Electrochemical Methods for Corrosion Behavior Characterization. Dr. Patrik Schmutz

Transcription

1 Electrochemical Methods for Corrosion Behavior Characterization Dr. Patrik Schmutz Head (a.i.) of Laboratory for Corrosion and Materials Integrity EMPA,Swiss Federal Laboratories for Materials Science and Technology Dübendorf, Switzerland contact: BioTiNet Workshop Ljubljana, 25 October 2011

2 Introduction - Corrosion and implant failure - Electrochemistry and implant surfaces Electrochemical potentiodynamic polarisation - Macroscale characterization of Ti alloys - Local electrochemistry by Microcapillary cell techniques Electrochemical Impedance Spectroscopy - Measurement principle - Characterization of biodegradable Metallic Mg implants Crevice and galvanic corrosion investigation - Static and dynamic electrochemical setups - Example of Stainless Steel and Co-Cr-Mo alloys Conclusions Outline

3 Acknowledgments Swiss Federal Laboratories for Materials Science and Technology Laboratory for Corrosion and Materials Integrity P. Schmutz Prof. Peter Uggowitzer Michael Schinhammer Physico-chemistry of reactive metal surface Dr. Alessandra Beni Dr. Giancarlo Pigozzi Dr. Magdalena Pawelkiewicz Dr. Jörn Lübben Dr. Emanuele Cardilli Dr. Ngoc Quach-Vu Noemie Ott Marianne Berg Micro- and nanocapillary electrochemistry Dr. Thomas Suter Dr. James DeRose Dr. Olga Guseva Mathias Breimesser Aurelien Tournier Corrosion management Dr. Markus faller Dr. Martin Tuschchmid Ronny Lay Nikola Gojkovic René Werner Urs Gfeller

4 Ti alloys - Corrosion resistant - Other problems related to fatigue or fretting In - Vitro testing of corrosion processes Less relevant Stainless Steel Co Cr based alloys - Pitting corrosion - Crevice corrosion - Galvanic corrosion Choice of testing solution chemistry Critical Mg alloys - Biodegradable Decreasing dependence on testing media Increasing corrosion resistance P. Schmutz et al., Interface, 17(2), (2008)

5 Example of corrosion related implant failure Ti6Al4V pin implanted in bone (failure after 6 months due to corrosion - fatigue) Cook S.D., et al., The in vivo performances of 250 internal fixation devices: a follow-up study. Biomaterial, 1987,8; p. 177 Hip joints: DLC coating/pe (delamination in vivo, Täger Group) Crevice corrosion: - 90 % of the plates and screws (26 months average) Cl - Metal Me ++ Metal Me ++ Me ++ O 2 Me ++ + H 2 O Me(OH) + + H + O 2 Me ++ Cl - Cl - Cl -

6 Accelerated tests and corrosion? In-vivo evidence of corrosion Years!!! Materials development and lifetime prediction Requires faster Feedback Ideally weeks/ month Accelerate the corrosion process by using more agg ressive media is an alternative but not always possible Electrochemical methods are a good alternative - Very small currents can be measured - It allows earlier detection of on-going degradation processes - Corrosion rates are directly linked to electrical charge flow - There is a very broad range of different information can be gained

7 Surface processes and electrochemistry on implants 4 SBF without buffer SBF Oxidation and Corrosion log Z [Ω.cm -2 ] phase angle log ω -20 Macroscale methods: - Open Circuit Potential - Potentiodynamic polarization Micro- and Nanocapillaries: - Local measurements - Solution analytics Dedicated artificial crevice setups: - Galvanic coupling - Mechanical sollication Electrochemical Impedance Spectroscopy (EIS) Electrochemical surface modifications: - Anodizing - Electropolishing Thick anodic oxide on Mg

8 Principle of microcapillary cell platinum wire holder electrolyte bridge (SCE) Full local electrochemical control for heterogeneous materials characterization Silicone sealing rubber sample d = 300 nm µm Setup designed by Dr. Thomas Suter at the Institute for Material Chemistry and Corrosion, ETH Zurich.

