Electroplating of metals using pulsed reverse current for control of hydrogen evolution

Abstract

Excessive evolution of hydrogen in electrolytic deposition of metals on a cathode substrate can be controlled by using a pulsed reverse current. Reverse current pulses interposed between the forward current pulses consume at least some of the nascent hydrogen and prevent the local pH at the cathode surface from becoming excessively alkaline. Control of hydroxide ion concentration by pulsed reverse current alleviates problems caused by reaction of metal-bearing-ions with hydroxide ions generated near the cathode by evolution of hydrogen. The method is useful in depositing functional chromium coatings on electrically conductive substrates from plating baths comprising aqueous solutions of trivalent chromium salts. In such a method the current comprises forward pulses having a duty cycle of from about 50% to about 90% and reverse pulses having a duty cycle of from about 5% to about 30%, and a frequency of from about 5 Hz to about 700 Hz.

Description

[0002] 1. Field of the Invention[0003] This invention relates to methods of electroplating metals onto a substrate and more particularly to electrodeposition of metals using pulsed reverse current for controlling evolution of hydrogen.[0004] 2. Brief Description of the Prior Art[0005] Electrodeposition of metal coatings onto a substrate is a process that is widely used in modern industry. Such electrodeposited coatings are usually applied to metallic substrates and are generally intended to provide enhanced surface properties to the base metal. For example, metal coating layers are applied to a base metal to prevent corrosion, enhance surface hardness, provide a smooth surface having a relatively low coefficient of friction, and the like.[0006] Metal coatings are ordinarily deposited by providing a plating bath which is an aqueous solution of metal-bearing ions, typically simple ions such as Cr.sup.3+, Zn.sup.2+, Au.sup.3+, Cd.sup.2+, which are generally present as aquo complexes, o...

AI Technical Summary

Benefits of technology

0036] A further object is to control the pH in the vicinity of

Problems solved by technology

The actual mechanism of the cathodic reduction and deposition can be complex, and is not well understood for many practical systems.

The hydrogen formed may itself present problems, such as hydrogen embrittlement of the deposited metal coating or interference with the metal deposition caused by bubbles.

A high pH in the plating layer may also produce problems such as formation of insoluble metal hydroxide layers on the cathode surface which also interfere with the transportation of the metal-bearing ions and the deposition of metal atoms on the surface.

Moreover, certain metals present problems in depositing layers having satisfactory properties such as uniformity, luster and hardness, especially at useful plating rates.

Some of these expedients have resulted in the use of plating baths that are hazardous to use and difficult to dispose of by environmentally benign procedures.

For example, it is has been found that the best results in gold electroplating are achieved using cyanide solutions which are evidently hazardous to use and difficult to remediate for disposal.

The workplace and environmental problems experienced with chromium plating are especially pressing because of the very extensive use of electroplated chromium coatings in industry.

However, the concentration of the additives is difficult to control because they are present in very small amounts.

Furthermore, the additives react and break down with the passage of time to form contaminants.

Consequently, the used Cr(III) bath and the rinse water from such plating operations cannot be replenished and/or recycled because the concentration of the contaminants would build up to unsatisfactory levels.

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