Corrosion-Kinetics.ppt

,,corrosion.kaist.ac.kr,4.Corrosion Kinetics,At the electrode/electrolyte interface,a charge separation between the metal surface and the electrolyte occurs.The spatial region corresponding to the charge separation is called the electrical double layer.It is usually separated into two parts,the,Helmholtz,layer or compact double layer and the,Gouy,-Chapman layer or diffuse layer.The charge at the interface establish an electric field.Within the compact layer,the electric field reaches the order of 10,8,to 10,9,V/m,and hence has an influence on the charge transfer reaction.Since corrosion is an electrochemical process involving the charge transfer reaction,its rate,is significantly influenced by the electrode potential or the electric field across the double layer.,.,+,+,+,+,+,M,x,-,-,-,-,-,M,S,S,x,-,-,-,+,+,+,+,+,-,-,-,-,-,-,-,-,-,-,Stuck ions,Scattered ions,M,S,M,S,Helmholtz,Model,Stern Model,4.1The Electrical analogue of double layer,The electrical double layer is characterized by two layers of opposite charge facing each other,as in a capacitor.The electrical current can,however,pass across the metal-solution interface,although there is some resistance to it.The electrode can then be represented by an electrical analogue composed of a capacitor parallel to resistance R,F,called,Faradaic,resistance.The R,F,is called also the polarization resistance or charge-transfer resistance.,e,-,e,-,e,-,e,-,e,-,M,+z,M,+z,Electrical double layer,Equivalent Circuit,C,dl,R,F,When an electrical current is impressed on the electrode,the R,F,must be overcome.This generates additional voltage and causes a shift in the electrode potential.At rest(open circuit),The electrode has a charged layer;in the absence of an electric current,the capacitor,C,dl,is charged.The current impressed on the electrode,i,t,is divided into two parts.i,t,=,i,F,+,i,ch,M,S,When,i,F,:,Faradaic,current,i,ch,:current of charge accumulated in the capacitorUsually,i,F,i,ch,.,The electrode potential is proportional to the charge Q of the double layer.Thus,the electrode potential changes under an electric current across the double layer;,E=,E,eq,+,(i),(i)=the additional voltage due to the current flow.,Electrode-Electric Analogue,C,dl,R,F,i,t,i,ch,i,F,4.2 Charge transfer,overpotential,or Activation potential,For charge transfer reaction in metal/solution interface:,M(in lattice),M,+z,(hydrated in sol.)+,ze,-,Actually,M,lattice,M,adsorbed,M,+z,sol.,The metal atoms on the electrode surface are in energy wells associated with the lattice structure,and in order to pass into the solution they have to overcome the activation energy.,Electrolyte,Metal,G,c,*,G,a,*,G,ch,M,+,G,elect,M,+,+,+,+,+,+,+,+,+,M,S,10,Helmholtz,Double Layer,Gouy,Chapman Layer,E=,M,-,s,G=,G,chem,+,G,electro,IHP,OHP,M,M,+,IHP,OHP,X,G,G,*,The chemical free energy change due to the dissolution or deposition of the metal is balanced by an equivalent