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The above discussion on MIC focused primarily on the biological component of steel corrosion. However, the chemical interactions between steel and sulphide are an integral part of the MIC process and it may be possible to learn much about the MIC reaction by studying steel corrosion/electrochemistry in the presence of inorganic

sulphide. Recently, Rickard et al. have summarized Fe-S marine chemistry in the context of iron sulphide formation processes and their kinetics [84, 85].

1.4.1 Thermodynamics of the Iron/Sulphur/Water System

Figure 1.15 shows a potential-pH diagram of the iron/sulphur/carbonate/water system [86]. The diagram shows zones of stability for a number of phases depending on the redox conditions. Within the blue box, mackinawite and siderite are the stable phases. For more oxidizing conditions Fe2O3 is the stable phase.

FIGURE 1.15: Pourbaix (E-pH) diagram for the Fe/S system, at a dissolved Fe2+

concentration of 1 × 10–6 moles kg–1,517 Torr CO2, and 26 Torr H2S [87]. The blue square encompasses natural anaerobic groundwater conditions.

1.4.1 Kinetics of Iron-Sulphide Systems

Unfortunately, there are few relevant studies investigating the formation of sulphides on iron and/or carbon steel. Electrochemical investigations performed by

Shoesmith et al. in the late 1970s to early 1980s remain the primary source of

information on FeS film formation on steel [88-91]. According to these authors, at pH 13, where the [OH–] dominates the ratio of [HS–]/[OH–] at the steel/solution interface, a Fe(OH)2/Fe3O4 film forms prior to a transformation to FeS [92]. As iron(III) oxy- hydroxides are known to be unstable in the presence of HS– [93], deposited oxides may subsequently be converted to FeS.

Surface-adsorbed bisulphide species are thought to influence the anodic process in a similar way to the influence of hydroxyl ions on iron corrosion [91]. These species can be formed via chemical (1.23) and electrochemical (1.24) reactions:

Fe + HS–aq→FeHS–ads (1.23)

FeHS–ads→ FeHS+ads + 2e– (1.24)

Subsequently, the reaction of FeHS+ads proceeds via two concurrent processes: the release of ferrous species into the liquid phase (1.25), and the formation of a surface sulphide film (1.26):

FeHS+ads → Fe2+aq + HS– (1.25) FeHS+ads → FeSads + H+ (1.26)

With a solubility product (KSP) for FeS of 8 × 10–19, the dissolution reaction (reaction 1.25) would lead very quickly to the precipitation of ferrous sulphide, primarily

[91]. The concentration of sulphide and pH govern which ferrous sulphide phases are formed. In contrast to the dissolution reaction (1.25), the direct formation of a surface phase leads to a mackinawite layer on the steel surface, reaction (1.26). Shoesmith et al. observed the presence of a surface mackinawite layer on carbon steel immersed in a saturated solution of hydrogen sulphide (pH 4-7), indicating that the process of sulphide phase formation, reaction (1.26), dominates over the dissolution step, reaction (1.25), in slightly acidic media [88]. The solution acidity promotes the dissolution of the surface sulphide film formed via reaction (1.27)

FeHS+ads + H3O+→ Fe2+ + H2S + H2O (1.27)

This would potentially explain the porosity and imperfection of the surface mackinawite phase observed at pH < 7 by Shoesmith et al. [91]. Generally, partial passivation of steel surfaces in sulphide containing solutions has only been observed at or above neutral pH, which implies that the mackinawite layer becomes more protective with increasing pH [91].

Shoesmith et al. showed, however, that HS– strongly adsorbs at active oxide sites, suggesting that HS– can displace OH– from the metal/oxide surface in part because of sulphide’s greater polarizability [92]. Mackinawite was shown to grow preferentially at fault sites on preformed iron oxides. Its rate of growth is controlled by the dissolution of a thin oxide film, which is catalyzed by an increase in local pH leading to oxidation of sulphide [92]. While Shoesmith et al. provide some useful kinetic information with regards to HS–, the alkaline conditions and high sulphide concentrations employed are

not representative of the near-neutral pH and low sulphide concentrations anticipated in dilute groundwater solutions [92].

