Study of the Fe ( II-III ) hydroxysalts transformation according to the pH and the concentration of the ions present

Sulphated green rust, GR (SO4), is one of the main corrosion products of carbon steel in marine environments. It is Fe (II)-Fe(III) hydroxylsalt in sheets, consisting of alternating layers of iron-hydroxide type Fe(OH)2, loaded positively due to the presence of the cations Fe(III) and negative interlayers consisting of anions and water molecules. This compound is strongly associated with the metabolism of sulphate-reducing bacteria, and can also evolve under cathodic protection. Thus, recently, GR (CO3) has been detected in place of GR (SO4) on already corroded ordinary steel, newly subjected to cathodic protection. This presence is due to the pH and[SO4 2−] [HCO3 −] ⁄ conditions imposed by the cathodic protection. In this paper, we chemically synthesize sulfated and carbonate green rust in a chlorinated medium; we then study their respective transformation according to the concentration [SO4] / [HCO3] ratio and pH. Our results show that from a GR (SO4), GR (CO3) is formed from a pH ≥8.2 for [SO4] / [HCO3] = 12 and without any change in pH for [SO4] / [HCO3] <12. Whereas from GR (CO3), GR (SO4) is formed for [SO4] / [HCO3] > 1 without any change in pH.


Introduction
One of the main corrosion products of carbon steel in the marine environment is Fe(II-III) hydroxysulfate, Fe(II − III) (Fe 4 II Fe 2 III (OH) 12 SO 4 .8H 2 O) 1,2 , better known as sulphated green rust.Green rusts (GRs)are hydroxysalts of Fe(II − III)) in sheets, established by the alternation of layers of a hydroxide of type Fe(OH) 2 , loaded positively due to the presence of the cations Fe(III) , and interlayers were established of anions and water molecules 3,1 .The thickness of the interlayer has been assumed to be defined by the intercalated anion.A compound containing spherical or planar anions, such as Cl − or CO 3 2− , produce similar X-ray diffraction(XRD) patterns, and as a group, they have been known as GR1.The tetrahedral anions produce larger basal plane spacing: these have been known as GR2 1,4,5 .There are various green rusts, in particular, those based on the main anions present in the sea water.
In the conditions of concentration of the sea water in these various anions, [SO 4  2− ] = 0.02824mol/l ; [HCO 3 − ] = 0.0023mol/l and [Cl − ] = 0.5368mol/l, only the sulphated variety is formed.To explain this peculiarity, experiments of a laboratory devoted to the oxidation by the air of a suspension of Fe(OH) 2 in the presence of SO 4 2− and Cl − ions showed that the GR(SO 4 2− ) formed instead of the GR(Cl − ) even in solutions with large[Cl − ] [SO 4  2− ] ⁄ molar ratios 6 .
Other experiments concerned this time the formation and the transformation of the GR(Cl − ) from an aqueous suspension of Fe(OH) 2 in the presence of SO 4 2− and CO 3 2− ions.Both anions were separately added at the end of the formation of the GR(Cl − ).In both cases, GR(Cl − ) was transformed into GR(SO 4 2− ) or into GR(CO 3 2− ), this is explained by the fact that the layered structure of GRs presents a strong affinity for divalent anions 7 .
The competition between the SO 4 2− and CO 3 2− ions have also been studied by the same authors.In every case, the GR(CO 3 2− ) of (Fe 4 II Fe 2 III (OH) 12 CO 3 .2H 2 O) formula was obtained instead of the GR(SO 4 2− ) .
The GR(SO 4 2− ) was obtained only when the CO 3 2− ion was insufficient to precipitate all Fe 2+ ions present.
The green rust is an unstable compound and that is quickly oxidized into oxyhydroxide of Fe 3+ ,(FeOOH) by the dissolved oxygen.So, at the beginning of the process of corrosion, the layer of rust formed on steel immersed in the sea water would consist of FeOOH near the electrolyte and GR(SO 4 2− ) near the steel.However, after 6-12 months of exposure, GR(SO 4 2− ) is consistently found to be associated with iron sulphide (FeS) and sulphatereducing bacteria (BSR) 2 .
The presence of iron sulphide (mackinawite, (FeS)) is a consequence of the metabolic activity of sulphate-reducing bacteria since sulfur is only present in seawater as sulphate 8 .SRBs are anaerobic microorganisms, and their presence among corrosion products confirms that anoxic conditions are established at the steel (GR(SO 4 2− )) ⁄ interface and inside the rust layer 9 .
The anoxic conditions are because the dissolved oxygen is consumed outside of the layer of rust by the aerobic microorganisms and by the GR(SO 4 2− ).This leads mainly to FeOOH.Thus, after some time, the kinetics of corrosion is no longer controlled by the transport of oxygen as this corrosion process is related to the activity of the microorganisms.Some authors suggest that the availability and transport of nutrients could be a limiting step 10 .
A recent study 11 has shown that SRBs can reduce sulphate ions from the GR(SO 4 2− ) structure.This phenomenon leads to the transformation of the GR(SO 4 2− ) to a variety of compounds including iron sulphide (mackinawite).
Cathodic protection is widely used to protect submerged carbon steel structures against corrosion 12 .In the range of applied potentials, the reduction of dissolved oxygen is done at the interface of the metal and is accompanied, for more cathodic potentials, by the formation of hydrogen 13,14 .These two reactions produce  − hydroxyl ions which alkalinize the medium locally.
This alkalinization of the medium leads to the change of the equilibrium of the inorganic carbon at the interface of the steel by promoting the formation of the carbonate ions (CO 3 2− ) at the expense of the bicarbonate ions ( 3 − ), according to the reaction (3).This leads also to the precipitation of CaCO 3 [13][14][15] .
The composition of the rust layers present on the surface of steel coupons immersed for 6 years and unprotected is similar to that of rust layers present under the calcareous deposits on protected and immersed coupons.These rust layers consist mainly of GR(SO 4 2− ), magnetite, mackinawite, and Fe(III) oxyhydroxide.The same composition has been reported for immersed coupons for 6-12 months 8 and 11 years 6 .Recently, the analysis of the corrosion products present on coupons subjected to cathodic protection during one year after 5 years of immersion without protection shows that it is not the GR(SO 4 2− ) that is present among the products of corrosion but the GR(CO 3 2− ) 15 .These authors assume that the presence of this compound is due to the transformation of the GR(SO 4 2− ) into GR(CO This transformation has the same origin as that leading to the formation of calcareous and occurs as the pH or the concentration of carbonate ions increases through reactions (1) or (2) due to cathodic protection.It involves anion exchange in the interlayer without dissolving the solid phase.The hydroxide layers are preserved while the interlayers are changed from GR2 to GR1 without the possibility of incorporation of an anion HCO 3 − 15, 16 .
In this study, we will synthesize GR(SO 4 2− ) and GR(CO 3 2− ), and we will study their relative stability according to the pH and on the presence of the HCO 3 − and SO 4 2− anions, in particular, to the concentration ratios of the sea water

