BENTONITE INTERACTION WITH CORRODED STEEL IN SALINE SOLUTION

Phenomena at the interface between metal oxides, clay minerals, and water play an important role in various areas such as metal corrosion, plant decontamination, heterogeneous catalysis, or environmental science. Studying corrosion of steels used for high-level radioactive waste containers buried in bentonite-rich environments is fundamental to prevent leakages that can be a potential danger for the environment. The goal of the research is to determine if the Fe ions from the corroded steel in contact with saline water are adsorbed into the bentonite interlayer or Fe oxides coat the clay particles using experimental corrosion test cases. Five batches were prepared: (i) a beaker contains a 5 cm long corroded steel bar rod submerse in a 1M saline solution; (ii) two beakers contain two brands of industrial grade bentonite (Sigma-Aldrich and Lovell), placed in contact with 1M saline solutions with a 5 cm long corroded steel bar rod, and (iii) two beakers contain industrial grade bentonite from Sigma-Aldrich and Lovell with only 1M saline solution. All beakers were covered with Parafilm to avoid air contamination. A sample from each batch was picked up with a pipette every two weeks to determine the way the bentonite reacted over time. X-ray powder diffraction (XRPD) was used to identify the minerals in bentonite and corroded phases in the steel rod. XRD pattern of the corroded coating on the steel rod show lepidocrocite, magnetite, and hematite. XRD pattern of Sigma-Aldrich bentonite show montmorillonite, illite, quartz, and plagioclase. XRD pattern of Lovell bentonite show montmorillonite, illite, quartz, plagioclase, and heulandite. Comparison of the XRD patterns of samples collected every two weeks show that peaks of lepidocrocite and hematite are observed in XRD pattern of both bentonite samples in saline solutions in contact with corroded steel rod collected after 6, 8, and 10 weeks. Scanning Electron Microscope (SEM) was used to determine the chemical composition and the morphology of bentonite and Fe-oxides. The SEM analyses show that Fe oxides particles from the corroded steel rod sit on bentonite flakes. XRD and SEM analyses show that Fe is not exchanged into the bentonite structure. The bentonite is not furthering the corrosion of the steel; however, there is a significant progressive corrosion of the steel at the clay-steel interface due to the high salinity of water solution.