US3563864A

US3563864A - Chromium-nickel plating

Info

Publication number: US3563864A
Application number: US451028A
Authority: US
Inventors: Arthur H Du Rose; Karl S Willson; Gustavo C Tejada
Original assignee: Kewanee Oil Co
Current assignee: HARSHAW/FILTROL PARTNERSHIP A PARTNERSHIP OF; Kewanee Oil Co
Priority date: 1965-04-26
Filing date: 1965-04-26
Publication date: 1971-02-16
Anticipated expiration: 1988-02-16
Also published as: DE1496829A1; GB1148433A; ES325133A1; NL6605580A; NL128956C

Abstract

THIS INVENTION COMPRISES A COMPOSITE OR LAMINATED METALLIC COATING, AND A PROCESS FOR MAKING SUCH COATING, COMPRISING A FIRST LAYER OF NICKEL AND AN OVERLYING LAYER OF CHROMIUM, THE LAMINATE BEING CRACKED IN A CRAZE PATTERN IN THE RANGE OF 300 TO 3000 CRACKS PER LINEAL INCH PREPARED BY ELECTRODEPOSITING ON A METAL SUBSTRATE A STRESSED LAYER OF NICKEL AND THEREAFTER ELECTRODEPOSITING ON THIS STRESSED LAYER OF NICKEL A STRESSED LAYER OF CHROMIUM ADHERENT TO THE STRESSED LAYER OF NICKEL AND HEATING THE RESULTANT LAMINATE. THE STRESSING IN THE RESPECTIVE LAYERS MAY BE EFFECTED BY MEANS OF AN ADDICTIVE IN THE ELECTROPLATING SOLUTION FROM WHICH IT IS PRODUCED. THE RESULTANT CRAZING GIVES AN IMPROVED PROTECTION AGAINST CORROSION.

Description

United States Patent 3,563,864 CHROMIUM-NICKEL PLATING Arthur H. Du Rose, Euclid, Karl S. Willson, Cleveland, and Gustavo C. Tejada, Euclid, Ohio, assignors, by mesne assignments, to Kewanee Oil Company, Bryn Mawr, Pa., a corporation of Delaware No Drawing. Filed Apr. 26, 1965, Ser. No. 451,028 Int. Cl. 'C23b 5/50, 5/52 US. Cl. 204-37 Claims ABSTRACT OF THE DISCLOSURE This invention comprises a composite or laminated metallic coating, and a process for making such coating, comprising a first layer of nickel and an overlying layer of chromium, the laminate being cracked in a craze pattern in the range of 300 to 3000 cracks per lineal inch prepared by electrodepositing on a metal substrate a stressed layer of nickel and thereafter electrodepositing on this stressed layer of nickel a stressed layer of chromium adherent to the stressed layer of nickel and heating the resultant laminate. The stressing in the respective layers may be effected by means of an additive in the electroplating solution from which it is produced. The resultant crazing gives an improved protection against corrosion.

This invention relates to composite, metallic coatings or laminated coatings comprising a first layer of nickel and an overlying layer of chromium. More specifically, the present invention relates to the discovery that when both layers of a two-layer nickel-chromium laminate are suitably stressed, the composite can be caused to become microcracked in a fine pattern and the resulting microcracked laminate has superior properties, especially in regard to protection against corrosion.

In accordance with the present preferred embodiment of the invention a nickel deposit having a high tensile stress is used, which, when combined with a suitable chromium overlying layer, produces coatings of improved corrosion resistance. Nickel deposits according to the in vention are stressed and, when combined with the chro mium deposit, which is also stressed, the stresses reinforce each other and the composite exhibits a microscopic crazing or cracking in a craze pattern which is made up of fine lines, from about 300 to 3000 cracks per lineal inch. Although still finer crack patterns are acceptable, it is preferred to produce 700 to 2000 cracks per inch. The deposits having patterns in the preferred range tend to give the best protection against corrosion. The nickel layer may be applied to a thickness from 0.03 mil to 0.5 mil, preferably 0.05 to 0.25 mil, and the thickness of the chromium layer being preferably from 10 millionths of an inch to 50 millionths of an inch.

Prior to the present invention it has been known to plate over bright nickel a layer of chromium from a solution containing selenium ion (Safranek and Miller, Plating 51 543) but, so far as known to applicants, the bright nickel referred to was not prestressed within the hereinbelow suggested values and does not give equivalent results to those obtained where both deposits are prestressed and cooperate to produce cracks within said lirnits'of 300 to 3000 cracks per lineal inch.

