ABSTRACT

Conceptual understanding of stress corrosion crack advance and ifs influential sub-processes has helped elucidate a variety of traditional and novel approaches that can be used to mitigate SCC in high temperature water. Among these, modifications to the environment are. the most useful for SCC mitigating in existing plants. This paper reviews the conceptual foundation of SCC in BWR environments and various mitigation approaches.

INTRODUCTION

A clear and strong conceptual framework of stress corrosion cracking (SCC) in high temperature water is indispensable for interpreting the diverse and scattered SCC dam for defining the processed parameters that control SCC for measuring, modeling, and predicting SCC behavior and to identify critical opportunities for most efficiently mitigating SCC. This paper delineates and substantiates a conceptual framework for SCC in BWR environments, and identifies a variety of mitigation approaches that are king developed bad on this framework.

CRACK ADVANCEPROCESSES

Conceptual understanding and modeling of the crack tip system has evolved by developing and testing a variety of hypotheses and critical concepts related to ionic current flow within cracks, the associated “crevice” chemistry within the crack, and metal oxidation/repassivation response. Concepts associated with these issues will be identified and the essential, supporting experimental data will be discussed.

While many aspects of our conceptual understanding of stress corrosion crack advance (e.g., the roles and interrelationship of potential and wafer purity in establishing a crack chemistry) are not directly linked to a specific crack advance mechanism, the underlying mechanism of crack advance is clearly a central ingredient in our conceptual understanding of SCC. We attribute environmental y assisted crack advance to a continuing sequence of local slip, film rupture, and oxidation / repassivation. In this mechanism, crack advance is related to dissolution / oxidation reactions at the crack tip where a thermodynamically stable, protective oxide is ruptured by increasing strain in the underlying matrix. Since electrochemical reactions (separate anodes and cathodes) are not essential to metal oxidation or passivation, this mechanism is equally applicable to gaseous environments (e.g., steam or hot gazes) where only chemical oxidation (not electrochemical reactions) can occur, although measurement of the metal oxidation during the repassivation transient that follows oxide rupture is much easier for electrochemical reactions.

The periodicity of tie local film rupture events is related to the local strain rate of the alloy which is, in turn, is controlled by either “creep” processes under constant load (“creep” in this broad context can include thermally activated creep, relation creep, or even strains resulting from the shifts in stress / strain fields induced by an advancing crack), or under applied strain rates from monotonically increasing or cyclic loading conditions. Thus, the model is potentially applicable not only to stress corrosion (constant stress) but also to corrosion fatigue over the range of stress amplitudes, mean stresses, frequencies, etc.

Following each localized oxide rupture event, very high initial reaction rates and subsequent repassivation occur (Figure 1).

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