High Local pH Research Articles

Iron release in aqueous environment by fresh volcanic ash from Mount Etna (Italy) and Popocatépetl (Mexico) volcanoes

In this study, we performed leaching experiments for timescales of hours-to-months in deionized water on fresh volcanic ash from Mt. Etna (Italy) and Popocatepetl (Mexico) volcanos to monitor Fe release as a function of ash mineral chemistry and size, with the aim of clarifying Fe release mechanisms and eventually evaluating the impact of volcanic ash on marine and lacustrine environments. To define sample mineralogy and Fe speciation, inclusive characterization was obtained by means of XRF, SEM, XRPD, EELS and Mossbauer spectroscopies. For Etna and Popocatepetl samples, glass proportions were quantified at 73 and 40%, Fe2O3 total contents at 11.6–13.2 and 5.8 wt%, and Fe3+/FeTot ratios at 0.33 and 0.23, respectively. Leaching experiments showed that significant amounts of iron, ~ 30 to 150 and ~ 750 nmol g−1 l−1 for pristine Etna and Popocatepetl samples, respectively, are released within the first 30 min as a function of decreasing particle size (from 1 to 0.125 mm). The Popocatepetl sample showed a very sustained Fe release (up to 10 times Etna samples) all along the first week, with lowest values never below 400 nmol g−1 l−1 and a maximum of 1672 nmol g−1 l−1 recorded after 5 days. This sample, being composed of very small particles (average particle size 0.125 mm) with large surface area, likely accumulated large quantities of Fe-bearing sublimates that quickly dissolved during leaching tests, determining high Fe release and local pH decrease (that contributed to release more Fe from the glass) at short timescale (hours-to-days). The fractional Fe solubility (FeS) was 0.004–0.011 and 0.23% for Etna and Popocatepetl samples, respectively, but no correlation was found between Fe released in solution and either ash Fe content, glass/mineral ratio or mineral assemblage. Results obtained suggest that volcanic ash chemistry, mineralogy and particle size assume a relevant role on Fe release mostly in the medium-to-long timescale, while Fe release in the short timescale is dominated by dissolution of surface sublimates (formed by physicochemical processes occurring within the eruption plume and volcanic cloud) and the effects of such a dissolution on the local pH conditions. For all samples, a moderate to sustained Fe release occurred for leaching times comparable with their residence time within the euphotic zone of marine and lacustrine environments (variable from few minutes to few hours), revealing their possible contribution to increase Fe bioavailability.

Local pH Effect in the CO2 Reduction Reaction on High-Surface-Area Copper Electrocatalysts

The local pH on electrode surfaces is known to play an important role in the electrochemical reduction of CO2, which could alter the chemical kinetics and molecular transport under the reaction conditions. Here we report the study of local pH effect on the catalytic performance of high-surface-area Cu electrocatalysts. The electroreduction of CO2 was systematically investigated on three types of Cu nanowires with distinct surface roughness factors and nanostructures. The measured electrocatalytic activities and selectivities were further correlated to the simulated local pH on the electrode surface. It was revealed that the high local pH induced by the production of hydroxide from the reaction beneficially suppresses the evolution of hydrogen and enhances the selectivity toward multi-carbon products, but detrimentally limits the transport of CO2 molecules at large current densities. An optimal range of local pH is determined for the electroreduction of CO2, which is insightful for improving the design of electrodes for more efficient energy conversion and chemical transformations.

Interplay of Mass Transfer and Local pH Effects in CO2 Reduction Electrocatalysis

Here we report on the study of mass transfer and local pH effects on CO2 reduction by using Cu nanowires as the electrocatalysts. Highly dense Cu nanowires were synthesized by first thermal oxidation of Cu mesh in air, and then reduction by applying a cathodic electrochemical potential or thermal annealing in a reducing atmosphere (see the Methods). Our previous report has shown that the electrochemically reduced (ECR) Cu nanowires are highly active for CO2 reduction, with favorable production of CO and formate at potentials more positive than ‒0.5 V (versus reversible hydrogen electrode; the same potential scale is used in the following discussion) and C2 species (ethanol, ethylene, and ethane) at more negative potentials. In this work, we performed more systematic studies in the high-overpotential region, i.e., potentials more negative than ‒0.5 V, where the mass transport of CO2 and high local pH near the electrode surface are believed to play a significant role in determining the electrocatalytic performance. We aimed to examine the interplay between the mass transfer and local pH effects in the electrocatalysis for CO2 reduction and elucidate their roles in the structure-performance relationships of the high-surface-area nanowire catalysts. The ultimate goal is to explore the optimal electrochemical conditions for CO2 reduction to produce valuable C2 products.

