US3464899A - Chromium plating process - Google Patents

Description

United States Patent 3,464,239 CHROMIUM PLATING PROCESS Hyman Chessin, Warren, Edgar J. Seyb, Jr., Oak Park, and Philip J. Smith, Jr., Royal Oak, Mich., assignors to M & T Chemicals Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Apr. 24, 1964, Ser. No. 362,457 Int. Cl. C23b 5/50, 5/06; C23f 17/00 US. Cl. 204-29 17 Claims ABSTRACT OF THE DISCLOSURE In accordance with certain of its aspects, the method of this invention for electrodepositing a corrosion-resistant chromium plate on the surface of a metal may comprise forming on the surface of said metal an adherent thin film of a preferably organic, hydrogen-acceptor, film-forming composition, and electroplating a chromium plate onto said metal bearing said film.

This invention relates to chromium plating. More specifically it relates to a novel process for obtaining deposits particularly characterized by their high resistance to corrosion.

As is well known to those skilled in the art, decorative chromium plating may be effected by various techniques. Although the prior art processes and the decorative plate produced thereby may be satisfactory for many purposes, platers have long appreciated that they were less than fully satisfactory. Specifically although it has long been desired to quickly produce decorative plate possessing a microcrack structure, it has not heretofore been possible to accomplish this in less than 12-16 minutes. Furthermore the prior art processes have not permitted attainment at low current density of thin chromium plate characterized by its microcrack or microporous structure and resulting high resistance to corrosion.

It is an object of this invention to permit attainment of bright decorative chromium plate which is highly resistant to corrosion. It is another object of this invention to obtain such plate in short plating time and even in low current density areas. Other objects will be apparent to those skilled in the art on inspection of the description which follows.

In accordance with certain of its aspects, the method of this invention for electrodepositing a corrosion-resistant chromium plate on the surface of a metal may comprise forming on the surface of said metal an adherent thin film of a preferably organic, hydrogen-acceptor, film-forming composition, and electroplating a chromium plate onto said metal bearing said film.

The basis metals which may be treated by the process of this invention may include metals such as iron, steel, brass, bronze, copper, zinc, aluminum, magnesium, nickel, etc. The preferred basis metal which may be plated may be steel, zinc, or brass, and preferably steel, zinc, or brass which has been plated with a plate of nickel, typically preceded by a first plate of copper.

The basis metal, preferably bearing a nickel plate, may be cleaned as by cathodically treating in an alkaline cleaner and rinsing in water prior to further treatment. The sotreatment metal may be dried or it may be further treated as 1s.

-In practice of certain aspects of this invention, the sotreated metal may be treated with an organic, hydrogen- 3,464,899 Patented Sept. 2, 1969 acceptor, film-forming composition. It has unexpectedly been found that use of preferably organic, hydrogen-acceptor, film-forming compositions permits attainment of the novel results hereinafter set forth. These compositions may be characterized by their ability to readily accept, i.e., to react with, hydrogen (particularly in the form of nascent hydrogen) in the presence of activated metal catalyst. They may be characteristically considered as hydrogen overvoltage poisons, i.e., when they are added to a system evolving hydrogen at a cathode, a higher potential is necessary to continue the hydrogen evolution at the same rate. They may be particularly characterized by their ability to raise the hydrogen overvoltage of a standard hydrogen electrode typically by at least about 0.20 volt. This may be observed by electrolyzing a solution which is one molar in acetic acid and one molar in sodium acetate at a current of 30 milliamps over an 8.25 cm. steel cathode. It will be found that the hydrogen overvoltage may be raised by at least about 020- volt when the composition is added in amount of saturation up to 0.0232 molar, q.v. Duwell, E.J., Jour. Electrochem. Soc., vol. 109, pp. 1013-1017 (1962).

The compositions used in practice of the process of this invention may be found to possess a high degree of adherence to and/ or adsorption on the basis metal cathode, particularly during the initial period (i.e., the first few seconds) of chromium plating.

Preferably the organic composition may be one containing at least one point of non-aromatic unsaturation e.g., nonaromatic double or triple bonds including, e.g., -CEC, C=C-, -CEN, --C=N, -C=S, -N=N--, and the C=O group as in quinone. A typical composition may include more than one point of unsaturation, e.g., it may contain at least two -C=C- bonds.

