Unveiling the transpassive film failure of 3D printing transition alloys

Chloride Induced Secondary Passive Film Failure for Laser Additive Manufacturing Nickel-Based Superalloys during Electrochemical Machining

“…It has been confirmed that in the NaNO 3 solution, SPF failure is mainly caused by the self-induced cracking on niobium-segregated phases and the erosion-induced degradation via shear stress on the film possibly assisted by cation vacancy condensation 23 and hence decohesion from the metal substrate. This suggests that the SPF ruptures from the metal/film interface incompletely in NaNO 3 solution, and thus the selective dissolution leads to an uneven surface shown in Figs.…”

“…In analogy with the primary passive film, the SPF is assumed to comprise a bi-layer structure with a point defective barrier layer that grows into the metal and a precipitated outer layer that forms from the cations transmitted through the barrier layer reacting with components of the solution, as indicated in our recent research. 23 In the case of a chromium-containing alloy with a sufficiently high content of Cr, the primary barrier layer is a defective Cr 2+x O…”

“…3,18,19 As a result, ECM is widely accepted as a better choice for the subsequent machining of LDED nickel-based superalloys, and many attempts have been made to improve the surface quality of the as-deposited components in NaNO 3 aqueous solution. 13,20,21 Our previous studies 4,9,19,22,23 have systematically investigated the anodic dissolution behavior of Alloy 718 by LDED in NaNO 3 solution (including the passive and transpassive dissolution behavior), and found that the surface micro-surface is rough even at high current density up to 40 A cm −2 . It is worth mentioning that the localized breakdown of the secondary passive film (also called transpassive film, generated in transpassive film after the primary passive film ruptures) formed during ECM deteriorates the surface quality by localized and selective dissolution.…”

Developing a New Flow Cell for Electrochemical Machining Anodic Dissolution Investigations by Simulation-Based Design and 3D Printing

“…These results are comparable to the results obtained previously using ex situ characterization methods such as X-ray photoelectron spectroscopy 40 and transmission electron microscopy. 38 Note that as a result of the hydrogen generation that occurs during cathodic polarization, accurate determination of the oxide layer thickness is difficult. However, it has been verified that the oxide layer thickness estimated using the method above is of the same order of magnitude as that obtained previously via other ex situ characterization methods.…”

“…7,26,27 The surface finish in ECM is closely related to the anodic interface structure, which has been demonstrated to be a metal/ supersaturated salt film structure in the NaCl electrolyte and a metal/ transpassive film/supersaturated salt film structure in NaNO 3 electrolytes for both Fe and SS304 steel under ECM conditions. 5,6,38,39 To characterize some aspects of the interface in detail, electrochemical methods including coulometry and the galvanostatic pulse technique were applied using the developed cell. After dissolution of Fe at 25 A cm −2 for 4 s, coulometry was conducted in situ by cathodic potential scanning from 0 to −2 V at a rate of 0.01 V s −1 .…”