Collect. Czech. Chem. Commun. 1997, 62, 1677-1689

Diagrams Er-mKOH for Iron at 25-125 °C and at Total Pressure of 1-30 Bar

Jan Baleja and Jiri Divisekb

a Consultant Bureau for Chemical Engineering, Johanniterstr. 28, 86609 Donauworth, Germany
b Institute of Energy Process Engineering (IEV), Research Center Julich (KFA), 52425 Julich, Germany


Taking into consideration the water activity aH2O, the equilibrium pressure of water vapour pH2O in or above KOH solutions of different molality (2-18 mol kg-1) and temperature (0-200 °C), the relations for the quantitative evaluation of the equilibrium conditions of individual reactions in the Fe-KOH-H2O system were derived in potential range from -0.3 to +1.8 V (RHE). With the aid of standard thermodynamic data for individual reaction components, Er-mKOH diagrams were constructed for iron at 25, 100 and 125 °C and a total pressure of 1, 10 and 30 bar (0.1, 1 and 3 MPa). Under standard conditions (aH2O = 1.0), Fe(OH)2(s) may result at temperatures of up to 63.1 °C, while at higher temperatures solid magnetite, Fe3O4, is directly formed as the thermodynamically stable primary corrosion product. The region of existence of solid Fe(OH)2 is influenced by the molality and temperature of KOH solutions in contact with metallic iron. For example, at 25 °C Fe(OH)2 can only arise as the primary corrosion product in KOH solutions mKOH ≤ 6.34 mol kg-1, which cannot be concluded from the usual Er-pH diagram after Pourbaix.

Keywords: Iron oxo compounds; Iron corrosion products.