Alloys and Alloying Elements

Chromium

Chromium is typically alloyed with cobalt in significant amounts to improve various properties of the metal, including corrosion and oxidation resistance. Cobalt superalloys generally contain 20-30% chromium, which contributes to their good oxidation and hot corrosion resistance. It also provides resistance to corrosion at lower temperatures, as well as higher temperature resistance to oxidation and hot corrosion. Moreover, chromium additions provide enhanced protection of cobalt-based alloys against corrosion in dilute nitric acid environments, but may also decrease the resistance to corrosion in high concentrations of nitric acid. Cobalt-based alloys with a significant chromium content are susceptible to corrosion in chromic acid environments. Cobalt-chromium alloys with a high carbon content also have good wear resistance, but carbon can also inhibit the beneficial effects of the chromium additions.

Nickel

Nickel additions improve the resistance of cobalt to corrosion in mineral acids, such as sulfuric and phosphoric acids. It also improves the resistance to SCC. Furthermore, nickel additions provide improved resistance to corrosion in caustic environments.

Tungsten

Tungsten additions can improve the resistance of cobalt to corrosion in general, but may lead to corrosion problems at temperatures above 980°C. Tungsten may also increase the corrosion resistance of cobalt-based alloys in chromic acid.

Other Alloying Elements

Copper additions improve the resistance of cobalt to corrosion in sulfuric and phosphoric acid conditions. Molybdenum additions can improve the resistance of cobalt to corrosion in general. Additions of vanadium and niobium can be detrimental to the cobalt alloys in terms of corrosion resistance, while additions of manganese, iron, yttrium and lanthanum can improve the alloy.