The preferred composition may be formed from at least one material selected from the polymer-forming group consisting of acrylate, acrylonitrile, butadiene and styrene. These compositions may be used in the form of monomers or preferably in the form of polymers. Typical compositions may be formed from inertly substituted materials including isoprene, i.e., 2-methylbutadiene, etc. Preferred compositions may include polymers and derivatives of acrylonitrile such as polyacrylonitrile and acrylic rubber, polymers of butadiene'including natural rubber (i.e., poly-Z methylbutadiene), and polymers of styrene including polystyrene. Copolymers such as butadienestyrene, butadiene-acrylonitrile, or acrylonitrile-butadienestyrene may be used. Modified polymers including modi fied butadieue-acrylonitrile may be employed wherein the acrylouitrile residue in the molecule may have been hydrolyzed to the acrylic acid or salt, i.e., the --CN groups may have been hydrolyzed to the -COOH group. A preferred composition may include methyl acrylate or ethyl styreneacrylonitrile together with a fatty acid emulsifier having a pH of 9.5 and an average particle size of about 400 Angstrom units (such as that sold by B. F. Goodrich under the trademark Hycar 1577);

(b) A latex containing a carboxylated copolymer of acrylonitrile-butadienestyrene together with alkyl aryl sulfonate anionic detergent having a pH of 8.0 and an average particle size of about 1200 Angstrom units (such as that sold by B. F. Goodrich under the trademark Hycar 1570-X20);

(c) A butadiene-styrene copolymer containing a fatty acid emulsifier having a pH of 10.0 and having an average particle size of 600 Angstrom units (such as that sold under the trademark Naugatex 2006 by Naugatuck Chemical 00.);

(d) A butadiene-acrylonitrile-carboxylic modifier latex (i.e., a polymer wherein the CN groups of the nitrile have been hydrolyzed to COOH groups) having an anionic emulsifier having a pH of 8.5 and an average particle size of about 1800 Angstrom units (such as that sold under the trademark Chemigum 520 by Good year Co.);

(e) A natural rubber latex having a pH of 10 and an average particle size of about 100200 Angstrom units; (f) A 0.5% solution of natural rubber in benzene.

Other illustrative compositions which may be used may include:

(g) A solution of 0.34 g. of Carters Rubber Cement in 100 ml. of petroleum naphtha (Carters Thinner sold by The Carters Ink Co.);

(h) A mixture of 45 ml. of water and 1 ml. of a commercial latex paint, Glidden Spred brand, Satin, garnet maroon 3438 containing synthetic rubber in a vehicle;

(i) A latex of styrene-butadiene such as that sold under the trademark SBR-2000 by B. F. Goodrich C0.

The preferred form in which these compositions may be employed may be in the form of a latex in aqueous medium having a concentration of 0.001%60%, say 0.05%. The preferred medium may be natural rubber latex which is an aqueous dispersion of 3% of rubber, i.e., predominantly Z-methylbutadiene.

Other organic, hydrogen-acceptor, film-forming, monomeric compositions which may be employed may include acetylenic compounds, i.e., compounds containing a CEC group such as phenyl acetylene; heptyne-l; heptadiyne-1,7; hexyne-3; 2,S-dimethyl-3-hexyne-2,5-diol; propargyl malondiamide; hexyn-5-ol; hexyne-Z; etc.

Other suitable hydrogen-acceptor, film-forming monomeric compositions which may be employed may include sulfur compounds including: sulfur; carbon disulfide; dicyclopentamethylene thiuram monosulfide; dicyclopentamethylene thiuram disulfide; dicyclopentamethylene thiuram tetrasulfide; 2,5 dimercapto-l,3,4-thiadiazole; phenyl-Z-mercaptobenzimidazol; piperidine pentamethylene dithiocarbamate; dibenzothiazyl dimethyl thiol urea; o-mercaptobenzoic acid; diethyldithiocarbamic acid (Na salt); tetramethyl thiuram disulfide; tetrabutyl thiuram disulfide; o-mercaptosuccinnic acid; 2 mercaptothiazoline; 2 mercaptoethanol; bis(2-hydroxyethyl)dithiocarbamic acid (K salt); bis(p-nitrophenyl)disulfide; and 2,2'-dithiobenzoic acid (K salt).

Other illustrative hydrogen-acceptor, film-forming compositions which may be employed may include: 2- nitrobenzene arsonic acid; arsenous acid; p-(phenyl selenyl)aniline; ethylene dicyclohexanol; hippuric acid; perthiocyanic acid (as Na salt); antimony oxide Sb O and arsenic oxide AS203.