Synthesis of green rust
The hydroxysulfate and hydroxycarbonate of iron (II)-(III) respectively GR(SO 4 2− ) and GR(CO 3 2− ) can be synthesized by oxidation of a precipitate of ferrous hydroxide in aqueous solution 6,17,18 .The simplest method is to precipitate Fe(OH) 2 from sodium hydroxide (NaOH) solution and ferrous sulfate solution to obtain GR(SO 4 2− ) 18 , but it is much more difficult to prepare GR(CO 3 2− ) from a ferrous carbonate since the latter is insoluble;in this case,Fe(OH) 2 is precipitated from the ferrous sulphate before adding sodium carbonate to obtain the GR(CO 3 2− ) 19 .An alternative method of GR(SO 4 2− ) preparation has been developed to simulate the formation conditions of GR(SO 4 2− ) in the marine environment 6 .This experimental approach, which derives from the fact that GRs have a high affinity for divalent ions 20 , consists of using ferrous chloride(FeCl 2 , 4H 2 O) to obtain Fe(OH) 2 and then adding (Na 2 SO 4 , 10H 2 O) or (Na 2 CO 3 , 10H 2 O) respectively, just after the precipitation to provide the SO 4 2− or CO 3 2− ions necessary for obtaining green rust respectively sulfated or carbonated.It is this last experimental approach that we use in this study.The concentrations used are: Obtaining green rust requires an excess of iron (II) 17 , so that an initial ratio [Fe 2+ ] [OH − ] ⁄ greater than or equal to 0.6 is suitable.In the case of this study, the[Fe 2+ ] [SO 4 2− ] ⁄ or [Fe 2+ ] [CO 3 2− ] ⁄ was set at 6, which allows incorporation into green rust of almost all of the sulphates or carbonates respectively, according to the following reactions, which describe the formation of green rust from Fe(OH) 2 : The synthesis of the GR is carried out in a beaker containing 200ml of an aqueous suspension of Fe(OH) 2 and dipping in a thermostatic bath at (25 ± 0.5)°C.The solution is vigorously stirred with a magnetic bar (stirring speed= 700rpm), constantly, to allow aeration homogeneity.Two electrodes introduced into this solution: a platinum electrode, and a saturated calomel electrode, although the Eh, electrode potential, is referred to the standard hydrogen electrode, to follow over time the evolution of the electrode potential of the solution.The chemical compounds used are provided by (Aldrich) and have a minimum purity of 99%.Once the green rust has been obtained, an event indicated by a rapid variation of Eh 6,17,18 (Figure 1), the precipitate is rapidly treated to prevent its oxidation by the ambient air, then either filtered for characterization or placed in a tightly closed bottle and covered with parafilm for preservation.

