Corrosion Behaviour of Niobium in Sodium Hydroxide Solutions

The electrochemical behaviour of niobium was investigated in sodium hydroxide solutions at different temperatures, using open-circuit potential (OCP) measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). OCP and polarization measurements show that Nb is spontaneously active in 10, 15 and 30 wt % NaOH at 25, 50 and 75 °C. The anodic polarization curves in all cases show a dissolution/passivation peak followed by a current plateau, corresponding to Nb₂O₅ formation. The spontaneous active corrosion of Nb leads to the formation of soluble niobates that precipitate to sodium niobates. The evaluation of the corrosion current densities obtained from Tafel extrapolation of polarization curves and the polarization resistance values determined from EIS measurements indicates that the corrosion rates of niobium increase with increasing NaOH concentration and temperature.     

Comparative study of Nb, Nb-10W, and Nb-16Ta-12W corrosion behavior in sodium hydroxide solutions

The electrochemical behavior of niobium, Nb-10W, and Nb-16Ta-12W alloys is investigated in sodium hydroxide solutions at different temperatures, using open-circuit potential (OCP) measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). OCP and polarization measurements show that the three materials are spontaneously active in 10, 15, and 30 wt.% NaOH at 25, 50, and 75 °C. The anodic polarization curves show in all cases a dissolution-passivation peak followed by a current plateau, corresponding to oxides formation. The spontaneous active corrosion of the three materials mainly leads to the formation of sodium niobates, as detected by X-ray diffraction analysis of the corrosion products. The evolution of the corrosion current densities obtained from Tafel extrapolation of polarization curves and the polarization resistance values determined from EIS measurements indicate that the corrosion rates of Nb, Nb-10W, and Nb-16Ta-12W alloys increase with increasing NaOH concentration and temperature. In all cases, the increasing order of corrosion resistance is: Nb<Nb-10W<Nb-16Ta-12W.     

Transport and conductivity properties of an advanced cation exchange membrane in concentrated sodium hydroxide

An electrochemical concentrator for application to the chlorine-caustic industry is currently under development. In it 30 to 35 wt % NaOH enters the anolyte and catholyte chambers and exits at 20 and 50 wt %, respectively. Consequently, in support of the electrochemical concentrator development, the conductance and transport properties of advanced cation exchange membranes in concentrated sodium hydroxide, are being investigated. The membrane voltage drop, sodium ion transport and water flux of these membranes in 20 to 35 wt % sodium hydroxide anolyte and 30 to 50 wt % sodium hydroxide catholyte at 75°C are presented. To better understand the behaviour of these membranes, electrolyte sorption measurements were conducted in the anolyte/catholyte environment appropriate for the electrochemical concentrator. The water uptake data appear to correlate well with the conductance data and the combined NaOH and water sorption data are consistent with the sodium ion transport data.     

Perfluorosulphonic acid (Nafion) membrane as a separator for an advanced alkaline water electrolyser

Nafion membranes of two different equivalent weights (eq. wt) were evaluated as a separator in an alkaline electrolyser with nickel screen electrodes in both KOH and NaOH electrolytes over the concentration range of 10–30 wt % and at temperatures from 25 to 160° C. For the same current densities, the cell voltage with 30% KOH electrolytes was more than twice that with 30% NaOH. This result correlates with the water content of the membrane which is almost twice as high in NaOH electrolytes. Thinner membranes and membranes of lower equivalent weight give lower cell voltages. Materials and performance considerations indicate that a membrane of 1000 eq. wt is the optimum separator for an alkaline electrolyser. Indications are that LiOH may be an even better electrolyte than NaOH for use with Nafion membranes. Further improvements in performance can be expected by membrane pretreatment such as exposing the membrane to elevated temperature in water. Nafion membranes have excellent physical and mechanical properties in alkaline electrolyte and can be used at temperatures up to 250° C.Work performed under the auspices of the US Department of Energy.     

