Anodic Region Research Articles

The effect of SiC content on the tribocorrosion performance of spark plasma sintered Al–SiC nanocomposites

As a follow-up study, the present work evaluates the tribocorrosion behavior of Al–SiC nanocomposites produced using spark plasma sintering. The tribocorrosion tests were carried out using a ball-on-disk tribometer with the sliding contact fully immersed in 3.5 wt% NaCl solution. Post-mortem characterization studies of the worn surfaces included scanning electron microscopy (SEM) coupled with an energy dispersive spectroscopy. From the results, oscillations in chronopotentiometry values characterize the in situ tribocorrosion behaviors of the composites. Shifts to the cathodic region at the beginning of sliding and immediate shift to the anodic region of the curve as sliding continues accounts for the oscillations in chronopotentiometry values. The SEM analyses infer the formation of adherent and modified tribolayers in the worn surfaces of the composites during sliding. The inert character and increasing resistance of the tribolayers enhance the tribocorrosion performance of the composites compared to the unreinforced Al alloy.

The Influence of KCl and HCl on the High-Temperature Oxidation of a Fe-2.25Cr-1Mo Steel at 400\xa0\xb0C

The influence of alkali- and chlorine-containing compounds on the corrosion of superheater alloys has been studied extensively. The current paper instead investigates the corrosive effects of KCl and HCl under conditions relevant to waterwall conditions. A low-alloy (Fe-2.25Cr-1Mo) steel was exposed to KCl(s), 500 vppm HCl(g) and (KCl + HCl) in the presence of 5%O2 and 20% H2O at 400 °C. The results indicate that alloy chlorination by KCl occurs by an electrochemical process, involving cathodic formation of chemisorbed KOH on the scale surface and anodic formation of solid FeCl2 at the bottom of the scale. The process is accompanied by extensive cracking and delamination of the iron oxide scale, resulting in a complex, convoluted scale morphology. Adding 500 vppm HCl to the experimental environment (KCl + HCl) initially greatly accelerated the formation of FeCl2 at the scale/alloy interface. The accelerated alloy chlorination is attributed to HCl reacting with KOH at the scale surface, causing the cathodic process to be depolarized. A rapid slowing down of the rate of chlorination and corrosion in KCl + HCl environment was observed which was attributed to the electronically insulating nature of the FeCl2 layer which forms at the bottom of the scale, disconnecting the anodic and cathodic regions.

Macro and microgalvanic interactions in friction stir weldment of AA2198-T851 alloy

The galvanic interactions within and between the friction stir weld zones of the AA2198-T851 alloy have been investigated using electrochemical and microscopy techniques. The parent material (PM) was the most anodic region and exhibited pronounced severe localized corrosion (SLC) both when coupled and isolated. The stir zone was the most resistant to corrosion and exhibited no SLC when coupled, but exhibited SLC when isolated. Profiles associated with dissolved oxygen consumption and hydrogen generation currents across the weldment were inversely related because the anodic (PM) region produced higher hydrogen bubbles and, interestingly, consumed more dissolved oxygen compared with the other regions.

Influence of Al2O3 and SiO2 nano-particles on hardness and corrosion resistance of aluminum alloy (Al-4.5%Cu-1.5%Mg)

In the current work, the influence of a single and hybrid addition of the nano-particles (Al2O3) and (SiO2) on the corrosion resistance and polarization of the aluminum alloy (Al-4.5 wt%Cu-1.5 wt%Mg) as base alloy was investigated. Many composites reinforced with an individual addition of (5 wt%) SiO2 or (5 wt%) (Al2O3) nano-particles and hybrid addition of (2.5 wt% SiO2 + 2.5% Al2O3) nanoparticles were fabricated using powder metallurgy route. The rate of corrosion was evaluated by the Tafel polarization extrapolation method. Results showed that the hardness of nano composites that reinforced with the nanoparticles was higher than unreinforced base material. It was found that rate of corrosion of the hybrid nano-composite was lower than nano-composites with single addition and base alloy sample in the (3.5%NaCl) solution. Also, the results revealed that the pitting corrosion resistance is improved with the Al2O3 and/or SiO2 nano-particles addition during the cyclic polarization by disappearing the loop of hysteresis from the anodic polarization region.