When the hydrogen-acceptor, monomeric, film-forming composition is employed in practice of this invention, it may preferably be employed in appropriate solution, typically of 0.005% in concentration up to saturation. Sulfur, if used, may be dissolved in isopropanol. Other compositions may be dissolved, suspended, etc., in appropriate medium typically water, alcohols such as methanol, ethanol, propanol, etc., hydrocarbons such as benzene, xylene, toluene, etc.

The composition may be deposited onto the metal by cathodic treatment. For example, arsenous acid, arsenic oxide, antimony oxide, etc., may be deposited by cathodizing the metal in a solution (e.g., of sulfuric acid) containing these materials.

It is a feature of this invention that the novel results attainable thereby may be enhanced, particularly when the organic, hydrogen-acceptor, film-forming composition is a latex of the polymer-forming group supra consisting of acrylate, acrylonitrile, butadiene, and styrene, by the use in the body of composition of a lyophilic protective colloid. Typical of such colloids may be gelatin, agar, gum tragacanth, karaya gum, ethyl cellulose, alginates, methyl ether of cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, pectin, etc. They may be added to the latex in amount of 0.005%10% or more.

A typical composition may include methyl ether of cellulose (Methocel brand) in amount of 4 g./l. in an aqueous latex containing 7.5% ethyl acrylate polymer.

Treatment of the basis metal (including plate metal previously deposited thereon, and typically steel bearing a first plate of copper and a second plate of nickel) with the composition in accordance with this invention may include dipping, spraying, electrolytically treating, swabbing, electrophoretically depositing, or dabbing the composition onto the basis metal. Preferably this may be effected by dipping the metal in a body of the latex. Preferably the metal surface may be maintained in contact with the composition at 10 C.50 0, say 20 C. for 1-60, say 5 seconds or longer and then withdrawn from contact. When the metal is introduced dry, the time may be very short. When the metal is introduced with a wet surface, the time may be longer and preferably the rate of introduction to the body should be less than about 10 centimeters per minute.

Preferably the metal surfaces bearing the films formed from the composition in latex form may be rinsed with water. Rinsing may remove the excess of composition and leave a film on the surface. These films may be dried to a uniform layer (that may be a monomolecular film) prior to rinsing.

It is a particular feature of the compositions noted that when applied to the metal surface, they may form a film. This film may be more-or-less continuous, i.e., substantially uniformly distributed on the entire surface of the metal. In case of certain compositions, e.g., sulfur, the sulfur may be uniformly but discontinuously distributed over the entire surface. In case of other compositions, e.g., a natural rubber latex, the rubber may form a very thin and substantially continuous film. Commonly such a film may have a thickness of -2000, say 400 Angstrom units and typically it may be a monomolecular film. The films, which may be invisible to the naked eye, remaining on the metal surface may be characterized by their high degree of adsorption and their adherence.

The so-developed film may preferably be cathodically treated, i.e., maintained as cathode for 1-60 seconds, say 30 seconds preferably in acid solution, typically 0.5%- 10%, say 2% by weight aqueous solutions of sulfuric acid. Preferably the cathode current density may be 0.5-8 a.s.d., say 4 a.s.d.

The metal surface bearing the film may then be placed in a chromium bath and chromium plating may be initiated. As plating proceeds onto the cathode surface bearing the film, the film may gradually during 3-180 seconds, typically 5-10 seconds be dissolved, dissipated, or etched off the cathode. The dissolution of the film may initiate at selected spots more-or-less uniformly distributed over the surface of the cathode at which the initial deposit of chromium may form. Substantially the entire film may be removed from the metal surface by the action of the evolving hydrogen at the cathode, and reaction with the chromium plating solution. It is a particular feature of these films that they are controllably removed under the peculiar'conditions prevailing at the cathode in a chromium plating bath, i.e., in the presence of large amounts of nascent hydrogen and of highly oxidizing chromic acid.- 7

Chromium plating may be effected at temperatures of 30 C.-60 C., say 50 C., and 5-50, say'30 a.s.d. for 0.5-15, say 1-9 minutes from a bath containing 100-500 g./l., say 250 g./l. of chromic acid and 1-5 g./l., say 2.5 g./l. of sulfate ion, typically derived from sodium sulfate. Other components including other chromium plating catalysts, e.g., fluoride or silicofiuoride, self-regulating compositions, fume suppressants, etc., may be present.

The chromium plate prepared by the process of this invention may be particularly characterized by its bright decorative appearance, its high corrosion resistance, and by its microcracked or microporous structure in a thin layer. When this plate is formed in decorative thickness, it may be found that it is characterized by unexpectedly superior properties.

