Stochastic Modeling of the Influence of Environment on Pitting Corrosion Damage of Radioactive-Waste Containers
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STOCHASTIC MODELING OF THE INFLUENCE OF ENVIRONMENT ON PITTING CORROSION DAMAGE OF RADIOACTIVE-WASTE CONTAINERS
Gregory A. Henshall, Lawrence Livermore National Laboratory, P. 0. Box 808, L-355, Livermore, CA 94551, USA.
ABSTRACT
A physically-based, phenomenological stochastic model for pit initiation and growth is presented as a potential tool for predicting the degradation of high-level radioactive-waste containers by aqueous pitting corrosion. Included in the model are simple phenomenological equations describing the dependence of the controlling stochastic parameters on the applied (or corrosion) potential, chloride ion concentration, and absolute temperature. Results from this model are presented that demonstrate its ability to simulate several important environmental effects on pitting. INTRODUCTION
A multiple barrier concept currently is being employed in the development of high-level radioactive-waste (HLRW) containers for use in the potential geological repository at Yucca Mountain. One of these barriers will be constructed of a highly corrosion resistant material, such as a Ni- or Ti-base alloy. Normally, such alloys are protected by a passive oxide film, but if they become wet and Cl- or other aggressive species are present the passive film can break down locally, causing pitting. Thus, the design of the corrosion resistant barrier must include an analysis of pitting. Modeling is a key element in this analysis because the required containment times are well beyond those for which experimental data can be collected. In particular, predictions of how changes in the container environment over these extensive time periods will affect pitting must be made. In this paper (sponsored by the Yucca Mountain Site Characterization project), a physically-based, phenomenological stochastic model of pit initiation and growth is presented. This model is based upon the theory [1] that small fluctuations in the local conditions (e.g. electrolyte chemistry, fluid flow rate, surface topography, surface metallurgy) cause local breakdown of the passive surface film, resulting in the "birth" of metastable pits or "embryos". Many of these embryos become unstable when the local conditions change and repassivation, or "death", of the embryo results. Once a surviving embryo reaches a critical size or age, it becomes a permanent, or "stable," pit and cannot die. Stable pits then grow at a rate that can be computed with deterministic, e.g. mass transport, laws or with a stochastic model. Several investigators have developed Monte Carlo computer codes based on this stochastic theory [2-4]. As discussed in detail elsewhere [4], these codes establish a unit area that is divided into individual "cells" to represent a metal surface in contact with an aggressive environment. Random numbers are generated and their values are compared with the Mat. Res. Soc. Symp. Proc. Vol. 353 0 1995 Materials Research Society
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prescribed birth probability, A,and death probability, g, to control the initiation, growth, and death of metastable pi
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