Potentiokinetic Curves Research Articles

Porous Carbon Materials for Elements in Low-Temperature Fuel Cells

Abstract The porosity, distribution of pores, shape of pores and specific surface area of carbon materials were investigated. The study of sintered graphite and commercial carbon materials used in low-temperature fuel cells (Graphite Grade FU, Toray Teflon Treated) was compared. The study covered measurements of density, microstructural examinations and wettability (contact angle) of carbon materials. The main criterion adopted for choosing a particular material for components of fuel cells is their corrosion resistance under operating conditions of hydrogen fuel cells. In order to determine resistance to corrosion in the environment of operation of fuel cells, potentiokinetic curves were registered for synthetic solution 0.1M H2SO4+ 2 ppmF-at 80°C.

USE OF MATERIALS FOR INTERCONNECTORS IN FUEL CELLS

This paper presents an analysis of the properties of sintered steels in terms of their practical use for production of interconnectors in fuel cells. The study covers measurements of mechanical properties, microstructural examinations and analysis of surface profile in the sintered samples. The porosity and contact resistance between interconnectors and gas diffusion layer are also examined. The main criterion adopted for choosing a particular material for the components in fuel cells is their corrosion resistance under operating conditions of hydrogen fuel cells. In order to determine resistance to corrosion in the environment of operation of fuel cells, potentiokinetic curves are registered for synthetic solution 0.1 M H2SO4 + 2 ppmF- at 80°C.

Sintered Stainless Steel for Interconnectors for PEM Fuel Cell

Polymer electrolyte membrane fuel cell performance strongly depends on properties of the fuel cell stack bipolar plates (BPs). Bipolar plates are a key component of fuel cells. Functions of materials used for fuel cells include even distribution of gas fuel and air, conduction of electricity between the adjacent cells, heat transfer from the cell as well as prevention of gas leakage and cooldown. Due to multifunctionality of fuel cell plates, choice of materials used for plates is immensely difficult. This paper presents opportunities of application of a new technology of powder sintering for creation of parts for electricity and heat generators. Sintered stainless steel 316LHD was investigated as a candidate material for bipolar plate materials. 316L powders were compacted with the following load: 700MPa, 550MPa, and 200MPa, and then sintered at the temperature of 1250 °C in hydrogen medium. The main criterion for selection of a particular material for components of fuel cells is their corrosion resistance in operating conditions of hydrogen fuel cells. In order to determine resistance to corrosion in the environment of operation of fuel cells, potentiokinetic curves (as a function of temperature) were registered in synthetic solution 0.1M H2SO4 + 2 ppmF- at 80°C. The investigations also covered measurements of mechanical properties and microstructural testing of sinters with austenitic structure.

Study of the deposition of cerium oxide by conversion on to aluminium alloys

This paper reports work aimed at finding an alternative treatment to chromate conversion coatings (CCC) that could meet all the requirements of industrial applications. A first step was made by carrying out the deposition of films of CeO 2·2H 2O on aluminium alloys in a few minutes at room temperature with or without a catalyst. Different types of pretreatment (acidic or alkaline) led to drastically different results. A model for the catalytic deposition mechanism is suggested which is consistent with scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) observations. The anticorrosive properties of these coatings were investigated by potentiokinetic curves measurements and by electrochemical impedance spectroscopy (EIS). Encouraging results were obtained though the performances still do not equal those of CCC. Some of the coating features (adhesion, sealing of the anodic sites, etc.) which should still be improved are highlighted.

Mechanistic Studies of the Corrosion of 2024 Aluminum Alloy in Nitrate Solutions

Nitrate ions, which are known as efficient inhibitors for the corrosion of aluminum, have been shown to become very aggressive in pitting of 2024 aluminum alloy in specific conditions. Plotting potentiokinetic curves and immersion tests revealed strong dissolution of 2024 alloy in nitrate solutions with or without chloride ions. Scanning electron microscopy observations showed that pitting of 2024 alloy mainly occurred on the intermetallic particles. Nitrate ions caused the dissolution of intermetallic particles whereas they acted as very good inhibitors for the aluminum matrix. However, local dissolution of the matrix has been found to occur around the Cu- and Mg-rich particles and has been quantified by using atomic force microscopy. Secondary ion mass spectroscopy and ultraviolet spectroscopy helped elucidate the mechanism of dissolution of the intermetallics. Reduction of nitrate ions to nitrite ions and then to ammonia on intermetallics induced pitting nucleation on these particles. Copper redeposition occurred and dissolution of the surrounding matrix was observed. © 2003 The Electrochemical Society. All rights reserved.

