Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom Microstructure and spatial distribution of corrosion products anodically grown on zinc in chloride solutions M. Prestat a,⁎ , L. Holzer b , B. Lescop c , S. Rioual c , C. Zaubitzer d , E. Diler a , D. Thierry a a French Corrosion Institute, 220 rue Pierre Rivoalon, 29200 Brest, France b Institute of Computational Physics, Zurich University of Applied Sciences, Wildbachstrasse 21, 8400 Winterthur, Switzerland c Lab-STICC, Université de Bretagne Occidentale, 6 Avenue Le Gorgeu, 29285 Brest, France d ETH Zurich, ScopeM, Auguste-Piccard-Hof 1, 8093 Zurich, Switzerland ARTICLE INFO Keywords: Simonkolleite Zinc oxide Microstructure Corrosion ABSTRACT Zinc substrates were electrochemically oxidized in NaCl solution to produce corrosion patinas. XRD, XPS and Raman analyses enabled the identification of simonkolleite and zinc oxide as the patina constituents. FIB-SEM imaging shows that the upper part of the patinas is a network of simonkolleite nanosheets with an open mi- crostructure that is unlikely to act as a significant barrier for corrosion processes. STEM investigations and Raman mapping measurements reveal the presence of a ca. 20–400 nm thin nanoporous ZnO-rich film below the simonkolleite and covering the zinc substrate. Under potentiostatic conditions, the reduced cathodic activity of the patina-covered zinc electrodes is assigned to this nanoporous ZnO layer. 1. Introduction The ability of zinc (and its alloys) to achieve self-protection with patinas formed by corrosion products (CP) has been an important topic of research [1–3,6]. Yet there is still a significant lack of knowledge regarding the microstructure of corrosion products (CP) and the influ- ence thereof on corrosion resistance of zinc. The latter is often ex- pressed in terms of “barrier effect” that may hinder the transport of electroactive species around the electrode/electrolyte interface [3,4,7]. The concept of physical barrier is therefore directly related to the CP microstructure. However, in the field of zinc corrosion, few in-depth investigations have been dedicated to that matter [8–9]. This stands in contrast to other fields of electrochemistry in which microstructure plays a paramount role and cross-sectional electron microscopy ana- lyses are regularly performed [10–13]. Simonkolleite (Zn 5 (OH) 8 Cl 2 ·H 2 O) is one of the main corrosion pro- ducts of zinc in chloride-containing environment [4,14–17]. However, its protective nature has not been unambiguously elucidated notably because limited microstructural data is available in the literature. Re- cently, Joo et al. investigated the corrosion of galvanized steel covered by synthetic simonkolleite obtained by dissolution of the zinc coating [4,5]. The pertinent rationale of their work was to study a single CP in order to better understand its protective properties. Yet ZnO was also detected along with simonkolleite and the microstructural analysis was limited to top-view and tilted SEM micrographs with relatively low magnification. Therefore the microstructure effect of each CP could not be clearly assessed. In this communication, zinc is anodically polarized in NaCl elec- trolyte. Stress is laid on cross-sectional electron microscopy analysis for gaining valuable insight on the microstructure, spatial distribution and barrier effect of the CP (simonkolleite and ZnO). 2. Experimental 2.1. Electrochemical cell All electrochemical experiments were carried out at room tem- perature using a three-electrode configuration in a Teflon cell at the bottom of which the working electrode was placed with a circular area (diameter 1.5 cm) exposed to the unstirred electrolyte. A zinc current collector was clamped below the substrate. All potentials were mea- sured versus a saturated calomel electrode. The counter-electrode was a titanium-based metallic grid placed parallel at ca. 2 cm above the disk. 2.2. Synthesis of patinas Zinc substrates (Rheinzink, > 99%) were grinded with SiC papers until grade P4000 with water lubricant and cleaned with ultrasound. Based on the work of Joo et al. [4], the simonkolleite layers were synthesized at -0.64 V (open circuit potential: -0.99 V) in 0.1 M http://dx.doi.org/10.1016/j.elecom.2017.06.004 Received 16 May 2017; Received in revised form 1 June 2017; Accepted 5 June 2017 ⁎ Corresponding author. E-mail address: michel.prestat@institut-corrosion.fr (M. Prestat). Electrochemistry Communications 81 (2017) 56–60 Available online 06 June 2017 1388-2481/ © 2017 Elsevier B.V. All rights reserved. MARK