CHARACTERISATION OF IRON OXIDE DEPOSITS ON NICKEL-BASED ALLOY TUBES

摘要

In nuclear pressurised water reactors (PWRs), steam generators (SGs) which are composed of thousands of tubes are used as heat exchangers between the primary (inside the tubes) and secondary (outside the tubes) circuits. On the SG secondary side, a fouling phenomenon occurs due to the deposition of corrosion products (mainly composed of iron oxides) at the surface of the tubes. In the long term, this effect could degrade thermal performances and may enhance risks of SG tube cracking. Several studies have been made on the characterisation of these deposits [1], on their formation mechanisms [2], and on their impact on the thermal transfer [3-5]. However, the relation between the deposit morphology and composition, the fouling mechanisms and their effect on the heat transfer efficiency in SG needs further investigations. The aim of this study is to get a detailed characterisation of the morphology, chemistry and crystallography of the deposits formed at the surface of SG tubes. Since PWR SG tube extraction is very difficult, experimental loops have been used to form fouling deposits on Ni-Cr based alloy (Inconel 600) tubes using representative secondary side thermodynamic conditions. As in a SG, two types of deposit conditions are considered: monophasic liquid and biphasic liquid-steam. In the present work, focus was made on morphological multi-scale characterisations. The observation of the surface of various sections of tubes using a FEI Nova NanoSEM 450 SEM and chemical analyses using a Bruker SDD EDS revealed that the deposits consist of geometrically-shaped crystallites containing Fe and O (Fig. 1) and allowed coverage estimations of about 82 %. Those observations were completed with u-Raman mappings to determine the molecular species present in the deposit. Deposit thickness measurements of various sections of tubes were also made from SEM observations in cross-sections to get statistical results giving a mean thickness of 0.97 ± 0.44 nm. As deposits are brittle and sensitive to oxidation, a water-free methodology has been developed for the metallographic preparation of samples, from cutting to polishing step including nickel-plating and cold resin-coating. Then, 3D electron tomography using a FEI Helios 660 dual column scanning electron microscope associated with a focussed ion beam (SEM/FIB) was used to get 3D reconstructions of the deposits over cubic micrometre volumes (Fig. 2). These reconstructions allowed the quantification of porosity, thickness distribution and coverage rate at the local scale.