Evaluation of the long-term corrosion behavior of dental amalgams: influence of palladium addition and particle morphology

Objectives: The purpose of this investigation was to evaluate the long-term corrosion behavior of experimental amalgams as a function of particle morphology and palladium content. Methods: Samples of four experimental high copper amalgams were prepared according to ADA specifications. Two of them had the same chemical composition but one had lathe cut particles (LCP) and the other had spherical particles (SP). The two others had spherical powders with an addition of 0.5 wt% of palladium (SP 0.5) and 1 wt% of palladium(SP 1) for the other. Corrosion resistance was evaluated by electrochemical techniques in Ringer's solution in a thermostated cell at 37 °C for samples aged 5, 8, 12, 16 months and 10 years. Potentiokinetic curves were drawn and the potential and the current density corresponding to the first anodic peak were registered. Results: For all the amalgam samples the corrosion behavior improves over the 10-year period. SP samples exhibit a better behavior than LCP. Palladium addition improves corrosion behavior as compared to samples without palladium. No real difference is found regarding the amount of palladium between 0.5 and 1%. The potentials progress from a range between 0 and 20 mV/SCE to a range of 60–80 after 10 years. The stabilization of the potential begins after only 16 months. Except for the LCP, all the values converge to the same level of 80 mV/SCE. Significance: The addition of no more than 0.5 wt% Palladium in a high copper amalgam powder improves the corrosion behavior of the amalgam up to a period of 10 years. The potential of the first anodic peak increases for each amalgam, probably in relation to the evolution of the structure of the material. Clinically, it is of interest to consider the good electrochemical behavior of older restorations when contemplating the repair or replacement of such fillings. At the same time, galvanic current can occur when a new amalgam restoration is placed in contact with an old one even if the same amalgam is used. In this situation, the new filling will be anodic and its degradation will be accelerated. The evaluation of the corrosion behavior of dental amalgams has to take into account the age of the samples.

Structure and properties of multicomponent and multilayer hard coatings obtained by the bias activated reactive evaporation method

The paper presents investigations of the structure and properties of multicomponent and multilayer hard coatings based on Ta, Mo, Ti, W, Al and their nitrides, obtained using standard and improved bias activated reactive evaporation methods. The results of secondary ion mass spectrometry investigation as well as X-ray diffraction and energy-dispersive analysis of X-rays are presented. The potentiokinetic curves of deposited layers are also shown.

Quantitative measurements of the degree of exfoliation on aluminium alloys

The paper deals with quantitative measurements of the degree of exfoliation on three aluminium-lithium alloys (Al-Li-Cu, 8090 and 2091) in under-aged and over-aged conditions and on 7075-T7351, 7175-T6511 and 2024-T351. A special device was used in this work to measure the forces developed by the corrosion products during exfoliation in Exco-solution. Potentiokinetic curves were obtained on these alloys in Exco-solution and sodium chloride solutions (0.6, 1, 2 and 4 M Cl −) at a sweep rate of 1000 mV min −1, to determine the pitting potential, E p , of each alloy. The curve obtained by plotting the forces developed by the exfoliation process as a function of time could be divided into two stages; an incubation period, t 1 , was followed by an increase in the forces to a level corresponding to a time of exfoliation equal to 240 h ( F 240h ). The results obtained were compared to those observed in the conventional Exco-test (ASTM G34). Finally, stress-corrosion cracking tests using the slow strain-rate tensile technique were carried out on the aluminium-lithium alloys, either in Exco-solution or in the sodium chloride solutions containing various chloride concentrations. The results obtained show that, when the time of incubation, t 1 , increases, the force of exfoliation, F 240h , decreases and the resistance to stress-corrosion cracking, R SCC , increases. This resistance is sensitive to the copper content, i.e. R SCC decreases also when the copper content increases. It is concluded that t 1 may be taken as an exfoliation criterion.

