Corrosion cobalt/chromium alloys

Cobalt—Chromium Alloys. Co—Cr and Ni—Cr alloys are used predominately for the casting of removable partial dentures fixed partial dentures (bridges), crowns, and inlays are also cast. Because of high hardness, corrosion resistance, and wear resistance cobalt-chromium alloys are used for bite adjustments and as serrated inserts in plastic teeth used in fliU dentures. These alloys are well tolerated by the body and also are used for dental implants and orthopedic implant alloys. 

The successful clinical use of titanium and cobalt-chromium alloy combinations has been reported Lucas etal. also investigated this combination using electrochemical studies based on mixed potential and protection potential theories. Verification of these studies was made by direct coupling experiments. The electrochemical studies predicted coupled corrosion potentials of -0.22 V and low coupled corrosion rates of 0.02 ft A/cm. Direct coupling experiments verified these results. The cobalt-titanium interfaces on the implants were macroscopically examined and no instances of extensive corrosion were found. Overall, the in-vitro corrosion studies and the examination of retrieved prostheses predicted no exaggerated in-vivo corrosion due to the coupling of these cobalt and titanium alloys. 

The effect of H2S concentration on total corrosion of selected alloys in 1000 hr at 1800°F is illustrated in Fig. 2. A variety of different oxidation-corrosion behaviors were observed. Ferritic alloys, like AISI 446, generally showed increased corrosion rate with decreasing H2S concentration, whereas 300 series austenitics typified by AISI 310 generally exhibited maxima at both 0.1 and 1.0 v/o H2S. IN-800 had high corrosion only above 0.5 v/o H2S. Aluminized AISI 310 and IN-800, IN-671, and several high-chromium alloys did not indicate a strong dependence of H2S concentration in 1000 hr total corrosion. Cobalt-base alloys also generally performed as shown for the aluminized AISI 310 and IN-671 specimen. 

It is worth mentioning that in addition to the added strength of cobalt-chromium alloy, its ability to resist corrosion is one of the driving factors for it wide-spread use, especially when molybdenum is added, as it contributes to higher stability [13]. 

The different nickel alloys are designated by numbers or letters, often preceded by the proper name of the suppher, such as Inconel 600 or Hastelloy C (Table 12.9). Because of their relatively high price, the use of these materials is limited to those applications that require a better corrosion resistance than stainless steels can provide. The corrosion behavior of cobalt alloys is similar to that of the corresponding nickel alloys. Because of their good corrosion resistance some cobalt-chromium alloys are used in biomedical implants. 

Merritt, K. and Brown, S. A., Release of Hexavalent Chromium from Corrosion of Stainless Steel and Cobalt-Chromium Alloys, Journal of Biomedical Materials Research, Vol. 29, 1995, pp. 627-633. 

Metals are used in the cardiovascular area including heart valves, heart pacemaker leads, and heart pacemaker cases. These metals include titanium, titanium aUojrs, cobalt-chromium alloys, and cobalt-nickel alloys. Metals used for aneurism clips include cobalt-chromium-molybdenum alloys, cobalt-nickel-chromium-molybdenum allojfs, and, previously, stainless steels were used. Metal seeds are used for fractionated hyperthermia treatment of prostate disease, and corrosion analysis showed the alloy, PdCo may be suitable for the seed implants [52],... 

Choosing a metal for implantation should take into account the corrosion properties discussed above. Metals which are in current use as biomaterials include gold, cobalt chromium alloys, type 316 stainless steel, cp-titanium, titanium alloys, nickel-titanium alloys, and silver-tin-mercury amalgam. 

Cobalt-chromium alloys, tike titanium, are passive in the human body. They are widely in use in orthopedic applications. They do not exhibit pitting corrosion. 

Cobalt-base alloys. The corrosion behavior of pure cobalt has not been documented as extensively as that of nickel. The behavior of cobalt is similar to that of nickel, although cobalt possesses lower overall corrosion resistance. For example, the passive behavior of cobalt in 0.5 M sulfuric acid has been shown to be similar to that of nickel, but the critical current density necessary to achieve passivity is 14 times higher for the former. Several investigations have been carried out on binary cobalt-chromium alloys. In cobalt-base alloys, it has been found that as little as 10% chromium is sufficient to reduce the anodic current density necessary for passivation from 500 to 1 mA cm". For nickel, about 14% chromium is needed to reduce the passivating anodic current density to the same level. 

Biegun T, Brilckman A, Mrowec S, High-temperature sulfide corrosion of cobalt-chromium alloys , OxldMet, 1978 12 157-172. 

The second approach, that of surface coating, is more difficult, and that means more expensive. But it is often worth it. Hard, corrosion resistant layers of alloys rich in tungsten, cobalt, chromium or nickel can be sprayed onto surfaces, but a refinishing process is almost always necessary to restore the dimensional tolerances. Hard ceramic coatings such as AbO, Cr203, TiC, or TiN can be deposited by plasma methods and these not only give wear resistance but resistance to oxidation and... 

Moberg, L. E. Long-term Corrosion Studies In Vitro of Gold, Cobalt-Chromium and Nickel-Chromium Alloys in Contact Acta Odontologia Scandanavia, 43, 215-222 (1985)... 

Edwards e/a/. carried out controlled potential, slow strain-rate tests on Zimaloy (a cobalt-chromium-molybdenum implant alloy) in Ringer s solution at 37°C and showed that hydrogen absorption may degrade the mechanical properties of the alloy. Potentials were controlled so that the tensile sample was either cathodic or anodic with respect to the metal s free corrosion potential. Hydrogen was generated on the sample surface when the specimen was cathodic, and dissolution of the sample was encouraged when the sample was anodic. The results of these controlled potential tests showed no susceptibility of this alloy to SCC at anodic potentials. 

Edwards, B. J., Louthan, M. R. and Sisson, R. D., Hydrogen Embrittlement of Zimaloy A Cobalt-Chromium-Molybdenum Orthopaedic Implant Alloy , in Corrosion and Degradation of Implant Materials, Second Symposium, (Eds) A. C. Fraker and C. D. Griffin, 11-29 ASTM Publication STP 859, Philadelphia (1985)... 

The most important application of chromium is in the production of steel. High-carbon and other grades of ferro-chomium alloys are added to steel to improve mechanical properties, increase hardening, and enhance corrosion resistance. Chromium also is added to cobalt and nickel-base alloys for the same purpose. 

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