The effect of microstructure on the anodic behaviour of Cu-Be-Ni-Zr alloy system

The effect of heat treatment on the anodic behaviour of the Cu-Be-Ni-Zr alloy in 65% phosphoric acid has been investigated by the potentiostatic method. Different anode states obtained by quenching from temperatures of 900,950 and 1000°C were examined under the microscope. It was found that the position and shape of the polarization curves, especially in the active and passive parts, are dependent upon the quenching temperature and colling rate. For pre-quenching temperatures up to 950°C, at which there are no appreciable changes in microstructure, the polarization curves are similar. For higher pre-quenching temperatures up to 1000°C where substantial microstructural changes occur, the active part of the polarization curves becomes more negative, by almost 100 mV, while the passivation decreases.     

Electrochemical behavior of centrifuged cast and heat treated Ti-Cu alloys for medical applications

The aim of this study was to evaluate the general electrochemical corrosion resistance of Ti-5, 7.1 and 15 wt.% Cu alloys with a view to medical applications. A centrifuged casting set-up and a solution heat treatment at 900 °C for 2 h were used to prepare the samples. Electrochemical impedance spectroscopy (EIS) and potentiodynamic anodic polarization techniques were used to analyze the corrosion resistance in a 0.15 M NaCl solution at 25 °C. An equivalent circuit analyses was also conducted. It was found that the corrosion rate increased with increasing Cu content. The results have shown that the addition of Cu has not stabilized the β phase. Martensite and Ti₂Cu intermetallic particles provided by casting and heat treatment processes, respectively, have important roles on the resulting impedance parameters and passive current densities of the Ti-Cu alloys.     

Corrosion behaviour of some stainless steels in chlorinated Gulf seawater

The corrosion resistance of conventional and high alloy stainless steels (specifically 316L, 317L, 904L, duplex 2205, 3127hMO, 1925hMO, 254SMO, 654SMO and Remanit-4565) was determined electrochemically in chlorinated and unchlorinated Arabian Gulf Seawater at 25 and 50 °C. The stainless steels 316L and 317L were used as reference alloys. The electrochemical potentiodynamic cyclic polarization method was used to determine the passive film break down potential (E _b), protection potential (E _prot) and maximum current attained on scan reversal (I _max). It was found that at 25 °C in chlorinated and unchlorinated seawater and at 50 °C in unchlorinated seawater, stainless steels 316L and 317L have poor resistance to corrosion, Stainless steels 904L and duplex 2205 at 25 °C in chlorinated and unchlorinated seawater showed good resistance to corrosion but at 50 °C these steels failed to resist. The high alloy stainless steels such as 3127hMO, 1925hMO, 254SMO, 654SMO and Remanit-4565 showed better corrosion resistance under all the test conditions.     

Insights into protection mechanisms of organic coatings from thermal testing with EIS

The proposition that corrosion rate is limited by the ionic resistance of an organic coating has been tested. Mild steel panels coated with an epoxy-phenolic paint were exposed to 3% sodium chloride solution at 50 °C for different periods and characterized by electrochemical impedance spectroscopy (EIS) across a range of temperatures (25–50 °C). Changes in the film resistance and charge-transfer resistance with temperature were analysed to deduce activation energies for the processes involved. It was found that the calculated activation energy from coating resistance is significantly lower than the activation energy for the charge transfer resistance. This suggests that the ionic resistance of the coating, as apparent in an AC measurement, cannot be controlling the corrosion rate. Coating resistances for free films of the same coating show even higher activation energy values, so that the resistance of un-degraded areas of the coating within the current path could be controlling the current flow. Potentiostatic pulse tests on coated metal have enabled iR-corrected polarization curves to be plotted at different temperatures that gave high activation energies from the estimated corrosion currents. This provides two possible explanations that can account for the results.     