Étude de la susceptibilité à la corrosion d’aciers inoxydables AISI 304 ET AISI 316 soumis à des contraintes mécaniques dans l’acide nitrique dilué bouillant en présence de chlorures

A detailed study of corrosion of stressed 304 and 316 stainless steels in boiling solutions of diluted nitric acid in presence of chloride is presented. After a chemical study of the electrolyte, the different kinds of corrosion observed are represented on HNO 3 concentration - Cl − concentration diagrams. A more fundamental study based on several electrochemical techniques (forward scan and return potentiodynamic curves, potentiokinetic curves with different scan rates, sample depassivation by rapid straining under potentiostatic control) is carried out. The results allow to confirm the observations and to explain them in terms of competition between anodic dissolution, depassivation, repassivation processes with a precise analyze of the role of the solution and of the mechanical stress

Three dimensional AE source location in metals : Kim, K.H.; Kishi, T. Nondestructive Testing Communications, Vol. 3, No. 3, pp. 75–88 (1987)

The paper deals with quantitative measurements of the degree of exfoliation on three aluminium-lithium alloys (Al-Li-Cu, 8090 and 2091) in under-aged and over-aged conditions and on 7075-T7351, 7175-T6511 and 2024-T351. A special device was used in this work to measure the forces developed by the corrosion products during exfoliation in Exco-solution. Potentiokinetic curves were obtained on these alloys in Exco-solution and sodium chloride solutions (0.6, 1, 2 and 4 M Cl−) at a sweep rate of 1000 mV min−1, to determine the pitting potential,Ep, of each alloy. The curve obtained by plotting the forces developed by the exfoliation process as a function of time could be divided into two stages; an incubation period,t1, was followed by an increase in the forces to a level corresponding to a time of exfoliation equal to 240 h (F240h). The results obtained were compared to those observed in the conventional Exco-test (ASTM G34). Finally, stress-corrosion cracking tests using the slow strain-rate tensile technique were carried out on the aluminium-lithium alloys, either in Exco-solution or in the sodium chloride solutions containing various chloride concentrations. The results obtained show that, when the time of incubation,t1, increases, the force of exfoliation,F240h, decreases and the resistance to stress-corrosion cracking,RSCC, increases. This resistance is sensitive to the copper content, i.e.RSCC decreases also when the copper content increases. It is concluded thatt1 may be taken as an exfoliation criterion.

Application of polarization curves for analysis of magnetic properties of Alnico 5 and Fe47·5Cr28Co23·5 Si1 magnets

Abstract Saturation induction and the volume fraction of the ferromagnetic α1 phase in Alnico 5 and Fe47·5Cr28Co23·5 Si1alloys are estimated by means of potentiokinetic curves of these alloys after heat treatment producing optimum magnetic properties. The origin of the high value of magnetic induction and maximum magnetic energy product of Fe47·5Cr28Co23·5 Si1 alloy lies in the greater difference in saturation induction occurring between the ferromagnetic and paramagnetic phases of that alloy as compared with those obtained for Alnico 5 alloy.

Polarization curves of Fe-28% Cr-24% Co-1% Si permanent magnet alloy in sulphuric acid solution

The potentiokinetic curves of anodic polarization of magnetically hard Fe-28% Cr-24% Co-1% Si alloy in sulphuric acid of pH 0 were plotted. The Fe-Cr-Co-Si alloy has been subjected to four different treatments that are usually used for generation of magnetic properties. Research has shown that the heat treatment at temperatures of the order of 640 °C applied for generation of two-phase structure causes the drastic decrease of corrosion resistance of the tested alloy.

