Title:
Electroless deposition of cobalt boron
United States Patent 3917464


Abstract:
Improved electroless deposits of cobalt-boron which find particular use in protecting basis metals against corrosive attack, the deposits being prepared from an acid bath having the following preferred composition:



Inventors:
Pearlstein, Fred (Philadelphia, PA)
Weightman, Robert F. (Philadelphia, PA)
Application Number:
05/381121
Publication Date:
11/04/1975
Filing Date:
07/20/1973
Assignee:
THE UNITED STATES OF AMERICA AS REPRESENTED BY THE SECRETARY OF THE ARMY
Primary Class:
Other Classes:
148/252, 427/131, 427/405, 427/438, 428/686, 428/926, 428/936
International Classes:
C23C18/34; (IPC1-7): C23B3/00
Field of Search:
117/13E,13B,71M 148
View Patent Images:
US Patent References:



Other References:

Brenner, A., "Electroless Plating," Modern Electroplating, John Wiley & S, Inc., New York, 1963, pp. 706-707..
Primary Examiner:
Kendall, Ralph S.
Assistant Examiner:
Wolfe Jr., Charles R.
Attorney, Agent or Firm:
Suga, Arthur Edelberg Nathan Gibson Robert M. P.
Claims:
We claim

1. A double layer deposit on steel for providing synergistic protection to said steel against corrosive attack in a saline environment, said double layer deposit comprising an initial electroless nickel-phosphorus layer and a coating thereover of electroless cobalt comprising by weight, 96.0% cobalt, 1.7% boron, 0.97% carbon, and 0.05% nitrogen.

2. Double layer deposit as described in claim 1 further characterized by said nickel-phosphorous layer being about 7.5 microns in thickness and said electroless cobalt layer being about 2.5 microns in thickness.

Description:
This invention relates to electroless deposition and more particularly concerns improved bath compositions for the electroless deposition of cobaltboron.

An object of the present invention is to provide an improved acid bath solution yielding an electroless cobalt-boron alloy deposit which is sound, decorative, and free of pits and cracks.

Another object of the invention is to provide such an alloy deposit which is particularly useful in protecting basis metals against corrosive attack.

Still another object of the invention is to provide such an alloy deposit which may readily be chromate treated to provide exceptional resistance to tarnishing.

Yet another object of the invention is to provide such an alloy deposit which is useful for providing magnetic deposits of low coercive force.

These and other objects of the invention will become apparent as the invention is further described hereinafter.

We have discovered an improved acid electroless cobalt-boron plating bath consisting of cobalt sulfate, sodium succinate, dimethylamine borane (hereinafter referred to as DMAB), and sodium sulfate. The succinate prevents formation of highly stressed, cracked deposits, which characterized prior art cobalt-boron deposits made from acetate-containing solutions. Even with the improved cobalt-boron deposits provided by succinate baths, however, pitted deposits resulted therefrom which offered only minimal resistance to corrosive attack of the basis or substrate metal. We have discovered that sodium sulfate very substantially reduces pitting in cobalt-boron alloy deposits.

More specifically, improved bath compositions depicting our invention are shown below:

TABLE I ______________________________________ Electroless Co-B Plating Bath Composition Preferred Effective Constituent Concentration g/l Range, g/l ______________________________________ Cobalt sulfate.7H2 O 25 15-45 Sodium succinate.6H2 O 25 10-30 Sodium sulfate 15 10-50 DMAB 4 1-6 ______________________________________

The bath will normally be used at a pH of about 5.0 and about 70C.

Electroless cobalt-boron deposition rates were only slightly affected by cobalt concentration within the range of 15 to 45 g/l; reduced deposition rates resulted beyond those limits.

DMAB concentration on deposition rate was determined from solutions containing 25 g/l cobalt sulfate .7 H2 O and 15 g/l sodium succinate .6 H2 O. The baths were adjusted to pH 5.0 with H2 SO4 solution and were operated at 80C. Deposition rates increased essentially linearly with DMAB concentration to a maximum of about 4 g/l. Above this concentration, catalytic particles of cobalt formed in the solution which settled and resulted in rapid bath decomposition.

The effect of succinate concentration on deposition rate was determined from solutions containing 25 g/l cobalt sulfate .7H2 O and 4 g/l DMAB at pH 5.0 and 80C. When no succinate was present, spongy cobalt formed within the solution accompanied by rapid bath decomposition and virtually no deposition was produced on palladium-activated copper test specimens. Satisfactory bath stability however was provided by the presence of 10 g/l, up to about 30 g/l, of the succinate in the bath. The succinate also provided buffering action, i.e., prevented rapid change in bath pH during deposition. Deposition rates were highest and essentially constant at succinate hexahydrate concentrations of about 15 to 30 g/l, with about 25 g/l appearing to yield the best deposition characteristics.

15 g/l or more of Na2 SO4 provided effective anti-pitting characteristics to the deposits. At Na2 SO4 concentration of 10 g/l or less, pitting could be observed in the final cobalt-boron alloy deposit.

