Some observations on the break-through of protective oxide films on iron-chromium alloys

Abstract

The break-through of thin protective films on pure iron-chromium alloys to give thick stratified scales has been studied by metallography and briefly by electron-probe microanalysis. During oxidation in air at 1000°C the interference colours produced initially soon disappear and the protective layer, consisting of a mat of small crystals, becomes apparent. This film thickens slowly at a declining rate and may persist in places for at least 420 min, rarely being more than 1μ thick. A few nodules of stratified scale are formed after 5–60 min but these become larger and more frequent after 60–120 min. The nodules vary widely in shape and size and are apparently formed at random sites on the surface. Eventually thick scale covers the whole specimen and all layers continue to thicken at roughly the same rate. Extensive internal oxidation is often observed behind thick scales or nodules but never behind protective oxide. Scales thicker than about 2μ may be usefully examined in cross-section by electron-probe microanalysis but thinner layers must be studied in plan. The thin protective films are extremely rich in chromium, probably consisting of Cr 2O 3 with a small quantity of Fe 2O 3, but after break-through virtually pure iron oxides form outer layers. The oxidation mechanism is discussed in light of the variation in distribution of chromium and iron in alloy and scale at various stages of oxidation.