Increasing phosphorus recovery from dewatering centrate in microbial electrolysis cells

BackgroundMicrobial electrolysis cells (MECs) use bioelectrochemical reactions to remove organic contaminants at the bioanode and produce hydrogen gas at the cathode. High local pH conditions near the cathode can also be utilized to produce struvite from nutrient-rich wastewater. This beneficial aspect was investigated using lab-scale MECs fed with dewatering centrate collected at a local wastewater treatment plant. The main objective was to improve phosphorus recovery by examining various cathode configurations and electric current conditions.ResultsThe stainless steel mesh (SSM) cathode was relatively inefficient to achieve complete phosphorus recovery because struvite crystals were smaller (a few to tens of micrometers) than the open space between mesh wires (80 µm). As a result, the use of multiple pieces of SSM also showed a limited improvement in the phosphorus recovery up to only 68% with 5 SSM pieces. Readily available organic substrates were not sufficient in the dewatering centrate, resulting in relatively low electric current density (mostly below 0.2 A/m2). The slow electrode reaction did not provide sufficiently high pH conditions near the cathode for complete recovery of phosphorus as struvite. Based on these findings, additional experiments were conducted using stainless steel foil (SSF) as the cathode and acetate (12 mM) as an additional organic substrate for exoelectrogens at the bioanode. With the high electric current (>2 A/m2), a thick layer of struvite crystals was formed on the SSF cathode. The phosphorus recovery increased to 96% with the increasing MEC operation time from 1 to 7 days. With the high phosphorus recovery, estimated energy requirement was relatively low at 13.8 kWh (with acetate) and 0.30 kWh (without acetate) to produce 1 kg struvite from dewatering centrate.ConclusionsFor efficient phosphorus recovery from real wastewater, a foil-type cathode is recommended to avoid potential losses of small struvite crystals. Also, presence of readily available organic substrates is important to maintain high electric current and establish high local pH conditions near the cathode. Struvite precipitation was relatively slow, requiring 7 days for nearly complete removal (92%) and recovery (96%). Future studies need to focus on shortening the time requirement.

Physicochemical characterization of lauryl glycinate-dodecyl sulfate equimolar complex: A base-triggerable catanionic liposomal system

A catanionic liposomal system made up of lauryl glycinate hydrochloride (LG·HCl) and sodium dodecyl sulfate (SDS) has been characterized by physicochemical techniques. Results of turbidimetric and isothermal titration calorimetric studies gave strong evidence the formation of an equimolar complex by the two amphiphiles in aqueous dispersion. Differential scanning calorimetric (DSC) studies suggested the two surfactants form an equimolar complex in the solid state also. Results of transmission electron microscopic experiments indicated that equimolar mixtures of LG·HCl and SDS formed spherical unilamellar liposomes in aqueous dispersion, whereas DSC studies showed that they underwent a phase transition at 52.2°C, with a transition enthalpy of 5.8 (±0.4)kcal/mol, which corresponds most likely to a lamellar-gel to a micellar phase transition. The catanionic liposomes are base labile and undergo disruption at high pH (>8.0). Powder X-ray diffraction studies on the LG-DS complex yielded a d-spacing of 36.81 (±0.29)Å, which is consistent with a lamellar bilayer packing of the complex. Overall the present studies strongly suggest that equimolar mixtures of LG·HCl and SDS form catanionic liposomes. This catanionic system can potentially be used to develop base-triggerable liposomal formulations for delivering therapeutic agents to tissues/organs that have a high local pH, e.g., colon.

