Stefano Frangini

Cr7C3-based cermet coating deposited on stainless steel by electrospark process: structural characteristics and corrosion behavior

Abstract The electrospark deposition (ESD) technique has been successfully applied to deposit at room temperature a cermet layer of Cr7C3 carbide bound with 20 wt.% Cr on a AISI 304 stainless steel substrate. It has been possible to obtain a fully dense, uniform and strong adherent coating layer of 10–30-μm thickness. The corrosion properties of this coating system have been evaluated in HCl electrolyte by conventional anodic polarization tests and impedance measurements. The results indicate an exceptional resistance of the coating to both general and localized corrosion attack. Some surface stress relief microcracks formed during the ESD deposit cycles do not impair the corrosion resistance of the coating even after long time immersion in HCl.

Sensitive Determination of Oxygen Solubility in Alkali Carbonate Melts

The solubility of O 2 in Li-K and Li-Na eutectic carbonate melts has been estimated in the range 600-700°C using a highly precise chemical titration method. In general, oxygen solubility is found to be distinctly higher in the Li-K than in the Li-Na eutectic with a tendency for such a solubility difference to disappear at 600°C. The precise determination of O 2 solubility values at different O 2 and CO 2 partial pressures allowed evaluation of plausible oxygen dissolution mechanisms by a data fitting procedure. Our results support the assumption of a predominant superoxide formation at 650 and 700°C in both melts, with the difference that in Li-Na carbonate oxygen appears to dissolve also through formation of peroxymonocarbonate ions. At 600°C, molecular oxygen is found to be the major oxygen component.

Sensitive assay for oxygen solubility in molten alkali metal carbonates by indirect flame atomic absorption spectrometric Cr(VI) determination.

A precise and accurate indirect analytical method for the assessment of O(2) solubility in molten (Li(0.62)K(0.38))(2)CO(3) and (Li(0.52)Na(0.48))(2)CO(3) is described. The method is based on the oxidation of Cr(2)(SO(4))(3) (added in excess to the melt) by the oxygenate species, which are formed inside the melt when it is in contact with oxygen gas, and subsequent determination of trace amount of Cr(VI) in withdrawn frozen melts by flame atomic absorption spectrometry (FAAS). The samples (1.0-2.0g) are dissolved in dilute hydrochloric acid at room temperature. The speciation of Cr(VI) is carried out by complexation with ammonium pyrrolidine dithiocarbamate (APDC), followed by extraction into methyl isobutyl ketone (MIBK), which is introduced directly into the flame. Optimisation of the flame composition provided maximum Cr signal in organic phase under lean acetylene-air flame. The separation and preconcentration parameters such as sample volume/extractant volume ratio, pH sample solution, chelating concentration and extraction time are evaluated. Under the optimised conditions Cr(VI) is efficiently separated from Cr(III), which exceeded 200-folds. The results of the analysis of synthetic samples using standards in MIBK medium give rise to recoveries of 98-99%. The Cr(VI) detection limit of 4x10(-6)gL(-1) using 12.5-fold preconcentration and relative standard deviation of 1% at the 0.10mgL(-1) level are obtained. The sampling-to-sampling reproducibility was typically 3-5% relative standard deviation. By changing the preconcentration factor and the dilution of the sample melt, it is possible to analyse oxygen concentrations in molten alkali metal carbonates as low as 1.5x10(-9)mol O(2) per gram melt.

The Dissolution of Oxygen in La2O3-Added 52 ∕ 48 mol % Li ∕ Na Molten Carbonate Determined by Gas Solubility Measurements

In this work the effect on the oxygen dissolution of adding La 2 O 3 to the binary eutectic 52/48 mol % Li/Na carbonate melt has been evaluated in the temperature range 550-650°C. The amount of dissolved O 2 has been determined by accurate chemical titration. The addition of 0.5 mol % La 2 O 3 to the carbonate salt has been found to result in a dramatic increase of O 2 solubility and also in a change of the activation energy (E a ) for the oxygen dissolution process. The experimental values for E a correspond closely to a peroxide mechanism in the La 2 O 3 -added carbonate as compared to a molecular oxygen or a prevalent superoxide in the pure Li/Na carbonate at 600 and 650°C, respectively. For a better understanding of the O 2 dissolution path, a study also has been conducted by O 2 solubility determinations at different CO 2 and O 2 partial pressures using a curve-fitting method. The peroxide mechanism has been confirmed at 650°C, while at 600°C simultaneous presence of a percarbonate dissolution path cannot be excluded. Overall, these O 2 solubility results can provide the basis for the explanation of the La electrocatalytic properties on the oxygen reduction reaction at the MCFC cathode.

