In oxidizing environments, electrocatalysis is highly effective in mitigating SCC, provided there is a stoichiometric excess of reductants over oxidants. This paper summarizes the mechanisms and criteria for effective SCC mitigation, with emphasis on the critical location for the catalyst in a crack and recent experimental support for these concepts. Optimization of electrocatalysis using an on-line process is described, and the experimental evidence for mitigation at = 0.1 ppb Pt is presented.
Most structural materials have suffered stress corrosion cracking (SCC) in boiling water reactors (BWRs) and other high temperature water environments. The crack growth behavior has been extensively evaluated for various stainless steels, alloy 600, alloy 182 weld metal, and low alloy and carbon steels under unirradiated and irradiated conditions [1-10]. From early studies it was clear that corrosion potential has a very strong influence on SCC in high temperature water (Figure 1), and methods to decrease the corrosion potential for SCC mitigation were pursued. Electrocatalysis can achieve the thermodynamically lowest possible corrosion potential, and this results in a low SCC growth rates with minimal negative impact on BWR operation [2,7,11-17]. This paper summarizes the mechanism by which noble metals act in high temperature water in terms of electrochemical kinetics. The kinetic requirement in typical gas phase catalysis, where reaction rates are very high, are more stringent than BWR systems, where surface reaction rates are much lower due to the presence of a liquid phase and lower concentrations of reactants (i.e., O2, H2O2, and H2). Thus, dilute noble metal alloys (e.g., stainless steel containing 0.1% Pd, Pt, etc.) and/or low surface loadings also exhibit catalytic behavior. Platinum group metals are electrocatalysts that efficiently recombine O2 and H2O2 with H2 on the metal surface by providing surface sites on which these species can dissociatively adsorb and readily undergo electron exchange reactions; the undissociated molecules are relatively stable when homogeneously dispersed at BWR temperatures. Electrocatalytic efficiency is expressed in terms of the exchange current density (i0) on a metal surface (Figure 2). Once a near-stoichiometric concentration of H2 is present for the formation of water (2H2 + O2 ? 2H2O), the surface concentration of O2 approaches zero and corrosion potential decreases to its thermodynamic minimum value of ? ?0.52 Vshe (in 288 °C pure water containing ? 0.01 ? 0.1 atm. H2). This nominally occurs at a 2:1 H:O molar ratio (Figure 3), which corresponds to a 1:8 H:O weight ratio, so that ?excess H2? exists if its concentration by weight is greater than one-eighth of the O2 value (e.g., in ppb). Slightly sub-stoichiometric H2 (e.g., 1:9 H:O) is sufficient because the diffusivity of H2 is higher than O2 or H2O2 in the stagnant liquid boundary layer. The oxidant levels from radiolysis are not very high (equivalent to about 1% oxygen in argon bubbled through 25 °C water), and because diffusion kinetics through liquid boundary layers are slow, the surface catalytic requirements are limited. Indeed, careful evaluation of surface loading requirements (Figure 3) by Kim et al [18] shows that adequately low corrosion potentials can be achieved at roughly 1% of a monoloayer, perhaps lower.
