US3449223A - Method for covering objects with a decorative bright nickel/chromium coating,as well as objects covered by applying this method - Google Patents

Description

United States Patent 2 9 Int. Cl. C23!) /06, 5/04, 5/50 US. Cl. 20441 14 Claims ABSTRACT OF THE DISCLOSURE This invention is directed to coating objects with a bright nickel/chromium coating having excellent corrosion resistance by first plating the object from a bright nickel plating bath having particles therein and tlfn plaing chromium over the nickel plating. In the nickel plating bath the concentration of the particles does not exceed, in grams per liter liquid bath, 20 times the average diameter of the particles in microns times the specific gravity of the particles.

This application is a continuation-in-part application of application Ser. No. 283,997, filed May 29, 1963, now abandoned.

This invention relates to a method for covering objects with a decorative bright nickel/chromium coating having excellent corrosion resistance and the products produced by the method.

More specifically, according to the present invention there is provided a method for coating an object with a bright nickel coating, comprising the step of electroplating said object with nickel from a bright nickel electroplating bath containing bath insoluble solid particles therein to produce on said object a bright nickel coating containing solid particles therein, said particles having a conductivity not exceeding the conductivity of graphite, the concentration of said solid particles in said bright nickel plating bath not exceeding, in grams per liter of liquid bath, 20 d/s., b being the average diameter of said particles in microns, and s being the specific gravity of said particles. This bright nickel coating having particles therein is then plated with chromium to produce a highly corrosion resistant bright nickel/chromium coating. The invention is further directed to certain preferred operating conditions, as Well as the resultant products. These and other aspects of the invention will now be described in the following detailed description of the invention.

In the normal bright nickel/chromium coating (i.e., a chromium layer over a nickel layer) the top chromium layer will always show pores and minute cracks which will lay the bright nickel surface partly bare. The bare surface will usually vary from 0% to something like 5%, in which 0% represents the ideal condition, which, however, even by applying special crack-free chromium baths, will never be achieved completely.

As the corrosion of the nickel (being a non-precious anode in respect to the more precious chromium cathode) and therefore the life of the object covered by the bright nickel/ chromium coating is dependent on the size of the bare surface, one tries to artifically enlarge thi bare surface so as to make the intensity of the corrosion current as small as possible per unit of surface. To this purpose a microcrack chromium has been developed (AES Proceedings 47 (1960), 215 .225), whereby the bare surface ice formed by the total amount of cracks comes to approximately 15-20%. By means of special intermediate treatment (see copending United States application Ser. No. 260,854, filed Feb. 25, 1963, now Patent No. 3,298,802, and Dutch Patent application No. 275,167) in galvanic baths containing solid particles of matter a thin intermediate layer containing particles is. deposited between the bright nickel layer and the chromium layer, and a bare structure of more than 50% bare surface is achieved, however, on the condition, that the intermediate layer does not become too thick, or, in other words, that the galvanic intermediate treatment will not last too long, as in that case the nickel/chromium coating would turn dull. According to the present invention, the intermediate layer of Patent No. 3,298,802 is eliminated, with the particles being incorporated into the base bright nickel layer.

Prior to the present invention it was generally accepted that in order to obtain bright nickel coatings and to avoid any dulling of the coating, the bright nickel plating bath had to be very carefully filtered to remove all particles therefrom. However, it has been surprisingly found that, under the proper conditions, it is possible to produce highly bright nickel coatings from bright nickel plating baths having solid particles therein. High brightness can even be obtained from entirely cloudy electrolyte baths. The coatings obtained from such bright nickel plating baths having particles therein are highly bright nickel coatings having particles embedded therein.

There will now be discussed the various factors which must be considered in order to avoid the dulling effect which prior art assumed would appear when using bright nickel plating baths containing even the slightest concentration of solid substances.

The most significant and the controlling factors which are involved in obtaining the bright nickel coating from particle containing bright nickel baths are: (1) The concentration of the solid particles; (2) The size (diameter) of the particles; (3) The specific gravity of the particles.