Microelectrode Investigation on the Corrosion Initiation at Lead-Brass Galvanic Interfaces in Chlorinated Drinking Water.

In this study, the effects of pH, dissolved inorganic carbon (DIC), and flow on changes in surface chemistry (pH, dissolved oxygen, and free chlorine) of lead-brass joints at initial stages of corrosion were investigated using microelectrodes. Surface measurements showed that the water chemistry at the metal surfaces was highly heterogeneous. At pH 7 and during water stagnation, local pH difference between anodic (leaded-solder) and cathodic (brass) regions differed by as much as 7.5 pH units. High DIC water under the water flowing condition showed minimal pH changes on the surface, whereas in low DIC water, a pH range of 7.6-5.4 (ΔpH 2.2) was observed over the surface. Free chlorine consumption near the lead-brass surface was greater under stagnation, regardless of bulk pH. It was also found that flow can move the low pH plume that originated at the anode. Overall, this study provides direct evidence for highly localized galvanic corrosion in a chlorinated drinking water environment.

The effect of electrode potential on stress corrosion cracking in highly sensitized Al–Mg alloys

The potential dependence of intergranular environmental cracking phenomena in highly sensitized AA5456-H116 is defined in Cl−-containing solution. Stress corrosion cracking (SCC) occurrence is compared with the propensity for corrosion attack, subsequent alloy degradation, and gaseous H evolution/H absorption phenomena as a function of potential in both the anodic and cathodic polarization regions. The SCC susceptibility is low (threshold stress intensity for SCC (KTH) approaching the fracture toughness) in Al–Mg under cathodic polarization conditions and high when anodically polarized (low KTH). The lack of environmental cracking susceptibility was directly proportional to low area normalized levels of H generation estimated to occur at the crack tip at each polarization level. Chemical blunting is shown to be an operative phenomenon at the crack tip to suppress cathodic crack growth by attenuating the mechanical driving force. This blunting phenomenon is demonstrated to have no effect on the threshold stress intensity or crack growth rate for SCC during anodic crack growth where the crack tip readily re-sharpens through the IGC/β phase dissolution process followed by H-induced crack advance.

Electrochemical Behavior of Polyaniline in the Presence of the Vanadate Anion

The chemical and electrochemical syntheses of polyaniline (PANI) were performed in a sulfuric electrolyte in the presence of the vanadate anion (VA). The resulting composite materials based on PANI and VA (PANI–VA) were characterized by IR spectroscopy, X-ray diffractometry, SEM, and elemental analysis. In the electrochemical trials, conditions were indicated under which the composite material retained 96% of its electrochemical capacitance (C) and the cycling of PANI–VA in the range of potentials extended to the anodic and cathodic regions did not lead to the degradation of the polymer material over 50 cycles.

The physiological response of Arundo donax and characteristics of anodic bacterial community in BE-CW systems: Effects of the applied voltage

The performance of bioelectrochemically assisted constructed wetland systems planted with Arundo donax were evaluated at applied voltages of 1, 2, 3 and 5 V. The RGR and Proline (Pro) results implied a positive response at the proper applied voltage. In the 3-V group, the RGR sharply climbed to a maximum over two stages, and the Pro reached a higher level of 256.25 μg/g at the end of the first stage without an additional organic carbon source. This result indicated that A. donax was attempting to adapt the 3-V stress via physiological self-regulation and the growth was nearly unaffected. However, in the 5-V group, the great increase in Pro and withering of the aboveground A. donax showed that the voltage stress had become lethal and the plant was not able to endure it through physiological regulation. In addition, Anodic microbial community compositions accumulating on graphite felts were analyzed via high-throughput sequencing. Results showed that there were two clear clusters at different applied voltages. One cluster was assembled from the anodic region of the 0-V and 1-V groups, and the other was collected from the 2-V and 3-V samples. At the phylum level, Proteobacteria, Firmicutes and Bacteroidetes were the most common phyla among the samples. However, at the genus level, the dominant genera varied with the applied voltage. The relative abundance of Desulfovibrio increased with increased applied voltage. The highest total nitrogen removal rate in the 2-V group was closely related to the high abundance of the Acinetobacter genus. Moreover, Exiguobacterium became dominant only in the 5-V sample, suggesting that this genus strongly depended on the interactive environment of exudes from A. donax and applied voltage.

