A. A. Olowe

The mechanism of oxidation of ferrous hydroxide in sulphated aqueous media: Importance of the initial ratio of the reactants☆

Abstract In simulated aqueous corrosion processes of iron in sulphated media, the most important factor is the ratio, R, of the initial concentration of Fe2+ and SO42− to OH− ions which separates the basic and acidic media at a value of 1 2 . Mossbauer spectroscopy, coupled with direct recording of the pH and the electrode potential, has identified six characteristic values of R and a basic sulphate, named sulphated ferrous hydroxide (SFH) and also determined the chemical formula of green rust 2 (GR2). Apart from GR2, two other ferrous-ferric intermediate compounds, a basic compound and a hydrated magnetite, the oxidation of which leads to delta FeOOH and magnetite respectively, are suspected. The various mechanisms of oxidation are established based on the rigorous analysis of the evolution of the composition of the end products of oxidation with respect to R.

Mössbauer study of the kinetics of simulated corrosion process of iron in chlorinated aqueous solution around room temperature: The hyperfine structure of ferrous hydroxides and Green Rust I

The hyperfine structure of the two ferrous hydroxides, Fe(OH)2, [2Fe(OH)2, FeOCl] and Green Rust I obtained during the oxidation of iron in chlorinated aqueous medium is determined. The structure of GRI is proposed. The kinetics of oxidation and the role of the chlorine concentration is thoroughly discussed.

MOESSBAUER EFFECT EVIDENCE OF THE EXISTENCE OF GREEN RUST 2 TRANSIENT COMPOUND FROM BACTERIAL CORROSION IN MARINE SEDIMENTS

Abstract Bacterial corrosion products obtained at the surface of mild steel samples are analyzed by Moessbauer spectroscopy, and x-ray and electron diffractions. The Green Rust 2 compound is identified as being the major species.

Hyperfine interactions and structures of ferrous hydroxide and green rust II in sulfated aqueous media

A sulfated ferrous hydroxide is obtained by mixing NaOH with melanterite depending on the R=[SO4−−]/[OH−] ratio and leading by oxidation to the green rust II transient compound. Hyperfine parameters are presented.

The influence of concentration on the oxidation of ferrous hydroxide in basic sulphated medium: Particle size analysis of goethite and δ-FeOOH☆

The evolution of the oxidation end product of ferrous hydroxide in the basic sulphated aqueous medium at R = {[FeSO4]/[NaOH]}initial = 0.2 is investigated at various initial concentrations. The end product is essentially well crystallized goethite at relatively high initial concentrations of the reacting chemicals. The crystal structure of goethite is modified as the initial concentration is decreased; at a critical low concentration, the orthorhombic structure is no longer observed. Inactive α′-FeOOH, which has a similar X-ray diffraction pattern to δ-FeOOH, is obtained. All these compounds are crystallographically related.

The simultaneous presence of green rust 2 and sulfate reducing bacteria in the corrosion of steel sheet piles in a harbour area

Mössbauer spectroscopy and X-ray diffraction analysis allow to detect the presence of green rust 2, the ferrous-ferric sulfated compound of composition, 4Fe(OH)2,2FeOOH,FeSO4,nH2O, mixed with magnetite at the surface of steel sheets corroded in a harbour area where the presence of sulfate reducing bacteria are also detected.

Mechanism of formation of magnetite from ferrous hydroxide in aqueous corrosion processes

The stoichiometric conditions for the formation of ferrous hydroxide Fe(OH)2, by mixing Fe2+ ions with caustic soda NaOH, leads by oxidation to magnetite, irrelevant of the foreign anions, e.g. Cl− or SO42−, as demonstrated from Mössbauer spectroscopy. The electrochemical potential Eh and pH value of the initial conditions correspond to the drastic change from basic to acidic medium, observed when varying the initial Fe2+/OH− ratio. Mössbauer analysis of the end products of oxidation at various temperatures shows that magnetite is only obtained at stoichiometry at very low temperature, but extends off stoichiometry at higher temperatures. The mechanism of formation of magnetite through an intermediate compound is discussed.

The influence of concentration on the oxidation of ferrous hydroxide in acidic sulphated aqueous medium: Particle size analysis of lepidocrocite☆

Abstract The evolution of the oxidation end products of ferrous hydroxide in the acidic sulphated aqueous medium at R = {[FeSO 4 ]/[NaOH]} initial = 0.5833 has been investigated at various initial concentrations of ferrous ions. The oxidation end products consist of lepidocrocite and goethite at relatively higher initial concentration and mainly of lepidocrocite at lower concentrations. The crystal structure of lepidocrocite is not modified as the initial concentration is decreased, contrary to what has been noticed for goethite in the basic medium.

Products obtained by microbially-induced corrosion of steel in a marine environment: Role of green rust two

An unusual low-water corrosion of steel sheet piles has been systematically investigated in a channel harbour (Boulogne sur Mer, France). An analysis of the environment reveals that all sampling of dark rust taken at different heights above marine sediments and kept in anaerobic conditions present unusual concentrations of sulfate-reducing bacteria. The rust products have been characterized by Mössbauer spectroscopy and X-ray diffraction, comprising the ferrous—ferric sulfated compounds of formula 4Fe(OH)2 · 2FeOOH · FeSO4 ·nH2O, called green rust 2, mixed sometimes with magnetite and a small amount of ferrous sulfide.

Mössbauer characterization of the corrossion products of steels in civil works: Suspension bridge and reinforced concrete

The rusting condition of the cables of suspension bridges is usually evaluated by self-induction measurements. This method assumes that rusts of same chemical composition have always the same magnetic properties. Unfortunately in some cases, this assumption has shown to be questionable and this study has demonstrated that Mössbauer spectroscopy supplies additional information on the nature of some detected defects. In the case of reinforced concrete, it has been empirically pointed out that the content of agressive element, e.g. Cl− ions, should be under a trigger. Mössbauer studies have demonstrated the physical meaning of this practical rule.