PASSIVATION AND REDOX PROCESSES OF REFRACTORY METALS IN CARBAMIDE-CHLORIDE MELT

The electrochemical behavior of refractory metals (Pt, Pd, W, Ti, Nb, Ta) in the lowtemperature carbamide–NH4Cl melt (texp=120 oC) and the structure of complex ions formed after anodic dissolution have been studied by cyclic voltammetry and spectroscopic methods. The features of the mechanism of metal electrodissolution and possibilities of the cathodic reduction of complexes formed in the melt are presented. A possible correlation between passivation processes on the metal surface, the mechanism of electrodissolution, composition of complexes and the electrochemical activity of metals and their ions are discussed. The data obtained reflect the possibilities to use the carbamide–NH4Cl melt for practical purposes in electroplating and electropolishing.


Introduction
The interest in carbamide-based molten mixture is due to its low melting point, its ability to dissolve many salts, ready availability and low cost, which is important in practice.Pure carbamide is not often used for investigation [1][2][3][4] because long operation and especially overheating (>150 0 C) lead to the polymerization and solidification of the melt.However, by adding to molten carbamide salts, whose ions form strong hydrogen bonds with carbamide molecules [5], it is possible to retard thermal destruction, and the carbamide-halide mixture becomes stable in time and suitable for long operation.The electrochemical properties of refractory metals, the equilibrium of metals with their ions in high-temperature fluoride, chloride and chloride-fluoride melts are fairly well studied [6-20, etc .].However, we failed to find literature data on the anodic dissolution of refractory metals in carbamide-containing melts except in our articles [21][22][23].
The investigation of the electrochemical behavior of the carbamide-NH 4 Cl melt at an inert electrode showed that the main cathodic process is the discharge of ammonium ion.Two processes are observed in the anodic part of polarization curves: the first corresponds to carbamide oxidation and the second one -to the oxidation of Cl - [24].At potentials higher than the Cl -oxidation potential a new product -the interaction product of NH 4 + , carbamide and Cl 2 -is formed.This is the case only in a NH 4 + -containing melt, and the product can be identified by the IR spectra of anodic gases and of the melt at 630 cm -1 .Our experimental data on the electrochemical behavior of some refractory metals in the carbamide-NH 4 Cl melt have already been published [21][22][23].In this work, new data on the electrochemistry of other metals in this melt are presented.But the main aim of this work is to analyze all the data obtained.

Experimental
The electrochemical study of the anodic behavior of metals in the carbamideammonium chloride melt was conducted by recording cyclic voltammograms with a PI-50-1 potentiostat.A metal electrode with an active surface area of 5.8x10 -3 cm 2 was used as the working electrode.The auxiliary electrode was a platinum plate.The potential was measured against silver reference electrode (Ag/carbamide + NH 4 NO 3 (16 mol.%) + 0.07 mol kg -1 AgNO 3 ).Experiments were carried out in the temperature range 120 to 140 0 C under argon.The state of metal ions in the CO(NH 2 ) 2 -NH 4 Cl (16.8 mol.%) melt was investigated by spectroscopic methods in the temperature range 120-140 0 C. UV/VIS spectra were taken during the anodic (j a =5 to 150 mA/cm -2 ) dissolution of metals with Specord UV/VIS and IKS-14 spectrophotometers adapted for high-temperature measurements.The valency of metal ions formed in the melts during anodic dissolution was determined from the spectra and also gravimetrically.Reagent grade salts and dried argon were used in the study.The salts were pre-dried in vacuum at 80 0 C.