When a selenium compound, e.g. sodium selenate, at a concentration of the order of 0.015 gram per liter is added to a chromium plating bath, the resulting chromium deposit, when plated over nickel to a thickness of, for example, 30 millionths of an inch will produce microcracking when the plated object is suitably heated, as, for example, by immersion in hot water. Said seleniumcontaining bath, however, does not have good throwing power and the deposit from such a bath has little or no chromium deposited in the low current density areas of the object being plated. In order to provide better coverage by the chromium and still retain the improved corrosion resistance of the microcracked chromium, it is customary to deposit in the order of 10 millionths of an inch thickness of chromium from a standard chromium bath (containing no selenium) and follow this with a deposit from the selenium-containing chromium bath of the order of 15 millionths or more in thickness. In addition to the reduced throwing power of the chromium bath containing the customary amount of selenium compound, the chromium deposits from such selenium-containing bath tends to have a bluish color which is undesirable. According to the present invention, the amount of selenium can be reduced or selenium can be omitted, as will be shown later, with increased throwing power as a result; also, the undesirable bluish color is reduced. The nickel deposit must be prestressed in order to get these good results.

Microcracking also has been produced in two-layer chromium systems by variations in the character of the separate chromium deposits due to differences in operating conditions or by variations in compositions of the plating baths (US. Pat. 3,157,585).

The present invention dilfers from the earlier methods of producing microcracking in that the two-layer composite responsible for the mircrocracking comprises a thin layer of nickel of special character, prestressed, and

a layer of chromium.

According to one method of producing a desirable microcracked coating of the present invention, a smaller than usual concentration of the selenium compound in the chromium bath can be used to produce the corrosion resistant coating. This is accomplished by plating first a stressed nickel deposit and then a layer of chromium from the chromium bath containing less than the usual concentrations of selenate. The first coating (nickel) is stressed but not cracked and the chromium layer from the selenium-containing bath is then laid down, thereby producing enough stress to crack the chromium when, for example, the deposit is heated as by dipping the coated substrate in hot water, e.g. boiling water or water at F. Certain selenium-free chromium plating solutions may be employed. The usual chromium plating solution produces a moderately stressed deposit and adding the selenium reduces throwing power. Adding less selenium gives better results as indicated above. The highly stressed nickel deposit makes possible omission of some or all the selenium without substantial loss in throwing power, still getting the improved throwing power as if selenium had not been used. The smaller than customary amount of selenium in the chromium bath does not result in the very large loss of throwing power caused by the normal amount of selenium. Further, the objectionable blue color of the chromium deposit from the usual selenium-containing bath is avoided or reduced.

According to one method of the present invention, a single additive (other than an antipitter which may be used if desired) to the Watts type nickel bath may be used to provide a stressed nickel of improved lustre. Small concentrations of certain additives yield a high degree of tensile stress in nickel deposits when sulfooxygen control agents are absent. Using a smaller than usual concentration of the selenium compound in the bath used for deposition of chromium over the stressed nickel, the lesser amount of selenium results in a laminate which will crack in a desirable fine pattern and good corrosion resistance will result.

In the laminates according to the present invention, it is important that the stress in both the nickel deposit and the chromium deposit be such that microcracking of the laminate occurs. Microcracking may occur spontaneously toward the end of or after electrodeposition of the chromium, or by immersion in hot water, or during outside exposure, or during accelerated corrosion testing.

Although the reasons for the appearance of stress in chromium deposits have been the subject of much experimentation and speculation, there is no general agreement as to the cause. It has been suggested that a codeposited chromium hydride or absorbed hydrogen may be responsible for the stress.

In our tests, we have found that when a stressed laminate of our present invention is inserted into boiling water, there is an immediate evolution of gas and the deposit is thereafter microcracked. A usual chromium deposit does not evolve gas under the same circumstances.

It is important that the microcracking show a pattern of the order of 300 to 3000 or more cracks per lineal inch, preferably from 700 to 2000 cracks per lineal inch. The crack pattern is controlled by the degree of prestressing. Numerous methods can be used to prestress the nickel deposits. Examples are hereinbelow stated.

A microcracked deposit as herein considered, within the specified limits of 300 to 3000 cracks per lineal inch, contains a random pattern of interlocking cracks which cannot be seen directly under the microscope using a magnification of the order of 150 times. In order to determine that microcracking has occurred, use is made of the Dubpernell test in which the composite electroplate including a top layer of chromium is made cathodic in an acid copper sulfate solution at low current density. Copper is deposited only at the microcracks and not on the uncracked area where it is believed the chromium is covered by an oxide film.

In the case of thin deposits of chromium over nickel Where microcracks develop, it is known that cracking extends to or into the nickel layer. With thicker deposits of chromium, the microcracking may not necessarily extend to the nickel deposit in all cases. Under these circumstances, copper is also deposited at the microcracks in the chromium using the Dubpernell test. In this case, it is believed that the oxide film does not cover the bottom of the crack.

