US2336568A - Method of metal electroplating - Google Patents

Description

Patented Dec. 14, 1943 METHOD OF METAL ELECTROPLATING Henry A. Holden Pray, Columbus, Ohio, asslgnor to Battelle Memorial Institute, Columbus, Ohio, a corporation of Ohio No Drawing. Application July 10, 1939, Serial No. 283,580

1 Claim.

This invention relates to a method of metal electroplating, and more particularly to a method of producing a bright metallic plate over an electrobuifed metallic surface.

When an article consisting of metals or alloys such as nickel, copper, or stainless steel, or an article having an electrodeposited or otherwise formed superficial layer of such metals or alloys, is made the anode in a suitable type of electrolytic bath and a current of proper density is passed through the bath, a highly polished, lustrous metallic surface can be obtained. Considerable difficulty, however, ha been experienced in obtaining clear metallic electrodeposits on such clectrobuffed surfaces. Ordinarily, unless the electrobuifed metallic urface is first subjected to a mechanical buiilng operation, any plate deposited thereon will have a milky cast, which is very undesirable. This applies particularly to chromium deposits on elcctrobuffed nickel surfaces. The cause for this difilculty is probably not a layer of nickel oxide formed as the result of the anodic process, for cathodic reduction treatments are not remedial. A more likely manner of causation for this trouble i outlined here inbelow.

Cold working, for example, by mechanical bufiing, of a metallic surface effects a degradation of the surface crystals so that the immediately superficial layer is either non-crystalline or of extremely fine grain size, forming a so-called Beilby layer. An electrolytically buffed surface, on the other hand, although highly polished and lustrous. is characterized by the absence of this Beilby layer and comprises crystal faces of the same size asthose of the underlying metal. When chromium or other metals are electrodeposited on such electrobuffed surfaces, the resulting deposit either continues the grain structure of the electrobuifed nickel or other metal or alloy or else deposition is effected selectively upon the relatively large exposed surface grains thereof. Either condition results in a chromium or other plate of undesirable color.

Regardless of what may be the correct explanation of the milky cast exhibited by metallic'plates deposited over electrobuifed metallic urfaces, I

v have now found that the milky appearance of such plates may be entirely obviated and a clear plate of chromium or other metal obtained by following the electrobufilng operation' with a flash metal electrodeposit applied directly to the electrobuffed surface. When chromium or other metals are then plated in the usual manner over the flash metal coating, the resulting plating has a clear color which in the case of chromium is superior to the best heretofore obtainable chromium plate. The metal used in the flash coating is, for electrobuffed nickel surfaces, preferably nickel itself, deposited from any conventional or usual nickel plating bath operated under conditions that will give a flne grained deposit.

' It is therefore an important object of this invention to provide a method of improving the color characteristics of metallic plates applied over electrobuffed metallic surfaces.

A further and more specific object of the present invention is to provide a satisfactory method of chromium plating over an electrobuifed nickel surface.

Another important, object of this invention is to provide a method of producing a bright, clear colored metallic plate, when plating over an electrobuifed metallic surface, the method including the step of applying a flash coating over the electrobufifed surface and then plating directly upon the flash coating.

Other and further important objects of this invention will become apparent from the follow- .ing description and appended claims.

The present invention may be broadly defined as involving the use of a sufliciently fine-grained deposit of any kind intermediate an electrobuffed metal and a subsequent metallic electrodeposit, to prevent crystal continuity between the electrobufied metal and the final metallic deposit.

The method of my invention is particularly applicable to the chromium plating of an anodically electrobuffed nickel surface, such as nickel or an alloy composed largely of nickel, or an electrodeposit of nickel. The electrobuffed surfaces in question, of course, do not include bright or mechanically buffed nickel surfaces, since. such nickel'surfaces need not be electrobuffed. Various satisfactory electrolytic baths and methods of anodic treatment to effect a highly polished, lustrous surface on nickel are well known, and, since they form no part of the present invention, re quire no further description. As explained above, however, if chromium is electroplated directly over an electrobuffed nickel surface, the resulting chromium plate has an undesirable milky cast, the elimination of which is an object of my invention.

