Corrosion Testing for Medical Device Validation

Effect of Corrosion on the Body

• Compatibility

• Tissue response

• Leach rates

• Toxicity

Corrosion Testing

Two aspects of in vivo corrosion:

1. How susceptible is implant material to corrosion in vivo?

2. What is the effect of any corrosion (even very small amounts) on the body?

Device Susceptibility: Corrosion Performance Validation

Selected corrosion tests used to validate medical devices:• ASTM F 1801- 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

Corrosion Testing

• Rest Potential

• Cyclic Polarization

• Galvanic

• Fretting

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 2129-06• 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 10993-15:2000

Rest Potential Monitoring• Provides an opportunity to measure release of leachable

substances, e.g., Ni, Cr, Co• Periodic solution analysis by ICP-MS

Nic

kel L

each

Rat

e (μ

g cm

-2t-1

)

Immersion time (hours)

Cyclic Potentiodynamic Polarization

• Preferred test method– ASTM F 2129-06

• 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 10993-15:2000 - not recommended

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

Cyclic Potentiodynamic Polarization

• No breakdown• Good resistance to localized corrosion

Rest Potential, Er

Vertex Potential, Ev

Pot

enti

al V

(S

CE

)

Current mA cm-2

Cyclic Potentiodynamic Polarization

Rest Potential, Er

Breakdown Potential, Eb

• Breakdown observed

Breakdown potential

Repassivation potentialRest potential

Pot

enti

al V

(S

CE

)

Current mA cm-2

Interpreting the Results

• Cyclic Potentiodynamic Polarization– ASTM F 2129-06 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?

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

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

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

ASTM F 2129Example of Typical Data Presentation:

Device Er Ezc Eb Ep Ev Eb-Er Ep-Er

Test 1 -305 -337 809 420 - 1104 725

Test 2 -410 -442 NB - 1200 - -

Test 3 -376 -393 NB - 1200 - -

Test 4 -311 -350 NB - 1200 - -

Test 5 -420 -449 740 332 - 1160 752

Test 6 -431 -452 NB - 1200 - -

Average -376 -404 775 376 1200 1132 739

All potential values are in mV

Er = rest potentialEzc = zero current potentialEb = breakdown potentialEp = repassivation potentialEv = vertex potentialNB = no breakdown

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