TABLE OF CONTENTS There are a number of cobalt-chromium alloys that are used as biomaterials, and their applications include as orthopaedic devices (femoral pins and heads) and as dental prostheses.26 Cobalt is usually the main component, and in addition to chromium there can be substantial amounts of molybdenum and/or nickel. All cobalt-chromium alloys are strongly passivated, as a result of the existence of the tenacious chromium oxide coating. However, corrosion resistance does vary slightly between the different alloys, with nickel conferring particularly good corrosion resistance. Cobalt-chromium alloys are generally reasonably inert in the body, especially over short time intervals. Like titanium alloys, though, there is a small but finite corrosion current in physiological saline, resulting in losses of about 30 ?g cm ?2 of metals into the surrounding tissues. Bulk cobalt-chromium implants tend to become surrounded by fibrous tissue and to show no evidence of acute or chronic inflammation or foreign body reaction.27
In general, cobalt-chromium alloys are resistant to pitting and crevice corrosion, even within the body.27 By contrast, relatively little is known about their susceptibility to stress corrosion cracking or corrosion fatigue. There have, though, been reports of fatigue failures in vivo of cobalt-chromium casting alloys for orthopaedics, an effect that may be assumed to arise from the presence of chloride ions in the surrounding body fluids under normal physiological conditions.
Cobalt-chromium alloys may undergo fretting corrosion quite readily.27 The process of fretting is a mechanical one and involves rubbing in the form of...