Tests have showed that where microcracking occurs within the limits herein specified, improved protection against corrosion is secured.

In this specification and in the appended claims nickel includes cobalt and nickel-cobalt codeposits, but nickel is preferred.

All plating solutions herein described are aqueous.

It is contemplated that the hereindescribed laminates of stressed nickel and chromium may be applied over a variety of substrates, including a usual bright nickel substrate or a semibright nickel substrate, said bright or semibright nickel substrates being electrodeposited over metallic surfaces generally, for example, iron or steel. The stressed nickel may be applied upon metals or conductive surfaces quite generally; for example, cobalt, nickel, copper, brass, or other metals, and alloys of two or more thereof.

A two-layer deposit according to the invention may be laid down by depositing a layer of stressed nickel on any of the substrates indicated above. On the stressed nickel layer there may be deposited a layer of stressed chromium. The nickel layer may be stressed by means of an additive to the solution. The chromium layer, likewise, may be stressed by an additive to the solution in which it is produced. The stressed nickel producing solutions may be produced, for example, by using as an additive an amine borane compound, or a pyridinium compound, or a quinolinium, or isoquinolinium compound. These additives in the nickel bath, and selenium compounds, for example, in the chromium bath cooperate to make a deposit plate more uniformly over the work be more free from objectionable bluish color. The selenium may be partly or completely omitted from certain chromium solutions while retaining many of the benefits of the invention. It is preferred, however, to incorporate a limited concentration of selenium into the chromium plating solution.

The process for producing the desirable laminates is critical in three respects. First, the chromium solution either with or without selenium must yield a stressed deposit, sufliciently stressed to cooperate with the nickel deposit to yield a microcracked laminate. Second, the stressed composite must yield a double stressed laminate capable of cracking in a fine pattern of from about 300 to 3000, or more, cracks per lineal inch. Third, the nickel deposit from the nickel plating solution must yield deposits which will be stressed such that the desirable crack pattern of from about 300 to 3000 or more cracks per lineal inch is obtained. Any additive to the nickel solution can be used to produce a nickel deposit which can be used in connection with a chromium deposit taken from a chrome solution comprising chromic acid and sulfate ion, provided the deposit is sufficiently stressed. The nickel deposit should be stressed prior to cracking to the extent of at least about 30,000 pounds per square inch (rigid strip method). The stressing of the deposits can be varied by adjusting the amount of additive in the solutions or as hereafter described.

In addition to the prior art above cited, attention may be called to US. Pat. No. 2,658,867 which describes a cracked deposit having of the order of 50 to 200 cracks per lineal inch when overplated with .01 mil of chromium from a standard chromium bath. U.S. Pat. No. 2,644,789 discloses the use of pyridinium compounds as brighteners in connection with sulfo-oxygen carriers, for the deposition of bright nickel. When such a bright nickel is overplated with a standard chromium deposit, a microcracked deposit does not develop even on heating to 200 F. No nickel deposits suitable for the present invention have been described in such disclosures so far as applicants are aware.

Sulfo-oxygen carriers (control agents), such as benzene sulfonate and saccharin, are undesirable in the solution used to produce stressed nickel of the present invention since they lower the tensile stress. If used in the bath, the concentration of the carrier should be lower than customary in a bright nickel bath and should not exceed 0.1 gram per liter.

The Watts type of nickel plating solution is preferred when stress-inducing agents are used. It may, for the purposes of this invention, consist of nickel sulfate in high concentration, nickel chloride in lesser concentration, and boric acid also in lesser concentration. Sodium lauryl alcohol sulfate may also be included but is not essential since the bath will function more or less well without it. In addition to the Watts bath, other baths ranging from all sulfate to all chloride solutions may be used. Baths containing sulfamates may be used. Alkaline solutions may be used as well as nickel fiuoborate, and many more as basic nickel plating solutions.

Two such groups of nitrogen compounds which may be suggested as preferable are the amine boranes and the pyridinium, quinolinium, and isoquinolinium compounds. A number of specific examples of these borane compounds are listed in Table I.

The invention contemplates preferably the two-layer laminate coatings, both layers together under stress high enough to allow microcracking and the process of producing the said laminates with suitable stress, and cracking such laminates so stressed. Further, more specific and preferred features of the invention are:

producing the laminates by the use of a suitable stressinducing agent in the chromium plating solution, and producing the laminates by the use of a Watts type bath containing a compound of the class consisting of the amine boranes, such as those listed in Table I, and pyridinium, quinolinium, and isoquinolinium compounds, such as those listed in Table II, and miscellaneous compounds, such as those listed in Table HI.

