First-principles Modeling And Application Of Electrochemical Corrosion Of Metals And Alloys

Corrosion of metals has always been a widespread engineering problem in materials sciences,and electrochemical corrosion is the most common behavior of corrosion of materials at room temperature.The main developing directions of corrosion sciences focus on detecting the characters of corrosion,elucidating the intrinsic mechanism of corrosion,and seeking good ways to improve the corrosion resistance of materials.However,current researches on corrosion of materials have been mainly performed in various experiments and are most limited to ellucidate the macroscopic changes of whole samples.There is a consensus that observed experimental results need to be interpreted from the atomic and electronic levels in analyzing underlying mechanisms of material corrosion.Nowadays,computational materials sciences are providing more powerful and reliable tools for the development of new materials and improvement of traditional materials.Although investigations on corrosion of materials have made great progress,to accurately compute and predict the corrosion-related degradation of materials in a given environment remains one of the most challenging issues in corrosion science.This is mainly because corrosion behavior is so complicated and multidisciplinary so that many factors(i.e.,surface state and environment)can affect the spontaneous occurrence of corrosion.To date,there have been no effective computational models to study the corrosion behavior of materials,highlighting great challenges.Within this context,the purpose of this thesis is to model electrochemical corrosion behavior of materials within the framework of first-principles calculations with the main aims of analyzing the corrosion mechanism and guiding the design of promising corrosion-resistant alloys.The main results are summarized as follows,(1)First-principles modeling of corrosion behaviors of metals and alloys in corrosive environments.In the first,by means of first-principles calculations,we proposed an ab initio model to simulate the crystallographic plane-dependent corrosion behavior of metallic electrodes for metals and alloys,and we found that the surface energy density,Esurf/p,and the work function,?,play important roles in determining the anodic dissolution behavior.In the second,we have further proposed an ab initio model to specify formula between hydrogen evolution rate and overpotential of hydrogen evolution reaction,concerning three different rate controlling steps mechanisms of the so-called Volmer reaction,the Tafel chemical desorption reaction and the Heyrowsky electrochemical desorption reaction.Within this modeling,the adsorption free energy(AGh*)of the hydrogen atom and the concentration of hydrogen ions in the solution have been included to correlate the exchange current density.By combining first-principles modelings of both the anodic dissolution and the hydrogen evolution reaction in electrochemical corrosion,we have realized to simulate the polarization curve in electrochemical corrosion using the first-principles calculation.Moreover,the models have been validated by available experimental results,evidencing their reliabilities.(2)Modeling analyses of the electrochemical corrosion behaviors of magnesium alloy.By computing the polarization curves of anodic dissolution and hydrogen evolution reaction of 18 crystalline faces of pure magnesium,it has been found that the corrosion current density(log[i0/(A/cm2)])and corrosion potential(Ecorr/VSHE)in a neutral solution is located in the ranges from-3.477 to-0.455 and from-1.360 to-0.892,respectively.These theoretical results are in good agreement with experimentally determined ranges of corrosion current density(from-4.265 to-4.012)and of corrosion potential(from-1.266 to-1.172)of pure magnesium in a 3.5 wt.%NaCl solution.The base surface(0001)exhibits the good corrosion resistance due to its lowest dissolution rate and lower hydrogen evolution rate,whereas the crystal face(2130)has worst corrosion resistance because of its fastest anodic dissolution rate and the hydrogen evolution rate among crystalline faces considered here.The effects of 3d-,4d-5d-and some p-type alloying elements on the electrochemical corrosion of magnesium matrix were studied systematically.It was found that,for the anodic dissolution reaction the alloying elements of Cr,Cd,Hg,Ga,In,As,and Sn reduce the anodic dissolution rate of the magnesium alloy.For the cathodic hydrogen evolution reaction Al,Cu,Zn,Cd,Hg,Ga,Sn and As reduce the free energy of the surface-adsorbed hydrogen atoms,thereby accelerating the rates of cathodic hydrogen evolution reaction.In agreement with experimentally measured potentiodynamic polarization curves of the Mg-1Zn and Mg-2Sn alloys,both Zn and Sn additions did accelerate the cathodic hydrogen evolution rate and reduced the anodic dissolution rate.We have elucidated that the intrinsic mechanism of microscopic galvanic corrosion behavior possibly exists between different crystal faces.It has been revealed that the work function and surface energy density are the key factors for galvanic corrosion between different crystalline planes.A crystalline plane with a small degree of atomic density exhibits a high surface energy density and a low work function,acting as an anodic phase and,conversely,a crystalline plane having a high degree of atomic density exhibits a low surface energy density and a high work function,acting as a cathodic phase during electrochemical corrosion of polycrystalline materials.(3)The effects of three twin boundaries on the electrochemical corrosion behavior.We have revealed that the existence of a twin boundary({1011},{1012},or{1013})on the surface of(1210)increases the surface energy density,thereby accelerating the self-corrosion rate of Mg from 1.18 to 1.82 times.The corresponding self-corrosion potential was about 7 mV to 27 mV,lower than the surface without twin boundaries.Furthermore,the addition of alloying elements(As,Cd,Hg,Zn,Sn)has been revealed to reduce the surface energy density,thereby slowing down the anodic dissolution rate of the magnesium alloy.They are beneficial to the corrosion resistance of magnesium alloys.