With regards to CoCrMo alloy, the passive layer is mainly Cr2O3 but I would like to know what is the passive layer for Co? Is it CoO? How strong is it?
The passive layer of Cr is Cr2O3 as the latter corresponds to the most stable oxidation state of Cr over a broad range of temperature. Oxidation state +II is generally the most stable for Co, however it is more sensitive to temperature in air than Cr is, and a fraction of Co(+III) in air may exist as well as non-stoichiometry. The answer thus depends on the temperature and on the partial oxygen pressure. The full range from Co1-xO to Co2O3 is possible, and this feature has a strong impact on the electrical conductivity of the oxide, which can be used as an oxygenmeter.
Alain
Hi Alain, thank you so much
I am using 37 C. for the oxygen is the oxygen pressure in the air.
The passivation layer of Co is first non stoichiometric CoO, which is converted into Co3O4 when the metal is heated under O2 below 900°C. Then, at higher temperatures, it decomposes again into stoichiometric CoO. But of course, in the case of an alloy, the passivation layer is obviously a mixture of the most stable oxides at the considered temperature and oxigen pressure, i.e. non stoichiometric CoO, Cr2O3 and MoO3, probably forming a complex mixed oxide. And surface hydration is of course quite probable.
Alain
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