Mackinawite formation from elemental iron and sulfur

Sulfur-assisted corrosion is a process known to material scientists for many decades now. Though the corrosion of iron in the presence of sulfur has been studied extensively, it has never been used to intentionally synthesize mackinawite. In contrast to the conventional preparation of mackinawite by precipitation, the synthesis from the elements can be carried out without additional ions. This makes it possible to investigate the influence of any dissolved salts on the mackinawite formation and its properties. We found that the addition of NaCl significantly accelerates the reaction and furthermore influences the Fe²⁺ ion content of the formed mackinawite itself. This finding leads us to propose a novel model of charged layers which can be used to explain some of the inconsistencies found in the literature regarding the structure and particle characteristics of nano-mackinawite.

Nanoparticulate mackinawite was synthesized from elemental iron and sulfur in a convenient and reliable reaction. The structure and composition of the products were characterized and a new model describing the particle characteristics is established.

Effective removal of silica and sulfide from oil sands thermal in-situ produced water by electrocoagulation

Effective removal of silica and sulfide from oil sands thermal in-situ produced water can reduce corrosion and scaling of steam generators, enhancing water recycling and reuse in the industry. The removal of these two solutes as well as calcium and magnesium (i.e., the solutes that can also cause scaling) from synthetic and authentic produced waters by electrocoagulation (EC) was investigated in this study. In Fe⁰-EC, the precipitation of FeS minerals resulted in a rapid removal of sulfide and adsorption of silica onto FeS. In Al⁰-EC, silica was removed via adsorption onto aluminum hydroxides, but sulfide was poorly removed. In both EC systems, Ca²⁺ and Mg²⁺ were removed from the organic-free synthetic produced water but not from the authentic water, likely due to the influence of organic species. Contaminant removals in Fe⁰-EC were controlled by charge density (q, C/L) but not current density (i, mA/cm²). Overall, this research suggests that EC can be a promising technology for the treatment of thermal in-situ produced water. Fe⁰-EC appears to be a better choice than Al⁰-EC considering that Fe⁰-EC was more effective at removing sulfide, and that Fe⁰ anodes are usually less expensive.

Test Conditions for Pipeline Materials Selection with High Pressure Sour Gas

Acid gases, such as CO2, H2S, and/or sulfur in oil industry’s production fluids, can be responsible for both general and localized corrosion, acting with different mechanisms, which depend on chemical and physical properties of the produced fluids. Materials selection for handling such fluids is performed by combining experience with suggestions from standards and regulations. A good deal of knowledge is available to predict corrosion rates for CO2-containing hydrocarbons, but the effect of high H2S pressure is less understood, mainly due to the difficulty of performing laboratory tests in such challenging conditions. For instance, the so-called NACE solution to assess SSC (Sulfide Stress Cracking) susceptibility of steels is a water-based solution simulating production fluids in equilibrium with one bar bubbling H2S gas. This solution does not represent environments where high gas pressure is present. Moreover, it does not take into account the corrosive properties of sulfur and its compounds that may deposit in such conditions. Besides, properties of high pressure gases are intermediate between those of a gas and those of a liquid: high pressure gases have superior wetting properties and better penetration in small pores, with respect to liquids. These features could enhance and accelerate damage, and nowadays such conditions are likely to be present in many production fields. This paper is aimed to point out a few challenges in dealing with high pressure gases and to suggest that, for materials selection in sour service, a better correspondence of test conditions with the actual field conditions shall be pursued.

Electrochemical and Microstructural Analysis of FeS Films from Acidic Chemical Bath at Varying Temperatures, pH, and Immersion Time

The corrosion resistance and corrosion products of 4130 alloy steel have been investigated by depositing thin films of iron sulfide synthesized from an acidic chemical bath. Tests were conducted at varying temperatures (25°C–75°C), pH levels (2–4), and immersion time (24–72 hours). The corrosion behavior was monitored by linear polarization resistance (LPR) method. X-ray Diffraction (XRD), Energy Dispersive X-ray (EDX) spectroscopy, and Scanning Electron Microscopy (SEM) have been applied to characterize the corrosion products. The results show that, along with the formation of an iron sulfide protective film on the alloy surface, increasing temperature, increasing immersion time, and decreasing pH all directly increase the corrosion rate of steel in the tested experimental conditions. It was also concluded that increasing temperature causes an initial increase of the corrosion rate followed by a large decrease due to transformation of the iron sulfide crystalline structure.

