Study On Electrochemical Characteristics And Corrosive Mechanism Of Sandy Soil Containing Soluble Sodium Salt

With the development of the national economy,the durability of a large number of underground space structures and underground facilities,such as integrated pipe corridors,has become the hot topic for scholars and engineers all over the world.Water content,salinity,acidity and alkalinity,etc.are all factors that can affect the durability of underground space structures and underground facilities.In this work,the electrochemical characteristics and corrosive mechanism of sandy soil containing soluble sodium salt are studied based on the main research line of the electrochemical theory of sandy soil,the electrochemical characteristics of sandy soil containing soluble salt and the application of electrochemical corrosion of X80 steel in sandy soil.Firstly,the particle morphology and pore structure characteristics of sandy soil were studied through the theoretical calculation,laboratory test and statistical analysis.And the electrochemical equivalent circuit model of sandy soil was established based on the electrochemical theory and the electrical conductivity characteristics.Secondly,the electrochemical behaviors of pore fluid,sand particles and sandy soil with different water contents and single soluble sodium salt?NaCl,Na₂SO₄ and NaHCO₃?were studied by experiments,revealing the influence of sand particles?solid phase?,pore solution?liquid phase?and soluble sodium salt on the electrochemical behavior of sandy soil.Finally,the corrosion polarization curves of X80 steel in different concentrations of easily soluble sodium salt solution and sandy soil with different contents of sodium salt,and the range and variance analysis based on macro and micro morphology observation and corrosion products confirmation were conducted.The effect of easily soluble sodium salt on the corrosion of X80 steel in sandy soil was studied,and the corrosion mechanism was elucidated.The main results are as follows,Basic physical and mechanical properties and electrochemical model of sandy soil have shown that the solid particles used in the test have smooth boundaries and there was no debris.The particles were mostly sub-circular,round and extremely round.The moisture content of sandy soil should be between 6%and 12%at moderate compactness and liquid bridge limit volume.The basic equivalent circuit of the sandy soil—electrode belonged to the equivalent circuit model?1?,R?C?R?Q?RW????.When the frequency was high,each path in sandy soil was in a conducting state,and part of the discontinuous phase path would be blocked at a low frequency.Unsaturated sandy soil exhibited resistance and capacitance characteristics,and the sandy soil presented electrical resistance as a whole as the water content reaches saturation.The electrochemical behavior of sandy soil with different water contents showed that the dry sandy soil without liquid phase had a slope of about-1 in the bode diagram?log|?5|-log1??,which was a nearly pure capacitive element.The Nyquist diagram of sand system with different water content was composed of flat semicircle in the high frequency region and nearly 45°oblique line in the low frequency region.The fitting was in good agreement with the extended equivalent circuit?2?,R?C?R?Q?RW?????CR?.As the water content increased,the radius of the capacitive anti-arc and the diffusion arc and the impedance modulus decreased,and the corrosion of the sandy soil increased.The particles of the sandy soil played a dispersion effect on pore fluid,and had increased the pore liquid resistance R_e by an order of magnitude.The hydraulic radius value per unit area of sandy soil was below 2 mS,while the magnitude of diffusion impedance representing the change of tortuosity?L?fluctuated between 10^-710^-8.The electrochemical behavior of sandy soil containing soluble sodium salt demonstrated that the EIS characteristics of pore fluid containing NaCl,Na₂SO₄and NaHCO₃ were consistent with that of sandy soils,but the shape and phase angle were different.The electrochemical process in the saline sand and the pore fluid was the same,the anode was dissolution of electrode,and the cathode was oxygen absorption process.In the electrochemical process,the migration and aggression of Cl^-were large,HCO₃^-contributed to the formation of a passivation film,and the charge of SO₄^2-was large.The charge transfer resistance(R_ct)could directly reflect the initial speed of electrochemical process in sandy soil.In sandy soil containing mixed soluble sodium salt,the order of the influence of ions on the R_ct was:HCO₃^->SO₄^2->Cl^-.Therefore,the corrosiveness of sandy soil could be judged according to the electrochemical impedance spectroscopy.The electrochemical mechanism studies of X80 steel in sandy soil contained a single soluble salt suggesting that as the increasing water content in the sandy soil,the corrosion rate of X80 steel gradually increased by two orders of magnitude.Both of NaCl and Na₂SO₄ accelerated the corrosion of X80 steel in sandy soil and pore liquid containing soluble salt from slight to moderate,and severe grade,in which NaCl action was more significant.While NaHCO₃ could weaken the corrosion of X80 steel and reduced the corrosion rate by 1² orders of magnitude.This was slightly different from the three-electrode system of the same material?copper?.Therefore,the electrochemical behavior of the sandy soil when the copper sheet was used as an anode can only reflect the corrosiveness of the sandy soil to a certain extent.In sandy soil,NaCl and Na₂SO₄ mainly affected the corrosion current density(I_corr)of X80 steel,and NaHCO₃ mainly affected the corrosion potential(E_corr).The corrosive mechanism researches of X80 steel in sandy soil containing mixed salt showed that the corrosion of X80 steel in sandy soils containing mixed salt was able to considered as local corrosion.There was significant interaction between Cl^-,SO₄^2-and HCO₃^-ions,and sand particles might also participate in the electrochemical process in an alkaline environment.According to the EIS and polarization test analysis,the corrosion of sandy soil to X80 steel was comprehensively judged,and it could been ranked from small to large,No.9<No.6<No.3<No.4<No.5<No.8<No.2<No.7<No.1.The influence mechanism of sand particles on electrochemical corrosion was due to the sand particles playing a role in promoting the low frequency process to hinder the high frequency process.The effect of sand particles on the corrosion current and corrosion potential of X80 steel was related to the soluble sodium environment.The sand particles accelerated the corrosion of X80 steel in NaCl environment,the sand particles had less influence on the corrosion of X80 steel in Na₂SO₄environment,and the sand particles slowed the corrosion of X80 steel in NaHCO₃environment.Comprehensive analysis of the electrochemical properties and corrosion mechanism of sandy soil containing soluble sodium salt confirmed that the inhomogeneity of solid—liquid—gas coexistence in sandy soil provided sufficient oxygen environment for cathodic reaction in electrochemical process and promoted the tendency of local corrosion.In the equivalent circuit,charge transfer resistance(R_ct)reflects the corrosiveness of sandy soil,solution resistance?R_e?and constant phase angle element(Q_dl)represented the state of the surface active region of the electrode,and the sand layer capacitance?C_s?and the resistance?R_s?corresponded to the state of particle size,water content and the conductive path,which was related to the physical and mechanical properties of the sandy soil.Both of the content and type of salt in sandy soil could affect the corrosiveness.Compared with pore fluid,X80 steel in sandy soil containing NaCl had a greater tendency of corrosion.The corrosion tendency of X80 steel in sandy soil containing Na₂SO₄ and NaHCO₃ was similar to that in pore fluid.Therefore,in the material selection,the pipe material with lower interface energy of the adhesion system should been adopted,and the uniformity of the soil around the pipe should be kept as much as possible in the pipe laying design.