Influence of Mg2+ Ions on the Formation of Green Rust Compounds in Simulated Marine Environments

Abstract

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(SO42−), 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(CO32−), 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 Mg2+ on the precipitation reaction of GR(SO42−) was investigated. For that purpose, Mg2+ was substituted, partially or entirely, for Fe2+. The GR was then prepared by mixing a solution of FeCl3·6H2O, Na2SO4·10H2O, NaCl, FeCl2·4H2O and/or MgCl2·4H2O 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 Fe3O4 at the detriment of GR(SO42−). The proportion of GR(Cl−) and Fe3O4 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 Mg2+ cations in the hydroxide sheets of the GR crystal structure is detrimental for the stability of the crystal structure of GR(SO42−) and favors the formation of other mixed valence Fe(II,III) compounds.