Passivate for tungsten alloy electroplating

A passivate for tungsten alloy electroplates. The passivate includes an effective quantity of CrO.sub.3 in an aqueous bath having a pH of from about 3.5 to about 7.5.

BACKGROUND OF THE INVENTION 
The present invention relates to tungsten alloy electroplate. More 
specifically, the present invention relates to passivation of tungsten 
alloy electroplates to increase tarnish and corrosion resistance. 
Chromium plating for decorative and functional plating purposes has always 
been desirable. Most often chromium plating is carried out in hexavalent 
chromium electrolytes. Functional coatings from hexavalent chromium baths 
generally range in thickness from about 0.0002" to about 0200" and provide 
very hard, lubrous corrosion resistant coatings. Decorative coatings from 
hexavalent chromium electrolytes, are much thinner, typically 0.000005" to 
0.000030", and are desirable because of their blue-white color and 
abrasion and tarnish resistance. These coatings are almost always plated 
over decorative nickel or cobalt or nickel alloys containing cobalt or 
iron. 
The imposition of government restrictions on the discharge of toxic 
effluents, including hexavalent chromium present in conventional chromium 
plating baths, has escalated in recent years. Some state and local 
government restrictions are extremely stringent. This is especially the 
case with regard to fumes generated during the electrolysis of hexavalent 
chromium baths. In some locals even minuscule amounts of airborne chromium 
is unacceptable. This has prompted the development of alternative 
electroplating baths intended to approach the color and the 
characteristics of chromium deposits. 
One possible solution is the electrodeposition of tungsten alloys. 
Typically, in such baths, salts of nickel, cobalt, iron or mixtures 
thereof are used in combination with tungsten salts to produce tungsten 
alloy deposits on various conductive substrates. In this case the nickel, 
cobalt and/or iron ions act to catalyze the deposition of tungsten such 
that alloys containing as much as 50% tungsten can be deposited, said 
deposits having excellent abrasion resistance, lubricity and acceptable 
color when compared to chromium. 
However, while such deposits have been desirable as replacements for 
chromium, the properties of resulting deposits and inherent manufacturing 
limitations in prior art processes have not allowed such deposits to 
replace decorative or functional chromium deposits. Typically, such 
processes have resulted in very slow plating times or the deposits are 
non-uniform, making them undesirable for decorative applications. 
However, with recent improvements in tungsten alloy electroplating the use 
of tungsten alloy electroplating in replacement applications for chromium 
plating has become closer to realization. For instance in my co-pending 
application entitled Additive for Tungsten Alloys filed Aug. 18, 1994, 
Ser. No. 08/292,610 I have taught a new bath for providing functional 
tungsten electroplates which are alloyed with nickel, iron, cobalt or 
mixtures of these. These baths operate at higher current densities and 
provide deposits with low internal stresses. These baths have provided a 
commercially feasible process for tungsten alloy electroplating. These 
tungsten alloys may be plated directly on a substrate such as steel, brass 
or copper. Additionally these alloys may be modified and plated directly 
over electroplates such as nickel and its alloys or, cobalt and its alloys 
to. 
However, in my experimentation with such tungsten alloy electroplate, I 
have found these electroplates tend to tarnish when exposed to high 
humidity and/or salt treatments, regardless of the substrate. Thus in 
applications wherein the plate is going to be exposed to high humidity or 
corrosive agents, tungsten alloy electroplate has been susceptible to 
tarnishing and corrosion. Thus it has been a goal to provide some type of 
treatment for the tungsten electroplate which would increase tarnish and 
corrosion resistance. 
In my U.S. Pat. No. 4,299,671 entitled Bath composition and Method for 
Electro Depositing Cobalt-Zinc Alloys Simulating a Chromium Plating the 
use of sodium dichromates and chromic acid is shown in the examples as a 
passivation treatment for cobalt zinc alloys. However, while passivation 
of such alloys as cobalt zinc, and cobalt tin is readily known in the art 
it has been readily accepted by those skilled in the art that passivation 
of tungsten and its alloys using chromates does not provide any 
significant beneficial effect to improve tarnish and corrosion resistance. 
Thus, when submitting tungsten plate to a solution of from 7.5 to 30 grams 
per liter CrO.sub.3 having a pH of approximately 2 there is no significant 
improvement in the resistance to neutral salt spray tests. This 
corresponds to the readily accepted teachings in the art that tungsten and 
its alloys cannot readily be passivated by the chromic acid treatments or 
the like which have been utilized in the past. 