In the process of manufacturing laminates according to the invention, the thickness of the deposits and the temperatures of electrodeposition are important although considerable and numerous variations can be utilized. For best results the nickel deposit should be plated to a thickness of from 0.03 to 0.52, preferably 0.05 to 0.25 mil at a temperature in the range of 60 F. to 160 F. The layer of chromium should be of a thickness from to 50 millionths of an inch applied in the temperature range of from 90 F. to 150 F. When the deposits have been applied, the laminate preferably should be heated to a temperature in the range from 180 F. to 450 F. in a period of time from 5 to 120 seconds. It may be desirable to alternately chill and heat the composite.

The following specific compounds as set forth below may be used in the production of the laminates above described:

TABLE I In general, the boranes listed in Table I, should be present in solution in concentrations from 0.5 to 2.0 grams per liter.

TABLE II B cranes Preferred concentration grams per liter 1. Trimethylamine borane (CH )3N:BH;

2. Dimethylamine borane (CH )2HN:BH

3. Tertiary butylamine borane (CH CNH2:BH

. Morpholine borane O HZBHQ Pyridine borane C5H5N:BH Picoline borane O H O NH:BH

7. Dimethyl propylamine borane Aniline borane C H NH2:BH Dimethyl amine dimethyl boraue (CH3)2NH:BH(CH3)2 10. Morpholine diethyl borane O 12. Dimethyl dodecyl amine borane 14. Piperazine diborane HEB :NH

15. Benzimidazole diborane The concentrations of the compounds listed in Table II hereof should be maintained in solution in the plating bath to the extent of from 0.01 to 1.0 gram per liter, the amount used for preferred results depending on the type of compound employed.

In addition to the above classes of compounds, various miscellaneous classes and type of stress producing agents may be used as listed in Table III with the preferred amounts shown.

TABLE III (These are representative, not restrictive) Grams per liter Gelatin .2-.5

Thiourea .01-.15

Acetonitrile .1-1.0 Ethylenecyanohydrin .1l.0 Succinonitrile .1-1.0

Lactonitrile .1-1.0

Cyanoethoxypropyne .005-.05 Thianaphthenedioxide .05l.5 3-thianaphthenone-l-dioxide 305-15 N-allyl-4-nitro-5 (-3'pyridinium)pyrazole iodide .005.015 H PO .33.0 S602 .2.5 TeO .2-.5 3-sulfolene 5-2.0 3-thiocyauopropane-l-potassium sulfonate .52.0 3-nitropropane sodium sulfonate .52.0 3-(dimethylsulfoxonium)propane sodium sulfonate .5-2.0 N-allyl quinaldinium bromide .006-.1 Polyalkylene amines (M.W. 100-1000) .01.2 ,Polyalkylene amine adducts with acrylonitrile,

acetylenic compounds .01-.2 5-nitroindazole .02-.05

NaNO;; 1 .5-1.0 2-butyne-1,4 diol 2 and ethoxylated butynediol 2 .9-1.5 5-amino-2-mercaptobenzimidazol .01.15 Polyglycols (M.W. 5 10,000) .02-1.0 Diethyleneglycolmonopropargyl ether .05-.2 p,p-Methylene-bis-triphenylphosphorium bro mide .02.2

1-amino-2 propyne .6l.5 2-styryl quinoline .006-.06 Ethyl-bis-(B-cyanoethyl)-sulfonium ethyl sulfate .005.

B,B'-thiodipropionitrile .003-.03 Diallylpropargylamine 2 .05.l Triethylpropargylammoniurn chloride .l-.5 3-(B-oxypropionamide) propyne-1 .l-5.

Nitrates in low concentrations (.05.3) may actually docrense tensile stress. Higher concentrations increase Ithe stress.

2 Deposits obtained by the use of acetylenics are frequently mlsplated or striated. These defects can be eliminated by the addition of certain other agents, but in so doing the tensile stress is frequently reduced. Therefore, when they are used to produce the highly stressed nickel needed in this invention, it is necessary to overcome these defects by the use of a high current density, i.e. minimum of 40 a.s.f.

Basic nickel plating solutions to which additions according to the present invention may be added are as follows:

TABLE IV All sulfate NiSo -7H O 100 to 400, preferably 200 to 300 grams Boric acid 0 to 60, preferably 10 to 40 grams.

H O to make 1000 cc.

8 High chloride NiSO -6H O 75 to 225, preferably to 200 grams. NiCl -6H O 50 to 150, preferably 75 to 150 grams. Boric acid 0 to 60, prefrably 10 to 40 grams.

H O to make 1000 cc.

Sulfate-chloride (Watts type) NiSo -7H O 100 to 400, preferably 200 to 300 grams.

