US3388049A - Method of electrodepositing microcrack chromium coatings - Google Patents

Description

Unite States Patent 3,388,il49 METHUD 0F ELECTRGDEPGSZTENG MTCRU- CRACK CHROMHJM COATINGS Gatan dc Coye de Castelet, Eiliancourt, France, assignor to Rcgie Nationale des Usines Renault, Eillanconrt, Seine, ..rance No Drawing. l iied Mar. 15, 1965, Eier. No. 440,019 (Claims priority, application France, Oct. 12, 1964,

991,133, Patent 1,447,976; Nov. 24, 196 5, 996,104;

Jan. 27, 18 65, 3,439

Mr Ciaims. (Cl. Edd -29) ABSTRACT 0 F THE DISCLGSURE Chromium plating with increased resistance to corrosion and flaking is produced by first electrodepositing on the substrate a nickel layer with a high internal stress level, sufiicient to produce micro-cracks therein during the electrodepositing of th chromium layer, and then electrodepositing the chromium layer. The electrolyte bath from which the cracking nickel is electrodeposited preferably includes (a) nickel chloride, nickel sulphamate or nickel fiuoborate and (b) acetic acid ammonium acetate or nickel acetate, or mixtures of them.

So-called decorative chromium deposits usually comprise, by way of terminal deposits, a deposit of nickel followed by a deposit of electrolytic chromium.

For many years it was thought that the presence of the chromium plating did not affect the overall protection but served only to improve the eye-appeal of the parts treated in this way.

More recently, an examination of anomalies noted in the resistance to corrosion of chromium-plated articles has led to the conclusion that the chromium plating can, on the contrary, greatly affect the protection, and that this role is not one of providing added protection but instead of speeding up the corrosion to a greater or lesser extent, depending on circumstances.

The physical condition of the chromium plating plays a predominant part. The metal may be deposited in the form of a tight coating, but because of the brittleness of electrolytic chroming, this tightness is destroyed fairly soon owing to the appearance of cracks or holes. Moreover, immediately after it is deposited, the chromium may be porous to some extent or embody minute cracks in a more or less large number per unit area, resulting in the formation of. individual elements placed more or less closely together.

It has been found that the depositing of chromium speeds up the corrosion of the nickel all the more as the density of the porosities or cracks is smaller; conversely, a large number of porosities or cracks results in a very slow nickel corrosion rate.

This fact has been used as the basis for certain methods proposed for improving the overall resistance of chromium-plated parts to corrosion. For instance, one way of achieving this is to obtain chromium with a microcrack structure (cracks spaced from one another by a few ten microns). This can be obtained either by a double-coating chroming process, which requires chromium thicknesses in excess of 0.5 micron, or by a single-coating chroming process in chromium baths containing a small quantity of selenium, in which the microcrack structure is produced with a chromium thickness of approximately 0.3 micron.

Another method aiming at obtaining microporous chromium deposits consists in applying, onto the terminal nickel coating (usually bright nickel), a thin nickel coat- 3,388,649 Patented June 11, 1968 ing about one micron thick containing non-conductive particle inclusions. The chromium coating deposited on this nickel is itself deposited in .porous form due to the presence of these non-conductive particles.

The present invention has for its object to provide a method of obtaining microcracked chromium under unusually simple, readily applicable conditions. This method consists in depositing on the metal, for instance on the nickel normally constituting the ultimate protective coating prior to the chromium coating, a thin metal coating under conditions such as to cause the metal to be highly stressed and barely ductile. During the subsequent chroming operation, this last coating of metal and the coating of chromium undergo simultaneous microcracking, thereby imparting excellent resistance to corrosion to the object treated in this way.

The thinner the coating of this intermediate metal deposited tor subsequent cracking, the more advantageous it will be for its electrochemical potential to lie between that of the subjacent metal and that of the terminal chromium plating (in the state of surface oxidation to which it is normally brought after a few hours or a few days have elapsed). By thin coating is to be understood a coating whose thickness is included between a fraction of a micron and approximately 3 or 4 microns.

