Stability of Surface Films Formed on Mg by Exposure Aqueous Solutions

Transcription

1 Stability of Surface Films Formed on Mg by Exposure Aqueous Solutions By: Mehdi Taheri Supervisor: Dr. Joseph Kish Walter W. Smeltzer Corrosion Lab McMaster University 1 2 nd MSE 702 Graduate Seminar Sept 2011

2 Outline Research Motivation Study Objectives DC Electrochemical Behaviour Film Structure & Composition Film Model Comparison & Evolution Aspects EIS Study of the Corrosion Mechanism Effect of Cathodic Polarization 2

3 Political Driver Canadian Government Agreement (2005): Canadian automobile manufacturers to reduce GHG s from vehicles by 5.3 Mt (6%) by 2010 Relevant Approaches: Green Technologies (Light Weight Mg Alloys) Major Drawback: Poor corrosion behaviour in aqueous solutions Mg + 2H 2 O Mg(OH) 2 + H 2 3

4 Mg Passivation Challenge ph Dependant: Weakly Passive at ph <10 Strongly Passive at ph 14 4 Formation Pathways: Dissolution-Precipitation; Mg(OH)2 film precipitates from a supersaturated solution Solid State; Mg(OH)2 film forms from direct anodic reaction Mg 2+ M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE, Houston, TX, 1974 Different Structure & Composition? Mg(OH) 2

5 Surface Film Model (Ex-Situ) Three layer film based on TEM a-mg(oh) 2 + c-mgo a-mg(oh) 2 + a-mgo Dual layer film based on XPS Yao et al. Model (2000) Diffuse MgO/Mg(OH) 2 bilayer structure based on AR-XPS study Santamaria et al. (2007) Model [& Liu et al. (2009)] Sharp MgO/Mg(OH) 2 bi-layer structure based on XPS study Constant native MgO layer thickness at short exposure times (up to 120 s) c-mgo + a-mg(oh) 2 Mg(OH) 2 MgO ~2-6 nm Nordlien et al., J. Electrochem. Soc., 142 (1995), Samples immersed in distilled water for 48hrs Mg substrate 5

6 Surface Film Model (In-Situ) Baril EIS model For Mg in Na 2 SO 4 (aq) Song EIS model For Mg in NaCl & Na 2 SO 4 (aq) Na 2 SO 4 (aq) NaCl (aq) then or: Inductive loop breakdown HF capacitive loop charge transfer & double layer LF capacitive loop concentration of Mg + 6

7 Hydrogen Charging Modified Mg-H 2 O system E-pH diagram by Perrault Cathodic Charging effect on Mg Corrosion after 1h galvanostatic exposure in 0.1M NaCl For i H < -1mA/cm 2 ; Mg + 2H + + 2e - MgH 2 For i H > -1mA/cm 2 ; Mg + 2H MgH 2 + V Mg 7 G.G. Perrault,, J. Electroanal. Chem., 51 (1974) T. Zhang et al, Corr. Sci., (2009) 51, 1883.

8 Research Objectives Compare & contrast structure and composition of surface film formed in aqueous environment Achieve by direct imaging of FIB prepared crosssections using STEM/EDS Mg immersed in pure H 2 O for 48 h Mechanistic analysis of the passivation behaviour at the interface using EIS results Evolution of cathodic charging as a self passivation improvement method 8

9 DC Electrochemical Behaviour 9

10 FIB & SEM Observations SEM image of the film plan view FIB preparation for TEM 10

11 TEM/STEM Observations Film Mg 11

12 12 EDS Results Pure H 2 O

13 13 Depth Profile Pure H 2 0

14 Diffraction Results Film Mg 50 nm Inner ring MgO {200} reflection Outer ring MgO {220} reflection SAD-DF image of interface for MgO {200} 14

15 Film Model Summary Direct consequence of both exposures Laboratory air (mechanical polishing) Immersion in H 2 O Formation of thin crystalline MgO during mechanical polishing From reported XPS studies Formation of Mg(OH) during immersion Initially from hydration of MgO inner-layer Finally from corrosion of Mg (ph 7) 15

16 EIS Study 16

17 H-Charging Impact on Mg Corrosion Effect of H-charging on Mg in 0.1M NaCl at -1mA/cm 2 17

18 Hydride in surface film 18

19 Hydride impact on surface film Mg + H 2 O + 2e - MgH 2 + 2OH - MgH 2 + 2H 2 O Mg(OH) 2 + 2H 2 MgH 2 Mg H + + 2e - Mg Mg + ads + e - Mg e - G.G. Perrault, Encyclopaedia of electrochemistry of the elements, Vol. VIII, Ed. A.J. Bard, pp , Marcel Dekker,

20 Conclusions Mg surface film composed of a diffuse bilayer structure in aqueous solutions: A thin crystalline MgO rich inner layer A Mg(OH)2 rich outer-layer on top of porous inner-layer Stability of MgO hydration controls anodic process. Pores in MgO showed evidence of breakdown in innerlayer in absence of aggressive ions e.g. Cl - and SO EIS results are consistent with ex-situ observation. H-charging was found to form MgH 2 on Mg and improve corrosion resistance. 20

21 Acknowledgments Dr. Joey Kish and SCM NSERC for financial support CANMET-MTL for providing pure Mg sheet CCEM for technical support Walter Smeltzer corrosion lab fellows 21

22 THANK YOU! QUESTIONS? 22