Critical cobalt-based alloys

Criticality after 1978-no reduction of cobalt-base alloys... 

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. 

The arrangement of the melting and vacuum spray chambers is critical for guiding the liquid metal to eject into the vacuum chamber. Difficulties exist in precisely controlling the expulsion of the liquid metal into the vacuum chamber. Therefore, flaky droplets may be formed in vacuum atomization. Although vacuum atomization was developed mainly for the production of high-purity nickel and cobalt based superalloy powders, it is also applied to atomize the alloys of aluminum, copper and iron. 

Table 2-16 shows the critical crevice and pitting temperatures for cobalt-based and other alloys. Data in Table 2-16 show that Ultimet alloy has an excellent resistance to localized attack, comparable with that of C-22 alloy. 

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