Note: Descriptions are shown in the official language in which they were submitted.
1~9~86
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CORRO S I ON/WEAR -RES I STANT METAL
COATING COMPOSITIONS
Backaround of the Invention
This invention relates to novel metal coatings
which exhibit exceptional resistance to corrosion and
wear. ~ore particularly this invention relates to metal
coatings containing nickel, cobalt, boron and thallium
and to the reductive deposition of said coatings on the
surfaces of substrate articles from aqueous solutions at
high p~.
The plating or deposition of metal alloys by
chemical or electrochemical reduction of metal ions on
the surface of an article to modify its surface
characteristics for both decorative and functional
purposes is well known in the art. Of particular
commercial significance is the deposition of metal/metal
alloy coatings on both metal and activated non-metal
substrates to enhance surface hardness and resistance to
corrosion and wear. Nickel-boron and cobalt-boron alloy
coatings are recognized in the art for their hardness
and associated wear-resistance. The patent literature
reflects an ongoing research and development effort in
the area of nickel-boron/cobalt-boron coatings with the
goal of producing still harder, more corrosion
resistance coatings. See, for example, U.S. Patents
3,738,849; 3,045,334; 3,674,447; and 2,726,710. 8ellis
U.S. Patent 3,674,447 describes nickel-boron and
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cobalt-boron coatings of improved hardness containing
controlled amounts of thallium dispersed throughout the
coatings. It has now been discovered that coatings
containing both nickel and cobalt in combination with
boron and thallium exhibit marked advantages over the
thallium-containing nickel/boron or cobalt/boron
coatings des~ribed by ~ellis. Mètal alloy coatings in
accordance with the present invention containing boron,
thallium and nickel and cobalt are more wear resistant
and remarkably more corrosion resistant than those
described in the prior art.
Electroless coatings containing both nickel and
cobalt are described in U.S. Patents 3,378,400 and
3,342,338. However in each of those patents a
hypophosphite, and not a boron-containing reducing agent
was used to deposit said coatings. Similarly
U.S. Patent 3,562,000 exemplifies deposition of a metal
coating from a bath containing both cobalt chloride and
nickel chloride using sodium hypophosphite. Although it
is disclosed in that patent that other suitable reducing
agents, including borohydrides, could be used in the
numbered examples in place of the preferred
hypophosphite, there is provided no description of the
improved coatings in accordance with this invention.
It is therefore a general object of this
invention is to provide improved metal coatings
containing both nickel and cobalt, boron and thallium.
A further object of this invention is to
provide an article of manufacture coated on at least a
portion of its surface with a hard, ductile, wear and
corrosion resistant metal coating comprising nickel and
cobalt, boron and thallium.
Still a further object of this invention is to
provide a heterogeneous electroless metal alloy coating
containing both nickel and cobalt, boron and thallium
having a metal concentration gradient in thickness
cross-section.
Another object of this invention is to provide
an electroless metal alloy coating presenting a
corrosion and wear resistant surface comprising
amorphous nodular deposits of nickel, cobalt, boron and
thallium.
Yet another object of this invention is to
provide coating baths from which a hard, ductile, wear
and corrosion resistant coating can be deposited on at
least a portion of the surface of a metal or activated
non-metal substrate.
Those and other objects of this invention will
be apparent to those skilled in the art from the
following summary and detailed description of the
invention.
SummarY of the Invention
According to the present invention there is
provided a novel metal alloy composition containing both
nickel and cobalt, boron and thallium. The alloy
composition is particularly useful for deposition on a
surface of an article of manufacture, which is subject
to exposure to corrosive conditions or one subject to
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sliding or rubbing contact with another surface under
unusual wearing and bearing pressures. The metal alloy
coating composition of the present invention comprises
about 67.5 to about 96.5 weight percent nickel, about 2
to about 15 weight percent cobalt, about O.S to about 10
weight percent boron and about 1 to about 8 percent
thallium. The weight ratio of nickel and cobalt in the
bulk coating is about 45:1 to about 4:1, more preferably
about 25:1 to about 5:1, respectively. It is remarkably
hard, yet ductile, and is highly corrosion and wear
resistant.