quantity of electrical work done by the ions in crossing the electric field imposed by the equilibrium electrode potential.,M,M+z+,ze,-,.at equilibrium or reversible potential.,Reaction rate=,k,x Concentration =amount produced per unit area per unit time=moles/cm,2,sec,The rate of metal dissolution or reduction may be expressed as a current density(A/cm,2,)according to the,Faradys,law,;,i=Q/At=,zFm,/At,w,here z=number of electrons,M=number of moles,F=,Faradys,constant(=96500 C/mole),A=Surface area on which reaction take places.,At the equilibrium electrode potential,the flux of charge through the double layer is the same in both directions and we call this the,exchange current density,i,o,.,At,equil,.,i,a,=,i,c,=,i,o,and anodic,reation,rate=cathodic reaction rate=,i,o,/zF,Anodic rx.rate=,k,a,x C,M,=f C,M,exp(-,G,*,/RT),where,k,a,=rate constant=f exp(-,G,*,/RT),C,M,=concentration of surface active atom,f=frequency of activated complex,10,12,/sec,exp(-,G,*,/RT)=Probability of reaction,k=,f(pool reactant species)(probability of reaction),Lets calculate C,M,approximately.,If the lattice constant is 3,the number of surface atoms:,N,s,=(10,16,2,/,cm,2,)/(,9,2,/atom),10,15,atoms/cm,2,Thus,the moles of atoms/cm,2,=N,s,/N,A,(,N,A,=Avogadro No.),However,surface atoms adjacent to the crystal defects have a higher probability of reaction than others or more active;,Fraction of surface atoms which are likely to be surface active,=10,-3,to 10,-4,depending on crystal structure,defect density and grain size.,3,Therefore,C,M,=,(N,s,/N,A,)=moles of surface active atoms/cm,2,i,a,=,i,o,=,zF,x(anodic reaction rate)=,zFf,C,M,exp(-,G,*,/RT),Cathodic rx.rate at,equil,.=,i,c,/zF,=,k,c,C,M,+z,V,L,=,fC,M,+z,V,L,exp,(-,G,*,/RT),where,C,M,+z,=conc.of metal ion at OHP V,L,=volume of the double layer/cm,2,10,-8,cm,3,Thus,i,c,=,i,o,=,zFfC,M,+z,V,L,exp,(-,G,c,*,/RT),G*can be changed in electrochemical reactions by externally applied potential(,E,app,).The change in electrode potential from the,equil,.value to acquire a net current(,i,e,externally measurable value)is called,polarization,.,i,e,=,f(E,app,-E,eq,)=f(,),M,M,+z,e,-,i,e,a,:,E,app,E,eq,1)Anodic polarization,M M,+z,+,ze,-,at,E,app,E,eq,i,e,a,=,i,a,i,c,=f(,a,),where,E,eq,=reversible potential or,equil,.potential,a,=,E,app,-E,eq,:anodic,overpotential,or anodic,overvoltage,i,a,i,c,2)Cathodic polarization,M M,+z,+,ze,-,.at,E,app,E,eq,i,e,c,=,i,c,i,a,=f(,c,),c,=,E,app,-E,eq,cathodic,overpotential,or cathodic,overvoltage,.,i,c,i,a,M,M,+z,e,-,i,e,c,:,E,app,0,G,*,Metal,Solution,M,M,+,IHP,OHP,G,0,z,a,F,1-,G,a,*,(1-),z,a,F,0,1,M,G,c,*,After anodic polarization,i,e,a,=,i,a,-,i,c,0,Solution,Metal,G,*,For anodic reaction,G*,a,=G*+(1-)zF,a,-,zF,a,=G*-,zF,a,For cathodic reaction,G*,c,=G*+(1-)zF,a,i,a,=,zFfC,M,exp(-G,*,a,/RT),=,zFfC,M,exp-(G,*,-,zF,a,)/RT,=,zFfC,M,exp-(G,*,/RT)exp(,zF,a,/RT),=,i,o,exp(,zF,a,/RT),i,c,=,zFfC,M,+z,V,L,exp,(-G,*,C,/RT),=,zFfV,L,C,M,+z,exp,(-G,*,/RT)exp-(1-)zF,a,)/RT,=,i,o,exp-(1-)zF,a,/RT,i,e,a,=,i,a,-,i,c,=,i,o,exp,(,zF,a,/RT)-exp-(1-,)zF,a,)/RT .,Volmer,-Butler equation,.,1)for,a,0.03 V.high field approximation.,i,e,a,i,a,=,i,o,exp(,zF,a,/RT),for,a,-0.03 V.cathodic polarization,i,e,c,=,i,c,-,i,a,i,c,=,i,o,exp-(1-,)zF,a,)/RT,At sufficiently large,overpotential,the ,i,e,relationship becomes exponential.,2)when,a,is very small,.