M.T Oropeza

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

Abstract Non-stoichiometric iron sulfide films (Fe x S y ) were formed electrochemically on a 1018 carbon steel/1 M (NH 4 ) 2 S, 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 (NH 4 ) 2 S, 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 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 cm 2 ) 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.

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

Abstract Corrosion films were prepared by applying cyclic potential pulses to the 1018 carbon steel–sour medium interface (1 M (NH 4 ) 2 S, 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, H 2 O and/or hydroxyl groups are incorporated into the film structure. The XPS spectra of Fe 2p show iron bonds with S as iron sulfide (FeS 2 and FeS) and the corresponding peak of O 1s shows those bonds with oxygen as Fe 2 O 3 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.

Electrochemical study on carbon steel corrosion process in alkaline sour media

Electrochemical impedance diagrams were obtained for the carbon steel/sour refinery medium interface to evaluate how the corrosion process depends on the immersion times. A corrosion product film was formed during the initial time and the corrosion velocity was high. During the first 7 h, the film thickness increases and then diminishes, after 50 h of immersion, growth and dissolution reach equilibrium. Moreover, different carbon steel damaged surfaces were prepared using the cyclic voltammetry, using as damaging parameter the inversion potential (Eλ+). Comparing the impedance diagrams for the interfaces above mentioned we established that the carbon steel surfaces prepared by cyclic voltammetry and those prepared by immersion time were equivalent. Additionally, Scanning Electron Microscopy was used to identify that at Eλ+=−0.7 and −0.6 V versus SSE, appears the first corrosion product over the carbon steel surface and its characteristics show a very compact (0.047 μm) and passive film. It was also determined that this corrosion film is the same obtained for 0 h of immersion time. A second corrosion product was formed with the cyclic voltammetry technique at more anodic potentials (Eλ+>−0.5 V vs. SSE). This new film had a greater thickness (0.4 μm) and an important electrochemical activity that leads to assume a porous layer, these properties are similar to those obtained when the carbon steel was immersed in a sour medium for times longer than 50 h. Concerning the impedance spectra analysis for the different carbon steel surfaces formed by the described strategies, we used the same equivalent circuit, assuming that the same steps were involved: charge transfer between the carbon steel/corrosion products interface, diffusion of Fe2+ from the interface to the corrosion products film (associated to the vacancies in the pyrrotite crystalline structure) and atomic hydrogen diffusion from the sour medium/corrosion products interface to the corrosion products film. The last one was supposed to be the most important step in the corrosion process of the carbon steel in the sour medium because it induces the blistering corrosion.

Electrochemical deposition of silver and gold from cyanide leaching solutions

A systematic voltammetric study developed in this work allows the determination of the potential range at which the selective deposition of gold and silver is carried out in the presence of a high content of copper. As a first approach, laboratory solutions prepared with a high content of cyanide and copper and low values of gold and silver are used; later, the methodology is applied to leaching solutions of industrial origin. The chemical speciation and microelectrolysis studies showed that copper deposition occurs at more negative potentials than deposition of gold and silver. Also, the voltammetric study of a cyanide solution containing low concentrations of Au(I) and Ag(I), free of and with high concentration of Cu(I) was carried out. The study shows the potential range at which Au(I) and Ag(I) are reduced despite the high concentration of the Cu(I) ions. The deposition of gold and silver was not interfered with by the high concentration of Cu(I) ions when the leaching solution was electrolyzed in a laboratory electrochemical reactor FM01-LC with a reticulated vitreous carbon (RVC) cathode.

