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Several attempts to develop zirconium alloys suitable for use in superheated steam have been described (1-4). Assessment (5,6) of the results suggested that although a number of alloying additions can have a beneficial effect, great variability confuses the results, which, moreover, are no better than would be achieved by unalloyed zirconium in dry oxygen. Further, it appeared that only a low concentration of the added elements, provided it was distributed appropriately in the oxide film, might suffice to give corrosion resistance (2); it was also suggested (6) that the significant film breakdown in steam occurred on a very small scale. The behavior of resistant alloys was pictured in terms of a balance between film growth and film breakdown, leading beyond weight gains of 100 mg/dm2 to a more or less linear rate of corrosion. This paper reports recent attempts to study these two processes more intimately, using thinner films on unalloyed zirconium, and exploring some newer techniques. It may be said that zirconium, quite apart from its nuclear interest, is a rather suitable metal for such studies, forming but one oxide and having only a single valence. Moreover, although the entry of small amounts of hydrogen into the oxide is important, the complication of hydrated oxides is absent. Inasmuch as the aim is to give an up-to-date report of current work and thought, loose ends and tentative explanations may be expected in the following account.
Wanklyn, J. N.
Atomic Energy Research Establishment, Harwell, Berkshire