Results for co-electrodeposition of copper and nickel as a function of pH at 40°C are presented in , and as a function of temperature at constant pH 4 in , without nitrogen sparging. In these plots, the data points are the measured values, and the curves are the model results for copper and nickel, respectively. As shown, the model results are in reasonable agreement with the data. Copper deposits first and nickel second, consistent with their respective standard reduction potentials. The behavior of the copper and nickel overpotentials, η = E – Ee, determined from the model, reflect these observations, exhibiting a larger, negative overpotential for copper than nickel over the entire experiment. A sharp change in the overpotential occurs for both metals over the region where copper deposition cuts off. For example, at pH 4, 40°C, 10A, copper initially deposits very rapidly up to about 110 min, which is where the nickel maximum occurs. Over this region, the overpotential for copper increases slightly (averaging about -230 mV), while that for nickel is positive and decreases slightly (averaging about +150 mV). Over the region where copper electrodeposition ceases (ca. 110 – 130 min), both the copper and nickel overpotentials are negative and decrease rapidly (i.e., become more negative) as nickel begins to electrodeposit. The model results also indicate that the copper electrodeposition rate constant and the mass transfer coefficient become approximately equal (i.e., kLa/k1=1) at about 110 min, while the same occurred for nickel (i.e., kLa/k2 = 1) at about 400 min.
Galvanostatic co-electrodeposition of copper and nickel in the spouted particulate electrode as a function of solution temperature at pH 4, 10A without nitrogen sparging.
Copper and nickel ion concentrations during “corrosion” studies with no applied current as a function of solution pH at 40°C, without nitrogen sparging.
For electrodeposition of just copper from acidic solutions in the same apparatus used in the current work, it was found that the net rate increased with increasing pH and decreasing temperature,15
while for solutions containing just nickel, the behavior with pH was similar, but the net rate increased
with temperature over the range of parameter values investigated.16
As shown in and , for co-electrodeposition, copper generally behaved as previously, except with a much more rapid net removal rate. Nickel behavior, however, was more complex, such that the maximum net electrodeposition rate occurred at about pH 4. Also, from the results in at constant pH 4, the maximum co-electrodeposition rates occurred at a temperature of about 30°C for copper and about 40°C for nickel.
The nickel maxima observed at relative concentration ratios (Ci/Ci0)
of about 1.05 - 1.2 in and are attributed to the metal displacement (oxidation) reaction of Ni0
]. Metal displacement reactions are well known29,30
and are used commercially in applications such as the “cementation” of silver,31
for example. These reactions can also result in significant quantitative and qualitative differences between the electrochemical removal of metals from single metal aqueous solutions and from solutions of multiple metal mixtures.
From the preceding, the metal displacement reaction between copper ion and nickel metal [Eq. (7)
] can effectively decrease/reverse the net rate of copper corrosion, accelerate the rate of copper removal, increase the net rate of nickel corrosion, at least initially, and also amplify the separation of the deposition regimes of the two metals in time. The latter suggests that the recovery of each metal as a relatively pure deposit layer may be possible by using such strategies as employing different sets of particulate electrodes at the appropriate times, for example.
In addition to the nickel maxima evident in and , the effects of the displacement reaction between copper and nickel are quite evident in the metal corrosion experiments as well. In are presented corrosion results at 40°C as a function of solution pH without nitrogen sparging. As noted, the concentration of nickel ion increases with time at all the pH values, while the concentration of copper ion also increases with time at low pH (2.5 and 3.0). With increasing pH, however, the rate (i.e., the slope) decreases to zero, and the copper ion concentration thereafter decreases with time with increasing pH; and increasingly so with pH.
Corresponding dissolved oxygen concentrations for the Cu/Ni co-electrodeposition data presented in and .
At pH 4, the copper concentration only decreases with time, and the rate (slope) becomes more negative with increasing temperature in the absence of an applied current. This behavior is directly attributable to the metal displacement reaction. With the current off, the electrodeposition rate terms are zero, and the resultant expressions can be solved to yield:
are, respectively, the copper and nickel cation concentrations at the point where the feeder current is turned off. From Eq. (14)
, it can be concluded that the copper cation concentration can exhibit two regimes of behavior, depending on the relative magnitudes of kc1
′: corrosion rate-control for kd
where the copper cation concentration increases with time after current shut-off; and displacement rate control for kc1 kd
′ where the copper cation concentration decreases with time after current shut-off. The results here demonstrate both types of behavior depending on temperature, pH, and copper cation concentration at the time of current shut-off. If the argument of the exponential is relatively small, the resultant behavior will be approximately linear in time; i.e.,
From , the corrosion data are indeed approximately linear in time, consistent with these expressions, and thus the kinetics are close to zeroth order, at least over this timescale.
Experimentally determined net corrosion rates for copper and nickel from experiments without nitrogen sparging are summarized in , along with the final “best fit” parameter values determined from the model, as described above. As shown, the experimental rates obtained by shutting off the current agree reasonably well with the values obtained from the model fits. The “fit” net corrosion values were consistently less than the measured values, but were quite close for the constant pH 4 data; however, they differ a bit more (by about a factor of 2-3 or so) for the isothermal 40°C data as a function of pH. This supports our conclusion that the essential elements of the system are reasonably well captured by the simple model.