Both physical and chemical analysis of
preferred electroless coatings of this invention reveals
significant heterogeneity in thickness cross-section,
the coatings comprising hard, amorphous alloy
micro-nodules of high nickel content dispersed or
~rooted" in a softer alloy matrix of high cobalt
content. The weight ratio of nickel and cobalt in the
micro-nodules of preferred coatings in accordance with
this invention is about 15:1 to about 45:1, respectively.
The present coating is preferably applied to a
substrate electrolessly by contacting the substrate with
a coating bath containing nickel ions, cobalt ions,
thallium ions, a metal ion complexing agent, and a
borohydride reducing agent at pH about 12 to about 14
and at an elevated temperature of about 180 to about
210F. However, the same baths used for electroless
coating in accordance with a preferred emhodiment of
this invention can be used at ambient temperature for
deposition of the present composition in an
electrochemical cell.
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The Drawings
Fig. 1 is an electron photomicrograph of the
outer corrosion and wear resistant surface of an
electroless coating of this invention.
Fig. 2 is an electron photomicrograph of the
substrate interface side of the coating shown in Fig. 1.
Detailed DescriPtion of the Invention
An article of manufacture in accordance with
this invention is coated on at least a portion of its
surface with a hard, ductile, wear and corrosion
resistant metallic coating comprising about 67.5 to
about 96.5 weight percent nickel, about 2 to about 15
weight percent cobalt, about 0.5 to about 10 weight
percent boron and about 1 to about 8 percent thallium.
Deposition of the metallic coating on suitable
substrates can be accomplished by contacting said
substrates with a plating bath comprising an aqueous
alkaline (pH about 12 to about 14) solution of nickel,
cobalt and thallium salts, a metal ion complexing agent
to maintain the metal ions in solution and a borohydride
reducing agent.
Suitable substrates are those with so-called
catalytically active surfaces including those composed
of nickel, cobalt, iron, steel, aluminum, zinc,
palladium, platinum, copper, brass, chromium, tungsten,
titanium, tin, silver carbon, graphite and alloys
thereof. Those materials function catalytically to
cause a reduction of the metal ions in the plating bath
by the borohydride and thereby result in deposition of
the metal alloy on the surface of the substrate in
contact with the plating bath. Non-metallic substrates
such as glass, ceramics and plastics are in general,
non-catalytic materials; however, such substances can be
sensitized to be catalytically active by producing a
film of one of the catalytic materials on its surface.
This can be accomplished by a variety of techniques
known to those skilled in the art. One preferred
lS procedure involves dipping articles of glass, ceramic,
or plastic in a solution of stannous chloride and then
contacting the treated surface with a solution of
palladium chloride. A thin layer of palladium is
thereby reduced on the treated surface. The article can
then be plated or coated with the metallic compositon in
accordance with this invention by contact with a coating
bath as detailed below. It is to be noted that
magnesium, tungsten carbide and some plastics have
e~ibited some resistance to deposition of the present
coatinqs.
A coating bath for deposition of the present
coatings comprises
(1) nickel ions, cobalt ions, and
thallium ions in the amounts indicated, expressed as
moles per gallon of coating bath: nickel ions,
about 0.4 to about 0.9; cobalt ions, about 0.1 to
about 0.4; and thallium ions, about 4 x 10 5 to
about ~ x 10
(2) chemical means for adjusting the pH
of the bath to between about 12 and about 14;
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(3) a metal ion complexing agent in an
amount sufficient to inhibit precipitation of said
ions from the highly alkaline coating bath; and
(4) about 0.025 to about 0.1 moles per
gallon of coating bath of a borohydride reducing
agent.
The borohydride reducing agent can he selected rom
among the known borohydrides having a good degree of
water solubility and stability in aqueous solutions.
