Corrosion of Cobalt and Cobalt Alloys

Abstract

Investigations are reviewed of the composition and structures of corrosion films and scales found on cobalt and on simple and complex cobalt-base industrial alloys. The corrosion resistance of these films and scales depends on metal and alloy composition, environment (an atmosphere containing one or more of CO2, S, H2S, O2, H2O, SO2, V2O3, NaCl, etc.), pressure, temperature, and time. Additional factors, alone or in combination, such as surface preparation, stress, thermal shock, geometry of the part, velocity, density, and flow pattern of the corroding environment also have an effect. Data on oxidation, sulfidation, and hot corrosion (a combination of the first two) of cobalt and its alloys are summarized in some detail, as are the known characteristics of the films and scales that are formed. Only the work that used techniques permitting determination of composition and crystal structure of these films and scales was reviewed. The effect of impurities in metals and alloys may be as significant as that of added alloying elements; impurities may alter the scales' chemical and physical properties—increasing, decreasing, or neutralizing corrosion rates. Until data are available on the intereffect on diffusion of elements present in scales, films, and metal surfaces, the mechanisms of corrosion proposed are only hypotheses.

Novel Co-rich high entropy alloys with superior tensile properties

“…It has been found that the Fe and Ni stabilize the γ phase in Co-based superalloys. In addition, independent reduction of Ni concentration or an increase in the Co concentration does significantly lower the SFE [18]. Therefore, the coordination lowers the SFE and reduces the γ phase stability, which will affect the deformation behaviors.…”

“…Furthermore, elements of Fe and Ni are γ phase stabilizers for the alloy while Cr is the phase stabilizer (i.e. alloying with Fe and Ni can stabilize the γ phase and increase the SFE, whereas alloying with Cr will stabilize the phase and lower the SFE of the alloys) [18]. In the present study, we aimed to design novel Co-rich HEAs with manipulated mechanical properties and deformation behaviors by regulating the SFE and phase stability, achieved by adjusting the elemental concentrations of the equiatomic CoCr-FeNi alloy based on the thermodynamics and ab initio calculations.…”

Laser Clad and HVOF-Sprayed Stellite 6 Coating in Chlorine-Rich Environment with KCl at 700 °C

“…Oxidation of Co-Cr alloys lead to creation of a two-layered oxide scale with appreciable porosity [13]. While the pores enhance oxidation, the Cr 2 O 3 and spinel CoCr 2 O 4 phases inhibit the oxidation and the CoO serves as the phase through which the Co ions diffuse [14].…”

“…The oxidation is, however, strongly dependent not only on oxygen pressure, but mainly on the amount of chromium in the alloy. Alloys with more than 25 wt% Cr have been reported to have both almost pure Cr 2 O 3 scales according to Phalnikar et al [15] and being of two-layer structure containing continuous layers of Cr 2 O 3 and spinel CoCr 2 O 4 [13] when oxidized in atmosphere with oxygen pressure above 13 kPa. The compactness of the scales, which improves with increase in Cr, is responsible for improved oxidation resistance [13] even though the overall spalling resistance was deemed as insufficient.…”

“…Alloys with more than 25 wt% Cr have been reported to have both almost pure Cr 2 O 3 scales according to Phalnikar et al [15] and being of two-layer structure containing continuous layers of Cr 2 O 3 and spinel CoCr 2 O 4 [13] when oxidized in atmosphere with oxygen pressure above 13 kPa. The compactness of the scales, which improves with increase in Cr, is responsible for improved oxidation resistance [13] even though the overall spalling resistance was deemed as insufficient. However, in deposit-induced high-temperature corrosion environments with chloride species, Cr 2 O 3 scale can be attacked by KCl deposits and K 2 CrO 4 is formed which destroys the protective ability of the alloy and coating [16,17].…”