Other factors which play a considerable, through less predominant influence are: (4) The current density applied during the coating; (5) The nature of the particles; (6) The liquids motion in the bath during the plating.

(l) The concentration of solid particles The higher the concentration of the solid particles in the plating bath, the greater will be the dulling effect in the nickel plating.

(2) The size (diameter) of the particles The dulling effect on the nickel coating increases as the size of the particles decreases. For example, if using two baths of the same concentration of particles having the same particles therein with the particles of one bath having a diameter of 0.05 micron and the particles of the other bath having the diameter of 1 micron, the dulling effect of nickel plating will be far greater in the case of the bath having particles whose diameter is 0.05 micron. In the case of particles having a size of 1 micron, a multiple of a concentration of the particles can be used before obtaining the same dulling eitect as would be produced when using particles of 0.05 micron. In this application, both in the specification and in the claims, when referring to particles which occur in a flat or plate-like form, such as aluminum oxide, it is taken for their diameter the diameter of a sphere having the same volume as the plate.

Since solid particles of size or diameter of approximately 10 microns produce the roughness known in the galvanotechnics, the particles used in the present invention do not exceed 10 microns in diameter. Otherwise, there is no limitation as to the size of the particles and the particles can therefore be any size from colloidal up to approximately microns. The preferred diameters of the particles to be used in the invention will be discussed later.

(3) The specific gravity of the particles The specific gravity of the particles plays a part insofar that, using particles with a higher specific gravity, a greater concentration may be applied with respect to the rate of the dulling effect.

(4) The current density applied in coating Though this is not of such importance for the dulling effect, with a slow building up of the bright nickel coating (i.e., at 0.1 A./dm. the bright nickel coating will already be dulled at even a relatively low concentration of solid substance as the supply of solid substance is relatively larger in respect to the electrolyte supply for the building up of the bright nickel layer. With an excessively fast building up of the bright nickel coating on the other hand (e.g. at -30 A./dm. the concentration of solid particles may be considerably higher before a dulling effect is observed. At the normal current density of 1-10 A./dm. the influence of this factor is, however, not as great.

(5) The nature of the particles The transparent character of the particles also plays a certain part, such, that in applying transparent materials (glass powder, quartz powder and diamond powder, etc.) the concentration may be slightly higher than is the case with substances like aluminum oxide, titanium oxide, filtering earth, iron oxide, chromium oxide or molybdenum oxide. Where the size of the particles is the same, at most twice as much may be added of the transparent materials as of the non-transparent solid substances.

However, the effect of the method according to the invention is achieved with solid particles of every kind, provided these are semior non-conductive, where graphite is to be named as the limit for semi-conductors. Which means that colloidal plastic materials can very well be applied.

(6) The motion of the liquid in the bath If the liquid in the bath is kept in motion, let us say by means of a pump for liquid, a stirrer or by blowing in air a maximum dulling effect is achieved when all the particles have been stirred up. At a stronger motion of the liquid, especially when air is blown in during the nickel plating process of an object, the dulling effect will decrease slightly. It is easy to understand that, when rigorous stirring takes place the growing together of the particles in the increasing nickel layer may be somewhat slowed down.

Discussion of conditions to produce the desired result I have discovered, after comprehensive research and testing many substances of various natures, the concentration, and diameter of the particles, that a bright nickel plating can be produced having particles therein so that a chromium layer can be plated over the nickel layer, wherein there is exposed bare nickel of sufficient amount to produce corrosion resistance. Since the brightness of the chromium layer depends upon the brightness of the nickel under layer, according to the invention, there can be produced a bright chromium layer. The bare surface may fall within the range of 20-30%. According to the invention, the underlying nickel layer having partticles therein may have any desired thickness. I have determined, experimentally and empirically, from this research that the maximum concentration of particles in the nickel plating bath is set forth in the following formula: I c=20 d./s.

in which d is the average diameter of the solid particles in microns, s is the specific gravity of the particles, and c is the concentration of the particles in the plating bath expressed in grams per liter of liquid bath.