Corrosion Inhibition of Mild Steel in Acidic Medium using 2-amino Thiophenoland 2-Cyanomethyl Benzothiazole

The corrosion inhibition characteristics of 2-amino thiophenol (ATP) and 2-cyanomethyl benzothiazole (CNMBT) on two types of steel in 1m HCl medium were investigated at different temperatures (25, 30, 35, 40 and 50°C). The pitting corrosion behaviour for the same system was studied using a potentiodynamic technique. The pitting corrosion resistance of steel samples increased with increase in concentration of the ATP and CNMBT. Some samples were examined by scanning electron microscopy. The effects of the inhibitors on the general corrosion of the two samples were investigated by using gravimetric and galvanostatic polarization techniques. The inhibition efficiencies increased with increase their concentration but decreased with increase in temperature. Free energies of activation, enthalpies and entropies for the inhibition processes were determined from rate constant data measured and different temperatures at different concentrations of ATP and CNMBT. Results were compared with fits obtained from the application of the Langmuir isotherm. Results were correlated to the chemical structure of the inhibitors. The inhibition efficiency of CNMBT is higher than that of ATP.     

Use of boron wastes in the production of heavy clay ceramics

Colemanite, ulexite and tincal are the main boron ores found in Turkey and account to 63% of the world estimated reserves. The production route of boron products results in significant amounts of different types of Boron Wastes, BW. Their open field disposal raises substantial environmental concerns in fear of leaching and groundwater pollution. The heavy clay ceramic industry can potentially absorb substantial quantities of BW. In the present study, BW produced from Kirka borax plants in Turkey, with 12.6 wt.% B₂O₃ was introduced in 0 wt.%, 5 wt.% and 15 wt.% in a heavy clay body mixture. Four peak temperatures, 800 °C, 850 °C, 900 °C and 950 °C, were examined for the dry pressed samples. The thermal behaviour analyzed by dilatometry and TGA shows that major loss in weight starts about 600 °C and continues to 700 °C approximately. For 5 wt.% BW addition and firing at 900–950 °C, the sintered bodies present comparable or improved physical and mechanical properties with respect to the reference formulation. The microstructure was analyzed by SEM whereas the main crystalline phases were identified by XRD. Samples fired at 900 °C with 5 wt.% BW present comparable properties with the reference ones. At 950 °C, the obtained properties were improved. Deformation occurs for samples with 15 wt.% BW when fired at >900 °C.     

Steel corrosion in alkaline batteries

During extended shelf storage and accelerated aging, low carbon steel may corrode producing soluble iron species that cause internal cell shorting and/or gassing (leakage), thus undermining battery reliability. Soluble iron may be generated from the active/passive and/or transpassive dissolution of the battery can steel. To minimize corrosion effects in commercial battery applications, NiCo plated steel is typically used. This report focuses on corrosion behavior of pure metals (iron, nickel, and cobalt) in 40% KOH at temperatures between 20 °C and 80 °C. A marked difference is found for the elements regarding the actual passivating behavior with time and temperature. As expected, nickel spontaneously passivates at all temperatures under study (20-80 °C). Cobalt on the other hand, is active at all temperatures but shows an active/passive-transition. Iron spontaneously passivates only at temperatures below 60 °C and activates at temperatures >60 °C. At sufficiently anodic potentials, passivity of Fe is lost by transpassive Fe dissolution to form ferrates [FeO₄]²⁻. The latter is considered the most likely cause for corrosion-induced failure in alkaline batteries.     

Hydrogen evolution reaction on Ni-Al-Mo and Ni-Al electrodes prepared by low pressure plasma spraying

The hydrogen evolution reaction was studied on vacuum plasma sprayed Ni-Al and Ni-Al-Mo electrodes in 1m NaOH and 25% KOH at 70°C. It was found that Ni-Al-Mo electrode is more active in 25% KOH whereas Ni-Al electrode was more active in 1m NaOH solutions. A.c. impedance measurements showed two semicircles on the complex plane plots. Two different equivalent models were used to explain this behaviour. The results indicate that the formation of the second semicircle is connected with the hydrogen evolution reaction.     

