Structure of Scale on Plain Carbon Steels

Abstract

THE classical picture of high-temperature scales on plain carbon steels as determined by Pfeil1 shows a three-layered structure. An outer layer of haematite, Fe2O3, a middle one of magnetite, Fe3O4, and an inner one of wustite, FeO, next to the parent metal is the normal composition of this layered structure in a scale that is continuous with the parent metal. (For practical and historical reasons, wustite is normally given as the stoichiometric FeO when in fact it is more accurately described as Fe1−yO, where Y is the concentration of vacancies.) Similarly, Paidassi2, in his recent work on the kinetics of oxidation of pure irons between 700° and 1,250° C., found that the three-layered structure is present on continuous scales. However, 15 min. was the minimim oxidizing time for the lower-temperature scales that he reported.