INVESTIGATION OF ADSORPTION ABILITY OF DIAMINOMALEODINITRILE ON NICKEL SURFACE BY ELLIPSOMETRIC METHOD

The difficulty of the discharge of nickel ions due to the adsorption of the addition of diaminomeleodinitrile on the electrode surface contributes to the formation of a fine crystalline precipitate with high reflectivity. Therefore, in this work, we studied the adsorption capacity of diaminomeleodinitrile on the surface of nickel from borate buffer solution in the cathodic and anodic regions. The values of its free adsorption energy were determined, which were at a potential of -0.65 V - 39.7 kJ / mol, and at 0.2 V - 66.2 kJ / mol. It was determined that diaminomeleodinitrile has adsorbed on the surface of nickel from borate buffer solution at pH = 7.4 mainly due to the forces of chemical interaction. It has been established that on the oxidized nickel surface, the adsorption of diaminomeleodinitrile begins in the region of lower concentrations compared to the “clean” surface. On average, a monolayer has formed within 60-75 min, while changes in the phase angle of the reflected light are small, due to the small size of the adsorbed molecule. It was determined that on the oxidized surface these changes are more than on the “clean” one. By method of reflective ellipsometry, which allows to measure and analyze the differences in the polarization parameters of a plane polarized light flux incident at an angle on two homogeneous media with different optical properties, the thickness of the resulting monolayer was determined. On an oxidized surface, it was ~ 0.5 nm, and on a “clean” surface, ~ 0.2 nm. When comparing these thicknesses with the size of the diaminomeleodinitrile molecule, it can be assumed that at a potential of 0.2 V, the diaminomeleodinitrile molecules are drawn out at an angle to the nickel surface, and at a potential of -0.65 V they are adsorbed flat on the nickel surface.

Corrosion Electrochemical Behavior of Nickel in the LiCl–KCl Melt Containing Lanthanum Trichloride

The effect of temperature on the corrosion of nickel (N1) in the range from 500 to 800°C during testing in the salt melt of the eutectic mixture of lithium and potassium chlorides with the addition of lanthanum trichloride in the amounts from 0.5 to 2 mol % is studied. The nickel corrosion rate increases with temperature, and lanthanum chloride additives decrease the corrosion rate at 500–650°C (inhibition of the shielding type due to the subsequent chemical reaction). The corrosion potential of nickel is approximately –0.5 V relative to the silver chloride electrode at 500°C, insignificantly decreases with temperature, and is independent of the concentration of lanthanum chloride. To specify the mechanism of corrosion damage, current–voltage curves are recorded at lanthanum chloride contents of 0.5 and 2 mol % and sweep speeds of 10 and 20 mV/s. X-ray diffraction analysis and X-ray spectral microanalysis are used. The sweep speed exerts no effect on the irreversible electrochemical oxidation process, and the addition of lanthanum chloride increases the current density in the anodic region tenfold or more. A comparison of the corrosion rates obtained by gravimetric and chemical analytical methods suggests an electrochemical corrosion mechanism.

Tuning molecular energy levels and band gap of two-dimensional benzo[1,2-b:4,5-b′] dithiophene and quinoxaline bearing polymers

Two novel donor-acceptor (D-A)-type two-dimensional (2-D) polymers of PBDTSeQ (P1) and PBDTFQ (P2) were synthesized and characterized where 4,8-bis[5-(2-ethylhexyl) thiophen-2-yl] benzo[1,2-b:4,5-b'] dithiophene (BDT) was used as the donor unit. 2,3-Bis(3,4-bis(octyloxy)phenyl)-5,8-dibromoquinoxaline was used as the acceptor moiety and selenophene and furan were utilized as π bridges. Optoelectronic properties of the polymers were examined by electrochemical and spectroelectrochemical characterizations. In cyclic voltammetry (CV) studies, both polymers were found to be ambipolar. In anodic region, oxidation potentials were observed as 1.45V and 1.15V for P1 and P2 respectively, while reduction potentials were 0.65V and 0.92V. In cathodic region, redox potentials were found to be −1.36V/−1.32V and −2.0V/−1.51V for P1 and −1.45V/−1.05V and −2.15V/−1.45V for P2. Both polymers showed good solubility in common solvents. The synthesized polymers were used as photoactive layers in solar cell devices. P1-based device (ITO/PEDOT:PSS/P1:PC71BM (1:3, w/w)/LiF/Al) provided the best performance with a PCE of 4.04%, a VOC of 0.67V, a JSC of 13.94mA/cm2, and a FF of 43.3%.

