Anodic treatment of strengthening the electrochemical coatings in electrolytes for electrochemical machining: 1. Micromachining of CoW coatings in nitrate and nitrate-alkaline solutions

Abstract

Polycrystalline CoW coatings (with a 5–6 at % content of W) were found to dissolve in a 2M NaNO3 solution with 100% current efficiency while reaching the conditions of thermokinetic instability (TKI) upon attaining the anode limiting currents resulting from the salt’s passivation. The anodic micromachining of nanocrystalline coatings (22–25 at % of W) in a nitrate solution also occurs with a 100% current efficiency, though at a very high degree of dissolution instability prior to attaining the TKI. The electrochemical micromachining of nanocrystalline coatings in a nitrate-alkaline solution (2M NaNO2 + 0.5 M KOH) at low current densities occurs at a current efficiency close to zero, but, in all the cases under the TKI conditions, the current efficiency (upon the coatings treatment with different W contents in different solutions) exceeds the 100% value. A procedure for the removal of the coating material under the TKI conditions is offered that suggests the formation of an oxide-salt film and its periodic destruction due to a thermal explosion. It is shown that the minimal surface roughness is registered after the coating dissolution under the TKI conditions. Results are reported that confirm the possibility of controlling the strengthening (weakening) process of a surface layer after micromachining in different electrolytes using constant and pulse currents.