Alkalinity Initiated Decomposition of Mediating Imidazolium Ions in High Current Density CO2 Electrolysis

AbstractAqueous ionic liquid electrolytes featuring the promising 1,3‐dialkyl‐ and 1,2,3‐trialkyl‐imidazolium cations are reported in CO2 electrolysis up to 200 mA cm−2. A close‐to‐application flow cell setup equipped with silver gas diffusion electrode to overcome CO2 mass transport limitations was used. Faraday efficiencies for CO as high as 68 % were achieved, while inhibiting the hydrogen evolution reaction. Particular attention is paid to the stability of the ionic liquids, which is determined by 1H NMR spectroscopic measurements. The root cause of decomposition is the formation of hydroxide ions through reduction reactions. A high local pH arises that depends on the current density. The water content is also found to play a key role. An overall mechanism is proposed from analysis of the decomposition products including methylamine, ethylamine, formate, acetate and N1‐ethyl‐N2‐methylethane‐1,2‐diamine. Methylation of the susceptible C2 position of the imidazolium ring fails to stabilize the system under high conversion rate electrolysis conditions.

Bulk pH contribution to CO/HCOO− production from CO2 on oxygen-evacuated Cu2O electrocatalyst

Controlling product selectivity is regarded as one of the main barriers in the application of Cu-based electrodes to electrochemical CO2 reduction and hence significant studies on experimental factors to improve the selectivity towards desired products. In particular, two pH parameters appear to be important in controlling product selectivity of the CO2 reduction process. We study how both bulk and local pH conditions affect product distributions during CO2 reduction on oxygen-evacuated Cu2O electrode. In contrast with previous studies of local pH effect on CO2 reduction using metallic Cu, our results suggest that not always is the preferential production of C2 compounds (C2H4+C2H6) achieved by high local pH condition irrespective of catalyst materials, but rather bulk pH condition could contribute to ability to control product selectivity. It is finally demonstrated that the bulk pH effect, often underestimated but related to carbonate speciation, is associated with the direct reduction of HCO3− to HCOO−.

Selective and Efficient Reduction of Carbon Dioxide to Carbon Monoxide on Oxide‐Derived Nanostructured Silver Electrocatalysts

In this work, the selective electrocatalytic reduction of carbon dioxide to carbon monoxide on oxide-derived silver electrocatalysts is presented. By a simple synthesis technique, the overall high faradaic efficiency for CO production on the oxide-derived Ag was shifted by more than 400 mV towards a lower overpotential compared to that of untreated Ag. Notably, the Ag resulting from Ag oxide is capable of electrochemically reducing CO2 to CO with approximately 80 % catalytic selectivity at a moderate overpotential of 0.49 V, which is much higher than that (ca. 4 %) of untreated Ag under identical conditions. Electrokinetic studies show that the improved catalytic activity is ascribed to the enhanced stabilization of COOH(.) intermediate. Furthermore, highly nanostructured Ag is likely able to create a high local pH near the catalyst surface, which may also facilitate the catalytic activity for the reduction of CO2 with suppressed H2 evolution.

Selective and Efficient Reduction of Carbon Dioxide to Carbon Monoxide on Oxide‐Derived Nanostructured Silver Electrocatalysts

AbstractIn this work, the selective electrocatalytic reduction of carbon dioxide to carbon monoxide on oxide‐derived silver electrocatalysts is presented. By a simple synthesis technique, the overall high faradaic efficiency for CO production on the oxide‐derived Ag was shifted by more than 400 mV towards a lower overpotential compared to that of untreated Ag. Notably, the Ag resulting from Ag oxide is capable of electrochemically reducing CO2 to CO with approximately 80 % catalytic selectivity at a moderate overpotential of 0.49 V, which is much higher than that (ca. 4 %) of untreated Ag under identical conditions. Electrokinetic studies show that the improved catalytic activity is ascribed to the enhanced stabilization of COOH. intermediate. Furthermore, highly nanostructured Ag is likely able to create a high local pH near the catalyst surface, which may also facilitate the catalytic activity for the reduction of CO2 with suppressed H2 evolution.