A voltammetric study concerning the structural stability of Li-overstoichiometric Mg-doped LiCoO2 powders

Abstract The effect of Mg-doping and Li overstoichiometry on the structural stability of LiCoO 2 powders has been investigated with emphasis to voltammetric properties. Microparticle cyclic voltammetry (CV) conducted in caustic NaOH to best simulate a non-aqueous electrolyte shows a marked improvement of the structure stability of doped LiCoO 2 . In contrast to the unsubstituted LiCoO 2 sample which shows voltammetric peaks associated to the well-known two-phase domain and monoclinic distortion reactions, in Li 1.08 Mg 0.06 CoO 2 , LiMg 0.06 CoO 2 and Li 1.08 CoO 2 samples these peaks are strongly suppressed providing direct evidence for the existence of a stable solid solution with negligible phase transitions in the reversible intercalation region (3.8–4.2 V vs. Li) as well as in the overcharged region. The effect is higher with Mg-doping, irrespective of the Li overstoichiometry. However, the concomitant presence of Mg and Li excess in the structure is important for obtaining small particle sizes. Since Mg-doping induces a quasi metallic behavior in the samples, whereas the Li excess may provide an higher initial capacity, it is suggested that the Li 1.08 Mg 0.06 CoO 2 composition may be of interest as positive cathode for advanced Li-ion batteries.

AC Impedance study of passive B2 FeAl intermetallic alloy in sulphuric acid

Abstract Impedance properties of passive films grown on a FeAl intermetallic alloy in sulphuric acid solution were investigated as functions of film thickness and electrode potentials in the range 0.5 ⩽ E ⩽ 1.5V. Data analysis of the impedance spectra allowed us to identify two overlapping time constants, which were attributed, respectively, to charge transfer processes at the film/solution interface and ion conduction within the film. The potential dependence of the resistive component of the loop observed in the lower frequency range was discussed in terms of ohmic or high-field contributions to the ion transport within the passive film.

Development of a New Aluminizing Method for Molten Carbonate Fuel Cell Wet Seals

A new aluminization method for molten carbonate fuel cell bipolar plate wet seals is described in this paper based on the application of the electrospark deposition (ESD) process. The molten carbonate corrosion behavior of single Al, FeAl, and a dual sFeAl/Ald diffusion coating applied onto a AISI 316L steel sheet of 0.5 mm thickness is examined. FeAl-containing coatings show low-rate corrosion after exposure to a sLi1Kd carbonate melt at 650 °C for 1000 h, whereas the single Al coating suffers excessive corrosion. Coatings degradation modes involve interdiffusion phenomena and transient formation of nodule iron–oxides. The dual sFeAl/Ald coating exhibits the best corrosion resistance due to its higher Al surface content. Characteristics of the ESD coatings in terms of thickness, uniformity, and composition are also reported. fDOI: 10.1115/1.1843119g

Repassivation rates of surgical implant alloys by rotating-disk scratching measurements

As the dissolution of metals ions in living tissues is an issue of primary importance in the field of surgical implant alloys, a rotating disk scratch technique has been used to examine this aspect by studying the potentiostatic repassivation behavior of pure titanium of commercial grade 11 and AISI 316L Stainless Steel (SS) samples in aqueous chloride solutions. By evaluating the effects of applied potential, pH and surface treatment on the current decay transients in the millisecond regime the higher tendency to passivate of titanium with respect to the stainless steel has been clearly demonstrated. An analysis conducted to estimate the efficiency of the oxide formation process has led to the conclusion that the majority of the measured transient current goes to the anodic dissolution process on both the materials investigated. Moreover, a repetitive process of film formation/breakdown on the AISI 316L surface related to pitting corrosion has been observed to strongly increase the ion release in the vicinity of the characteristic pitting potentials.

Solutions for Material Corrosion Problems in MCFC

It is widely recognized that metallic corrosion of the cathode current collector is a key technological problem that must be fully resolved before Molten Carbonate Fuel Cells can be commercialized on a more competitive basis. This paper presents a short overview on the corrosion mitigation strategies that appear more appropriate for MCFC current collectors. As alternative to the current use of the 300-series austenitic stainless steels, specialty high-Mn stainless steels, corrosion-resistant Ni-based alloys and sol-gel coatings of thin conductive spinel or perovskite ceramic layers are seen as the most promising corrosion solutions for the cathode-side environment. The use of basic additives into electrolyte for inhibiting molten carbonate corrosion is a further mitigation option yet with less practical perspectives due to the high constraints on the electrolyte properties. Recent and current studies conducted at ENEA on MCFC corrosion solutions are also mentioned.

Effect of Various Electrolyte Compositions on the NiO Degradation in Molten Carbonates

In the present paper we report the NiO solubility in a variety of alkali carbonate compositions in presence of MgO, CaCO3 , BaCO3 and La2 O3 additions under different conditions of gas composition and temperature. Two types of binary eutectic melts were chosen, namely the standard electrolyte (62–38) (Li-K)2 CO3 and (52–48) % (Li-Na)2 CO3 mixtures. The ternary eutectic melt chosen was (43.5-31.5-25.0) mol % (Li-Na-K)2 CO3 . The observed reduction of NiO solubility induced by the additives can be explained in terms of a higher basic character of all the modified electrolytes, although the NiO dissolution still follows an acidic dissolution mechanism in all the conditions under study.Copyright