US2415724A - Chromium plating - Google Patents

Description

Patented Feb. 11, 1947 UNITED STATES 1936, Serial No.

105,799. Divided and this application November 6, 1939, Serial No. 303,07 7

The invention relates to the electrodeposition of chromium and is more particularly concerned with a process employing a novel chromium plating solution and special preparation of the metal surfaces preliminary to the plating operation per se.

This application is a division of my copending ap lication Serial No. 105,799, filed October 15, 1936, now Patent No. 2,182,244, dated December 5, 1939.

The object of the invention is to provide a process of chromium plating which is an improvement on present practices particularly in being less sensitive to variations in the factors involved in chromium plating and in the quality of the chromium plate produced.

A common practice in chromium plating is to use a plating bath composed essentially of a water solution of chromic acid (CIOs) and a sulphate or sulphuric acid, the content of chromic acid varying, in general, from 150 to 450 grams per liter and the sulphate, estimated as S04, being preferably about 1% of the CrOa content. The extreme theoretical range for the ratio of S04 to CrOs is from 1 to 50 to 1 to 250, i. e. the S04 amounts to from 2% to 0.4% of the CrOs content.

4 Claims. (Cl. 204-32) But in actual practice the ratio of S04 to CrOx is between 0.66% and 1.15%. Seemingly slight departures from the prescribed conditions of the S04 to (3103 ratio, the concentration of the C103, and the temperature result in defective plating.

In contrast with the generally accepted practice, I have found that by the use of other metals than chromium and of their soluble salts of chromic acid, particularly bichromates, a chromium plating process is produced of lower sulphate content which has the desired advantages over known methods. In the following description I will refer specifically to salts oiv sodium, such salts being more readily available commercially, but it should be understood that it is not intended to confine the invention to the use of sodium salts, as soluble salts of potassium, ammonium, Zinc, and other bases are suitable. The essential characteristic of the additional metal is that it forms with the chromic acid a soluble bichromate, the metal of which does not deposit or the salt precipitate or form a colloidal solution. Bichromates of limited solubility can be mixed with those of greater solubility.

There are several ways in which the plating bath can be made up. A preferred method, disclosed in my prior application, Serial No. 686,069, filed August 21, 1933, of which the parent application Serial No. 105,799, above referred to, is in part a continuation, comprises mixing the following:

Chromic acid (CrO3) pounds 100 Sodium carbonate (NazCOz) do 36 /2 Alumina hydrate (commercial ounces 19 Water pounds 8 The chromic acid and alumina hydrate are mixed first and then the sodium carbonate and water added in alternating increments, the final product being about 125 lbs. of a yellowish powder. (The final content of S04 inthis mixture as shown by analyses is about .21-.22% of the powder or about .27% of'the total CiOs.) The plating bath is made up by dissolving the mixture in water, about three pounds to the gallon being preferred.

The composition may be obtained by direct admixture of CrOs and sodium bichromate or equivalent chromate, the choice of reagents to convert the C103 into bichromates or to supply the chromate or bichromate being merely a question of relative cost of materials of the necessary purity.

The alumina hydrate employed is a commercial product containing a fairly definite proportion, about 12% of S03. The alumina hydrate, being a bulky material, facilitates uniform mixing. The commercial CrOs also contains sulphate as an impurity as may also the NazCOs. But in the actual practice of my invention, the total sulphate is preferably around or below 0.1 oz. per gallon of bath as compared with three or four times as much in the conventional high acid bath. The above mixture will give about 0.1 oz. per gallon of plating solution containing 3 lbs. of the mixture. Of course, other sulphate carriers than the alumina hydrate can be used, such as sodium sulphate in proper proportions, the essential ingredients being free CI'Os, the salt of. ClOs and the catalytic acid radical preferably S04.

