engleski

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

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 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 oC, 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 a 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 oC generated Fe2(OH)2CO3 and magnetite. A certain analogy between the precipitation of Fe(III)-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 (Fe3-xO4) and in some cases it was difficult to distinguish between Fe3-xO4 and maghemite ( -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 amorphous fraction, which is in agreement with XRD. A difference between the results of 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.

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

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