Mechanisms of LiOH Degradation and H3BO3Repair of ZrO2 Films

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.

The Distribution of Li Ions in the Oxide Film Formed on Zircaloy-4 Corroded in Lithiated Water at 633 K

“…The findings of this study help to better understand the promotion of the enlargement or extension of the pores in the oxide film formed on the specimens corroded in lithiated water, as suggested by Cox et al [1][2][3], and also the Li ions could modify the characteristics of the inner barrier layer of the oxide film to prompt the disappearance of the dense barrier layer, proposed by Pêcheur et al [7,8], can also be better understood in light of such a pattern of Li ion distribution in oxide.…”

“…Up to now, several mechanisms regarding the effect of lithiated water on the corrosion resistance of zirconium alloys have been proposed. For example, Cox et al [1][2][3] suggested that m-ZrO 2 or t-ZrO 2 were present prior to dissolution in LiOH solution and where many pores formed in the oxide film, then the corrosion rate of zirconium alloys in lithiated water was accelerated. However, Liu et al [4] raised a query where the driving force for the transport of dissolved ZrO 2 to the solution came from.…”

Effect of Acidic and Alkaline Solutions on Electrochemical Corrosion of Zircaloy-2

“…1), the corrosion of Zircaloy was considered to proceed by the diffusion of oxygen ions across the oxide layer [18]. Accelerated corrosion of Zircaloy by porosity generation was suggested by others [11,19]. For the corrosion tests performed in 1M NaOH and 1M KOH as compared to LiOH, there was no accelerated corrosion observed with the addition of NaOH or KOH.…”

“…In order to explain the accelerated corrosion of cladding in reactor, several hypotheses were suggested such as corrosion hydrogenprecipitation [4], radiation effects in the oxide films [5], metallurgical variables(tin and intermetallic particles) [2,6,7], ZrO 2 film thermal conductivity (oxide thicknesses effect) and thermal feedback [2,8], and primary coolant chemistry (lithium, boron, and oxygen content) [6,8,9]. The argument for the possibility of an increase in rate of corrosion due to lithium in-reactor isbased on many hyposises [3, 6, and 10].Chemical concentration process which extracts water from the solution in the pores to form additional ZrO 2 also act as a concentration mechanism [11]. Many investigations have been documented; evidence from these studies for a lithium-related acceleration of zirconium alloys is somewhat conflicting.…”

Effect of pH on Hydrogen Pick-Up and Corrosion in Zircaloy-4

“…77,78 Figure Cox reported tests to determine how adding boric acid to LiOH solutions affected the corrosion. In low LiOH solutions boric acid was found to have little effect on the corrosion 78,79 , but in 1.0 M LiOH the presence of boric acid inhibited the generation of deep pores. It was suggested that this is due to the precipitation of a complex LixZryBz salt, which plugs deep pores as they form and prevents a deep pore network forming.…”