In vitro studies of hydrogen peroxide treated titanium for biomedical applications

Pure titanium substrate was subjected to chemical treatment with different concentrations of hydrogen peroxide and subsequently heat treated to produce a titania gel layer with anatase nature. The surface modified substrates were then immersed in simulated body fluid for the formation of calcium phosphate layer over the surface. Titanium treated with 15 wt.% of hydrogen peroxide produced a uniform porous layer, which was found to induce the calcium phosphate formation more easily. However, titanium treated with 5 wt.% and 25 wt.% of hydrogen peroxide exhibited inhomogeneous surface for the growth of calcium phosphate layer. Further, the corrosion behaviour of the untreated and hydrogen peroxide treated specimens in simulated body fluid was evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy. The results have shown that the surface modified titanium using 15 wt.% of hydrogen peroxide exhibited higher corrosion resistance compared to specimens treated with other concentrations of hydrogen peroxide.     

Effects of curing temperature and NaOH addition on hydration and strength development of clinker-free CKD-fly ash binders

Effects of curing temperature and NaOH addition on hydration and strength development of cement kiln dust (CKD)-fly ash (FA) binders were investigated. Pastes made with 50% CKD and 50% FA, having 0, 2, and 5% NaOH addition, and cured at temperatures of 24, 38, and 50 °C were evaluated. The hydration products of the binders were examined by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) tests. The results indicate that the major crystalline hydration product of the CKD-FA binders is ettringite, and the ettringite is stable in the CKD-FA system at age over 100 days. Curing at elevated temperature is more effective for CKD-FA binder strength improvement than NaOH addition, the later often depressing ettringite formation in a CKD-FA system. At a proper curing temperature (38 °C), addition of a small amount of NaOH (2%) may increase CKD-FA binder strength; while at a high curing temperature (50 °C), addition of NaOH (2%) may reduce the binder strength.     

Interpretation des diagrammes E-pH du systeme Fe-H2O en relation avec la fragilisation caustique des aciers

Resume Les diagrammes E-pH du système Fe-H₂O ont été traces pour 60, 100 et 150° C en utilisant de nouvelles valeurs thermodynamiques. La zone de formation de Fe(OH)₂ solide déterminée d 25° C par Silverman se rétrécit avec une augmentation de la température pour disparaître aux températures au-delà de 65° C. Ce changement est relié à la susceptibilité de l'acier à la fragilisation caustique, observée en pratique et au laboratoire. La relation entre la nature de la couche passive et la corrosion sous tension des aciers dans les solutions de NaOH est discutée. La formation de Fe(OH)₂ à basses températures et son oxydation subséquente en FeOOH contribuent à la réparation du film passif. Toutefois, à hautes températures la formation de Fe₃O₄ au pic II entraine la susceptibilité à la corrosion sous tension des aciers.

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E-pH diagrams for the Fe-H₂O system are presented at temperatures of 60, 100 and 150° C, employing new thermodynamic data. At 25° C, the formation of a solid Fe(OH)₂ zone, determined by Silverman, was found to reduce with increasing temperature and at temperatures above 65° C the zone disappeared. Previous work has related this change in the electrochemical behavior of steel in NaOH solutions at + 65° C to caustic embrittlement, observed in practice and in the laboratory. The relation between the nature of the passive film and stress corrosion cracking of steel in NaOH solutions is discussed. The formation of Fe(OH)₂ at low temperatures and its oxidation to FeOOH allow the repair of the passive film. However, at high temperatures, the formation of the oxide film (Fe₃O₄) at peak II leads to the susceptibility of steels to stress corrosion cracking (SCC).

