US1864014A - Chromium plating process - Google Patents

Description

Patented June 21, 1932 UNITED. STATES PATENT OFFICE DWIGHT r. nwme, or near msma-nrcmeaiw CHBOMIUH PLATING PROCESS No Drawing.

The invention relates 'to electrolytes forelectrodeposition of chromium and to a process for electroplating chromium in a form suitable for commercial requirements.

In my copending application Serial No. 126,094, filed J uly 30, 1926, I have-described a method for rendering a solution of chromic acid suitable for electrodeposition of chromium by the addition of certain compounds of 4 metals having oxidizing and reducing propof chromium and the electrolyte is renderedsuitable for depositing chromium by the addition of a cerium compound, such for example as cerium, oxalate, cerium carbonate, etc. Although chromium may be plated from such solutions the results at times are I irregular, due to the formation of a brownish film in spots upon the surface of the chromium. Where such areas are found the deposits of chromium seem to be thinner and under certain conditions the film, upon drying the plated object, isquite adherent. and bufling is required for its removal. The formation of some of thefilm may be prevented to a large extent by washing the objects to be plated in a certain manner, but I have discovered another method to prevent its formation which consists in adding to the electrolyte certain additional substances. While there are a large number of different "sub stances which have the property of prevent- (a ing the brownish film formation, I have found it most advantageous to usea salt of a metal having alow. hydrogen over-voltage. The

' quantity of this material which-should be used can not be well stated in terms of weights and volumes because it is-a function of such factors as current density, temperature of Application filed January 3, 927. Serial No. am.

bath, concentration of chromic acid, proportion of cerium oxalate or other compound added to the chromic acid, the anode material and the shape of the object to be plated.- Therefore the amount of the film-preventing substance which should be added tothe elec-" trolyte can best be found'by trial, :although it may be stated briefly that the amount used should be that quantity which is just necessary to prevent the formation of the film when the operator is working with a clean piece of work, such as an object of brass, steel or nickel. If substantially more than this amount is added, the efiiciency of the plating process is somewhat lowered. As an example of some of the substances which have been found to prevent the formation of the brownish film are the following: hydrofluo-' silicic acid, hydrofluoboric acid, or their soluble salts, sulphites or persulphates. 7 As an example of a specific electrolyte suitable for commercial requirements and the process of preparing the solution, the fol lowing formula is 'givenf Add to a liter of water 200 to 300 grams of commercial chromic acid. When this has dissolved, 10 grams of commercial cerium oxalate is added, with stirring, and dissolved. This solution is now treated with a dilute solution of sodium fluosilicate about 1 gram 30 per 100 cc. solution until upon making a trial under the best current density by plating a" brass-or nickel object (area about 1 square decimeter) no brown film is formed. The

temperature of the bath may be about 20 Instead of incorporating the cerium oxalate addition-agent in a chromicacid solution it'may be used in other chromium-ca ing electrolytes where the chromium is in t e hexavalent state substantially free from trivalent chromium, For example, under certain conditions good deposits can-be obtained from a solutioncontaining per liter of solution 290 grams sodium-chromate' Nacrolmmo and 10 cerium oxalate.

Whilein the above formma Ihzwesuggested the useofcerium oxalataas a-su stance for rendering the bath p suitable for-electroplating, it is to be under-' stood'thatmy invention can also be carried out: by adding the film-preventing substance [to a chromic acid' bath having incorporated therein other compounds of cerium such as the oxides,earbonates,sulphates, etc. and in the broader aspects of the inyentionthe film- 'preventing compound may be addedto any" posit being'of uniform thickness, bright andlustrous and free from any brownish film. It'is believed that-one of the reasons .why

go-sodium fluosilicate and other fluosilicates have such abeneficial actionon the electrolyte is due to the fact that they are inorganic colloidal substances. It is believed essential. that the substance be inorganic in character abrather than organic because the presence of chromic acid in the electrolyte introduces such strong oxidizing properties that organic matter present is destroyed.

I In carrying out the process of electrodep 3Q osition of chromiuimutilizing the improved electrolyte above described, it has been found very advantageous to accurately determine:

the amount of current in the, working circuit, especially when thebathis being used 3 for objects of differentshapes or materials. It is therefore desirable to-have .an ammeter arranged in the workingcircuit and byusing the ammeter'I have discovered. amethod bywhich the efliciency of the electroplating 40 process maybe increased and the proper current density determined for any particular object which is to be plated. My method con- '.sists in placing the object inthe electrolyte with a small current flowing in the working;

circuit and gradually increasing the efi'ec-' tive voltage on the bath by rheostats or other pound Shaving valent-state substantially free from trivalent is desired to obtain bright polished chromium surface it is preferable to have a polished under surface on whichthe deposition takes place to avoid subsequent smoothing of the extremely hard chromium plate.

hat I claim as my invention is': 4 1. Anelectrolyte for electrodep'osition' of chromium comprising a solution containing principally chromic acid, together with small amounts of cerium oxalate and sodium hydrofluosil-icate.

principally chromic acid and a relatively.

small amount of a cerium compound.

3. Anelectrolyte for electrodeposition of chromium comprising. a. solution containing principally chromic. acid and a relatively small amount of cerium' oxalate."

4.'An electrolyte for electrodeposition'of chromium comprising a solution containing as-the principal ingredient chromic acid, a relatively. small amount of a cerium compound and a soluble fluosilicate.

v5. An electrolyte for electrodeposition ofchromium comprising .a solution containing having oxidizing and reducing properties -2. An electrolyte for electrodeposition of chromium comprising a solution containing relatively small amount of a cerium compound and sodium fluosilicate.

[-6. An electrolyte for electrodeposition ofas the principal ingredient. a chromium comthe chromlum 1n the hexachromium and a serium compound. Intestimony whereof I aflix my signature.

DWIGHT T. EWING.

suitable means and carefully noting'the resulting increase of current flowing throu h thef circuit. It has been found that as the voltage is adually. increased the current .als o 1s gra ually' and uniformly increased -'unt1l a certain criticalpoint is reached where there isa sudden and appreciable drop in the amperage asin'dicatedby the ammeter. It has been found thatwhen the object isplated with the current corresponding tothe critical,

point ora current just slightly greater than the crltlcaljpomt, a very satisfactory deposit of. chromium .is obtained and the efii--' 00 ciency of -th e; ath is higher than; with other current dens1ti'es.'-

. It fis'to be understood that a. electroplatingprocess asabove. described can be carried 2' out onsteel, copper, or nickel surfaces which. 6 are either pohshed or unpolished,;but if it chromium comprising a solution containing