Effect of Corrosion on the Body • Compatibility • Tissue response • Leach rates • Toxicity
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?
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 Nickel Leach Rate (μg 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
Vertex Potential, Ev Rest Potential, Er Cyclic Potentiodynamic Polarization • No breakdown • Good resistance to localized corrosion Potential V (SCE) Current mA cm-2
Breakdown Potential, Eb Rest Potential, Er Cyclic Potentiodynamic Polarization • Breakdown observed Breakdown potential Potential V (SCE) Rest potential Repassivation potential 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 2129 Example of Typical Data Presentation: 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