NiCl -6H O 10 to 60, preferably 25 to 40 grams.

Boric acid 0 to 50, preferably 15 to 40 grams.

H O to make 1000 cc.

The basic chromium solution may be a water solution of chromic acid with sulfate ion to the extent of about 0.6 to 1.5 percent of the chromic acid. To this may be added, to produce microcracking in combination with a stressed nickel, a selenium compound, such as Na SeO or an organic compound, such as AS203.

Other baths, for example those containing strontium and fluosilicate ions may be used.

The following chromium solution (See Table V) produces a good microcrack with prestressed nickel but not with unstressed nickel. The stress in the nickel deposit was obtained by use of a bis-pyridinium compound additive in the Watts bath. The bis-pyridinium compound may be used at a concentration of preferably from 0.1 to 1.0 gram per liter.

CrO 250-375 grams per liter H 80 2.5 grams per liter Na SeO 0.005 grams per liter Temperature F.

Cathode current density a.s.f. for 10 minutes This solution may also be used to produce microcracking over stressed nickel stressed by use of an amine borane.

Sodium selenate, for example, may be used in the above solution. It may be added in concentration from no selenium to about 0.10 gram per liter: preferbaly, about 0.0025 to 0.0075 gram per liter should be used. (Note: sodium selenate of the order of 0.015 gram per liter should be used to produce microcracking over ordinary bright nickel.)

Groups of additives have been set forth by way of examples as to how to produce the stressed nickel deposits of the invention. One of these groups is the amine boranes; another group is the bis-pyridiniums, bis-quinoliniums, and bis-isoquinoliniums. There is also a miscellaneous group (Table III.) The plating conditions have been stated suificiently to enable the chemist skilled in the art to produce the stressed deposits. The nickel is first deposited having requisite stress, after which there is applied a coating of chromium having the requisite stress. The chromium solution may contain selenium as above stated. Having applied both layers, with proper stress, the composite will microcrack during the chromium plating step, shortly thereafter on mild heating, or on corrosion testing, such as the Corrodkote test, or on outdoor exposure (which may take an undesirably long time.) Heating with hot water apparently is the most practical cracking step.

It is generally agreed that cracking of a chromium deposit is due not only to the stress, ductility, and tensile strength of the chromium deposit but is also affected by the stress of the immediately underlying substrate. There is also evidence that to a large extent, the stresses are additive, so that the stress in the chromium deposit may not be high enough to fracture the chromium but the additive effect of the stresses in chromium and nickel will cause cracking of the chromium. It is well known, therefore, that when a standard chromium deposit is applied .over a higher than normally stressed nickel deposit (20,000'-30,000 p.s.i.) cracking or crazing occurs but this is macrocracking and is visible and unsightly. We have discovered, however, that when both the nickel and chromium of this laminate are highly stressed the macrocracking is avoided and the desired microcracking is obtained.

The values obtained for stress of electrodeposits are affected by nonreproducibility of calculated stress values when measured by different methods, the nature of the substrate, the thickness of the deposit, and the cracking of the deposit during the plating process which relieves the apparent stress.

For a given Watts solution, the rigid strip method of measuring stress will give values of 12,000 to 17,000 p.s.i.; the helical contractometer method for the same solution gives values of 20,000 to 27,000 p.s.i. For an all chloride solution, the following values may be obtained:

P.s.i. Rigid strip 33,000-41,000 Contractometer 53,000-62,600

If there is no cracking, the stress of a deposit will depend on its thickness primarily because of the effect the substrate structure has on the deposit structure. This eflect may disappear after 500 angstroms or still be present at thicknesses of .1 to .2 mil. In most cases (see C. Williams, Met. Finishing J. 8 (85) (1962)) for nickel or chromium on their normal substrates (Fe, Cu, Ni), the tensile stress will decrease with increase in thickness. It is possible, however, for the reverse effect to occur (see 11. Watkins, J. Electrochem Soc., November 1961.)

It is well known that measurement of stress in chromium deposits is fraught with difficulties due to the cracking of the chromium deposit during deposition. Brenner (Proc. Am. Electroplaters Soc. p. 32, 1947) reports 80,000 p.s.i. for very thin uncracked deposits, but 17,000 p.s.i. for thicker deposits which have evidently relieved themselves by cracking during the plating process. This same effect can occur with nickel deposits which are highly stressed. For example, an 0.25 mil deposit of nickel from the nickel solution of Example II has a stress of about 40,000 p.s.i., but a 1 mil deposit shows a stress of only 15,100 p.s.i. and this is primarily because fine cracking has occurred in the latter case.