Corrosion of iron by sulfate-reducing bacteria: new views of an old problem

About a century ago, researchers first recognized a connection between the activity of environmental microorganisms and cases of anaerobic iron corrosion. Since then, such microbially influenced corrosion (MIC) has gained prominence and its technical and economic implications are now widely recognized. Under anoxic conditions (e.g., in oil and gas pipelines), sulfate-reducing bacteria (SRB) are commonly considered the main culprits of MIC. This perception largely stems from three recurrent observations. First, anoxic sulfate-rich environments (e.g., anoxic seawater) are particularly corrosive. Second, SRB and their characteristic corrosion product iron sulfide are ubiquitously associated with anaerobic corrosion damage, and third, no other physiological group produces comparably severe corrosion damage in laboratory-grown pure cultures. However, there remain many open questions as to the underlying mechanisms and their relative contributions to corrosion. On the one hand, SRB damage iron constructions indirectly through a corrosive chemical agent, hydrogen sulfide, formed by the organisms as a dissimilatory product from sulfate reduction with organic compounds or hydrogen ("chemical microbially influenced corrosion"; CMIC). On the other hand, certain SRB can also attack iron via withdrawal of electrons ("electrical microbially influenced corrosion"; EMIC), viz., directly by metabolic coupling. Corrosion of iron by SRB is typically associated with the formation of iron sulfides (FeS) which, paradoxically, may reduce corrosion in some cases while they increase it in others. This brief review traces the historical twists in the perception of SRB-induced corrosion, considering the presently most plausible explanations as well as possible early misconceptions in the understanding of severe corrosion in anoxic, sulfate-rich environments.

Sulfate-reducing bacteria lower sulfur-mediated pitting corrosion under conditions of oxygen ingress

The effect of oxygen ingress into sour water containing dissolved sulfide on the production of sulfur and polysulfide (S-PS) and associated iron corrosion was investigated. Biotic (active SRB present), abiotic (autoclaved SRB present), and chemical (no bacteria present) conditions were compared. Under biotic conditions formation of S-PS was only seen at a high ratio of oxygen to sulfide (R(OS)) of 1 to 2.4. General corrosion rates increased 10-fold to 0.10 mm/yr under these conditions. Under abiotic and chemical conditions S-PS formation increased over the entire range of R(OS) with general corrosion rates reaching 0.06 mm/yr. Although general corrosion rates were thus highest under biotic conditions, biotically corroded coupons showed much less pitting corrosion. Maximum pit depth increased to 40-80 μm with increasing R(OS) for coupons incubated for 1 month under abiotic or chemical conditions but not for biotically incubated coupons (10 μm). This appeared to be related to the properties and size of the sulfur formed, which was hydrophobic and in excess of 10 μm under chemical or abiotic conditions and hydrophilic and 0.5 to 1 μm under biotic conditions. Hence, perhaps contrary to expectation, SRB lowered pitting corrosion rates under conditions of oxygen ingress due to their ability to respire oxygen and produce a less aggressive form of sulfur. Microbial control, which is usually required in sour systems, may be counterproductive under these conditions.

Marine sulfate-reducing bacteria cause serious corrosion of iron under electroconductive biogenic mineral crust

Iron (Fe⁰) corrosion in anoxic environments (e.g. inside pipelines), a process entailing considerable economic costs, is largely influenced by microorganisms, in particular sulfate-reducing bacteria (SRB). The process is characterized by formation of black crusts and metal pitting. The mechanism is usually explained by the corrosiveness of formed H₂S, and scavenge of ‘cathodic’ H₂ from chemical reaction of Fe⁰ with H₂O. Here we studied peculiar marine SRB that grew lithotrophically with metallic iron as the only electron donor. They degraded up to 72% of iron coupons (10 mm × 10 mm × 1 mm) within five months, which is a technologically highly relevant corrosion rate (0.7 mm Fe⁰ year⁻¹), while conventional H₂-scavenging control strains were not corrosive. The black, hard mineral crust (FeS, FeCO₃, Mg/CaCO₃) deposited on the corroding metal exhibited electrical conductivity (50 S m⁻¹). This was sufficient to explain the corrosion rate by electron flow from the metal (4Fe⁰ → 4Fe²⁺ + 8e⁻) through semiconductive sulfides to the crust-colonizing cells reducing sulfate (8e⁻ + SO₄²⁻ + 9H⁺ → HS⁻ + 4H₂O). Hence, anaerobic microbial iron corrosion obviously bypasses H₂ rather than depends on it. SRB with such corrosive potential were revealed at naturally high numbers at a coastal marine sediment site. Iron coupons buried there were corroded and covered by the characteristic mineral crust. It is speculated that anaerobic biocorrosion is due to the promiscuous use of an ecophysiologically relevant catabolic trait for uptake of external electrons from abiotic or biotic sources in sediments.