A Corrosion Localization Assessment of the Mild Steel Used for Nuclear Waste Package
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A CORROSION LOCALIZATION ASSESSMENT OF THE MILD STEEL USED FOR NUCLEAR WASTE PACKAGE MASATSUNE AKASHI, TAKANORI FUKUDA AND HIROSHI YONEYAMA Ishikawajima-HarimaHeavy Industries Co., Ltd., Research Institute, 3-1-15 Toyosu, Koto-ku, Tokyo 135, Japan
ABSTRACT This paper describes a study of corrosion behavior of a mild steel as a candidate of the high-level nuclear waste package in the geological disposal situations, conducted to establish a model for estimating the corrosion allowance requirement to achieve the 1,000 year lifetime for the package. In several series of galvanostatic tests, the maximum penetrationdepth and the depth distribution were measured for each specimen with a sophisticated ultrasonic inspection technique. The Gumbel distribution model was successfully used in analyzing each set of data for the maximum penetration depth. Relations among the average penetration depth, the maximum penetration depth, and the corrosion allowance requirement were discussed.
INTRODUCTION The high-level nuclear waste packages must be assured of a service lifetime of at least 1,000 years. If mild steels are to be used for the container of such wastes, essentially the only as well as the greatest detriment for their integrity is corrosion by the ground water, so that studies of long-term integrity of mild steels in ground water are of the prime importance. Insofar as ordinary ground water is, even though there are special cases, a neutral solution of pH = 7 or thereabout, deep level ground waters that contain almost no dissolved oxygen can be regarded as essentially a reducing environment. Although mild steels are so fated as to be corroded even by such a mild environment, yet they would tolerate 1,000 years' corrosion if an appropriate corrosion allowance, i.e. the corrosion allowance requirement, is set to them, because their corrosion rates are sufficiently small. Conversely, this means in turn that, before a mild steel is applied to the service, accurate prediction of its corrosion rate and rational determination of the corrosion allowance requirement are both mandatory. The present paper, then, intends to examine the localization process of corrosion in mild steels through galvanostatic tests as a first step toward the goal set forth above, because it is not by the average penetration but by the extremes in the localized corrosion penetration that the corrosion allowance requirement should be determined.
BACKGROUND Figure 1 schematically presents the dependence of initial corrosion rate on pH. As may be noted in the figure, mild steels cannot be used in the acidic domain, where the low pH of natural water, such as ground water, is due to presence of dissolved carbon dioxide, [C0 2 -aq], because then the rate of Mat. Res. Soc. Symp. Proc. Vol. 176. @1990 Materials Research Society
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corrosion by the so-called wet CO2 attack will expectedly be high [1]. In the alkaline domain, on the other hand, mild steels will passivate themselves readily, meaning that the pitting/crevice corrosion or stress corrosion cracki
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