For the above reasons, it is difiicult to specify stress values which are desirable for the stressed nickel and stressed chromium of this invention. The stressed nickel fiash is best described operationally as one which will effect microcracking when a higher than normal stressed chromium deposit is applied over it and will not give microcracking when standard chromium is applied over it. Likewise, the stressed chromium deposit is described operationally as one which will give microcracking when applied over a higher than normal stressed nickel depcsit but will not give microcracking when applied over a normal nickel deposit such as that from a Watts bath at pH 3.5 or such as that from a bright nickel solution containing a sulfo-oxygen control agent.

The stressed nickel deposit should not be too thick; if greater than 0.15 to 0.2 mil, it may tend to crack before chromium plating and give undesirable macrocracking visible to the naked eye. Provided the nickel flash is under enough stress, finer microcracking is obtained, after chromium plating, for thin nickel deposits. Stressed nickel deposits in the order of 0.1 mil or less are preferred. The stress of the nickel deposit when measured by the rigid strip method and at 0.2 mil should be at least 30,000 p.s.i.

Another practical reason for the use of thindeposits is that in some cases, especially where the stress inducing agents are not also brightening agents, or where special 10 addition agents are not even used, the deposit will tend to become dull with increasing thickness. In these cases, the stressed nickel deposit should not be thicker than 0.05 mil.

Addition agents are not always needed in order to obtain high tensile stresses in the nickel deposit. In general it has been found that the stress in the nickel deposit, as evidenced by the ease of obtaining microcracking when the stressed chromium is applied, increases with increase in chloride content of the solution, with decrease in temperature, and with increase in current density. The pH should be either high (greater than 5.0) or low (less I than 2.0). Also, the acid solutions should not be too highly buffered. Alkaline nickel solutions, such as described in U.S. 2,773,818, US. 2,069,566, British 512,484, and British 880,786 may be used, although simple solutions containing only nickel citrate or tartrate are as effective as some of those solutions proposed for heavier nickel deposits. The addition of acetylenic type brighteners, such as butynediol to alkaline solutions, further increases the tensile stress in the nickel flash and excellent corrosion results have been obtained.

Representative nickel solutions which may be used without stress inducing agents are given in Table V:

TABLE V NiSO -6H O grams per liter. H BO 7 grams per liter. pH 5.5. Temperature 40 C. C.D. 100 a.s.f.

NiSO -6H O- 50 grams per liter. pH 1.7. Temperature 30 C. C.D. 50 a.s.f.

N1Cl '6H O 300 grams per liter. H BO 7 grams per liter. pH 1.7. Temperature 30 C. C.D. 200 a.s.f.

NiCl 6H O 200 grams per liter. H BO 22 grams per liter. pH 5.5. Temperature 30 C. C.D. 200 a.s.f.

NiSO- '6H O 200 grams per liter. NiCl '6H O 60 grams per liter. H BO 30 grams per liter pH 5.8. Temperature 50 C. C.D 20 a.s.f.

NiSO '6H O 200 grams per liter. NiCl '6H O 60 grams per liter. H BO- 30 grams per liter. (NH SO 35 grams per liter. pH 1.5. Temperature 30 C. C.D 50 a.s.f.

NiSQ -6-H O 100 grams per liter. Na citrate 200 grams per liter. pH 6.5. Temperature 50C.

C.D 40 a.s.f.

TABLE VCont-inued NiSO -6H O 100 grams per liter. Na citrate 66 grams per liter. NH Cl grams per liter. EDTA 50 grams per liter. Triethanolamine 50 ml. per liter. Ni(NO 20 grams per liter. pH 9.0. Temperature 120 F. OD. 50 a.s.f.

While the microcracked composite coating may be applied directly to a basis metal, such as steel, copper, etc., we believe it will find its greatest use when applied over a substrate of bright or semibright nickel coatings, or combinations of semibright and bright nickel electrodeposits. Thus, the composite coating of the invention may be deposited over a bright nickel electroplate, such as deposited according to U.S. Pat. 2,712,522, or preferably over a duplex nickel deposit comprising a semibright electroplate according to, for example, U.S. Pat. 2,635,076, followed by a bright electrodeposit according to, for example, U.S. Pat. 2,978,391. The composite according to the present invention may also be used with a triplate nickel coating according to U.S. Pat. 3,090,733.

The following specific examples will help to illustrate the invention:

EXAMPLE I Over a 1 mil thick electrodeposit of bright nickel on steel, an additional electrodeposit of stressed nickel was laid down (to a thickness of 0.1 mil.) This stressed nickel was deposited from an aqueous solution made up as follows:

NiSO -6H O 240 grams per liter. NiCl '6I-I O 40 grams per liter. H BO 40 grams per liter. pH 3.5.

Cathode current density 40 a.s.f. Temperature 140 F. Trimethylamine borane 1.0 ml. per liter.