Sodium and potassium borohydrides are preferred. In
lS addition, substituted borohydrides in which not more
than three of the hydrogen atoms of the borohydride ion
have been replaced can be utilized. Sodium
trimethoxyborohydride [NaB~OCH3)3H] is illustrative
of that type of compound. Sodium cyanoborohydride has
been found to stabilize electroless coating baths
utilizing other borohydride reducing agents (U.S. Patent
3,738,849).
The coating bath is prepared to have a pH of
about 12 to about 14. Best results have been observed
when the pH of the bath is maintained during the coating
process within that range and more preferably at about
pH 13.5. Adjustment of bath pH can be accomplished by
addition of any of a wide variety of alkaline salts or
solutions thereof. Preferred chemical means for
establishing and maintaining bath pH are the alkali
metal hydroxides, particularly sodium and potassium
hydroxide, and ammonium hydroxide. Ammonium hydroxide
offers an additional advantage in that the ammonium ion
~ ~9~ 8~i
can function to assist metal ion complexation in the
coating bath.
Due to the high alkalinity of the coating bath,
a metal ion complexing or sequestering agent is required
in the bath to prevent precipitation of the nickel and
cobalt hydroxides or other basic salts. Importantly,
too, the metal ion complexing agent functions to lower
metal ion reactivity; the complexed or sequestered metal
ions have minimal reactity with the borohydride ions in
the bulk solution but do react at the catalytic surfaces
of substrates in contact with the solution. The term
catalytic surface refers to the surface any article
composed of the aforementioned catalytic materials or to
the surface of a non-catalytic ~aterial which has been
sensitized ~y application of a film of said catalytic
materials on its surface.
The complexing or sequestering agents suitable
for use in this invention include ammonia and organic
complex-forming agents containing one or more of the
following functional groups: primary amino, secondary
amino, tertiary amino, immino, carboxy and hydroxy.
Many metal ion complexing agents are known in the art.
Preferred complexing agents are ethylene diamine,
diethylene triamine, triethylene tetramine, the organic
acids, oxalic acid, citric acid, tartaric acid and
ethylene diamine tetraacetic acid, and the water soluble
salts thereof. Most preferred for use in the present
coating bath are ethylene diamine, the water soluble
salts of tartaric acid, ammonia and combinations thereof.
9~
About 2 to about 8 moles of complexing agent
are used per gallon of coating bath. Best results have
been obtained when about 3 to about 5 moles of
complexing or sequestering agent is used for each gallon
of coating bath.
The nickel, cobalt and thallium ions in the
coating bath are provided by the addition to the bath of
the respective water soluble nickel, cobalt and thallium
salts. Any salts of those metals having an anion
component which is not antagonistic to the subject
coating process is suitable. For example salts of
o~idizing acid such as chlorate salts are not desirable
since they will react with the borohydride reducing
agent in the bath. Cobalt, nickel, and thallium
chlorides, sulfates, formates, acetates, and other salts
whose anions are substantially inert with respect to the
other ingredients in the alkaline coating bath are
satisfactory.
The coating bath is typically prepared by
forming an aqueous solution of the appropriate amounts
of nickel and cobalt salts, adding the complexing
agent(s), adjusting the pH to about 12 to about 14,
heating to about 195F, filtering and finally,
immediately before introducing the substrate into the
bath, adding the required amounts of thallium salt and
sodium borohydride (typically in aqueous alkaline
solution).
The article to be coated or plated using a bath
in accordance with this invention is prepared b~
mechanical cleaning, degreasing, anode-alkaline
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cleaning, and finally pickling in an acid bath in
accordance with the standard practice in the
metal-plating art. The substrate can be masked if
necessary to allow deposition of the metal alloy coating
only on selected surfaces. Although the present
coatings in general exhibit excellent adhesion to
properly prepared substrate surfaces, in instances where
coating adhesion is critical or where some adhesion
problems are experienced, coating-adhesion can often be
enhanced by depositing a nickel strike electrochemically
on the substrate surface prior to applying the present
coating.
