TABLE OF CONTENTS When metal loss is occurring at a uniform rate across the entire exposed surface, the metal is considered to be undergoing general or uniform corrosion. For metals capable of forming passive films, Figure 3 7 shows that general corrosion can occur in both the active and passive regions of the polarization curve. In other words, the anodic and cathodic currents can cross in either of these regions. However, corrosion rates found within the active region are normally too high for the metal to be of interest in medical applications. In the passive region the general corrosion is sometimes referred to as passive corrosion, and in the long term its rate depends on a balance between the rates of growth and dissolution of the passive film. For a successful implant material, the long-term general corrosion rate should fall to less than 1 m per year. For almost any other application, such low corrosion rates would be considered insignificant. However, even at these rates it has been reported that after eight years of implantation, the levels of nickel, chromium and cobalt in surrounding tissues can be five times higher than the normal values [19]. It has also been shown that the presence of metal ions suppresses cell growth of human gingival fibroblasts [20].
As mentioned earlier, stable oxide films formed on many metals in neutral and alkaline environments reduce corrosion rates to very low levels. However, in the long term, even these very...