The exact proportion of the hexavalent chromium present in the form of the salt (considering the salt to be bichromate) is not critical, 70% being generally sufficient. Solutions in which 80% is in the combined form have a slightly higher throwing power but lower conductivity. Much above 80% results in difficulties in obtaining a good plate. Operating conditions will therefore affect the ratio selected, for example, the relative desirability of lower resistance and of higher throwing power. In general, 70% is recommended and less than is not preferred. Thepreferred method outlined above gives neutralization of chromic acid to bichromate of approximately Variations in concentration of the bath content are also possible. These variations are lim-' ited on the upper side by solubility of the compound and on the lower side by a darkening or lack of uniformity of the deposit. I have, therefore, found it desirable to maintain the mixture at not less than 30 oz. per gallon. The concentration of free CrOa, however, should be maintained at not less than oz. per gallon even when lower concentrations of total chromium are used.

I have found that variations in the sulphate concentration in this plating solution over a very wide range when operated as hereinafter described give results that produce a flat bright plate characteristic curve. This curve from a concentration of about .25 ounceof S04 per gallon on down to the lower limit practically possible by perating with commercial CrOs shows a substantially uniform bright plate range. The range, however, is somewhat wider at the lower S04 concentrations. It was for this reason and also for a minimum etching action of the solution that I prefer the .1 oz. S04 per gallon concentration, which concentration can be conveniently maintained with average commercial CrOs even when taking into account the consumption factor which tends to build up S04 from this impurity in the CIOs consumed. With much above the .25 oz. S04 per gallon, however, the bright plate range rapidly narrows, as reduced chromium oxides are formed at a faster rate than they are dissolved by the solution. This difficulty is of course greater at lower mixture concentrations, which is the reason for choosing the approximately 48 oz. per gallon solution concentration as the preferred one. While higher solution concentrations possess certain advantages, the drag out factor also increases, causing increased cost.

I have found that, by varying the proportions of chromic acid salts, the ratio of hexavalent chromium to S04 can be widely varied and platingsolutions obtained which are very low in S04 content. For example, a solution of a high grade of chromic acid containing 0.085% S04, which without addition of more sulphate will give no indication of plating at normal current densities, can be made to plate upon neutralizing a substantial part of the chromic acid with sulphatefree" NaOH. In such solution the ratio of hexavalent chromium to S04 is about 1180 to 1.

Good results with such solution have been obtained when the CrOs was neutralized to about 80 (estimating the neutralized CrOa to be in the form of sodium bichromate) leaving 20% of the Ci'Oa as free acid, with the total original ClOs content about 40 oz. to the gallon. Consistent plating has been obtained with the neutralization from 75% to 85% but if as much as 90% of the CrOa is neutralized no plating at all is obtained. It will be noted that, with 75% neutralization, the S04 is 0.34% of the free or unneutralized C1'O3, and at 90% neutralization the S04 is 0.85% of the free CI'O3.

It is my belief that the salts thus introduced serve to dissolve or to maintain in solution the salts or oxides of reduced chromium which otherwise deposit along with metallic chromium and cause iridescence, splotches, or dark deposits. The bath differs, however, from all former chromium plating baths, of which I am aware, in the following particulars: first, the total sulphate radical relative to the total CrOa radical present is lower than is practicable in any known commercial bath. (This feature lowers the acidity and reduces the oxidizing and etching effect on the metal surfaces to be plated); second, the sulphate radical present has no definite relation either to the neutralized CrO; present or to the unneutralized radical present.

The maintenance of the solution is very simple. The proportions are much less critical than in the high acid baths in present use and commercial baths have been controlled over long periods by tests of the Baum gravity only, by merely adding material of the original composition plus a percentage of C10: to compensate for the consumption of the CrOa. Frequent chemical analyses are unnecessary.

In plating with my low acid bath it has been found that the deposits are whiter, i. e., they are free from the bluish cast of conventional chromium plate. Also the deposit is less brittle and there is a practically complete absence of line cracks commonly experienced with higher acid ratios.

The characteristics of my bath of low acid content not only enable it to plate chromium directly upon articles, such as zinc-base die casting's, or articles of lead, pewter or silver, that cannot be satisfactorily plated in the usual bath, but also enables it to plate successfully on nickel, copper, brass etc. after the work has been subjected to preliminary cleaning and etching operations which are adapted to the particular metal surface to be plateda The bright plate range is not controlled by the chemical composition of the solution and temperature alone, but is very largely dependent at the lower S04 concentrations on the preliminary treatment of the metal surface.

