Green rust compounds (GR), i.e., Fe(II-III) layered double hydroxides, are important transient compounds resulting from the corrosion of steel in seawater. The sulfated variety, GR(SO
42−), was reported as one of the main components of the corrosion product layer, while the
[...] Read more.
Green rust compounds (GR), i.e., Fe(II-III) layered double hydroxides, are important transient compounds resulting from the corrosion of steel in seawater. The sulfated variety, GR(SO
42−), was reported as one of the main components of the corrosion product layer, while the chloride variety, GR(Cl
−), was more rarely observed. The carbonate variety, GR(CO
32−), is favored by an increase in pH and forms preferentially in the cathodic areas of the metal surface. Since Mg(II) is abundant in seawater, it may have a strong influence on the formation of GR compounds, in particular as it can be incorporated in the hydroxide sheets of the GR crystal structure. In the present work, the influence of Mg
2+ on the precipitation reaction of GR(SO
42−) was investigated. For that purpose, Mg
2+ was substituted, partially or entirely, for Fe
2+. The GR was then prepared by mixing a solution of FeCl
3·6H
2O, Na
2SO
4·10H
2O, NaCl, FeCl
2·4H
2O and/or MgCl
2·4H
2O with a solution of NaOH. The precipitation of the GR was followed or not by a 1-week aging period. The obtained precipitate was characterized by X-ray diffraction. It was observed that Mg(II) favored the formation of chloride green rust GR(Cl
−) and magnetite Fe
3O
4 at the detriment of GR(SO
42−). The proportion of GR(Cl
−) and Fe
3O
4 increased with the Mg(II):Fe(II) substitution ratio. Without Fe(II), the precipitation reaction led to iowaite, i.e., the Mg(II)-Fe(III) compound structurally similar to GR(Cl
−). It is forwarded that the presence of Mg
2+ cations in the hydroxide sheets of the GR crystal structure is detrimental for the stability of the crystal structure of GR(SO
42−) and favors the formation of other mixed valence Fe(II,III) compounds.
Full article