STP1295

    Mechanisms of LiOH Degradation and H3BO3Repair of ZrO2 Films

    Published: Jan 1996


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    Abstract

    During a program to elucidate the mechanisms by which LiOH accelerates the corrosion of zirconium alloys and boric acid inhibits this effect, specimens were exposed to 0.01, 0.1, and 1.0 M LiOH solutions at 300°C (573 K) or 360°C (673 K) with and without the addition of boric acid. Results showed that local dissolution of the ZrO2 films formed pores whose depth was a function of the LiOH concentration and probably also of the temperature, alloy composition, and structure. Below a critical LiOH concentration, only superficial porosity was developed in short experiments. Above this critical concentration (which lies between 0.1 and 1.0 M LiOH for Zircaloy-2 at 300°C) porosity develops throughout the initially impervious oxide and no pretransition corrosion kinetics are observed. Below this critical concentration, no effect of LiOH is observed on the pretransition oxidation kinetics until pores and cracks start to develop in the oxide prior to the oxidation rate transition. At this point (1.5 to 1.8 μm) LiOH can concentrate in the freshly developed pores by chemical extraction of water from the solution in the pores to form new ZrO2. Once the critical LiOH concentration is reached in the pores, enlargement or extension of the pores can occur by dissolution. This process should occur beneath the relatively untouched pretransition oxide when the bulk solution is not concentrated enough to attack the oxide surface. Hydrothermal redeposition of much of the dissolved ZrO2 occurs on specimen surfaces or within the porous oxide. Boric acid has no effect on ZrO2 dissolution by LiOH. It is considered that in concentrated solutions the solubility product of some complex lithium zirconate borate can be exceeded and this can plug the pores. In dilute solutions, therefore, boric acid can only operate inside the porous oxide film, where the chemical concentration mechanism should be equally effective for both LiOH and H2BO3. Any Li or B found subsequently in the film will be there as a consequence and not a cause of the corrosion process. It would be expected to occur in at least two forms, Li or B within pores or adsorbed on pore walls, and Li or B that is incorporated in hydrothermally deposited oxide. These two forms of doping would be “leachable” and “non-leachable,” respectively.

    Keywords:

    zirconium alloys, corrosion, high-temperature water, LiOH, boric acid


    Author Information:

    Cox, B
    professor of nuclear engineering and chairman of the Centre, Centre for Nuclear Engineering, University of Toronto, Toronto, Ontario

    Ungurelu, M
    professor of nuclear engineering and chairman of the Centre, Centre for Nuclear Engineering, University of Toronto, Toronto, Ontario

    Wong, Y-M
    professor of nuclear engineering and chairman of the Centre, Centre for Nuclear Engineering, University of Toronto, Toronto, Ontario

    Wu, C
    professor of nuclear engineering and chairman of the Centre, Centre for Nuclear Engineering, University of Toronto, Toronto, Ontario


    Paper ID: STP16170S

    Committee/Subcommittee: B10.01

    DOI: 10.1520/STP16170S


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