Electrochemical characterisation of sulfur species formed during anodic dissolution of galena concentrate in perchlorate medium at pH 0

Abstract An electrochemical study of galena concentrate in perchlorate medium at pH 0 was carried out using carbon pasteelectrode-galena (CPE-galena). It has been reported that anodic dissolution of galena is inhibited by the formation of elementalsulfur on the mineral. Electrochemical characterisations of sulfur species performed in this work complement the results previouslyreported. Using the cyclic voltammetry technique it was determined that for E 0.6 V vs. SCE, elemental sulfur and Pb(II) wereproduced, whereas for E 0.6 V vs. SCE, thiosulfate and sulfate ions were produced. Through voltammetric characterisation ofthe species produced in the interface after microelectrolysis at constant potential, the presence of elemental sulfur and PbS 2 O 3 andPbSO 4 species was detected. Chronoamperometric studies showed that elemental sulfur on the galena surface inhibits theelectrodissolution of this mineral concentrate, emphasising that the presence of Pb(II) ion in the interface gives a greater porosityto the formed film. In addition, it was demonstrated that thiosulfate and lead sulfate species are also porous and only partiallyinhibit the electrodissolution of galena.

The role of different surface damages in corrosion process in alkaline sour media

Abstract By applying different electrochemical methodologies to 1018 carbon steel/different electrolytic media interfaces, surfaces with generalised and blistered damages, commonly present in the oil refinery catalytic plants, were obtained. Afterwards, a freshly polished carbon steel surface as well as the damaged surfaces were characterised by electrochemical impedance spectroscopy and scanning electronic microscopy (SEM). The SEM results reveal that the damaged surfaces present films with different physical properties. In spite of the surface modification, the corrosion mechanism of these interfaces in a medium simulating catalytic plants condensates of the refineries in Mexico (0.1 M (NH4)2S, 0.4 mM NaCN (10 ppm CN−), pH=8.8) turned out to be the same. It was also found that the corrosion process presents the following steps: charge transfer resistance of steel oxidation in the metal/corrosion product film interface and the diffusion processes of Fe2+ and Ho ions through the corrosion product film. It was demonstrated that when there was no damage on the surface (freshly polished surface), a homogeneous film was formed instantaneously upon introducing carbon steel into the sour media. The corrosion process in this film occurs in three stages. On the surface with generalised corrosion the three stages are favoured, particularly that of atomic hydrogen process (inducing blistering). Meanwhile, the blistered surface presents a higher charge resistance of steel oxidation and the Fe2+ and atomic hydrogen diffusions through the corrosion products are slower than those presented on the other surface states.

Anion influence in lead removal from aqueous solution by deposition onto a vitreous carbon electrode

We investigated the electrolytic removal of Pb(II) from aqueous solutions containing different electrolytes (nitrate, chloride or sulfate), by electrolysis onto reticulated vitreous carbon electrode (RVC). The efficiency of the electrolytic process of lead removal was found to be a function of electrolyte composition. The chloride containing electrolyte, provided the highest efficiency of lead removal, while removing Pb(II) from the sulfate electrolytes turned out to be a very difficult and high energy consuming process. Cyclic voltammetry and the atomic force microscopy (AFM) were used to characterize lead deposits on RVC and fractured vitreous carbon (FVC) electrodes surface. Our study showed that in the chloride solution, a significantly larger amount of lead deposit was formed than in the sulfate electrolyte. Since the same phenomenon was observed with both electrode types, the FVC electrode is established as an appropriate laboratory model for studying the RVC, which is often used in industrial applications. AFM analysis revealed that lead deposits formed from different electrolyte solutions possess different surface morphologies, indicating different mechanism of formation or different kinds of interactions between the metal adlayer and the adsorbed anions. A highly dense lead deposit, spread all over the electrode surface, in the form of a 2-D film, was found when using the chloride-containing electrolyte. This was attributed to the surface annealing effect and the increased number of nucleation sites due to chloride coadsorption at the electrode surface. Deposits formed from the sulfate electrolyte consisted of numerous, isolated and rather small lead clusters, indicating that deposition from sulfate solutions was inhibited by the formation of the passivated salt adlayer over the lead clusters.