It is a particular characteristic of the novel deposit that it does not, in contrast to prior art plate deposited directly on the basis metal, acquire the stress pattern of the basis metal. Rather it acquires a low stress which is characteristic of the novel deposited plate.

The novel deposit is unexpectedly characterized by excellent corrosion resistance at very low thickness, typically as'low as 0.1 micron and simultaneously by a microcrack pattern attained at thickness as low as 0.25-0.75 micron, say 0.3 micron, not heretofore attainable. The unexpected ability of this novel process to permit attainment of the noted crack pattern at such low thickness is particularly outstanding. The plate is normally characterized by presence of fine cracks, typically in amount of 20- 800 or higher, say 400 per centimeter. Even at thicknesses below which a crack pattern is attained, the chromium possesses an unusual microporosity which during corrosion testing develops into a disconnected microcrack pattern in the forms of stars or crowfeet.

This novel product when tested for corrosion as conducted under ASTM Specification B368-61T (CASS- Test) and B-117-62 (neutral salt spray) may be found to be substantially superior to a standard chromium plate of the same thickness when prepared by conventional methods.

The novel chromium plate of this invention may be characterized by its unique combination of low thickness, as low as 0.1-0.75 micron, commonly 0.3 micron, and its fine crack pattern in thickness as low as 0.25 micron. As the plate is deposited and goes from to 0.25 micron, the chromium deposit may be characterized by a plurality of holes each typically of diameter of 0.01-0.2 micron, say q 04 micron and spaced at a distance from each other of 50-200, say 100 microns. Typically'als'o the plate may include (as demonstrated by testing in the CASS test followed by copper plating in the standard Dupernell test, to reveal crack structure) a plurality of nonconnected, nonintersecting, very fine attenuated crack lines, each 1-4 microns, say 2 microns long and radiating from the said holes. Typically each hole may have less than about 5 crack lines radiating therefrom; most commonly 3 cracks may be present in the form of a star or a crowfoot. The number of holes present in this form may be 1000-4000, say 2000 per square centimeter.

As the thickness of the plate increases to the preferred thickness of 0.25-0.75 micron or more, the crack pattern which develops may include a plurality of intersecting microcracks, typically in amount of 20-800 per centimeter or higher, say 400 per centimeter.

The novel chromium plate of this invention, which may be particularly characterized'by its crack pattern of typically 20-800 cracks per centimeter in decorative thickness of 0.25-0.75 micron, may be found to possess unexpectedly superior corrosion-resistant properties in these decorative thicknesses. When tested as by the CASS test 6 supra, it will be found that at a decorative thickness of, e.g., 0.25 micron, the novel plate is completely satisfactory while a control plate of the same thickness produced by prior art techniques is totally unsatisfactory with respect both to appearance of the chromium plate and attack of the basis metal.

The novel chromium plate of this invention, which also may be unexpected characterized by microporosity in thickness as low as 0.1 micron may be found to possess unexpectedly superior corrosion resistant properties in these decorative thicknesses. When tested as by the CASS test supra, it will be found that at a decorative thickness of, e.g., 0.125 micron, the novel plate is satisfactory while a control plate of the same thickness produced by prior art techniques is totally unsatisfactory with respect to appearance of the chromium plate and attack of the basis metal.

It appears that the unexpected properties of this novel plate may possibly be related to the hydride content of the initial layer of chromium deposit. According to the theory of Cloyd Snavely (Trans. Electrochem. Soc., vol. 92, 537-577, 1947), it appears that chromium hydride may be codeposited with chromium during electrodeposition, and the stress in the chromium deposit may arise from the conversion of this hydride to chromium metal and the simultaneous release of hydrogen. Cracking in chromium deposits may appear at this time because the newly formed chromium lattice is smaller than the lattice of the deposited chromium hydride. The novel process of this invention appears to permit increase in the hydrogen overvoltage during the initial stages of deposition so that a larger proportion of hydride is formed. Thus, when conversion to chromium metal occurs, the initial stress which causes cracking may occur sooner, i.e., when the deposited plate is thinner.

Practice of the process of this invention may be more apparent from inspection of the following examples wherein all parts are parts by weight unless otherwise specified.

In one standard example, a brass panel 75 mm. high x mm. wide which had been bright-nickel plated was employed. It was subjected to the following steps:

(a) Cold Water rinse.

(b) Dip into 2% sulfuric acid by volume.

(c) Cold water rinse.

((1) Hot water wash.

(e) Blow dry.

