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, Switzerlandcontact: [email protected]
BioTiNet Workshop
Ljubljana, 25 October 2011
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
Prof. Peter Uggowitzer
• Michael Schinhammer
• Dr. Giancarlo Pigozzi
• Dr. Magdalena
Pawelkiewicz
• Dr. Jörn Lübben
• Dr. Emanuele Cardilli
• Dr. Ngoc Quach-Vu
• Noemie Ott
• Marianne Berg
Swiss Federal Laboratories for Materials Science an d Technology
Laboratory for Corrosion and Materials Integrity
P. Schmutz
Physico-chemistry of reactive metal surface
Dr. Alessandra Beni
Micro- and nanocapillaryelectrochemistry
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
Acknowledgments
Ti alloys
- Corrosion resistant
- Other problems related to fatigue or fretting
Stainless SteelCo – Cr based alloys
- Pitting corrosion
- Crevice corrosion
- Galvanic corrosion
Mg alloys
- Biodegradable
Incr
easi
ng c
orro
sion
res
ista
nce
Decreasing dependence on testing media
Choice of testing solution chemistry
Less relevant
Critical
In - Vitro testing of corrosion processes
P. Schmutz et al., Interface, 17(2), 35-40 (2008)
Hip joints: DLC coating/PE (delamination in vivo, Täger Group)
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
Crevice corrosion:
- 90 % of the plates and screws (26 months average)
Example of corrosion related implant failure
Metal
Metal
Me++
Me++ Me++
Me++Me++ + H2O → Me(OH)+ + H+
Cl-
Cl-
Cl-
Cl-
O2O2
In-vivo evidence of corrosion
Years !!!
Materials development andlifetime prediction
°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 degradatio n processes- Corrosion rates are directly linked to electrical charge flow
- There is a very broad range of different informati on can be gained
Requires faster Feedback
Ideally weeks/ month
Accelerated tests and corrosion ?
Micro- and Nanocapillaries:- Local measurements
- Solution analytics
Electrochemical ImpedanceSpectroscopy (EIS)
Electrochemical surface modifications:
- Anodizing- Electropolishing
Thick anodic oxide on Mg
-20
0
20
40
60
80
-2 -1 0 1 2 3 4 5 6
1
2
3
4
phas
e an
gle
log ωωωω
SBF without buffer SBF
log
Z [ ΩΩ ΩΩ
.cm
-2]
Macroscale methods:- Open Circuit Potential
- Potentiodynamic polarization
Dedicated artificial crevice setups:
- Galvanic coupling- Mechanical sollication
Oxidation and
Corrosion
Surface processes and electrochemistry on implants
Setup designed by Dr. Thomas Suter at the Institute for Material Chemistry and Corrosion, ETH Zurich.
Full local electrochemical control for heterogeneous materials characterization
plat
inum
wire
holder
sample
Silicone sealing rubber
d = 300 nm - 1000 µm
elec
trol
yte
brid
ge(S
CE
)
Principle of microcapillary cell
12 µm
bulk interface
MnS
SEM before
10 -2
10 -1
100
101
102
103
-500 0 500 1000
10 -6
10 -5
10 -4
10 -3
10 -2
Potential [mV] (SCE)
Cur
rent
den
sity
[µA
/cm
2 ]
d area
= 2.5 µm
bulk
MnS
interface
1 M NaCl
Cur
rent
[nA
]
Pitting
Localized corrosion characterization
Stainless Steel : 18Cr/10Ni Electrochemical polarization measurements
Stainless Steel implants
• Plastically deformed and scratched areas are
very susceptible to localized corrosion
Ti alloys and electrochemical Polarization
International standard ISO 10271:2001 (Dental Implants)
Lactic acid solution (pH 2.2)5.85 g/l NaCl + 10 g/l C3H6O3
Ti grade 4 + transfer piece: Ti 6Al 7Nb(b)
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
-0.5 0 0.5 1 1.5
Potential vs. SCE (V)
Log(
i) (A
/cm
^2)
Ti6Al4V SLA
Ti6Al4V SL
(b)
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
-0.5 0 0.5 1 1.5
Potential vs. SCE (V)
Log(
i) (A
/cm
^2)
Ti Grade4 SLATi Grade4 SL
Ti and Ti alloys
• No corrosive attack
under static conditions
• Some ionic release
detected by ICP-MS
Am
plitu
de
E
I
Simple model- for electrochemical Interface
Rs
Rp