In order to mimic the relevant chemistry, while avoiding biological complications, Newman et al. [94] used Na2S as the source of HS– in order to develop a mechanistic understanding of the complex interactions between steel and sulphide under various in redox conditions. In their work, polished electrodes exposed to 1.5 x 10–2 mol L–1 HS– would passivate (polarization resistance [RP] > 50,000 ohm cm2) within eight days. RP represents the film resistance, which is inversely proportional to the CR. However, electrodes pre-corroded in a low sulphide solution (6 x 10–4 mol L–1 HS–) and then later exposed to higher concentrations (1.5 x 10–2 mol L–1 HS–) would not passivate (RP < 2,000 ohm cm2) [94].

Regardless of sulphide source and iron sulphide film morphology, Raman analyses indicate that mackinawite is the dominant iron sulphide phase formed both biogenically and inorganically under anaerobic conditions [95-97]. Bourdoiseau et al. recently monitored the oxidation of Fe(II)-mackinawite to an Fe(III)-containing mackinawite, FeII₁₋₃xFeIII2xS, in air [96];

2FeIIS + xO2 + 2xH2O → 2FeII1−3xFeIII2xS + 2xFe2+ + 4xOH− (1.28) 4FeII1–3xFeIII2xS + (3−3x)O2 + (2–2x)H2O → (4−4x)FeIIIOOH + 4S0 (1.29)

Any unreacted mackinawite can further react with sulphur to form greigite (Fe3S4) [98],

which may further convert to form Fe(III) oxy-hydroxides and pyrite, FeS2, in air [96],

3FeIIFeIII2S4 + 2O2→ FeIIFeIII2O4 + 6FeIIS2 (1.31)

The significance of the investigations by Bourdoiseau et al. [95, 96]are that they indicate the possible evolution of phases expected under dry out conditions on pipelines. These anaerobic to aerobic transformations may be very important in determining the very rapid CR under aerobic MIC conditions.

Despite the efforts discussed above, the mechanism that governs FeS formation on steel at near-neutral pH under anaerobic conditions remains unclear.

1.4.2 Iron Oxide to Sulphide Conversion Kinetics

The reduction kinetics of various iron oxy-hydroxides in the presence of inorganic sulphide in aqueous systems have been well studied at near-neutral pH [93, 99-101], and shown to be influenced by the presence of SRB [102]. These studies are important geochemically since hydrogen sulfide is an important reductant of iron oxides, and may be the major reagent for the reductive dissolution of iron oxides in sulphide-containing marine sediments [99]. For example, γ-Fe2O3 may be slowly reduced to FeS and S0 [93]:

A similar reaction appears to occur on oxidized iron surfaces, and Hansson et al. have characterized the transformation of a “magnetite”-like oxide film on steel to mackinawite (Fe1-xS) using in situ Raman spectroscopy [67].

The mechanism of FeIII oxide reduction in the presence of sulphide involves a surface-controlled process, originally proposed by dos Santos Afonso and Stumm [103]. Initially, HS– rapidly displaces hydroxide from an FeIII surface site,

≡FeIIIOH + HS– → ≡FeIIIS– + H2O (1.33)

followed by electron transfer from sulphide to iron (reaction (1.34)), and the generation of a S•– radical (reaction (1.35)):

≡FeIII

S–→ ≡FeIIS (1.34)

≡FeIIS + H2O → ≡FeIIOH2+(ads) + S•– (1.35)

This leads to the release of Fe2+ and the generation of another reaction site,

≡FeII

OH2+(ads)→ new surface site + Fe2+(aq) (1.36)

The S•– radical (reaction (1.35)) can then rapidly reduce additional FeIII species leading to

the formation of elemental sulphur (reaction (1.37)) [99, 104],

≡FeIII

Dissolved Fe2+ may then react with additional HS– to yield FeS (reaction (1.18)).

In a recent publication by Hansson et al., the authors briefly describe the nature of iron sulphide films and how HS– aggressively attacks unprotected regions of the steel surface [67]. Unfortunately, while this article offers relevant information about iron sulphide formation, the time frame of the experiments performed were on the order of minutes, which is much too short for direct comparison to studies performed in the field over long time periods. What remains unknown is how iron oxide and sulphide phases form, and the inter-conversion between phases, on steel at near-neutral pH and at sulphide concentrations anticipated in groundwaters. It is unclear from the literature whether an iron oxide intermediate is required before forming iron sulfide on steel.