A Computer Program for Constructing Stability Diagrams in Aqueous Solutions at Elevated Temperatures

When a metal is placed into an aqueous solution containing various chemicals, corrosion problems are often encountered under the high temperature and high pressure conditions present in nuclear power systems. Potential-pH diagrams of metals in aqueous solutions are a valuable means of predicting these corrosion problems. A computer program has been developed for calculating electrode potentials and pHs of reactions and delineating the predominant area of each chemical or ionic species in aqueous solutions. The free-energy values of solids, liquids and gases are from standard sources and those of dissolved ionic species are from an empirical relationship known as the Criss-Cobble Correspondence principle. The program is easy to use. The input is kept to a minimum through an expandable library of species (established as a data base), allowing the retrieval, by name, of thermodynamic and free-energy data. Presently, the library contains various compounds of elements Fe, S, Ni, Cr, C, N, Ti, B, Zr, and Cu. The free-energy value can be retrieved at any temperature from 25 to 300 C. The stability diagram can be obtained continuously from room temperature upto 300 C and plotted on a digital plotter. A combination of comprehensive tabular and graphical output makes the program an efficient and effective tool in the study of stability diagrams to help explain corrosion or water-chemistry-related problems.