(f) Treat the lower 25 mm. of the plate as set forth in column 2 of Table I. This is the experimental portion. The 25 mm. immediately above this experimental portion is the control portion.

(g) Cathodize entire panel in a Hull cell containing 5% sulfuric acid by volume for 30 seconds at 5 amperes.

(h) Cold water rinse.

(i) Chromium plate in a standard cell by making immediate contact and quickly raising the current to 7.5 amperes for three minutes at 42 C.-43 C. in a bath typically containing:

G./l. cro 200 so.,= 1.5 SiF 2.0

In each example, the nickel plated basis metal was treated in manner generally similar to that set forth in the standard example except that in the step there designated as step (f), the lower portion of the panel was treated in the manner set forth in Table I.

' After the plating had been effected from a bath such as that typified by the standard example, the number of cracks per centimeter was determined for the upper or control portion of the plate and for the lower or experimental portion of the plate. The number of cracks per centimeter for the control was determined by measuring across a line generally perpendicular to the plurality of parallel gross cracks which characterized the substrate on which the control is deposited. These Y- or grosscracks reflect the surface structure of the basis metal rather than that of the microcrack pattern; and in fact no microcrack pattern was observed in the control areas. The number of cracks for the experimental was determined by measuring along a randomly positioned line.

TAB LE I Example Treatment Cracks/cm.

C ontrol Experimental 1 Swab with a solution made by thinning 1 ml. of rubber cement (Carters Rubber Cement containing rubber in petroleum naphtha) with 30 ml. of petroleum ether.

2 Swab with a solution made by adding 1 ml. of latex paint (Glidden Spred Satin, garnet maroon 343B) containing synthetic rubber emulsion in vehicle, to 45 ml. of water.

3 Swab with natural rubber latex (B. F. Goodrich 00.).

4 Spray with a synthetic polymer latex (B. F. Goodrich Co. Hycar 1577) formed from styrene-butadiene-ac y onitrile;

5 Dip into a solution of 5 ml. of

synthetic polymer latex (B. F. Goodrich Co. SEE-2000 latex) formed from styrene-butadiene, ml. distilled water and ml. acetone.

6 Dip into solution formed by adding 90 ml. distilled water to 10 ml. of a synthetic polymeric acrylonitrile latex (B. F.

Goodrich Co. Hycar 1552 meduim acrylonitrile type latex).

7 Dip for 60 seconds into solution formed by mixing equal parts of distilled water and a synthetic polymeric (B. F. Goodrich Co. Hycar 1570-X-) carboxylated acrylonitrile latex.

8 Dip for 30 seconds into a solution formed by mixing equal parts of distilled water and a synthetic polymeric (Goodyear Co. Chemigum Latex 520) carboxylic modified butadieneacrylonitrile latex.

9 Spray with a solution formed from 10 ml. ethynylbenzene, 60 ml. isopropanol, 30 ml. distilled water.

10 Swab with a solution made by boiling with excess isopropanol, 5 g. of a plastic material formed by polymerizing trimethyl dihydroquinoline with 1% by weight conc. sulfuric acid.

11 Swab with a solution made by boiling for 4 minutes an aqueous solution containing 14.8 g./l. of sodium hydroxide and 5.8 g. ll. of a synthetic (General Aniline Gantrez AN-3132) ethyl half ester of a copolymer of methyl vinyl ether and maleic anhydride (Gantrez AN 119).

12 Swab with a solution in isopropanol of 5 g./l. of a synthetic (General Aniline Gantrez AN- 3391) methyl halt ester of a copolyrner of methyl vinyl ether and maleic anhydride (Gantrez AN-139).

13 Swab with an aqueous saturated solution of hippuric acid, -C O-NH-CHz-COOH, let dry.

14 Swab with hot solution containing 0. 15 g. of dicyclopentamethylene thiuram monosulfide in 50 ml. isopropanol.

15 Swab with hot solution containing 0. 15 g. of dicyclopentamethylene thiuram disuliide in 50 ml. isopropanol.

16 Immerse into solution containing 100 ml. distilled water, 2 ml. sulfuric acid, and 0.1 g. p- (phenyl selenyl) aniline lrP-CBH5NH2.

17 Swab with warm isopropauol solution oi product prepared by heating at 105 C. for 7.5 hours, 2.2 g. of dicyclopeutamethylene thiurani disullldc.

TAB LE I (Continued) Cracks/cm.