Therefore, it has been a goal in the art to provide a passivation of 
tungsten alloy electroplate which will allow the use of these new 
electroplates in highly corrosive atmospheres, to provide more 
advantageous replacement of decorative and functional chromium alloys. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is provided a method for 
passivativing a tungsten alloy electroplate. In contrast to the teachings 
of the prior art the inventor of the present invention has discovered that 
the tungsten alloy electroplate may be passivated with an effective 
quantity of CrO.sub.3 in a bath which has critical pH parameters of from 
about 3.5 to about 7.5. It has been found that operating in this range of 
pH results in a significant and unexpected increase in passivation of 
tungsten alloy electroplates. This results in a significant increase in 
tarnish and corrosion resistance as demonstrated by tungsten neutral salt 
spray tests. The increase in corrosion resistance and tarnish resistance 
versus the use of prior art passivating baths or untreated tungsten 
electroplate is significant and unexpected based on the teachings in the 
art. 
Thus in accordance with the present invention the inventor has discovered 
that by use of the baths herein described, tungsten alloy electroplates 
may be passivated to improve corrosion and tarnish resistance, thereby 
allowing further uses for such electroplates in replacements for chromium 
deposits. 
Other advantages and benefits of the present invention will be readily 
appreciated by those skilled in the art in light of the following 
description of the preferred embodiments, when taken in conjunction with 
the examples given below and the claims appended hereto. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In its broad aspects the present invention is accomplished by contacting a 
tungsten alloy electroplate with a passivating quantity of CrO.sub.3 in a 
bath having a pH of from about 3.5 to about 7.5. The inventor has found 
that by controlling the pH of the bath, containing an effective amount of 
chromate, provides the unexpected result of greatly increasing the tarnish 
and corrosion resistance of tungsten alloy electroplates verses the use of 
standard chromate baths which typically have pH's of approximately 2 or 
lower. Examples exemplifying this unexpected result are set forth below. 
In order for passivation to be accomplished it is first necessary to 
provide a substrate which includes a tungsten alloy electroplate thereon. 
Typically substrates such as steel, brass or copper may be plated over 
with tungsten alloy electroplates. However the present invention will also 
be useful when the tungsten electroplate is provided over other 
electroplates such as nickel an its alloys or cobalt and its alloys. 
As stated above tungsten alloy electroplates are designed to replace 
decorative or hard chromium and therefore are typically used on substrates 
which are going to be exposed to corrosive conditions such as high 
humidity, salt or other corrosive agents, extended outdoor exposure or the 
like. While the present method is useful in many types of tungsten alloy 
electroplates, the present invention is particularly suitable for use in 
tungsten electroplate which is alloyed with iron, cobalt, nickel or 
mixtures of these. A suitable process for providing the tungsten 
electroplate is set forth in co-pending U.S. application Ser. No. 
08/292,610, entitled Additive for Tungsten Alloys filed on Aug. 18, 1994, 
which is hereby incorporated herein by reference thereto. While the method 
of the present invention is useful in any tungsten alloy electroplate, the 
method is particularly suitable for use with tungsten cobalt alloys since 
these alloys seem particularly susceptible to tarnishing and corrosion. 
The bath, in which the substrate having a tungsten alloy electroplate is 
passivated, must include an effective amount of CrO.sub.3 in an aqueous 
solution. The source of the CrO.sub.3 may come from chromic acid, 
potassium dichromate, sodium dichromate and mixtures thereof. Generally, 
these additives are contained in baths of the present invention in 
quantities of from about 1.8 to about 45 g/l. Such quantities of chromates 
have been found to provide effective passivating of tungsten alloy 
electroplate when following the teachings of the present invention. 
Typically baths in accordance with the present invention, include from 
about 7.5 to about 30 g/l and preferably from about 11 to about 19 g/l 
CrO.sub.3 in the solution. 
As stated above, baths of the present invention operate best in the general 
range of pH of from about 3.5 to about 7.5. Preferably the pH of 
passivating baths of the present invention will range from about 5 to 
about 6. It is critical in order to provide proper passivating, to 
maintain the pH in a range of from about 3.5 to about 7.5 during 
contacting of the substrate having the tungsten alloy electroplate with 
the bath. Typically, baths containing the chromate contents set forth 
above, must be adjusted to the operable range of pH. This may be 
accomplished by the addition of a source of a hydroxide ion in the bath. 
Thus any number of basic substances may be used as is known to those 
skilled in the art. Preferably, additions of sodium hydroxide, ammonium 
hydroxide, carbonates or mixtures thereof are added to the bath for 
adjusting of the pH into the critical range. 