The cleaned or otherwise surface-prepared
article is immersed in the hot (about 180 to about
210F) coating bath to initiate the coating process.
The process is continued until deposition of the coating
has progressed to the desired thickness or until the
metal ions are depleted from solution. Deposition rates
vary under the conditions of the present process from
about .1 mil (1 mil = one one-thousandth of an ;nch) to
about 1 mil per hour.
A preferred concentration range for each of the
metal ion components of the present coating bath is as
follows: nickel ions, about 0.5 to about 0.8 moles per
gallon; coba.lt ions, about 0.15 to about 0.3 moles per
gallon; and thallium ions, about 8 x 10 5 to about
4 x 10 moles per gallon. A range of about 0.3 to
about 0.8 moles per gallon of borohydride reducing agent
is preferred. The ratio of nickel, cobalt, boron and
thallium in the present coatings can be adjusted by
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varying the relative amounts of the metal salt
components and borohydride in the coating bath.
Under normal usage conditions of the coating
baths in accordance with the present invention, thallium
ions and borohydride reducing agent are added to the
coating bath hourly in amount equivalent to their usage
in preparation of the bath initially. The need to
replenish the present coating baths with thallium and
borohydride depends on the ratio of coating bath volume
to the surface area being coated. Thus replenishment of
thallium and borohydride to the present coating bath
would not be required where but small surface areas are
being treated. One gallon of bath prepared in
accordance with the preferred embodiment of the present
invention will coat approximately 700 square inches to a
2~ thickness of 1 mil where the bath is replenished in
accordance with the above description with thallium and
borohydride ion as those components are depleted from
solution.
The pH of the coating bath will tend to drop
during the coating process and should be checked
periodically to assure that it is within the preferred
pH range of about 12 to about 14. It has been found
that any problems with pH maintenance throughout the use
of a coating bath can be minimized simply by using a
highly alkaline (concentrated sodium hydroxide) solution
of borohydride to replenish the borohydride content of
the bath as required. The coating deposition rate from
the present electroless coating bath is about 0.1 to
about 1 mil per hour and is dependent on bath
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temperature, pH, and metal ion concentration~ The
deposition rate on most metal substrates from freshl~
prepared coating baths at a preferred temperature of
about i85 to about 195F is approximately 1 mil per hour.
The practical aspects carrying out electroless
coating processes are well known in the art. Such
processes are disclosed generally in U.S. Patents
3,338,726 issued to Berzins on August 19, 1967;
3,096,182 issued to Berzins on July 2, 1963; 3,045,334
issued to Berzins on October 1, 1958; 3,378,400 issued
to Sickles on April 16, 1968; and 2,658,841 issued to
Gutzeit and Krieg on November 10, 1953; the disclosures
of which are hereby incorporated by reference.
The electroless coating bath of this invention
can also be used for electrolytic deposition of coatings
comprising about 67.5 to about 96.5 weight percent
nickel, about 2 to about 15 percent weight cobalt, about
0.5 to about 10 weight percent boron and about 1 to
about 8 percent thallium. The bath is prepared as
described above and is used at ambient temperatures as
the electrolyte in an electrolytic cell using, for
example, a nickel anode and the substrate as the
cathode. The cell is connected to a 12-volt DC power
source and current flow through the cell is adjusted to,
for example, abGut 50 amps per square foot, and current
flow is maintained until the metal alloy is deposited on
the substrate cathode to the desired thickness.
The preferred electroless metal alloy coatings
of the present invention exhibit unprecedented hardness
and concomitant wear resistance. They are highly
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ductile allowing the coating to flex with the substrate
while maintaining a strong bond to the coated material.
The present coatings are nonporous and exhibit
remarkably enhanced corrosion resistance over nickel
boron coatings previously known in the art.
The electroless metal alloy coatings of this
invention present a wear and corrosion resistant surface
comprising hard, amorphous nodular deposits of metal
alloy. Hardness of the present coatings can be
increased by heat treatment of the coated articles.
