Raman Spectroscopy of Mackinawite FeS in Anodic Iron Sulfide Corrosion Products

Abstract

Raman spectroscopy in a confocal microscope was used to study electrochemically synthesized corrosion products from sour gas experiments. When exposed to oxygen-containing atmosphere, the initial mackinawite FeS corrosion product transformed under laser irradiation to hematite, Fe 2 O 3 . Measurements with a thin water layer on top of the corrosion products prevented the transformation, as drying was prevented. In situ Raman measurements of mackinawite formation avoided the problem of transformation completely. In situ and operando, the initially formed mackinawite showed two Raman peaks in the wavenumber range >180 cm −1 centered around 200-215 cm −1 and 285-300 cm −1 . On an empirical basis, these modes were assigned to a B 1g mode of the iron sublattice and an A 1g mode of the sulfur sublattice, respectively. A comparison with a literature assignment for aged mackinawite suggests that the aging observed involves significant changes in the sulfur sublattice. Corrosion of iron in H 2 S containing solutions is a general problem in crude oil and natural gas production, and is generally referred to as sour corrosion. Aqueous H 2 S solutions promote corrosion of steels, [1][2][3] but the exact nature and mechanisms of corrosion strongly depend on the reaction conditions. 4-7 While the process has been widely investigated for pure iron, [8][9][10][11][12] and carbon steels, [13][14][15][16][17] there is still a lack of understanding of the reaction path and electronic properties of the corrosion products. 18,19The chemistry of the corrosion products formed during H 2 Striggered corrosion is rather complex, as there are many different solids consisting only of iron and sulfur. 20 However, mackinawite has been found to be the initial, 8,21 and probably most important corrosion product, of iron in aqueous sulfide solutions as it was observed in reactions carried out over a wide range of pH and temperature. 11,19,[22][23][24] It possesses a tetragonal, layered crystal structure, [25][26][27] and can be synthesized by precipitation from solutions containing Fe 2+ and S 2− solutions, 28,29 or by reaction of sulfide solution with metallic iron. 19,21,26,30 In the field of microbially induced corrosion, sulfatereducing bacteria are known to transform sulfate compounds into iron sulfides, e.g. mackinawite. [31][32][33][34][35][36] Raman spectroscopy has become an important tool for identification of corrosion products. [37][38][39][40][41][42][43][44][45] Some major problems may still occur due to laser heating, fluorescence, or low sensitivity as a consequence of the small cross-sections of Raman scattering. However, the fact that glass and water are both very weak Raman scatterers makes this technique suitable for in situ measurements in aqueous environments. 46-48The presence of water has an additional positive effect on the process as it decreases the heating from the laser. The use of (in situ and operando) Raman spectroscopy for study corrosion process studies of iron in sulfide rich environments has been already repor...

Role of molybdenum in corrosion of iron‐based alloys in contact with hydrogen sulfide containing solution

“…This is different from previous observations, where nanocrystalline mackinawite was already identified as the initial corrosion product after much shorter exposure times than the shortest used here . Overall, an initial, possibly nanocrystalline mackinawite then typically recrystallises to form crystalline mackinawite . A thermodynamic analysis of the system showed that pyrite is the expected, thermodynamically stable corrosion product in certain ranges of the predominance diagram , in general at higher electrode potentials and lower pH compared to mackinawite.…”

“…Two sharp Raman bands were detected in the measurements at 209 and 290 cm −1 (shoulder). The bands are similar in position to those observed on bare iron and discussed in detail elsewhere . These bands are seen as an indication of microcrystalline mackinawite .…”

“…The thickness of the layer was determined to be 7 μm. Raman spectra of dried corrosion products always transform to hematite as a consequence of laser light heating in the presence of oxygen . Figure d shows a Raman spectrum of the corrosion products formed on the FeMo surface when the sample is still covered by a thin water layer during the measurements, protecting the sample from possible transformation due to laser heating .…”

“…XRD and Raman spectroscopy show no evidence for the presence of a corrosion product of pure molybdenum, e.g., MoS 2 or Mo 2 S 3 . The Raman spectra show no peak at 1300 cm −1 , indicating that no oxidation of the corrosion products took place .…”

X‐ray micro‐computed tomography analysis of accumulated corrosion products in deep‐water shipwrecks

“…It has been found that the spectra of the dark inner portions, as shown in Figure , might consist of lepidocrocite, goethite, magnetite, maghemite and organic compounds. With regard to magnetite, it is essentially characterized by band around 670 cm −1 . Moreover, the Raman peaks found at 359 and 505 cm −1 can be attributed to maghemite .…”