The invention accordingly contemplates the use of certain preliminary treatments adapted to and in combination with the subsequent plating in my low acid bath. By these treatments I apparently invisibly etch or roughen the metal surfaces so that the proper bond is produced in my plating solution to which the work is subsequently subjected.

In determining the ingredients of the etch solution, the products of their reaction should be soluble in the etch solution itself.

For nickel I use a mixture of an oxidizing reagent such as CrO'a and an acid such as I-ICl. A properly etched surface, free from insoluble oxidation products, is obtained for the above preferred plating bath by the following solution:

Ounces per gallon CIQs 2 NaCl .04 to .06

Ounces per gallon Sodium bichromate 20 C1O3 2 NaCl .04 H2SO4 .1

The .1 of an ounce of H2804 is substantially 5% of the 20 oz. of the sodium bichromate used.

acrea e This is the normal sulphate impurity in commercial sodium bichromate.

Instead of adding sodium bichromate, equivalent amounts of soda ash and chromic acid may be added. As commercial soda ash contains sodium chloride as an impurity, the employment of this combination would operate to add additional sodium chloride.

The procedure for the plating of a nickel sur-- face is:

(a) Clean work in cleaning solution and rinse with water.

(17) Etch for about ten seconds in nickel etch solution at room temperature. Time is .not critical. Then rinse in water.

Enter work in the chromium tank, making sure that it is well wet with the plating solution before making contact with the cathode bar.

The plating solution of my invention possesses a good bright plate range on nickel, even at ternperatures of 110 F. but the best general range on this metal is 80-90 F.

While I have outlined difierentetching solutions which I have found preferable for my preferred plating solution, I must make clear that these exact proportions for the etching solutions will not be the optimum for the entire range of variation in plating solutions having sulphate radical concentrations below approximately .65 percent, but would have to be'varied according to the departures from the preferred plating solu-- tions and directions given above. It should understood that it is not intended to confine the invention to the use of the preferred nickel etching solution described above. Since I have found that the salts of both hydrochloric and sulphuric acid reactas acids through ionization in the presence of weak acids, such as chromic acid, it is obvious that hydrochloric or sulphuric or other strong acids such as hydrofluoric acid can be used in combination with an oxidizing reagent, such as chromic acid to make suitable nickel etching solutions. In the substitution of other strong acids for hydrochloric acid, it should be observed.

that the most suitable proportions of acid Will not depend upon the comparison with hydrochloric acid upon the basis of hydrogen ion concentration. Diiferent acids have different etching char acteristics in the presence of an oxidizing reagent, such as chromic acid. The proper proportions for other strong acids will easily be found. When the action of the etching bath is too strong, unplated areas form on the work in the plating operation; when too weak, the throwing power is poor and the resulting plate hazy.

The etching action of the preferred plating solution on nickel is practically negligible; but if higher ratios of S04 to total ClOs are used, the etching action is not negligible and must be taken into account and allowed for in determining the degree of etching action desired in the preliminary etching operation.

While I have described the use of plating baths containing bichromate and low in S04 concentration with my etching baths, it should be understood that the invention is not to be confined to the use of my etching baths with such plating solutions, for my etching baths can be usefully employed with plating solutions containing little or no bichromate (representing neutralizations of CIOs from 60 per cent down to zero per cent) but containing S04 in lower concentrations than .65 per cent. Although such plating solutions have a greater etching action than similar baths containing substantial amounts of bichromate as described above. they are still useful for plating with my etching solutions. If the S04 concentration of such baths is reduced, the etching action is correspondingly reduced until it practically ceases at the lowest concentrations. Higher S04 concentrations though still below .65 per cent result in an etching action which must be taken into account and allowed for in determining the degree ofetchiug action desired in the preliminary etching operation. This etching action of the higher S04 concentrations (though still below .65 per cent) is little affected by small amounts of bichromate. But if the proportion of bichromate is increased, the effect is to diminish the etching action until atthe preferred percentages of neutralization, the etching action is reduced to minimum proportions.