Heat treatment is accomplished at a temperature of about
375 to about 750F for a period of about one to about 24
hours. Shorter times, about one to two hours, is
preferred for the higher temperatures of between about
550-750F while longer heat treatment times have been
shown to be advantageous at the lower temperature ranges
of between about 375 to about 450F.
X-ray analysis of the metal alloy coatings
prepared in accordance with the preferred embodiments
show that the hard, amorphous nodular deposits lie in a
somewhat softer metal alloy matri~. See Figs. 1 and 2.
X-ray analysis (using a *"JEOL" scanning electron
microscope with a computerized *"EDAX" analyzer) also
revealed that the coating is heterogenous in thickness
cross-section having a metal concentration gradient with
higher cobalt concentrations at the interface of the
coating and the surface of the substrate. The corrosion
and wear resistant surface (the hard nodular deposits)
of several coatings prepared in accordance with
preferred embodiments of this invention were shown to
f~ *trade-mark
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comprise zbout 86 to about 92 percent nickel, about one
to about five percent weight cobalt, about one to abou~
eight percent boron, and about one to about five percent
thallium. Analysis of those same coatings at the
interface of the coating and the surface of the
substrate was shown to have high cobalt concentrations
(as high as about 95 weight percent cobalt).
The nodular deposits making up the wear and
corrosion resistant surface presented by the present
coatings are believed to be amorphous as deposited from
the electroless coating bath. With heat treatment in
accordance with the above description, X-ray data showed
crystalline domains of metal borides selected from
nickel boride and cobalt boride dispersed in the
amorphous metal alloy matrix. The formation of hard
crystalline domains of metal borides within the nodular
structures is believed to be responsibl~e for the high
hardness levels which have been measured for the present
heat-treated coatings. Heat-treated coatings in
accordance with the present invention have been found to
have a Knoop hardness value of between about 1230 and
about 1300. These values are more than 20 percent
higher than the best hardness values reported previously
for nickel boron electroless coatings.
Because of the heterogeneity in thickness of
cross-section observed for preferred coatings in
accordance with the present invention, the actual bulk
weight percent content of any of the four components in
any given coating depends to a some extent on coating
thickness. The surface-presented nodules are high
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nickel-low cobalt content while the softer alloy matrix
for the nodules formed immediately at the surface of the
substrate (i.e., the first deposited component of the
present coatings) is of high cobalt and low nickel
content. Thus the thinner deposits of the present
coating have a higher overall weight percent cobalt.
Thicker coatings in accordance with the present
invention have a greater percentage of their thickness
in the form of the amorphous nodules and, therefore have
lower overall bulk weight percent cobalt content.
The present coatings have a wide range of
applications which will be recognized by those skilled
in the art. They have particular utility for coating
surfaces of articles which under normal use are
subjected to highly abrasive, rubbing, or sliding
conditions under high temperatures/pressures. Such high
wear conditions are found at many points in construction
of tools, internal combustion engines including gas
turbine engines, transmissions and in a wide variety of
heavy equipment construction applications.
The following examples provide details of bath
compositions, process conditions, and coating
compositions and properties representative of the
present invention. The examples are illustrative of the
invention and are not in any way to be taken as limiting
the scope thereof.
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EXAMPLE 1
A five (5) gallon batch unit of coating bath
was prepared as follows. Nickel chloride (0.9 pounds,
3.15 moles) was combined with sodium tartrate (2.5
pounds, 4.93 moles) in about two gallons of distilled
water having a resistance of approximately ten megohms.
To that solution was added 0.25 pounds of cobalt
chloride (0.85 mole) and 3.0 pounds or reagent grade
(99.5% pure) ethylene diamine (17.4 moles), 3.5 pounds
of reagent grade sodium hydroxide (39.7 moles) and l.0
pound of concentrated ammonium hydroxide solution. The
volume of the resulting mixture (pH about 13.5) was
adjusted to five gallons by the addition of distilled
water, and the solution was heated to 180F and filtered
20 into electroless plating tank capable of continuous
filtration, heating and agitation of the bath
composition. The temperature of the bath was raised to
about 185F.
