This slide describes about the electrodeposition and it features along with the discussion of corrosion in FeCoNiCu/Cu electrodeposition of nanowires

1. Electrodeposition and Corrosion Mechanism in FeCoNiCuQuaternary system PRAJON RAJ SHAKYA CMEN 557 Instructor: Dr. Despina Davis Louisiana Tech University Ruston , LA 71272 USA

2. Electrodeposition Electrochemical rxn - applied potential; occurs at –ve potential region. Mn++ ne--> M ; and rate , rc = - ic/nF = -kc(CsMn+)PM Where, kc = kc,o .exp (-αcF.E/RT) =kc,o .exp (-bk. E); αcF/RT: Tafel slopes ic = -io .exp (-αcF.η/RT) ; η = E-Erev. ; η =Erev. when ic =0 Where, io = nF. kc,o .exp (-αcF.η/RT) (CsMn+)PM When E low; kinetic control => Cs = Cb ; deposition rate depends on E in exponential manner. Mass transport : Cs< Cb; gradient of ion develops. η increase : r increase and Cs decrease. When Cs = 0 ; conc. gradient maximum and r can not increase further. Then, complete transport control -> current: limiting current. More negative Erev. , less noble the rxn => metal cation is more difficult to reduce.

4. If conc. and kinetic rxn. rates of all metals similar => when I is passed=> alloy rich in Cu produced.

5. So, conc. Cu low and deposition rate -> Mass transport control and others kinetically controlled.

6. Low I -> only Cu deposited.

7. High I -> alloy rich in Fe group elements deposited.

8. Fe group elements: do not follow standard equilibrium potential instead undergoes anomalous behavior.Mass Transport Control Kinetic Control Fig: Current potential relationship [1]

9. Table showing standard equilibrium potential of reactions in the alloy solution (vs. NHE)

10. Anomalous co-deposition Fe group elements -> anamolous behavior in NiCo, NiFe and CoNi. Many theories : to explain behavior. Vagramyan and Fatueva (1963): NiFe: retard of Ni and facilitate of Fe. Dahms and Croll: hydrolysis key : Fe(OH)2 absorbed on surface before reduced; stable high pH; Ni redn. Inhibit -> less free surface available. Romankiw: surface pH not high ->(MOH) precipitation; both when deposited as single and as alloy. Similarly, many studies as of Hessami and Tobies; Grade and talbot. Sasaki and Talbot: NiCo and CoFe. In 1993 Matlosz: 2 step rxn mechanism: adsorbed monovalent intermediate ion. Mj++ + e- ↔Mj+,ads (reduced and absorbed) Mj+,ads + e- ↔ Mj (metal state) Fe monovalent intermediate : difference betn. Tafel constants for electrosorption of two elements : inhibition of Ni.

11. Anomalous behavior contd…. Zech et al.: catalytic mechanism – enhancement of less noble metal. M2++ + M1++ + e- ↔ [M2M1]ads+++ (adsorbed mixed metal complex) [M2M1]ads+++ +e- ↔ M2 + M1++ Also, Fe, Co and Ni : exhibit anomalous behavior as of binary. Cu : non-interactive with others: not generate adsorbed intermediate. Zhuang and Podlaha: combined Matlosz and Zech et al. – inhibition and enhancement in ternary system. The metal reduction reaction that takes place are: Fe++ + e- ->Fe+ads Fe++ + Co++ + e- -> [FeCo]+++ads Fe+ads + e- -> Fe [FeCo]+++ads + e- -> Fe + Co++ Co++ + e- -> Co+ads Fe++ + Ni++ + e- -> [FeNi]+++ads Co+ads+ e- -> Co [FeNi]+++ads + e- -> Fe + Ni++ Ni++ + e- -> Ni+ads Co++ + Ni++ + e- -> [CoNi]+++ads Ni+ads + e- -> Ni [CoNi]+++ads + e- -> Co + Ni++ Cu++ +2e- ->Cu

12. Anomalous behavior and deposition…. Similarly, the side reactions that takes place are: O2 + 4H+ + 4e- -> 2H2O H+ + e- -> Hads Hads+ Hads -> H2 H2O + e- ->1/2 H2 + OH- Due to O2 reduction; low efficiency at less negative potential and increases as reduction of Cu - significant. After Cu redn. -> mass transport; proton redn. occurs -> efficieny drops. When H2 partial current density -> limit value; efficiency increase with increase in Fe group metal redn. rate with potential. Very high potential -> redn. of water; efficiency lowers. After Cu reaches limiting current=>partial current density is constant. High overpotential; dep. rate of Fe group - increases and Cu – decrease. Higher conc. of more noble ions enhances catalytic step of less noble ions.

13. Corrosion Corrosion – also electrochemical rxn. ; at positive potential region. Occurs : oxidation of metals as: M ↔Mn+ + ne- at anode Due to ppt of O2 as: O2 + 2H2O + 4e- -> 4(OH)- Usually, it is the case of steel and Cu alloys. M+ + H2O -> MOH + H+ Also, with hydrolysis; production of H+ ions; more corrosive i= icorr. {exp. (αaF.ζ/RT) – exp(-αcF.ζ/RT)} (Stern); Where; ζ = E – Ecorr.and Ecorr. : determined by kinetics. In case of FeCoNiCu; many ways metals get corroded. Stress corrosion - NaOH , NH3 and amine for Cu and Ni alloy, resp. Acid corrosion: HNO3/alcohol ; FeCl3/HCl; K2Cr2O7/H2SO4 for Cu and Co- rich alloys expt. Less noble metal are prone to corrosion. Corrosion rate in HNO3/alcohol slower than others.

14. Corrosion in FeCoNiCu (cont…) Ecorr. Cu > Ecorr. Co; positive for Cu. More severe corrosion in other two than HNO3/alcohol despite short etch time. High corrosion rate – short etch time – difficult to control. Fe is less corrosion resistant as easily forms Fe(OH)2complex. Ni-based alloys : hot corrosion: 700-800 0C: air drawn traces of NaCl: acts with SO2 from combustion forms Na-sulfate – dissolves oxide –diffuses; catastrophic oxdn. – finally metal break. Hot corrosion-protect by Cr rich coatings. Similarly, use of coatings or paintings; changing the corrosion environment , applying inhibitors etc. References: [1]. http://etd.lsu.edu/docs/available/etd-12192003-015815/unrestricted/Huang_dis.pdf <Qiang Huang, “Electrodeposition of FeCoNiCu Quaternary System,” PHD Dissertation, Department of Chemical Engineering, Louisiana State University, May 2004> THANK YOU 