Green rust treatments
Two series of experiments were carried out: at different pH.After treatment, the GR is analyzed by infrared spectroscopy and X-ray diffraction.

Instrumentation and sample preparation
The diffractograms were made using a Bruker-AXS D8 Advance diffractometer, whose Cu Kα radiation has a wave length λ = 0.15406 nm, and used in Bragg-Brentano geometry [θ − 2 θ].The precipitates are first filtered using a vacuum pump on a filter paper and the paste obtained is then rapidly deposited on the sample holder and covered with glycerol in order to limit the oxidation during the analysis.
The spectroscopic analysis were carried out using a Nexus Fourier Transform Infrared Spectrometer equipped with the ATR Smart MIRacle accessory and a capsule allowing to work in a controlled atmosphere.The precipitate is first filtered and rinsed with water and then with ethanol before being placed on the crystal.There, it is dried by a flow of nitrogen through the capsule.
In (Figure 2a), the GR(CO 3 2− ) and GR(SO 4 2− ) spectra are superimposed.Each has a wide absorption band between 3300cm −1 and 3500cm −1 , which is associated with stretching vibrations and the hydrogen bonding of water, and an absorption band at 1650cm −1 which is attributed to the water vibration of deformation.On the spectrum (1) the band at 1350cm −1 is attributed to the symmetric stretching mode of CO 3 2− 17 .
On the spectrum (2), the band at 1100cm −1 , supported by an another one at 1140cm −1 , corresponds to the symmetric stretching mode of SO 4 2− 16,24 .Finally, the absorption bands at 779cm −1 and 840cm −1 are attributed to the deformation of Fe − OH (crystal lattice vibration) and are the signature of green rust 16 .
The (Figure 2c) shows the X-ray diffractogram of the mixture of equal parts of the two sulfated and carbonated GRs.It presents the three main intense line located at 2θ ~8°; 16° and 24°of GR(SO 4 2− ),as well as the two main intense rays located at 2θ~12° and 23.6°of GR(CO 3 2− ) 8,15 .
We also synthesized a suspension of GR(CO 3 2− ) under the same conditions as the previous one, but from a mixture of the three anions to confirm the formation of the GR(CO 3 2− ) at the expense of those based on the present anions.(Figures 3a,b are localized at 2θ = 11.8° and 23.7° 8,15 .These results will serve as a reference to identify the transformation of GR(SO 4 2− ) into GR(CO 3 2− ) and vice versa in subsequent treatments. ) are added.The pH increases by about 3 10 ⁄ of the unit and becomes equal on average to 7.53.NaOH is then added gradually to obtain suspensions at well-defined pH.Table 1 summarizes the results obtained.
At pH ≤ 8.2, we do not obtain GR(CO 3 2− ), the FTIR spectroscopy analysis of the samples at pH = 7.6 and 8.2, respectively without and with pH modification, has a spectrum identical to that of the GR(SO 4 2− ), as shown in the spectrum1of (Figure 2a).The X-ray diffraction analysis of these samples reveals the same GR compound and its spectrum is identical to the X-ray diffraction pattern of a GR(SO 4 2− ) alone as shown at the (Figure 4a) 22 .
From a pH = 8.33 we observe on the usual FTIR spectrum of the GR(SO 4 2− ) a small absorption at 1350cm −1 corresponding to the absorption wave number of the CO 3 2− group of the GR(CO 3 2− ) (Figure 4b).This absorption is not due to a CO 3 2− ion adsorbed but to a CO 3 2− that is well incorporated in a green rust structure.This phenomenon is observed because, at a pH = 8.90 and more, the shape of the peak is precise, the absorption is important, indicating the formation of a larger quantity of GR(CO 3 2− ).It should be noted, however, that the pH change must be progressive to prevent the dissolution of the precipitate.At pH = 10.36, the absorption is not greater than at pH = 9.27.The transformation of GR(SO 4 2− ) into GR(CO 3 2− ) is depending on the pH, but quantitatively it is not very important because [HCO 3 − ] is very small (0. OO23 mol.l −1 ).For a given pH, if the transformation is complete, a maximum of 0. OO23 mol.l −1 of GR(CO 3 2− ) would be formed and the ratio between the two GRs concentrations is about 8 in favor of GR(SO 4 2− ).Furthermore, after 14 days of aging, the pH of the sample was found to have decreased slightly from its earlier value of 8.2 [8.18], and its FTIR spectrum reveals a small absorption indicating the formation of a GR(CO 3 2− ) (Figure 4c).The transformation does not necessarily occur in real time even at pH > 8.40.Thus the compound was detectable half a day later.The (Figures 4 c,d) respectively show FTIR and X-ray diffraction spectra of a sample which has a pH of 8.65 and which revealed the presence of GR(CO 3 2− ) only after 24h of aging.From these results, it can be assumed that the transformation of GR(SO 4 2− ) into GR(CO 3 2− ) is possible at pH close to 8.2.