Two strips of steel 15 mil thick by 1/2 inch in
25 width were degreased and prepared for immersion in the
coating bath by successive anodic alkaline oxidation
followed by acid pickling.
To the heated coating bath was added 0.023
pounds of sodium borohydride (.11 moles~ and 0.20 grams
of thallium sulfate (4 x 10 4 mole). The coating
solution was agitated for about 3 minutss prior to the
immersion of the prepared steel strips into the bath. A
third steel strip was immersed in the bath without
pretreatment.
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The steel plates were removed from the coating
bath after about 1.5 hours. Each had an electroless
coating in accordance with the present invention about 1
mil (l/l,OOOth of an inch) thick. Electron microscopic
examination (x 6000) of the surface on the coated steel
strips showed the surface revealed nodular metal alloy
deposits having a cauliflower-like appearance. See
Fig~ 1. Using scanning electron microscopy (SEM) the
nodular deposits at their outermost surface were found
to have the following composition: about 90 weight
percent nickel, about 5 weight percent boron, about 2
weight percent cobalt, and about 3 weight percent
thallium.
The third steel strip which did not have its
surface properly prepared for optimum adhesion of the
electroless coating was bent and creased so that the
coating was purposely fractured, and a small sample
separated from the steel substrate surface. Analysis of
the substrate interface side of the coating deposited on
the steel surface revealed that it contained in excess
of 95 weight percent cobalt. (See Fig. 2)
Interestingly, analysis of apparent holes in the
interface side of the coating showed lower cobalt levels
and much higher nickel levels. Similarly x-ray analysis
of the "valleys" between the nodules on the outer
surface of the coating showed nickel levels lower than
those in the upper surfaces of the nodules and higher
cobalt levels. These results indicate that the coating
prepared in accordance with preferred embodiments of the
present invention are heterogeneous in thickness
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cross-section having a higher cobalt concentration at
the interface of the coating and the substrate surface.
In sum, it appears that the high nickel alloy nodules at
the outer surface of the coating are imbedded in a
softer high cobalt alloy matrix deposited during the
early stages of the electroless coating process.
A coated steel strip was tested for surface
hardness using a Knoop hardness measuring device (KH
100) and found to exhibit a Knoop hardness of 1100 which
surpasses that of commercial grade hard chrome.
Following heat treatment at 725F. for 90 minutes the
same surface was found to have a Knoop hardness of
approximately 1240. Electroless coatings deposited from
a bath prepared in accordance with the example have also
shown exceptional corrosion resistance under laboratory
test conditions: ASTB B117 Salt Spray-1200 hours.
EXAMPLE 2
The same procedure was followed as in Example 1
except for variation of the relative amounts of the bath
constituents: nickel chloride, 0.9 pounds (3.12 moles);
cobalt chloride, 0.3 pounds (1.05 moles); thallium I
sulfate, 0.05 gram (1 x 10 4 mole); sodium borhydride,
0.0275 pounds (0.33 moles); ethylene diamine, 3.0 pounds
(17.4 moles); sodium hydroxide, 6.0 pounds (68 moles);
concentrated ammonium hydroxide, 0.75 pounds; sodium
tartrate, 2.5 pounds (5 moles). X-ray analysis of the
nodules at the wear and corrosion resistant surface of
the coated steel strips showed the nodules to contain
~ ~j92~6
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about 88 weight percent nickel, about 3 weight percent
cobalt, about 8 weight percent boron, and about 1 weight
percent thallium in an alloy matrix or layer containing
cobalt in excess of about 95 weight percent. Several
coated substrates were heat treated at 725F. for 90
minutes and others were treated at 550F. for 12 hours.
The coatings on the heat treated substrates were found
to have a hardness of approximately 1300 Knoop.
