US2069566A - Nickel plating solutions and processes - Google Patents

Description

Patented Feb. 2, 1937 PATENT OFFICE NICKEL PLATING SOLUTIONS AND PROCESSES Robert L. Tuttle, Woodbridge, N. .L, assignor to Zialite Corporation, York No Drawing.

a corporation of New Application July 11, 1931, Serial No. 550,289. Renewed July 14, 1934 2 Claims.

This invention relates to improvements in the electro-plating of various metals with nickel, and in nickel-plating solutions. It is an object of the present invention to provide means for they efiicient electro-deposition of nickel on various other metals and especially to provide processes and plating solutions whereby zinc may be plated with nickel economically and efliciently, in any desired practical density, and with a deposit of even, adherent, and ductile character, so that many articles of utility may be made of zinc with its desirable and economical properties and yet render them non-corrosive, durable, and to give them a practically permanently fine appearance and luster.

It is a further object of providing such a character of nickel-plated surface which may be readily, evenly, and efficiently chromium plated. Chromium plating upon a nickel-plated surface requires an especially good nickel deposit, because any subsequent chromium plate will tend to uncover any defect in the nickel deposit. Other ob- .jects of the inventiomand of various steps thereof, will become further apparent as the disclosures of the present specification are better understood.

The art of nickel-plating zinc as now practiced does not produce entirely uniformly satisfactory results, for all purposes. The causes for such short-comings appear to be due to a combination of electro-chemical factors which the past and current art have been only partially able to overcome. With the known methods there is a tendency to make the deposited nickel dark, spongy, non-adherent, and not dependably suitable for chromium plating.

The desirable optimum qualities in a nickelplating solution, for zinc, are that it should:- Substantially prevent the solution of zinc, thus to eliminate the deposition of nickel by immersion; deposit nickel at a low current density per unit area, i. e. 7-10 amperes per square foot, without causing black striae on the metal to be plated; deposit the coating of nickel without treeing out on edges or building up unevenly, during electrolysis for any desired practical length of time at a relatively low current density, even aslow as say 7-10 amperes per square foot; have good anode corrosion, conductivity, and throwing power; maintain a fairly constant pH value and metal content; hold in solution iron compounds or other impurities which may be present in anodes or nickel salts; have practical stability of solution under control at all times; deposit a coating which is adherent and ductile regardless of thickness even up to 0.005"; and deposit a coating easily polished to a clear white lustre.

The nickel-plating of metals generally by the old processes has been reasonably satisfactory. One.of the chief short-comings encountered with 5 some metals is the peeling of the nickel plate from the base metal, especially-in the case of zinc, lead, and aluminum. My improved method, however, produces superior results with practically all metals and especially with the more 10 difficult types. The nickel coatings deposited from ordinary solutions are apt to'be irregular, peel because of internal strain or faulty deposition, and at times show other defects, because the solutions become unbalanced, deposit nickel 15 by immersion, and permit more or less extensive occlusion of hydrogen within the tieposited metal, etc.

These untoward results as to the character of the deposit have been theoretically ascribed in the case of certain metals to one or more of a number of factors such as, for instance:

(1) High nickel ion concentration in the plating bath, which causes nickel to be deposited on immersion in the solution already, instead of substantially only by electro-deposition. This causes the nickel to deposit unevenly on difierent contours of the articles to be plated, and tends to prevent a uniform homogeneous deposit throughout over the surfaces to be plated.

(2) Zinc going into solution in substantial amount, by substitution in reaction with the nickel salt present in the solution, thus replacing the nickel and furthering the undesirable deposition of the latter by immersion, especially 35 when the nickel-ion concentration is high.

(3) Occlusion of hydrogen within the film of the deposited nickel. This has a tendency to make the thin nickel film porous, reduces the native ductility and toughness of the nickel, and 40 makes it less adherent to the base metal, which affects the workability and permanence of the plate.

In the eifort to improve the solutions for nickel-plating zincto overcome these untoward effects, the experimental work seems to have been especially directed to limiting or reducing the nickel-ion concentration. It has also been recommended to employ high initial current densities so that heavier deposits of nickel may be formed at once, and before the solutions may become unbalanced.

Some preferred staridard solutions, which are more or less generally employed in the nickel- 55 I plating of zinc by the present methods, are based on the following formulas:-

Room temperature.-Initial current density 30 amperes per square foot for 2 minutes, to prevent black striation. Subsequent plating, current density 12-18 amperes per square foot; pH limits to be controlled between 5.8 and 6.2.

One.

