![Fe group elements: do not follow standard equilibrium potential instead undergoes anomalous behavior.Mass Transport ControlKinetic ControlFig: Current potential relationship [1]](https://image.slidesharecdn.com/cmen557-13087603143231-phpapp02-110622115721-phpapp02/85/Electrodeposition-and-Corrosion-Mechanism-on-FeCoNiCu-Cu-Quaternary-system-Class-presentation-on-Corrosion-Engineering-8-320.jpg)
Electrodeposition and Corrosion Mechanism on FeCoNiCu/Cu Quaternary system: Class presentation on Corrosion Engineering
- 1. Electrodeposition and Corrosion Mechanism in FeCoNiCuQuaternary system PRAJON RAJ SHAKYACMEN 557Instructor: Dr. Despina DavisLouisiana Tech UniversityRuston , LA 71272USA
- 2. ElectrodepositionElectrochemical rxn - applied potential; occurs at ve potential region.Mn++ ne--> M ; and rate , rc = - ic/nF = -kc(CsMn+)PMWhere, kc = kc,o .exp (-留cF.E/RT) =kc,o .exp (-bk. E); 留cF/RT: Tafel slopesic = -io .exp (-留cF.侶/RT) ; 侶 = E-Erev. ; 侶 =Erev. when ic =0Where, io = nF. kc,o .exp (-留cF.侶/RT) (CsMn+)PMWhen E low; kinetic control => Cs = Cb ; deposition rate depends on E in exponential manner.Mass transport : Cs< Cb; gradient of ion develops.侶 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.
- 3. Electrodeposition in FeCoNiCu In case of FeCoNiCu; Cu has the most positive Erev; first reduced.
- 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 ControlKinetic ControlFig: Current potential relationship [1]
- 9. Table showing standard equilibrium potential of reactions in the alloy solution (vs. NHE)
- 10. Anomalous co-depositionFe 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- -> 2H2OH+ + 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 corrosivei= icorr. {exp. (留aF.龍/RT) exp(-留cF.龍/RT)} (Stern); Where; 龍 = 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