FeS Layer Research Articles

Highly efficient FeS/Fe3O4 @ biomass carbon bifunctional catalyst with enriched oxygen vacancies for heterogeneous electro-Fenton catalysis

Low H2O2 production, narrow adaptive pH range and slow Fe(II) regeneration on the cathode still limit efficiency of electro-Fenton (EF) and its application. Herein, we designed a bifunctional catalyst with FeS and Fe3O4 nanoparticles dispersed on porous carbon (CFeS@C) using template of sodium alginate (SA)/FeSO4 hydrogel mixed with carbon black (CB), which presented high H2O2 generation efficiency and outstanding tetracycline degradation efficiency under wide pH ranges (3-8) with a low energy consumption of 19.6 kWh/kg total organic carbon (TOC). The introduction of CB created abundant oxygen vacancies in CFeS@C, promoting the oxygen adsorption and the electrochemical generation of H2O2, which further boosted the formation of •OH due to the interaction with Fe2+ on the cathode surface. Simultaneously, the reaction between the outer layer of FeS and Fe3+ not only accelerated iron cycling but also reduced the solution pH. It was verified that •OH and 1O2 played a dominant role in organics degradation. The system maintained stability after 10 cycles and effectiveness in the treatment of pharmaceutical wastewater. This study would offer a new strategy to develop an efficient and durable bifunctional catalyst for heterogeneous EF system working in wide pH conditions for wastewater treatment.

Thermal and magnetic evolution of Mercury with a layered Fe-Si(-S) core

Elucidating the structure and composition of Mercury is important for understanding its interior dynamics and evolution. The planet is characterised by unusual chemical characteristics and a weak magnetic field generated in a large metallic core, and its early evolution was also marked by the presence of a magnetic field, widespread volcanism and global contraction. Here we develop a parameterised model of coupled core-mantle thermal and magnetic evolution considering a layered Fe-Si(-S) core structure with chemical and physical properties of the mantle and the core based on previous laboratory studies. We seek successful solutions that are consistent with observations of Mercury's long-lived dynamo, total global contraction, present-day crustal thickness, and present-day interior structure. Successful solutions have a mantle reference viscosity >1021 Pa s (corresponding to a present-day bulk mantle viscosity >2×1020 Pa s), a silicon concentration in the core >13 wt%, a present inner core radius of ∼1000−1200 km and a thermally stable layer ∼ 500−800 km thick below the core-mantle boundary. Our results show that if present, a molten FeS layer atop the core has minimal effect on Mercury's long-term thermal and magnetic evolution. Predictions from our models can be tested with upcoming Bepi-Colombo observations.

Corrosion Evolution mechanism of N80 tubing steel under supercritical CO2 and H2S environment

The corrosion evolution mechanism of N80 tubing steel in 8 MPa supercritical CO2 and 0.1 MPa H2S environment was investigated. The results show that although the corrosion rate of N80 steel decreases with the prolonged corrosion time, it still maintains a high level of about 1.06 mm/y after a long period of 360 h. As the corrosion progresses, the corrosion products change from FeS to a mixture of FeS and FeCO3. The corrosion form of N80 steel changes from uniform corrosion to localized corrosion. The origin credited for localized corrosion is the detachment of large particles of FeS in the early corrosion stage. A double-layer film consisting of an outer layer of FeS and an inner layer of FeCO3 forms in the area of corrosion pits after a prolonged period of corrosion, which provides protection for the substrate, thereby causing the decrease of localized corrosion rate of N80 steel.

Enhancement of norfloxacin degradation by citrate in S-nZVI@Ps system: Chelation and FeS layer

