Towards the modeling of microgalvanic coupling in aluminum alloys : the choice of boundary conditions

Transcription

1 Presented at the COMSOL Conference 2008 Boston COMSOL Conference BOSTON November Towards the modelng of mcrogalvanc couplng n alumnum alloys : the choce of boundary condtons Ncolas Murer *1, Nancy Mssert 2, Roland Oltra 3, Rudy Buchhet 1 1 Fontana Corroson Center, Oho State Unversty, 2041 College Road, Columbus, OH Sanda Natonal Laboratores, 1515 Eubank SE, Albuquerque, NM Insttut Carnot de Bourgogne, UMR 5613 CNRS, Djon Cedex * Correspondng author: murer.3@osu.edu

2 Phenomenology of localzed corroson Envronment Pure alumnum s passve n neutral, low conductve envronments. 2

3 Phenomenology of localzed corroson Durng the fabrcaton process of structural alumnum alloys, mcrostructural heterogenetes are formed n the materal. 3

4 Phenomenology of localzed corroson The presence of the partcles modfes the composton of the oxde layer and reduces ts passve character. 4

5 Phenomenology of localzed corroson Corroson s lkely to take place at the nterface between the ntermetallc partcle and the matrx. (More precsely at the nterface between the alumnum oxde and the ntermetallc oxde) 5

6 Phenomenology of localzed corroson Galvanc couplng Partcle oxde layer Intermetallc partcle (IMP) I e - Matrx oxde layer Matrx In low conductvty envronments, galvanc couplng takes place between the two phases. In most cases, IMP s the cathode and the matrx s the anode. 6

7 Phenomenology of localzed corroson Galvanc couplng : Redox reactons OH - O 2 OH - OH - OH - Partcle oxde layer Intermetallc partcle (IMP) Al 3+ Matrx oxde layer Matrx At the cathode : O 2 + 4e - + 2H 2 O 4OH - At the anode : Al Al e - 7

8 Phenomenology of localzed corroson Effects of alkalzaton O 2 OH - OH - OH - OH - Intermetallc partcle (IMP) Al 3+ Matrx oxde layer Matrx Dssolve oxde layers Increase the dssoluton rate of the substrate 8

9 Why modelng? The current and ph dstrbutons n ths type of confguraton are very dffcult to reach. The former phenomena have only been qualtatvely expermentally hghlghted

10 Why modelng? The current and ph dstrbutons n ths type of confguraton are very dffcult to reach. The former phenomena have only been qualtatvely expermentally hghlghted 1-4. Two man obstacles prevent the collecton of knetc data : the small scale (tens of mcrometers) the passve nature of alumnum and alumnum alloys 10

11 Why modelng? The current and ph dstrbutons n ths type of confguraton are very dffcult to reach. The former phenomena have only been qualtatvely expermentally hghlghted 1-4. Two man obstacles prevent the collecton of knetc data : the small scale (tens of mcrometers) the passve nature of alumnum and alumnum alloys A predctve model s requred f we want to quanttatvely descrbe localzed corroson. 11

12 OBJECTIVES PRIOR TO USE THIS MODEL MUST BE EXPERIMENTALLY VALIDATED 12

13 OBJECTIVES PRIOR TO USE THIS MODEL MUST BE EXPERIMENTALLY VALIDATED We must use a model system that mmc real geometres and for whch couplng currents can be montored. 13

14 Expermental : the model system Cu slands dameter s 50 μm Electrolyte s NaCl 10 mmol.l -1 All the Al/Cu channels are coupled. Each net current orgnatng from an Al/Cu channel can be montored and compared to the smulated current profles. 14

15 Governng equatons Nernst Planck equaton n statonary regme assumng electroneutralty.. N =.( z u Fc φ D c ) = N s the total flux densty of speces (mol.m -2.s -1 ) φ s the potental n the doman.e. n the soluton (V) u s the moblty of the speces (m.s -1 ) c s the concentraton of the speces (mol.m -3 ) z s the charge number of the speces R s the producton term,.e. the flux of speces due to reactons that are occurrng n the bulk soluton (.e. subdoman) (mol.m -2.s -1 ) R Speces : Na +, Cl -,OH -,H +, Al 3+,O 2 15

16 Governng equatons Electroneutralty n z c = 0 Electroneutralty assumpton (.e. nfnte dlute solutons assumpton) allows potental dstrbuton calculaton and complete solvng of Nernst Planck equaton. Currents are due to the moton of the charged speces = F z N 16

17 Boundary Condtons Two actve boundares : the anode and the cathode. Two dfferent types of boundary condtons are possble to descrbe galvanc couplng. 1. non-lnear sem-emprcal Butler-Volmer based current-potental relatons 5 2. O 2 dffuson lmted cathodc reducton reacton

