Hello Carlos

Did you  list given at standard ASTM E 407? there is reagent for pure Co, for hard facing and tool metals and for high temperature alloys. for example:

5 ml HNO3

200 ml HCl

65 g FeCl3

Use a certified and tested hood. Immerse few seconds.

or you can try with 

10 g CuSO4

50 ml HCl

50 ml water

Immerse or swab 5-60 s. made more active by adding few drops of H2SO4 just before use.

Good luck

According to signalled reference, the samples can be etched electrolytically in HNO3 (conc.) + H2O (1:1 vol/vol; 9 V; 3 s): https://www.degruyter.com/view/j/afe.2012.12.issue-4/v10266-012-0118-6/v10266-012-0118-6.xml

You may find also useful the following paper: http://www.sciencedirect.com/science/article/pii/S1044580305002688