Chemical Characterization of Corrosion Films Electrochemically Grown on Carbon Steel in Alkaline Sour Environment

Surface reactivity and the chemical nature of corrosion films electrochemically formed on carbon steel in a I M (NH 4 ) 2 S and 500 ppm CN - medium for different growth times were studied by scanning photoelectrochemical microscopy (SPECM) and X-ray photoelectron spectroscopy (XPS). SPECM diagrams corresponding to films grown for times 15 min these films showed surface activity and homogeneity in the characterization medium. The XPS spectra of Fe 2p, S 2p, and O Is on the film surface indicated that FeS 2 and FeO are present in the protective (heterogeneous) films; meanwhile, Fe(OH) and FeS are the chemical species present in nonprotective (homogeneous) films. The O Is peak analysis using a mix of Gaussian and Lorentzian functions indicated that during film growth both H 2 O and/or hydroxyl groups are incorporated into the film structure. These results prove that the corrosion film composition was modified as a function of electrochemical oxidation time and this composition is directly related to the passive properties of these films.

Lead deposition onto fractured vitreous carbon: influence of electrochemical pretreated electrode

We evaluated the electrochemical deposition of Pb(II) onto Fractured Vitreous Carbon (FVC) electrodes from solutions containing very low concentrations of lead in different electrolytes (sulfate or chloride). To examine how the FVC surface state influences the lead deposition efficiency, the electrodes were subjected to different electrochemical pretreatments prior to the actual deposition process. The FVC electrode was used as a representative model of the vitreous carbon (VC) bulk, avoiding the polishing procedure that could change the surface. Electrochemical pretreatment was carried out by cyclic voltammetry in electrolytes containing chloride or nitrate anions and in some cases, ferrocyanide. Before and after the electrochemical pretreatment, the electrode surface morphology was assessed using Atomic Force Microscopy (AFM) imaging. The quantity of lead deposited on the FVC electrode surface (lead deposition efficiency) in each experiment was estimated from the charge under the lead anodic, dissolution peak. Electrochemical pretreatment of electrodes in chloride or nitrate electrolytic baths consistently reduced the lead deposition efficiency. A detailed analysis, correlating lead deposition efficiencies to surface roughness and fractal dimension of the freshly prepared and electrochemically pretreated FVC electrodes, indicated that the decrease in efficiency corresponded to the change in electrode surface geometry. The greater efficiency of lead deposition observed in the chloride-containing electrolyte was due to the interaction between chloride and deposited lead rather than a chloride interaction with the FVC substrate.

Oxidation of Mineral Species as a Function of the Anodic Potential of Zinc Concentrate in Sulfuric Acid

The anodic dissolution of a zinc concentrate [63.4% sphalerite (ZnS), 20.1% pyrite (FeS 2 ), 5% chalcopyrite (CUFeS 2 ), 0.33% galena (PbS), 0.45% tetrahedryte (Cu 12 Sb 4 S 13 ), and 0.4% arsenopyrite (FeAsS)] derived from the flotation process was studied in 1.7 M H 2 SO 4 . An electrochemical strategy (based on common electrochemical techniques) was proposed to find the potential range at which each mineral species could be oxidized. Anodic potential pulses were applied to carbon paste electrodes prepared with zinc concentrate (CPE-zinc concentrate). The resulting solutions were analyzed by anodic stripping voltammetry. In addition, the surface of each modified CPE-zinc concentrate was characterized by cyclic voltammetry. The final surface state of each electrochemically modified electrode and the solution composition indicated that sphalerite oxidized in the range 0 ≤ E ≤ 600mV vs. saturated sulfate electrode (SSE); similarly, galena dissolution took place in the same range. In the case of chalcopyrite, it was determined that nonstoichiometric copper sulfur was formed at 200 < E ≤ 440 mV, whereas covellite species appeared at 460 ≤ E ≤ 600 mV vs. SSE. The oxidation of pyrite to iron polysulfur took place at 480 < E ≤ 500 mV vs. SSE, while Fe(III) and sulfate ions were formed at 500 < E ≤ 600 mV vs. SSE. The progressive anodic dissolution of zinc concentrate in the whole potential range (0 ≤ E appl ≤ 600 mV) was significant. The results obtained in this paper demonstrate an alternative for hydrometallurgical processing of complex zinc concentrates.