Biocompatibility, in Vitro Antiproliferative, and in Silico EGFR/VEGFR2 Studies of Heteroleptic Metal(II) Complexes of Thiosemicarbazones and Naproxen.

Eight heteroleptic nickel(II) and copper(II) complexes of the type [M(L1-4)(nap)2] (1-8), where L1-4 = 2-(1-(4-substitutedphenyl)ethylidene)hydrazinecarbothioamide, nap = naproxen, and M = Ni(II) or Cu(II), have been synthesized and characterized. UV-vis and EPR spectral studies showed distorted octahedral geometry around metal(II) ions. The cyclic voltammogram of complexes 1-8 displayed an irreversible one-electron transfer process in the cathodic region (Epc = -0.66 to -1.43 V), and nickel(II) complexes 1-4 displayed an irreversible one-electron oxidation process in the anodic region (Epa = 0.75 to 1.10 V). The obtained magnetic moment values (1.82-1.93 μB) for copper(II) complexes 5-8 indicate distortion from octahedral geometry, which is further supported by EPR studies. The geometry of the complexes is retained in both solid and solution phases as evidenced from UV-vis and EPR studies. All the complexes showed stability for almost 72 h in biologically relevant solutions. The reducing ability of the copper(II) complexes in the presence of ascorbic acid was analyzed by UV-vis and cyclic voltammetry techniques, which indicates the reduction of the copper(II) to a copper(I) center, and possible interaction within the cells. An in vitro antiproliferative study revealed the nontoxic nature of complexes to normal human dermal fibroblast (NHDF) up to a concentration of 100 ng/mL. The antiproliferative activity of the complexes was tested against three cancerous (human breast adenocarcinoma (MCF-7), hepatoma (HepG2), and lung (A549)) cell lines using MTT reduction assay, which showed enhanced activity for complexes 4 and 8 containing the hydrophobic substituent. Apoptotic and cellular uptake studies showed that complex 8 is readily taken up by HepG2 cell lines and induces ROS-mediated mitochondrial and caspase-dependent apoptosis. In silico studies indicated hydrogen bonding, hydrophobic, and π-pair (π-π, π-σ, and π-cation) interactions between the complexes and EGFR/VEGFR2 kinase receptors.

Electrochemical surface modification of carbon for enhanced water electrolysis

This study reports the activity of porous carbon paper as an efficient oxygen-evolving catalyst. Toray Carbon paper, consisting of carbon fibres, exhibited high current density towards water oxidation after repeated cycling in the anodic region. This activity was due to electrochemical surface modification. X-ray photoelectron spectroscopy analysis showed that the carbon oxidizes to form oxygen rich functional groups, assisting the water oxidation. The surface was analysed by non- destructive Kelvin probe (work function) and Raman spectroscopy to support the enhanced electrocatalytic activity. Surface charge measurements (Zeta potential) were performed to support the surface oxidation. Obtained Zeta potentials were accounted for the surface carbon oxidation. It was found that increased work function of carbon paper after continual cycling in the anodic region increased the water splitting ability of carbon electrode. Herein, we observed that functional groups existing on carbon electrode after electrochemical oxidation exhibit excellent catalytic activity and may be a promising low cost material for OER.

Use of thermally sprayed aluminium (TSA) coatings to protect offshore structures in submerged and splash zones

In this work, the behaviour of arc-sprayed aluminium (1050) coatings was investigated under full artificial seawater immersion and compared with simulated splash zone conditions under droplets of artificial seawater exposed to controlled conditions. To gain a better insight into the mechanism of corrosion of thermally sprayed coatings, tests were also performed on 1050 aluminium sheet. The effectiveness of TSA coatings was evaluated using electrochemical techniques and corrosion products were examined by SEM/EDX and Raman spectroscopy. Sulphur containing corrosion products, such as felsobanyaite, were found on the coating as well as on the Al sheet. This highlights the importance of using seawater, and not NaCl solutions, as a corrosive medium simulating marine environment. Moreover, it was observed that cathodic and anodic regions on thermally sprayed coatings were not easily distinguishable, whereas on Al sheet, cathodic areas were located in the spreading region, where carbonate corrosion product (dawsonite) was detected. Full immersion studies revealed the need for pre-exposing samples before electrochemical testing, in order to predict the long-term behaviour of the coating in marine service.

