Diagrams Er-mKOH for iron at 25-125 °C and at total pressure of 1-30 bar

Taking into consideration the water activity a(H2O), the equilibrium pressure of water vapour p(H2O) in or above KOH solutions of different molality (2-18 mol kg(-1)) and temperature (0-200 degrees 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, E-r-m(KOH) diagrams were constructed for iron at 25, 100 and 125 degrees C and a total pressure of 1, 10 and 30 bar (0.1, 1 and 3 MPa). Under standard conditions (a(H2O) = 1.0), Fe(OH)(2)(s) may result at temperatures of up to 63.1 degrees 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 degrees C Fe(OH)(2) can only arise as the primary corrosion product in KOH solutions m(KOH) less than or equal to 6.34 mol kg(-1), which cannot be concluded from the usual E-r-pH diagram after Pourbaix.