3)The carbon dioxide corrosion behaviors of the studied steel are investigated by analyzing corrosion morphology characteristics and composition of corrosion products under different immersion period experiment.The high temperature and high pressure autoclave and oil temperature machine are the main equipment,the test electrolyte is 3.5 wt%NaCl solution,and is heated to 75 ?,the total pressure is 1.2 MPa with partial pressure 0.64 MPa CO2 and 0.56 MPa N2.The results show that the low alloy steel with ferrite and bainite microstructure exhibits good corrosion resistance,corrosion product film can be divided into three layers,and the denser inner layer can resist eroding further.(4)The comprehensive properties of corrosion resistance steel in NaCl solution containing H2S and CO2 environment are investigated.The test electrolyte is 3.5 wt%NaCl solution.and is heated to 75?,the total pressure is 1.2 MPa with partial pressure 0.64 MPa CO2,0.009 MPa H2S and 0.551 MPa N2.The corrosion product scales are analyzed by the scanning electron microscopy(SEM),X-ray diffraction(XRD)and energy dispersive spectrometer(EDS).The results show that H2S corrosion plays a significant role,the numerous FeS(mackinawite)are observed,the studied steel with ferrite and bainite exhibits good corrosion resistance.According to the change characteristics of the corrosion product film,the H2S/CO2 corrosion process is divided into three periods.The corrosion product film is forming at the beginning of corrosion experiment,the metal substrate is severely eroded.During the intermediate stages,corrosion product film is steady growth,it has protective effect on metal substrates.At the later stage,dissolution rate is greater than the generation rate of corrosion product,the corrosion layer is damaged.(5)The average corrosion rate is measured using weight loss method through corrosion simulation of COT bottom plate experiment.Corrosion production morphology and phase composition are observed,the corrosion mechanisms are analyzed,the corrosion regularity is obtained.The results show that the corrosion rate is 0.211-0.305 mm/a,which have reached the classification of corrosion resistant steel national standard requirement,the studied steel with ferrite and bainite microstructure exhibits good corrosion resistance.(6)The electrochemical corrosion behavior of corrosion resistance steel with ferrite and bainite microstructure is researched using electrochemical workstation,the influence of cooling rate and immersion time on corrosion property is studied by the polarization and electrochemical impedance spectroscopy method.The results show that the electrochemical corrosion rate gradually decreases and small angle grain boundary ratio gradually increases with the increase of cooling rate.The corrosion current density increases,the polarization resistance decreases and the corrosion rate rises with increase of immersion time.