The effect of bath pH on deposition rate at 80C was determined. Deposition rate increased markedly with an increase in bath pH, but above 5.5, baths tended to be unstable and deposits formed on the walls and the bottom of the beaker.

It is interesting to note that baths at pH 4.5 and lower, increased in pH during the deposition tests, while baths at pH 5.5 and above, decreased. Normally, electroless plating baths decrease in pH during use because hydrogen ions will be formed as a product of the electroless reaction. DMAB, however, is subject to acid catalyzed hydrolysis: (CH3)2 HNBH3 +3H2 O+H+➝C2 H6 NH++ H3 BO3 + 3H2

Indeed, we have found that even when no electroless plating was occurring, baths (80C) at pH 4.5 and below increased in pH with passage of time and that gassing was evident within the solutions. At bath pH of 5.0, our baths were very stable, and little or no change in pH resulted during deposition. Unless otherwise noted, all bath compositions hereinafter used and tested had a pH of 5.0.

The rate of cobalt-boron deposition increased with increasing bath temperature in a manner typical of most electroless plating baths. At 90C or above, the bath was subject to spontaneous production of cobalt-boron particles and rapid decomposition. At 80C, the bath was stable during one hour plating tests. A tendency for catalytic particle formation in the bath was observed during more extended plating periods at 80C causing bath decomposition. The bath will be less susceptible to decomposition if trace amounts of a catalytic poison such as 1 to 2 mg/l of lead acetate trihydrate, or thiorea, are introduced therein. However, at bath temperatures of 80C and above, consumption of DMAB by hydrolysis is significantly wasteful and it is thus advisable, from a practical standpoint, to operate the bath at lower temperatures unless very high deposition rates are needed. At 70C, the bath is operable without the necessity for addition of catalytic poison stabilizers and hydrolysis losses are minimized.

Deposition rates are comparatively low at bath temperatures of 40 C and lower. The baths are usable even at room temperatures of 22 to 27C for applying thin, electrically conductive deposits to palladium-activated nonconductors, provided the bath pH is increased to about 6.3.

Substrates that are sufficiently catalytically active to spontaneously initiate deposition by immersion in our preferred acid electroless cobalt-boron plating bath, at pH 5.0 and 70C, are steel, electroless nickel, palladium and gold. Aluminum is spontaneously plated by a displacement deposit of cobalt initially formed on the metal during immersion in the bath.

Copper, brass, silver, platinum, titanium, stainless steels and nonconductors usually do not initiate electroless cobalt-boron deposition unless one of the following steps is taken:

a. activation of the surfaces by nucleation with a catalytically active metal such as palladium, for example.

b. contacting the metal in the electroless plating bath with an actively plating metal which initiates deposition by galvanic action.

c. momentary application of sufficient cathodic current to the metal in the electroless plating bath to apply a thin cobalt-boron film electrolytically, thus initiating deposition by electrolytic action.

Our electroless cobalt-boron alloy deposit, when made from our acid bath, pH 5.0, 70C, of preferred concentrations, has the following composition, by weight:

Co 96.0% B 1.7% C .97% N .05%

Carbon and nitrogen indicate the presence of organic or organometallic compounds in the deposit.

Our electroless cobalt-boron deposits possessed a hardness of 270 kg/mm2 (Vickers) in the as-plated condition. After heating the deposit however, for 24 hours at 250C, hardness of the deposit increased to 480 kg/mm2. Additional heating at 250C further increased hardness to a maximum of about 640 kg/mm2 after a total heating period of 4 days.

The coercive force of our electroless cobalt-boron deposits was generally constant, at about 8 oersteds, over the range of thicknesses tested, i.e., at 17500 to 47100 Angstroms.

The salt spray corrosion resistance of steel plated with our deposits is compared to that of electroless nickel-phosphorus plated steel, also highly resistant to salt spray, and, is presented in Table II below:

TABLE II __________________________________________________________________________ Salt Spray Exposure Test Corrosion Rating* Deposit After Exposure to Salt Spray Deposit Thickness, Microns 24hrs 48hrs 72hrs 96hrs 168hrs __________________________________________________________________________ Electroless Co-B 3 5 4 3+ 3(sl.E) 2(E) 5 5 5 4 3+ 3(sl.E) 10 5 5 5 5 3+(sl.E) Electroless Ni-P 3 4(E) 4(E) 4(E) 3+(E) 2+(E) (from acid-type bath) 6 5(E) 5(E) 5(E) 5(E) 5(E) 12 5 5(E) 5(E) 5(E) 5(E) __________________________________________________________________________ *Ratings: 5 = no basis metal attack 4 = traces of basis metal attack 3 = slight basis metal attack 2 = moderate basis metal attack 1 = considerable basis metal attack E = edge corrosion sl. E = slight edge corrosion

All results are expressed as an average of 3 tested specimens. Salt spray exposure test is described in ASTM Designation B-117-64, 31 August 1964, Standard Method of Salt Spray Testing.

