Several mechanism-related aspects of the corrosion of zirconium alloys have been investigated using different examination techniques. The microstructure of different types of oxide layers was analyzed by transmission electron microscopy (TEM). Uniform oxide mainly consists of m-ZrO2 and a smaller fraction of t-ZrO2 with columnar grains and some amount of equiaxed crystallites. Nodular oxides show a high open porosity and the grain shape tends to the equiaxed type. A fine network of pores along grain boundaries was found in oxides grown in water containing lithium. An enrichment of lithium within such oxides could be found by glow discharge optical spectroscopy (GD-OES) depth profiling. In all oxides, a compact, void-free oxide layer was observed at the metal/oxide interface. Compressive stresses within the oxide layer measured by an X-ray diffraction technique were significantly higher compared to previously published values. Electrical potential measurements on oxide scales showed the influence of the intermetallic precipitates on the potential drop across the oxide. In long-time corrosion tests of Zircaloy with varying temperatures, memory effects caused by the cyclic formation of barrier layers could be observed. It was concluded that the corrosion mechanism of zirconium-based alloys is a barrier-layer controlled process. The protective properties of this barrier layer determine the overall corrosion resistance of zirconium alloys.

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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.

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The corrosion behavior of Zircaloy-type alloys with different tin contents of 1.55, 0.70, and 0.55 wt% was studied at 350°C and 17 MPa in an environment similar to PWR primary water. For this non-interrupted test, a special autoclave system was used that was equipped with electrical feed that allowed followup on the growth of oxide layers by impedance spectroscopy and corrosion potential measurement at high temperature and pressure. As a reference electrode, a platinum wire was used that works as a hydrogen electrode according to the hydrogenated environment established during the start-up procedure. The test ran without interruption for 471 days. Impedance spectra were taken at time intervals and evaluated for thickness and morphology of the oxide layer as well as for its electrical resistance. The tests without any temperature and pressure cycling showed similar oxidation behavior with repeated transitions as in discontinuously performed standard autoclave tests. Early in the pre-transition range, a dense oxide layer is formed, and fast changes of corrosion potential and electrical resistance are observed. The dense layer increases in thickness and homogeneity up to the transition, where a sudden breakdown occurs. Abrupt changes of the corrosion potential and electrical resistance were observed also at those points. After transition, a new dense layer is built up. The corrosion potential changes are caused by a decrease of the electrical corrosion current with increasing oxide layer thickness, by the formation of a potential drop over the high-resistance dense oxide layer, and by structural changes at the transition points. In general, alloys with different tin contents show similar behavior. However, they show differences in the times to transition, the kinetic constants deduced from their impedance spectra, and in the ionic and electronic resistance of the dense inner layer controlling corrosion.