Electrochemical Study For The Corrosion Of Nickel-base Alloy 718 In The Simulated Sour Gas Well Environment

The proportion of sour oil&gas well is increasing as mining depth deepens.Good mechanical properties and corrosion resistance of metal materials are demanded to meet the service requirement of the high temperature high pressure?HTHP?down-hole sour environment.Nickel-base alloy 718 is one of the typical age-hardened austentic alloys,and has important application in the oil&gas industry due to its good mechanical properties,corrosion resistance and non-magnetic property.The corrosion cracking of the alloy 718 in the sour well environments has been known according to the field operation accumulation and the laboratory tests for years.However,the corrosion crack initiation and early propagation rules behind the corrosion tests are still poorly understood,inasmuch as the inaccessible downhole corrosion failure process by the in-situ observation and tracking,which influence the security evaluation of the materials application directly.Adopting the simulated sour well environments assisted by the electrochemical measurements makes the material's corrosion process becoming relatively transparent.By using the autoclave,this paper systematically investigates the corrosion process and mechanisms of the alloy 718 in the simulated sour well environments,based on the in-situ electrochemical measurements together with the surface examination after corrosion in the laboratory scale.To detect and characterize the localized corrosion the cross recurrence analysis approach has been proposed to elucidate the electrochemical noise?EN?signals and has been successfully applied in the system under study.The main research contents and innovations are as follows.?1?According to the comparison of the electrochemical behavior of the conventional CO₂ and H₂S corrosion in the oil field for the alloy 718,their different corrosion resistant mechanisms were studied.The electrochemical impedance spectra?EIS?of the CO₂ corrosion presents two capacitive loops arose from the oxygen passive layer and the interface impedance;while the EIS of the H₂S corrosion has the characteristic diffusive impedance at the low frequency domain,and the sulfur-containing corrosion product tarnish film may contribute to the diffusive impedance which plays important role in the corrosion resistance of the alloy.The defect of the passive film would increase when the temperature or CO₂/H₂S pressure enhances,so as to raise the uniform corrosion rate or induce the localized corrosion;the diffusive impedance would disappear when the sulfide dissolves under lower pH and the high frequency resistance increases at the same time.?2?The electrochemical noise measurements performed on the alloy 718 in the sulfur-containing high temperature saline presents characteristic transients when localized corrosion takes place.The cross recurrence quantification analysis?CRQA?on the current and potential noise would discriminate the initiation,formation and propagation process for the localized attack.The cross recurrence plot?CRP?patterns as well as the synchronization index further indicate the kinetics transition during the localized corrosion process.The recurrence plot-based dynamic analysis is more suitable for the short-term certain and long-term uncertain,as well as the non-stationary aperiodic noise signals arose from the localized corrosion than the conventional time-frequency analysis methods.?3?Several types of the current noise transient corresponding to different localized corrosion rates are extracted from the EN signals of the alloy 718 in the high temperature saline containing sulphur.A qualitative equivalent circuit model based on the heterogeneous cathodic reactants and the localized corrosion mechanism induced by the molten sulfur is proposed to elucidate the forms of the different current noise type.?4?The stress corrosion cracking?SCC?tests of the alloy 718 in the HTHP CO₂-H₂S-Cl^-environment indicate the H₂S diffusion to the crack front is the essential condition of the crack propagation.The electrochemical noise monitoring of the alloy in the high temperature sulfur-containing saline indicates the temperature,surface roughness and soaking time may be the key influence factors to the SCC crack formation of the alloy 718 in the test conditions,for enough time should be taken to induce the crack under the localized corrosion.