As is apparent from Table II, each of the electroless cobalt-boron or nickel-phosphorus deposits were protective to steel for 48 hours salt spray exposure. With longer exposure times, the protective value of the coatings tended to improve as coating thickness increased. Edge corrosion of the substrate was very pronounced with the electroless nickel-phosphorus plated steel whereas the electroless cobalt-boron deposits provided good resistance to substrate edge corrosion. Good edge corrosion resistance of substrate metals is vital when used in sensitive mechanisms such as fuses and other military items where any dislodging of corrosive products could readily cause serious malfunctioning. Secondarily, discoloration due to edge corrosion, incipient or advanced, might be considered aesthetically unappealing.

The corrosion potential of electroless cobalt-boron deposits, immersed in 50 g/l NaCl solution for 24 hours is -0.60 volts versus the saturated calomel electrode (S.C.E.). In the same solution, steel has a corrosion potential of -0.63 volts (S.C.E.) and electroless nickel-phosphorus has a corrosion potential of -0.35 volts (S.C.E.). Our cobalt-boron deposit, with a corrosion potential of -0.60 volts (S.C.E.) is substantially more compatible electrochemically with steel than the electroless nickel deposit having a corrosion potential of -0.35 volts (S.C.E.). Parenthetically, a deposit having a corrosion potential identical with the steel under these conditions would not accelerate corrosion of the steel substrate. Thus, our cobalt-boron deposit, having a more favorable corrosion potential than electroless nickel deposits, will tend to better resist corrosion of basis metal exposed at any coating defect or pore by adverse galvanic cell action.

A double-layer deposit of electroless nickel-phosphorus followed by a coating of electroless cobalt-boron will provide synergistic protection to the basis metal because of the ability of the cobalt to sacrificially protect the nickel layer from corrosive penetration. Double-layer deposits, i.e., 7.5 microns of electroless nickel and 2.5 microns of electroless cobalt-boron, on a substrate of steel were found capable of preventing basis metal attack (even at edges) after 168 hours salt spray exposure and were superior to the same total thickness of either layer alone.

Electroless cobalt-boron deposits tarnish rapidly during salt spray exposure to produce a tenacious, mottled blue-brown film. Some success was achieved in providing a black non-reflective surface so often desired in military equipment by immersing the deposit for 10 minutes in a 10 g/l potassium persulfate solution at 25C.

Immersion of the cobalt-boron deposit in a solution consisting of 200 g/l Na2 Cr2 O7.2H2 O and 6 ml/l H2 SO4, sp. gr. 1.84, greatly improved the resistance of the deposit surfaces to tarnishing, the surfaces remaining bright and untarnished after 72 hours salt spray exposure. The above solution however, is mildly corrosive to the cobalt-boron deposit and will remove about 0.1 micron of deposit thickness during a 10 second immersion. No visible film is produced by this treatment.

In the production of our improved cobalt-boron alloy deposit, the following steps should be observed:

a. Remove any organic contaminants such as oils, greases, etc. from the metal substrate surfaces by soak-alkaline cleaning, or if heavy contamination exists, a vapor degrease prior to soak-alkaline cleaning may be desirable. Electro-cleaning operations may also be applied.

b. Oxide or corrosion products will now be removed from the substrate surfaces, such as by immersion in a 50% (vol) HCl solution. A 10% (vol) H2 SO4 dip, or a chemical polishing operation, is satisfactory for copper or copper alloy substrates. Aluminum may be immersed in 50% (vol) HNO3 for removal of oxides and a thin zinc immersion deposit on aluminum by treatment in zincate solution is desirable for rapid, uniform coverage with cobalt.

c. The cleaned substrate metal may now be directly immersed in our solution, or if the metal does not initiate electroless cobalt-boron deposition, then one of the steps outlined above for this class of substrates should be followed. Deposit thickness will depend, among other things, upon the amount of time the metal is permitted to remain in the bath.

There is set forth hereinbelow for purposes of illustration, examples of our cobalt-boron deposits prepared under varying conditions:

EXAMPLE I ______________________________________ Bath composition Preferred Substrate Steel Bath temperature 70C Bath pH 5.0 Deposition time 1 hour Thickness of deposit 13 microns (about 0.5 mil) EXAMPLE II Bath composition Preferred (1 mg/l lead acetate added) Substrate Copper Bath temperature 80C Bath pH 5.5 Deposition time 1 hour Thickness of deposit 25 microns (about 1.0 mil) ______________________________________

It is to be noted that substantially identical deposit thicknesses will result after a given amount of time, regardless of the substrate metal used, so long as the metal is prepared in accordance with the aforementioned teachings, i.e., cleaning and insuring that the metal is made receptive to electroless cobalt-boron deposition.

EXAMPLE III ______________________________________ Bath composition Preferred Substrate Palladium-activated glass (glass initially abrasive blasted for better deposit adhesion) Bath temperature 25C Bath pH 6.3 Deposition time 30 min. Thickness of deposit 0.3 micron (approx) ______________________________________

We wish it to be understood that we do not desire to be limited to the exact details described for obvious modifications will occur to a person skilled in the art.