While baths of the present invention start showing beneficial results at 
temperatures of about 90.degree. F., it is preferred to operate the baths 
of the present invention at elevated temperatures of from about 
100.degree. F. to about 180.degree. F. This provides commercially 
practical treatment times, while providing maximum passivation of the 
alloys. While brief exposures to the bath will result in increased 
corrosion and tarnish resistance, typically contact with the bath will 
range from about 30 seconds to one minute, at temperature for providing 
optimum results. However, it has been found that retention times in the 
bath of up to 4 minutes may be accomplished with substantially no surface 
attack of the tungsten alloy electroplating. Preferably, the baths of the 
present invention are maintained at a temperature of from about 
130.degree. to about 160.degree. F. for optimum results. It will be 
readily appreciated by those skilled in the art that time and temperature 
of the contacting of the substrate may be varied depending on the 
concentration of chromium in the bath and depending on the results desired 
to be obtained. 
The substrates of the present invention may be contacted with the above 
described bath in any number of readily available ways such as immersion, 
spray application or any other method which provides contact of the bath 
with the surface. 
The passivation method of the present invention allows use of tungsten 
alloy electroplates in high humidity or highly corrosive environments. 
Thus, for instance the electroplates, as treated in the present invention 
are useful in milling tools and/or hand tools and other equipment which 
require a hard surface and require properties of tarnish resistance and 
corrosion resistance which approach or are equal to chromium deposits.

Further understanding of the present invention will be had by reference to 
the following examples, which are presented herein for purposes of 
illustration but not limitation. 
EXAMPLE I 
A four liter cobalt-tungsten bath was prepared as follows: 
______________________________________ 
Cobalt Sulfate Heptahydrate 
40 g/L 
Sodium Tungstate Dihydrate 
8 g/L 
Citric Acid 50 g/L 
Sodium Sulfate 25 g/L 
2 Ethyl Hexyl Sulfate 0.4 g/L 
Ammonium Hydroxide to pH 7.5 
Temperature 140.degree. F. 
______________________________________ 
The plating cell contained stainless steel anodes and was connected to a 6 
V DC rectifier. Typically, the alloy plated from this solution will be 
about 30-35% tungsten and the remainder cobalt. 
A chromium containing passivate was made up as follows: 
______________________________________ 
Chromic Acid 15.0 g/L 
pH Variable 
Temperature 160.degree. F. 
______________________________________ 
4".times.3" polished steel panels were plated in a commercial bright nickel 
bath to an average thickness of 0.00020". The nickel plated panels were 
then plated in the above-described alloy bath at about 15 ASF to an 
average tungsten cobalt plate thickness of about 0.000010". The panels 
were then immersed in the passivate described above for about 30 seconds 
to 1 minute. The pH of the passivate was varied to determine the effect of 
pH on corrosion protection. The panels were placed in a neutral salt 
spray(NSS) cabinette to evaluate resistance to surface staining and 
rusting of the steel substrate. This method of corrosion testing is 
described in ASTM Designation B 117. The test solution is 5% sodium 
chloride, pH 6.5-7.2, sprayed at a temperature of about 95.degree. F. The 
results of the varied pH passivate are set forth below in Table 1 
TABLE I 
______________________________________ 
pH v. APPEARANCE AFTER NSS EXPOSURE* 
pH 16 hrs 24 hrs 48 hrs 72 hrs 120 hrs 
______________________________________ 
1.5 sss sss, rr -- 
2.0 sss sss, rr -- 
2.5 sss sss sss, rr 
-- 
3.0 ss sss sss, rr 
-- 
3.5 lss ss sss sss, lrr 
-- 
4.0 lss sss sss sss, lrr 
-- 
4.5 ok ok ok lss sss 
5.0 ok ok ok lss lss 
5.5 ok ok ok vlss lss 
6.0 ok ok ok ok lss 
6.5 ok ok ok lss sss 
7.0 ok lss sss sss, rr 
-- 
7.5 lss sss ss, rr -- 
8.0 sss sss, rr -- -- 
______________________________________ 
*CODE: vlss -- very light surface stain, lss -- light surface staining, s 
-- surface staining, sss -- severe surface staining, lrr -- light red 
rust, rr -- red rust. 
EXAMPLE II 
Panels were plated and immersed in the passivate as described in Example I. 
However, this time the pH of the passivate was kept constant at 5.5 and 
the temperature of the passivate varied to determine the effect of 
temperature on corrosion protection. The results of the test are set forth 
below in the Table II. 