I have found that when the concentration c of the particles does not exceed 20 d./s., the nickel plated from such bright nickel plating bath will be a bright nickel layer having particles embedded therein.

When, because of unfavorable conditions as previously discussed (low current density, a strongly non-transparent substance, a maximum of stirring in the sense previously mentioned), there is a loss of brightness, let us say in the concentration of 15 d./s., the loss of brightness can be avoided by increasing the current density, or by increasing the amount of conventional brightening agent in the bath, such as butynediol, propargyl alcohol, chinaldine-ethyliodide, etc.

Nevertheless, an optimal brightness can be obtained in all cases, even under unfavorable conditions, when the concentration of particles is about 25% of the maximum permitted. In other words, even under unfavorable circumstances, satisfactory brightness can be obtained with a good open chromium coating when the concentration 0 of the particles does not exceed 5 d./s.

At a much lower concentration of particles, for example at c=d./s., or still less, a more or less favorable influence is still to be found, especially with particles whose specific gravity is fairly large. As a practical matter, the concentration c of the particles should not be less d.s. in order to obtain desired corrosion resistance together with the brightness. However, even at smaller concentrations where there may only be traces of solid particles, such particles will enlarge the bare surface as compared to no particles at all, and some corrosion resistance, al-

" though not very significant, will be obtained.

Generally, a concentration c should be between about d./s. and 10 d./s., and a preferred concentration should be about 5 d./s.

In the following table there are set forth numerous exemplary substances which can be used in the invention, together with the desirable concentrations and other properties.

TABLE Average diameter (s ze of Most particles) Maximum favorable of solld concentraconcensubs tance Specific tion O=20 tration (in Substance in microns gravity ds (in g./1.) g./l

Quartz powder- 1 2. 6 52 12. 5 MOS 0.05 4.8 4.8 1.25 0. 3 3. 5 21 5. 25 0.2 2. 6 10. 4 2. 5 0. 3 7. 3 43. 8 l1 2 2. 5 25 0. 4 5. 2 41. 6 10. 5 0. 3 5. 2 31. 2 7. 5 0. 2 2. 6 10. 4 2. 5 1 3. 1 62 15 0. 05 1. 8 1. 8 0. 45 O. 1 4. 5 9 2. 25 0. 1 4. 1 8. 2 2

l Obtained at pH=6, by precipitating calcium chloride in a nickel sulfate-tree bath, by means of sodium carbonate.

1 OEtarned by add ng barium chloride to a normal nickel sulfate bath. l O, llilalled by addmg N aOH to a nickel plating bath until a pH=6-6. 5 15 was e The invention is not limited to the exemplary substances mentioned. In fact any bath insoluble material having a conductivity not exceeding the conductivity of graphite can be used.

There will now be set forth specific examples of the invention.

5 EXAMPLE 1 An iron object is nickel plated in a nickel bath of the following composition:

Substance: Concentration, g./l. Nickel sulfate (NiSO '6H O) 300 Nickel chloride (NiCl -6H O) 60 Boric acid (H BO 40 Saccharine 1 2 Butyne2-diol-1,4 (HOH2CCE-CH2OH) 0.2 Aluminum oxide (A1 (average d=0.3 microns) 5 g./l. Chromic oxide (Cr O 300 Sulphuric acid (H 50 3 at a temperature of 40 C. and with a current density of 15 A./d.m.

In order to determine how large the surface of the open pores is, the pattern of the open pores is developed by copperplating in an acid copper bath to render the pores more readily visible. In a microscopic examination with a magnification of at least 300 times it is found that approximately 20% of the surface will lie bare.

In order to determine which is the resistance against corrosion, the Corrodkote test (ASTM B 380-61 T) is applied. Here a strongly improved resistance against corrosion is found in respect to that of objects having been treated under similar circumstances in a similar bath, though without adding solid substance.

