Effect Of Hydrostatic Pressure On The Stress Corrosion Of Ti-6Al-4V Alloy

With the improvement of perception and mobility in the deep ocean,more and more major marine engineering equipment has been built and put into service on the seabed.Recently,the development of the manned submersible Fendouzhe and its sea trials indicated China's capability of carrying out scientific exploration and research in the deepest parts of the world's oceans as well as its comprehensive strength in marine high technology.During the effective exploration and development of deep-sea resources,the safety of scientific and technological equipment is the basic guarantee.However,there is no systematic theory about the metal corrosion in the deep sea,so how to ensure the safety of equipment in service in the extreme environment will face important scientific and technological challenges.Therefore,it is of great significance to study the mechanism of metal corrosion in the whole ocean depth for maintaining the safety of marine equipment and predicting the remaining service life of submarine components.According to the results that hydrostatic pressure affects the double electric layer at the solid/liquid interface.Pure iron was studied in priority,as metal dissolution is the essential reaction in corrosion.The effect of hydrostatic pressure on the reaction kinetics of the anode electrochemical reactions was investigated by differential capacitance measurement and molecular dynamics simulation.After that,Ti-6A1-4V alloy was taken as the major research object,and the in-situ electrochemical system in the simulated deep sea environment autoclave and the slow strain rate testing(SSRT)system were established in the laboratory.The influence of hydrostatic pressure on the interaction between stress and electrochemical reactions were systematically conducted by electrochemical testing methods such as potentiodyamic polarization,and EIS,combined with characterization methods such as XPS,SEM,and TEM-EDS.The aim in the paper is to study the common problem of hydrostatic pressure affecting the corrosion of active and passive metals.Referring to the current research results,hydrostatic pressure has a significant effect on the anodic reaction kinetics in corrosion processes,and the dissolution mechanisms of pure iron is clearest among the alloys.In this study,the potentiodynamic polarization and differential capacitance of iron were measured from 0.1 MPa to 15MPa in 3.5%NaCl solution and 0.6 mol/L Na2SO4 solution,respectively.The results show that the electrochemical corrosion of iron is still promoted by hydrostatic pressure in the solution without Cl-.Meanwhile,the EIS results and molecular dynamics simulation showed that hydrostatic pressure reducing the distance from the electrode surface to the outer Helmholtz layer,which changes the electric double layer structure and the ?1 potential during the anodic reactions.Therefore,hydrostatic pressure promotes the anodic reactions by compressing the electric double layer.According to the polarization curves,hydrostatic pressure was introduced into the dynamic equation of electrode process,and the quantitative relationship among hydrostatic pressure,electrode potential,and the current density was determined.After that,the corrosion of Ti-6Al-4V alloy under tensile stress and hydrostatic pressure was studied in detail.First,the influence of hydrostatic pressure on its passivation process was studied in 3.5%NaCl solution.The pitting resistance and repassivation of the titanium alloy are significantly reduced under the interaction of the applied stress and hydrostatic pressure.The U-bend Ti-6A1-4V alloy was polarized by a given constant potential to form a passive film at 0.1 MPa and 20 MPa hydrostatic pressure,respectively.According to the Mott-Schottky,XPS and TEM-EDS,the point defect density of the passive film formed increased,and the thickness decreased at 20 MPa.The passive film/substrate interface also shows a fluctuating structure at hydrostatic pressure.Given that hydrostatic pressure affects the electrode reactions through the double layer,the dissolution processes of the titanium substrate and the passive film were enhanced during the passive film development,because the double layer only exists at the passive film/solution and the substrate/solution interfaces.As a result,the passive film is always in a state of passive film breakdown-repassivation,which is the main reason for the deterioration of the corrosion resistance of passive film at hydrostatic pressure.After studying the mechanism of hydrostatic pressure on the active dissolution reactions and the passivation processes.The slow strain rate tensile tests were conducted on Ti-6Al-4V alloy at 0.1 MPa and 20 MPa hydrostatic pressure,respectively,to study its stress corrosion in the deep-sea environment.It was found that the passive film could not be repaired in time after its breakdown at 20 MPa,which made a large amount of hydrogen ions in the environment enter into the titanium alloy.As a result,hydrogen-enhanced localized plasticity(HELP)occurs and titanium hydrides are formed near the phase boundary.Under hydrostatic pressure,the titanium alloy exhibited hydrogen-induced cracking behavior and the sensitivity of stress corrosion increased.Finally,to study the mechanism of hydrogen induced cracking at hydrostatic pressure,the cracking morphology of U-bend Ti-6A1-4V alloy after immersing in 3.5%NaCl solution at 0.1MPa and 20 MPa for 300 d were characterized by EBSD and TEM,respectively.The results showed that the crack along the ?/? phase boundary is mainly influenced by the HEDE and AIDE mechanism,and the transgranular crack in a phase is mainly influenced by the mechanism of HELP and hydride formation.