1. Corrosion Testing for Medical Device Validation

2. Effect of Corrosion on the Body
Compatibility
Tissue response
Leach rates
Toxicity

3. Corrosion Testing Two aspects of in vivo corrosion:
How susceptible is implant material to corrosion in vivo?
What is the effect of any corrosion (even very small amounts) on the body?

4. Device Susceptibility: Corrosion Performance Validation
Selected corrosion tests used to validate medical devices:
ASTM F Practice for Corrosion -Fatigue Testing of Metallic Implant Materials
ASTM F 1875 – Practice for Fretting Corrosion Testing of Modular Implant Interfaces: Hip Femoral Head-Bore and Cone Taper Interface
ASTM F 2129 – Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implants
ASTM G71 - Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes
ASTM F 746 – Test Method for Pitting or Crevice Corrosion of Metallic Surgical Implant Materials

5. Corrosion Testing
Rest Potential
Cyclic Polarization
Galvanic
Fretting

6. Rest Potential Monitoring
Addressed by several standards
ISO 16429:2004
Implants for surgery – Measurements of open-circuit potential to assess corrosion behaviour of metallic implantable materials and medical devices over extended time periods
ASTM F
Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices
Alternative standards
ISO 10271:2001 for dental materials
ISO :2000

7. Rest Potential Monitoring
Provides an opportunity to measure release of leachable substances, e.g., Ni, Cr, Co
Periodic solution analysis by ICP-MS
Nickel Leach Rate (μg cm-2t-1)
Immersion time (hours)

8. Cyclic Potentiodynamic Polarization
Preferred test method
ASTM F
Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices
Extract potential data
Rest potential (Er)
Breakdown potential (Eb)
Alternative test methods
ISO 10271:2001 for dental materials
ISO : not recommended

9. ASTM F 2129 General Procedure:
Typically performed in saline environment at 37°C
PBS, 0.9% NaCl, simulated bile, etc.
Monitor rest potential (Er) for 1 hour
Potentiodynamic polarization to 0.8 or 1 volt vs. SCE
If breakdown, record potential (Eb)
Reverse potentiodynamic polarization
record repassivation potential (Ep)
reformation of the passive layer

10. Cyclic Potentiodynamic Polarization
No breakdown
Good resistance to localized corrosion
Vertex Potential, Ev
Potential V (SCE)
Rest Potential, Er
Current mA cm-2

11. Cyclic Potentiodynamic Polarization
Breakdown observed
Breakdown potential
Breakdown Potential, Eb
Potential V (SCE)
Rest potential
Repassivation potential
Rest Potential, Er
Current mA cm-2

12. Interpreting the Results
Cyclic Potentiodynamic Polarization
ASTM F is a deliberately aggressive test
General consensus that no breakdown up to 0.8 V (SCE) will provide sufficient resistance to localized corrosion in vivo
But if breakdown has been observed
How do we treat the data?
How good is good enough?

13. Interpreting the Results
Neither ASTM F 2129, nor the FDA (or other regulatory agencies) provide specific guidance as to what constitutes an acceptance criterion
Two approaches using Eb
Compare with threshold for ‘optimum corrosion resistance’
Criterion is independent of material and environment
Compare with that of a predicate device
Assumes suitable device is available
The breakdown potential alone, however, is not a good measure of localized corrosion resistance

14. Interpreting the Results
Er and Eb are not intrinsic properties of a metal or alloy
For a given alloy, Eb and Er are influenced by -
The environment, e.g., pH, solution chemistry, temperature
Surface finish, e.g., mechanical polish vs. electropolish
Immersion time
Eb is also influenced by the test method
Potentiodynamic scan rate
Faster scan rates can increase the measured value of Eb

15. Interpreting the Results
Consider the gap between the breakdown potential and the rest potential
Thus, a measure of an alloy’s susceptibility to localized corrosion is given by Eb - Er
The gap Eb - Er can be used to evaluate both pitting and crevice corrosion for a finished device
Because breakdown will occur at the most susceptible location whether it be a crevice or a pit-initiation site

16. ASTM F 2129 Example of Typical Data Presentation: Device Er Ezc Eb Ep
Eb-Er
Ep-Er
Test 1
-305
-337
809
420 -
1104
725
Test 2
-410
-442 NB
1200
Test 3
-376
-393
Test 4
-311
-350
Test 5
-420
-449
740
332
1160
752
Test 6
-431
-452
Average
-404
775
376
1132
739
All potential values are in mV
Er = rest potential Ezc = zero current potential Eb = breakdown potential Ep = repassivation potential Ev = vertex potential NB = no breakdown

17. Galvanic Corrosion
Perform ASTM G 71 tests on galvanic couples and individual anodes
Measure and compare steady corrosion rates (current densities)
Current increases of more than an order of magnitude are considered signficant
Also can compare coupled and un-coupled leach rates in longer-term leaching tests