Corrosion Behavior and Sacrificial Properties of Zn and Zn-Al Coatings in Conditions Simulating Deep Geological Disposal of Radioactive Waste at 80 °C

Abstract

In France, it is planned to manage high-level radioactive waste via deep geological disposal. The carbon steel overpacks containing the waste will face corrosion processes specific to the unusual environment of the disposal. To protect these overpacks against corrosion, the use of metallic sacrificial coatings was considered as a possible solution. Therefore, the corrosion behavior of thermally sprayed Zn and Zn-15wt.%Al coatings (500 µm thick) was studied in a solution simulating the specific environment of the disposal. Galvanic corrosion experiments were performed with a coated steel electrode connected to a bare steel one, which simulated a coating defect. First, it was observed that the Zn coating had lost its sacrificial properties after ~2 months of coupling. XRD and µ-Raman analysis carried out after the experiment demonstrated that the coating was covered with a protective hemimorphite (Zn4Si2O7(OH)2·H2O)-rich layer. In contrast, the Zn-15wt.%Al coating kept its sacrificial properties all along the 6-month experiment. It proved to be covered with a layer mainly composed of Zn and Zn-Al hydroxysulfates, which did not contain hemimorphite. However, SEM cross-sectional observations showed that the electrolyte had seeped into the coating and had even reached the steel surface. This infiltration led to the oxidation of the splats inside the coating, even close to the steel surface, and induced a large swelling of the coating. Its thickness was increased by 50%.