Mg Corrosion Control By Biopolymer-Polyelectrolyte Membranes

Mg and Mg alloys offer a great potential for use as a biomedical material for temporary implant applications: its lack of toxicity, ability to easily dissolve in aqueous environments, and its bone-like bulk properties. One of the key drawbacks however, that prevents the more widespread use of magnesium-based materials is their fast and often uncontrollable corrosion within the human body that impedes its utilisation in a wide range of orthopaedic components1,2. The possibility to control this dissolution is vital if Mg alloys are to be considered as a practical, temporary biomedical implant materials. In this study we detail how this aim can be attained by the use of biocompatible polymeric membrane materials. For example, by using a polyelectrolyte like poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) to modify cellulose acetate (CA) membranes it is possible to achieve Mg dissolution control. Commercially pure magnesium surfaces were spin-coated with differing concentrations of PDMAEMA to create a number of cellulose based surfaces that were subsequently exposed to an aqueous solution. On immersion it was observed that the CA film distends and begins to function like a membrane that can control both ion flow and H2 gas evolution. Electrochemical measurements (open circuit potential, polarisation and linear sweep voltammograms) demonstrate that by varying the CA:PDMAEMA ratio, the rate of Mg corrosion can be controlled. Additional measurements of pH and with ICP-OES clearly demonstrate that the accumulation of corrosion products between the membrane and the sample also reduces the undesirable effects of H2 formation and high local pH that takes place during the corrosion process3. Furthermore, SECM studies (with ferrocenemethanol as a redox mediator and using a custom-made cell set-up) were performed in order to determine membrane permeability: knowledge over the permeability of the membrane is crucial when designing the membrane for dissolution control. Overall, these findings demonstrate that the presence of polyelectrolyte modified cellulose membranes have the potential to control Mg corrosion in physiological environments and that the permeability of the membrane can be controlled by additions of a cationic polyelectrolyte like PDMAEMA. (1) F. Witte, N. Hort, C. Vogt, S. Cohen, K. U. Kainer, R. Willumeit, F. Feyerabend, Current Opinion in Solid State and Materials Science, 2008, 12, 63-72. (2) S. Virtanen, Mater. Sci. Eng. B, 2011, 176, 1600-1608. (3) K. Yliniemi, B. P. Wilson, F. Singer, S. Höhn, E. Kontturi S. Virtanen, ACS Applied Materials and Interfaces, 2014, 6, 22393-22399.

Nickel-Cobalt Double Hydroxide Decorated Carbon Nanotubes via Aqueous Electrophoretic Deposition towards Catalytic Glucose Detection

In this work, we present a facile technique based on electrophoretic deposition (EPD) to produce transition metal hydroxide decorated carbon nanotubes (CNT) for electrochemical applications. We specifically explore the performance of nickel-cobalt hydroxides given their high activity, conductivity and stability as compared to the individual hydroxides. We exploit the high local pH at the negative electrodes during water-based EPD to form nanoparticles of nickel-cobalt hydroxides in situ on the CNT surface. We focus our work here on obtaining functional and conductive deposits on CNTs. The hydrophilic binderless deposits of Ni-Co double hydroxide decorated CNTs obtained here are used for non-enzymatic glucose detection. XPS data and electrochemical testing reveal difference in the deposited double hydroxide based on chronology of charging salt addition even at the same ratio. When cobalt and nickel salts are sequentially added at a ratio of 1:1, the deposited double hydroxides show excellent glucose sensitivity of ~3300μA/mM.cm2 at applied potential of 0.55V vs. Ag/AgCl reference electrode.

Mechanisms of enhanced total organic carbon elimination from oxalic acid solutions by electro-peroxone process

Electro-peroxone (E-peroxone) is a novel electrocatalytic ozonation process that combines ozonation and electrolysis process to enhance pollutant degradation during water and wastewater treatment. This enhancement has been mainly attributed to several mechanisms that increase O3 transformation to OH in the E-peroxone system, e.g., electro-generation of H2O2 from O2 at a carbon-based cathode and its subsequent peroxone reaction with O3 to OH, electro-reduction of O3 to OH at the cathode, and O3 decomposition to OH at high local pH near the cathode. To get more insight how these mechanisms contribute respectively to the enhancement, this study investigated total organic carbon (TOC) elimination from oxalic acid (OA) solutions by the E-peroxone process. Results show that the E-peroxone process significantly increased TOC elimination rate by 10.2–12.5 times compared with the linear addition of the individual rates of corresponding ozonation and electrolysis process. Kinetic analyses reveal that the electrochemically-driven peroxone reaction is the most important mechanism for the enhanced TOC elimination rate, while the other mechanisms contribute minor to the enhancement by a factor of 1.6–2.5. The results indicate that proper selection of electrodes that can effectively produce H2O2 at the cathode is critical to maximize TOC elimination in the E-peroxone process.

Dissolution control of Mg by cellulose acetate-polyelectrolyte membranes.