18 The Choce of the Boundary Condtons To know whch knd of condtons we have to use for the predctve model, we must compare the smulated current dstrbutons obtaned for ether boundary condton, wth the expermental current dstrbuton. 18

19 Model System used n COMSOL Channel 5 : anodc net current Channels 1, 2, 3, 4 : cathodc net currents Smplfcaton of the geometry 1 mm : expermental electrolyte layer thckness Subdoman Electrodes : 150 μm Insulaton : 50 μm Al/Cu systems 19

20 Boundary Condtons 1. Butler Volmer based potental-current relatons. Cathodc channels (Cu) : presence of a cathodc current plateau C = 1+ c C ( c exp(e C C exp(e (V C m (V dff Anodc channels (Al) : charge transfer regme exp(e (V V)) / β ) m C V)) / β )) V)) / A = c A A m αa All parameters are determned wth expermental polarzaton curves of Al and Cu n correspondng meda. C ) 20

21 Boundary Condtons 2. Galvanc couplng controlled by dffuson-lmted cathodc reacton. At each cathode j : cj = -4*F*J dff O 2 (Four moles of electrons are requred to reduce one mole of O 2 ) At the anode : Sum of the currents denstes ntegrated on each cathode dvded by the length of the anode. a = C O2 = 0 dx)/ a Wth a the anode length and c the cathode length ( j c 0 cj 21

22 Boundary Condtons 3. Expermental currents Cathodes : for each channel, the current montored wth tme was averaged and dvded by the surface of the channel. Each current densty was then appled as a boundary condton. Anode : the sum of the cathodc current denstes was the current densty appled at the anodc boundary. 22

23 Results 23

24 Results Boundary condtons that are the most accurate to descrbe galvanc couplng are based on the assumpton of a cathodc control by O 2 dffuson. 24

25 Results Boundary condtons that are the most accurate to descrbe galvanc couplng are based on the assumpton of a cathodc control by O 2 dffuson. Ths assumpton wll be used to descrbe galvanc couplng at the nterface between a cathodc partcle and an anodc matrx. Partcle oxde layer Intermetallc partcle (IMP) Matrx oxde layer Matrx 25

26 Results Boundary condtons that are the most accurate to descrbe galvanc couplng are based on the assumpton of a cathodc control by O 2 dffuson. Ths assumpton wll be used to descrbe galvanc couplng at the nterface between a cathodc partcle and an anodc matrx. Partcle oxde layer Intermetallc partcle (IMP) Matrx oxde layer Matrx The ph dstrbuton at the surface of ths system can be predcted. 26

27 Applcaton : ph dstrbuton 6 Axal symmetry Alkalzaton s confrmed. Knowng the dssoluton current assocated wth ph (j dssoluton = f(ph)), we can deduce the ph contrbuton to the damage. 27

28 Applcaton : effect of ph 6 Dssoluton depth (µm) Galvanc contrbuton (Faraday law appled to COMSOL surface current densty dstrbuton) ph contrbuton (Faraday law appled to j dssoluton obtaned through j dssoluton vs ph) 28

29 Conclusons Nernst-Planck applcaton mode wth electroneutralty has been proven able to descrbe tertary current dstrbutons takng place durng galvanc couplng. The desgned model system allowed to determne that the most sutable boundary condtons must be based on the assumpton of a mass transport lmted cathodc control of the galvanc couplng. The actual alkalzaton occurrng over the partcle/anodc matrx nterface has been hghlghted. Its role n damage accumulaton was shown to be less mportant than expected. 29

30 Perspectves Introducng tme : galvanc couplng n ths confguraton s NOT a statonary phenomenon. The role of galvanc couplng can be reduced wth tme as the electrolyte n the vcnty of the surface becomes more conductve. Consderng the oxde layers : the cathodc reacton seems to be correctly modeled, but the effect of the ph on the anodc dssoluton depends on the tme-dependent stablty of the matrx oxde layer, whch s a semconductor. 30

31 References 1. Llevbare, Schneder, Scully, Kelly, J. Electrochem. Soc., 151 (8) B465- B472 (2004) 2. Park, Pak, Alkre, J. Electrochem. Soc., 143 (8) L174-L176 (1996) 3. Park, Pak, Huang, Alkre, J. Electrochem. Soc., 146 (2) (1999) 4. Alodan, Smyrl, J. Electrochem. Soc. 145 (1998) P. Bucalle, R. Oltra and T. Warner, Theoretcal and expermental studes of galvanc corroson between alumnum and Al-4% Cu alloys, Materals Scence Forum, 242, (1997) 6. Murer, Oltra, Neel, Vullemn, J. Electrochem. Soc., submtted 31

32 Thank you for your attenton 32