Fabrication of Ni–Al composite coating on stainless steel using hybrid gas-phase coating technique and its corrosion study

Working in high-concentration acidic environments is required in some important applications. One of the most important determinants of using stainless steel is the breakage when working at high acidic concentrations, where failure can occur by a mechanism similar to chloride attack that leads to stress corrosion cracking. On the other hand, superalloys containing nickel are chosen to be used in certain applications because of their good surface stability, corrosion resistance, and oxidation resistance. In this work, a composite coating based on Ni–Al phases was performed by using a hybrid gas-phase coating technique on the surface of 316 austenitic stainless steels. The deposition temperatures were ranging from Td = 650–950 °C, where the Ni–Al-based vapors was obtained in an evacuated chamber by using pure metal powders in such system. The structural characterization for the deposited Ni–Al coating was performed by using XRD technique and the scanning electron microscopy. The scratch test was performed to determine the thin-film adhesion with the substrate by a diamond indenter that was drawn across the coated surface under a specific load. Electrochemical corrosion tests have been performed to obtain the anodic polarization curves for the base metal and the coated surfaces. From the electrochemical test, the Ni–Al coating surfaces show a great future in reducing the current density of the steel surface in the anodic region, which indicating improved pitting resistance for all Ni–Al coating samples. No pitting corrosion was observed after the test in all the coated surfaces were deposited at 750, 850, and 950 °C with Inters. Pot.: − 495.1 (mV) Inters. Cur.: 18.35 µA, Inters. Pot.: − 661.5 (mV) Inters. Cur.: 11.94 µA, and Inters. Pot.: − 344.4 (mV) Inters. Cur.: 7.19 µA, respectively, at potential test that reaches up to 2000 mV (i.e., no breakdown potential).

Enhanced visible light photoelectrochemical performance of β-Bi2O3-TiO2/ITO thin films prepared by aqueous sol-gel

Semiconductor heterojunction of β-Bi2O3-TiO2/ITO thin films was prepared by aqueous sol-gel and their photoelectrochemical and photoelectrocatalytic properties were evaluated on the oxidation of salicylic acid. The β-Bi2O3 sol precursor properties were characterized by DLS, SEM, and X-ray diffraction. The composition and morphology of the films were characterized by SEM-EDS. The photoelectrochemical characterization was performed by linear sweep voltammetry with chopped light irradiation and photocurrent measurements. The photoelectrocatalytic activity of the films under UV-Vis and visible light irradiation was conducted on the oxidation of salicylic acid. Results show that the films are composed of a TiO2 layer with Bi2O3 particles dispersed throughout the surface. X-ray diffraction results confirm that the Bi2O3 obtained from the sol precursor prepared corresponds to β-tetragonal phase. The linear sweep voltammetry shows that the β-Bi2O3-TiO2/ITO films have a higher photocurrent in the anodic region than TiO2/ITO films used as reference. The photoelectrocatalytic activity of the β-Bi2O3-TiO2/ITO films on the oxidation of salicylic acid under visible light irradiation was higher than TiO2/ITO films. The results show that the β-Bi2O3-TiO2/ITO films are active under visible light and can be used in photoelectrochemical cells.