EXAMPLE 3
The same procedure was followed as in Example 1
excspt that the coating bath constituents were utilized
in the following amounts: nickel chloride, 1 pound (3.5
moles); cobalt chloride, 0.375 pounds (1.3 moles);
thallium I sulfate, 0.25 gram (5 x 10 4 moles); sodium
borohydride, 0.0175 pounds (0.21 moles); ethylene
diamine, 2.5 pounds (14.5 moles); sodium hydroxide, 5
pounds (57 moles); ammonium hydroxide, 0.75 pounds;
sodium tartrate, 4 pounds (7.9 moles). X-ray analysis
of the surface nodules presented by the deposited
electroless coating showed them to contain about 90
weight percent nickel, about 4 weight percent cobalt,
about 1 weight percent boron, and about 5 weight percent
thallium. Several coated steel plates were heat treated
at 725F. for 90 minutes while others were treated at
550F. for 12 hours. Hardness testing of the coated
articles both before and after heat treatment showed a
hardness of approximately 1,000 Knoop. While that value
is somewhat less than those measured for the coatings
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prepared in Examples 1 and 2 above, it is nonetheless
comparable to hard chrome and much more corrosion
resistant. It appears that the present coating having a
higher ratio of thallium to boron and a marginally
higher cobalt content in the surface nodules would find
particular application where corrosion resistance is
more important than hardness value.
EXAMPLE 4
An electroless coating bath having a volume of
one gallon was prepared as follows: 81 grams of nickel
chloride (0.625 mole); 34 grams of cobalt chloride (0.26
moles), 227 grams of ethylene diamine (2.9 moles), and
136 grams of sodium tartrate (0.59 moles) were combined
in about 3 quarts of distilled/deionized water. The pH
of the solution was adjusted to about 13.S by the
addition of 181 grams of sodium hydroxide (4.5 moles)
and 68 grams of concentrated ammonium hydroxide
solution. The volume of the resulting mixture was
adjusted to about one gallon by the addition of
distilled water. The coating bath mixture was then
heated to approximately 190F. and filtered into an
electroless heating bath tank having means for
continuous filtration, heating and agitating of the bath
mixture. Two case hardened steel pins measuring about
nine inches in length and 2.5 inches in diameter were
degreased, and subjected to anodic alkaline and acid
cleaning treatments and washed thoroughly with distilled
water.
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Immediately before immersing the pretreated
steel bars into the coatlng bath, 0.04 grams of thallium
I sulfate (8 x 10 5 moles) and 2 grams of sodium
borohydride (0.053 moles) were added to the hot, stirred
electroless coating bath. After about 5 minutes the
substrate steel bars were lowered into and suspended in
the electroless coating bath. Hydrogen evolution at the
surface o the bars was noted immediately. After about
1 hour an additional 0.4 grams of thallium sulfate and
10 millileters of a sodium borohydride solution ~0.83
pounds sodium borohydride in 1 gallon of water
containing also about 400 grams of sodium hydroxide).
After 2 hours the coated substrates were removed from
the coating bath, washed and scanned by x-ray for
surface nodule elemental content and found to have about
90 weight percent nickel, about 2 weight percent cobalt,
about S weight percent boron, and about 3 weight percent
thallium. The coating exhibits exceptional hardness and
corrosion and wear resistance.
EXAMPLE 5
The coating bath of Example 4 is used to apply
an electroless metal strike before and after application
of nickel plates to prepared metal substrates. It was
found that deposition of a thin metal strike either
before or after the nickel electroplating process
significantly decreased the porosity, and therefore
enhanced the corrosion resistance, of the plated
substrates. An electroless nickel alloy strike
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utilizing the coating baths of the present invention is
particularly effective to improve corrosion resistance
of electroplates when it is applied to the electroplate
as an overcoat.
While there have been described what are at
present considered to be certain preferred embodiments
of this invention, it will be understood that various
modifications may be made therein, ànd it is intended to
cover in the appended claims all such modification as
fall within the true spirit and scope of the invention.
