| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 1471336 | Corrosion Science | 2008 | 9 Pages |
Iron(II) hydroxide and hydroxychloride precipitates were obtained by mixing FeCl2 · 4H2O and NaOH aqueous solutions with various concentration ratios R′ = [Cl−]/[OH−] = 2 [FeCl2]/[NaOH] at [NaOH] = 0.4 mol L−1. They were analysed by Infrared spectroscopy after 24 h of ageing at room temperature. Fe(OH)2 was obtained alone only for the smallest values of R′, typically R′ ⩽ 1.16. β-Fe2(OH)3Cl formed as soon as R′ ⩾ 1.40 and was obtained alone for R′ ⩾ 2.25. The initial precipitates were oxidised by addition of a small amount of hydrogen peroxide (5 mL of an aqueous solution containing approximately 30 vol% H2O2) instead of O2. The action of H2O2 on Fe(OH)2 gave rise to δ-FeOOH as already reported. Its action on Fe(II) hydroxychlorides gave rise to akaganéite β-FeO1−2x(OH)1+xClx. A transformation of the two-phase system found at R′ = 1.5 after long ageing times (6 months) was observed and β-Fe2(OH)3Cl remained alone. This slow transformation of Fe(OH)2 into β-Fe2(OH)3Cl may explain why β-Fe2(OH)3Cl was only reported as a corrosion product on iron archaeological artefacts. Finally, the respective domains of stability of Fe(OH)2 and β-Fe2(OH)3Cl were demarcated and an estimation of the standard Gibbs free energy of formation of β-Fe2(OH)3Cl could be given: ΔGf∘(β-Fe2(OH)3Cl)=-923.5±0.9kJmol-1.