9 Localized corrosion characterization Stainless Steel : 18Cr/10Ni Electrochemical polarization measurements SEM before MnS interface bulk Current density [µa/cm 2 ] M NaCl interface MnS Pitting bulk Current [na] 12 µm 10-2 d area = 2.5 µm Potential [mv] (SCE) 10-6 Stainless Steel implants Plastically deformed and scratched areas are very susceptible to localized corrosion

10 Ti alloys and electrochemical Polarization International standard ISO 10271:2001 (Dental Implants) 1.E-04 1.E-05 (b) Lactic acid solution (ph 2.2) 5.85 g/l NaCl + 10 g/l C 3 H 6 O 3 Log(i) (A /cm ^2) 1.E-06 1.E-07 1.E-08 1.E-09 Ti Grade4 SLA Ti Grade4 SL 1.E Potential vs. SCE (V) Ti grade 4 + transfer piece: Ti 6 Al 7 Nb Ti and Ti alloys 1.E-04 1.E-05 (b) No corrosive attack under static conditions Some ionic release detected by ICP-MS L o g (i) (A /cm ^2) 1.E-06 1.E-07 1.E-08 1.E-09 Ti6Al4V SLA Ti6Al4V SL 1.E Potential vs. SCE (V)

11 EIS: Electrochemical Impedance Spectroscopy Measurement at corrosion potential Voltage perturbation (10 mv) is applied - The current response in function of frequency is measured Simple model - for electrochemical Interface R p Amplitude E I Ohm s law gives a simple relation U = I * R R s C Rs : solution resistance Rp : polarization resistance of the surface C: capacitance of the surface When AC signal are applied, the relation is E ac = I ac * Z Z: impedance For Mg: Rp= 1000 Ohm/cm 2 Corrosion rate ~ 220 µm/year

12 EIS: data representation Nyquist plot Imaginary vs. Real component -Z Φ Z 0 Φ Z Impedance (Ohm) Rp Time Rs Phase (deg) Bode plot Frequency (Hz) 12 - Frequency dependent representation

13 Mg alloys as degradable implants Magnesium is not only biocompatible... It is an important element in hundreds of metabolic processes. Suggested daily ratio: 350 mg/day Pins Mg-Y-Re alloys are preferred to of AZ91/AZ71. A defined degradation sequence during the first 3-6 months is aimed at Cardiovascular application Osteosynthesis Stents: ph of blood weight:4mg

14 Experimental strategy : alloys and solutions 3 alloys with systematic variation of Y, Zn for screening experiments Alloy Mg Y RE Zr WE43 Bal Alloy Mg Zn Y Ca Mn In vivo study: very complex media WZ21 Bal ZW21 Bal Detailed description of WE43 dissolution In-vitro SBF Solutions considered: SBF 27 as base (concentration in mmol/l) NaCl, 4.0 KCl, 27.0 NaHCO 3, 1.0 MgSO 4 * 7H 2 O, 2.5 CaCl 2 * 2H 2 O, 1.0 KH 2 PO 4, TRIS Buffer (ph 7.4) Matrix of solutions with combination of ionic species model solution matrix for corrosion mechanisms

15 log Z [Ω.cm -2 ] EIS: a kind of spectroscopic method Immediate immersion of WE43 in SBF 27 with and without buffer agent SBF without buffer SBF log ω Distinction between uniform or localized corrosion Double layer / surface oxide dielectric properties Diffusion and adsorption processes can be monitored at low frequency phase angle Frequency resolved electrochemical impedance measurements allows to characterize reaction mechanisms ph 7.4 increased ph Uniform corrosion - slower Reaction rates localized attack - fast process

16 Hydroxide products growth and redeposition WE43 in100 mm NaCl (buffered) Direct Charge transfer Porous corrosion products Mg Alloy -Z (1) (2) Inductive effects (3) related to ionic adsorption from solution -Z 1 Z 2 3 Adsorption and Integration of ionic species (inductive effects) can be distinguished from purely growth of hydroxides as a function of time Degradation properties of Mg alloys 16

17 Nature of corrosion products: from ZW21 to WE43 NaCl, Tris Transition from porous non protecting to more compact corrosion products Important influence of Yttrium Full SBF WE ZW, WZ WE WZ No significant changes in the nature of the corrosion products ZW21 220

18 EIS and porous oxides characterization Osteosynthesis application: Thick porous Mg-hydroxides Electrochemical Impedance Spectroscopy log(z) Intact coating Time dependant 0 h 2 h 10 h 4 Solution contact to metal 3 through pores log(f) Defective coating model (applicable to other coatings like DLC) % of open pores can be determined Initial stage of degradation can be followed as function of time Corrosion rate Inert Oxide Alloy Polymer monolayer T1 T2 T3 Implantation time