Onto the above cited stressed nickel deposit, we plated chromium to a thickness of 20 millionths of an inch, from a bath containing:

CrO 250 grams per liter. H 80 2.5 grams per liter. Na SeOg, .005 grams per liter. Temperature 100 F.

Current density 140 a.s.f.

EXAMPLE II Over a bright nickel deposit on steel, as cited in Example I, we electroplated a 0.1 mil deposit of stressed nickel from a bath as cited in Example I except that the amine borane was replaced by 0.4 gram per liter of N,N-trimethylene-bis-pyridinium bromide. Over the stressed nickel was then deposited chromium as cited in Example I. Results were similar to those reported for Example I.

EXAMPLE III Using the bright nickel on steel followed by the stressed nickel electrodeposit of Example II, we deposited 30 12 millionths of an inch thickness of chromium from the following bath:

CrO 375 grams per liter. NaOH 50 grams per liter. Cr O 7 grams per liter. H 0.7 grams per liter. Temperature F.

Current density a.s.f.

A good microcracked pattern was secured on dipping for one minute in water at 200 F.

No microcracking occurred when chromium from the above bath was deposited over bright nickel directly, omitting the stressed nickel deposited from the bath containing the pyridinium bromide compound.

EXAMPLE IV Using the bright nickel on steel followed by the stressed nickel deposit of Example II, we deposited 30 millionths of an inch of chromium from the following bath:

CrO 200 grams per liter.

K Cr O 36.8 grams per liter. SrCrO 4.5 grams per liter.

K SiFe 10.5 grams per liter. SrSO, 6.0 grams per liter.

Temperature 120 F.

Current density 250 a.s.f.

The resulting composite gave a good microcrack pattern after dipping for 2 minutes in water at 200 F.

EXAMPLE V EXAMPLE VI Using the composite nickel electrodeposit on steel, including the stressed nickel layer of Example II, chromium was deposited to a thickness of 30 millionths of an inch in the following bath:

CrO grams per liter. H 80 2.5 grams per liter. As O 3.95 grams per liter. Temperature 118 F.

Current density a.s.f.

After treating the deposit in water at F. for two minutes, microcracking was secured. When the stressed nickel layer was omitted, microcracking did not occur.

EXAMPLE VII Steel plated with 1 mil bright nickel was further plated with 0.1 mil of stressed nickel at 40 a.s.f. and 140 F. from a Watts bath (pH containing 0.015 gram per liter of 2-oxydiethylene-bis-isoquinolinium chloride. When a 30 millionth of an inch deposit of chromium was applied from the bath cited in Example I, microcracking was produced by the hot water treatment and good corrosion resistance was secured.

EXAMPLE VIII Results similar to those of Example II were secured when the pyridinium bromide compound of Example II was replaced by 0.2, 0.4, or 0.6 gram per liter of N,N oxydimethylene-bis-pyridinium chloride.

EXAMPLE IX Excellent corrosion results were secured when 1 gram per liter of morpholine borane replaced the trimethyl amine borane used in depositing the stressed nickel layer 13 in Example I and millionths inch thickness of chromium was deposited from the bath cited in Example I.

EXAMPLE X Excellent corrosion results were secured when 1 gram per liter of morpholine borane replaced the trimethyl amine borane used in depositing the stressed nickel layer in Example I and 10 millionths inch thickness of chro mium was deposited from the bath cited in Example I, when the stressed nickel layer was deposited at room temperature and a.s.f.

EXAMPLE XI Composites showing excellent resistance to corrosion were secured using the following electrodeposits successively over steel.

(a) Semibright nickel 0.8 mil (b) Bright nickel 0.4 mil (c) Stressed nickel 0.1 mil from a Watts bath containing 2 grams per liter of morpholine borane at room temperature and 15 a.s.f.

(d) 10 millionths of an inch of chromium from the bath cited in Example I EXAMPLE XII Same as Example XI except that in (c) the stressed nickel was deposited from a Watts bath containing 0.2 gram per liter ethylene-bis-pyridinium bromide at 140 F. and 40 a.s.f., and the chromium deposit thickness was millionths of an inch.

EXAMPLE XIII Onto a bright nickel deposit on steel, an 0.1 mil deposit of stressed nickel was electroplated from a Watts bath containing 0.5 gram per liter of N,N'-methyl-N'-piperazino-l hydroxy butene, at 140 F. and 40 a.s.f. After further electroplating 20 millionths of an inch of chromium from the bath of Example I, and heating the deposit at 190 F. when microcracking occurred.

EXAMPLE XIV Same as Example XIII except that the Watts bath contained 0.4 gram per liter of 1,6-dimethyl piperazine instead of the piperazino compound.

