Experimental investigaiion of corrosion rates and mechanisms in Boral

Abstract

Boral is a material finding wide application in the nuclear industry as a neutron shielding medium. It is produced as a laminate comprising two aluminium plates enclosing a centrat layer of boron carbide and aluminium granules, fused together and fused to the aluminium plates. Many appiications of Boral bring it into contact with water and other plant materials, often stainless steel; it is in this context that the corrosion of Boral has been identified as needing investigation. The corrosion of Boral has been found to proceed mainly by a specialized mechanism, where microcircuits are set up in the laminate since the exposed boron carbide and aluminium cladding are mutually anodic and cathodic, respectively. When set in a corrosion couple with stainless steel, the cathodic production of hydrogen is found to occur almost exclusively at the stainless steel surface, while the cathodic reduction of oxygen occurs at the boron carbide surface. The corrosion currenss which may be deduced as flowing in microcircuits greatly exceed the corrosion currents observdd to flow in an external circuit with stainless steel. This finding, and in particular the localized nature of the procssses implied, indicates that the consequences of the corrosion of Boral may be more severe than has previously been recognized. Comparisons are presented for the rates of formation of hydrogen and removll of oxygen, in air-ullaged† system. The effects of using an inert ullage gas, such as argon, and demineralized or boronated water are discussed; the findings are consistent with recently developdd theory for modelling corrosion and radiolysis in laboratory experiments.