In this example, the aluminum oxide has the following sieve anaylis:

Micron Percent From this, the average particle size d is calculated as follows:

so d=0.3 micron.

Sodium hydroxide is added until 4 grams of nickel hydroxide per liter of bath liquid have been precipitated. The suspension of nickel hydroxide is kept in motion by means of a stirrer. The average diameter of the nickel hydroxide particles is 0.1 micron.

An iron object which has been nickel plated in this bath for 30 minutes at a current density of 5 A./dm. and at a temperature of 60 C. will have a highly bright nickel layer, notwithstanding the fact that the bath in appearance is dull and cloudy.

After chromium plating in a normal chromium bath as used in Example l, and after the copper plating needed for the developing of the pore pattern, one observes under the microscope at a magnification of 300 times that approximately 25% of the total surface will be copperplated, viz. will lie bare after the chromium plating. In testing the resistance against corrosion by means of the Corrodkote test a highly improved resistance against corrosion will be found (in respect to those objects treated under the same circumstances in the same bath but without any solid substance).

Though as basic material for applying the bright nickel/ chromium coating iron has been mentioned exclusively, it will be clear that also other conducting materials or materials which can be made conductive can serve as basic material, such as copper, brass, zinc and other metals and alloys, or non-conductive material such as wood and plastic which have been made conductive by coating it with carbon.

According to the present invention, the particles can be added to any standard bright nickel plating bath. It is possible to add the particles to any other normal type of nickel plating bath, even those not producing a bright plate and obtain resistance to corrosion, but the essence of the invention is not only to produce resistance to corrosion, but to obtain a bright finish which, according to the prior art, was hitherto considered unobtainable.

EXAMPLE 3 This example compares the brightness and corrosion resistance produced according to the invention with that obtained from plating baths outside the scope of the invention. In this comparison a number of iron covers were plated with a nickel layer 10 microns thick, using the following baths and then the nickel was subsequently overplated with chromium from a standard chromium plating bath. The following baths were used:

Bath No.: 6/1.

A-A bright nickel bath containing:

Nickel sulphate 300 Nickel chloride 60 Boric acid 40 Saccharine 2 Butyne-2-diol-l,4 0.2 B -Bath A+barium sulphate 2 B -Bath A-i-barium sulphate 10 B -Bath A-i-barium sulphate 40 C -Bath A+kaolin 1 C -Bath A+kaolin 10 C -Bath A+kaolin 40 D -A bright nickel bath containing:

Nickel chloride 200 Boric acid 40 Saccharine 2 Butyne-2-diol-l.4 0.2 Calcium carbonate 0.4 D -Bath D but with calcium carbonate 10 g./l.

The barium sulphate had an average diameter of 0.1 micron and a specific gravity of 4.5, the kaolin an average diameter of 0.1 micron and a specific gravity of 2.6, and the calcium carbonate an average diameter of 0.05 micron and a specific gravity of 1.8.

In baths D and D the calcium carbonate was added to a sulphate-free nickel bath to prevent the formation of a calcium sulphate precipitate, and at a pH of 6 to prevent the calcium carbonate from being dissolved.

The coating produced from each of the baths is identified herein with the same letters as the baths used to produce the coating. Coatings A, B C and D all show a uniform high brightness. Of these layers, A represents a control, since no particles were used in bath A. Coatings B C and D are within the scope of the present invention, while the remaining ones are outside the invention. Covers B and C showed a uniform satiny lustre.

Covers B C and D had no uniform appearance, but were cloudy in places.

When the coated covers were tested by means of the Corrodkote test, the percentage of rust on cover A was about and on the other covers less than 1%. Accordingly, the corrosion resistance of covers B C and D was much higher than that of cover A (control), and was about the same as the corrosion resistance of the remaining covers which are outside the scope of the invention, insofar as uniform brightness is concerned.