Whilst various metals, or alloys based on nickel or cobalt in particular, can be employed, a coating of nickel is entirely suitable for the deposit designed to become cracked, provided its potential is slightly higher than that of the previous metal coating. Furthermore, this intermediate coating must not detract from the final appearance of the chromium-plated part.

Two distinct processes for obtaining a chromium deposit of microcracked structure according to the principle of the present invention will now be more particularly described by way of non-limitative examples of the subject method of this invention. It is, however, to be clearly understood that the examples given hereinafter do not describe such preliminary operations involving degreasing and depositing metals such as copper, brass, zinc, levelling nickel, and so fourth, as may be considered useful for reasons unconnected with the obtainment of the microcracking.

Example 1 Principal protective nickel plating, for example bright nickel plating Rinsing operations Depositing of the cracking nickel under the following conditions:

Optimum Anhydrous nickel acetate, 50 g./l. to

saturation 100 g./l. Hydrated nickel chloride, 100 g./l. to

saturation 375 g./l. pH, 2 to 6 5 to 5.5. Current density, 1 to 15 a./dm. l0 a./dm. Duration, 15 sec. to 2 min. sec. Temperature (below C.) 20 to 25 C.

Agitation optional (with compressed air, say) Rinsings Chromium plating Rinsings Drying Example 2 for example) Rinsings 3 Depositing of cracking nickel, in the following solution:

Optimum Hydrated nickel chloride, 100 g./l. to

saturation 400 g./l. Anhydrous ammonium acetate, 50 g./l.

to saturation 100 g./l. pH, 2 to 6 to 5.5. Current density, 1 to 15 a./dm. 10 a./dm. Duration, sec. to 2 min. 30 sec. Temperature (below 35 C.) to C.

Agitation optional (using compressed air, say) Rinsings Chromium plating Rinsings Drying The cracking-nickel-plating baths used in the conditions specified in the two foregoing examples give, for plating times of less than two minutes, a coating of brightness such as to ensure the subsequent obtainment of very bright chromium plating.

In certain special cases, however, it may be necessary to extend the duration of the cracking-nickel-plating operation for a considerably greater time. This may be the case, for instance, when the parts to be protected have heavily recessed portions, since the thickness will increase less rapidly in such places than on the raised portions. It may also happen that certain installations, by their very design, will not permit of reducing the duration of the crackingnickel-plating to 2 minutes or less.

It is then necessary to add to the bath one or more suitable brightening substances. Such brightening substances must not appreciably aiiect the potential of the cracking-nickel deposit, so that the electrochemical protective process may be maintained as described precedingly. Nor must they reduce the propensity for cracking of the ultimate coat of nickel which generates the microcracking in the subsequent chromium plating.

By way of brightening agents it is possible to utilize the sulfonates, the sulfonamides and the sulfonimides of aromatic compounds. More specifically, the sodium salt of sulfonimide ortho-benzoyl acid (saccharine) can be added in proportions ranging from 0.5 to 3 g./l., the optimum being around 1 g./liter, through this is by no means critical.

If added to the cracking-nickel-plating baths, butyne 1-4 diol will enable very bright deposits of cracking nickel to be obtained, regardless of the thickness of the deposits. This compound is added to the bath in the proportion of 0.1 to 3 g./liter.

Other brightening agents which will enable particularly bright cracking-nickel deposits to be obtained, regardlessof their thickness, are the amino derivatives of heterocyclic compounds such as 2-aminothiazole.

Lastly, when cobalt is added to the cracking nickel baths, it also enables the brightness of the deposit to be enhanced for concentrations ranging from 1 to g./ liter.

