During corrosion of Zr alloys in pressurized water at high temperatures a fraction of the corrosion-hydrogen is picked up by the metal. Long-term out-of-pile corrosion experiments have shown that chemical composition of Zr alloys and the size of second-phase particles (SPP) in Zircaloy-4 (Zry-4) affect the corrosion and the corrosion-hydrogen pickup fraction. The mechanism of hydrogen pickup is not well understood, although several influencing parameters were evaluated or discussed in the literature. One of the parameters that might influence hydrogen pickup is the electrical potential gradient that develops over the oxide during corrosion. Long-term electrochemical measurements of Zry-4 samples with different SPP sizes and Fe content and of Zr-2.5Nb in pressurized water at 350°C with and without polarization were used to check this influence.

The potential difference between the reaction interface and the oxide surface is due to the oxidation reaction of the Zr metal resulting in electrons that have to move through the highly resistive oxide to the surface. Tests without polarization showed the potential difference proportional to the corrosion rate and depending on metallurgical aspects as the alloy composition and the SPP size. The lowest potential difference has been found for Zry-type material with large SPP and for Zr-2.5Nb.

A negative polarization voltage of the samples against a Pt-reference electrode increases the H pick up and even leads to an accelerated corrosion at large potential differences. Analysis of H pickup clearly shows that, besides corrosion-H, H from the electrochemical surface reaction is also picked up.

Samples with oxide layers exhibiting high electrical resistance pick up relatively more H than samples exhibiting oxide layers with low resistance. Zr-2.5Nb forming a very low-resistant oxide layer picks up only very little H. The effect of the SPP sizes can, at least partially, be explained by their influence on the electrical resistance of the oxide layer.

The results of this study identify the potential gradient formed over the oxide layer as an important parameter for the relative amount of H pickup.