Rezumat
The features of electrochemical anodic micromachining of hardening Co-W coatings with various W concentrations (in poly- and nanocrystalline form) in nitrate and nitrate-alkaline solutions were investigated by the method of rotating disk electrode (RDE) in the form of "recessed" RDE (RRDE). It was found that the polycrystalline coating (5–6 at.% W) dissolved in 2M NaNO3 solution with 100% current efficiency before the conditions of thermokinetic instability (TKI) were achieved as the effect of anodic limiting currents due to the salt passivation. Anodic micromachining of nanocrystalline coatings (with 22–25 at.% W) in the nitrate solution also occurs with 100% current efficiency but with a very high degree of dissolution heterogeneity before the TKI conditions are settled. Electrochemical micromachining of nanocrystalline coatings in nitrate-alkaline solution (2M NaNO3 + 0,5M KOH) at low current densities occurs with the nearly zero current efficiency, but in all cases under the TKI conditions current efficiency for coatings with various W concentration in different solutions exceeds 100% value. The mechanism assuming the formation of oxide-salt film and its periodic destruction due to thermal explosion was proposed for explanation of coating material removing the under TKI conditions. It is shown that the minimum surface roughness is observed after coating dissolution under TKI conditions. Some examples of coating micromachining in different electrolytes using constant and pulsed currents illustrate possibility to control process of the surface layer hardening-softening..