Corrosion, Electrochemistry, and Metal Ion Release of MP35N Material

MP35N is a multi-phase Co-based alloy, containing approximately 35% Ni, 20% Cr, 10% Mo, and 35% Co.  MP35N possesses a combination of high strength and excellent corrosion resistance and fatigue life, which makes it one of the most widely used materials for implantable medical device application.  Even though MP35N has been used as cardiac leads for over 40 years and no reported failure has been directly attributed to corrosion of MP35N material, the question remains: is MP35N susceptible to corrosion?

The answer is “YES”.  When MP35N, low Ti or standard grade, is subjected to anodic potential higher than 0.5 V (vs. SCE), electrochemical reaction(s) causes significant dissolution of MP35N.  Cyclic polarization per ASTM F2129 on MP35N typically shows a large current increase above 0.6 V (SCE) and another large current increase at higher anodic potential (pH dependent) which is due to oxygen evolution.  SEM analysis on MP35N samples post cyclic polarization experiments revealed that sample surface is often covered with a relative thick layer of corrosion products with “mud-cracking” morphology.  The corrosion product layer exceeds a few microns in thickness.  We believe the electrochemical reaction at 0.5 V and above involves metal oxide and elemental metal direct reaction with H₂O, forming metal hydroxyl and hydroxide such as M(OH)_x.nH₂O.  Cyclic voltammetry was performed to better understand the redox mechanisms over wide potential ranges.

In this presentation, we shall report data generated by a variety of analytical techniques, including electrochemical methods, SEM and X-ray mapping, XPS surface analysis and depth profiling, and metal ion release study.  Utilization of aforementioned techniques yielded valuable information which improved our understanding on degradation of MP35N material.