Pulse Off-times Research Articles

Pulse Electrodepositon of Ni-W-P-CeO<sub>2</sub>-SiO<sub>2</sub> Composite Coatings

Square-wave pulse current was used to electrodeposit Ni-W-P-CeO2-SiO2 composite coatings. The influence of pulse on-times, off-times and peak current densities on properties of the composite coatings was researched. The microhardness, chemical compositions and microstructures were evaluated by EDX, SEM and microhardness tester. The results show that the composite coatings possessed higher microhardness and fine-grained structure. The crystal sizes of the composite coatings were found initially to decrease with the rise of pulse on-times before it started to increase with further increasing pulse on-times. Decreasing pulse off-times or increasing pulse peak current densities resulted in considerable refinement of crystal sizes.

Pulse-plating of copper–cobalt alloys

Pulse-plating of copper–cobalt alloys was studied theoretically and experimentally. The deposits were produced from a citrate electrolyte using a recessed rotating cylinder electrode (rRCE) and an inverted recessed rotating disk electrode (IrRDE), respectively. A mathematical model developed previously by the authors was applied to the prediction of alloy composition as a function of pulse off-time. The displacement reaction between copper and cobalt during the pulse off-time was simulated using step functions for the characterization of the coverage of the surface by a copper layer. Comparison of simulated and measured alloy compositions showed that a displacement reaction occurred even at the longest pulse off-times studied. Potential responses measured during pulse off-time suggest that the displacement reaction leads to incomplete coverage of the alloy surface by copper. Investigation of deposit cross-sections with the scanning electron microscope showed that distinct phases rich in copper and in cobalt form a columnar structure. This is thought to be the reason that a displacement reaction could be observed even at long pulse off-times.

A surface coverage model for pulse-plating of binary alloys exhibiting a displacement reaction

Models for the prediction of the composition of pulse-plated copper-nickel alloys in the presence and absence of a displacement reaction during the pulse off-time are reviewed and critically evaluated. For very long and very short pulse off-times the composition of copper-nickel alloys deposited from electrolytes containing an excess of nickel ion is entirely controlled by mass transport of copper ion. At intermediate pulse off-times, however, the kinetics of nickel dissolution play a role. A modified model is proposed which takes into account the role of surface coverage by copper for the rate of nickel dissolution. Assuming a step function for the variation of surface coverage with off-time the model predictions can be fitted to experimental data using the critical off-time which corresponds to a change in surface coverage from zero to one as adjustable parameter. The model is used to estimate for different experimental conditions the amount of copper leading to a suppression of the displacement reaction. More copper is deposited by the displacement reaction in sulfamate than in citrate electrolytes.

Pulse-plating of copper–nickel alloys from a sulfamate solution

In pulse-plating of alloys, a displacement reaction can take place during the off-time, involving dissolution of the less noble component and deposition of the more noble component. The extent of the reaction is dependent on the length of the off-time. The influence of the displacement reaction between copper and nickel on the composition of copper–nickel alloys pulse-plated from a sulfamate electrolyte has been studied and the results are compared with those previously obtained for citrate solutions. Copper–nickel alloys were pulse-plated on a rotating cylinder cathode using a wide range of pulse-plating parameters and the resulting deposit composition was measured by X-ray fluorescence. Experimental results were compared with the predictions of two theoretical models, previously described in the literature, assuming a mass-transport-controlled displacement reaction and the absence of displacement reaction, respectively. Mass transport properties needed for this comparison were measured by independent techniques. The data show that a displacement reaction similar to that observed in citrate solution occurs between copper and nickel. Mass-transport-controlled corrosion of nickel is observed up to longer pulse-off times than in citrate. This excludes the hypothesis that, in sulfamate solution, passivation or an ‘irreversible mechanism’ prevents dissolution of nickel in the presence of copper(II) species.

Modelling and Analysis of Pulse Electrochemical Machining (PECM)

A small interelectrode gap in Electrochemical Machining (ECM) results in improved dimensional accuracy control and simplified tool design. However, using a small gap with conventional ECM equipment adversely affects the electrolyte flow or mass transport conditions in the gap, leading to process instability. The most remarkable breakthrough in this regard is the development of ECM using pulsed current. Pulse Electrochemical Machining (PECM) involves the application of a voltage pulse at high current density in the anodic dissolution process. PECM allows for more precise monitoring and control of machining parameters than ECM using continuous current. Small interelectrode gap, low electrolyte flow rate, gap state recovery during the pulse-off times and improved anodic dissolution efficiency features encountered in PECM lead to improved workpiece precision and surface finish when compared with ECM using continuous current. This paper presents mathematical models for the PECM process which take into consideration the nonsteady physical phenomena in the gap between the electrodes, including the conjugate fields of electrolyte flow velocities, pressure, temperature, gas concentrations, current densities and anodic material removal rates. The principles underlying higher dimensional accuracy and simpler tool design attainable with optimum pulse parameters are also discussed. Experimental studies indicate the validity of the proposed PECM models.

Palladium electrodeposition from Ammonia-free electrolyte

The electrodeposition of palladium from an ammonia-free electrolyte with both direct current and pulse current is discussed. The polarization experiments show that the electrode reactions include oxygen reduction, hydrogen evolution and palladium deposition. The oxygen reduction is always mass transfer-controlled. Deposits obtained with current densities higher than the limiting one are powdery. The current efficiency also drops under these conditions. The variation of the electrode potential during current pulses reveals that hydrogen desorption occurs during pulse-off periods. This hydrogen desorption refreshes the electrode surface and facilitates hydrogen reduction in the next pulse. It explains the severe drop in current efficiency in pulse plating with long pulse-off times. This explanation is also supported by the morphology observations.