It is emphasized that according to the present invention, when utilizing the concentrations of particles as herein specified, the brightness of the resulting plating is substantially the same 'as that which would have been produced by utilizing the same bath without the particles, but the utilization of the particles Within the ranges of the invention produced a corrosion resistance significantly greater than that which the plating has when produced in the absence of particles.

In this application and in the art, numerous terms are used to describe the appearance of the nickel plate, such as, bright, satin, semi-bright, smoky (or cloudy), dull, etc. The difference between these various terms is well known to a worker skilled in the art, and it is well known that the term bright is different in kind from the other terms, in that a bright coating is me which is a specular coating, which the others are light diffusing coatings. A bright coating is a uniform, mirror-like coating which reflects images. One way to evaluate the brightness of a coating is to place the coating about -30 cm. from the eye and observe the mirror image of the iris. In a bright coating the image of the iris can be clearly seen, and is a sharp image. The light diffusing coatings do not reflect a clear image of the iris.

The particles as used in the present invention serve to produce the corrosion resistance of the chromium plate. When nickel is plated from a bath containing particles, some of the particles become embedded in the nickel layer as the latter is formed, and also cover part of the surface thereof. Since the solid particles are nonconductive, or semi-conductive, no nickel will be deposited on these particles, so that pores will be formed in the nickel layer, which may be closed again during further formation of the nickel layer. Accordingly, the surface of the ultimate nickel layer will show pores "and non-conductive or semi-conductive particles on which no chromium is deposited during the subsequent chromium plating, so that a microporous chromium layer is formed, the porosity or openness of the chromium layer imparting the corrosion resistance.

The production of corrosion resistance in this manner was been described in United States Patents Nos. 3,152,971, 3,152,972 and 3,152,973. However, in these patents the concentration of the particles is such that the nickel plating is satin finish, i.e., light diffusing. This is consistent with the prior knowledge that particles in a plating bath destroy the brightness of the layer plated from the bath. These patents do not recognize that by selecting the correct concentration of particles, bright finishes can be produced, despite the fact that demand for bright finishes greatly exceeds the commercial demand for satin finishes.

These patents emphasize that to produce a satisfactory finish, the concentration of the particles cannot be less than the minimum disclosed therein, and state when a concentration is less than this minimum, the plate has a smoky, non-uniform appearance.

There is no gradual transition from a satin nickel/chromium coating to a bright nickel/chromium coating as the concentration of the particles is decreased. Instead, between the two desirable coatings, namely satin and bright, the coatings are cloudy or smoky. Normally,

it would be expected that when the concentration of the particles is decreased from that giving a uniform satin coating, there would be two posibilities. The first possibility is that as the concentration is lowered, there would be produced a coating having an undesirable smoky or cloudy appearance until the particles concentration is reduced to zero. The second possibility would be that at a given low particles concentration in the bath, no particles became embedded in the nickel layer, and thereby there is produced a coating having a normal brightness but no corrosion resistance. Prior practice points in the direction of the first possibility, for, in the past, it has always been recommended for the galvanic bath to be carefully filtered since solids suspended in the bath tend to affect the brightness of the coating. Both of these possibilities are incorrect.

It will be appreciated that the number of particles which are embedded in a nickel layer depend primarily upon the particles concentration in the nickel bath. Obviously, if there is a high cencentration, relatively more particles will be embedded in the nickel layer which will produce a uniformly light diffusing layer. As the concentration of the particles is lowered, normally it would be thought that the embedding of particles would be non-uniform (caused by various factors, such as locally varying current density and/or non-homogeneous distribution of the particles in the bath), and therefore a smoky or non-uniform coating would be produced and these are the results predicted by the above mentioned three patents. Nevertheless, as has been discovered according to the present invention, there is a concentration range for solid particles, said concentration range being intermediate zero concentration and the concentration range which produces a smoky appearance, at which the solid particles are embedded in the layer in sufiicient amount to produce corrosion resistance and at the same time, without affecting the brightness of the layer.