The degradation of organic pollution by sulfur-modified nano zero-valent iron(S-nZVI) combined with advanced oxidation systems has been extensively studied. However, the low utilization of nZVI and low reactive oxygen species (ROS) yield in the system have limited its wide application. Herein, a natural organic acid commonly found in citrus fruits, citric acid (CA), was combined with the conventional S-nZVI@Ps system to enhance the degradation of norfloxacin (NOR). The addition of CA increased the NOR removal by about 31% compared with the conventional S-nZVI@Ps system under the same experimental conditions. Among them, the enhanced effect of CA is mainly reflected in its ability to promote the release of Fe2+ and accelerate the cycling of Fe2+ and Fe3+ to further improve the utilization of nZVI and the generation of ROS; it also promotes the dissolution of the active substance (FeS) on the surface of S-nZVI to further improve the degradation rate of NOR. More importantly, the chelate of CA and Fe2+ (CA-Fe2+) had higher reactivity than alone Fe2+. Free radical quenching and electron spin resonance (ESR) experiments indicated that the main ROS for the degradation of NOR in the CA/S-nZVI@Ps system were SO4•- and OH•. CA-bound sulfur-modifying effects on NOR degradation was systematically investigated, and the degradation mechanism of NOR in CA/S-nZVI@Ps system was explored by various techniques. Additionally, the effect of common anions in water matrix on the degradation of NOR in CA/S-nZVI@Ps system and its degradation of various pollutants were also studied. This study provides a new perspective to enhance the degradation of pollutants by S-nZVI combined with advanced oxidation system, which can help to solve the application boundary problem of S-nZVI.

Sulfidation of nano zero-valent iron for enhanced hexavalent chromium removal performance

Nano zero-valent iron (nZVI) and sulfidation-modified nZVI (S-nZVI) were synthesized and used to remove hexavalent chromium (Cr(VI)) in wastewater. Characterization of the products showed that the sulfidation process significantly changed the morphology of nZVI, with enhanced crystallinity. The effects of S/Fe ratio, pH value, and reaction temperature on Cr(VI) removal were studied. A S/Fe ratio of 0.5 was the most appropriate parameter, with a removal efficiency of 98% in the condition of pH = 2. The effects of anions (Cl−, NO3−, CO32−, and SO42−) and cations (Ca2+ and Mg2+) on the Cr(VI) removal efficiency were investigated, and the relevant mechanisms were discussed. The Cr(VI) removal efficiency of S-nZVI was significantly greater than that of nZVI, mainly owing to the existence of FeS layers that could protect Fe0 cores and prompt electron transfer. The aging and cycling experiments showed that S-nZVI could maintain its reactivity when facing the corrosion of water, and showed cycling stability. Thus, S-nZVI is an effective and feasible agent for the remediation of Cr(VI)-contained wastewater.

Internal differentiation and volatile budget of Mercury inferred from the partitioning of heat-producing elements at highly reduced conditions

Understanding the behavior of elements under highly reduced conditions is fundamental to explain the differentiation, crust formation, and volatile budget of Mercury. Here we report experiments on a synthetic composition representative of the bulk silicate Mercury (BSM), at pressure up to 3 GPa, temperature up to 1720 °C, and under highly reduced conditions (∼IW − 8 to ∼IW − 1, with IW the iron-wüstite oxygen fugacity buffer). We determined partition coefficients for >30 minor and trace elements between silicate melt, metal melt (FeSi), sulfide melt (FeS), and MgS solid sulfides. Based on these results and published literature, we modeled the behavior of heat-producing elements (HPE: U, Th, and K) during Mercury's early differentiation and mantle partial melting and estimated their concentrations in the mantle and crust. We found that U, K and especially Th are principally concentrated in the BSM and did not partition into the core because they are not siderophile elements. Uranium is chalcophile under highly reduced conditions, and so our model suggests that an FeS layer at the core-mantle boundary formed during Mercury's primordial differentiation would likely have incorporated large amounts of U, significantly increasing the Th/U ratio of the BSM. However, this is inconsistent with the chondritic or slightly sub-chondritic Th/U ratios of Mercury's lavas. In addition, the likely presence of mantle sulfides, such as MgS, would have also fractionated U and Th, increasing the mantle Th/U. It is possible to have an FeS layer if Mercury formed under less reduced conditions, or if the building blocks of Mercury had Th/U ratios close to the lower end of chondritic data. If, as suggested by our model, no FeS layer formed during differentiation, it means that the majority of HPE are concentrated in Mercury's thin silicate part. Based on the compatibility of U, Th and K, we also show that surface K/Th and K/U ratios are respectively 2–4 times and 3–6 times lower than expected for initial K/Th and K/U ratios similar to enstatite chondrites, implying that the planet suffered an important volatile loss via mechanisms that remain undetermined.

