Microscopic investigations of electrochemical machining of Fe in NaNO3

Abstract Electrochemical micro machining (ECMM) is a promising process in microsystem technologies. The work-pieces of steel and other metals are structured in neutral NaNO 3 solution by anodic dissolution at large current densities (about 100 A/cm 2 ) and high electrolyte flow rates (some m/s). Accordingly, an identification of the processes, the structure and the current distribution at the interface of the work-piece is critical. Since available simulation software for ECMM neglects details of the work-piece—electrolyte interface and needs detailed information on rate determining processes, two new strategies are presented. In a first approach, the scanning droplet cell was modified to enable such large current densities and flow rates in a three-electrode arrangement under potentiostatic conditions. A new flow-through concept was developed to realize flow and to suppress effects of side reactions like oxygen bubble formation and precipitation of products. A simultaneous product analysis yields the Fe 2+ /Fe 3+ ratio. The second approach concerns (super-) saturated solutions of Fe(NO 3 ) 3 and Fe(NO 3 ) 2 , which we expect at the metal surface. These liquids are meta-stable without crystallization for days and form highly viscous honey-like surface layers, which represent the transition from solutions to melts. Conductivities and viscosities are measured to determine diffusion coefficients under these conditions to get parameters for modeling of the process in future finite element simulations. On the basis of our experiments, a two-layer interface is assumed: a modified solid passive film and a viscous adherent layer of supersaturated iron nitrates.