Zirconium alloys such as Zircaloy-2 and -4 are used as fuel cladding materials in nuclear power plants. These alloys have adequate corrosion resistance in high temperature aqueous conditions. For cladding applications it is desirable that Zr alloys form a thin adherent lustrous oxide covering the entire surface of the component. However, under abnormal conditions spots of accelerated oxidation (corrosion) may develop. The two main anomalous mechanisms are nodular corrosion and shadow corrosion. Shadow corrosion may develop on Zr alloy components that are "facing" in close proximity other materials such as stainless steels or nickel alloys. The mechanism of shadow corrosion is still highly debated but is appears to be similar to a process of galvanic corrosion. Under irradiation the conductivity of the oxide film in Zr alloys increases, and its free corrosion potential drifts in a direction opposite to the direction of the stainless steel or nickel based alloy. This separation in potential may create a singular environment between the two different metals, which may lead to the development of shadow corrosion. That is, shadow corrosion may be a consequence of the unique photo-electrochemical behavior of the oxides that develop over the two types of materials (i.e. Zr alloys and stainless steels, for example). This article presents a critical review of the photo-electrochemistry characteristics of the passive film on Zr alloys and its impact on the modes of corrosion ofZr alloys.
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