The following compositions and procedures may be mentioned by way of complementary examples of the depositing of cracking nickel according to the invention:

Example 3 Bath with addition of butyne 1-4 diol: Optimum Anhydrous nickel acetate, g./l. to

saturation 100 g./l. Hydrated nickel chloride, 100 g./l. to

saturation 375 g./l. Butyne 1-4 diol, 0.1 g./l. to 3 g./l. 0.2 g./l. pH, 2 to 6 5 to 5.5. Current density, 1 to 15 a./drn. 10 a./dm.

Duration, 15 sec. to 5 min. Temperature (below 35 C.) 20 to 25 C.

4 Optional agitation (with compressed air or by displacing the cathodes).

Example 4 Bath with addition of Z-amino-thiazole: Optimum Anhydrous nickel acetate, 50 g./l. to

saturation g./1. Hydrated nickel chloride, lOO g./l. to

saturation 375 g./l. pH, 2 to 6 4 Z-arnino-thiazole, 5 mg./l. to 100 mg./l l0 mg./l. Current density, 1 to 15 a./dm. 10 a./drn. Temperature (below 35 C.) 20 to 25 C.

Optional agitation (with compressed air or by displacing the cathodes).

Example 5 Optional agitation (with compressed air or the cathodes).

by displacing The thickness of the cracking-nickel deposit can be considerably increased by the aid of the brightening agents utilized in accordance with the invention. This enables the method to be used in existing installations with minimum or no modification.

Indeed, this leads to the possibility of a simplified application: for when the electrolysis baths ermit of obtaining bright and cracking deposits of a thickness appreciably greater than a few microns, for instance 10 to 20 microns, then in accordance with the present invention these deposits can be substituted for the terminal protective nickel coating instead of being superimposed thereon.

The following may be mentioned by way of non-limitative examples:

(a) The sequences customarily consisting of a superimposed copper (or brass) deposit and a conventional nickel deposit, in which the latter deposit is replaced by thick bright deposit of cracking nickel or nickel alloy.

(b) The sequences comprising a coating of dull or levelling nickel followed by a coating of bright nickel, this double nickel coating being obtained by the so-called binickel" or duplex method, in which the customary coating of bright nickel is replaced by a thick bright coating of cracking nickel.

It is to be noted that this operating sequence permits of combining the advantages offered by duplex nickel plating and microcrack chromium-plating; moreover, this sequence can be executed without the need to seek a more noble cracking nickel than the subjacent coating of levelling nickel.

Lastly, in accordance with the present invention, in the case of certain bright or scmibright nickel-plating baths in which the additives may result in a degree of surface passiveness, perfect adhesion of the cracking-nickel deposit is ensure by initially depassivating the bright or semibright nickel deposit. This is achieved by means of the following operations:

Careful rinsing of the bright or semibright nickel deposit;

Treatment in an alkali cyanide-base depassivation bath, for which the following formulation represents a nonlimitative example:

Caustic soda 45 g./l. Anhydrous sodium carbonate 45 g./l. Sodium cyanide 20 g./l. Temperature: Ambient Current density 5 to a./dm. Duration of cathode pass to 30 sec.

Careful rinsing before insertion into the bath, thereby ensuring a deposit of cracking nickel.

In cases where the initial nickel deposit is strongly passivated, provision may be made for an anode pass lasting approximately 5 seconds prior to the cathode pass.

While cracking-nickel-plating baths as a whole are no more sensitive to impurities than nickel-plating baths in general, recourse may be had, in order to maintain such baths in good condition, to intermittent or continuous filtering, or else to some method well known per se such as electrolysis on low-current-density cathodes for eliminating foreign metal, or neutralization and filtering on active charcoal in order to get rid of undesirable organic substances and the like.

It should be noted that the deposit of cracking nickel cracks slowly even in the absence of subsequent chromium plating. It is therefore necessary to avoid unduly long waits between the deposit of cracking nickel and the chromium plating, for otherwise the final chromium plating would mask the cracks already formed in the nickel, which would run counter to the aim sought.