Comportement de l'alliage Cuivre-Nickel [formula omitted] dans une solution de NaCl A 3%

The corrosion behaviour of 70 30 copper-nickel alloys in 3% NaCl solution has been investigated through the use of potentiokinetic curves, measure of the Cu Ni ratio in the reaction products and surface analysis by E.D.X. The anodic current-potential curves were measured after free corrosion (for up to 10 days) and it was shown that there were one or two Tafel slopes depending on the time of free corrosion. At the corrosion potential, copper was found to exist in a high concentration in solid corrosion film products and nickel in a high concentration in the solution. The percentage of copper in the total reaction products was higher than in the alloy substrate. In the Tafel region, E.D.X analysis has shown that there was a nickel enrichment of the surface of the alloy which could be related to a dealloying of the alloy.

Potentiokinetic Criteria relevant to the pitting corrosion of stainless steels in chloride solutions

AbstractThe adequacy of 7 parameters of potentiokinetic curves for evaluating the resistance to pitting corrosion in sodium chloride solutions was quantitatively assessed by reference to immersion testing in ferric chloride solutions for 11 type CrNi (MoTiCu) stainless steels on the basis of 314 experimental runs.

ChemInform Abstract: INFLUENCE OF NITRATE IONS ON SILVER ELECTROCRYSTALLIZATION

AbstractPotentiokinetische und chronopotentiometrische Untersuchungen sowie Impedanzmessungen ergaben, daß bei der Reduktion von Ag ‐Ionen aus einer AgNO3 ‐Lösung an der Ag‐Kathode nicht der Ladungstransport der geschwindigkeitsbestimmende Schritt sein kann und daß eine Zunahme der NOg‐Konz.

Influence of nitrate ions on silver electrocrystallization

The influence of nitrate anions on the reduction of silver ions on a silver cathode from a silver nitrate solution was studied with three different experimental methods: potentiokinetic curves, chronopotentiometry and impedance measurements. It was concluded that charge transfer of the Ag + ions is certainly not the rate determining step of the reaction. It was clearly shown with the three methods that an increase in the nitrate ion concentration activates the reduction of the Ag + ions; the overvoltage of the reaction decreases markedly. With the aid of an equivalent circuit it is possible to quantify the activation of the electrode, in particular through the determination of the charge transfer resistance and the resistance and the capacity of adsorbed species.

Action d'oxydations et de réductions électrochimiques successives sur la structure superficielle d'électrodes d'or polycristallines: Étude optique et életrochimique

Flat, mechanically polished gold electrodes have been subjected to several hundred cycles of electrochemical oxidation and reduction (0.4 to 1.9 V NHE) in normal sulfuric acid. The evolution of these electrodes has been followed by electrochemical measurements (capacity and potentiokinetic curves) and optical (ellipsometry) measurements, and the surfaces have been examined by metallographic microscopy. After a number of cycles the electrochemical treatment transforms the originally disturbed surface into a structured surface, with appearance of grain boundaries and faceting analogous to thermal faceting. Considering the work of Vetter and Schultze, we interpret the restructuring of the electrode surface as being due to two facts: mobility of metal atoms, because of their delocalization during the oxygen desorption phase and the anisotropy of the surface energy, owing to the presence of adsorbed oxygen. The complex evolution of the optical properties of the metal cannot be accounted for simply by facetting, and thus we have been led to postulate an accumulation in the metal of traces of undesorbed oxygen.

Study of the electrochemical behavior of gold dental alloys.

The corrosion of some gold base alloys was studied in artificial saliva through the use of electrochemical methods, primarily potentiokinetic curves. The effects of various alloying elements were investigated.

Etude du degagement d'oxygene sur une electrode de nickel

The anodic behaviour of nickel in sulphuric acid solution is studied in the transpassive range. At potentials between 800 and 1800 mV(sce), the potentiokinetic curves of nickel show a current maximum and a minimum, and a considerable hysteresis. The slope of the Tafel line for oxygen evolution is linked to the polarization parameters. These depend on the fraction of the electrode surface covered with intermediate adsorbed species and on the transfer coefficient of the reaction.