Standard Electromotive Force (EMF) Series. Outline. Concentration and Temperature Effects. Example

Transcription

1 Standard Electromotve Force (EMF) Seres Outlne Bref Revew: Corroson Concentraton and Temperature Effects Example Corroson Rates Polarzaton Corroson Rate - Actvaton - Concentraton Passve behavour For any gven metal, say Fe 2+, anythng above t on the table would act as a cathode relatve to the ron and cause t to corrode. Smlarly, anythng lsted below ron would act as an anode relatve to ron and cause t to plate out. To fnd the net emf for any gven par of electrodes, smply take the dfference between the values lsted. MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/1 These are for the reducton reactons; for the oxdaton reactons, the drecton of the reacton s reversed and the sgn of the voltage changed. Example: Cu Cu e - V o = V MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/2 Concentraton and Temperature Effects EMF seres s under deal, standard condtons - pure metals Changng the purty ( ), - 1M solutons temperature or the concentraton of ons - 25 o C. etc. n the electrolyte l t changes the potental t E.g. M 1 + M 2 M 1 + M 2 Standard cell potental s: ΔV 0 = V V 1 General formula s gven by Nernst Equaton: o o RT ΔV = ( V V 2 1 ) ln nf [ M ] 1 [ M ] 2 (96,500 C/mol - charge of 1 mole of electrons). At 25 o C, and usng log 10 rather than natural logs, ths becomes: Where: n s the number of electrons transferred [M ] = Molar on concentratons R s the gas constant F s the Faraday constant o o ΔV = ( V V 2 1 ) log n [ M ] 1 [ M ] 2 Example One half of an electrochemcal lcell conssts of a pure nckel electrode n a soluton of N 2+ ons; the other s a cadmum electrode mmersed n a Cd 2+ soluton. (a) If the cell s a standard one, wrte the spontaneous overall reacton and calculate the voltage that s generated. (b) Compute the cell potental at 25 C f the Cd 2+ and N 2+ concentratons are 0.5 and 10-3 M, respectvely. Is the spontaneous reacton drecton stll the same as for the standard cell? ( Note: If concentratons are 1 M, then Eqn smplfes to ΔV 0 = V V 0 1 ) MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/3 MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/4

2 Effect of Soluton Concentraton Ex: Cd-N cell wth Ex: Cd-N cell wth standard 1M solutons non-standard solutons o V N o VCd = o V N V Cd = V N o RT VCd nf ln X Y + n = #e - per unt oxd/red Cd 25 C N Cd T N reacton (=2 here) 1.0 M 1.0 M Cd 2+ soluton N 2+ soluton X M Y M Cd 2+ soluton N 2+ soluton Reduce VN VCd by - ncreasng X - decreasng Y MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/5 Corroson Rates Real systems are not at equlbrum and short-crcutng often occurs. EMF potentals gve ndcaton of relatve tendences to dssolve etc but no nformaton on... Corroson rates can be measured as Corroson Penetraton Rate (CPR), or thckness of materal lost per unt tme. For most applcatons a CPR of 0.5mm/yr s acceptable (unform corroson). KW CPR = ρat Where: K s a constant (For SI unts, K = 87.6 and then CPR s gven n mm/yr) W s weght loss (mg) after tme, t (hrs). ρ s the materal densty (gcm -3 ) A s the area (cm 2 ) exposed. We can also express corroson rate n terms of current densty (current/unt surface area corrodng). r = nf Where: r s corroson rate n mol.m -2.s -1 F s Faraday s constant s current densty n A.m -2 n s the number of valence electrons nvolved. MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/6 Corroson rates and Polarzaton Actvaton Polarzaton Because of short-crcutng these cells wll not be at equlbrum and wll not have the standard emf values. The dfference between standard and real value s called and the value s called the overvoltage (η). E.g. f actual potental of znc electrode s and standard value s , η s V. Ths dfference n voltage can take place by two mechansms: - Actvaton polarzaton - Concentraton polarzaton Electrochemcal cell consstng of standard znc and hydrogen electrodes that has been short-crcuted. Ths refers to condton when reacton rate s controlled by the slowest step n the process. I.e. there s an energy barrer related to ths slowest step. Ths may be affected by the composton of the metal, etc. Schematc representaton of possble steps n the hydrogen reducton reacton, the rate of whch s controlled by actvaton polarzaton. MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/7 MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/8

