Iron oxides formation

Where caustic deposits occur, the resultant corrosion of steel by caustic gouging or stress corrosion cracking (SCC) mechanisms produces particulate iron oxides of hematite and magnetite. It is common to see white rings of deposited sodium hydroxide around the area of iron oxide formation. 

Fig. 15.6 Schematic representation of iron oxide formation at a textural (= Eh) discontinuity in unconsolidated clastic sediments.

Soils - a unique environment for iron oxide formation in terrestrial ecosystems... 

The physical and chemical parameters which influence iron oxide formation vary with time and space, e. g. through changing water/air content. Microenvironments exist in pores of different sizes and with different degrees of filling. For example, hematite was identified in coatings at the (dry) surface of a basalt boulder, whereas goethite occurred in a nearby (moister) crack (Bender-Koch et al., 1995 a). In another case, goethite was the dominant oxide next to the root surface, whereas lepidocrocite predominated a few mm away from it (Schwertmann Fitzpatrick, 1977). Often, however, the exact conditions under which Fe oxides form are difficult to determine. 

Schwertmann, U. Cambier, P. Murad, E. (1985) Properties of goefhites of varying crystallinity. Clays Clay Min. 33 369-378 Schwertmann, U. Carlson, L. Fechter, H. (1984) Iron oxide formation in artifidal groundwaters. Schweiz. Z. Hydrol. 46 185— 191... 

By varying the conditions of synthesis much can be learned about the experimental parameters which govern iron oxide formation during weathering and soil formation (pedogenesis) (Schwertmann, 1985 1988a Fitzpatrick, 1988 Schwertmann and Taylor, 1989). 

This chapter consists of a 45 minute lecture/demonstration of iron oxide formation in vitro and in situ. Several simple bench syntheses of iron oxides are carried out and explained. The lecture may be concluded with a short video which illustrates iron oxide occurrence in a real landscape. 

Schwertmann, U., ftchtcr, H., Taylor, R. M. and Stanjek, H. (1995) A lecture and demonstration for students on iron oxide formation. In G.J. Churchman, R.W. Fitzpatrick and R.A. Eggleton (Editors) Clays Controlling the environment. Proc. lO" Int. Clay Conf, Adelaide, Australia, 1993,. CSIRO Publishing, Melbourne, Australia, p. 11-14. 

Plate II. Iron oxide formation in the environment Ferrihydrite formed by oxidation of Fe in ferriferous spring in Iceland. (Courtesy L.Carlson, University of Helsinki). 

Several photosynthetic bacteria have been discovered which oxidize ferrous ion anaerobically using light energy (Ehrenreich and Widdel, 1994 Heising et al., 1999). The investigation of these bacteria suggests that the banded iron oxide formation can occur even in the absence of molecular oxygen. 

Residual elements, such as nickel, copper and tin, are normally found in the steels made from or substantially from scrap steels. The presence of these elements in the steels had some effects on their oxidation kinetics but these were not significant [97,98]. The main concern with the presence of copper and tin is surface hot shormess, which is caused by the presence of a molten copper-rich phase on the steel surface as a result of steel oxidation. This molten copper-rich phase can penetrate austenite grain boundaries and cause surface cracking during subsequent hot rolling. Nickel is beneficial in alleviating surface hot shortness but it can cause sticky scale as a result of network-like internal iron oxide formation. Further discussion of surface hot shortness behaviour is beyond the scope of the current review. 

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