]I.Nickel sulphate 32 Nickel chloride 2 Boric acid 2 Sodium citrate 8 Temperature 68-86" F. Initial current density, 28-47 amperes per square foot for at least 1 or 2 minutes. Subsequent plating, current density 15-20 amperes per square foot; pH about 5.4

Both of these solutions, andv modifications thereof, usually require current densities varying from 28-47 amperes per square foot, applied for the initial strike to assure a preliminary heavy coating of nickel. The current density is subsequently lowered and the operation proceeds at a current density 12-20 amperes per square foot. The time of plating is usually 8-10 minutes. The time of application with these processes should not greatly exceed such time, as otherwise the nickel film will, 'in process of depositicn, tend to build up unevenly on edges of objects being plated. Black striae may also become apparent.

The employment of sodium sulphate and sodium citrate in such plating solutions does reduce somewhat the deposition of nickel on zinc "by immersion", but only to a limited extent, and the result is not entirely'satisfactory under all conditions. The quality of the nickel deposited electrolytically from these-solutions on zinc is not comparable to coatings of nickel on brass, steel or copper. I

I have found that these untoward effects can be practically overcome by the appropriate use of sodium pyrophosphate in place of the standard sodium sulphate, in formula 1, above, for instance, with, of course, appropriate modifications in the other adjuvant ingredients, as will appear from the further disclosures herein.

It is my theory, that the superiority of my process and formulae over the prior art is probably due to the more certain substantial prevention of the solution of zinc. I, therefore, for convenience, refer to this characteristic ingredient, the sodium pyrophosphate, in the combinations as used, as a zinc retardant. In any event, and whatever may be its actual function, my formula insures the formation of an unstriated, even nickel deposit which is substantially hydrogen free and non-porous, and promotes better adherence to the base metal. In-

my experimental work, while developing my process, I was guided bythe theory that, for the satisfactory electro-deposition of nickel on zinc, it was of greater practical importance to "retard" or prevent the solution of zinc and the occlusion of hydrogen in the deposited nickel than to attempt mereLv to reduce the nickelion concentration.

Nevertheless, on theoretical grounds, it is believed that the metallic nickel-ion concentramay, in conjunction with the other considerations involved, explain the improved results obtained in preventing" the solution of the zinc and the deposition of the nickel "by immersion. Whatever the correct explanation, the practical effects are attained.

The process herein described as applicable to the nickel-plating of zinc is also substantially adaptable to superior plating of other metals such as iron, steel, copper, brass, and in fact to practically all metals generally, but its outstanding advantages are particularly manifest with such a difficult metal as zinc.

In thecase of some metals as with aluminum, some further modifications in the solution of the plating bath are found to be expedient. Aluminum is placed higher in the electro-motive series than zinc, and, therefore, should have a greater tendency to" precipitate nickel "by immersion. Actually this does not appear to be true. This is probably due to an anomalous passivity 'of that metal. Because of such passivity of aluminum, it is, therefore, comparatively easy to deposit a coating of nickel upon aluminum satisfactorily enough for ordinary purposes, and so far as general appearance is concerned, but still the plating so produced is not usually sufficiently adherent for all practical uses. The coating so deposited may be even, tough and ductile, yet, unless thefaluminum has been preliminarily subjected to a special etching treatment, the deposited coating will not be uniformly and dependably free from peeling throughout,

With lead,also, while it takes a nickel deposit readily enough with solutions now generally used, the plating is not always satisfactorily adherent throughout. Here, too, a more dependable result is obtained by my process.

What has been said of zinc, aluminum, and lead is also appropriately applicable to alloys thereof, within limits, as will be readily understood by experienced electroplaters of metals.

It has been found that, especially in the chromium-plating of nickel-plated surfaces, it is highly desirable that the deposited nickel coating be strongly adherent to the sub-surface metal and that the deposited nickel film he hydrogefifree, and thus retain the inherent toughness anddeposited nickel is also so substantially hydrogenfree as to permit a fine character of chromium plate to be deposited upon the nickel by' the usual known methods without peeling of the nickel and chromium from the base metal.

However, in the development of my process and solutions, it was found desirable to modify the plating-bath for certain special purposes as, for instance, in the nickel-plating of various grades of aluminum and its alloys, and it is my purpose hereafterto file continuation applications to cover such improvements and modifications for such special purposes, especially to form such nickel-plated surfaces on aluminum articles as will withstand the rigor of subsequent chromium plating.

