INTERPRETATION OF E-PH DIAGRAMS OF THE SYSTEM FE-H2O IN RELATION TO CAUSTIC EMBRITTLEMENT OF STEELS

E-pH diagrams for the Fe-H2O system are presented at temperatures of 60, 100 and 150-degrees-C, employing new thermodynamic data. At 25-degrees-C, the formation of a solid Fe(OH)2 zone, determined by Silverman, was found to reduce with increasing temperature and at temperatures above 65-degrees-C the zone disappeared. Previous work has related this change in the electrochemical behavior of steel in NaOH solutions at +65-degrees-C to caustic embrittlement, observed in practice and in the laboratory. The relation between the nature of the passive film and stress corrosion cracking of steel in NAOH solutions is discussed. The formation of Fe(OH)2 at low temperatures and its oxidation to FeOOH allow the repair of the passive film. However, at high temperatures, the formation of the oxide film (Fe3O4) at peak II leads to the susceptibility of steels to stress corrosion cracking (SCC).