Treatment Control Experimental Swab with solution prepared 20-30 by saturating boiling isopro- 19 Swab with solution prepared by 4-8 120 dissolving in 25 m1. of isopropanol0.1 g. of 2, 5-dimercapto- 1, 3, 4-thiadiazole, let dry.

20 In a Bull cell, cathodize the lower half of the panel for 30 seconds at 5 amperes in an aqueous solution containing excess arsenous acid and hydrochloric acid to lower the pH to 1.2.

21 In a Hull cell, cathodize the lower half of the panel in an aqueous 2% solution of sulfuric acid saturated with arsenous acid AS203.

22 u In a Hull cell, cathodize the lower half of the panel in an aqueous 2% solution of sulfuric acid saturated with antimony oxide SbzOg.

23 Swab squalene onto the lower portion of the panel.

24 Swab hydroxypropyl methacrylate monomer onto the bottom of the panel.

25 Swab allyl propionate onto the lower portion of the panel.

26 Swab styrene onto the lower portion of the panel.

27 Dip the lower portion of the panel into a solution containing 25% by volume of a synthetic polymer latex (B. F. Goodrich C0. Hycar 1577) formed from styrene-butadiene-acrylonitrile and 75% by volume of a solution of 4. 1 g.ll. agar-agar in water adjusted to pH 10 with ammonium hydroxide.

28 Dip the lower portion of the panel into a solution containing 15% by volume of a synthetic polymer latex (B. F. Goodrich Go. Hycar 1577) formed irorn styrene-butadiene-acrylonitrile and by volume of an aqueous solution of 4.7 g./l. of karaya gum.

29 Dip the lower portion of the panel into a solution containing 15% by volume of a synthetic polymer latex (B. F. Goodrich Co. Hycar 1577) formed from styrene-butadiene-acrylonitrile and 85% by volume of an aqueous solution of 4.7 g./l. gum tragacanth. 30 Dip the lower portion of the panel i into a solution containing 14 ml.

of a synthetic polymer latex (B. F. Goodrich Co. Hycar (1577) formed from styrenebutadieneacrylonitrile, 43 ml. of a solution of 25 g./l. of gelatin which was adjusted to a pH of 9. 5 by addition of ammonium hydroxide, to which distilled water was added to make min In another example, No. 31, a nickel plated panel was treated in the standard manner supra'through steps (e). For step (f), the lower 25 mm. of the panel was dipped into-a 20% by volume solution of an acrylate (B. F. Goodrich Hycar 2600 X94). Then the panel was water rinsed andplated 'at 48 C. in the standard Hull cell at 10 amperes for seven minutes in the following chromium plating solution: CrO 250 g./l. and S0 2.5 g./l. There was no cracking in the untreated control area, while the crack count on the test area was 50 cracks per centimeten.

Inspection of the results of the typical illustrative examples hereinbefore set forth reveals that, inall cases, treatment of the basis metal prior to plating in accordance with the novel process of this invention permits attainment of a crack pattern characterizedby the. presence of substantially more cracks per centimeter than that of the control. In one case, Example 22, the number of cracks was increased by a factor of as much as,l00 times. As will be apparent, an increase by a factor of 5-10 is common.

.9 More significant however is the fact that practice of the novel process of the instant invention gives a microcrack pattern which permits attainment of a degree of corrosion resistance heretofore unattainable in decorative plate of such low thickness.

In order to illustrate the high degree of superiority of the novel plate produced by the process of this invention, comparative corrosion tests were run as follows:

CORROSION TESTS In this comparative test, 100 mm. x 150 mm. steel panels were vapor degreased, anodically. cleaned, scrubbed, water rinsed, dipped in 6% aqueous solution of hydrochloric acid, water rinsed, and nickel plated for 80 minutes to produce thereon a bright nickel plate having a thickness of 1.0 mil. The panel was then water rinsed, dipped in 2% aqueous sulfuric acid, water rinsed, and dipped into a solution containing 50% by volume of the synthetic polymer latex (B. F. Goodrich Co. Hycar 1577) formed from styrene-butadiene-acrylonitrile and 50% water. The experimental panel was removed from this bath and water rinsed. A similar control panel was treated in identical manner in all respects except that it was not dipped into the styrene-butadiene-acrylonitrile latex solution.