The composition of mackinawite

Abstract The composition of a mineral is a defining characteristic. The various compositions listed for mackinawite in current mineralogical databases and reference books, such as Fe(Ni)S and Fe1+xS, are both wrong and misleading. Statistical analyses of over 100 mackinawite compositions reported over the last 50 years show a mean composition of Me1.0S where Me = Fe + Co + Ni + Cu. Mackinawite is stoichiometric FeS. As with many sulfide minerals, Ni-, Co-, and, possibly, Cu-rich varieties occur in addition to the simple iron monosulfide. These varieties are best referred to as nickelian mackinawite, cobaltian mackinawite, and cupriferous mackinawite. The results confirm that these metals substitute for Fe in the mackinawite structure rather than being contained in the interstices between the Fe-S layers. Most compositional data on mackinawites derive from electron probe microanalyses of small grains in magmatic/hydrothermal associations. The result means that there is no dichotomy between the composition of ambient temperature synthetic mackinawite (which is supposed to be equivalent to sedimentary mackinawite) and mackinawites from higher temperature associations. The correct representation of the composition of mackinawite has implications for a wide swathe of fundamental science, including the origin of life, the genesis of magmatic ore deposits, the provenance of meteorites as well as industrial applications such as water treatment and steel corrosion. The stoichiometric formulation permits the mackinawite formula to be balanced electronically using conventional Fe and S ionic species. It also enables simple, balanced chemical equations involving mackinawite.

Chromium on Mercury: New Results From the MESSENGER X‐Ray Spectrometer and Implications for the Innermost Planet's Geochemical Evolution

AbstractMercury, the innermost planet, formed under highly reduced conditions, based mainly on surface Fe, S, and Si abundances determined from MESSENGER mission data. The minor element Cr may serve as an independent oxybarometer but only very limited Cr data have been previously reported for Mercury. We report Cr/Si abundances across Mercury's surface based on MESSENGER X‐Ray Spectrometer data throughout the spacecraft's orbital mission. The heterogeneous Cr/Si ratio ranges from 3.6 × 10−5 in the Caloris Basin to 0.0012 within the high‐magnesium region, with an average southern hemisphere value of 0.0008 (corresponding to about 200 ppm Cr). Absolute Cr/Si values have systematic uncertainty of at least 30%, but relative variations are more robust. By combining experimental Cr partitioning data along with planetary differentiation modeling, we find that if Mercury formed with bulk chondritic Cr/Al, Cr must be present in the planet's core and differentiation must have occurred at log fO2 in the range of IW‐6.5 to IW‐2.5 in the absence of sulfides in its interior and a range of IW‐5.5 to IW‐2 with an FeS layer at the core‐mantle boundary. Models with large fractions of Mg‐Ca‐rich sulfides in Mercury's interior are more compatible with moderately reducing conditions (IW‐5.5 to IW‐4) owing to the instability of Mg‐Ca‐rich sulfides at elevated fO2. These results indicate that if Mercury differentiated at a log fO2 lower than IW‐5.5, the presence of sulfides whether in the form of a FeS layer at the top of the core or Mg‐Ca‐rich sulfides within the mantle would be unlikely.

Corrosion behavior of heat-resistant steel T91 in high-temperature supercritical carbon dioxide with impurity O2, SO2 or H2S

This study investigated corrosion of T91 in supercritical carbon dioxide (sCO2) with impurity of 100 ppm O2, SO2 or H2S at 550 ℃ and 15 MPa. O2 slightly increased mass gain of corroded T91, and the corrosion kinetics both in pure sCO2 and in sCO2 with O2 impurity followed parabolic law. Fe2O3 was formed in outer corrosion layer due to enhanced partial pressure of oxygen. SO2 or H2S remarkably exacerbated T91 corrosion. For test in sCO2 with SO2 impurity, outer corrosion product was magnetite, and the inner was composed of Fe-Cr spinel and FeS, different from that in pure sCO2. Addition of H2S caused an external FeS layer and internal Fe-Cr spinel. Introduction of all the three impurities reduced carburization in the underlying alloy substrate.

Corrosion of Fe-Cr-Si Alloys in Oxidizing and Sulphidizing-Oxidizing Atmospheres

To clarify the mechanism of the third-element effect in sulphur-containing and sulphur-free oxidation environments, the corrosion behaviours of four kinds of Fe-xCr-ySi (x = 5, 10 at.% and y = 5, 10 at.%) alloys were studied at 600 °C in a H2-CO2 and a H2-CO2-H2S gaseous mixture with the same oxygen partial pressure. The results showed that, in the pure oxidizing atmosphere, thin and slowly growing protective oxide layers were formed on the alloys surfaces. Conversely, all alloys formed a corrosion product layer with an outer layer of FeS and an inner layer of a mix of oxides and sulphides in the oxidizing-sulphidizing atmosphere, which meant that adding Cr into the alloy as the third element had less of an effect on improving the alloy in the harsh sulphidizing-oxidizing environment. The oxidation and sulphidation mechanism as well as the effects of chromium and silicon on the corrosion resistance of the alloys was discussed.

