Electrodeposition of chromium from trivalent chromium using modulated electric fields

Abstract

A layer of chromium metal is electroplated from trivalent chromium onto an electrically conducting substrate by immersing the substrate and a counter electrode in a electroplating bath and passing a modulated electric current between the electrodes. In one embodiment, the current contains pulses that are cathodic with respect to said substrate and in another embodiment the current contains pulses that are cathodic and pulses that are anodic with respect to said substrate. The cathodic pulses have a duty cycle greater than about 80%.

Description

GOVERNMENT RIGHTS[0001]The experimental work leading to this invention was funded in part by EPA Phase I SBIR program. Contract No. EP-D-11-044. The U.S. Government may have certain rights in the invention.FIELD OF THE INVENTION[0002]This invention relates to electrodeposition of chromium metal and more particularly to electrodeposition of chromium metal from a trivalent chromium electroplating bath as contrasted with conventional carcinogenic hexavalent chromium electroplating bath.BACKGROUND[0003]The US Environmental Protection Agency identified hexavalent chromium as one of 17 “high-priority” toxic chemicals based on their known health and environmental effects, production volume, and potential for work exposure. Typical thick, hard chrome coatings have been produced from a hexavalent chromium bath using a direct current deposition process. One problem associated with this type of coating process is worker exposure to the hexavalent chromium during plating, which is overcome by r...

AI Technical Summary

Benefits of technology

[0004]A range of process conditions that allow for the scalable production of dense hard chrome coatings from a trivalent chromium plating bath have been developed. In one embodiment the coatings have a Knoop microhardness values of at least about 804 KHN and up to about 1067 KHN, and about 947 KHN on average. Using a pulsed waveform and a trivalent chromium electroplating bath, the processes produce chrome coatings exhibiting bond strength, porosity, hardness, and wear resistance demonstrating the potential of the developed coatings competitive with conventional chrome coatings produced from a hexavalent chromium bath. Representative processing conditions that improve the visual uniformity and density of the coating are summarized below:

[0011]Each of the foregoing processing parameters has been shown to enhance the visual uniformity of the coating across shafts of various diameters.

[0012]The present invention provides a process for producing dense, scalable hard chrome coatings from a trivalent chromium plating bath. The process involves controlling the electric field during electrodeposition to plate the substrates, e.g., a steel landing gear, with a chrome coating that is as hard and wear resistant.

[0014]To improve the coating uniformity to long length scales, the electric field applied between the substrate and the counter electrode may be interrupted or the magnitude maybe varied during the electrodeposition process such that the electric field is turned on and off many times or intensity is varied across the substrates surface. Additionally, the polarity of the substrate upon which the chrome coating is to be formed may be reversed during the pulsing of the electric field during the electrodeposition process such that the deposition substrate becomes anodic for a period of time and the counter electrode becomes cathodic for a period of time. A schematic illustration of a pulse reverse waveform used in one embodiment is provided in FIG. 1, which consists of a cathodic pulse current density, ic, a cathodic on-time tc, an anodic pulse current density ia, an anodic on-time, ta, and an off-time to. The reverse portion of the waveform in FIG. 1 may not be included, such that a pulse waveform that only consists of the cathodic pulse current density, ic, a cathodic on-time tc, and an off-time, to is used for electrodeposition and said FIG. 1 is not limited by such. The sum of the cathodic and anodic on-times and the off-time is the period, T, of the pulse reverse waveform and the inverse of the period is the frequency, f. The cathodic γc and anodic γa, duty cycles are the ratios of the respective on-times to the period. The average current density or net electrodeposition rate cathodic (forward) is given by:Electrodeposition rate=icγc−iaγa

Problems solved by technology

One problem associated with this type of coating process is worker exposure to the hexavalent chromium during plating, which is overcome by replacing the carcinogenic material with a benign trivalent chromium plating electrolyte.

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