According to my present process, an electrobuffed nickel surface, prior to chromium plating, is given a flash coat of nickel or other suitable metal, such as cobalt, nickel and cobalt, cadmium,

copper, or the like. By the term flash coating or plate is meant a very rapid plating operation that results in a very thin electrodeposit. As indicative of the thickness of a flash metal plate, but without limitative effect upon the scope of the term, the thickness will ordinarily be in the neighborhood of ti of an inch, or less.

Where a flash nickle plate is employed, the nickel may be deposited from any suitable nickel plating bath in a manner well known to those skilled in the art. For instance, the flash nickel plate may be applied from a so-called bright nickel bath, or from an ordinary gray nickel bath, such as the Watts bath, modified to operate at a low pH. As typical of the compositions of these two types of bath, the following are given:

Bright nickel bath 240 g./l NiSO4L6HzO 45 g./l NiCl2.6H2O 3O g./1 H3303 6.75 g./l Naphthalene-1,5 di-sulfonic acid 1.5 g./1 ZnSO4.7H2O pI-I 2.5

With the above bath, using a current density of about 40 amperes per sq. ft. and a, temperature of 140 F., the time required for effecting a satisfactory flash plate is not more than from 30 seconds to 3 minutes, with 30 seconds generally sufficient.

Ordinary or gray nickel plating bath 262 g./l NiSO4.6H2O 6O g./l NiC12.6H2O 10.5 g./1 H3303 pH... 1.0

The above plating bath is a modified Watts type of bath, having a rather low pH and optionally containing sodium lauryl sulfate or other wetting agents. The length of time required to effect a flash plate is of the same order of magnitude as that required when using the bright nickel plating bath. Excellent results have been obtained with the modified Watts type bath by operating the bath at a temperature of 130 F.- *10 F. and by using to 30 amperes per sq. ft. for 30 secs. to 3 min. with air agitation of the bath or simple oscillating movement of the work.

After the electrobuifed nickel surface has been coated with a flash or strike plate of nickel, the

alysis, containing in the neighborhood of 250 g./l. of chromlc acid (CrOa) and 2 g./1. of sulfuric acid. The temperature and cathode current density should be regulated within the well known limits required to obtain a mirror-like chromium plate. A bright, clear colored chromium plate, without any milky appearance, is directly obtained when following the process of this invention.

One of the advantages of applying a flash plate of nickel, or other metal, over the electrobuffed nickel surface, rather than resorting to other chemical or to mechanical means for eliminating the milky cast of the chromium plate, is that the flash plating operation is relatively inexpensive and rapid. Even where ordinary gray nickel, as contrasted with so-called bright nickel, constitutes the flash plate, it is so thin and fine-grained as not to have any noticeable effect on the brightness of the after applied chromium plate. This is noteworthy, since ordinary gray nickel when deposited in the usual thickness employed as an undercoat from a chromium electroplate, must first be buffed, either electrolytically or mechanically, if the subsequently applied chromium plate is to have a mirror-like finish. Thus, the advantages of electrobuflin can be realized by following the process of my invention, without the difliculties heretofore experienced with respect to the milky cast imparted to the after applied chromium plate.

It will, of course, be understood that various details of the process may be varied through a wide range Without departing from the principles of this invention and it is, therefore, not the purpose to limit the patent granted hereon otherwise than necessitated by the scope of the appended claim.

I claim as my invention:

In the method of metal electroplating wherein a nickel surface other than bright or mechanically buifed nickel is first electrobuifed and then chromium plated, the improvement whereby a clear chromium plate is obtained, which comprises electrodepositing on the electrobuffed nickel surface a flash coat of fine-grain nickel sufficiently thin to retain the characteristically lustrous surface produced by electrobufiing, and then chromium plating directly upon said thin nickel flash coat.

HENRY A. HOLDEN PRAY.