Ball milling sulfur-doped nano zero-valent iron @biochar composite for the efficient removal of phosphorus from water: Performance and mechanisms

This study successfully prepared a novel sulfur-doped nano zero-valent iron @biochar (BM-SnZVI@BC) by modifying corn stover biochar with Fe0 and S0 using a mechanical ball milling method for effective phosphorus (P) adsorption in the waterbody. Batch experiments revealed that BM-SnZVI@BC (BC/S0/Fe0 = 3:1:1) reached a Qmax of 25.00 mg P/g and followed PFO and Langmuir models. This work had shown that electrostatic attraction, surface chemical precipitation, hydrogen bonding, and ligand effects all contributed to P removal. Since the FeS layer mitigated the oxidation-induced surface passivation of nZVI, sulfidation significantly extended the lifetime of BM-SnZVI@BC, removing 84.4% of P even after 60 d aging in air. The regeneration experiments of composites showed that re-ball milling destroyed the surface iron oxide layer to improve the properties of the recovered material. This is an essential step in the design of P-removal agents to implement anti-aging and commercialization of adsorbents for engineering applications.

Distinctive aging and inhibiting effects of microplastics between fresh and sulfidated nano-zero valent iron for various metal adsorption

Nano zero-valent iron (nZVI) is one of the most widely applied nanomaterials in environmental engineering for heavy metal and organic pollutants removal. However, its in-situ application is often compromised to the increasingly complex environmental systems, such as dissolved oxygen, coexisting (oxy)anions and emerging pollutants such as microplastics (MPs). To figure out measures to overcome the inhibition by MPs, sulfidation of nZVI was chosen as a solution to these drawbacks as sulfidation could enhance surface conductivity and electron transfer from the core of nZVI, and improve the electron selectivity. Batch experiments were carried out, indicating polyvinyl chloride (PVC) inhibited the metal sequestration efficiency of nZVI and resulted in desorption of metals (especially in Zn (II) system). After sulfidation, however, such inhibition on Zn (II) from PVC was only observed before equilibrium, while inhibition completely disappeared on the other metals. With the employment of S-nZVI, Cu (II), Pb (II), Zn (II) could reach a complete (100%) removal within 24 h even under the interference of PVC. Therefore, sulfidation assisted to overcome the inhibition of PVC on nZVI, which was attributed to enhanced reducing, stronger adsorption, higher affinity of metals with FeS layer, and the anti-ageing characteristics presented by S-nZVI. These findings suggest sulfidation is a promising method to overcome the inhibition of PVC MPs. Mechanisms identified between S-nZVI and metal ions in the presence of MPs may trigger studies to examine further insights to promote heavy metal removal using S-nZVI in the face of similar intricate surrounding environments.

Time- and temperature-dependence of the anticorrosion effect of sodium sulfide on Q235 steel for post-combustion CO2 capture system

As a heat stable salt, sodium sulfide remains thermally-unregenerable during the regeneration process for post-combustion capture. Therefore, it became important to undertake a full assessment of its potential presence in the system. The paper investigates the mechanism and influence of sodium sulfide on the corrosion behavior of Q235 steel in amine-based post-combustion CO2 capture technology with respect to exposure time and temperature. The inhibitive influence of sulfide in the system was first confirmed by electrochemical techniques, while gravimetry was used to evaluate the effect of time and temperature on the inhibitive actions. SEM/EDX and XPS were used to characterize the corrosion product films on the steel surface. The behavior of sulfide was largely dependent on its concentration as its inhibition ability decreased with increasing concentration, and this is credited to the presence of FeS corrosion film layer. Time-dependent corrosion studies of steel conducted at 50 °C for the solution with 0.05 M and 0.1 M sulfide showed that the inhibition efficiency decreased from 89.1% and 75.0% to 82.4% and 68.6%, respectively, after 7 and 14 days immersion. At 80 °C, the inhibition efficiency of 0.05 M sulfide decreased from 89.1% to 48.9% after 7 days immersion and 82.4% to 46.1% after 14 days immersion. Together with FeS layer, FeCO3, Fe2O3 were also confirmed on the steel surface by XPS measurement. The research provides practical information on the influence of sulfide in the absorber and rich-lean heat exchanger sections of a CO2 capture unit.