Results
The investigation of the electrochemical behavior of refractory metals (Pt, Pd, W, Ti, Nb, Ta) in the carbamide-NH 4 Cl melt (t exp =120 o C) and the spectroscopic study of complex formation of metal ions being in the melt after electrochemical dissolution with melt constituents allowed us to divide the metals investigated into three groups according to main characteristics.
Group I. Metals that are not passivated in the melt (Pt, Pd).Platinum is electrochemically insoluble in the carbamide and carbamide-NH 4 Cl melts.According to the results of investigation of the melt and anode gases composition [24], the wave appearing in the anodic curve in the carbamide-NH 4 Cl melt at Pt and glassy carbon electrodes (Fig. 1) corresponds to the anodic oxidation of carbamide.The electrochemical dissolution of Pt takes place only if the carbamide-NH 4 Cl melt contains corresponding complex salts: K 2 PtCl 6 , K 2 PtCl 4 [25].These salts form ammonia complexes after being dissolved in the melt.The metal ions are electroreduced reversibly to metal (Fig. 2), which is corroborated by the results of X-ray phase analysis of the deposits obtained.In the case of electrochemical dissolution in the carbamide-NH 4 Cl melt, platinum forms complexes of different structure with ammonia chloride, and the Cl -content of the complexes increases during electrolysis.Palladium dissolves electrochemically in the carbamide and carbamide-chloride melts without passivation (Fig. 3).Its ions can be reduced at the electrode to the metal both in pure and K 2 PdCl 4 -containing melts.In all cases, the rate of electroreduction is very high, and the metal is deposited as a fine-crystalline powder.Like platinum, palladium forms ammonia and ammonia-chloride complexes when being dissolved electrochemically in carbamide-contaning melts.
Other metals (Ti, Nb, Ta [21,22], W) are passivated in the carbamide and carbamide-NH 4 Cl melts, but the degree of their passivation is different, which is associated with the chemical properties of the metal, thickness and composition of the passive film, as well as with the solubility of the compounds forming the passive film in the melt.In should be noted that the composition of the film does not remain constant but undergoes changes on electrode polarization (Auger spectroscopic method).The electrochemical dissolution of metals that form a passive film in the carbamide-NH 4 Cl melt proceeds by the solid-phase mechanism of refractory metal dissolution [26], but the main characteristics of their electrochemical behavior differ from each other due to different degree of passivation.Metals that are slightly passivated in the melt (W, Ti) should be classified as Group II.These metals are slightly but electrochemically soluble in pure carbamide.There is no dissolution wave at the forward anodic sweep, but at the backward sweep a small wave appears, which increases gradually from cycle to cycle (Fig. 4).By using gravimetry and UV/VIS spectroscopy it was shown that ions of the highest oxidation state (W(VI) and Ti(IV)) in the form of isocyanate complexes of the respective metal are formed in the melt.
In the carbamide-NH 4 Cl melt, a W and Ti dissolution wave is observed during the very first cycle in the anodic part of the polarization curve (Fig. 5).The Cl -ion, being a relatively weak depassivator, is able to dissolve the metals.Titanium is transferred into the melt as Ti(II) ions (Fig. 6, curve 1) and tungsten as W(V) ions.Further oxidation with the formation of ions in the highest oxidation state takes place upon approaching the solvent decomposition potential.The formation of W(VI) can be observed only by Group III comprises metals with a strong passive film (Nb, Ta).They are totally passivated in the carbamide melt.Their passivation in the carbamide-chloride melt is also rather strong and the depassivating action of Cl -ions is insufficient to dissolve these    The passivation of a Ta electrode is more pronounced than that of a Nb electrode.One pass to the transpassive region is insufficient for the Ta dissolution wave to appear in the cyclic voltammogram, i.e. the amount of depassivator that is formed within one cycle at potentials >0.1 V is insufficient to depassivate the Ta surface.To accumulate the depassivator, either repeated cycling or decreasing the polarization rate or pre-electrolysis at transpassive region potentials is needed.In these cases, the metal dissolution wave can be observed, which corresponds to the formation of ions in the highest oxidation state (gravimetry, spectroscopy).However, the electrochemical reduction of Ta(V) takes place neither in the bulk of the melt nor on the electrode surface (Ta, Pt, glassy carbon).During the electrochemical dissolution of tantalum in the carbamide-NH 4 Cl melt, chloride complexes [TaCl 6 ] -are formed.

Discussion
Comparing the results of the investigation of the mechanism of electrode processes and the state of refractory metal ions formed upon electrochemical dissolution in the carbamide-NH 4 Cl melt, we can point out the following: During the electrochemical dissolution of metals that are not passivated (Pt, Pd) in the carbamide-NH 4 Cl melt (in the presence of complex chlorides of these metals), ammonia chloride complexes are formed and then they are discharged at the cathode to deposit pure metals [27].
In the case of weak passivation (W, Ti), the electrochemical dissolution of metals involves the formation of their carbamide (chloride) complexes in the medium and highest oxidation state, the reduction of these complexes to metal proceeds stepwise via the formation of intermediates.The cathodic deposit consists mainly of oxy-chloride compounds of metals in the lowest oxidation state, while the quantity of pure metals is negligible.
The electrochemical dissolution of metals that are characterized by strong passivation (Nb, Ta) involves the formation of chloride complexes of ions in the highest oxidation state, whose reduction either does not take place at all (Ta(V)) or the recharge of metal ions Nb(V) → Nb(IV)) takes place to form a compound in lower oxidation state, which is soluble in the melt.Therefore, the formation of any cathodic deposit is ruled out in this particular case.
The passivation ability of Ti, Nb, Ta in the carbamide-NH 4 Cl melt was used to develop the electropolishing of these metals [28][29][30].

Conclusions
Based on the experimental results obtained and established the correlation between passivation effects and the electrochemical properties of metals it is possible to make conclusions about possibilities to use the carbamide-ammonia chloride melt both for electroplating and electropolishing.The melt can be used to obtain pure metals in the form of powders or electrodeposits if the metal is not passivated, and for the electrochemical treatment and surface polishing in the case of metal surface passivation.

Figure 4 .
Figure 4. Cyclic voltammogram of a Ti electrode in the carbamide melt, t=130 0 C, scan rate 0.1 V/s.

Figure 6 .
Figure 6.The anodic part of the cyclic voltammogram taken at a Ti electrode in the carbamide-NH 4 Cl melt, t=130 0 C, scan rate 0.1 V/s.

Figure 7 .
Figure 7.The anodic part of the cyclic voltammogram taken at a Nb electrode in the carbamide-NH 4 Cl melt, t=130 0 C, scan rate 0.1 V/s.