Structure and Adhesion of Oxide Scales on Iron-Aluminium Alloys

Abstract

The influenceof aluminium (up to 0·2 wt.-%) on the structure and high temperature adherence of scales formed on pure iron has been studied over the temperature range 850–1050° for simulated burnt town's gas atmospheres, with and without additions of 5 vol.-% of free oxygen. Weight-gain measurements revealed that whilst additions of up to 0·045 wt.-% of aluminium tended to produce an increase in the oxidation rate of pure iron, the presenceof 0·2wt.-% of aluminium conferred some protection. The former effect was particularly marked at 850–950°, and the latter at 850°. Both effectsweremost apparent during oxidation in mixtures of carbon dioxide and water vapour. Scale adherence was assessedby means of a hot bend test carried out in situ within a controlled atmosphere furnace. The development of tensile stresses in the scale/metal interface region of the strip specimens employed successfully revealed the significantly weaker adhesion of scales which formed on alloys containing o·1–0·2 wt.-% of aluminium by oxidation at 950–1050°. Weak adhesion, which was more evident in thicker scales containing a higher proportion of structural defects, was governed by aluminium-rich oxidation products at the scale/metal interface and within adjacent regions of the scale.