Question

Identify the likely corrosion mechanisms at the following locations: a. Hip stem made of titanium coupled with a cobalt chromium head b. Nitinol stent c. Fracture fixation plate made of stainless steel

Answer 1

Corrosion mechanism of Hip stem made of titanium coupled with a cobalt chromium head:

Titanium has excellent corrosion resistance, however subject to fretting and wear, with particles of the alloy found in close tissue, instead of precipitated corrosion product due to uniform or localized corrosion. A current drawback associated with orthopaedic alloys is corrosion at the taper connections of standard joint replacement parts. With the massive and increasing variety of total joint styles that embody metal-on metal round shape taper connections, the impact of crevices, stress and motion defy increasing importance. Retrieval studies have shown severe corrosion attack will occur within the crevices shaped by these tapers in vivo.

Laboratory studies where Co-Cr-Mo alloy has been immersed in a simulated body fluid (Hank’s salt solution) showed that Co dissolved from the surface and therefore the remaining surface consisted of Cr oxide (Cr+3) containing Mo oxide (Mo+4, Mo+5, and Mo+6). XPS analysis of the samples therein study discovered that chromium and molybdenum were additional widely distributed within the inner layer than within the outer layer of the oxide film. In body fluids, cobalt is completely dissolved, and the surface changes into Cr oxide containing a little amount of Mo oxide. Coleman, et al. reported an increase within the level of metal within the blood in the initial year when implantation of metal solid Co - Cr- Mo hip prostheses. Wear at the bearing surfaces seems to be accountable for generating unleash of the cobalt, however corrosion of the implant materials or of the wear and tear particles may also contribute to the release of Co into the surrounding tissue fluid. However, there has been a correlation between elevated metal levels within the blood due to corrosion in patients having mixed-alloy standard metal-on synthetic resin hip implants.

Corrosion mechanism of Nitinol stent:

Nitinol, consists of close to equi-atomic amounts of Nickel and Titanium. Since 1970 it has found widespread clinical use as dentistry material and additional recently as tube-shaped structure stents thanks to its exceptional mechanical characteristics and its high biocompatibility. Many studies have highlighted the variation in the corrosion performance of Nitinol relying upon the surface condition of the test specimens used and also the surface condition given. Since heat treatment is concerned throughout the producing method, the passivating oxide on Nitinol is crystalline in nature, and has been found to exhibit severe pitting and crevice corrosion, whereas surface treatment to make amorphous compound leads to glorious corrosion resistance. Different surface treatments, like electrochemical polishing, has conjointly been found to be a decent surface treatment before implantation, leading to considerably increased corrosion resistance and very low levels of metal dissolution, well below the calculable average dietary intake levels of 201–299 μg per day.

Corrosion mechanism of Fracture fixation plate made of stainless steel:

Surgical grade implants corrode within the body setting and unleash Fe, Cr and Ni ions and these ions are found to be powerful allergens and carcinogens. Studies on retrieved implants show that over 89% of the failure of implants of stainless-steel are due to pitting and crevice corrosion. This reality alone deems that a better material be used for even temporary implant devices.

Pitting.—Pitting is the most common form of corrosion arising from the breakdown of the passivating oxide film, which can be enhanced by the presence of proteins in the tissue fluid and serum.

Crevice corrosion.—Stainless steel is highly susceptible to crevice corrosion attack as compared to the other implant alloys. The occurrence of corrosion on the bone plate and screws made of stainless steel at the interface between the screw heads and the countersink holes is a common feature.