CRs : solution resistanceRp : polarization resistance
of the surfaceC: capacitance of the surface
For Mg: Rp= 1000 Ohm/cm 2
Corrosion rate ~ 220 µµµµm/year
Measurement at corrosion potential
Voltage perturbation (10 mV) is applied
- The current response in function of frequency is measured
Ohm’s law gives a simple relation
U = I * R
When AC signal are applied, the relation is
Eac = Iac * Z Z: impedance
EIS: Electrochemical Impedance Spectroscopy
Nyquist plot
Φ
Φ
Z0
Z’
-Z’’
12
EIS: data representation
10 1
10 2
10 3
10 4
10 5
-80
-60
-40
-20
010 -1 10 0 10 1 10 2 10 3 10 4 10 5
Impe
danc
e (O
hm)
Phase (deg)
Frequency (Hz)
Time
Rp
Rs Bode plot
- Frequency dependent representation
Imaginary vs. Real component
Pins
Stents:
weight:4mg
• Magnesium is not only biocompatible... It is animportant 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 first3-6 months is aimed at
pH of blood
Mg alloys as degradable implants
Cardiovascular application Osteosynthesis
• 3 alloys with systematic variation of Y, Zn for screening experiments
• Detailed description of WE43 dissolution
Solutions considered:
• SBF 27 as base (concentration in mmol/l)
100.0 NaCl, 4.0 KCl, 27.0 NaHCO 3, 1.0 MgSO4 * 7H2O, 2.5 CaCl2 * 2H2O, 1.0 KH2PO4, TRIS Buffer (pH 7.4)
• Matrix of solutions with combination of ionic species
In vivo study: very complex media
In-vitroSBF
model solution matrixfor corrosion mechanisms
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
Experimental strategy : alloys and solutions
-20
0
20
40
60
80
-2 -1 0 1 2 3 4 5 6
1
2
3
4
phas
e an
gle
log ωωωω
SBF without buffer SBF
log
Z [ ΩΩ ΩΩ
.cm
-2]
Immediate immersion of WE43 in SBF 27 with and without buffer agent
• Distinction between uniform or localized corrosion
• Double layer / surface oxide dielectric properties
• Diffusion and adsorption processes can be monitored at low frequency
Reaction rates
localized attack- fast process
Uniform corrosion- slower
Frequency resolved electrochemical impedance measurements allows to characterize reaction mechanisms
EIS: a kind of spectroscopic method
pH 7.4
increased pH
Inductive effects (3) related to ionic adsorption from solution
• Adsorption and Integration of ionic species (inductive effects) can be distinguished from purely growth of hydroxides as a function of time
-Z’’
16Degradation properties of Mg alloys
WE43 in100 mM NaCl (buffered)
-Z’’
1
3
2
Z’
Hydroxide products growth and redeposition
Mg Alloy
Direct Charge transfer
Porous corrosionproducts
(1) (2)
NaCl, Tris Full SBF
ZW, WZ WE
WE433000
WZ211000
ZW21220
Transition from porous non protecting to more compact corrosion products
Important influence of Yttrium
No significant changes in the nature of the corrosion products
Nature of corrosion products: from ZW21 to WE43
Defective coating model (applicable toother coatings like DLC)
• % of open pores can be determined
• Initial stage of degradation can befollowed as function of time T1 T2 T3
Cor
rosi
on r
ate
Inert Oxide Alloy
Polymermonolayer
Implantation time
2
3
4
5
6
7
8
-2 -1 0 1 2 3 4 5 6
log(f)
log(
Z) 0 h
2 h
10 h
Osteosynthesis application:Thick porous Mg-hydroxides
Electrochemical Impedance Spectroscopy
Intact coating
Solution contact to metal through pores
EIS and porous oxides characterization
Time dependant
Screw:SS, CCM or TiAlNb
SS Plate
Crevice Corrosion
Material combination in implant fixation
Corrosion of metallic implant materials
Stainless steel (SS) is very susceptible tocrevice corrosion problems
Cobalt Chromium Molybdenum (CCM) ismore corrosion resistant but can release“toxic” ionic species when depassivated
FDA concern issued February 2011
Titanium Aluminum Niobium alloys showvery stable passivation but are thanprone to fatigue-corrosion problems,inducing rapid breaking of the implant.
