Study On The Hazard Mechanism Of Mercury On The Gathering System Of KS Gas Field

Mercury is easily condensed and adsorbed on the inner wall of pipelines and equipment.Adhesion to the inner wall of the equipment for a long time will certainly cause some damage to the metal.The KS mercury content in domestic mercury-bearing gas fields is relatively high.At present,there are accidents where mercury-induced equipment failure occurs in many gas fields.The mercury-containing gas gathering system has high temperature and pressure and contains a variety of corrosive media.The corrosive environment is harsh.To ensure the safe and effective operation of the KS mercury-contained gas gathering system,this paper do some analysis of corrosion samples caming from KS mercury-bearing gas fields from the aspects of mercury corrosion and mercury penetration and conclud corrosion mercury corrosion mechanisms and mercury penetration mechanisms under various corrosive media,eventually propose corresponding mercury protection measures.According to the results of investigations over the corrosion of the KS gas field gathering pipelines and equipment over the years,four typical corrosion samples were retrieved,which are from the KS102 gas gathering branch line,the KS2 gas gathering trunk line,the Monel alloy pressure gauge and the stainless steel pressure gauge.From the corrosion macroscopic appearance and corrosion position to preliminarily assess the corrosion samples and found that the cause of corrosion of the gathering pipeline should be related to the liquid corrosive medium,the cause of the failure of the pressure gauge should be related to the vapor phase corrosive medium.Through the analysis of the gas-liquid two-phase corrosion media,the mercury content in the single-well vapor corrosion media was between 537 and 7220μg/m3,CO2 content was 0.679 mol%,and a small amount of water vapor.The content of Cl-in the liquid corrosive medium is high,reaching a maximum of 29892.95 mg/L,and the mercury content is from 55μg/L to 213 μg/L,and the main form is Hg2+.Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)were used to observe the micro-morphology of the corrosion components.The micro-area elements were analyzed by the energy spectrometer on the corrosion area.The metallographic microstructure and grain size of the corrosion area were analyzed by the metallographic microscope to determine the effect of mercury on the corrosion of the KS gathering system.Mainly reflected in three aspects,Firstly,the penetration of mercury into the grain boundary of Monel leads to the reduction of the toughness of the Monel spring tube,penetrative cracking below the yield stress,and the second is the penetration of mercury into the inner wall of the 304 stainless steel pressure gauge cause the amalgamation of chromium in the matrix metal of the region,and the existence of mercury in the crack region may also accelerate the generation of new cracks;thirdly,the Cl-in the gas field water causes pitting corrosion in the heat affected zone of the pipeline weld.Hg2+in the gas field is involved in the cathodic reaction around the pit,accelerating the corrosion of the metal in the anode region inside the pit.SEM and EDS were used to analyze the elemental surface distribution and elemental line distribution on the inner wall surface of the gathering pipeline and along the section direction respectively.Combined with the tensile performance analysis of the 316L lining of the gathering pipeline and the elemental analysis,the result was carried out that there is no mercury permeation on the inner wall of the pipeline,and it is inferred that the mercury penetration mechanism is that elemental mercury physically adsorbs on the sulfide or corrosion product layer on the surface of the stainless steel inner wall and reacts with the sulfide layer or the corrosion product and adheres tightly to the matrix metal oxide.On the film,once the oxide film is broken,the mercury vapor molecules adhering to the oxide film diffuse into the grain boundary or in the vapor phase,and the mercury vapor molecules gather at the grain boundaries,lowering the binding force at the grain boundaries,and lowering the plasticity of the material.On the basis of mercury hazard mechanism and corrosion control theory,combined with on-site process,three effective mercury protection measures are proposed,use of mercury-resistant brittle metals such as 316L austenitic stainless steel,titanium alloy,22Cr duplex stainless steel,and nickel Cobalt alloys,etc.Electroless nickel plating,General Mercaplate’s HCR coating and NEDOX coating and ceramic coating can effectively cut off the direct contact between mercury and metal.Mercury treatment of pipelines and equipment can reduce the chance of mercury being in contact with the inner wall of the equipment.