|,a,|0.01 V.low field approximation,By using:e,x,=1+x+x,2,/2.=1+x ,when x,1,i,e,a,=,i,o,exp(,zF,a,/RT)-exp-(1-,)zF,a,)/RT,=,i,o,(,zF,a,/RT).a linear relationship between,a,and,i,e,a,.,i,i,a,i,c,i,e,a,i,e,c,4.3 Polarization Diagram(Evans diagram),o.Polarization:An electrode is no longer at,equil,.when a net current flows from or to the surface.The extent of potential change caused by net current to or from an electrode,measured in volts,is,overpotential,().,=f(,i,e,),1)For anodic polarization,i,e,a,=,i,a,-,i,c,=,i,o,exp(,zF,a,/RT)-exp-(1-,)zF,a,)/RT,=,i,o,exp(,zF,a,/RT)at high field approx.,a,=2.3RT/zF log,i,e,a,-2.3RT/zF log,i,o,=,B,a,log,i,e,a,/i,o,.,Tafel,equation,.,2)For cathodic polarization,i,e,c,=,i,c,-,i,a,i,c,=,i,o,exp-(1-,)zF,a,)/RT at high field approx.,c,=-2.3RT/(1-,)zF,log,i,e,c,/i,o,=,B,c,log,i,e,c,/i,o,where,i,o,=exchange current density,E,r,=,equil,.potential,or rest potential,B,a,B,c,:,Tafel,constant,-0.05 V,B,0.15 V,a,c,E,R,log,i,o,log|,i,o,|,E,Anodic current,Cathodic current,B,a,=2.3RT/,zF,=0.059/,z,B,c,=-2.3RT/(1-,),zF=-0.059/(1-,),z,4.4 Concentration Polarization,One of the assumptions in the derivation of the,Volmer,-Butler,eq,.is the uniformity of concentration near the electrode.This assumption fails at high current densities because migration of ions towards the electrode from the bulk is slow and may become rate determining.A larger,overpotential,is then needed to produce a given current because the supply of reducible or,oxidizable,species has been depleted.This effect is called,conc.polarization.,For reduction reaction:,M,+z,+,ze,-,M,E,r,=E,+RT/ZF,ln,C,B,.(1),When a passage of external current is made through the cell,the interfacial conc.changes to a value of C,S,resulting in change of electrode potential.,M,+z,+,ze,-,M,E,p,=E,+RT/,zF,ln,C,S,.(2)Conc.,overvoltage,c,=,E,p,-,E,r,=RT/,zF,ln,C,s,/C,B,.(3),Conc.,C,B,C,S,M,+Z,M,:,Nernst,Layer,IHP,OHP,Ficks,1st law:J=-D,C/,x=-D(C,S,-C,B,)/,=,i/zF,where D=diffusivity of M,+z,ion,=,Nernst,layer,about 0.1mm.,therefore,i=,DzF,/,(C,B,-C,S,).(4),When C,S,0,i,i,L,:limiting current density,Thus,i,L,=,DzFC,B,/,.(5),(5),(4)i=,i,L,(1-C,S,/C,B,),C,S,/C,B,=(1-,i/i,L,).(6),(6),(3),c,=RT/,zF,ln,(1-,i/i,L,)=0.059/z log(1-,i/i,L,),at 298 K.