Influence of Desulfovibrio sp. biofilm on SAE 1018 carbon steel corrosion in synthetic marine medium

This work assessed the effect of an enriched culture medium and synthetic seawater on the growth and production of exopolymeric substances (EPS) of a Desulfovibrio sp. strain, isolated from a Mexican oil well. The EPS (mainly consisting of proteins) growth was only achieved after exposing sulfate-reducing bacteria to culture media under dissimilative conditions that predominantly promoted the growth of the biofilm and a small concentration of microorganisms. Once this EPS film was obtained, the evolution of SAE 1018 carbon steel/biofilm/synthetic seawater (VNNS medium) interface was further studied using electrochemical impedance spectroscopy technique (EIS). This study revealed strong adhesion of the biofilm during the formation of iron sulfide (pirrotite) on carbon steel surface. The biofilm inhibits the accelerated damage of the steel for some time exhibiting impedance values of 30 000 Ω. However, at longer times the chemical environment around the biofilm, as a result of microbial metabolism, may become quite corrosive to steel.     

Effect of Heat Treatment on the Electrochemical and Mechanical Behavior of the Ti6Al4V Alloy

Metallurgical and Materials Transactions A - The effects of heat treatment on the hardness and electrochemical behavior of the Ti6Al4V alloy were studied. Two heat treatments were performed: one...     

Electrochemical characterization of the different surface states formed in the corrosion of carbon steel in alkaline sour medium

In this study the different surface states that manifest in the corrosion process of 1018 carbon steel in alkaline sour environment, solution prepared specifically to mimic the sour waters occurring in the catalytic oil refinery plants of the Mexican Oil Company (PEMEX) (0.1 M (NH₄)₂S and 10 ppm NaCN at pH 9.2) were prepared and characterized. The surface states of the carbon steel were formed by treating the surface with cyclic voltammetry at different switching potentials (E_λ+), commencing at the corrosion potential (E_corr=−0.890 V vs sulfate saturated electrode, SSE). The surface states thus obtained were characterized using electrochemical impedance spectroscopy and scanning electron microscopy techniques. It was found that for E_λ+=−0.7 and −0.6 V vs SSE a first product of corrosion formed, characterized by a high passivity. Moreover, it was very compact (with a thickness of 0.047 μm). However, at more anodic potentials (E_λ+>−0.5 V vs SSE) a second corrosion product with non-protective properties (porous with a thickness of 0.4 μm and very active) was observed. The diffusion of atomic hydrogen (H⁰) was identified as the slowest step in the carbon steel corrosion process in the alkaline sour media. The H⁰ diffusion coefficients in the first and second products that formed at the carbon steel–sour medium interface were of the order of 10⁻¹⁵ and 10⁻¹² cm²/s respectively.     

Investigations of corrosion films formed on API-X52 pipeline steel in acid sour media

Corrosion films formed by voltammetry using different switching potentials and by immersion on API-X52 pipeline steel in simulated acid sour media (NACE ID182) have been characterized using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Linear Polarization and Electrochemical Impedance Spectroscopy (EIS) techniques. XRD and EDS analysis showed that the films are mainly composed of sulphide compounds (mackinawite, troilite, marcasite and pyrite) as well as iron oxides, as steel damage increases. Across SEM micrographs the corrosion films formed by potentiodynamic and immersion tests are very similar, covering most of the steel. Polarization and EIS results corroborate poor behavior against corrosion.     

Analysis of the formation of Ta2O5 passive films in acid media through mechanistic modeling

Electrochemical impedance spectroscopy (EIS) analyses are carried out to evaluate the passive features of tantalum oxide films (Ta₂O₅) formed at different potentiostatic conditions (0.5, 1.0, 1.5 and 2.0 V vs SSE). A supporting electrolyte of 0.1 M H₂SO₄ (pH 1) has been used to emulate acidic corrosive conditions for the growth of films with an n-type electronic character. A modification of the point defect model (PDM) accounting for the formation of molecular hydrogen (blistering damage) is used to fit the experimental EIS diagrams, and obtain the kinetic parameters that best describe the semiconductive behavior of the passive films. After this analysis, diffusivities in the order of 5.37 ± 1.6 × 10⁻¹⁷ and 1.98 ± 1.4 × 10⁻²⁰ cm² s⁻¹ were obtained for the oxygen (DVO) and hydroxyl vacancies (DVOH), respectively. These findings show the capabilities of the EIS and the physicochemical modeling to account for the formation of valve-metal oxide films on a different range of conditions.     

Photocatalytic Degradation of Mixed Dyes in Aqueous Phase by MgAlTi and ZnAlTi Mixed Oxides

Topics in Catalysis - This study reported the synthesis and evaluation of mixed metal oxides MgO/Al2O3–TiO2 and ZnO/Al2O3–TiO2 as heterogeneous photocatalysts for the potential...     

