Question

An engineer is designing a total hip implant. She intends to make the femoral stem out of titanium because it forms a good interface with bone. However, since titanium does not have optimum wear properties she intends to make the ball component out of stainless steel and attach it to the stem using a press fit. Should she be worried about corrosion? If so, what types of corrosion could take place?

Answer 1

Yes. She should be worried about corrosion.

• The 18-8 stainless exhibits intergranular corrosion due to high (0.08%) carbon content and gross pitting due to low molybdenum content.
• Of all the stainless steels, only the austenitic molybdenum-bearing 316 was of any use, even though it was described as inherently corrodible. 316L alloy, having a much lower carbon content (0.03%), greatly reduced the risk of intergranular attack.

Types of corrosion:

1. Intergranular corrosion: Due to the formation of chromium carbides. Requires very low carbon content (< 0.03%)
2. Pitting.—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.
3. Fretting: Repeated relative surface motion. Example: Orthopedics
4. Crevice corrosion: Between screw holes and surface of contacts
5. Galvanic corrosion: Cyclic loads with combination of disimilar metal (in this case titanium and stainless steel) is involved
6. Stress corrosion: Happens as a prt of intergranular corrosion