Innovations in Corrosion and Materials Science   Send Orders for Reprints to reprints@benthamscience.ae 108 Innovations in Corrosion and Materials Science, 2018, 8, 108-112 RESEARCH ARTICLE The Mechanism of Sulphide Film Growth on Copper in Anaerobic Sulphide Solutions Under Natural Corrosion Conditions J. Chen * , Z. Qin and D.W. Shoesmith Department of Chemistry and Surface Science Western, Western University, London, ON, N6A 5B7, Canada  A R T I C L E H I S T O R Y Received: February 08, 2018 Revised: September 12, 2018 Accepted: October 19, 2018 DOI: 10.2174/2352094909666181126152106 Abstract: Background and Method: The growth mechanism of sulphide films formed on copper in anaerobic 0.1 M NaCl + 5 x 10 -4 M Na2S solution has been investigated under natural corrosion con- ditions for exposure periods up to 1691 hours using scanning electron microscopy, focused ion beam cross-sectioning, and a Au marker procedure. Results and Conclusions: The film formed by a chemical deposition process via an outward growth mechanism. This process was controlled by cuprous ion transport in the film combined with sulphide diffusion in solution. Keywords: Sulphide film, copper, corrosion, diffusion, crystal growth, microscopy. 1. INTRODUCTION Since copper is stable under aqueous anaerobic condi- tions, it is a primary candidate material for the fabrication of high level nuclear waste containers to be emplaced in deep geologic waste repositories. Once the oxygen trapped in the groundwater on repository closure is consumed, the only available oxidant would be sulphide, present in groundwater due to either mineral dissolution (i.e. pyrite, FeS 2 ) or pro- duced by the action of sulphate reducing bacteria. Corrosion would then be controlled by either the diffusive transport of sulphide through the compacted clay buffer surrounding the canister, or by the expected protective properties of the cop- per sulphide layer on the canister surface [1]. This makes a knowledge of the growth mechanism of the sulphide film formed on the copper surface under natural corrosion condi- tions essential for the performance assessment of copper nuclear waste canisters under deep geologic repository con- ditions. While studies on copper corrosion in anaerobic sulphide solutions under either electrochemically-controlled [2-7] or natural corrosion conditions [1, 8, 9] indicate that the sul- phide film formed on Cuin aqueous sulphide solutions grows at the film/solution interface, direct experimental evidence has not been presented. In this paper, we use a Au-marker procedure coupled with scanning electron mi- croscopy (SEM) on corroded surfaces and focused ion beam (FIB) cut cross sections, to determine the sulphide film growth mechanism on copper corroding in anaerobic aqueous sulphide solutions. *Address correspondence to this author at the Department of Chemistry and Surface Science Western, Western University, London, Ontario, Canada, N6A 5B7; E-mail: jchen496@uwo.ca 2. EXPERIMENTAL PROCEDURES Experiments were conducted using phosphorous-doped (40-60 ppm), oxygen-free copper provided by the Swedish Nuclear Fuel and Waste Management Co. (SKB), Stock- holm, Sweden. The working electrode was a copper disk, with a diameter of 1 cm, threaded into a stainless steel shaft. The electrode and shaft were painted with non-conductive lacquer to prevent contact of the Cu/steel junction with the aqueous solution. After painting, the electrode was heated at 60°C for 12 h to promote adhesion between the paint and the sample. The exposed Cu surface was polished successively with 240, 600, 800, 1000, 1200 grade SiC paper and then to a mirror finish using 1 μm, 0.3 μm, and 0.05 μm Al 2 O 3 sus- pensions. Prior to experiments, the electrode was washed with Type I water (resistivity of 18.2 MΩ⋅cm) purified using a Millipore-Q plus unit, ultrasonically cleaned in methanol, washed again with Type 1 water, and finally dried using ar- gon gas. All experiments were performed within an argon purged anaerobic chamber (Canadian Vacuum Systems Ltd.) main- tained at a positive pressure (2-4 mbar) by an MBraun glove box control system to ensure anoxic conditions. The oxygen concentration in the chamber was analyzed with an MBraun oxygen probe with a detection limit of 1.4 mg/m 3 . The an- aerobic chamber was maintained at a total oxygen concentra- tion ≤ 4.2 mg/m 3 , which includes the oxygen in both air and vapor. The actual oxygen content of the solution would have been less than this value. Even though a trace amount of oxygen may have been present, copper sulphide is more sta- ble in an aqueous sulphide solution than copper oxide based on thermodynamic data (ΔG° = -101.46 kJ/mol for the con- version of Cu 2 O to Cu 2 S in sulphide solutions at 298 K [10]: 2352-0957/18 $58.00+.00 © 2018 Bentham Science Publishers Personal Use Only Not For Distribution