Pitting Corrosion of High Strength Alloy Stimulation Electrodes under Dynamic Conditions

Abstract

Pitting corrosion is a major cause of in vivo failure of high strength alloy intramuscular (IM) stimulation electrodes. Avoidance of pitting corrosion can prolong the life expectancy of stimulation electrodes. Corrosion behavior of two candidate IM electrode materials, 316LVM and MP35N, was studied in a saline solution using fast sweep cyclic voltammetry with potential sweep rates ranging from 0.005 to 500 V/s. The potential at which pitting was observed to occur increased with increasing sweep rates, and was found to be related to sweep rate by the empirically fitted equations defines the maximum potential to which an electrode can be polarized under nonequilibrium conditions before pitting corrosion occurs. A state diagram analysis method was developed to predict operating limits for stimulation electrodes under pulsed conditions. These predictions were tested by carrying out 1h in vitro stimulation experiments with biphasic, rectangular, charge balanced, cathodic first current pulses for the two materials, and good agreement between experimental results and predictions was obtained. This study indicates that pitting corrosion processes are kinetically limited at the high rate of potential change encountered with electrodes used to excite neural tissue electrically. The results further suggest that an asymmetrical or charge imbalanced waveform may be the optimal choice to avoid pitting corrosion of stimulation electrodes.