DEVELOPMENT AND PROPERTIES OF URANIUM-BASE ALLOYS CORROSION RESISTANT IN HIGH TEMPERATURE WATER. PART III. CORROSION MECHANISM OF URANIUM-BASE ALLOYS IN HIGH TEMPERATURE WATER

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Abstract

The factors affecting corrosion resistance both of bare and of clad uranium-base alloys are reviewed and a mechanism proposed for their corrosion behavior. For unclad gamma-phase uranium alloys exposed to a high temperature water corrodent, it is proposed that the corrosion rate is determined primarily by the oxidation of the alloy by water. This behavior is contrary to that of alpha uranium in which the corrosion rate is primarily determined by the formation and subsequent oxidation of a nonadherent hydride layer. In gamma- phase alloys the hydrogen released by the corrosion reaction, rather than forming the thermodynamically stable UH/sub 3/ phase, dissolves (at least in part) in the base metal where it precipitates as a metastable hydride. The amount of hydrogen absorbed by the metal and hence precipitating as the metastable hydride may be markedly reduced by the addition of hydrogen depolarizers such as nickel or platinum to the water or to the metal. Similarly the amount of absorbed hydrogen may be reduced by introducing sinks that preferentially absorb hydrogen. Both alpha uranium and zirconium were shown to be suitable sinks. Precipitated hydride hardens and embrittles the matrix and by its preferential corrosion eventually leads to discontinuous failure. The hydride may be made to precipitate in a less harmful manner by heat treatments which precipitate nucleating and hardening impurities. The mode of the precipitation is shown to be sensitive to stress; in fact, the assumption of elastic stress as contributing to hydride precipitation is considered necessary to explain the distribution of the precipitate during corrosion. In order to apply a gammaphase fuel alloy as a fuel element material, it is necessary primarily to reduce the amount of hydrogen absorbed by the fuel. This can be done by cladding the fuel with a zirconium- base alloy. Under such conditions it has been shown that fuel element lives in excess of 4 years should be obtainable with properly fabricated fuel elements before they are subject to corrosion failure. Optimum fuel element corrosion life can be achieved by decreasing the general corrosion rate, increasing the hydrogen solubility, increasing the hydrogen diffusion rate, and maintaining a proper clad-fuel bond. For uranium-base alloys such as U/sub 3/Si that do not form a hydride during hot water corrosion, use of a Zircaloy clad is unnecessary. Therefore, the development of alternate cladding materials such as Al for corrosion resistant fuel elements is predicted upon the development of uranium- base alloys that do not corrode by a hydride mechanism. In addition to U/sub 3/ both Zr-U alloys and the strained alpha uranium-base alloys do not appear to corrode by a hydride mechanism. (auth)

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