US1993186A - Process in the electrolytic deposition of chromium, and the preparation of solutions therefor - Google Patents

Description

Patented Mar. 5, 1935 UNITED STATES PROCESS IN THE ELECTROLYTIC DEPOSI- TION OF CHROMIUM, AND THE PREPARA- TION F SOLUTIONS THEREFOR Wilfred W. Scott, Attilio A. Bissiri, and William 0. Gregory, Los Angeles, Calif.; Harriet E. Scott executrix of said Wilfred W. Scott, de-

- ceased No Drawing. Application April 22, 1932,

Serial No. 607,012

6 Claims.

In the electrolytic deposition of chromium, theuse of chromic acid as the source of chromium ions has been universally accepted. It is a well recognized scientific fact that if an ordinary low voltage, direct current, is used in the process of chromium electroplating, a bright surface of metallic chromium can not be deposited on the cathode unless some other substance in addition to chromic acid and water, is used in the electrolytic bath. The use of such addition agents has been accompanied in some cases by a certain degree of success from the standpoint of commercial application, but all such processes at present in use are at their best more or less unreliable, causing electroplaters, at times, considerable trouble.

Our invention provides an entirely new process which is a great improvement over all other existing processes of chromium plating.

According to our invention the electrolyte is composed of a solution of chromic acid and varying amounts of selenium or tellurium, or a combination of the two, added in any form that is, or becomes, soluble in the water solution of chromic acid. Small amounts of sulphates, as for instance is sometimes present in coimnercial grades of chromic acid, are not detrimental to the successful deposition of chromium, according to our invention.

In introducing the selenium or tellurium in the electrolytic bath, it is immaterial whether it is as the element, or as a compound of the element selenium or tellurium. Repeated and careful experiments, both on a small scale and on a commercial scale, have disclosed the fact that the presence of even small amounts of either selenium or tellurium results in the deposition of crystals of metallic chromium at the'cathode' of such a nature that a beautiful bright and hard finish is obtained. The exact nature of the chemical reaction or'reactions taking place is very difi'icult to determine, but it is possible that the addition of either selenium or tellurium results in the formation of complex salts of chromium and selenium, or chromium and tellurium, which complex salts,. when present even in small' amounts, bring about the desired crystal formation of metallic chromium at the cathode.

When tellurium is used, the addition of a small amount of seleniumnmakes possible the desired good results using a total amount of selenium and tellurium which is less than the amount that would have been required if only tellurium had been used. The'same condition has proved to hold true when a small amount of tellurium is used in conjunction with the selenium.

The actual results have shown also a decided improvement in the throwing power.

The process of our invention is characterized 5 by a high degree of flexibility as to what concerns temperature of the solution, current density, concentration of chromic acid, and the ratio of selenium or tellurium to chromic acid. A ratio of selenium or tellurium to chromic acid 10 varying from a minimum of 1 part of selenium or tellurium to 10,000 parts of chromic acid, and a maximum of 1 part of selenium or tellurium to 50 parts of chromic acid, has been found to give the desired results. However, the optimum l5 ratio is obtained when a minimum of 1 part of selenium or tellurium to 1000 parts of chromic acid, and a maximum of 1 part of selenium or tellurium to 250 parts of chromic acid is observed. tion can be easily obtained by empirical methods, thus greatly simplifying the process, and increasing its commercial value. When a new solution is made, the preferred ratio of selenium or tellurium to chromic acid is obtained by weighing the materials, or measuring them if in solution. but after that, the physical properties (brightness and hardness) of the objects plated will tell the operator when more selenium or tellurium must be added. If desired, ordinary methods of quantitative analysis may also be used.

What is claimed is:

1. A process which includes the step of depositing chromium electrolytically from a solution of chromic acid, in the presence of an amount of compounds of selenium or tellurium dissolved in the chromic acid to provide a proportion of one part of selenium or tellurium to from 250 to 1000 parts of chromic acid.

2. A process which includes the step of electro- 40 depositing chromium from a chromic acid solution in which is dissolved a compound or compounds of selenium providing a ratio of 1 part of selenium to from 50 to 10,000 parts of chromic. acid.

3. A process which includes the steps of dissolving in a chromic acid solution a compound or compounds of tellurium to provide the proportion of 1 part of tellurium to from to 10,000 50 parts of chromic acid, with the optimum proportion being 1 part of tellurium to from 250 to 1000 parts of chromic acid and electrodepositing chromium from said solution.

4. A process which includes the step of electro- The control of the electrolytic solu- 20 depositing chromium from chromic acid in the presence of both selenium and tellurium.

5. A process which includes the steps of making an electrolyte containing in solution chromium and both selenium and tellurium and electrolytically depositing chromium therefrom.

6. A process for the continuous electrolytic deposition of chromium from chromic acid which includes the introduction into the electrolyte of both compounds of tellurium and compounds of selenium in the proportion of 1 part of tellurium and selenium together to from 250 to 1000 parts of chromic acid and the electrolytic deposition of chromium therefrom.

WILFRED W. SCOI'I. ATTILIO A. BISSIRI. WILLIAM C. GREGORY.