| CO2/H2S corrosion is common damage in oil and gas exploitation and transportation.Corrosion leads to serious safety accidents,such as casualties,economic losses and environmental pollution.Although stainless steel and anti-corrosion alloy have excellent corrosion resistance,the high cost limits their application in pipeline transportation.Because the onshore and offshore oil and gas reservoirs are usually distributed in remote areas,it is economical and effective to use carbon steel in the oil and gas transportation.However,CO2/H2S corrosion often occurred on carbon steel pipeline.It is reported that the addition of 1～5wt.%Cr in low Cr alloy steel could significantly reduce CO2 corrosion rate,and the corrosion resistance mechanism has been researched.With the exploitation of the harsh oil and gas environment,H2S corrosion has become a serious problem.Exploring the corrosion behavior and mechanism on low Cr alloy steel in CO2-H2S aqueous environment could expand the application and development field of low Cr alloy steel.Moreover,in CO2-H2S-O2 interaction moisture corrosion environment and the exploitation environment containing dissolved oxygen and silty sand,corrosion resistance of low Cr alloy steel should be discussed,which could also affect the application and development of low Cr alloy steel.The corrosion behaviors of low Cr alloy steel in the conditions of high temperature and high pressure CO2-H2S,CO2-H2S-O2 coexistence moisture,CO2 aqueous environment with dissolved oxygen and silty sand were studied in this paper.The corrosion mechanism of low Cr alloy steel in different conditions was analyzed by electrochemical test,surface morphology analysis,phase analysis and physical testing method.The effect of chromium on the formation and impacting mechanism of corrosion product film was discussed.The specific conclusions were as follows:1.Under high temperature and high pressure CO2-H2S environment,the corrosion resistant of low Cr alloy steel was better than pipeline steel.With the increasing of temperature and CO2,H2S partial pressure,FeCO3 in corrosion product film was turned to FeS.Due to the hydrolytic action of Cr3+,pH value decreased on electrode surface,which delayed the formation of FeCO3 layer.Cr compounds and mackinawite FeS made a major contribution to inhibit further corrosion.Meanwhile,mackinawite FeS deposition rate affected the integrity of Cr-rich layer.With the increasing of H2S partial pressure,the effect of element Cr on corrosion resistance gradually diminished.2.Under the moisture CO2/H2S/O2 environment,the corrosion resistant of low Cr alloy steel was better than pipeline steel,especially in the condition containing O2.In CO2-H2S-O2 coexistence moisture environment,corrosion product on pipeline steel was continually changed by H2S reduction and O2 oxidation,forming a porous corrosion product film containing sulfide,iron oxide and elemental sulfur.For low Cr alloy steel,pH value in condensate film increased by oxygen reaction,and therefore,Cr-rich layer was formed to increase the corrosion resistance and not affect by H2S.The corrosion rate of low Cr alloy steel was lower than that of pipeline steel.3.In CO2 environment containing silty sand,the size of silty sand in the adsorption layer could affect the mass transport in cathodic process,as well as the corrosion product film density mixed with silty sand.In 325 mesh silty sand(45μm)condition,silty sand adsorption layer prohibited the transport of H2CO3 and the diffusion of Cr3+,and Cr-rich layer was formed under the sand adsorption layer,exhibiting high corrosion resistance of low Cr alloy steel.In 5000 mesh silty sand(2.6μm)condition,Cr-rich layer was formed in the sand adsorption layer.Silty sand had little effect on the integrity of Cr-rich layer,and therefore,the corrosion rate in 5000 mesh silty sand condition was lower than sand-free condition.In 1000 mesh silty sand(13μm)condition,silty sand damaged the integrity of Cr-rich layer,resulting in the increasing corrosion rate and local corrosion.4.In CO2 environment containing silty sand and dissolved oxygen,the size of silty sand in the adsorption layer could affect the transport of dissolved oxygen,as well as the change of corrosion product from ferrous carbonate to ferric oxide.For pipeline steel,1000 mesh silty sand inhibited the transport of O2 and promoted the formation of ferrous carbonate,increasing corrosion resistance of corrosion product film.5000 mesh silty sand had little effect on the transport of O2 and promoted the formation of ferric oxide,resulting in low protective and local corrosion.For low Cr alloy steel,1000 mesh silty sand damaged the integrity of Cr-rich layer,resulting in the increasing corrosion rate and local corrosion.5000 mesh silty sand had little effect on the transport of O2,which was beneficial to form Cr-rich layer.5.In CO2 environment containing 1000 mesh silty sand and dissolved oxygen,a mixture film of silty sand and ferric oxide was formed preferentially on low Cr alloy steel surface.With the increasing corrosion time,ferric oxide increased in the film and silty sand was easily to fall off due to low cohesion.A local Cr-rich area was formed under ferric oxide-rich area.Gradually,the mixture film of silty sand and ferric oxide had little protective and was fall off completely.The local Cr-rich areas expanded and formed a complete Cr-rich layer,which had effective corrosion resistance. |
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