INTRODUCTION
ABSTRACT The passivity behavior of Alloy 22 and Grade 7 titanium has been studied at 95°C in a high pH salt environment characteristic of concentrated Yucca Mountain groundwater. Measurements of corrosion potential (CP) versus time, potentiostatic polarization (PP) and cyclic potentiodynamic polarization (CPP) behavior were conducted to evaluate the passivity of these alloys. The characterization of passive films was also analyzed by x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) to obtain the chemical composition and cross-sectional view of the metal, interface, and oxide layers.
It was observed that the oxide layer responsible for passivity of Alloy 22 consisted of chromium oxide (Cr203) containing Ni. The surface analysis showed that the passive films formed on Alloy 22 at high anodic potentials (> 0 mV vs. SCE) contained more Mo and W than ones formed at lower anodic potentials (<0 mV vs. SCE). However, no visual evidence of localized corrosion on Alloy 22 after potentiostatic polarization measurements was observed.
This program is designed to examine the oxide characteristics formed on Alloy 22 and Ti-grade 7 as disposal-canister and drip-shield corrosion resistant materials, respectively for the Yucca Mountain Project [ 1]. The waste package and drip shield are essential elements of the engineered barrier system, and the ability to provide very long waste package lifetimes that can be predicted with confidence is a central factor in the calculated release rate of radionucleides from the mountain.
General and localized corrosion, and stress corrosion cracking represent the most likely degradation modes for the corrosion-resistant materials comprising the waste package and drip shield [2- 5]. One key to demonstrating and predicting waste package lifetimes lies in characterizing the local environment that forms on the waste package. This is particularly important for temperatures above = 75 °C, where the heat flux through the waste package is higher, the environments are more concentrated, and the material susceptibility to corrosion degradation is highest. The waste package is always hotter than its surrounding environment, probably by several degrees C at > 70 °C.