US1928284A - Process of electrodepositing chromium - Google Patents

Description

Patented Sept. 26, 1933 UNITED STATES PATENT} OFFICE CHRO MIUM

Colin G. Fink, New York, N. Y., and Hugh David McLeese, Detroit, Mich.

No Drawing. Application January 14, 10:: Serial No. sasaoo 4 Claims.

This invention relates to processes of electrodepositing chromium, and aims to provide improvements therein.

The invention provides a novel process of de- 5 positing chromium, by which a good quality of plate, both as to structure, brightness, and color may be obtained, and with good efllciencies for chromium plating.

The invention further provides a process of electroplating chromium which may be used without etching polished surfaces of articles to be plated.

According to the present invention, chromic acid is used as the electrolyte. The catalyst is a compound of fluorine, usually a fluorid, such as chromic or sodium fluorid. We have obtained satisfactory bright plates using as catalysts the following: chromic fluorid, sodium fluorid, fluosilicic acid, fluoboric acid (I'IBF4) and hydrofluoric acid, the substances being arranged substantially in their order of efliciencies.

Temperatures from 15 C. to 35 C. are quite suitable, and current densities from $4 to 1 ampere per square inch are suitable, the lower current densities being preferred for bright deposits.

The efliciency of the current in depositing chromium varies with the amount of fluorid used in.

the bath. With a solution containing 250 grams per liter of chromic acid the best current efilciency has been found tobe around 3 grams per liter of the fluorine compound, calculated as chromium fluorid, the efliciencies decreasing above and'below this amount, the fluorine radical being considered the constituent of importance. In amounts up to about 10 grams per liter of the fluorid the solution does not etch polished surfaces of articles (steel, iron, brass, for example) placed therein for electroplating.

A mode of procedure is as followsz-A bath is made up containing 250 grams per liter of chromic acid (Croz) and about 2 grams per liter of hydrofluoboric acid (made by adding hydrofluoric acid to boric acid4I-IF+HaBO3- HBF4+3H2O) or an equivalent amount of a soluble fluoboric compound. The article tobe plated, a polished piece of flatware (a spoon) for example, is connected to the cathode busbar and a current of. about ampere per square inch passed through thesolution to the cathode at room temperature.

Good bright chromium plate generally is obtained at bath temperatures. 15 to 35 C. The temperature may be kept approximately constant at a desired temperature by suitable heating and cooling means.

55 The process may be carried out by other modes of procedure than that herein specifically described. As a guide to such other modes of procedure it may be stated that chromic acid baths containing the fluoboric compound follow the general rule of the catalyst compound being. proportional to the concentration of chromic acid in the bath; and of the current-density increasing with temperature for maintaining a given current efliciency.

In the present process, involving the use of fluoboric compounds we have found that chromium plate is best obtained at a bath concentration of 150 grams per liter chromic acid. In such a solution the fluoboric acid content may be 0.39 to 2 grams per liter, the optimum varying somewhat with temperature, being approximately as follows:-

20 C.0.88 grams per liter fluoboric acid 30 C.-0.86 grams per liter fluoboric acid 40 C.--1.15 grams per liter fluoboric acid 50 C.-1.43 grams per liter fluoboric acid Mirror plate is best obtained by the use of baths of lower chromic acid concentration, as for example the 150 gram per liter baths described above, with which mirror plate may be obtained through a considerable range of temperature and current density, as for example- 20 C.-% amp/sq. inch 30 C. amp/sq. inch 40 C.--% 'amp./sq. inch 50 C.-1 amp/sq. inch At 450 grams per liter of chromic acid, chromium plates show a milkiness at 20 C. to 50 C. over a wide range of current densities and fluoboric acid content. Y

The range of fluoboric content is not quite as wide as in the case of the chromium fluoride.

Subject-matter claimed in this-case has been divided from our application, Serial No. 49,820, flied August 12, 1925, now Patent 1,844,751, this application being a continuation in part thereof.

What is claimed is:-

' 1. A process of electroplating chromium comprising passing an electric current through a solution containing chromic acid and a soluble fluoboric compound, the amount of fluoboric compound being 3 grams per liter, or less of fluoboric acid for a solution containing 250grams per liter CrOa, or equivalent of other fluoboric compound, and a proportionately higher or lower fluoboric acid content for a higher or lower CrOa content.

2. A process of electroplating chromium comprising passing an electric current through a solution containing chromic acid and a soluble n fluoboric compound. the amount of fluoboric com-.

4. A process according to claim 3, wherein the relations of the following are substantially as follows:

Fluoboric acid Temperature Current density 0.86 grams per liter 20 C V4 amp. per sq. in. 0.86 grams per liter 30 C M amp. per sq. in. 1.15 grams per liter 40 C 96 amp. per sq. in. 1.43 grams per liter 5( C 1 amp. per sq. m.

COLIN G. FINK. HUGH DAVID MCLEESE.