Interfacial Phenomena during Salt Layer Formation under High Rate Dissolution Conditions Joshua A. Hammons, † Alison J. Davenport, ‡ S. Majid Ghahari, ‡ Mehdi Monir, ‡ Jean-Phillipe Tinnes, † Mahrez Amri, † Nick Terrill, § Federica Marone, ∥ Rajmund Mokso, ∥ Marco Stampanoni, ∥ and Trevor Rayment* ,§ † School of Chemistry, University of Birmingham, B15 2TT, Birmingham, United Kingdom ‡ School of Metallurgy and Materials, University of Birmingham, B15 2TT, United Kingdom § Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom ∥ Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen, Switzerland * S Supporting Information ABSTRACT: Interfacial phenomena occurring during high metal dissolution rates, in an environment with diffusion-limited transport of dissolution products, have been investigated using time-resolved X-ray diffraction (XRD), small-angle X-ray scattering (SAXS) and fast radiography. Time resolved SAXS data reveal that highly anisotropic interfacial X-ray scattering always precedes salt nucleation. The correlation between the interfacial scattering the presence of salt crystals indicates that the interface is between the metal electrode and the concentrated NiCl 2 electrolyte and can therefore be interpreted as reflectivity or Porod scattering. Using fast radiography, we show that continued crystal nucleation and growth results in formation of a crystal-containing salt layer, which initially extends far from the interface (>20 μm), until the NiCl 2 concentration decreases below saturation. Dissolution of this thick salt layer occurs mainly at the furthest boundary from the interface until, the salt layer thickness decreases to a steady state value, resulting in a steady state limiting current. These results show that the presence of a crystalline salt layer at a dissolving interface causes microscopic roughening which has implications for understanding both the role of salt films in pitting corrosion and electrochemical processing. ■ INTRODUCTION This study focuses on interfacial phenomena that occur during rapid metal dissolution under diffusion-limited conditions. To achieve this, nickel is used as the metal of choice because it is a single component metal, has a relatively high density, and is also commonly used. Metals can dissolve at high rates during electrochemically driven processes such as electropolishing and electrochemical machining, and in forms of localized corrosion such as pitting, in which cavities propagate under metal surfaces. Electrochemical dissolution of metal is an autocatalytic process, since the resulting local high concentration of metal ions next to the surface forms a highly acidic solution (through hydrolysis reactions) and presence of a high concentration of anions, both of which further increases the rate of dissolution. 1 However, in circumstances where transport becomes diffusion- limited (which is the case with occluded cavities formed during pitting corrosion), the solution adjacent to the dissolving interface becomes supersaturated, and a granular solid salt layer 2 precipitates. This layer can substantially decrease the rate of dissolution since current is carried through the salt layer only via electrolyte present in between salt crystals. Thus the composition and the porosity of the film will strongly influence electropolishing and corrosion rates. Furthermore, salt films also play a significant role in the stability of corrosion pits by providing a “reservoir” of metal ions that can maintain an aggressive acidic solution chemistry near the interface even in the presence of a high rate of ion transport out of pits, for example after the rupture of a protective lacy cover over a corrosion pit. 3 Subsequent growth of the pit may then be controlled by ion transport through the salt film. 4 “Brightening” and “leveling” which are important properties of electro- polished and electromachined processes are known to be controlled by the presence of a salt film. 5 In the presence of a salt layer, the current density is independent of increasing applied potential since the rate of transport of metal ions away from the dissolving interface is diffusion limited. 6 It has been proposed that in such situations for iron 4,7 and nickel 4,8−10 increases in potential lead to an increase in potential drop solely across the salt layer which in turn implies an increase in layer thickness. The principal assumption in this hypothesis 6,7 is that no concentration gradient exists inside the film, as the concentration is considered to be at saturation; most often, it is assumed that there is a bulk salt layer that is uniformly composed of salt and Received: November 27, 2012 Revised: April 17, 2013 Published: April 18, 2013 Article pubs.acs.org/JPCB © 2013 American Chemical Society 6724 dx.doi.org/10.1021/jp311666w | J. Phys. Chem. B 2013, 117, 6724−6732