19 Material combination in implant fixation Corrosion of metallic implant materials Screw: SS, CCM or TiAlNb Stainless steel (SS) is very susceptible to crevice corrosion problems Cobalt Chromium Molybdenum (CCM) is more corrosion resistant but can release toxic ionic species when depassivated FDA concern issued February 2011 Titanium Aluminum Niobium alloys show very stable passivation but are than prone to fatigue-corrosion problems, inducing rapid breaking of the implant. Besides, the intrinsic corrosion problems, dissimilar materials will add the problem of galvanic coupling SS Plate Crevice Corrosion

20 Crevice corrosion and galvanic coupling Crevice conditions Dissimilar materials Mechanical damage active surface in the crevice Low migration of OH - ions increase of H + in the crevice Differential aeration Damaged surface Potential difference between crevice and external surface Galvanic corrosion Passive film instability Passive film stability Cyclic damage ph decreases Cl - in crevice increases Crevice corrosion Potential difference between crevice and external surface increases Repassivation Passive layer instability, active surface in the crevice Investigation procedure is divided into: - Static tests - Dynamic tests

21 Nanoscale crevices at coating interface DLC-Si Si 101 patients DLC/PE 101 patients Al 2 O 3 /PE TiAlV 8.5 year follow-up 50% of DLC/PE failed FIB cuts: It seems that the first 50 nm above the TiAlV are more or less totally corroded away (not a crack growth) Si interlayer is very susceptible to crevice corrosion in-vivo

22 Crevice corrosion and galvanic coupling Aeration cell: stabilize the anodic reaction in the crevice ( < 100 µm) O 2 e - e - e - RE WE1 All the potential and the currents flowing between electrodes can be measured I 1 I 2 I 3 WE2 Differential aeration Cathode ½ O + 2 H 2 O + 2e - = 2OH - Anode Me Me e - Me H 2 O Me(OH) 2 + 2H + Ch1 Ch2 Ch3 ph Additional passive panels to increase differential aeration Additional cathode Crevice Additional cathode Passive or grinded sample in the crevice

23 Electrochemical potentials of CCM and SS Stainless Steel: Cr 17% - Ni 13% - Mo 2.5% - Mn 2% - Fe Bal CCM: Co 66% - Cr 28% - Mo 6% 9 g/l NaCl, 0.4 g/l KCl, 0.5g/l CaCl 2, 0.2g/l NaHCO 3 There is a risk of galvanic incompatibility between SS and CCM especially in case of damaged surfaces

24 Influence of ph on SS surfaces in crevice Lactic acid solution Ringer s solution Crevice corrosion has been monitored in lactic acid condition and a significant current measured for the coupling SS-SS It takes 8 days for localized corrosion to take place and then the current is progressively increasing In Ringer s solution, the environment is too mild to initiate an attack in a reasonable time

25 SS CCM surfaces in crevices 8.00E-07 Galvanic current densities Lactic acid solution Current density (A/cm^2) 7.00E E E E E E E E+00 SS(E) Panel Grinded SS Crevice Grinded CCM Crevice CCM(C) Panel -1.00E Time (day) CCM fully repassivates very fast at low ph (dominating influence of chromium) SS is unable to repassivate in crevice conditions The introduction of CCM in the combination SS-SS increase the risk of crevice corrosion for the steel

26 In vivo degradation of CoCrMo hip implants Study under dynamic conditions: According to ASTM-F75-92 Total Hip Replacement (THR) in 12 sheeps; Euthanasia after 8.5 months - S. Virtanen, A. Hogson (ETHZ) - B. Von Rechenberg, Tierspital Zürich - S. Mischler (EPFL) CCM (66% / 28% / 6%) - Clinical Analysis - Corrosion - Wear

27 Dissolution processes studied by ICP-MS Characterization methods Static immersion or Online Microcapillary flow system coupled to ICP-MS Spectrometer CCM (66% / 28% / 6%) Extremely high dissolved ions concentration is found in the tissues next to the implant when micro-motion is present!

28 Summary and conclusions Macro- and microelectrochemical polarization allow to characterize the intrinsic corrosion resistance of materials in aggressive media. The method is ideal in relation with materials development (structure, defects) Electrochemical Impedance Spectroscopy is the preferred method for a detailed investigation of complex corrosion processes at the OCP. The frequency dependent spectroscopic information allow to track different corrosion processes (localized, uniform) occurring in parallel on surfaces and coatings Electrochemical crevice and galvanic coupling setups are necessary to simulate the aggressive local chemistry that is responsible for most of the implant failures Dynamic characterization in crevice and galvanic conditions will be the most relevant electrochemical tests in the case of Ti alloys that are highly corrosion resistant in static conditions (not shown in the webversion) Leaching of metallic ions that can be investigated by ICP methods is a corrosion related aspect that should not be neglected