Besides, the intrinsic corrosion problems,dissimilar materials will add the problemof galvanic coupling
Passive layerinstability, active
surface in the crevice
Differential aerationLow migration of OH- ionsincrease of H+ in the crevice
pH decreasesCl- in crevice increases
Galvanic corrosion
Potential differencebetween crevice
and external surfaceincreases
Potential difference betweencrevice and external surface
Crevicecorrosion
Damaged surface
Mechanicaldamage
active surfacein the crevice
Repassivation
Passive film
stabilityC
yclicdam
age
Pas
sive
film
inst
abili
ty
Creviceconditions Dissimilar
materials
Passive layerinstability, active
surface in the crevice
Differential aerationLow migration of OH- ionsincrease of H+ in the crevice
pH decreasesCl- in crevice increases
Galvanic corrosion
Potential differencebetween crevice
and external surfaceincreases
Potential difference betweencrevice and external surface
Crevicecorrosion
Damaged surface
Mechanicaldamage
active surfacein the crevice
Repassivation
Passive film
stabilityC
yclicdam
age
Pas
sive
film
inst
abili
ty
Creviceconditions Dissimilar
materials
Investigation procedure is divided into:
- Static tests- Dynamic tests
Crevice corrosion and galvanic coupling
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
TiAlV
DLC-Si
Si
Nanoscale crevices at coating interface
101 patients DLC/PE101 patients Al 2O3/PE
8.5 year follow-up50% of DLC/PE failed
Cathode½ O 2 + H2O + 2e - = 2OH -
Anode
Me Me2+ + 2e-
Me2+ + 2H2O Me(OH)2 + 2H+
pH
O2
Diff
eren
tial a
erat
ion
e - e -
e -
Cathode½ O 2 + H2O + 2e - = 2OH -
Anode
Me Me2+ + 2e-
Me2+ + 2H2O Me(OH)2 + 2H+
pH
Anode
Me Me2+ + 2e-
Me2+ + 2H2O Me(OH)2 + 2H+
pH
O2
Diff
eren
tial a
erat
ion
e - e -
e -
RE
WE1
WE2
I1 I2 I3
Ch1 Ch3Ch2
Additional cathode Additional cathode
Crevice
RE
WE1
WE2
I1 I2 I3
Ch1 Ch3Ch2
Additional cathode Additional cathode
CreviceAdditional passive panelsto increase differential aeration
Passive or grinded sample in the crevice
Aeration cell:
stabilize the anodic reaction in the crevice ( < 100 µµµµm)
Crevice corrosion and galvanic coupling
All the potential and the currents flowing between electrodes can be measured
• There is a risk of galvanic incompatibility between SS and CCM especially in case of damaged surfaces
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
• 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
Ringer’s solution
Lactic acid solution
Influence of pH on SS surfaces in crevice
• 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
Galvanic current densities
-1.00E-07
0.00E+00
1.00E-07
2.00E-07
3.00E-07
4.00E-07
5.00E-07
6.00E-07
7.00E-07
8.00E-07
0 2 4 6 8 10 12 14 16 18
Time (day)
Cur
rent
den
sity
(A
/cm
^2)
SS(E) Panel
Grinded SS Crevice
Grinded CCM Crevice
CCM(C) Panel
Lactic acid solution
SS – CCM surfaces in crevices
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
In vivo degradation of CoCrMo hip implants
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 !
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 setup s are necessary to simulate the aggressive local chemistry that is responsible for most of the implant failures
Dynamic characterization in crevice and galvanic con ditions 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
Summary and conclusions
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