TABLE II 
______________________________________ 
TEMPERATURE v. APPEARANCE AFTER NSS EXPOSURE 
Temp 16 hrs 24 hrs 48 hrs 72 hrs 120 hrs 
______________________________________ 
80 sss sss, rr -- 
90 ok sss sss, rr 
-- 
100 ok ok sss, lrr 
sss, rr 
-- 
110 ok ok sss sss, srr 
sss, rr 
120 ok ok lss sss sss, rr 
130 ok ok ok lss ss, lrr 
140 ok ok ok lss ss 
150 ok ok ok vlss lss 
160 ok ok ok vlss lss 
170 ok ok ok vlss lss 
______________________________________ 
The above results indicate that as the temperature of the passivate 
increase overall corrosion protection improves. 
EXAMPLE III 
Panels were plated and immersed in the passivate as described in examples 1 
and 2. However, this time the concentration of the chromic acid was varied 
to determine its effect on corrosion protection. The pH of the passivate 
was adjusted to 5.5 and the temperature was maintained at 160.degree. F. 
The results of this test are set forth below in Table III. 
TABLE III 
______________________________________ 
CONCENTRATION v. APPEARANCE AFTER NSS EXPOSURE 
CONC** 16 hrs 24 hrs 48 hrs 72 hrs 120 hrs 
______________________________________ 
0.5 ok iss sss sss, rr 
sss, rr 
1.0 ok ok iss sss sss, lrr 
1.5 ok ok ok lss lss 
2.0 ok ok ok vlss lss 
3.0 ok ok ok ok lss 
4.0 ok ok ok ok vlss 
______________________________________ 
**Concentration is set forth in ounces of chromic acid/gallon of solution 
 
These test results indicate that as the concentration of the chromic acid 
increases corrosion protection improves. 
EXAMPLE IV 
A nickel tungsten plating bath was made up as follows: 
______________________________________ 
Nickel Sulfate Hexahydrate 
10.0 g/L 
Sodium Tungsten Dihydrate 
55.0 g/L 
Citric Acid 60 g/L 
Ammonium hydroxide to pH of 8.0 
Temperature 140.degree. F. 
______________________________________ 
The plating cell contained stainless steel anodes and was connected to a 10 
V DC rectifier. Typically, the alloy plated from this solution contains 
about 35-40% tungsten, the remainder nickel. 
In 4".times.3" polished steel panels were plated in the nickel tungsten 
alloy bath to an average thickness of about 0.00020". A pair of panels 
were air dried with no passivate. The rest of the panels were then 
passivated via immersion in the passivate described in example 1 with the 
pH adjusted to 5.5. In this example the nickel tungsten was not plated 
over nickel. Thus, the resulting coating is not decoratively appealing. It 
is, however, suitable for functional applications as a replacement for 
electroless nickel and hard chromium. The corrosion resistance was then 
evaluated in the NSS cabinette. The results of the test are set forth 
below in Table IV. 
TABLE IV 
______________________________________ 
PASSIVATED NICKEL TUNGSTEN ELECTROPLATE 
APPEARANCE AFTER NSS EXPOSURE 
No Passivate 24 hrs 48 hrs 72 hrs 96 hrs 
______________________________________ 
1 NONE sss sss, rr 
-- 
2 NONE sss sss, rr 
-- 
3 YES ok lss lss sss, rr 
4 YES ok ok lss sss 
5 YES ok vlss lss sss, rr 
6 YES ok ok vlss sss 
______________________________________ 
These test results indicate that the passivate also improves the corrosion 
properties of nickel tungsten coatings. 
The above examples indicate the relatively dilute solutions of hexavalent 
chromium salts will provide outstanding tarnish resistance and base metal 
corrosion protection when applied to alloys of tungsten and nickel, 
tungsten and cobalt or combinations thereof within the parameters 
specified above. 
EXAMPLE V 
An Iron Tungsten alloy plating bath is prepared as follows: 
______________________________________ 
Ferrous Sulfate Heptahydrate 
40 g/l 
Sodium Tungstate 50 g/l 
Citric acid 66 g/l 
Salycilic acid 150 mg 
pH 8.0 
Temperature 140.degree. F. 
______________________________________ 
A 3".times.4" steel hull cell panel is plated in a 1 liter Hull Cell 
containing the above electrolyte. The panel is plated at 5 amps for 30 
minutes. The resulting panel is lustrous. 
The above panel is then passivated by immersion in the passivate described 
in Example 1 above. The resulting product is placed in the NSS salt spray 
booth and the panel is found to be resistant to tarnishing or staining for 
about 48 hours. 
While the above specification and exemplification was given for purposes of 
disclosing the preferred embodiments of the present invention, it is not 
to be construed to be limiting of the present invention. 
Therefore, it will be readily appreciated by those skilled in the art that 
the present invention can be practiced other than as specifically stated. 
Thus, the invention may be subject to modification, variation and change 
without departing from the proper scope and fair meaning of the 
accompanying claims.