What are the kinetic and passivational roles of carbonate during the corrosion of iron (or steel) in a carbonate-based environment?
Fe (pipe) +0.5 O2 (in soil) + H2O (in soil) ↔ Fe^+2 +2OH^-1 .
Also
Fe^+2 +2OH^-1 +0.25 O2 (in soil) +0.5H2O (in soil) ↔ Fe (OH) 3 (s) .
Fe^+2 + CO3^-2(in soil) ↔ Fe (CO3) [Siderite] .
2Fe (OH) 3 ↔ Fe2O3+3H2O .
3Fe (CO3) +0.5O2 ↔ Fe3O4 +3CO2.
4Fe3O4 +O2 ↔6Fe2O3(s) .
The following points may also be taken into consideration:
1. As one or two or all the three types of the scale/s like Fe (CO3), Fe2O3, Fe3O4 can form, so it is quite heterogeneous.
2. O2 concentration can have varying effect on corrosion and increases with the increasing concentration of O2, iron concentration and the type/s of the scale/s formed.
3. Siderite can form a protective layer and may stop/slow the reaction/s.
4. Green rust is a generic name to such a scale containing both Fe (2) and Fe (3) salts.
5. The extent of corrosion is measured in terms of CORROSION INDEX postulated by different workers named Lange lier Index[LI], Rzynar Index[RI], Aggressiveness Index[AI], Momentry Excess[ME], Driving Force Index[DFI] and Calcium Carbonate Precipitation Potential[CCPP].
Thank you Manohar for your valuable mechanistic information. They contain a lot of what I was looking for.
Cheers,
Faysal
What is the pH of the soil. At pH above 9 carbonate ions become the predominant species. At pH less than 7 Bicarbonate ions are the predominant species. Note that Ferrous carbonate is not soluble in water while Ferrous bicarbonate is soluble..
Thank you Mahmud, the pH in the hydrated soils is at least 8. It seems that the corrosive environment is a mixture of the carbon-carrying species of bicarbonate, carbonate (and chloride and sulfate actually).
As regards to the rusting of iron, I submit the following observations already reported in the literature:
1. There are two types of the green rusts called Green Rust-1[GR-I] and Green Rust-II [GR-II].
2. The former contains mainly carbonate ions [GR-CO3] and the latter mainly contains both sulphate/ chloride ions. [GR-SO4].
3. The former has the general formula Fe (II) 4Fe (III) 2(OH) 12 CO3•mH2O and the latter Fe (II)4Fe(III)2(OH)12SO4•mH2O.
4. It will be over simplification to assume that all rusts contain carnonate/ sulphate/ chloride ions.
5. The rust containg chloride ion starts at pH=6.8 and the sulphate ions at 7.0 while the rust containg carbonate ions at appxo at 8(even some what more).
6. It is only at pH=8.3-8.4 that both types of green rusts are present.
7. The competitive formation of green rust 1[GR-CO3] and green rust- II [GR-SO4] depends upon the relative ratio (R) of bicarbonate and sulphate concentrations. When R is equal to or more than 0.17, only [GR-CO3] was formed whereas when R less than 0.17, a mixture of GR-I [GR-CO3] and GR-II [GR-SO4] was obtained.
8. The presence of Fe (II] and Fe (III) was detected by Mossbauer spectral studies.
Regarding the comments of Manohar Saghal, GR was not able to form in soil (see work from Ph. Refait on GR or P. Dillmann on archeological corrosion products). The corrosion products found in soil are magnetite, siderite, chuckanowite and iron silicate.
Sir, I had already answered another question (Can we differentiate between FeCO3 and Fe2O3, visually, in a cross section of of a corrosion layer formed on a CO2-saturated solution? ) by the same author and wanted to write more on the general properties of RUST, GR-1 and GR-11; but it got wrongly placed below this question. Good or bad, you may find answers (at first place) to both these questions. Please note that I have already given a mechanism for the formation of sidesite/magnetite in my answer for your kind perusal.
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