Corrosion of beryllium in EDM-1 fluid after γ pre-irradiation

Beryllium is one of the most important materials in particle physics and nuclear physics experiments. Among these applications, it is used as the material in particle collision tubes, such as the beam pipe in the operational Beijing Electron and Positron Collider (BEPC Ⅱ) and in the Circular Electron Positron Collider (CEPC) currently in the planning stage. High-speed particles produce large amounts of γ irradiation and impose a heat load on the beam pipe. The beam pipe must be cooled by the scouring fluid to maintain a stable temperature for particle detection; this cooling process will induce fluid erosion of the beam pipe. The corrosion properties of materials in contact with the oil No. 1 for electric discharge machining (EDM-1) fluid under irradiation are not yet known. A device for testing pipeline corrosion was built to study the corrosion of beryllium in EDM-1 fluid after γ pre-irradiation. The mass of the sample was measured by an electronic balance, and the surface morphologies and composition were examined by scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The results show that the corrosion of beryllium in EDM-1 is affected by two corrosion mechanisms:erosion and chemical corrosion. Erosion is mainly influenced by the surface morphology of the sample, whereas the chemical corrosion is mainly influenced by the dose of γ irradiation, impurity elements in the sample, and organic sulfides in EDM-1. Measurements of the sample before and after irradiation reveal that the mass decreases, then increases, and then decreases again under the combined effects of the two kinds of corrosion. The corrosion rate increases substantially with increasing radiation dose, and γ pre-irradiation promotes pitting nucleation and the formation of pitting holes in beryllium in EDM-1. After 2880 h of corrosion, the sample not subjected to pre-irradiation exhibits only obvious pitting nuclei, whereas some of the pitting nuclei on the sample subjected to 200 and 100 kGy of pre-irradiation develop pitting holes; the diameter of the pitting holes in the former case is approximately twice that of the pitting holes in the latter case. The larger the radiation dose, the earlier the pitting occurs and the larger the diameter of the corrosion holes. Impurity elements (e. g., Al, Si, Fe, Cr, and Ti) and S appear in the pitting nuclei and pitting holes. The impurity elements in the beryllium samples are important factors to induce pitting. The chemical reactions of organo-sulfur compounds produce SO₂ and SO_x(SO₂, SO₃, and SO₄) in the pitting holes by physical and chemical adsorption, which promotes the formation and expansion of pitting.