Cellulose acetate (CA)-based membranes are used for Mg dissolution control: the permeability of the membrane is adjusted by additions of the polyelectrolyte, poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA). Spin-coated films were characterized with FT-IR, and once exposed to an aqueous solution the film distends and starts acting as a membrane which controls the flow of ions and H2 gas. Electrochemical measurements (linear sweep voltammograms, open-circuit potential, and polarization) show that by altering the CA:PDMAEMA ratio the dissolution rate of Mg can be controlled. Such a control over Mg dissolution is crucial if Mg is to be considered as a viable, temporary biomedical implant material. Furthermore, the accumulation of corrosion products between the membrane and the sample diminishes the undesirable effects of high local pH and H2 formation which takes place during the corrosion process.

Evaluating the corrosion behaviour of Magnesium alloy in simulated biological fluid by using SECM to detect hydrogen evolution

Scanning electrochemical microscopy (SECM) in surface generation/tip collection mode is investigated as an assessment tool for studying the corrosion behaviour of magnesium in simulated biological fluid. The technique provides a local map of hydrogen (H2) evolution which alone can be used as a direct measure of corrosion. The H2 generated during corrosion of magnesium is oxidized at the probe(i.e. a Pt ultra micro-electrode);with the magnitude of the current generated due to oxidation being indicative of the intensity of H2 evolution at a local scale on the magnesium surface. This method was calibrated using a cathodically polarized Pt disk to simulate H2 evolution in a controlled condition on a homogeneous surface. Potential interference from dissolving Mg or high local pH was also investigated. The technique was implemented for studying H2 evolution at the surface of AZ31 as a model Mg alloy.SECM results combined with SEM-EDX and profilometry data revealed that local domains of higher H2 evolution on the surface of AZ31 are in close proximityof the observed pitting sites.

Phosphorylation of Carbonic Anhydrase IX Controls Its Ability to Mediate Extracellular Acidification in Hypoxic Tumors

In the hypoxic regions of a tumor, carbonic anhydrase IX (CA IX) is an important transmembrane component of the pH regulatory machinery that participates in bicarbonate transport. Because tumor pH has implications for growth, invasion, and therapy, determining the basis for the contributions of CA IX to the hypoxic tumor microenvironment could lead to new fundamental and practical insights. Here, we report that Thr443 phosphorylation at the intracellular domain of CA IX by protein kinase A (PKA) is critical for its activation in hypoxic cells, with the fullest activity of CA IX also requiring dephosphorylation of Ser448. PKA is activated by cAMP, which is elevated by hypoxia, and we found that attenuating PKA in cells disrupted CA IX-mediated extracellular acidification. Moreover, following hypoxia induction, CA IX colocalized with the sodium-bicarbonate cotransporter and other PKA substrates in the leading edge membranes of migrating tumor cells, in support of the concept that bicarbonate metabolism is spatially regulated at cell surface sites with high local ion transport and pH control. Using chimeric CA IX proteins containing heterologous catalytic domains derived from related CA enzymes, we showed that CA IX activity was modulated chiefly by the intracellular domain where Thr443 is located. Our findings indicate that CA IX is a pivotal mediator of the hypoxia-cAMP-PKA axis, which regulates pH in the hypoxic tumor microenvironment.

Dioxygen Reduction Affects Surface Oxide Growth and Dissolution on AA2024-T3

The influence of dioxygen reduction on oxide growth and dissolution at the surface of AA2024-T3 has been investigated in near-neutral, unbuffered conditions. Dioxygen reduction at Fe and Cu also was examined to compare with the behavior of intermetallic particles in AA2024-T3. Electrodes were exposed alternately to -saturated or -saturated solution in a characterized wall-jet flow cell. Limiting current densities associated with reduction on Fe and Cu range between 1.5 and , whereas AA2024-T3 displays significantly lower values. AA2024-T3 samples also display potential-dependent current density transients when the solution is alternated between -saturated and -saturated supporting electrolyte. The data show that the current density transient behavior is due to competition between interfacial etching of oxide due to high local pH, which is a result of the cathodic oxygen reduction reaction (ORR) at intermetallic particles, and anodic oxide growth on the alloy, which is driven by the electric field produced at a given potential. Scanning electron microscopy shows significant differences in oxide thickness with radial distance from the center of the jet impingement region. Energy-dispersive spectroscopic imaging reveals an increase in near-surface Cu and Fe in areas that experience high current densities.

