Formation and Breakdown of Oxide Films in High-Rate Anodic Dissolution of Chromium–Nickel Steels in Electrolytes for Electrochemical Machining

Abstract

It is shown that, in high-rate pulsed galvanostatic anodic dissolution of type CSN17335 and AISI 304 chromium-nickel steels in electrolytes for electrochemical machining (ECM) (chloride, nitrate, and mixed chloride-nitrate solutions with a conductivity of 0.15 S/cm) using microsecond pulses with a duration of 20-2000 μs and current densities in the range of 1-100 A/cm 2 , a substantial fraction of charge (up to ~40%) is spent on the formation of a passivating oxide film with a semiconducting behavior. The electrochemical treatment therefore directly involves the oxide film, not the alloy. As a consequence, the current efficiency of ECM of these materials is ~60-70%, depending on the alloy composition. When using direct current, the rate of machining increases as a result of the oxide film breakdown due to its thermokinetic instability ("thermal explosion") caused by a rise in the surface temperature.

Investigating the Impact of Physical Vapour Deposition (PVD)-Coated Cutting Tools on Stress Corrosion Cracking Susceptibility in Turning Super Duplex Stainless Steel

“…The surface machined with the AlCrN/TiSiN-coated insert had the thinnest oxide film, which suggests that this coating had a better resistance to oxidation than the other coatings. Overall, the oxide film on the machined SDSS surfaces was determined by the chemical composition, phase distribution, and surface finish of the substrate, as well as by the machining parameters, insert coatings, and environmental conditions [50]. The oxide film was composed of a mixture of chromium oxide (Cr2O3), iron oxide (Fe2O3), and nickel oxide (NiO), with different ratios depending on the oxidation kinetics and the availability of the alloying elements.…”