Abstract
Numerous approaches have been developed for mitigating stress corrosion cracking in existing BWRs. Among these, reduction of the corrosion potential provides the most efficient, consistent, and dramatic decrease in the crack growth rate of unirradiated and irradiated materials of all types. Historically, reduction in corrosion potential has been accomplished by adding H2 to the feed water to decrease the dissolved O2 and H2O2 concentrations. However, H2 concentrations greatly in excess of the stoichiometric amount are required, and factors such as the cost of H2, increased N16 in the steam ("turbine shine"), and enhanced Co60 deposition must be addressed.
By comparison, noble metal technology provides a unique opportunity for achieving the thermodynamically lowest possible corrosion potential and therefore the lowest possible crack initiation and growth rates with minimal negative impact on BWR operation. Noble 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.
Once a near-stoichiometric concentration of H2 is present for the formation of water (2H2 + O2 → 2H2O), the corrosion potential decreases to its thermodynamic minimum value of ≈ -0.5 Vshe. This nominally occurs at a 2:1 H:O molar ratio, which corresponds to a 1:8 H:O weight ratio, so that "excess H2" exists if its concentration is ≥ 1/8th of the O2 value (e.g., in ppb). However, slightly sub-stoichiometric H2 (e.g., 1:10 to 1:12 H:O) is sufficient because the diffusivity of H2 in the liquid boundary layer is higher than O2 or H2O2. This catalytic effect has been shown for a broad range of O2 concentrations as well as H2O2 concentrations, for which the appropriate molar ratio is calculated from an equivalent O2 concentration based on its decomposition (2H2O2 → 2H2O + O2); thus, the equivalent O2 is 0.47 times H2O2 in ppb.
A variety of techniques have been developed for creating catalytically active surfaces on structural materials using noble metals, including electro- and electro-less plating, sputtering, etc. of pure Pd or Pt. Because of the comparatively low oxidant concentrations in BWR water (e.g., <0.01 bar O2) and comparatively low transport rates (from low diffusivity in liquids and the presence of the liquid boundary layer), it has also been shown that dilute alloys (e.g., stainless steel containing <0.03% to 10% Pd or Pt) also behave catalytically. Based on this knowledge, other coating techniques have also been developed, and this paper summarizes the use of the plasma spray (PS) and hyper-velocity oxy-fuel (HVOF) thermal spray techniques to coat with noble metal doped powders. Powders of various compositions of stainless steel and nickel-base alloys containing various Pd or Pt levels were prepared, applied, and evaluated. Thermal spray coating characteristics include excellent adhesion and wear resistance, low heat input (especially important for irradiated or cracked components), proven capability for field application, absence of aggressive chemicals required for plating, etc. Coatings can also be directly applied underwater, and all coatings (depths up to 25 meters have been evaluated) show excellent properties and catalytic performance.
The catalytic performance of the thermal spray coating was evaluated for stress corrosion cracking response using slow strain rate and compact type specimens in 288°C water. These data are compared with other data obtained on Pd-plated and Pd-alloyed specimens.