(7),i,L,is increased by,higher solution concentration,C,B,;,higher temperature which increases diffusivity,D;,higher solution agitation,which decreases,.,Concentration polarization only becomes important when the current density approaches,i,L,For anodic dissolution process:,M M,+z,+,ze,-,During corrosion,conc.polarization for anodic dissolution can be ignored because an unlimited supply of metal atoms is available at the interface.But,at high corrosion rates,the conc.of M,+z,ion in the,anolyte,is significantly increased and exert a back e.m.f.which results in anodic conc.polarization.,Tafel,Region,i,L,A,log,i,a,+,+,+,Tafel,(Linear Kinetics),M,+,+,+,Conc.polarization,+,+,+,+,+,+,+,+,+,+,M,4.5 Combined Polarization,T,=,ct,+,c,At low reaction rates,ct,controls the reaction rates.,At high reaction rates,c,controls the reaction rates.,1)for anodic dissolution,c,is small;,diss,.,=,B,a,log,i/i,o,2)for reduction process;,red.,=,B,c,log,i/i,o,+2.3RT/zF log(1-,i/i,L,),E,log,i,c,i,o,B,a,E,r,ct,C,i,L,4.6 Resistance,Overvoltage,Resistance,Overvoltage,(,R,),arises from the passage of electric current through an electrolyte solution(with low conductivity)surrounding the electrode.This,iR,drop may be offset by closing the tip of,Luggin,capillary to the surface of the electrode.,Significant when surface oxide films forms on the electrode surface as a result of electrochemical reaction.,R,=,i(R,electrolyte,+,R,film,),4.7 Mixed Potential Theory,-presented by Wagner and,Traud,in 1938.,-based on the following hypotheses;,1)All free corrosion reactions involve at least one anodic and one cathodic process.,2)During the corrosion of an electrically isolated metal,the total partial anodic current must equal to the total partial cathodic current:,A,a,i,a,=,A,c,i,c,at E=,E,corr,3)For metals,the,electrical(Galvani,)potential of the metal at the anodic site is equal to that at the cathodic site due to their very low,resistivities,.This may not be valid for semiconductors,particularly if the anodic and cathodic sites are widely separated.,Consider Zn undergoing active corrosion in a deaerated,HCl,solution at 25C:,Anodic reaction:Zn,2+,+2e,-,Zn,E,r,Zn,=E,Zn,2+,/Zn,+0.059/2 log a,Zn,2+,=0.763+0.059/2 log 10,-6,=0.763 0.177=0.94 V.,Cathodic reaction:2H,+,+2e,-,H,2,E,r,H,2,=E,H,2+,/H,+0.059/2 log P,H,2,/,a,H,+,=0.059 V at P,H,2,=1,atm,and pH=1,Zn,2+,Zn,2e,-,2H,+,H,2,H,+,Cl,-,For hydrogen reduction reaction,E,r,H,2,=0.059V,i,o,H,+,/H,2,(on Zn)=10,-11,A/cm,2,B,c,=0.12 V/decade,For Zn oxidation,E,r,Zn,=0.94V,i,o,Zn,2+,/Zn,=10,-7,A/cm,2,B,a,=0.06 V/decade,A mixed electrode is an electrode or metal that is in contact with two or more oxidation-reduction systems.,Corrosion,potential(E,corr,)and corrosion current,density(i,corr,)are determined at the point where the total rates of oxidation and reduction are equal.,i,corr,=the rate of Zn dissolution=i,H,2,(Zn)at,E,corr,i,corr,E,corr,E,r,H,2,=,0.059,E,r,Zn,=,0.94,i,o,H,+,/H,2,(Zn),i,o,Zn,2+,/Zn,2H,+,+2e,-,H,2,Zn,Zn,2+,+2e,-,10,-10,10,-8,10,-6,10,-4,10,-2,10,-0,+0.2,0,-0.2,-0.4,-0.6,-0.8,-1.0,C,A,E(SHE)V,log|i|A/cm,2,4.8 Importance of exchange current density,Consider the corrosion of iron in deaerated acid solution.,1)For Fe oxidation:,Fe,2+,+2e,-,Fe.E,Fe,2+,/Fe,=,0.44 