     

Electrochemical and time-of-flight secondary ion mass spectrometry analysis of ultra-thin metal oxide (Al2O3 and Ta2O5) coatings deposited by atomic layer deposition on stainless steel

Ultra-thin (5–50 nm) layers of aluminium and tantalum oxides deposited by atomic layer deposition (ALD) on a stainless steel substrate (316L) for corrosion protection have been investigated by electrochemical methods (linear scan voltammetry, LSV, and electrochemical impedance spectroscopy, EIS) and time-of-flight secondary ion mass spectrometry, ToF-SIMS. The effects of the deposition temperature (250 °C and 160 °C) and coating thickness were addressed. ToF-SIMS elemental depth profiling shows a marked effect of the organic and water precursors used for deposition and of the substrate surface contamination on the level of C and OH trace contamination in the coating, and a beneficial effect of increasing the deposition temperature. The polarization data show a decrease of the current density by up to four orders of magnitude with increasing coating thickness from 5 to 50 nm. The 50 nm films block the pitting corrosion in 0.8 M NaCl. The uncoated surface fraction (quantified from the current density and allowing a ranking of the efficiency of the coating, also confirmed by the capacitance and resistance values extracted from the EIS data) was 0.03% with a 50 nm thick Al₂O₃ film deposited at 250 °C. The correlation between the porosity values of the coatings and the level of C and OH traces observed by ToF-SIMS points to a marked effect of the coating contaminants on the sealing performance of the coatings and on the corrosion resistance of the coated systems.     

Localized corrosion behavior of scratches on nickel-base Alloy 690TT

Localized corrosion of scratched Alloy 690TT was studied using scanning electrochemical microscopy (SECM) in 10 wt.% NaOH solution and atomic force microscopy (AFM) after short-time oxidation tests in high temperature high pressure water at 250 °C. The results show that the scratches detected by SECM displayed more extensive electrochemical activity. The scratch groove and the whole deformed region caused by scratching as anode corresponded to the peak of tip current. The oxides formed in 250 °C high purity pressurized water nucleated preferentially at the scratch groove and grew faster than non-scratched surface. The nano-grained scratch groove has more electrochemical reactivity than mechanical twins at scratch banks. The intense localized corrosion could contribute to crack initiation at the bottom of scratch groove.     

Effect of temperature on performanceof LaNi4.76Sn0.24 metal hydride electrode’

The effect of temperature on the performance of a LaNi4.76Sn0.24 metal hydride electrode was investigated in the temperature range of 0 to 50°C. The electrode showed a maximum discharge capacity at 25°C. The total resistance increases with a decrease of temperature from 50°C to 0°C. The apparent activation enthalpies at different states of charge were determined by evaluating the polarization resistance at different temperatures. It was found that the apparent activation enthalpy is an indicator of the relative reaction rate of the charge-transfer reaction and hydrogen absorption.     

Corrosion behavior of alumina-based ceramic core materials in caustic alkali solutions

Corrosion behaviors of the porous alumina-based ceramic core materials in KOH and NaOH solution were investigated. Corrosion tests were carried out at 100°C, 150°C, and 200°C, and the concentration of KOH and NaOH was 50, 67, and 75 wt%, respectively. The results indicated that the optimal concentration was 67 wt% for KOH solution and 50 wt% for NaOH solution, respectively. Increasing corrosion temperature and prolonging corrosion time were helpful to enhance the corrosion effect, and temperature played an extra important role during the whole corrosion process. NaOH solution was better than KOH solution for corrosion at the same temperature and concentration.     

Studies on the anodic and cathodic polarization of amalgams—V. the behaviour of tin amalgams in alkaline solutions

Tin-amalgams of varying concentrations were oxidized galvanostatically in 0·01, 0·10 and 2·00 N NaOH solutions at 25°C. For each amalgam and solution composition a number of constant polarizing currents were used and cyclization was repeated until the oxidation (reduction) features of the amalgam became apparent. Two different oxidation patterns were distinguished. The first describes the behaviour of all amalgams in 0·01, 0·1 N NaOH, and of concentrated amalgams in 2 N NaOH, and the second for the behaviour of dilute tin amalgams in 2 N NaOH solutions.