Investigating the Influence of Pre-Grown Copper Oxide Film on Sulfide-Induced Corrosion of Copper
Authors: Bergendal Erik , Salehi Alaei Elham , Lilja Christina , Chen Jian , Behazin Mehran , Shoesmith David , Noël James J. 
Published: February 2024

Used nuclear fuel should be confined safely and permanently due to its detrimental effect on human health and the environment. The universally proposed plan for the permanent disposal of used nuclear fuels is to bury the containers containing the nuclear used fuels in a multi-barrier system known as a deep geological repository (DGR) at a depth 500 m underground. The container is the key engineered barrier that can withstand long-term mechanical loads and the corrosive environment. The proposed container in some countries will be designed with either a cylindrical Cu shell containing a nodular cast iron insert (Sweden, Finland) or a Cu‐coated carbon steel vessel (Canada). After emplacement, the container will experience a range of conditions underground, evolving from warm, humid, and oxidizing to cool, dry, and anoxic over time.
During the initial oxidizing period, the entrained oxygen, trapped in the DGR upon sealing, and the water-radiolysis will lead to the formation of an oxide/hydroxide film on the Cu container surface. Once anoxic conditions are eventually established, sulfide ions produced by the action of sulfate-reducing bacteria remote from the container will become the main potential oxidant.
So far, considerable efforts have been devoted to investigating exclusively either the oxic period or the anoxic period. However, the significant gap that the current research aims to address is how the early period of oxide growth affects the later stages of the repository evolution, at which time the primary threat to the integrity of the Cu container would be sulfide ions within the groundwaters. In this series of experiments, various methods were applied to create different types of copper oxide/hydroxide layers with known compositions and unique structures, to investigate their role in the subsequent sulfide-induced corrosion of the underlying Cu under de-aerated conditions. The morphology of the surface oxide film and the nature and concentration of sulfide species will influence the possible manners in which the sulfide species could interact with the oxide film; for example:

Chemical conversion of copper oxide to copper sulfide

Galvanically-coupled processes between the copper oxide reduction and Cu oxidation by sulfide

Direct corrosion of Cu by sulfide species arriving at the metal surface

Our results have shown that regardless of the composition and nature of the oxide film, either a single-phase oxide film or a dual layer of combined copper oxides, the pre-grown oxide film on the Cu surface is partially converted to a copper sulfide film via chemical conversion and/or galvanically-coupled processes. Moreover, an unreacted remnant of the oxide layer was detected on the surface, surviving for longer than the duration of our experiments. This remnant oxide is non-protective, and permits direct corrosion of Cu by sulfide species arriving at the metal surface.
Oxide films were grown either electrochemically, radiolytically, or chemically. The electrochemically-grown oxides, upon exposure to sulfide solutions, underwent quick conversion to copper sulfide (Cu2S) as shown by the corrosion potential (Ecorr) measurements and cathodic stripping voltammetry (CSV) at different immersion times. The same results were observed for the radiolytically-grown oxide with a multi-layer structure composed of Cu(I) and Cu(II) oxides. The thickness of the two mentioned oxides was on the nanometre scale.
Chemically grown oxides were thicker than the electrochemically- and radiolytically-grown oxides. The results of CSVs showed that some parts of the oxide film and the sulfide film formed on top of the oxide film could not be cathodically stripped from the surface and remained in the form of a defective film, and that direct corrosion of Cu occurred, driven by sulfide species reaching the metal surface. This suggests that conversion of oxide to sulfide films is fastest when an electrochemical pathway is available, and much slower if it must depend on the chemical pathway. The regions of the oxide which were not connected electrically to the surface could only undergo chemical conversion. The results obtained from immersion of copper oxide powder in sulfide solution support the idea that the chemical conversion is not as quick as the electrochemical reaction in the presence of copper metal.