Combining electrokinetic treatment with modified zero-valent iron nanoparticles for rapid and thorough dechlorination of trichloroethene

In situ injection of nanoscale zero-valent iron (nZVI) slurry is a promising method to treat chlorinated solvents represented by trichloroethylene (TCE) in groundwater. In this study, the effects of sulfidation and emulsification treatment on the performance of nZVI reductive dechlorination of TCE under enhancement by an external electric field were evaluated. The hydrophobic oil film on the surface of sulfidized and emulsified zero-valent iron (S-EZVI) can sequestrate more than one-fifth of the unreacted TCE in the early stage of the experiment (at 5 min). The FeS layer formed on the surface of S-EZVI can facilitate the electron-transfer process and reduce the degree of corrosion of Fe0 with water by 94.0%. Electric-field-enhanced S-EZVI technology can remove more than 93.1% of TCE in the pH range 6.0–9.0, and the performances in overly acid and overly alkali environments both improved. Under the optimal conditions, the TCE removal rate and reaction constant of the applied electric field group reached 96.7% and 1.6 × 10−2 L g−1 min−1, respectively, which were much higher than those of the group without an electric field (53.2% and 3.3 × 10−3 L g−1 min−1) owing to rapid concurrent hydrogenolysis of dichloroethenes and vinyl chloride, or another transformation pathway, such as direct oxidation by the anode. Thereby, this method avoids accumulation of chlorinated intermediates, especially toxic vinyl chloride. This work shows that combination technology has many characteristics that are favorable for field application, and it is expected to provide a new reference and have application value for development of in situ efficient and thorough treatment of TCE-contaminated groundwater.

New insights of the interaction of H2S with mackinawite FeS in a wet environment: An ab initio molecular dynamics study

Mackinawite FeS is the most common corrosion product in the early stage of steel in H2S environment and also has an effective catalysis for hydrodesulfurization, hydrogen release reactions and heavy metal ion removal since its high specific surface area and reactive surface like other van der Waals materials. In this paper, ab initio molecular dynamics (AIMD) is used to study the interactions between small molecules (H2S, H2O and H) and FeS at 300 K. The calculation results show that the primary dissociation of H2S only occurs on the FeS(111) surface and H2S is chemically adsorbed on the (011) and (100) surfaces but physically adsorbed on the (001) surface. It will dissociate and generate H atoms when different amounts H2S or H2O molecules appear in the interlayer of FeS, in which H2S is more prone to dissociation. The dissociated H atoms will be “captured” by Fe/S atoms in mackinawite FeS, leading to the shrink of layer. Moreover, H atoms could combine into H2, which suggests that layered FeS has great potential for hydrogen generation. The diffusion coefficient of H atoms in mackinawite FeS layer is estimated about 1.67 × 10−8 m2/s. These findings have significant meaning for application of mackinawite FeS in corrosion science, hydrogen generation, hydrogen storage and other fields.

Hydrazine intercalated iron sulfide (N2H4)1-xFe2+δS2: Synthesis and characterization

The discovery of superconductivity in anti-PbO-type FeS has attracted great interest in the intercalation compounds of FeS. However, it is much more difficult to synthesize intercalates of FeS than to synthesize intercalates of FeSe. Here, we report a novel intercalation compound of FeS with chemical composition of (N2H4)1-xFe2+δS2 (x ​~ ​0.25, δ ​~ ​0.1), which is synthesized via a combination of self-flux solid state method and hydrothermal route. The successful intercalation of hydrazine was confirmed by X-ray diffraction, thermogravimetric analyses and attenuated total internal reflection-Fourier transform infrared spectroscopy, while the maintenance of anti-PbO-type FeS layer in the structure was verified by transmission electron microscopy and Raman spectroscopy. Magnetic and resistivity measurements show that (N2H4)1-xFe2+δS2 is a ferromagnetic metal rather than a superconductor. The low-temperature ferromagnetism may be caused by the intercalated hydrazine molecules or interlayer iron ions and the absence of superconductivity should be closely related to the ferromagnetic ordering.

