Research On Dissolution Behavior And Mechanism Of High Plastic Mg-Gd Based Solution Magnesium Alloy

The development and application of magnesium alloys as a degradable material in the field of oil and gas extraction is gradually emerging due to their poor corrosion resistance.For example,in the process of unconventional energy extraction,the application of soluble fracture temporary plugging tools can greatly improve the efficiency of shale gas extraction.A large number of conventional magnesium alloys are currently available on the market,but their degradation rates are hardly sufficient for practical engineering applications,so more and more researchers are developing new magnesium alloys with high degradation rates.For example,in this paper,a new high plasticity Mg-Gd based soluble magnesium alloy prepared by alloying with rare earth element Gd is studied.The soluble material shows a very high degradation rate during the dissolution process,and its use as a temporary plugging tool can be dissolved and rendered ineffective after oil and gas extraction is completed and used without removing it from the ground and directly passing it into the KCl solution.However,few studies have been conducted on the dissolution behaviour and dissolution mechanism of this material.Therefore,in this paper,the as-cast and extrusion and perforation deformation（EP）Mg-Gd-based soluble magnesium alloys were studied in terms of their microstructure and electrochemical properties.On this basis,immersion experiments were carried out at different temperature gradients,different Cl^-concentrations and different initial organisational states to investigate the corrosion dissolution behaviour of the alloys.In addition,in order to further investigate the dissolution process of the highly plastic Mg-Gd based soluble magnesium alloy,immersion weight loss experiments and electrochemical tests were carried out by melting the matrix phase and the rare earth-rich second phase Mg₅Gd,respectively.Finally,the effects of temperature,Cl^-concentration and initial tissue state on the dissolution mechanism were modelled through the analytical study of microstructure and dissolution behaviour,respectively.The specific studies are as follows:（1）The morphology,size and distribution of the second phase and the specific composition of the second phase of the as-cast and metamorphic Mg-Gd based alloys were investigated using metallographic microscopy（OM）,scanning electron microscopy（SEM）,energy spectroscopy（EDS）and X-ray diffraction（XRD）tests,respectively.（2）The surface roughness and surface volt potential of the as-cast and EP deformed Mg-Gd based alloys were investigated by SKPFM tests,and the surface roughness and surface volt potential values of the as-cast and EP deformed alloys were obtained;the Tafel polarisation curves and AC impedance spectra of the as-cast and EP deformed alloys were analysed and studied by electrochemical tests,and an equivalent circuit model for the electrochemical reaction process was fitted by the test results.（3）Immersion weight loss tests were carried out for Mg-Gd based alloys at different temperatures（25°C,50°C,75°C and 100°C）,different Cl^-concentrations（1%,3%,5%and 7%）and different initial tissue states（as-cast and EP deformed）,respectively,and the corrosion dissolution behaviour was investigated in terms of the macroscopic morphology,corrosion rate and corrosion products of the specimens after corrosion dissolution.Finally,the relationship between different temperatures and chloride ion concentrations and corrosion rates was modelled by the test results and experimentally verified and analysed.（4）The second phase and matrix phaseα-Mg of the Mg-Gd based alloy were calibrated by EDS and XRD tests,and their main components were determined to be Mg₅Gd and Mg,respectively;on this basis,the second phase specimen matrixα-Mg was melted,and immersion tests and electrochemical tests were carried out,respectively,to investigate how the rare earth element Gd affects the corrosion and dissolution mechanism of the Mg-Gd based alloy,and to investigate its high The dissolution rate mechanism based corrosion types were studied in depth.（5）By studying the effect of different initial tissue states on the corrosion mechanism of cast and EP deformed Mg-Gd-based soluble magnesium alloys,the corrosion mechanism models of cast and EP deformed alloys were finally established respectively.