The advantages of my process claimed herein over the present high-acid method are as follows:

(1) 0n nickel the ease with which a true bright plate may be obtained even with irregular hand operation in the plating bath.

(2) No changes take place in the etching solution to upset the equilibrium so that control is greatly simplified.

(3) The plating solution is much less sensitive to the proportions of the chemical components present, e. g.. Na, CIGs, and S04, so that its maintenance is much less critical. There is no necessity to maintain a substantially constant ratio of S04 to either the total CI'O3 present or to the free CrOa present.

(4) The temperature may vary over a considerable range without affecting the results.

While I have referred above to the sulphate content, it should be understood that the invention is not limited to such acid radical additional agent or catalyst. Other such addition agents known in the art, fluorides, fluosilicates, etc. may be employed in reduced amounts in accordance with the present invention, The impurity present in the CrOs and other materials, however, must be taken into account and corresponding reduction made in the additions of such catalysts. As sulphate is already present in the commercial raw material in amounts that roughly correspond with that used, the use of such equivalent known catalysts would largely depend on obtaining a raw material of greater purity than presently commercially available.

I claim:

1. The process of preparing the surface of nickel for electrodeposition of chromium thereon 'comprlsingsubjecting the surface to the nonelectrolytic action of a solution containing chromic acid, bichromate, sulphate and sodium chloride, said solution being composed substantially as follows: 20 oz. per gallon sodium bichromate, 2 oz. per gallon CrOs, .04 oz. per gallon NaCl, and .1 oz. per gallon H2s04.

2. The process of plating chromium on a surface composed essentially of nickel which comprises; first subjecting the surface to the nonelectrolytic action of a solution containing chromic acid as an oxidizing agent and the acid radical of a strong acid to prepare the said metal surface for plating chromium thereon; washing said prepared surface; thereafter immersing the same in a plating bath containing an aqueous solution of chromic acid in part free and in part combined as an alkali salt of chromic acid, the

total chromic acid being at least about 30 oz.

per gallon of solution and the free acid constituting approximately 20 to 40% of the total, and sulphate catalyst to the amount of not over approximately .65% of the total chromic acid; and

surface composed largely or entirely of nickel which includes; subjecting the surface to the non-electrolytic action of a solution containing chromic acid as an oxidizing agent and the acid radical of a strong acid such as hydrochloric acid, sulphuric acid and hydrofluoric acid to prepare the said metal surface for plating chromium thereon; washing said prepared surface; thereafter immersing said surface in a plating bath containing an aqueous solution of chromic acid in part free and in part combined as a metal salt of chromic acid, the metal of which does not deposit or the salt precipitate to form a colloidal solution, the total chromic acid being at least about 30 oz. per gallon of solution and the free acid constituting approximately 20 to 40% of the total, and a catalyst consisting of at least one of the following class of catalysts, sulphates, fluorides, and fiuosilicates to the amount of not over approximately 55% of the total chromic acid; and supplying plating current to said surface.

4. The process of plating chromium upon a metal surface composed largely or entirely of nickel which includes; treating the metal surface in a solution containing about 20 oz. per gallon sodium bichromate, about 2 oz. per gallon Croi, about .04 oz. per gallon NaCl, and about .1 oz. per gallon HzSOr; washing said treated surface;

thereafter immersing the surface in a plating bath comprising an aqueous solution containing free chromic acid, a soluble bichromate, and a sulphate catalyst, the total chromic acid being about to oz. per gallon of solution, the free chro-' mic acid being about 30% of the total chromic acid and the sulphate catalyst being approximately .1 oz. per gallon; and supplying plating current to said surface.

FRANK H. BEALL.

REFERENCES CITED The following references are of record in the file of this patent:

McGraw-I-Iill Bk. Co., N. Y., pp. 293, 306. in Div. 56.)

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