Original scientific paper

The Effect of Bicarbonate/Carbonate Ions on the Formation of Iron Rust

Svetozar Musić ; Ruđer Bošković Institute, P. O. Box 180, HR-10002 Zagreb, Croatia
Israel Nowik ; Racah Institute of Physics, The Hebrew University, IL-91094 Jerusalem, Israel
Mira Ristić ; Ruđer Bošković Institute, P. O. Box 180, HR-10002 Zagreb, Croatia
Zvonko Orehovec ; Ruđer Bošković Institute, P. O. Box 180, HR-10002 Zagreb, Croatia
Stanko Popović ; Department of Physics, Faculty of Science, University of Zagreb, P. O. Box 331, HR-10002 Zagreb, Croatia

Abstract

The effect of bicarbonate/carbonate ions on the formation of iron rust in an aqueous medium has been investigated using XRD, Mössbauer and FT-IR spectroscopies. Corrosion of carbon steel in doubly distilled water was monitored as a reference process. Lepidocrocite as the dominant phase and magnetite were detected in rust generated in doubly distilled water in a period from three weeks to one year at room temperature. At 90 °C, magnetite and small amounts of hematite/goethite were formed in a period from one day to three weeks. In these rust samples, FT-IR spectroscopy detected a very small amount of lepidocrocite, which dissolves and thus occurs in overall phase transformations in rust. On the other hand, goethite was found as the dominant phase in rust formed in natural water containing a significant amount of bicarbonate ions, whereas magnetite and lepidocrocite were minor phases. In 0.005 M Na₂CO₃, goethite and magnetite were generated at room temperature, and after one month of corrosion, magnetite was present only in traces. Corrosion in 0.005 M Na₂CO₃ at 90 °C generated Fe₂(OH)₂CO₃ and magnetite. A certain analogy between the precipitation of FeIII-oxyhydroxides from iron-salt solutions containing carbonates and the formation of goethite and lepidocrocite in rust was shown. Mössbauer spectroscopy showed that microstructural properties of the oxide phases also varied. Magnetite showed substoichiometry (Fe_3–xO₄) and in some cases it was difficult to distinguish between Fe_3–xO4 and maghemite (γ-Fe₂O₃). Mössbauer spectra, recorded as a function of temperature, showed poor crystallinity and/or superparamagnetic behavior of goethite particles. Also, the Mössbauer spectra of the rust formed in carbonate solutions provided evidence of the amorphous fraction, which is in agreement with XRD. The difference between the results of the present investigation and those obtained by other researchers can be attributed to the electrochemical stimulation of steel (iron) surfaces in bicarbonate/carbonate aqueous media and the shorter corrosion times used in their experiments.

Keywords

iron rust; carbon steel; bicarbonate/carbonate ions; lepidocrocite; goethite; hematite; magnetite; basic iron carbonate; Mössbauer; FT-IR; XRD

Hrčak ID:

102658

URI

https://hrcak.srce.hr/102658

Publication date:

31.5.2004.

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