Iron oxide dissolution

C) for cast iron and up to 140 °F for marstenitic SS (60 °C). It is widely used where silicates are present with the iron oxides. Typically, 5 to 7.5% HC1 is employed. The ammonium bifluoride normally is present at 0.5%, but it may be increased to a maximum of 1.5% for a boiler that has not been cleaned for many years. The presence of hydrofluoric acid (HF), which is formed by the reaction of ammonium bifluoride with HC1 (see equation), tends to increase the rate of iron oxide dissolution and reduce the corrosion rate of exposed steel, when compared to using HC1 alone. This is due to the stability of the hexafluoroferric ion (FeFg3 ), which prevents the ferric ion from corroding exposed steel. 

As the overall concentration of copper and copper oxides in the boiler deposit increases, however, less thiourea is required. This is because, as ferric ions are generated during the iron oxide dissolution process, they oxidize the plated copper, which can then be removed from the boiler by forming a complex with thiourea. Conversely, if ferric ions are not generated, the plated copper remains and no complexing can take place. 

Except for phthalic acid, all other carboxylic acids studied induce considerable increases in the light compared to the dark values (the relatively high rate of iron oxide dissolution induced by oxalic acid has been extensively studied (5,8). Phthalic acid actually appears to stabilize the iron oxide against photodissolution despite the solution phase complex exhibiting some photoactivity. 

Iron oxide dissolution can proceed by a variety of pathways, viz. protonation, com-plexation and reduction, photochemical and biological. 

Chiarizia, R. Horwitz, E.P. (1991) New formulations for iron oxides dissolution. Hydrometallurgy 27 339-360 Childs, C.W Wilson, A.D. (1983) Iron oxide minerals in soils of the Ha apai Group, Kingdom of Tonga. Aust. J. Soil Res. 21 489-503 Childs, C.W. (1992) Ferrihydrite A review of structure, properties and occurrence in relation to soils. Z. Pflanzenemahr. Bodenk. 155 441-448... 

Malengreau, N. Bedidi, A. Muller, J.-P. Her-billon, A.J. (1996) Spectroscopic control of iron oxide dissolution in two ferralitic soils. 

Hersman L. E. (2000) The role of siderophores in iron oxide dissolution. In Environmental Microbe-metal Interactions (ed. D. Lovley). ASM Press, Washington, DC, pp. 145-157. 

Various chemical extraction techniques have been introduced in order to selectively remove metals from the different adsorption or complexation sites of natural sediments (e.g., Tessier et al, 1979 Erel et al, 1990 Leleyter et al., 1999). It is, for example, shown by Leleyter et al. (1999) that between 20% and 60% of REE in various suspended river sediments are removed by successive extractions by water, by Mg(N03)2 (exchangeable fraction), sodium actetate (acid-soluble fraction), NH2OH - - HCl (manganese oxide dissolution) ammonium oxalate (iron oxide dissolution) and a mixture of H2O2 + HNO3 (oxidizable fraction). The complexity of... 

Initiation of iron oxide dissolution, humic acid complexation of trace metals and trace metal co-precipitation with FeS... 

Kraemer, S. M. (2004). Iron oxide dissolution and solubility in the presence of siderophores. Aquat. Sci. 66, 3-18. 

K. Barbeau, J.W. Moffett (20(X)). Laboratory and field studies of colloidal iron oxide dissolution as mediated by phagotrophy and photolysis. Limnol. Oceanogr., 45, 827-835. 

Bradley and Smith reported that corrosion rates of exposed steel were much lower when mixtures of HF and HCI were used to dissolve magnetite, as compared with the use of HCI alone. In both cases, an organic corrosion inhibitor was used to control the attack of the acid on the steel. Lower corrosion rates also were reported for HF solutions compared with similar concentrations of HCI. Similar results were noted by Alfandry et al. Poff also described the benefits of adding ammonium bifluoride (ABF) to acidic solvents. This compound decreased the corrosion rates of the acidic solvents. " Fluorides also were reported to increase the rate of iron oxide dissolution in several solvents. Discussions of possible mechanisms are included in Chapter 4. Bieller described the advantages of the use of HF as a boiler solvent. These include lower corrosion rates and increased cleaning rates compared with the use of HCI solvents. 

The purpose of this section is to review recent literature on the kinetics and mechanisms of iron oxide dissolution. Particular emphasis is placed on pure iron oxides and iron oxides on steel surfaces. The limited scope of this review precludes complete coverage of the vast amount of work on passive film formation and dissolution. However, some aspects of passivation that apply directly to chemical clening are reviewed in Chapter 4. 

Compared with the large amount of work on the mechanism of free iron oxide dissolution, there is relatively little Information available on the dissolution of oxides from steel surfaces. " A major contribution of these studies is confirmation... 

Colhode 2Fe 2r— 2Pe Figure 1. Iron oxide dissolution mechanism. 

Rate Constants for Iron Oxide Dissolution in Ammonium Chelant Solutions... 

Figure 3. Iron oxide dissolution tests in ammonium chelates at a pH of 9.2 tests were run at 320°F (160°C).

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