EXAMPLE XV Onto a bright nickel substrate, there was deposited an additional deposit of stressed nickel according to Example I except that the amine borane was replaced by 1 gram per liter of H PO (phosphorus acid). Onto this stressed nickel was deposited 0.03 mil of stressed chromium from a bath made up and used as follows:

, CrO 80 grams per liter. H 80 0.5 gram per liter. Temperature 130 F.

Cathode current density 100 a.s.f.

After immersing for 1 minute in hot water (100 F.), the deposit was microcracked.

The invention claimed is:

1. A method of depositing a corrosion resistant microcracked duplex coating on a substrate, said duplex coating consisting of chromium over nickel which comprises electrodepositing a smooth continuous nickel layer having high internal stress on a substrate and thereafter electrodepositing a chromium layer on said smooth continuous nickel layer, said chromium layer being sufiiciently stressed so that it interacts with said smooth continuous nickel layer to cause both said nickel and chromium layers to crack into a micro-cracked pattern during or subsequent to the electrodeposition of said chromium layer.

2. A method of depositing a corrosion resistant microcracked duplex coating on a substrate as stated in claim 1 wherein said micro-cracked duplex coating has a crack density in the range of from about 300 to 3,000 cracks per lineal inch.

3. A method of depositing a corrosion resistant microcracked duplex coating on a substrate as stated in claim 2 wherein said nickel layer is of the thickness of from 0.03 to 0.5 mil and said chromium layer is of a thickness from 10 millionths to 50 millionths of an inch.

4. A method of depositing a corrosion resistant microcracked duplex coating on a substrate as stated in claim 3 wherein said micro-cracking is accelerated by moderately heating said duplex coating such as by contacting the same with hot water.

5. A method of depositing a corrosion resistant microcracked duplex coating on a substrate as stated in claim 4 wherein said nickel layer is electroplated from an aqueous nickel plating solution comprising a nickel compound supplying nickel ions and an internal stress producing addition agent selected from the group consisting of amine boranes, pyridinium compounds, quinolinium compounds and isoquinolinium compounds.

6. A method of depositing a corrosion resistant microcracked duplex coating on a substrate as stated in claim 5 wherein said chromium layer is electroplated from an aqueous plating bath comprising chromic acid, sulfate ion and selenate ion.

7. A method of depositing a corrosion resistant microcracked duplex coating on a substrate as stated in claim 4 wherein said chromium layer is electroplated from an aqueous plating bath comprising chromic acid, sulfate ion and selenate ion.

8. A method of depositing a corrosion resistant microcracked duplex coating on a substrate as stated in claim 1 wherein said nickel layer is electroplated from an aqueous nickel plating solution comprising a nickel compound supplying nickel ions and an internal stress producing addition agent selected from the group consisting of amine boranes, pyridinium compounds, quinolinium compounds and isoquinolinium compounds.

9. A method of depositing a corrosion resistant microcracked duplex coating on a substrate as stated in claim 8 wherein said chromium layer is electroplated from an aqueous plating bath comprising chromic acid, sulfate ion and selenate ion.

10. A method of depositing a corrosion resistant microcracked duplex coating on a substrate as stated in claim 1 wherein said chromium layer is electroplated from an aqueous plating bath comprising chromic acid, sulfate ion and selenate ion.

11. A method of depositing a corrosion resistant microcracked duplex coating in a substrate as stated in claim 1 wherein said chromium layer is electroplated from an aqueous plating bath comprising chromic acid, sulfate ion and silicofluoride ion.

12. A method of depositing a corrosion resistant microcracked coating on a substrate which comprises electrodepositing 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 sufiicient to produce micro-cracks therein during the deposition of the chromium layer which in turn produces a micro-crack pattern in the chromium layer.

13. The method in accordance with claim 12, in which the said deposit of nickel is applied to a previous metallic deposit.

14. The method in accordance with claim 13, in which the said previous metallic deposit is selected from the group of electrolytic coatings consisting of semi-bright nickel plating and bright nickel plating.

15. The method according to claim 12 in which the cracking nickel layer is deposited from said aqueous acid nickel bath at a current density of between 15 and 200 amps per square foot.

References Cited UNITED STATES PATENTS Lind et a1. 20449 Nachtman 20437 Shenk, Jr. 20449 Schaer 20437 Mikulski 1061 Zirngiebl et a1. 204-49X Odekerken 20441X de Castelet 20441X l6 FOREIGN PATENTS 9/1961 Canada 106-1 3/1958 France 20451 5 OTHER REFERENCES Graham, A. Kenneth, Electroplating Engineering Handbook, p. 178, 1955.

GERALD L. KAPLAN, Primary Examiner 10 US. Cl. X.R.