For many years, nickel-free cobalt-chromium (Co-Cr) alloys have been used as the gold standard in the substructure of metal-supported crowns and bridges. These alloys are very durable due to their high mechanical resistance and low flexure. In terms of biocompatibility, when cobalt-chromium is placed in the mouth, it forms a very thin, passive chromium-oxide layer on its surface, isolating itself from oral fluids and resisting corrosion. However, the oral environment is a highly aggressive electrochemical field due to constantly changing pH values (acidic foods), fluoride toothpaste, and the presence of different metals. In Co-Cr alloys that remain in the mouth for many years (10-15 years), micro-level corrosion may begin, and trace amounts of metal ions may be released into surrounding tissues. This situation has limitations as it can trigger local gingival reactions in sensitive or allergic individuals.