We also noticed during these tests, that the pH decreases during aging.If it reaches a value of less than about 8, the GR(CO 3 2− ) formed by transformation after alkalinization treatment disappears.It seems that the GR(CO 3 2− ) compound is unstable at pH ≤ 8 in a sulphated environment and that the opposite reaction would be possible.According to reaction (4) a drop in pH causes the reaction to shift in the direction of GR(SO 4 2− ) formation.towards the formation of GR(CO 3 2− ) by an exchange of anions, as shown in the reaction (4) 15 .
For a ratio of [SO 4 2− ] [HCO 3 − ] ⁄ = 12, we saw that the GR(SO 4 2− ) did not convert to GR(CO Indeed, if the concentration of [HCO 3 − ] ion increases this means that for a concentration ratio of [SO 4 2− ] [HCO 3 − ] ⁄ ≤ 6, there is the formation of GR(CO 3 2− ) from a GR(SO 4 2− ) (Figure 5).Thus, the transformation of GR(SO 4 2− ) into GR(CO 3 2− ) in the presence of SO 4 2− and HCO 3 − ions seem to depend on the value of the concentration ratio of the two species of anions 23 for a pH < 8.2 in our operating conditions.For a ratio of concentration equal to or less than unity, so an amount of NaHCO3 bigger than that of SO 4 2− involved in the structure of GR(SO 4 2− ), the transformation of GR(SO 4 2− ) to GR(CO 3 2− ) is not complete since the analysis of the sample reveals the presence of the two GRs after a week of aging.For an initial formation with an equal concentration of anions, that is to say, for a ratio equal to 1, it is only the GR(CO 3 2− ) which is formed 1 .
No GR The GR(CO 3 2− ) suspension prepared has an average pH of 7.70.This increases by one to twotenths of a unit (1 10 ⁄ to 2 10 ⁄ ) after the addition of sodium sulphate and sodium bicarbonate.The pH is then adjusted to values close to neutrality to allow the GR(CO 3 2− ) to transform itself or not into GR(SO 4 2− ).The GR(CO 3 2− ) alone, treated with HCl, is stable to pH = 6.90.At pH = 6.80, it has been dissolved under the effect of H + ions and we do not observe GR on the infrared spectrum.
The GR(CO 3 2− ) sample in the presence of SO 4 2− and HCO 3 − ions and treated with H + did not reveal any GR compound at pH = 6.90; this same sample analyzed 24 hours later revealed the presence of the two GRs, the GR(CO 3 2− ) and the GR(SO 4 2− ), (Figure 6).The GRs have probably dissolved and reformed during aging.Note that the main (2θ = 8°) line of the GR(SO 4 2− ), is as important as that the GR(CO 3 2− ); (2θ = 11.7°).These results also show that GR(CO 3 2− )is not stable at pH < 6.90 in an environment with or without SO 4 2− ions.
The results show that, for the concentration ratios considered, the GR(CO 3 2− ) is transformed into GR(SO 4 2− ) when it is in an environment where the concentration in its anion is 6 to 12 times lower than that of the sulfate ion([HCO 3 − ] < 6[SO 4 2− ]), this ratio would be less than 4 according to 25 , so GR(SO 4 2− ) is not found when R is 1 and 0.5.(Figure 7a) shows the infrared spectra of the GRs obtained at different R. The absorption due to GR(SO 4 2− ) decreases when R decreases.For R = 12 the sample was analyzed a second time 24h after the treatment, its diffractogram, shown in (Figure 7b), is that of a mixture of the two GRs, where the intensity of the main line of the GR(SO 4 2− )is ¼ of that of the GR(CO 3 2− ).This shows that the transformation of the GR(CO 3 2− ) into GR(SO 4 2− ) is only partial!For R = 1 the sample was analyzed after one week of aging, its X-ray diffractogram presented in (Figure 7c) is that of a GR(CO 3 2− ) alone.Samples adjusted to pH = 8.2 gave similar results to those whose pH was unchanged.These results show that the transformation of the GR(CO  2− ion engaged in a green rust structure and the free CO 3 2− anion in solution.This exchange is related to the value of the concentration of the sulphate to bicarbonate species ratios which are present and to a lesser extent to the pH of the medium.For a [0.0282] [0.0023] = 12 ⁄ which is that of seawater, the transformation of GR(SO 4 2− ) into GR(CO 3 2− ) is related to the pH of the solution.For ratios of concentrations lower than 12, the transformation from GR(SO 4 2− ) to GR(CO 3 2− ) no longer depends on pH and becomes dependent only on the value of this ratio.We have seen that the inverse transformation, from GR(CO 3 2− ) to GR(SO 4 2− ) is also possible, but only for R ≥ 6.A drop in pH would favor this transformation; thus, for R = 12, the transformation is greater at pH = 6.90 than at pH = 7.72!.Whatever the initial green rust, the transformation is only partial for all the pH and concentration ratios considered in this study.
) respectively show the evolution of the potential E of the electrode as a function of time during the formation and oxidation of the GR(CO 3 2− ), and the X-ray diffractogram obtained during the analysis of the product collected at time tg.The diffractogram is typical of a GR(CO 3 2− ) whose two main intense lines