Measured and “fit” net copper and nickel corrosion rates without nitrogen sparging.
For the isothermal copper data (@40°C) at low pH (3.0 and 3.5), the corrosion rate is greater than the metal displacement rate, while the reverse is true at higher pH (4.0 and 4.5) where the corrosion rates are less. Consequently, the resultant slopes for the isothermal “corrosion” data for copper exhibit a transition from positive to negative with increasing pH. As a function of temperature at a constant pH of 4, the copper behavior is dominated by metal displacement, such that the “corrosion” curves exhibit only negative slopes. These slopes become increasingly negative with increasing pH, reflective of the Arrhenius temperature dependence of the displacement rate constant. This differs from the behavior observed in acidic solutions containing just copper15
in which it was found that the corrosion rates were always positive; i.e., the metal ion concentrations always increased with time when the current was turned off. For nickel, however, both the corrosion and displacement reactions act in the same direction to monotonically increase the concentration of nickel ion in solution, such that the resultant slopes can only be positive, as previously discussed. As shown in , the corrosion rates for nickel increase monotonically with temperature and decreasing pH.
Fricoteaux and Douglade32
found that Cu2
O formation can occur at pH ≥ 3.0 via:
The deposited cuprous oxide would then further inhibit the metal displacement reaction. The higher the pH, the more facile the production of copper oxide, which may at least partially explain why the nickel displacement reaction rate decreases with increasing pH. A power law fit of the displacement reaction rate constant data as a function of pH from , indicates that the rate constant is proportional to [H+]-0.72 at 40°C. Also, from the results in , the metal displacement reaction rate constant is Arrhenius temperature-dependent with an apparent activation energy of 47.2 kJ/mol at a constant pH of 4.
The behavior of the DO concentration data with time, corresponding to the electrodeposition data in and , is presented in . As shown, upon initiation of electrowinning, the DO concentration increases rapidly, exhibits a maximum, and then gradually decreases again. This behavior is similar to that observed for nickel electrodeposition alone.16
The DO concentration increases up to pH 4, and then decreases with increasing pH beyond that point. Also, as shown, it first increases with temperature (at constant pH 4) to 40°C, and then decreases thereafter. This behavior can be understood in terms of the model presented here, as discussed elsewhere.16
Figure 5 Normalized copper and nickel concentrations at a constant solution temperature of 40°C as a function of solution pH with nitrogen sparging. The bold solid and dashed curves without data symbols are the data for copper and nickel removal/recovery (more ...)
In it is noted that the net copper electrodeposition rate increases with increasing
pH and decreasing
temperature. This was also noted in previous work with just copper in solution.15
This differs somewhat from the behavior of solutions with just nickel present, where the electrodeposition rate increases with both increasing
pH and temperature.16
These observations are attributed to the relative effectiveness of the net electrodeposition rate in comparison to the corrosion rate. In the case of nickel, the corrosion rate increases with both temperature and decreasing pH, but not as rapidly with temperature as observed for copper.
As was found for both copper and nickel solutions, sparging with an inert gas reduces the dissolved oxygen concentration and, consequently, the corrosion rate.15,16
In are presented co-electrodeposition data with nitrogen sparging for copper/nickel solutions as a function of pH at 40°C, 10A. The data from for pH 4, 40°C, 10A, are also presented for comparison. As shown, nitrogen sparging considerably increases the observed net electrodeposition rates for both metals – considerably more so than was observed from solutions with just the single metals alone.15,16
Normalized sum of copper and nickel ion concentrations from as a function of pH at 40°C, 10A.
Corrosion rate results following co-electrodeposition of copper and nickel are summarized in . As shown, the total corrosion rates for both metals were reduced by about a factor 2-3 with nitrogen sparging. The reduction in corrosion rates increases the negative slopes of the copper curves, and decreases the positive slopes of the nickel curves, decreasing the inhibition of the displacement reaction which accelerates copper and nickel removal rates to a greater degree than shown in and without nitrogen sparging.
Measured net copper and nickel “corrosion” rates with nitrogen sparging.
In Figure 7 are presented the sum of the copper and nickel cation concentrations as a function of time (at 40°C, 10A) with pH as a parameter from the results in . Summation of the differential mass balances for the two cations [Eqns. (9)
)] eliminates the metal displacement reaction terms, so that these curves represent the net effect of just metal electrodeposition and corrosion. As shown, the resultant curves are approximately linear over the entire range, with a slope that gradually increases with pH, except for the data with pH 4.5. This behavior is attributed to galvanostatic operation under conditions where the operating current is less than the limiting current. Under these conditions, it has been shown that the concentration of the metal cations decreases linearly with time.20,34
The increasing (negative) slope with pH is attributed to diminution of the net corrosion rate with pH, with the exception of the slope for pH 4.5, where the effects of corrosion are minimal, but where the net metal electrodeposition rate is also slowing.