Recent studies have suggested that hydride precipitation at the metal/oxide interface could play a detrimental role on the waterside corrosion rate. Nevertheless, the mechanism of that detrimental role is not completely understood, and two hypotheses were investigated to understand the mechanism that controls the role of the hydrides. The first hypothesis is based on a mechanical effect: the hydrides precipitate at the metal/oxide interface and destroy the physical integrity of the “barrier” oxide layer. The second hypothesis is a modification of the transport properties of the oxide grown on the hydrided metal.
The detrimental role of hydrides on the corrosion rate was studied by charging unirradiated Zircaloy-4 cladding material with hydrogen to a level higher than the limit of solubility at 400°C. Both gaseous and cathodic charging techniques were used. Static corrosion tests were carried out in autoclave with steam at 400°C on an as-received and hydrided sample. The detrimental role of hydrides is confirmed for the post-transition corrosion rate, and that effect is more significant for high cathodic charging.
The results of the metallurgical examinations are discussed to provide an understanding of the mechanism. No relationship between hydrides, physical defects in the oxide, and local corrosion rate enhancement was found. Therefore, our results do not support the hypothesis of a mechanical effect at the scale of the performed examinations, but more detailed work is required to confirm this.