Preparation and Characterization of Stable Ionomers and Ionomer Membranes for Fuel Cells

AbstractMembranes based on sulfonated arylene polymers and their cross‐linked blends were prepared. Embedding the ionomers into textile or porous PTFE matrix and/or doping with ZrP nanoparticles reduced the swelling, and improved the thermal stability. PTFE reinforcement led to an excellent single cell performance, which is promising for PEMFC applications. Some of the membranes performed better than Nafion®. The net methanol crossover correlates with the membrane thickness and composition, but surprisingly, suppression of the methanol crossover alone, does not enhance U/I performance. ESR experiments of oxidative degradation of the monomer by OH• at pH > 10 leads to phenoxyl and semi‐quinone radicals. The study focuses on the case where O2•– or 1O2 are the damaging species, i.e. above pH 11.7, and the dominant reaction is that involving the aromatic ring. Under conditions of fuel starvation, the acid protons get depleted, which may lead to high local pH values, even in an overall acidic environment. It is therefore important to be aware of the entirely different form of degradation chemistry at high pH. In the presence of both HO• radicals and oxygen, complete degradation of the aromatic rings can be observed, within hours. In view of this, perfluorinated membranes are more inert. Cross‐linked arylene polymer blend membranes and PTFE‐reinforced arylene ionomer membranes can be regarded as promising for further development, due to their high stability and excellent PEMFC performance.

Products formed during cobalt cementation on zinc in zinc sulfate electrolytes

The observed removal of Co 2+ during cementation on zinc is due to two reactions. One reaction is formation of a Co alloy or Co in solid solutions. The second reaction is formation of basic cobalt salts, and is a function of local high pH at the zinc surface and the concentration of Zn 2+. The high local pH close to the zinc surface is created by evolution of hydrogen. This hydrogen evolution stops when the core of metallic zinc in the cemented particle is consumed, and basic cobalt salts in the particle may redissolve. Basic cobalt salts redissolve more easily than Co alloys and is the major reason for cobalt redissolution during the technical purification of the zinc sulfate electrolyte before zinc electrolysis.

Control of Alkaline Stress Corrosion Cracking in Pressurized-Water Reactor Steam Generator Tubing

Abstract Outer-diameter stress corrosion cracking (ODSCC) of alloy 600 (UNS N06600) tubings in steam generators of the Kori-1 pressurized-water reactor (PWR) caused an unscheduled outage in 1994. Failure analysis and remedy development studies were undertaken to avoid a recurrence. Destructive examination of a removed tube indicated axial intergranular cracks developed at the top of sludge caused by a boiling crevice geometry. A high ODSCC propagation rate was attributed to high local pH and increased corrosion potential resulting from oxidized copper presumably formed during the maintenance outage and plant heatup. Remedial measures included: (1) crevice neutralization by crevice flushing with boric acid (H3BO3) and molar ratio control using ammonium chloride (NH4Cl), (2) corrosion potential reduction by hydrazine (H2NNH2) soaking and suppression of oxygen below 20 ppb to avoid copper oxide formation, (3) titanium dioxide (TiO2) inhibitor soaking, and (4) temperature reduction of 5°C. Since application o...

Adsorption of sulfate and chloride ions on aluminum

Radioactive labeling method, electrochemical techniques and X-ray photoelectron spectroscopy (XPS) were applied to study sulfate and chloride adsorption/incorporation at the passive films on pure aluminum in 0.1 mol dm −3 NaClO 4 containing sulfate or chloride ions. We found that the anion adsorption was pH dependent and occurred over a broad electrode potential range. Under open-circuit conditions, the adsorption of sulfate anion is controlled by solution pH, surface charge and the stability of the aluminum-oxide film. The surface concentration of chloride is smaller than that of sulfate and also depends on pH, but slightly. The degree of adsorption irreversibility between the two anions is different, most likely due to a dissimilar extent of the incorporation of sulfate and chloride in the oxide film. We demonstrate that the relative amount of irreversibly bonded sulfate increases with the solution pH and exposure time. Under well-controlled potential conditions, the sulfate coverage increases with the electrode potential until the passive film breakdown occurs. When aluminum undergoes pitting, sulfate removal from the surface is observed. In contrast to sulfate, the surface coverage of chloride increases with anodic polarization even above the pitting potential. The cathodic polarization reduces both the sulfate and chloride coverage due to the high local pH and the destabilization of the passive film.