Samples of the control panel and the experimental panel were each independently chromium plated in a first chromium plating bath containing 190 g./l. CrO 1.03 g./l. SO and 1.64 g./l. SiF (from potassium silicofluoride) or in a second bath containing 240 g./l. CrO 0.9 g./l. SO and 1.9 g./l. SiF Plating in each case was effected at 43 C. and 15 a.s.d. Selected panels, control and experimental, which had been each plated for 1 minute, 2 minutes, and 4 minutes to produce respectively plates of 0.25 micron, 0.50 micron, and 1.0 micron thickness were each tested by the 24-hour CASS test in standard manner to determine-corrosion. The results are measured in standard manner on a -10 scale wherein 10 is perfect (no corrosion) and 0 is totally unsatisfactory. A value of 7 or less is considered commercially unacceptable and a value of 9 or 10 is acceptable. The results of the tests are as follows:

TABLE II.-FIRST BATH Rating Thickness Experimental Control TABLE TIL-SECOND BATH Rating Thickness Experimental Control Selected panelsmay be found to have crack patterns, expressed in cracks per centimeter, as follows, these correspondingto Table III supra. z

" TABLE IV.SECOND BATH Thickness Experimental Control 10 16 hours in the CASS test and rated for corrosion as before. The results are given in Table V.

. TABLE V Thickness Experimental Control After the accelerated corrosion test, the test panels did not show a continuous connected crack pattern but rather a fine porosity described previously in which some of the pores had developed 2-5 radiating cracks in the form of stars or crowfeet. These average approximately 2000 per square centimeter.

Two irregularly shaped steel parts (license plate brackets) were plated in the same fashion as the panels described above. The test parts were dipped in the latex solution while control parts were not. Both were plated in the second bath supra at amperes for nine minutes. The chromium thickness in the recess was approximately 0.1 micron and 2 microns at the protuberances. The crack density was found to be about 750 cracks per centimeter on the protuberances of the experimental part. These parts were exposed in the CASS test for 76 hours. Particular note was made of the recesses where the chromium was thin. With the experimental parts, one part showed no corrosion at all (rating of '9) in the recesses. With the control parts, the rating was 0 in the recesses for both parts. All parts rated 10 where the chromium was thick.

Another set of parts were treated as before and plated in the second bath supra but with a higher current of 300 amperes for the nine minutes. The thickness range was from about 0.25 microns in the recesses to about 5 microns on the protuberances. The crack density was found to be about 1000 cracks per centimeter on the protuberances of the experimental part. The parts showed no corrosion (rating of 10) both in the recesses and where the thickness was greater, after 76 hours CASS testing.

From Tables II and III, it will be apparent that the corrosion resistance of the novel plate is superior to that of the control in the decorative thicknesses deposited. For example a thickness of 0.25 micron was found to be totally satisfactory when achieved by the process of the present invention whereas one prepared by the control process which was twice as thick, 0.50 micron, was found to be unsatisfactory from the point of view of corrosion.

Furthermore, it is apparent from Table IV that the novel product is characterized by the unique combination of microcrack pattern at the unusually low decorative thickness of less than about 1 micron. It is also apparent from Table V, and the example using parts, that at very low chromium thicknesses where continuous chromium cracking does not occur, outstanding corrosion protection is still attained. It will be further apparent that the novel plate of this invention may be deposited in greater thickness in the typical decorative range up to about 5 microns or more as illustrated by the parts plated at the greater chromium thicknesses, but that the advantages herein disclosed are peculiarly outstanding at lower thicknesses.

Although this invention has been described by reference to certain specific examples, it will be apparent to those skilled in the art that various changes may be made therein which fall within the scope of this invention.

We claim:

1. A novel chromium plating process for plating onto a metal a decorativechromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns having substantially increased crack density and a corrosion rating by the CASS test of at least 8, which comprises forming on the surface of said metal an adherent thin film of a hydrogen-acceptor, film-forming composition, and electroplating from an aqueous bath a chromium plate onto said metal bearing said film said hydrogen-acceptor being capable of raising 11 the hydrogen overvoltage of a standard hydrogen electrode by at least 0.20 volt.

2. A novel chromium plating process as claimed in claim 1 for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1- microns wherein said hydrogen-acceptor, film-forming composition is an organic composition containing at least one point of nonaromatic unsaturation.

3. A novel chromium plating process as claimed in claim 1 for plating onto a metal a decorative chromium plate characterized 'by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns wherein said hydrogen-acceptor, film-forming composition is formed from at least one material selected from the group consisting of acrylate, acrylonitrile, butadiene, and styrene.

4. A novel chromium plating process as claimed in claim 1 for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns wherein said hydrogen-acceptor, film-forming composition is formed from ethyl acrylate.

5. A novel chromium plating process as claimed in claim 1 for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns wherein said hydrogen-acceptor, film-forming composition is formed from natural rubber.

