TEM characterizations of iron oxides of mild steel formed at high-temperature and high-concentration NaOH

In the Bayer process of alumina refining, high temperature (>200ºC) high pH sodium hydroxide (NaOH) solutions are required to dissolve aluminia from bauxite ores. Corrosion of the piping vessels and components, which are mostly made of mild steels, exposed to these solutions poses a serious challenge to industry, especially within heat exchangers where high flow rates are also encountered. The corrosion resistance of steels ultimately depends on the material’s ability to form a tightly adherent, non-porous oxide film across its surface, which serves as a barrier between the metal and environment. Previous studies showed that oxide growth occurs by the formation of a duplex magnetite (Fe3O4) film which grows both inwardly at the metal-oxide interface to form a fine-grained tightly packed structure, and outwardly at the oxide-solution interface via precipitation from the solution to form a coarse-grained porous structure [1-8]. However, the microstructures of the oxide film, in particular at high-temperatures (250ºC to 280ºC) and high-concentration NaOH (10M), have not been fully understood. The aims of the studies were to better understand the role of temperature and oxide dissolution in non-passive oxidation behaviour. Samples were prepared by first cutting ~25mm x 15mm x 4mm pieces from a thick-walled A106B (<0.18%wt C, <0.63%wt Mn) mild steel tube, which were then polished to a 6µm finish. These samples were then placed directly on a Teflon base and submerged in a 10M NaOH solution which had been partially deoxygenated by bubbling nitrogen through it for ten minutes. The autoclave was then sealed and heated to the desired temperature for the experimental trial (one of 130ºC, 250ºC, 280ºC).XRD, SEM and TEM studies found only magnetite phase exists in the oxide layer from the 130ºC sample with grain size ranging from several microns to tens of microns in diameter in consistent with the previous observations[1-8]. XRD pattern from the 250ºC sample indicates the presence of both magnetite and maghemite phases in the oxide layer. TEM images and diffraction patterns (Figure 1) show that magnetite grains in the oxide layer are typically several microns in size; whereas the nano-sized (20 – 40 nm) maghemite grains exist as clusters randomly distributed among magnetite grains in the oxide layer. Both the magnetite and maghemite are common forms of iron oxide and have an inverse spinel structure. Magnetite (Fe3O4) belongs to the space group Fd3m (N0.227) with lattice constant a = 0.8396 nm (JCPDS No. 19-0629) while maghemite belongs to the space group P4132 with lattice constant a = 0.8352 nm (JCPDS No. 25-1402). TEM study of the oxide layer from the 280ºC sample shows that the oxide layer composes oxide grains of 20 – 40 nm in diameter throughout the layer. The grains in the bottom part of the layer are compact and belong to maghemite phase as identified by SAED pattern. The grains in the upper part of the layer are loosely packed and also contain magnetite phase as identified by SAED and XRD patterns (Figure 2). Our studies reveals, for the first time, that the oxide layers formed on A106B mild steel in 10M NaOH solution at 250ºC and 280ºC contain maghemite as well as magnetite. The microstructures of the samples at 250ºC and 280ºC have been characterized by TEM imaging and diffraction.