Electrochemical Production of Bismuth in the KCl–PbCl2 Melt

An anode dissolution of binary metallic lead–bismuth alloys with different concentrations of components has been studied in the KCl–PbCl2 molten eutectic. The dissolution of lead is found to be a basic process for the alloys of Pb–Bi (59.3–40.7), Pb–Bi (32.5–67.5), Pb–Bi (7.0–93.0) compositions in the whole interval of studied anode current densities. A limiting diffusion current of lead dissolution was observed at 2 A/cm2 and 0.1 A/cm2 for the alloys of Pb–Bi (5.0–95.0) and Pb–Bi (3.0–97.0) compositions, respectively. The dissolution of bismuth takes place at the anode current densities exceeding the mentioned values. The number of electrons participating in the electrode reactions is detected for each mechanism. Based on the theoretical analysis, the experimental electrolysis of bismuth was performed in the laboratory-scale electrolytic cell with a porous ceramic diaphragm. The final product contained pure bismuth with a lead concentration of 3.5 wt.%.

Copper deposits obtained by pulsating overpotential regime with a long pause and pulse duration from sulfated solutions

The morphologies of the copper deposits obtained by pulsating overpotential regime with prolonged pulse and pause durations from the solution of 0.15 M CuSO4 in 0.50 M H2SO4 at overpotentials lower, higher and belonging to the plateau of limiting diffusion current density were compared with those obtained by the same electrodeposition regime from solutions of 0.075 and 0.30 M CuSO4 in 0.50 M H2SO4 and 0.15 M CuSO4 in 0.25 and 1.00 M H2SO4 at overpotentials outside the plateau of limiting diffusion current density. These samples were characterized by scanning electron microscopic (SEM) analysis and the cathodic polarization characteristics from solutions compared. Increasing the Cu(II) concentration led to an increase in the limiting diffusion current density. Decreasing the H2SO4 concentration shifts both beginning and the end of the plateau of the limiting diffusion current density towards higher electrodeposition overpotentials. Also, electrodeposition in solutions of 0.15 M CuSO4 in 0.25 and 1.00 M H2SO4 led to the formation of morphological forms of copper deposits characteristic for electrodeposition of copper from higher CuSO4 or lower H2SO4 in solution at some higher overpotentials.

Relationship between cathodic protection current and limit diffusion current as an additional criteria of cathodic protection

The influence of the temperature and oxygen concentration in NS4 solution on the electrochemical characteristics of X70 steel for pipelines and the relationship of cathodic protection current density to limiting diffusion current density, in the normalized DSTU 4219 range of protective polarization potentials from -0.75 V to -1.05 V (сh.s.е.) were investigated. It has been established that with increasing of the temperature from 20 to 80 °C, the corrosion activity of X70 steel increases, which is confirmed by more negative values of corrosion potential and an increasing of the corrosion rate, and indicates on the prevailing effect of temperature on the corrosion process compared to the effect of oxygen concentration. In the temperature range considered, the limiting diffusion current has a maximum value at 40 °C and decreases with increasing of temperature, correlating with a decreasing of the oxygen concentration. Under conditions of free oxygen access to the NS4 solution in the normalized DSTU 4219 range of protective polarization potentials, the ratio of cathodic protection to limiting diffusion current varies from 0.4 to 1.3 at temperatures (20-40) oС, at temperatures (60-80) oC the ratio is less than 1. Under conditions of limited oxygen access, the ratio increases from 1.1 to 4.7. Using calculated and experimental results, it was shown that, at various values of relationship , conditions for excessive hydrogen evolution can be created, which represents a threat to the safe operation of the pipeline over time. Data on the ratio values should be taken into account when analyzing the protection state of pipelines.

Estimation of the exchange current density and comparative analysis of morphology of electrochemically produced lead and zinc deposits

The processes of lead and zinc electrodeposition from the very dilute electrolytes were compared by the analysis of polarization characteristics and by the scanning electron microscopic (SEM) analysis of the morphology of the deposits obtained in the galvanostatic regime of electrolysis. The exchange current densities for lead and zinc were estimated by comparison of experimentally obtained polarization curves with the simulated ones obtained for the different the exchange current density to the limiting diffusion current density ratios. Using this way for the estimation of the exchange current density, it is shown that the exchange current density for Pb was more than 1300 times higher than the one for Zn. In this way, it is confirmed that the Pb electrodeposition processes are considerably faster than the Zn electrodeposition processes. The difference in the rate of electrochemical processes was confirmed by a comparison of morphologies of lead and zinc deposits obtained at current densities which corresponded to 0.25 and 0.50 values of the limiting diffusion current densities.

The shape of the polarization curve and diagnostic criteria for control of the metal electrodeposition process

The simulated shapes of the polarization curves were correlated with the type of metal electrodeposition process control in a function of the exchange current density to the limiting diffusion current density (j0/jL) ratios. Diagnostic criteria based on the j0/jL ratios were established. For j0/jL> 100, the system is under the ohmic control. In the range 1 < j0/jL ? 100 there is the mixed ohmic-diffusion control. The pure diffusion control appears in the range 0.1 < j0/jL ? 1. For j0/jL ? 0.1, the system is activation controlled at the low overpotentials. The proposed diagnostic criteria were verified by comparison of the simulated curves with experimentally recorded ones and by morphological analysis of deposits obtained in the different types of metal electrodeposition process control.