Research On The Internal Corrosion Behavior Of 316L Clad Pipes In Tarim Typical CO₂ Gas Fields

Corrosion has always been a major obstacle to the safe development of oil and gas fields.In recent years,in order to cope with increasingly stringent corrosion conditions,a number of Tarim CO2 gas fields,including the Kela 2 gas fields and Dina 2 gas fields,are beginning to use 316L stainless steel as the material of pipes.However,several failure accidents have occurred due to internal corrosion of 316L clad pipe in practical engineering applications,causing a serious impact of the normal operation of the gathering and transportation.Based on the results of field investigation,the internal corrosion behavior of 316L clad pipe in simulated Tarim CO2 gas field environment was studied systematically by using high temperature and high pressure static coupon experiment,electrochemical experiment and slow strain rate tensile test,etc.The main conclusions are as follows:(1)The corrosion type and corrosion degree of 316L stainless steel were studied by high temperature and high pressure static coupon experiment.The results show that temperature and Cl' concentration have a great effect on the pitting corrosion behavior of 316L and the corrosion rate increases with the increasement of temperature and Cl-concentration;increasing CO2 partial pressure has no significant effect on the corrosion rate of 316L,but has a inhibitory effect on the pitting degree in some extent.By calculating the corrosion rate and observing the macroscopic/microscopic corrosion morphology of the subjects,it is found that the corrosion of the weld area is much more serious than that of the parent metal.(2)The effects of CO2,temperature and Cl-concentration on the pitting mechanism of 316L were studied by conventional electrochemical method.The results show that the increasement of temperature and Cl-concentration could both weaken the pitting resistance of 316L stainless steel,but the mechanisms are different.The effect of CO2 on the pitting corrosion of 316L has a dual effect,but mainly plays a role in suppressing it.Under the same experimental conditions,the pitting corrosion resistance of the parent electrode is better than that of the weld electrode.The results of electrochemical experiments are in good agreement with those of high temperature and high pressure static coupon experiment.(3)The effects of CO2 partial pressure,temperature and Cl' concentration on the corrosion cracking resistance of 316L stainless steel were investigated by slow strain rate tensile test.The experimental results show that the influence of CO2 partial pressure,temperature and Cl-concentration on the stress corrosion cracking resistance of 316L stainless steel are significantly different.The increase of temperature and Cl-concentration has a acceerative effect on the stress corrosion cracking.The change of CO2 partial pressure has little effect on the experimental results.The macroscopic and microscopic analysis(SEM/EDS)of the fracture of the tensile specimen show that the fracture site is in the weld and heat affected zone.With the increase of temperature or Cl-concentration,fracture characteristics is changing from the typical mechanical toughness to brittle fracture.(4)To use quick-slow scan rate electrochemical method to study the stree corrosion cracking behavior of 316L stainless steel.The three sensitive potentials of 316L chloride stress corrosion cracking are the cathodic protection-activation transition zone,activation-passivation transition zone and blunt transition zone.The addition of CO2 can inhibit the stress corrosion cracking of 316L.The increasing of temperature not only enhances the stress corrosion sensitivity but also widens the stress corrosion sensitive potential.Increasement of the Cl-concentration also promotes chloride stress corrosion cracking.Compared with ordinary samples,316L stainless steel U-shaped curved specimen has a greater sensibility of chloride stress corrosion cracking.(5)The degree of sensitization of 316L stainless steel was evaluated using an improved DL-EPR method.The experimental results show that the degree of sensitization of the weld is obviously higher than that of the parent material,and local corrosion is more likely to occur.The metallographic scan of the electrode after the test showed that the passivation film on the surface of the parent material is complete.Meanwhile,the sensitization phenomenon has occured at the grain boundary of the weld.