3 Actvaton Polarzaton Concentraton Polarzaton For actvaton polarzaton: η = ±β log Where β and o are constants for the partcular halfcell a o Dffuson of reducng speces controls rate o s the exchange current tdensty o At equlbrum: rred = roxd = n F o s determned expermentally and t vares from system to system Slope = +ββ oxdaton For a hydrogen electrode, plot of Slope = -β reducton actvaton polarzaton overvoltage versus logarthm of current densty for both oxdaton and reducton reactons. MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/9 As concentraton falls, more polarzaton s needed to make the current flow Buld-up or depleton of ons near electrode can affect reactons because the current densty s reduced. If reacton rate s low and/or concentraton of fons s hghh then there s always a good supply of H + ons near the electrode surface. If reacton rate s hgh and /or concentratons are low, wll get a.. around electrode so reducton reacton s slowed dd down (f freducton reacton s slowed - so must be the oxdaton reacton) MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/10 Corroson Rates from Polarzaton Data Corroson Rates from Polarzaton Data Rates of oxdaton and reducton. so only possble where two lnes ntersect. Ths s the corroson potental, V C, and the corroson current densty, C. These values can then be used to predct corroson rate. Recall that corroson rate s proportonal to current densty. (So the hgher C s, the faster the corroson rate.) r = nf Both concentraton and actvaton polarzaton are possble for reducton reactons. Under these crcumstances, the total overvoltage s just the sum of both overvoltage contrbutons. Schematc electrode knetc behavor for metal M; the reducton reacton s under combned actvaton concentraton concentraton polarzaton control.. polarzaton polarzaton Lmtng current MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/11 MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/12

4 Passve Behavor Some metals cease to be reactve under the rght condtons 1. Actve Behavor 2. Passve Behavor 3. Transpassve Under certan envronments some metals/alloys become nert (passve). E.g. Chromum, nckel, ttanum, ron Usually by formaton of thn,,p protectve, surface oxde flm. Stanless steels (>11%Cr) develop Cr 2 O 3 layer whch n many envronments s protectve. Alumnum forms Al 2 O 3 layer. If damaged (scratched etc) normally reforms rapdly. In some envronments ths layer breaks down and materal becomes actve agan. Passvty Schematc polarzaton curve for a metal that dsplays an actve passve transton. MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/13 MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/14 Curve 1 ntersects the oxdaton polarzaton curve n the actve regon at pont A, yeldng a corroson current densty C (A). The ntersecton of curve 2 at pont B s n the passve regon and at current densty C (B). The corroson rate of metal M n soluton 1 s greater than n soluton 2 snce C (A) s greater than C (B) Ths dfference n corroson rate between een the two solutons may be sgnfcant (several orders of magntude) when one consders that the current densty scale n s scaled logarthmcally. Passvty Reducton polarzaton curves Oxdaton polarzaton curve Demonstraton of how an actve passve metal can exhbt both actve and passve corroson behavor. MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/15 Example - Corroson of Iron An ron contaner 10 cm 10 cm at ts base s flled to a heght of 20 cm wth a corrosve lqud. A current s produced as a result of an electrolytc cell, and after 4 weeks, the contaner has decreased n weght by 70 g. Calculate (1) the current and (2) the current densty nvolved n the corroson of the ron. SOLUTION: 1. The total exposure tme s: MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/16

5 Example - Copper-Znc Corroson Cell Suppose that n a corroson cell composed of copper and znc, the current densty at the copper cathode s 0.05 A/cm 2. The area of both the copper and znc electrodes s 100 cm 2. Calculate (1) the corroson current, (2) the current densty at the znc anode, and (3) the znc loss per hour. SOLUTION 1. The corroson current s: 2. The current n the cell s the same everywhere. Thus: Next tme: Contnue Corroson 3. The atomc mass of znc s g/mol. From Faraday s equaton: MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/17 MECH 321 Mech. Eng. Dept. - Concorda Unversty lecture 16/18