Investigation On Corrosion Of Pipeline Containing CO₂ In Slug Flow

Slug flow is a very common flow regime in oil and gas pipelines.Due to the special flow characteristics of slug flow,the pipelines containing CO₂ will suffer more severe corrosion under this regime.In this paper,the CO₂ corrosion behavior of X60 pipeline steel in the conditions of static soaking and scouring in slug flow were studied by using the homemade slug flow simulation equipment,and then synergistic rule of erosion and corrosion in slug flow were studied,and the corrosion mechanisms were discussed.The effects of temperature,CO₂ partial pressure and Cl-on corrosion rate as well as erosion-corrosion rate of X60 pipeline steel were studied by using weight loss method through static soaking and slug flow scouring experiments.The results of static soaking experiment show that with the increasing of temperature the CO₂ corrosion rate will increase firstly,then decrease,and at 75?,the corrosion rates get maximal.With the increasing of CO₂ partial pressure,the corrosion rate will increase firstly,and then decrease.With the increasing of Cl-content,the CO₂ corrosion rate will increase firstly,and then decrease and the maximum value appeared at 35000mg/L.The results of scouring experiment show that with the increasing of temperature the erosion-corrosion rate will increase firstly,then decrease,and at 65?,the rates get maximal.With the increasing of CO₂ partial pressure,the erosion-corrosion rate will increase firstly,and then decrease,finally increase,and the maximum value appeared at 0.45 MPa.With the increasing of Cl-content,the erosion-corrosion rate will increase firstly,then decrease,and the maximum value appeared at 35000mg/L.Using weight loss method to study the combined effects of temperature and CO₂ partial pressure on erosion-corrosion of X70 pipeline steel.The results showed that,the effect of changes in CO₂ partial pressure on erosion-corrosion rate at high temperature is not obvious.At higher partial pressures,the magnitude of erosion-corrosion rate increases with temperature increasing and rate reduced significantly at higher temperatures.The macro morphology and structure of the corrosion product film as well as its effects on corrosion rate and the sample matrix morphology are studied by using macro observation methods after static soaking and slug flow scouring experiments.The results of static soaking test show that with the increasing of temperature the thickness and compactness of corrosion product film increase.At high temperature,product film has high density,but vulnerable to local corrosion failure.In the range of higher CO₂ partial pressure,the effect of increased partial pressure on density of corrosion product film is not obvious and the corrosion rate is high.When Cl-content varies from 45000mg/L to 60000mg/L,the corrosion reaction rate is low with less corrosion products covering the surface of the sample.At the concentration of 35000mg/L,pitting appears because of the porous film.The results of scouring test show that in the range of lower temperature from 40? to 65?,localized corrosion such as pitting or grooving corrosion is severe on the matrix of sample,but reduce at higher temperature.In the range of lower CO₂ partial pressure from 0.1MPa-0.15 MPa,the product film is difficult to form because of the less product caused by lower reaction rate.At 0.45 MPa,product film on the surface of sample has double-layer with the upper and lower layer forming a clear separation.At low Cl-concentration,the sample surface peeling occurs and makes erosion-corrosion more severe while peeling disappears at higher Cl-concentration,but the tendency of localized corrosion becomes larger.Based on the conclusions of soaking and flow slug scouring test,the synergistic rule of erosion and corrosion at different temperatures,CO₂ partial pressure and Cl-concentrations are studied.The results show that at different temperatures synergistic effects of erosion and corrosion differ a lot.At different CO₂ partial pressure is,the difference of synergism between erosion and corrosion is small and the maximum value appears at 0.55 MPa because of good protection of corrosion product film.At different Cl-concentration,synergistic effects of erosion and corrosion differ a lot as well.When Cl-concentration varies from 4000mg/L to 25000mg/L,the contribution to the erosion-corrosion rate made by corrosion is increasing,while the contribution of synergies decreasing,concentration of 35000mg/L to 60000mg/L in contrast.The synergistic effect on erosion-corrosion rate at high flow rates is greater than low flow rate.