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In-situ Synchrotron Studies of the Effect of Nitrate on Iron Artificial Pits in Chloride Solutions: I. On the Structures of Salt Layers

DOI: 10.1149/2.0581506jes DOI Help

Authors: Weichen Xu (University of Birmingham) , Steven Street (University of Birmingham) , Mahrez Amri (DTU Riso) , Fred Mosselmans (Diamond Light Source) , Paul Quinn (Diamond Light Source) , Trevor Rayment (Diamond Light Source) , Alison Davenport (University of Birmingham)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Journal Of The Electrochemical Society , VOL 162 (6) , PAGES C238 - C242

State: Published (Approved)
Published: February 2015

Open Access Open Access

Abstract: The effect of nitrate on the salt layers in iron artificial corrosion pits in acidic chloride solutions has been studied using in-situ synchrotron X-ray diffraction. During dissolution in 1 M HCl, there is a salt layer of FeCl2.4H2O on the electrode surface, which is isotropic. With addition of trace nitrate, the salt layer remains FeCl2.4H2O and no nitrate phase is observed, but the diffraction pattern becomes anisotropic, consistent with the formation of platelets with (1 2 0) planes settling horizontally. In nitrate solution containing trace of chloride (0.1 M HNO3 + 10 mM HCl), a salt layer is formed that is isostructural with Co(NO3)2.6H2O, and therefore assumed to be Fe(NO3)2.6H2O. This is the first reported crystal structure of ferrous nitrate. The salt layer is also found to give an anisotropic diffraction pattern, consistent formation of platelets with (0 2 0) planes settling horizontally. It is well known that salt layers can form at the bottom of growing corrosion pits due to supersaturation of metal salts,1–4 and that the presence of salt layers is important for continued pit growth.5 This information is significant both in the field of electrochemical machining (ECM), where nitrate solutions are often used,6–8 and in corrosion of steel in radioactive waste solutions.9–11 These salt layers are a slurry of crystallites that form on a dissolving metal surface12 when the rate of metal ion production (dissolution) is greater than the rate that they can diffuse from the interface, leading to supersaturation and thus crystallite nucleation. The equilibrium thickness of the layer is determined by a self-regulating process.13 The formation of the salt layer leads to a resistance to ion flow in the electrolyte since ions can only flow in channels between the crystallites. This resistance decreases the interfacial potential, decreasing the dissolution rate. The steady state thickness of the salt layer is such that the rate of metal ion production is equal to the rate of escape. Investigations of salt layers in pits can only be carried out in situ, since they dissolve as soon as the interfacial potential driving dissolution is removed. Studies are often carried out in “artificial” corrosion pits, in which a wire or foil is embedded in resin and dissolved back to give a one-dimensional cavity,13–15 which is simpler to study and model. Electrochemical impedance measurements on the salt layer of iron in a chloride solution have led to the suggestion that the salt layer is duplex, comprising a compact semiconducting inner film and a porous outer film.16–18 Rayment et al.12 carried out an in-situ synchrotron study on salt layers on Fe and stainless steel in HCl-containing artificial pits, and in both systems, the salt layers were found to be FeCl2.4H2O, but the size of the crystallites on Fe was much smaller than that on stainless steel. However, despite their practical importance, salt layers in nitrate-containing solutions have received scant attention except in electrochemical machining (ECM) studies. Surface brightening was reported to involve salt precipitation on an iron surface in a nitrate-containing electrolyte.19 It was proposed that, during ECM processing of iron in NaNO3 (potential above 2 V, current density up to 80 A/cm2), a salt layer with a duplex structure was present on iron surfaces, comprising a solid oxide inner film and a supersaturated outer layer, which is a meta-stable viscous solution or molten salt of Fe(NO3)3.9H2O/Fe(NO3)2.6H2O due to Joule heating at high current densities.7,20,21 The lack of free water in the outer layer was presumed to suppress oxygen evolution.8 In this work, the salt layer composition and structure in nitrate-containing solutions have been characterized using in-situ synchrotron X-ray diffraction combined with electrochemical measurements on iron artificial pits

Subject Areas: Chemistry, Materials

Instruments: I18-Microfocus Spectroscopy