V,E,r,Fe,=E,Fe,2+,/Fe,+0.059/2 log a,Fe,2+,=,0.44+0.059/2 log10,-6,=,0.62 V,i,o,Fe,2+,/Fe,=10,-6,A/cm,2,B,a,=0.08 V/decade2)For hydrogen reduction:2H,+,+2e,-,H,2,E,r,H,2,=,0.059 pH=,0.059 V at pH=1,i,o,H,+,/H,2,(on Fe)=10,-6,A/cm,2,B,c,=0.15 V/decade,H,+,H,2,O,2H,+,H,2,Fe,2+,+0.2,0,-0.2,-0.4,-0.6,-0.8,10,-10,10,-8,10,-6,10,-4,10,-2,10,-0,i,corr,E,corr,E,r,H,2,E,r,Fe,i,o,H,+,/H,2,i,o,Fe,2+,/Fe,H,2,2H,+,+2e,-,2H,+,+2e,-,H,2,Fe,Fe,2+,+2e,-,Fe,2+,+2e,-,Fe,E,corr,=corrosion potential,rest potential,open circuit potential,mixed potential,Hydrogen,overvoltage,=,E,corr,E,r,H,2,=,E,corr,(,0.059pH),Comparison of corrosion behavior between Zn and Fe,1),E,r,Zn,=,0.94V,i,corr,Zn,This is due to the lower exchange current density for hydrogen evolution on Zn compared to that on Fe.i,e i,o,H,+,/H,2,(Zn)i,o,H,+,/H,2,(Fe),The exchange current density(i,o,H,+,/H,2,)for hydrogen evolution reaction is highly sensitive to the nature of the metal substrate on which the reaction occurs,and are markedly reduced by the presence of trace impurities such as arsenic,sulfur,and antimony compounds.,For corrosion of metals in acid.,H,2,=,E,corr,E,rH,+,/,H,2,=,E,corr,+0.059pH,H,2,(Zn)R,F,(Pt),Exchange current density,i,o,(A/cm,2,)depends on:,electrode composition,redox,reaction,temperature,surface roughness,4.9 Effect of oxidizer,The driving for corrosion is increased by the addition of a stronger oxidizer that is a,redox,system with a half cell electrode potential much more noble than that of any others present.,Consider the corrosion of metal M in an acid containing Fe,3+,.,Fe,2+,Fe,3+,Fe,2+,Fe,3+,2H,+,H,2,H,+,H,2,O,2e,-,E,corr,is determined by the point at which the total rate of oxidation equals the total rate of reduction.,i,corr,=,i,M,M,+,=i,Fe,3+,Fe,2+,+i,H,+,H,2,The addition of an oxidizer to an acid solution:,1)shifts corrosion potential in the noble direction,2)increases the corrosion rate from,i,corr,to,i,corr,3)decreases hydrogen evolution from,i,corr,to i,H,+,H,2,.,The effect of an oxidizer on the corrosion rate is dependent on its,redox,potential and its exchange current density.,4.10 Effect of concentration polarization,At low cathodic polarization the reduction process is activation controlled,but at high polarization it is diffusion or concentration controlled.,diss,.,=,B,a,log,i/i,o,red.,=,B,c,log,i/i,o,+2.3RT/zF log(1-,i/i,L,),A common example of corrosion controlled by concentration polarization is iron or steel in dilute aerated sea water.The cathodic process is reduction of dissolved oxygen.The maximum solubility of dissolved oxygen in water is relatively low,about 8ppm at ambient temperature.,O,2,+2H,2,O+4e,-,4OH,-,i,L,=100,A/cm,2,In this system,i,corr,=,i,L,depending on conc.,temperature and velocity.,E,corr,B,E,corr,A,E,corr,C,i,oC,+,/C,i,oB,+,/B,i,oA,+,/A,i,oH,+,/H,2,(M),log|i|,i,L,For different metals;A,B and C,i,corr,is equal to,i,L,.,4.11 Experimental Polarization Curves,