This delay will vary with operating conditions and with the structure and stress specific to the subjacent nickel deposit. By way of indication, however, it may be stated that spontaneous cracking in the ultimate nickel deposit egins after approximately a quarter of an hour has elapsed.

By applying the method according to the invention, the microcracked structure can be developed over all the chromium-plated surfaces, using conventional chromiumplaiing baths comprising, as catalysts, the sulphuric, hydrofluoric, fluosilicic, and like ions.

It must be added that the use of special chromiumplating baths containing, say, selenium or additive particles insoluble in the intermediate-deposit solution, while not bringing marked benefits to the method, is not incompatible with the use of the cracking deposits specified hcreinabove.

Vlhat is claimed is:

1. A method of depositing a corrosion resistant microcracked coating on a substrate which comprises electro depositing nickel on said substrate from an aqueous acidic bath and thereafter electrodepositing chromium on said nickel, said nickel being deposited with a high internal stress level, said stress level being suit'icient to produce micro-cracks therein during the deposition of the chromium layer which in turn produces a micro-crack pattern in the chrom um layer.

2. The method in accordance with claim 1, in which the said deposit of nickel is applied to a previous metallic deposit.

3. The method in accordance with claim 2, in whizh the deposited nickel has an electrochemical potential between that of the surface on which the said nickel is deposited and that of chromium.

4. The method in accordance with claim 3, in which said aqueous acidic bath includes a brightening agent enabling the duration of the treatment to be increased.

5. The method in accordance with claim 4, in which the said brigten-ing agent is selected from the group consisting of the sulphonates, sulphonarnides, and sulphoninsides of aromatic derivatives, butyne 1-4 diol, cobalt salts, the amino derivatives of hetcrocyclic compounds such as Z-aminothiazols, and mixtures of at least two of these agents.

6. The method in accordance with claim 2, in which the said previous metallic deposit is copper.

7. The method in accordance with claim 2, in which said previous metallic deposit is brass.

8. The method in accordance with claim 2, in which the said previous metallic deposit'is selected from the group of electrolytic coatings consisting of dull nickel plating, levelling nickel plating, serni-bright nickel plating and bright nickel plating.

9. A method in accordance with claim 8, in which the previous metallic deposit is a semi-bright or bright nickel plating, and this nickel plating is subjected to a depassivation operation before the deposit of cracking nickel t-here- 10. The method in accordance with claim 1, in which cobalt is deposited simultaneously with said nickel.

11. The method in accordance with claim 1, in which the said electrodcposited nickel coating is a duplex nickel plating consisting of a first deposit of dull or levelling nickel and a second deposit of a coating of cracking nickel.

12, The method in accordance with claim 1, in which said aqueous acidic bath consists essentially of (a) nickel chloride, and (b) a member of the group consisting of ammonium acetate, nickel acetate, and mixtures thereof.

13. The method according to claim 1, in which the cracking nickel layer is deposited from said aqueous acid nickel bath at a current density of between 1 and 15 amp/dnfi.

14. The method according to claim 13, wherein the bath contains (a) a compound from the group consisting of nickel chloride, nickel sulphamate and nickel fluoborate and (b) a member of the group consisting of acetic acid, ammonium acetate, nickel acetate and mixtures thereof.

References Cited UNITED STATES PATENTS 2,678,908 5/1954 Tucker 204-4 XR 3,282,810 11/1966 Odekerken 204-41 3,298,802 1/1967 Odekerken 20441 XR OTHER REFERENCES Lovell, W. E, et al.: Experience in the Operation and Performance of Dual Chromium Systems, Proc. of the American Electroplaters Society, vol. 47, pp. 215-225, 1960.

Seyb, E. J.: Corrosion Protection with Decorative Chromium, Proc. of the American Electroplaters Society, vol. 47, pp. 269-214, 1960.

Romanotf, F. P.: Ductility and Adhesion of Nickel Deposits, Trans. of the Electrochemical society, vol. 65, pp. 385-399, 1934.

JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

G. KAPLAN, Assistant Examiner.