| In our country,it is a promising route to exploit the oil and gas in ocean because ocean owns rich resource and the oil and gas in land become shorter.Compared with common pipeline steel,there are many merits for oceanic flexible pipe,such as high stability,excellent corrosion resistance and large deflection.Therefore,ocean flexible pipe are the preferential choice for the exploitation of oil and gas in ocean.However,in our country,the fabrication of oceanic flexible pipes is at the early stage.Armour layer is one of the key components of oceanic flexible pipe.Armor layer,mainly bearing the pressure of transporting oil and gas,is made by low alloy high strength steel.Armor layer is easily failed by severe corrosion,such as high concentration acid and others' corrosion electrolytes.The armor layer with high strength can decrease the self-weight of oceanic flexible pipes and increase the flexibility.However,the steel with high strength is 'sensitive to sulfide stress corrosion cracking.There are two necessary demands for oceanic flexible pipe,namely high strength and good corrosion resistance to complex environments.Based on the knowledge of metallurgy and electrochemistry,it is our goal to fabricate armor layer steel with high strength of oceanic flexible pipes.It is expected that this steel is beneficial for independently fabricating oceanic flexible pipes by our country.The demand of mechanical properties for armor layer steel with high strength of oceanic flexible pipes is that yield strength ReL ? 700 MPa,tensile strength Rm? 780 MPa and elongation A ? 5%.The tested steel is fabricated by hot rolling,cold forming and heat treatment.The tested steel should own excellent corrosion resistance to hydrogen induced cracking(HIC),sulfide stress corrosion cracking(SSCC),hydrogen embrittlement(HE),high-temperature and high-pressure CO2 corrosion,high-temperature and high-pressure H2S/CO2 corrosion and seawater corrosion.The main content in our paper and the innovative results are listed below:(1)Based on the principle that the content of alloy elements should be low,the chemical compositions of tested steels are designed.The tested steel meeting the demand of mechanical properties was fabricated by melting,hot rolling,cold forming and heat treatment.The standard corrosion experiments were carried out to study the corrosion behaviors of tested steel fabricated by different process of heat treatment,such as HIC,SSCC and HE.The experimental results demonstrate that the mechanical properties of tested steel with tempered martensite are ReL=780 MPa,Rm=812 MPa and A=15%.The tested steel meets the corresponding standard demands of HIC,SSCC and HE,demonstrating that the tested steel has excellent corrosion resistence to hydrogen induced corrosion cracking.The immersion experiment was conducted to study the corrosion behaviors of tested steel exposed to high-temperature and high-pressure CO2 environment,high-temperature and high-pressure H2S/CO2 environment and seawater environment.The corrosion rates are 1.72 mm/y?1.44 mm/y and 0.046 mm/y,respectively.Therefore,surface corrosion rate of tested steels is low,meeting the demand of practical environment of armor layer steel with high strength of oceanic flexible pipes.The experimental study demonstrates that the armor layer steel with high strength of oceanic flexible pipes with excellent corrosion resistance to hydrogen induced corrosion cracking and surface corrosion is successfully fabricated.(2)Based on the standards of NACE TM 0284,NACE 0177 and ASTM F519,the corrosion experiments of HIC,SSCC and HE,respectively,are carried out to study the corrosion behaviors of tested steel fabricated by different processes of heat treatment.The experimental results demonstrate that the annealing tested steel has bad corrosion resistance to HIC and SSCC.The tested steel with tempered martensite microstructure meets the standard demands of SSCC and HE,indicating the good corrosion resistance to hydrogen induced corrosion cracking.The corrosion mechanism of tested steel fabricated by different processes of heat treatment is explained.(3)The immersion experiments were carried out to systematically study the corrosion behaviors and mechanism of tested steel exposed to high-temperature and high-pressure CO2 environment and high-temperature and high-pressure H2S/CO2 environment.The experimental results indicate that the main corrosion products of tested steel exposed to high-temperature and high-pressure CO2 environment is cubic FeCO3.The one exposed to high-temperature and high-pressure H2S/CO2 environment is FeS.FeS exhibits two styles of crystals,mackinawite and iron sulfide.H2S dominates the chemical reaction of high-temperature and high-pressure H2S/CO2 environment because the ratio of H2S partial pressure and CO2 partial pressure is high.At high-temperature and high-pressure CO2 environment and high-temperature and high-pressure H2S/CO2 environment,the corrosion products containing chromium and molybdenum is prior to be formed on the coupons surface.At high-temperature and high-pressure CO2 environment,it takes long time to completely cover coupons surface with corrosion products because FeCO3 easily dissolves into corrosion electrolyte.While,at high-temperature and high-pressure H2S/CO2 environment,it takes short time to completely cover coupons surface with corrosion products because FeS is easily formed on the coupons surface.Therefore,at early corrosion stage,the corrosion rate of high-temperature and high-pressure CO2 is higher than that of high-temperature and high-pressure H2S/CO2 because the corrosion products on the coupons surface prevents the ions in corrosion electrolyte from accessing into steel substrate.The corrosion rate of high-temperature and high-pressure CO2 is approximate to the one of high-temperature and high-pressure H2S/CO2 when the corrosion products completely cover the coupons surface.H2S promotes the fast formation of corrosion products on the coupons surface.(4)At high-temperature and high-pressure CO2 environment,chromium,molybdenum and chloride are accumulated in the inner layer.The iron concentration in the outer layer is higher than that in the inner layer.At high-temperature and high-pressure H2S/CO2 environment,iron is distributed homogeneously in corrosion products,and sulfur tend to accumulate in the inner layer.Chromium and molybdenum is aggregated in the inner layer.The corrosion mechanisms of tested steel exposed to high-temperature and high-pressure CO2 environment and high-temperature and high-pressure H2S/CO2 environment are proposed.(5)The immersion experiment was carried out to systematically study the corrosion behavior and mechanism of tested steel exposed to seawater environment.The experimental results indicate that corrosion rate decreases with prolonging immersion time.The corrosion process consists of three stages,and the main corrosion products are a-FeOOH??-FeOOH and Fe3O4.The corrosion products containing chromium and molybdenum are prior to be formed on the coupons surface.(6)Based on the study results in laboratory,the chemical compositions are optimized.The industry-trial experiment in factory is carried out.The corrosion behaviors of tested steel,such as SSCC,HE and seawater corrosion,were studied.The mechanical properties of industry-trial steel fabricated by quenching and tempering meet the designed demand.The industry-trial steel meets the standard demand of SSCC and HE,demonstrating that the industry-trial steel has excellent corrosion resistance to hydrogen induced corrosion cracking.The corrosion rate of industry-trial steel exposed to seawater environment is low,meeting the demand of practical environment of armor layer steel of oceanic flexible pipes.The experimental results demonstrate that the industry-trial steel is suitable for the practical environment.(7)The armor layer steel of oceanic flexible pipes with high strength(ReL? 700 MPa)is successfully fabricated through the studies in laboratory and factory.The armor layer steel has excellent corrosion resistance to hydrogen induced corrosion cracking and surface corrosion. |
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