Investigations at Teddington (Oxidation of Iron around 200° C.)

Abstract

An earlier investigation¹ on the oxidation of abraded mild steel sheet in air at atmospheric pressure revealed the existence of a 'critical temperature' at c. 200°. Thus (i) interference colours, the characteristic sequence of which is a function of film thickness above 200°, do not appear below 200°, although the corresponding oxygen uptake may be appreciably exceeded; (ii) oxidation - time curves, characteristically parabolas above 200°, show marked nonconformity below 200°; (iii) the temperature-coefficient curve shows an inflexion at c. 200°. Recent advances in film-stripping technique at the Chemical Research Laboratory having facilitated the study ol films removed from the substrate, the opportunity has been taken to investigate the phenomenon morf closely. Films have been removed from mild steal sheet heated for various times at 180° and 225°. The orystal structure, as observed by electron diffraction (work carried out in the Metallurgy Division, National Physical Laboratory), has been compared with the composition of the film as determined by chemical analysis, the relative thickness being assessed from the total iron content of the film.