Geoff Shannon

  Source:  Today’s Medical Developments

The most commonly used medical device materials are stainless steel 304 and 17-4. These materials have a natural passive corrosion-resistant layer, consisting of chromium oxide, which resists repeated sterilization and has an inert surface that will not react inside the body.

During the manufacturing process, multiple machining steps can remove or degrade this passive surface by embedding iron chips and particles into the surface. The material must then be put through passivation to rebuild the passive layer – removing iron from the part’s surface, which also removes potential corrosion sites.

The style of mark required by the medical device industry is called a dark or annealed mark. This mark does not remove any material from the part, avoiding any potential for any contamination to collect. When handling the part, the dark or annealed mark must not be able to be felt on the surface of the material. The heat input needed to build up this oxide layer tends to degrade the passive layer on the marked surface and can cause local migration of alloying elements.

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