In the formula used in this application defining the useable concentration range the controlling factors are the specific gravity and the particle size and it is not necessary to consider the number of particles in the bath as a separate factor. In fact, the number of particles is not a separate factor, but inherently is defined by the specific gravity and the particle volume in conjunction with the concentration. The particle volume can be calculated from the particle diameter.

It is emphasized that when plating using the range of concentrations disclosed herein, corrosion resistance (as compared to using baths without particles) is materially increased, while on the other hand, as shown in Example 3, corrosion resistance is of the same order as the corrosion resistance obtained in satin finishes using large concentrations of particles. Stated differently, as the concentration of the particles is reduced from that required to produce a satin finish, through that which produces a smoky finish, to that which produces a bright finish, the corrosion resistance does not significantly vary when compared with the improvement of the corrosion resistance over that obtained when using a bath having no particles therein.

It has been previously mentioned that the diameter of the particles used in the present invention fall within the range of colloidal up to about 10 microns. Preferably, the size of the particles should not exceed about 5 microns. These figures refer to the diameters of substantially each particle used. On the other hand, generally the average particle diameter should not exceed about 2 microns, and preferably the average diameter should not exceed about 1 micron, the optimum average particle diameter should be about 0.1-0.5 micron. The formula for the maximum concentration, namely,

applies to all the diameters of particles useable in the invention.

I claim:

1. A method for coating an object with a bright nickel/ chromium coating, comprising the steps of electroplating said object with nickel from a bright nickel electroplating bath containing bath insoluble solid particles therein to produce on said object a bright nickel coating containing solid particles therein, said particles having a conductivity not exceeding the conductivity of graphite, the concentration of said solid particles in said bright nickel plating bath not exceeding, in grams per liter of liquid bath, 20 d./s., d being the average diameter of said particles in microns, and s being the specific gravity of said particles, and then electroplating chromium over said bright nickel coating containing solid particles therein, the concentration of said particles being at least d./s.

2. A method according to claim 1, wherein the diameter of said particles does not exceed 10 microns.

3. A method according to claim 2, wherein the diameter of said particles does not exceed about 5 microns.

4. A method according to claim 3, wherein the average diameter of said particles does not exceed about 2 microns.

5. A method according to claim 4, wherein the concentration of said particles does not exceed about 10 d./s.

6. A method according to claim 5, wherein the concentration of said solid particles in said bright nickel bath is about 5 d./s.

7. A method according to claim 6, where in the average diameter of said particles is about 0.1 to 0.5 micron.

8. A method according to claim 7, wherein the particles are selected from the class consisting of quartz powder, MoS, A1 0 China Clay, CeO B C, Fe O Cr O glass powder, SiC, CaCO BaSO and Ni(OH) 9. An object comprising a base having thereon a bright nickel plating containing solid particles, said particles having a conductivity not exceeding the conductivty of graphite, and a bright chromium plating on said nickel plating, said object having been plated according to the method of claim 1.

10. An object according to claim 9, wherein the diameter of said particles does not exceed 10 microns.

11. An object according to claim 10, wherein the diameter of said particles does not exceed about 5 microns.

12. An object according to claim 11, wherein the average diameter of said particles does not exceed about 2 microns.

13. An object according to claim 12, wherein the average diameter of said particles is about 0.1 to 0.5 micron.

14. An object according to claim 13, wherein the particles are selected from the class consisting of quartz powder, MoS, A1 0 China Clay, CeO B C,, Fe O Cr O glass powder, SiC, CaCo BaSO and Ni(OH) References Cited UNITED STATES PATENTS 3,061,525 10/ 1962 Grazen 204-9 3,152,971 1 0/1964 Tomaszewski et al. 204-41 3,152,972 10/1964 'Brown et a1 204-41 3,152,973 10/1-964 Tomaszewski et a1. 204-41 HOWARD S. WILLIAMS, Primary Examiner.

G. L. KAPLAN, Assistant Examiner.

US. Cl. X.R. 29-1912, 194, 196.6; 204-38, 40, 49