(Invited) Diamond Electrodes for Electro-Chemical Synthesis

Boron doped diamond (BDD) electrodes have remarkable electrochemical properties, they are enabling novel electrochemical processes, from advanced oxidation liquid waste treatment to new types of electrochemical sensors and emerging new environmental processes. As an allotrope of carbon the properties of diamond electrodes have been shown to be a function of their crystallinity, boron doping concentration and non diamond (graphitic) carbon content. Optimized solid free-standing (not a coating) electrodes with high crystallinity and heavily boron doped can have solvent windows > 4 V (versus SCE) combined with metal like conductivity. Many industrial chemical processes can benefit from the ability of diamond electrodes to deliver high oxidation and reduction potentials while suppressing the electrolysis of water. In the anodic region due to their high physical, chemical and electrochemical stability the hydroxyl and sulfate radical can be formed enabling direct or indirect electrochemical oxidation of dissolved species in the electrolyte. At the anode species such as nitrates can be reduced to nitrogen and since water electrolysis is suppressed in some conditions these processes can be carried out at near 100% Faraday current efficiencies Diamond electrodes are being used in electrochemical advanced oxidation processes (AOPs) for removal of recalcitrant organic pollutants in the industrial liquid wastes by mineralizing them to CO2 and H2. Hydroxyl radicals have a high oxidation potential (2.80V) when compared to ozone (2.08V) and hydrogen peroxide (1.27V) and since the formation takes place directly at the anode by electrochemical oxidation, these systems have been found to be highly effective and universally applicable. In environmental processing and recycling electro oxidation is emerging as a way of making closed cycle chemical processes for e.g. materials recovery from wastes and regeneration of process effluents. Diamond electrodes can be used to simultaneously remove dissolved organics contaminants and restore the oxidation potential of a process effluent. Enables a reduction of the need for chemical dosing and the disposal of chemical wastes. In this talk we will explore some examples of these new electrochemical processes and discuss the potential for new processes that exploit the cathodic properties of diamond electrodes.

The semiconductor properties of passive films and corrosion behavior of stainless steel reinforcing bars in simulated concrete pore solution

The vulnerability of concrete reinforcing steels to corrosion when depassivation occurs, typically in the presence of chloride, makes it important to understand the nature of the steels’ passive films. In Part I of the study, electrochemical techniques and Mott–Schottky analysis were used to investigate these films formed on five different grades of stainless steel and carbon steel reinforcing bars exposed to simulated concrete pore solution. The influence of the steel composition and surface finish on Mott–Schottky plots and the electronic properties are discussed in relation to the steels’ corrosion resistance. A p-type semiconductor behavior was observed in the stainless steel alloys in the cathodic potential regions and an n-type in the anodic potential regions. The n-type behavior is similar to that observed in the carbon steel. The electronic and electrochemical properties of the austenitic grades were superior to the duplex grades. The molybdenum-containing grades, 316LN and 2205, did not show the expected superior properties compared to molybdenum-free grades, 304L and 2304. Also, the replacement of nickel by manganese in the 24100 alloy was not found to be detrimental. The as-received bars showed electronic and electrochemical properties that are more realistic to field conditions than those of the polished cross-sections.

Evidences of the Electrochemical Production of Sulfate Radicals at Cathodically Polarized TiO2 Nanotubes Electrodes

This work presents a novel approach for cathodically polarized TiO2 nanotube electrodes in electrochemical advanced oxidation processes generating sulfate radicals. Although TiO2 do not offer electrochemical response at the anodic region in the dark, its simple self-doping can enable oxidation capacity. Cathodically polarized TiO2 electrode (CP-TNT) was applied in the electrolysis of p-nitrosodimethylaniline dye in sulfate and nitrate aqueous electrolytes, showing much better decolorization efficiency in the presence of sulfate ions. Another evidence of SO4•− generation was the detection of persulfate ions (SO4•− + SO4•− → S2O82−) formed during bulk electrolysis of a sulfate aqueous solution (38 μmol L−1 S2O82− after 30 min). These results open the way for application of CP-TNT electrodes in EAOPs as an alternative for highly cost anodes for electrochemical generation of active sulfate species for degradation of organic contaminants.

Measurement of local galvanic surface corrosion using scanning electrochemical microscopy on ductile cast iron

The application of scanning electrochemical microscopy has increased recently in corrosion studies. The electrochemical activity image is usually generated indirectly by measuring the rate of reaction of a mediator which is not a part of the studied system. This work aims at developing an experimental technique to use scanning electrochemical microscopy without using electrochemically active species. Galvanic corrosion of ductile cast iron was studied by pulsing both the sample and tip electrode to cathodic and anodic regions. By tuning the pulse parameters, the current response of the tip differs in the vicinity of iron and graphite, leading to the imaging of the oxidation/reduction processes.