Figure 5 .
Figure 5. FTIR spectra of analysis of GR(SO 4 2− ) at R variable and without pH modification

3 2 −
) in to GR(SO 4 2− ) for this ratio do not depend on the pH of the solution.In general, GR(CO3  2−  ) is formed by alkalinization of a GR(SO 4 2− ) from pH = 8.2 if R = 12, and without pH change for R ≤ 6.The GR(SO 4 2− ) is formed from a GR(CO 3 2− ) for R ≥ 6 whatever the pH (under our test conditions: 7.7 ≤ pH ≤ 8.2).

Table 2 .
3  2−) if the pH is not increased by 8.2, therefore this reaction would be possible only by the consumption of H + ions.For R < 12, the amount of NaHCO 3 to be added is calculated relatively to [SO 42− ] = 0.0282 mol.L −1 Treatment of GR(SO 4 2− ) with [Na 2 SO 4 ] = 0.0282 mol.L −1 + [NaHCO 3 ] variable.

Table 4 .
In this part, we did two types of tests: in the first test we treat the GR(CO 3 2− ) with SO 4 2− and HCO 3 − ions in a variable ratio ranging from 12 to 0.5 ; while in the second test, we adjusted the pH to 8.28 to recall the physicochemical conditions of the sea water; Table4summarizes the results obtained : Treatment of GR(CO 3 2− ) with [Na 2 SO 4 ] = 0.028 mol.L −1 + [NaHCO 3 ] variable.