Effect of Low-temperature Heat Treatment on the Potentiodynamic Corrosion Performance of Metallic Stents

There is anecdotal evidence that heat treatments performed at temperatures below 400 °C in air can have deleterious effect on the potentiodynamic corrosion performance of Nitinol- and Cobalt-alloy-based stents and other implantable medical devices. Such heat treatments are performed when applying a polymer encapsulation to the devices and occasionally the encapsulation is removed prior to corrosion testing so as to expose the bare metallic surface of the stents. We hypothesize that the low temperature heat treatments do not produce a deleterious effect on the corrosion behavior. Rather, a poorer corrosion performance is observed in stents which have a damaged surface oxide layer caused by the removal of the encapsulation prior to corrosion testing or damage to the surface due to contact between the bare metal struts of the stent. We test the hypothesis through an experimental study. Our test protocol consists of: (i) manufacturing stents with various starting materials, surface areas, and heat treatments, (ii) Applying and removing a polymer encapsulation on some of these stents, and (iii) Performing potentiodynamic corrosion testing per ASTM F2129 on groups of these stents. These results can guide best practices in performing corrosion testing on encapsulated medical devices.