By color, or morphology, can we detect the difference between Fe2O3 and FeCO3 in the cross section of a corrosion layer formed in CO2 solutions?
As already proved in our previous discussion, that there are two ways to look at the corrosion (rustig) – [1]containing only Fe2O3 (formed in the presence of O2/moisture;[2] containing both Fe2O3 and FeCO3(siderite) formed by presence of O2/CO2/moisture.
Fe2O3 is formed from Fe (OH) 2 which, itself, is practically white, but even traces of O2 impart a greenish tinge. If the solution was not deoxygenated and the iron reduced, the precipitate can vary in color starting from green to reddish brown depending on the iron (III) content. This precipitate is also known as "green rust". In the crystal lattice of iron (II) ions are easily substituted by iron (III) ions produced by its progressive oxidation. In the presence of O2, the color changes quickly. They belong to Layered Doubled Hydroxide (LDH).
In second type of rusting, the Fe2O3 and FeCO3 react in moisture to form [(Fe^2)4(Fe^3)2(OH) 12][CO3.2H2O]. This rust is called GRI [Green Rust I] which is also green; of course the intensity of green color is different from that of Fe (OH) 2 which in fact exists as Fe2O3 in rust.
The conclusion is that rust, Fe2O3 and FeCO3 are difficult to distinguish with the naked eye as there is always the formation of the above written compound in situ . In a way, there is always a possibility of all the three –Fe2O3, FeCO3 and [(Fe^2)4.(Fe^3)2.(OH) 12][CO3.2H2O] existing together in varing proportions at fifferent intervals of time.
Of course, there are many spectral techniques which may be used for their distinction.
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