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

Abstract

MUZIC-2 was the second phase of a large international collaboration between industry and academic institutions seeking to understand the fundamental mechanisms behind aqueous corrosion and hydrogen pick-up of the zirconium alloys used for fuel cladding in Light Water Reactors. The collaboration was led by Westinghouse, and the industrial partners were: the

A high-resolution characterization of the oxide-metal interface in Zircaloy-4 and its relation to the oxidation and hydrogen pickup mechanisms

“…Transition, however, has a marked effect on the hydrogen pickup rate in the two environments, as shown in Figure 16. In most of the pretransition period, the hydrogen picked up is minimal, and similar in the two environments; close to transition, the hydrogen pickup rate increases in both, but becomes around five times higher in pure water than in the high pH environment [32]. This relation (or lack of a relation) between oxidation, pH and hydrogen pickup provides an insight into the nature of the species which carries hydrogen across the oxide into the metal substrate.…”

“…The counts were normalised to wt. ppm using calibrated measurements provided by the Paul Scherrer Institute and Westinghouse performed with Hydrogen Hot Gas Extraction on a selection of samples [35].…”

“…The first environment is pure water at 360°C and the second is replica PWR water with KOH added to increase the environmental pH (i.e. reduce the proton availability in the environment) and, hence, to reduce the hydrogen pickup rate [32] in order to observe any correlations between the substrate oxidation process and hydrogen pickup. KOH was chosen to modify the pH rather than increasing the LiOH content as KOH has been found to affect the oxidation process less than LiOH [33].…”

“…The pH itself has been reported to have no effect on the oxidation rate, unless it is extremely low or extremely high (below 1 or above 13) [6]. It does, however, affect the hydrogen pickup fraction as described in an interim report on this project [32]. Using the two different environments is intended to clarify whether the suboxide interacts with the hydrogen pickup process.…”

“…= 0.42 0.31 = 0.53 0.31 (1) Where ypH is the oxide thickness (in µm) for the high pH environment, ypw is the oxide thickness (in µm) in pure water, and t is the time in days (note that this analysis differs slightly from that in [32]).…”