Breakaway oxidation of 15-18% Cr ferritic stainless steels occurring in water vapour is described in the temperature range 800-1000°C. The failure of the protective chromia scale leads to iron oxide(s) nodule formation with accelerated kinetics. Characterisation of the (Fe,Cr)₂O₃ initial oxide scale by Raman spectroscopy and photoelectrochemistry shows chemical evolution with oxidation time, with increasing Cr/Fe ratio before haematite suddenly appears at the steel-oxide interface. The mechanisms for such a phenomenon are discussed, first on a thermodynamic point of view, where it is shown that chromium (VI) volatilisation or chromia destabilisation by stresses are not operating. It is rather concluded that mechanical cracking or internal interface decohesion provide conditions for haematite stabilisation. From a kinetic point of view, rapid haematite growth in water vapour compared to chromia is thought to be the result of surface acidity difference of these two oxides.