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.

Finally, decorative plating from a Cr(III) bath suffers from low current efficiency.

Currently, functional chromium coating from a Cr(III) bath is not commercially practical because it is difficult to plate thick chromium coatings with appropriate properties.

Furthermore, the low current efficiency and low plating rate of Cr(III) baths lead to unfavorable economics.

Attempts to plate gold from non-cyanide solutions have also experienced difficulties.

This is undesirable for high speed operation, or for applications requiring selectivity.

1) It requires a hydrogen-relief bake post-treatment to eliminate hydrogen embrittlement.

2) It is difficult to control the composition of the alloy as deposited.

Zinc alloy plating suffers from what is known in the plating industry as anomalous deposition.

According to one of the leading proposed mechanisms the problem is caused by the formation of a zinc hydroxide film within the double layer adjacent to the cathode surface that inhibits the electrodeposition of the more noble metal.

Attempts have been made to correct the anomalous deposition by adjusting the composition of the plating bath, but the results have not permitted zinc alloys to replace cadmium extensively.

The treatment process requires about six hours to complete and it is difficult to control the chemical treatment step.

In addition it is difficult to control the Ce--Mo composition and coating distribution due to the chemical treatment process.

However the conventional electrolytic method, which uses direct current (DC), involves a large amount of hydrogen evolution due to the very negative reduction potential of Ce.sup.3+ (-2.335 V vs Standard Hydrogen Electrode (SHE)).

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