Having thus described the general objects of my invention, I give the following example of a preferred application thereof, by way of illustration, but not limiting myself to the precise quantities. manner. and means shown. To those iii! Nickel sulphate ounces 1'6 Sodium'pyrophosphate do 8% I Citric acid do 2 Salt do 4 Sodium bisulphite do l4; Ammonia water 28%) do 4-8 Water gallon 1 Dissolve the nickel sulphate in A; of the quantity of water, hot, add other components in order, and dissolve the precipitated phosphate compound with the required amount of ammonia water (28%). Time of application is to be continued according to the desired thickness of coating, as will be well understood by those skilled in the art. Temperature, 20-30 C.; current density, 7-10 amperes per square foot. High initial current density is unnecessary, although, if it is desired to, expedite the operation, an appropriately higher density may be employed. Surface preparation and cleaning: Wash the metal to be plated in mild alkaline cleaner, rinse in clear cold water; then dip in 2% hydrochloric acid solution, and rinse in cold water and plate..

In the plating of zinc especially, for the best attainable results, as heretofore explained, it is desirable to make the plating bath at least weakly but definitely alkaline. It may be necessary occasionally to adjust the solution by adding more or less ammonia. On the other hand, it is also desirable to avoid excessive alkalinity, as such a condition will tend to burn the plated articles at the edges. In empiric shop practice, such excessive alkalinity usually can be detected promptly by the escape of bubbles of ammonia gas, and whenever too much ammonia has been used inadvertently, this may be corrected by adding small quantities of sulphuric or citric acid until such bubbling definitely ceases.

When the solution made-up is acid in reaction it is turbid; but by still further increasing the acidity, it again becomes clear. Such acid solu tions are still available for certain kinds of plating, but the nickel deposit then assumes a hard crystalline character. Indeed, for certain classes of work, like small cheap pieces, made up in large quantities, which can be cheaply finished in a tumbling barrel and where such a hard nickel coating may be desirable, even the acid solution will serve advantageously enough.

Summarized, the prescribed advantages of my formula, solution, and process over one or the other of the known methods are that, especially in nickel-plating zinc, it is possible, more certainly:-

1) To deposit electrolytically for a more or less prolonged period, thus making it possible to plate with a thicker filmof nickel, as may be desired for any given purpose;

(2) To thus deposit the nickel without marked concurrent precipitation by chemical interaction, yielding a more even plating throughout;

(3) To deposit the nickel substantially hydrogen-free, thus retaining the non-porous, tough and ductile character of the pure metal;

(4) To deposit the nickel uniformly and definitely in a more adherent bond to the base metal, greatly minimizing the tendency to peeling;

(5) To deposit the nickel with a continuously lower current and without requiring any initial high potential, producing a more uniform and denser deposit; and

(6) To prevent darkening or striation of the deposit.

Some 01' these advantages and effects are interdependent as to their causation, but may in their entirety by theoretically ascribed to the retardation of zinc solution to the substantial prevention of hydrogen occlusion, and to the control of the metallic nickel-ion concentration, as suggested elsewhere.

The several ingredients in the formula exemplified may, therefore, for convenience, be referred to in terms of their respective assumed predominant functions as i'ollows:-- nickel sulphate, initial source of nickel content; pyrophosphate, zinc retardant; sodium citrate, hydrogen neutralizer; citric acid, buffer ag nt; salt, conductivity agent and anode corrodent; bisulphi-te, conductivity and nickel throwing agent; ammonia, phosphate. solvent and alkaline medium. The functions of a number of these overlap to a degree and are manifestly not susceptible of precise short definition, but references to them in these terms, in the claims to follow,.will be readily understood with appropriate limitation in view of what has been said.

What I claim as new and for which I pray the issuance of Letters Patent is:-

1. An alkaline nickel-plating solution for metals, especially zinc base metals, containing nickel sulphate to furnish nickel and sulphate ions; having sufiicientpyrophosphate ions, with relation to the sulphate ions, to prevent deposition of nickel by immersion; sufficient citrate ions to provide adequate buffering effect and to neutralize the absorption of hydrogen by the deposited nickel; a small amount of sulphite ions being present to provide satisfactory nickel-throwing power; sufficient chloride ion to provide proper anode corrosion; the general combination serving to produce a smooth, ductile plate of nickel, and being characterized in its most desirable composition and proportions, but admitting of reasonable variations therefrom, by a bath' of the following composition:-

Sodium sulphate ounces 16 Sodium pyrophosphate do 8 Sodium bisulphite do A Sodium citrate do 8 Citric acid e do 2 Salt do 4 Ammonia water (28%) do 4-8 Water gallon 1 serving to produce a smooth, ductile plate ofnickel, and being characterized in its most desirable composition, but admitting of reasonable variations therefrom, by a bath composed of the following ingredients: Nickel sulphate, sodium pyrophosphate, sodium bisulphite, sodium ci' trate, citric acid, salt, ammonia water, water.

' ROBERT L. TUI'ILE.