Peter J. Millett

Tungsten/Tungsten Oxide pH Sensing Electrode for High Temperature Aqueous Environments

The reliable determination of pH in high temperature aqueous environments is of primary importance for the development of corrosion prevention technologies in the thermal power industry. A knowledge of pH is also important for developing an understanding of various aspects of high temperature water chemistry and geochemistry. The tungsten/tungsten oxide electrode was investigated as a pH sensor over a wide range of pH at temperatures from 200 to 300 C and was compared with the yttria-stabilized zirconia membrane electrode. The tungsten/tungsten oxide electrode shows a Nernstian pH response within the pH range of 2 to 11 and exhibits stable and reproducible potentials. In solutions with pH values lower than 2 were observed deviations from Nernstian behavior. The potential of the electrode is not significantly affected by the redox properties of the system, as established by the presence of oxygen or hydrogen in the solution.

Transport processes in steam generator crevices—I. General corrosion model

A new model for describing transport processes in Pressurized Water Reactor (PWR) Steam Generator (SG) tube/tube support plate crevices has been developed. The internal and external systems are coupled using the principle of charge conservation, recognizing that anodic and cathodic reactions may occur at any location. This model includes the influence of convective transport, diffusion, and migration of species in the crevice on the evolution of crevice properties. Calculations show that under normal operating conditions after an initial time period, the concentrations of all species and potential of the metal reach limiting values at the center of the crevice. After this period, the space in the cavity can be subdivided into two parts: (1) a region near the crevice mouth where concentration and potential gradients exist, and (2) a region near the crevice center where the concentrations of the various species and metal potential attain constant values. In the latter, the cathodic current density compensates for the corrosion current. The limiting values of the specie concentrations and the metal potential do not depend on the corrosion potential of the metal, but do depend on the available superheat and the bulk composition of the solution. For a sufficiently negative corrosion potential, the net current in the cavity can be negative, which must be balanced by a positive current on the external surface.

Transport processes in steam generator crevices. II. A simplified method for estimating impurity accumulation rates

An approximate, yet accurate method for calculating average volume concentrations of impurities and corrosion products in Pressurized Water Reactor (PWR) Steam Generator (SG) tube/tube support plate crevices has been developed. The method is based on the assumption that the real concentration profiles can be approximated by ideal step functions. The method yields analytical expressions for such important parameters as the rate of accumulation of impurities, corrosion current density, liquid velocity, saturation, transition time, and wetted length. It is shown that, under normal operating conditions of a PWR SG, we can estimate the corrosion rate even when only meager information is available for the physical parameters of the system. The method also permits prediction of the accumulation of impurities in crevices for long times, if the corresponding experimental measurements are available only for short times. Long-term predictions from the model describe the experimental data with great accuracy.

High-Temperature, Aqueous-Phase Diffusion of NaCl through Simulated Deposits of Corrosion Products ✫

Abstract Aqueous-phase effective diffusion coefficients were measured for sodium chloride in simulated porous corrosion products using a modified diaphragm cell technique at temperatures from 25°C to 280°C. The effect of the pore structure on the measured diffusion coefficient was determined from the bulk diffusion coefficient at the experimental temperature. A tortuosity factor was calculated for each corrosion product as a function of temperature. The tortuosity factor was used with a bulk diffusion coefficient and porosity to estimate the transport rate of a solute specie in corrosion products at elevated temperatures and pressure.