A Study into the Localized Corrosion of Magnesium Alloy Magnox Al-80 Ronald N. Clark, ‡, * James Humpage,** Robert Burrows,* Hugh Godfrey,*** Mustufa Sagir,**** and Geraint Williams** Magnesium (Mg) nonoxidizing alloy, known as Magnox, was historically used as a fuel cladding material for the first generation of carbon dioxide gas-cooled nuclear reactors in the United Kingdom. Waste Magnox is currently stored in cooling ponds, pending final disposal. The corrosion resistance of Mg and its alloys is relatively poor compared to modern cladding materials, such as zirconium alloys, so it is important to have a knowledge of the chloride concentration/pH dependence on breakdown and localized corrosion characteristics prior to waste retrievals taking place. These results show that Magnox exhibits passivity in high-pH solutions, with charge transfer resistance and passive film thicknesses showing an increase with immersion time. When chloride is added to the system, the higher pH maintains Magnox passivity, as shown through a combination of potentiodynamic and time-lapse/post-corrosion imaging experiments. Potentiodynamic polarization of Magnox reveals a -229 mV/decade linear dependence of breakdown potential with chloride ion concentration. The use of the scanning vibrating electrode technique enabled the localized corrosion characteristics to be followed. At high pH where Magnox is passive, at low chloride concentrations, the anodes that form predominantly couple to the visually intact surface in the vicinity of the anode. The high pH, however, means that visually intact Magnox in the vicinity of the anode is less prone to breakdown, restricting anode propagation such that the anodes remain largely static. In high-chloride concentrations, the higher conductivity means that the anode and cathode can couple over greater distances, and so propagation along the surface can occur at a much faster rate, with the visually intact surface acting as a distributed cathode. In addition, the chloride anion itself, when present at high concentration, will play a role in rapid passive film dissolution, enabling rapid anode propagation. KEY WORDS: chloride ion concentration, electrochemical impedance spectroscopy, focused ion beam, magnesium, magnox, pH, potentiodynamic, scanning electron microscopy, scanning vibrating electrode technique, temperature, x-ray diffraction INTRODUCTION A key aspect to the safe cleanup and decommissioning of the Magnox Swarf Storage Silo (MSSS), located at Sellafield in the United Kingdom, is the retrieval, sorting, and repacka- ging of a large quantity of legacy nuclear waste currently stored within the silo. The bulk of the legacy waste is a magnesium (Mg) nonoxidizing alloy, more commonly known as Magnox, which was used to clad uranium (U) metal fuel from the first gener- ation of United Kingdom carbon dioxide (CO 2 ) gas-cooled nuclear reactors. These reactors were originally constructed in the 1950s, and the last ceased its operations in 2015. Mg was used as a cladding material in these reactors for a number of reasons, such as: a low-neutron cross section (transparent to neutrons), resistance to oxidization in CO 2 , chemical com- patibility with U, and sufficient ductility to withstand the stresses caused by the fuel during irradiation. These are in addition to the ease of machining and welding. The major drawback of using Mg for cladding fuel is its poor corrosion resistance during aqueous storage at the end of useful service life in reactor. In 2019, the United Kingdom completed defueling of all Magnox-type reactors, and in line with the U.K. Magnox operating plan, all spent fuel from these reactors is to be reprocessed by the end of 2020. 1-2 Before the fuel can be reprocessed, the Magnox cladding is stripped from the spent fuel. Currently, the waste cladding, which is known as Magnox swarf, is encapsulated in cement. From the mid-1960s to around the late 1980s, however, the swarf was stored underwater at MSSS. To decommission MSSS where Magnox and its corrosion products are stored, the waste will need to be retrieved and repackaged into modern duplex stainless steel storage con- tainers, and then interim stored until a U.K. geological disposal facility (GDF) becomes available. The expected corrosion rate over the interim storage period, and therefore the waste expansion due to the formation of voluminous corrosion products within the containers, will need to be considered before the containers are filled, so that the waste can be stored safely. There is an opportunity to optimize the waste fill levels, such that the number of containers required may be reduced. To do so, however, further information on the future corrosion rate of Magnox is required. Mg is thermodynamically very active; it oxidizes rapidly, spontaneously forming a thin Mg oxide (MgO) film in dry oxygenated environments. 3 Based on thermodynamics, the oxidized states of Mg (Mg 2+ , MgO, and magnesium hydroxide [Mg(OH) 2 ]) are more stable than that of Mg metal, and of these, Submitted for publication: April 23, 2020. Revised and accepted: August 21, 2020. Preprint available online: August 21, 2020, https://doi.org/10.5006/3574. ‡ Corresponding author. E-mail: ronald.clark@uknnl.com. * National Nuclear Laboratory, Unit 102B, Sperry Way, National Nuclear Laboratory, Stonehouse, GL10 3UT, U.K. ** Swansea University, Materials Research Centre, Bay Campus, Fabian Way, Crymlyn Burrows, Swansea, SA1 8EN, Wales, U.K. *** National Nuclear Laboratory, Workington Laboratory, Havelock Road, Derwent Howe, Workington, Cumbria, CA14 3YQ, U.K. **** Sellafield Limited, Hinton House, Birchwood Park Avenue, Risley, Warrington, Cheshire, WA3 6GR, U.K. SCIENCE SECTION 168 FEBRUARY 2021 • Vol. 77 • Issue 2 ISSN 0010-9312 (print), 1938-159X (online) © 2021 NACE International. Reproduction or redistribution of this article in any form is prohibited without express permission from the publisher. CORROSIONJOURNAL.ORG Downloaded from http://meridian.allenpress.com/corrosion/article-pdf/77/2/168/2750018/3574.pdf by China Metallurgical Standardization Research Institute user on 25 October 2022