Influence Of Pore Structure On Corrosion Behavior Of Different Porous Metals And Its Mechanism

Porous metals possess the advantages of low bulk density,high specific strength and stiffness,good thermal conductivity,good energy absorption and damping effect,and are widely used in fluid separation and infiltration,mass and heat transfer,damping adsorption and other fields.As a type of metal material,its failure caused by corrosion is one of the main factors limiting its application performance.Existing studies have shown that the introduction of pore structure affects the corrosion behavior of porous metals.However,the corrosion behavior of porous metals,especially the corrosion law inside the pores,is still not clear.Therefore,three typical metals（TC4 titanium alloy,Q235 carbon steel,T2 pure copper）were selected in this work for preparation of porous metal materials by mechanical processing technology.Then the influence of pore structure characteristics such as pore size,porosity and open pore state on the corrosion behavior of the porous metals in static and dynamic neutral solutions was discussed by electrochemical technology.In particular,the corrosion behavior of the pore interior was studied by the scanning Kelvin probe technique,and the corrosion mechanism of the three porous metals was discussed.The main research work in this paper includes:（1）Under static conditions,the corrosion behaviors of the porous metals in 3.5 wt%NaCl solution and 15 wt%NaCl solution were investigated by using potentiodynamic polarization curve,electrochemical impedance spectrum and SEM characterization.The results show that the TC4 titanium alloy specimens’corrosion gradually increased with the increase of the number of pores in different NaCl solutions.With the same number of pores,the corrosion resistance of TC4 titanium alloy decreased with the rise of Cl^–concentration.However,the corrosion of Q235 carbon steel and T2 pure copper samples gradually decreased with the addition of the number of pores in different NaCl solutions.The corrosion process of Q235 carbon steel in NaCl solution was mainly controlled by anodic polarization,and the formation of surface passivation film caused corrosion slowdown.The corrosion process of T2 pure copper specimen in NaCl solution was controlled by the anodic polarization and cathodic polarization jiontly,and the corrosion was affected by the oxygen diffusion in the solution and the formation of oxide film on the metals’surface.（2）The corrosion behaviors of the porous metals in dynamic 3.5 wt%NaCl solution and 15 wt%NaCl solution were investigated by using self-made dynamic circulation system,using potentiodynamic polarization curve,electrochemical impedance spectroscopy and SEM characterization.The results show that the TC4 titanium alloy specimens gradually increased with the rise of the number of pores in different NaCl solutions.With the same number of pores,the corrosion resistance of TC4 titanium alloy increased with the increase of Cl^–concentration,mainly because the high concentration of Cl^–promoted the addition of dense Ti O₂ content in the oxide film,effectively isolating further erosion of Cl^–.Corrosion of Q235 carbon steel intensified in NaCl solution with the increase of the number of pores.Under dynamic conditions,the diffusion of Cl^–and dissolved oxygen in solution was enhanced.The corrosion process of Q235 carbon steel was controlled by the reduction and depolarization of oxygen in cathodic,and the metal surface was not easy to form a passivation film,so the corrosion resistance was significantly reduced.The corrosion of T2pure copper samples in different NaCl solution first rose and then decreased with the increase of the number of pores.When the number of pores was 24,the sample corrosion was the most serious.In the flow solution,the increase of the number of pores,on the one hand,promoted the rise of the reaction area between pure copper and aggressive Cl^–in the solution,resulting in metal dissolution and accelerated corrosion.On the other hand,the introduction of pore structure provided more diffusion channels for oxygen,which was conducive to the formation of oxide film on the copper surface and reduces corrosion.The combined effect of these two aspects determined the corrosion rate of porous copper.（3）The effects of different pore sizes（φ=1mm andφ=2mm）,porosity,through holes and sealed holes on the corrosion behavior of the porous metals in static 3.5 wt%NaCl solution were investigated by electrochemical methods.The results show that under the sealing condition,the corrosion resistance of porous TC4 titanium alloy specimens increased with the increase of the number of pores.When the porosity is the same,the corrosion resistance of through-hole TC4 titanium alloy decreased with the addition of the exposed area of the specimen.After the pores of porous Q235 were sealed,the passivation phenomenon on the surface of the sample disappeared,resulting in aggravation of its corrosion.When the porosity was the same,the corrosion of through-hole Q235 carbon steel reduced with the rise of the exposed area of the specimen.Under the sealing condition,the corrosion resistance of T2 pure copper specimen in 3.5 wt%NaCl solution increased with the addition of the number of pores.When the porosity was the same,the corrosion resistance of through-hole T2 pure copper was less affected by the exposed area of the sample.（4）The scanning Kelvin probe technology was used to test the changes of the outer surface of the specimen and the surface of the inner wall of pore before and after immersion of the porous metals in NaCl solutions with different concentrations,so as to evaluate the difference in the corrosion behavior inside and outside the pores.The results show that after the porous TC4 titanium alloy was immersed,the inner wall of pore had obvious micro-anode and micro-cathode formation,showing the characteristics of local corrosion,which ultimately leaded to the reduction of the corrosion resistance of the metal matrix.After the corrosion of Q235 carbon steel,the average Kelvin potential of the inner wall of pore was higher than that of the surface of the sample,and the corrosion products on the inner wall of pore are more prone to accumulate in and around the pore after formation,which blocks the reaction of the aggressive ion Cl^–with the metal,and the corrosion resistance of Q235 carbon steel was improved.After T2 pure copper corrosion,the positive degree of Kelvin potential on the inner surface of pore was greater than that of the Kelvin potential on the outer surface of the metal,that is,the corrosion products formed in the pore were denser and thicker,and there was no obvious anode region and cathode region formation on the inner wall of the pore,the potential difference was reduced compared with before corrosion,and the corrosion tendency was reduced.