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:
[email protected]
BioTiNet Workshop Ljubljana, 25 October 2011
Outline 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
Acknowledgments
Swiss Federal Laboratories for Materials Science and Technology
Laboratory for Corrosion and Materials Integrity
Prof. Peter Uggowitzer
P. Schmutz
•
Physico-chemistry of reactive metal surface
Micro- and nanocapillary electrochemistry
Dr. Alessandra Beni
Dr. Thomas Suter
• Dr. Giancarlo Pigozzi • Dr. Magdalena Pawelkiewicz • • • •
Dr. Jörn Lübben Dr. Emanuele Cardilli Dr. Ngoc Quach-Vu Noemie Ott
• Marianne Berg
• 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
Michael Schinhammer
Ti alloys - Corrosion resistant
- Other problems related to fatigue or fretting
Choice of testing solution chemistry
Stainless Steel Co – Cr based alloys
Critical
- Pitting corrosion - Crevice corrosion - Galvanic corrosion
Mg alloys In - Vitro testing of corrosion processes
- Biodegradable
Decreasing dependence on testing media P. Schmutz et al., Interface, 17(2), 35-40 (2008)
Increasing corrosion resistance
Less relevant
Example of corrosion related implant failure Ti6Al4V pin implanted in bone (failure after 6 months due to corrosion - fatigue) Hip joints: DLC coating/PE (delamination in vivo, Täger Group) Cook S.D., et al., The in vivo performances of 250 internal fixation devices: a follow-up study. Biomaterial, 1987,8; p. 177 Crevice corrosion:
Cl
Metal
- 90 % of the plates and screws (26 months average)
O2
Me++
Cl -
Cl Me++
-
Metal
Me++
Me++ + H2O → Me(OH)+ + H+
-
O2 Me++
Cl -
Accelerated tests and corrosion ?
In-vivo evidence of corrosion
Materials development and lifetime prediction
Requires faster Feedback Years !!! 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
Surface processes and electrochemistry on implants SBF without buffer SBF
4
80
3 40 2
20
phase angle
-2
Oxidation and Corrosion
log Z [Ω .cm ]
60
0 1 -2
-1
0
1
2
3
4
5
6
-20
log ω
Macroscale methods: - Open Circuit Potential
Electrochemical Impedance Spectroscopy (EIS)
- Potentiodynamic polarization
Micro- and Nanocapillaries:
Dedicated artificial crevice setups:
- Local measurements - Solution analytics
- Galvanic coupling - Mechanical sollication
Electrochemical surface modifications: - Anodizing - Electropolishing
Thick anodic oxide on Mg
electrolyte bridge (SCE)
platinum wire
Principle of microcapillary cell
holder
Full local electrochemical control for heterogeneous materials characterization
Silicone sealing rubber sample d = 300 nm - 1000 µm
Setup designed by Dr. Thomas Suter at the Institute for Material Chemistry and Corrosion, ETH Zurich.
Localized corrosion characterization Electrochemical polarization measurements 10
3
Current density [µA/cm
2
]
1 M NaCl
SEM before
MnS interface bulk
12 µm
10 2
10
MnS
interface
10
-2
10
-3
10
-4
10
-5
1
bulk
10 0
Pitting 10
10
Current [nA]
Stainless Steel : 18Cr/10Ni
-1
d area = 2.5 µm
-2
-500
0
500
10 -6
1000
Potential [mV] (SCE)
Stainless Steel implants • Plastically deformed and scratched areas are very susceptible to localized corrosion
Ti alloys and electrochemical Polarization (b)
International standard ISO 10271:2001 (Dental Implants)
1.E-05 Log(i) (A /cm ^2)
Lactic acid solution (pH 2.2) 5.85 g/l NaCl + 10 g/l C3H6O3
1.E-04
1.E-06 1.E-07 1.E-08 Ti Grade4 SLA Ti Grade4 SL
1.E-09 1.E-10 -0.5
0
0.5
1
1.5
Potential vs. SCE (V)
Ti grade 4 + transfer piece: Ti6Al7Nb
(b) 1.E-04
Ti and Ti alloys
detected by ICP-MS
L o g (i) (A /cm ^2)
• No corrosive attack under static conditions • Some ionic release
1.E-05 1.E-06 1.E-07 1.E-08 Ti6Al4V SLA
1.E-09
Ti6Al4V SL 1.E-10 -0.5
0
0.5 Potential vs. SCE (V)
1
1.5
EIS: Electrochemical Impedance Spectroscopy Simple model
Measurement at corrosion potential Voltage perturbation (10 mV) is applied
- for electrochemical Interface
Rp
Amplitude
- The current response in function of frequency is measured
Rs
E
C
I Rs : Rp :
Ohm’s law gives a simple relation
C:
solution resistance polarization resistance of the surface capacitance of the surface
U=I*R When AC signal are applied, the relation is
For Mg: Rp= 1000 Ohm/cm2
Eac = Iac * Z
Corrosion rate ~ 220 µm/year
Z: impedance
EIS: data representation Nyquist plot
-Z ’’
Imaginary vs. Real component
Z0 Φ Φ
Z’
10 5 10 4
Rp -60
10 3
Time -40
Rs
10 2 10 1 10 -1
Phase (deg)
Impedance (Ohm)
-80
Bode plot
-20
10 0
10 1 10 2 10 3 Frequency (Hz)
10 4
0 10 5
12
- Frequency dependent representation
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
•
Mg-Y-Re alloys are preferred to of AZ91/AZ71.
