Research Of Corrosion Behavior For Buried Gas Steel Pipeline

At present,the domestic buried gas pipelines are mainly steel pipes,which are prone to leakage or failure due to internal or external corrosion in service process,mainly because of exposure to gas impurities and soil,as well as inappropriate design or construction.For special detrimental characteristics of gas such as combustibility,explosive and toxic feature, erosion related accidents may bring more serious harms comparing with other transportation medium:not only causing great economic loss but also threatening people's safety.So it is necessary to carry a thorough investigation of the corrosion behavior of buried gas steel pipelines.The service environment of buried gas steel pipelines can be divided into two categories: internal and external environment,which are separated by the tube wall.Based on the analysis of buried gas steel pipeline's corrosion environment,internal corrosion behaviors and soil corrosion behaviors of Q235B steel and L245 steel are studied in this paper by means of potentiodynamic polarization curves,electrochemical impendence spectroscopy,SEM,XRD and weight loss test.Concentration of S^2-,temperature and pH value are three factors for the inner corrosion.The results show that:(1) In manufactured gas pipeline inner-seeper simulation solution for Dalian city,Q235B steel and L245 steel have similar thermodynamic stability;with the increase of concentration of S^2-,the corrosion potential of Q235B steel and L245 steel become negative then positive, and the corrosion rates of them first increase then decrease,and the largest corrosion rate appears whenρ(S^2-) equals 20 mg/L;at lower concentration of S^2-,pitting corrosion is obviously presented while uniform corrosion is found at high concentration of S^2-.In the primary stage,the film formed by corrosion products plays a protective role on Q235B steel and L245 steel,which means high concentration of S^2- has corrosion inhibition effect to carbon steel to some extent;the corrosion resistance of L245 steel is more stronger than Q235B steel in the same internal environment of the pipe.(2) In manufactured gas pipeline inneroseeper simulation solution,with the increase of temperature,the corrosion potential decreases gradually,and approches the anode's equilibrium potential,which means the corrosion reaction is controlled by cathodic process. Both of the cathodic and anode mass transfer process are promoted with the increase of temperature,which also enhances the corrosion rate;lower pH values contribute to positive corrosion potential of Q235B and L245 steel,and help to accelerate the cathodic process, which turns into a decisive factor for overall corrosion reaction.Therefore,the promotion of cathodic process gives rise to the corrosion rate.(3) In water saturated soil in Dalian city,the corrosion potential of L245 steel is about 20mV more negative in compassion with Q235B steel.In the initial stage,both of their EIS are composed of three parts:high frequency small capacitive reactance,intermediate frequency large capacitive reactance and low frequency small inductive reactance.Then low frequency small inductive reactance degrades before disappearing finally.The corrosion rate of Q235B steel increases at first then decreases slowly for 31 days,,while the corrosion rate of L245 steel decreases with a constantly quickening pace.After 31 days,the corrosion rate of Q235B steel is 0.01927mm/a,and the corrosion rate of L245 steel is 0.01763mm/a;Both of weight loss test and EIS test reveal that corrosion resistance of L245 steel is much stronger than Q235B steel in the same external soil environment.This paper analyses the electrochemical corrosion behavior of Q235B and L245 steel in internal and external environment of gas pipe,and puts forward some suggestions to gas pipe's anticorrosion on the base of experiments,which has certain theoretical and practical significance of ensuring buried gas pipe's safety and reliable operation.