G. Picard

Mechanism of corrosion of AISI 304L stainless steel in the presence of nitric acid condensates

Corrosion of stainless steels in the presence of nitric acid condensates is far more severe than in the liquid bulk and can lead to intergranular attack even on non-sensitized steels. Parametric corrosion tests were performed in order to determine the effect of various factors on this type of corrosion. Specific apparatus was designed to control and characterize nitric condensates. An electrochemical study of the reduction of nitric acid on a platinum electrode and on a stainless steel electrode allows linking of the results of the corrosion tests to the process of reduction of nitric acid.

A chemical and electrochemical study of titanium ions in the molten equimolar CaCl2+NaCl mixture at 550°C.

Abstract The chemical and electrochemical properties of solutions of titanium chlorides in the fused CaCl 2 +NaCl equimolar mixture were studied at 550°C using different working electrodes. We determined the stability range of the various oxidation states of titanium and calculated the standard potentials of the Ti(III)/Ti(II) and Ti(II)/Ti(0) redox couples, the solubility products of the titanium oxides, the kinetic parameters of the electrochemical systems at the different electrodes and the diffusion coefficients of the electroactive species. We have also studied the titanium electrodeposition process by potential step measurements which indicated instantaneous nucleation of titanium at tungsten substrates. All these data allowed us to build-up the potential-pO 2− diagram which summarizes the properties of Ti-O compounds in the melt studied and can be used (together with the E-pO 2− diagram of chlorinating gaseous mixtures) to predict operating conditions for the process of industrial production of metallic titanium from molten salt systems.

Oxoacidity reactions in molten LiCl+KCl eutectic (at 470°C): Potentiometric study of the equilibria of exchange of O2− between Al(III) systems and carbonate and water systems

Abstract The reactions between aluminium chloride and oxide anion were studied in molten LiCl+KCl eutectic at 470°C. For that purpose, a pO2− indicator electrode made with a membrane in yttria-stabilized zirconia has been used and tested by means of a carbonate ion whose dissociation constant, 10−2.15 atm, was determined. Then, the electrode has allowed us to obtain the formation constant of AlO+ (solvated by Cl−) and the solubility product of Al2O3 (S): 1010.7 and 10−27.4, respectively (molality scale). The equilibrium constant of the following system: 2HCl (g)+O2− agH2O (g)+2Cl− has also been determined: 10−9.93 atm mol kg−1. The conditional solubility of alumina in LiCl+KCl melt is discussed.

Acidity and purification of the molten zinc chloride (33.4 mol%)-sodium chloride (66.6 mol%) mixture

Abstract The behavior of an yttria-stabilized zirconia electrode in the ZnCl 2 2NaCl melt has been investigated between 450 and 500°C. The response of the electrode was nernstian and limited in oxobasic media by the solubility of zinc oxide. Although it is more acidic than LiClKCl eutectic melt, the chlorozincate can be purified towards oxide ions by gaseous reagents such as HCl and Cl 2 .

Americium chemical properties in molten LiCl-KCl eutectic at 743 K

Abstract The chemical and electrochemical properties of americium chlorides have been studied in the fused LiCl–KCl eutectic at 743 K using a tungsten working electrode and a pO 2− indicator electrode. The standard potential of Am (III)/Am (II) and Am (II)/Am redox couples were determined using cyclic voltammetry. Titration of Am (III) chlorocomplex by O 2− demonstrated the existence of the soluble americium species AmO + and the precipitation of AmOCl and Am 2 O 3 . The electrochemical behaviour of AmO + was then studied by cyclic voltammetry. All these data allowed us to draw the potential–pO 2− diagram which summarises the properties of americium species in the melt.

Electrochemical study of the properties of iron ions in ZnCl2 + 2NaCl melt at 450°C

The electrochemical properties of Fe(II) in the ZnCl2 + 2NaCl melt at 450°C have been studied using a glassy carbon electrode. Iron oxidation states II and III have been shown to exist in the mixture and the stability of metallic iron has been confirmed. The kinetics of the electrodeposition and electrocrystallization of iron were studied finding that the process is quasi-reversible. In the same way, the Fe(III)Fe(II) exchange was found to be quasi-reversible. The values of the kinetic parameters, ko and α for both reactions were obtained. Mass transport towards the electrode is a simple diffusion process. The diffusion coefficient was found ((1.3 ± 0.4) × 10−5 cm2 s−1. Potential step measurements indicate instantaneous nucleation and growth of iron (I against t12 followed by diffusion control (Cottrell equation).

Industrial aluminas and hexahydrated aluminum chloride chlorinations by phosgene in LiClKCl eutectic melt at 470°C

Abstract The variations of pO 2− during the chlorination by phosgene of industrial aluminas and hexahydrated aluminium chloride were observed by means of an yttria-stabilized zirconia electrode. The curves obtained showed that chlorinations carried out in a continuous flow reactor, take place rapidly (30 min with a phosgene rate of flow of 157 cm 3 min −1 and a simple bubbler). Transformation of oxychloride to tetrachloroaluminate is the slow step; its specific rate constant k s and its order α have been determined ( k s = 0.19 mol −1 kgs −1 ; α = 2). The latter value is interpreted by postulating the involvement of the complex Al 2 OCl − 5 . The catalytic effect of the presence of aluminum chloride on the chlorination of the alumina has also been illustrated.