6. A novel chromium plating process as claimed in claim 1 for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns wherein said hydrogen-acceptor, film-forming composition is a latex.

7. A novel chromium plating process as claimed in claim 1 for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns wherein said hydrogen-acceptor, film-forming composition includes a latex of natural rubber.

8. A novel chromium plating process as claimed in claim 1 for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns wherein said hydrogen-acceptor, film-forming composition is an acrylonitrile-butadiene-styrene polymer.

9. A novel chromium plating process as claimed in claim 1 for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns wherein said hydrogen-acceptor, film-forming composition is an acetylenic compound.

10. A novel chromium plating process as claimed in claim 1 for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns wherein said hydrogen-acceptor, film-forming composition is an acetylenic compound selected from the group consisting of heptyne-l; heptadiyne-1,7; hexyne-B; 2,5-dimethyl-3- hexyne-2,5-diol; propargyl malondiamide; hexyne-S-ol; and hexyne-Z.

11. A novel chromium plating process as claimed in claim 1' for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns wherein said hydrogen-acceptor, film-forming composition is a sulfur-containing composition.

12. A novel chromium plating process as claimed in claim 1 for plating onto a metal a decorative chromium 7 plate characterized by its high resistance to corrosion in .typical decorative thicknesses of 0.1-5 microns wherein said hydrogen-acceptor, film-forming composition is selected from the group consisting of sulfurt'carbon disulfide; dicyclopentamethylene thiuram monosulfide; dicyclopentamethylene thiuram disulfide; dicyclopentamethylene thiuram tetrasulfide; 2,5-dimercapto-1,3,4-thiadiazole; phenyl-Z-mercaptobenzimidazol; piperidine pentamethylene dithiocarbamate; dibenzothiazyl dimethyl thiol urea; o-mercaptobenzoic acid; diethyldithiocarbamic acid; tetramethyl thiuram disulfide; tetrabutyl thiuram, disulfide; o-mercaptosuccinic acid; 2-mercaptothiazoline; Z-mercaptoethanol; his(2-hydroxyethyl)dithiocarbamic acid; bis(pnitrophenyl)disulfide; and 2,2'-dithiobenzoic acid.

13. A novel chromium plating process as claimed in claim 1 for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns wherein said hydrogen-acceptor, film-forming composition is selected from the group consisting of Z-nitrobenzene arsonic acid; arsenous acid; p-(phenyl selenyl)aniline; ethylene dicyclohexanol; hippuric acid; perthiocyanic acid; antimony oxide Sb O and arsenic oxide As O 14. A novel chromium plating process as claimed in claim 1 for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns wherein said adherent thin film of composition is formedin the presence of and contains a lyophilic protective colloid.

15. A novel chmomium plating process as claimed in claim 1 for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns wherein said lyophilic protective colloid is selected from the group consisting of gelatin, agar, gum tragacanth, karaya gum, ethyl cellulose, alginates, methyl ether of cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, and pectin.

16. A novel chromium plating process for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosionin typical decorative thicknesses of 0.1-5 microns having substantially increased crack density and a corrosion rating by the CASS test of at least 8, which comprises forming on the surface of said metal an adherent film of a hydrogen-acceptor, filmforming composition, drying said film thereby forming a dry adherent thin film of hydrogen-acceptor, film-forming composition, and electroplating from an aqueous bath a chromium plate onto, said metal bearing saidfilm, said hydrogen-acceptor being capable of raising the hydrogen over-voltage of a standard hydrogen electrode by at least 0.20 volt. r

17. A novel chromium plating process as claimed in claiml for plating onto a metal a decorative chromium plate characterized by its high resistance to corrosion in typical decorative thicknesses of 0.1-5 microns including the step of cathodically treating said film prior to chromium plating.

References Cited UNITED STATES PATENTS "2,846,380 8/1958 Brown 204-51 3,041,257 6/1962 Cope et al 204 51 1,283,973 11/1918 Thum 6121 204-29 1,839,905 1/1932 Tainton 1 204-51 2,899,367 8/1959. Veeder 204-32 3,129,149 4/1964 Johnson 204-29 3,175,964 3/1965 Watanabe 1 al. 204-37 FOREIGN PATENTS 1,105,683 4/1961 Germany.

. 1.117.96 il G rm y JOHN H. MACK, Primary Examiner W: B. VAN S ISE, Assistant Examiner US. Cl. X.R.' 204---38, 51 l