DIAGRAMS E-R-M(KOH) FOR IRON AT 25-125 DEGREES-C AND AT TOTAL PRESSURE OF 1-30 BAR

Taking into consideration the water activity a(H2O), the equilibrium p ressure 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 rela tions for the quantitative evaluation of the equilibrium conditions of individual reactions in the Fe-KOH-H2O system were derived in potenti al range from -0.3 to +1.8 V (RHE). With the aid of standard thermodyn amic data for individual reaction components, E-r-m(KOH) diagrams were constructed for iron at 25, 100 and 125 degrees C and a total pressur e 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 de grees C, while at higher temperatures solid magnetite, Fe3O4, is direc tly formed as the thermodynamically stable primary corrosion product. The region of existence of solid Fe(OH)(2) is influenced by the molali ty 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 corr osion product in KOH solutions m(KOH) less than or equal to 6.34 mol k g(-1), which cannot be concluded from the usual E-r-pH diagram after P ourbaix.