Novel core-shell sulfidated nano-Fe(0) particles for chromate sequestration: Promoted electron transfer and Fe(II) production

Sulfidated nanoscale valent iron in form of FeS/Fe (0) shell-core nanoparticle has the aptitude to be a promising remediation material toward reductive removal of metal oxyanions. However, disrupted contact between Fe (0) core and FeS shell by thick iron oxides limited its reactivity improvement, and its mechanism of electron transfer remains unveiled. In this study, a novel sulfidated nZVI core-shell particles (FeS/Fe (0)) was fabricated via a modified post sulfidation approach to achieve a more uniform coverage of FeS for aqueous Cr(VI) sequestration. SEM and STEM tests confirmed the formation of the core-shell FeS/Fe (0) structure with a more solid interaction between FeS layer and Fe (0) core. The highest Cr(VI) removal rate was offered at optimal S/Fe molar ratio of 1/25 that the most chelated Fe2+ was also observed. The improved performance was due to that FeS shell with greater electronegativity could significantly accelerate the corrosion of Fe (0), facilitate the electron transfer form Fe (0) core to FeS shell according to the electrochemical tests. Moreover, FeS shell provided a protective layer for Fe (0) core so as to alleviate its anoxic passivation in water that FeS/Fe (0) had a better longevity for Cr(VI) removal than nFe (0). Characterizations of STEM and XPS revealed that Cr(VI) was reduced to Cr(III) and evenly coprecipitated with surface Fe(II)/Fe(III).

Influence of H2S on the general corrosion and sulfide stress cracking of pipelines steels for supercritical CO2 transportation

This article presents a literature review and corrosion testing outputs to fill the knowledge gaps about the effect of H2S on corrosion and sulfide stress cracking (SSC) of pipeline steels in supercritical CO2 (s-CO2) environments. In H2O-saturated s-CO2, H2S facilitates the formation of a protective FeS layer with a threshold content less than 50 ppm. Increasing temperature or adding O2 destroys the integrity of FeS layer and remarkably enhances corrosion. The corrosion is likely to be controlled by gas chemical reactions instead of electrochemical reactions. The SSC susceptibility of strained pipeline steels is lower than expected.

NaOH-Intercalated Iron Chalcogenides (Na1-xOH)Fe1-yX (X = Se, S): Ion-Exchange Synthesis and Physical Properties.

The discovery of the (Li1-xFexOH)FeSe superconductor has aroused significant interest in metal hydroxide-intercalated iron chalcogenides. However, all efforts made to intercalate NaOH between FeSe and FeS layers have failed so far. Here we report two NaOH-intercalated iron chalcogenides (Na1-xOH)Fe1-yX (X = Se, S) that were synthesized by a low-temperature hydrothermal ion-exchange method. Their crystal structures were solved through single-crystal X-ray diffraction and refined against powder X-ray and neutron diffraction data. Different from the (Li1-xFexOH)FeX superconductors that crystallize in a tetragonal space group P4/nmm with Z = 2, (Na1-xOH)Fe1-yX belong to an orthorhombic space group Cmma with Z = 4. The structural solution also reveals that there are vacancies in both Na and Fe sites and there are not iron ions in the (Na1-xOH) layer. This is probably why both Fe(II) and Fe(III) species exist in the title compounds, as detected by X-ray photoelectron spectroscopy. Based on magnetization and electrical resistivity measurements, the two compounds were found to be paramagnetic semiconductors. The absence of superconductivity should be closely related to the iron vacancies in the Fe1-yX layer. Theoretical calculations suggest that inducing superconductivity in (Na1-xOH)Fe1-ySe is promising due to the similarity of the electronic structures between stoichiometric (NaOH)FeSe and the (Li1-xFexOH)FeSe superconductor.

An in-situ Raman study on the oxidation of mackinawite as a corrosion product layer formed on mild steel in marginally sour environments

An in situ Raman flow loop with flow cell was developed to enable observation of the mackinawite corrosion product layer generated in marginally sour environments with traces of oxygen ingress. As a result, the mackinawite corrosion product layer was found to be oxidized into magnetite in the solution when [O2](aq) > 3 ppb(w), which was linked to the pitting initiation of mild steel (API X65) in marginally sour environments. The 2D precursor of nano-crystal mackinawite, the chemisorption FeS layer, Sads (Fe), was detected by ex situ Raman microscopy.