Structures of oxyfluoroaluminates in molten cryolite-alumina mixtures investigated by DFT-based calculations

Abstract Aluminum is produced throughout the world using the Heroult-Hall process, which consists in the electrolysis of alumina dissolved in cryolite melts. This dissolution of alumina gives rise to the formation of oxyfluoroaluminates, the structures of which vary with the composition of the electrolyte mixture. Up to now there has been no real consensus as to the structural entities of these fluoroaluminate ions. The lack of knowledge of the ionic species arising from the dissolution of alumina in molten cryolite has led us to undertake structural, energetic and vibrational studies of these species using the methods of computational chemistry. Conformational and vibrational analyses of the ionic complexes were performed with the DGauss software on Cray C98 and C916 machines located at Cray Research, Eagan, MN, USA. A systematic study was carried out on the complexes [ AlOF x ] 1− x (1 ≤ x ≤ 5), [ Al 2 OF x ] 4− x (4 ≤ x ≤ 9), [Al 2 O 2 F x ] 2−x (2 ≤ x ≤ 7), which are those most commonly suggested for interpreted experimental measurements. The Al 4 OF 10 and Al 4 OF 8 2+ complexes were also investigated because of their possible occurrence in the electrorefining process of aluminum. Some of the studied complexes are much more stable than the others and so are the complexes most probably involved in the electrowinning process of aluminum. These complexes are AlOF 2 − , Al 2 OF 6 2− and Al 2 O 2 F 4 2− . Their infrared spectra have also been calculated. Concerning the two complexes Al 4 OF 10 and Al 4 OF 8 2+ , we find that oxygen is three-coordinated and not fourcoordinated. The last atom of aluminum is stabilized by two fluoride bridges. This computational approach has enabled us to eliminate the majority of the structures suggested in the literature for oxyfluoroaluminate ions. Currently we are examining in our laboratory the effects of cations on the anionic complexes.

Effect of the addition of fluoride on the conditional solubility of alumina in LiCl-KCl eutectic melt

Abstract Owing to its well-known high complexing power toward Al 3+ ion, fluoride ion is able to increase the solubility of alumina in alkali chloride melts. To determine the extent of this effect, the formation of aluminium(III) fluoro-complexes was studied potentiometrically in LiCl-KCl eutectic at 470°. But the sodium fluoride addition appeared to produce not only the complexation effect but also a mineralization effect on alumina. So, the thermodynamical stability of alumina formed in this melt by precipitation from aluminium chloride with carbonate ion (oxide anion donor) depends on the fluoride ion concentration. These two effects explain the solubility variation of alumina in the LiCl-KCl eutectic + NaF mixtures. A pF − - p O 2− diagram, which represents the stability area of the various aluminium (III) species is established, and leads to some conclusions concerning the electrowinning of aluminium from molten chloride melts. The cumulative formation constants of the aluminium(lII) fluoro-complexes (AlF 3−i i ) have been obtained, whose values are the following: 2.5 ± 0.4, 4.7 ± 0.6, 5.7 ± 0.5, 7.5 ± 0.4, 8.0 ± 0.5, 9.0 ± 0.6, respectively for i = 1, 2, 3, 4, 5 and 6. It has been shown that oxyfluoride species such as AlOF 1− j i does not exist. The solubility products of gamma and alpha-alumina have been determined and are equal to 10 −42.9 and 10 −44.0 respectively (all the constants are given in the molality scale). They differ widely from the solubility product of the alumina obtained in the absence of fluoride ion, ie 10 −27.4 .

Chemical and electrochemical behaviour of indium ions in the ZnCl22NaCl melt at 450 °C

The stability of indium chloride and oxide as well as the electrochemical behaviour of the In3+ and In+ ions have been studied in the ZnCl22NaCl melt at 450 °C by X-ray diffraction (XRD), potentiometry, cyclic voltammetry, chronoamperometry and chronopotentiometry. The standard potential of the redox couple In(III)/In(I) and the solubility products of indium oxides have been determined (E ° In(III)/In(I) = −1.006 ± 0.001 V (vs. Cl2(1 atm)/Cl−), pKs = 12.9 ± 0.2 in molality scale), finding that In(O) is not stable in these mixtures according with the reaction: ZnCl2(dis) + 2In(liq) ⇌ 2InCl(dis) + Zn(liq) These results have allowed the construction of E-pO2− equilibrium diagrams. Using a tungsten electrode, we observed that the In(III)/In(I) exchange is quasireversible; log(ks0/cm s−1) and a values for this reaction are (−1.92 ± 0.09) and (0.51 ± 0.06) respectively at 450 °C. The average diffusion coefficients DIn(III) and DIn(I) are (5.7 ± 0.7) × 10−6, and (6.2 ± 0.9) × 10−6 cm2 s−1 respectively.