Evolution of non-stoichiometric iron sulfide film formed by electrochemical oxidation of carbon steel in alkaline sour environment

Non-stoichiometric iron sulfide films (Fe_xS_y) were formed electrochemically on a 1018 carbon steel/1 M (NH₄)₂S, 500 ppm CN⁻ interface, using cyclic chronoamperometry for different time intervals. The films showed great stability in medium typical of the catalytic plants of PEMEX Mexico (0.1 M (NH₄)₂S, 10 ppm CN⁻ as NaCN, pH 8.8). Characterization of the films by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) show two different behaviors depending on the growth time. For films grown at times <15 min, oxidation of the film was continuous, while oxidation ability for the film formed at times >15 min was lower. Film characteristics were more clearly defined by EIS experiments, as the Nyquist diagrams show depressive loops with high, real impedance values (Zr>1 kΩ cm²) for films grown at longer times. Structural characterization by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) correlated electrochemical behavior with topographical changes and chemical composition of the films formed. The abundance of sulfur and pyrrhotite is evident in the samples grown for increasing times, and is likely due to electrochemical/chemical oxidation of iron sulfides during film growth. The sulfur-rich layer is responsible for the passive character of these films. The equivalent electrical circuit describing the EIS spectra for films formed over longer times has fewer elements than that used to model EIS spectra for films grown at shorter times. In particular, diffusion of atomic hydrogen is not apparent in sulfur-enriched films, and the charge transfer is carried out at the metal/film interface with values that are insensitive to film thickness and chemical nature.     

The influence of different surface conditions in the corrosion process of carbon steel in alkaline sour media

Different electrochemical methodologies were established to induce general corrosion and blistering on homogeneous and heterogeneous carbon steel surfaces similar to the corrosion damage in a catalytic oil refinery plant. In one case, the film porosity and the iron sulphide stoichiometry were modified and in other case, the surface conditions were changed with sulphur films and microblisters. Additionally, we studied the influence of 1018 carbon steel surface conditions on the corrosion process in a medium simulating the average composition of sour waters in catalytic plants of PEMEX Mexico (0.1 M (NH₄)₂S, 10 ppm CN^– as NaCN, pH 8.8). Using the impedance spectra, from 10 kHz to 0.01 Hz, it was possible to qualitatively identify the carbon steel surface condition in an alkaline sour environment and to suggest the same corrosion process steps for this system, despite different surface conditions: charge transfer resistance of steel oxidation in the metal/corrosion product film interface and Fe²⁺ ion and H° diffusion through the corrosion product film. Finally, scanning electron microscopy of a freshly polished surface showed the formation of a homogeneous film immediately after introducing the carbon steel into the sour media. The other surface changes depended on the induced corrosion process and corroborated the electrochemical impedance predictions.     

Electrochemically grown passive films on carbon steel (SAE 1018) in alkaline sour medium

Corrosion films were prepared by applying cyclic potential pulses to the 1018 carbon steel-sour medium interface (1 M (NH₄)₂S, 500 ppm CN⁻) for 1 min. Electrochemical behavior and surface morphology of these films were determined using electrochemical impedance spectroscopy (EIS), scanning electron microscopy, and scanning photoelectrochemical microscopy (SPECM). EIS diagrams and SPECM images show the passive properties and homogeneity of the films. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to characterize their chemical nature and structure. XPS results show that different oxide and sulfur structures were developed during the electrochemical oxidation of carbon steel in concentrated sour media. The analysis of O 1s data indicated that, during film growth, H₂O and/or hydroxyl groups are incorporated into the film structure. The XPS spectra of Fe 2p show iron bonds with S as iron sulfide (FeS₂ and FeS) and the corresponding peak of O 1s shows those bonds with oxygen as Fe₂O₃ and/or FeO. XPS depth profile analyses for the film showed that the ratio of FeS and FeO increases from film surface to film-carbon steel interface. This corroborates the diffusion of iron ions through the film during its electrochemical growth. The chemical shift through the film for the peak associated with Fe 2p signal proves that transport mechanism of iron ions through the film is carried out by chemical diffusion.     

Characterization of different allotropic forms of calcium carbonate scales on carbon steel by electrochemical impedance spectroscopy

Calcium carbonate scales with allotropic forms of aragonite, calcite and a mixture of both, were selectively induced onto carbon steel substrates in a synthetic cooling water. Electrochemical impedance spectroscopy (EIS) was successfully applied to characterize these scales. The resistance and capacitance of the high frequency loop of impedance diagrams for the aragonite and aragonite—calcite scales were related to the morphology of the deposits, and gave information which is in good agreement with scanning electron microscopy (SEM) observations. The impedance response of the scale with a calcite—aragonite mixture was dominated by the aragonite constituent. However, the calcite constituent has a strong influence on the scale morphology, so that the calcite crystals merged laterally with the aragonite agglomerates to form a more coherent and thicker scale, compared to that for a scale consisting of aragonite alone. For scales containing aragonite and a calcite-aragonite mixture, increasing the formation time lead to less porous and thicker deposits.