•
A defined degradation sequence during the first 3-6 months is aimed at
Osteosynthesis
Cardiovascular application
pH of blood
Stents: weight:4mg
Pins
Experimental strategy : alloys and solutions •
•
3 alloys with systematic variation of Y, Zn for screening experiments Alloy
Mg
Y
RE
Zr
WE43
Bal.
4.0
3.5
0.5
Alloy
Mg
Zn
Y
Ca
Mn
WZ21
Bal.
0.9
1.7
0.25
0.15
ZW21
Bal.
2.0
0.8
0.25
0.15
Detailed description of WE43 dissolution
In vivo study: very complex media
In-vitro SBF
Solutions considered: •
SBF 27 as base (concentration in mmol/l) 100.0 NaCl, 4.0 KCl, 27.0 NaHCO3, 1.0 MgSO4 * 7H2O, 2.5 CaCl2 * 2H2O, 1.0 KH2PO4, TRIS Buffer (pH 7.4)
•
Matrix of solutions with combination of ionic species
model solution matrix for corrosion mechanisms
EIS: a kind of spectroscopic method Immediate immersion of WE43 in SBF 27 with and without buffer agent SBF without buffer SBF
4
Frequency resolved electrochemical impedance measurements allows to characterize reaction mechanisms
80
3 40 2
20
phase angle
-2
log Z [Ω.cm ]
60
increased pH
0 1 -2
-1
0
1
2
3
4
5
6
-20
pH 7.4
log ω
localized attack - fast process
• Distinction between uniform or localized corrosion • Double layer / surface oxide dielectric properties • Diffusion and adsorption processes can be monitored at low frequency
Uniform corrosion - slower
Reaction rates
Hydroxide products growth and redeposition Direct Charge transfer
WE43 in100 mM NaCl (buffered)
Porous corrosion products
-Z ’’
Mg Alloy
-Z ’’
Inductive effects (3) related to ionic adsorption from solution
(1)
1
(2)
2
Z’ 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
Nature of corrosion products: from ZW21 to WE43 NaCl, Tris
Transition from porous non protecting to more compact corrosion products
Full SBF WE43 3000
Important influence of Yttrium
WZ21 1000 ZW, WZ
WE
No significant changes in the nature of the corrosion products
ZW21 220
EIS and porous oxides characterization Osteosynthesis application: Thick porous Mg-hydroxides
Electrochemical Impedance Spectroscopy 8
Intact coating
7
Time dependant
log(Z)
6
0h 2h 10 h
5
4 3 through pores Solution contact to metal 2 -2
-1
0
1
2
3
4
5
6
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
log(f)
Inert
Oxide
Alloy
Polymer monolayer
T1 T2 T3 Implantation time
Material combination in implant fixation Screw: SS, CCM or TiAlNb
Corrosion of metallic implant materials Stainless steel (SS) is very susceptible to crevice corrosion problems
Crevice Corrosion SS Plate
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
Crevice corrosion and galvanic coupling Crevice conditions
Low migration of OH- ions increase of H+ in the crevice
Dissimilar materials
Mechanical damage active surface in the crevice
Differential aeration Damaged surface
in sta bil ity film Pa ss ive
ge ma da
pH decreases in crevice increases
Potential difference between crevice and external surface increases
c cli Cy
Cl-
Crevice corrosion
ity bil sta film
Galvanic corrosion
ive ss Pa
Potential difference between crevice and external surface
Repassivation
Investigation procedure is divided into: Passive layer instability, active surface in the crevice
- Static tests - Dynamic tests
Nanoscale crevices at coating interface
DLC-Si Si TiAlV 101 patients DLC/PE 101 patients Al2O3/PE 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
Crevice corrosion and galvanic coupling Aeration cell:
All the potential and the currents flowing between electrodes can be measured
stabilize the anodic reaction in the crevice ( < 100 µm) ee-
RE
e-
O2
WE1
I1
I3
I2
Differential aeration
WE2 Cathode ½ O 2 + H2O + 2e - = 2OH
-
Ch1
Ch2
Ch3
Anode Me
Me2+ + 2e-
Me2+ + 2H2O
Me(OH)2 + 2H+
pH
Additional cathode
Additional cathode
Additional passive panels to increase differential aeration
Crevice
Passive or grinded sample in the crevice
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 CaCl2, 0.2g/l NaHCO3
• There is a risk of galvanic incompatibility between SS and CCM especially in case of damaged surfaces
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
SS – CCM surfaces in crevices Galvanic current densities Lactic acid solution 8.00E-07
Current density (A/cm^2)
7.00E-07 6.00E-07 5.00E-07
SS(E) Panel
4.00E-07
Grinded SS Crevice
3.00E-07 Grinded CCM Crevice
2.00E-07 CCM(C) Panel
1.00E-07 0.00E+00 -1.00E-07 0
2
4
6
8
10
12
14
16
18
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
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
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 !
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
