Anode configuration for nickel-phosphorus electroplating

An anode configuration is provided particularly for a bath that is used for electrolytically plating a substrate with a nickel and/or cobalt phosphorus alloy. The anode comprises a plurality of widely spaced portions of material, preferably of platinum or rhodium. The anode configuration is such that the anode has a very high current density in use--at least 200 amperes per square foot and preferably 500 amperes per square foot. The wide spacing of the anode portions may be provided in a number of different ways. A platinum wire may extend between titanium screws attached to a pair of parallel spaced titanium buses, in a zig-zag manner. A platinum wire may be welded at its opposite ends to a titanium bus, and helically wrapped around the bus, with the welding junctions covered by an insulating material (e.g. vinyl). A platinum tube may be shrink fit onto a titanium bus.

BACKGROUND AND SUMMARY OF THE INVENTION 
The utilization of electrolytically deposited nickel phosphorus, cobalt 
phosphorus, and nickel cobalt phosphorus coatings having an amorphous 
structure has been found to be useful in a wide variety of circumstances. 
For instance a fluid jet orifice plate having enhanced utility, by 
electrolytically coating the substrate metal of the orifice plate with an 
amorphous nickel phosphorus alloy, may be produced. The production of 
electrical contacts, and other products, utilizing such a coating 
procedure, also has been recognized. While the plated objects so produced 
have a number of distinct advantages over like but non-coated articles, to 
date there has not been a truly significant commercialization of a wide 
variety of nickel and/or cobalt phosphorus coated articles. This may be 
due, in part, to the relatively quick destruction of baths used in the 
plating processes. 
According to one conventional procedure, in order to obtain an amorphous 
nickel and/or cobalt-phosphorus coating, the major phosphorus component of 
the bath is provided by phosphorus acid, with the nickel provided by 
NiCl.sub.2 to various degrees when a cobalt component of the alloy is also 
desired. Plating can be practiced without any phosphoric acid, but 
typically a small amount of phosphoric acid (compared to the amount of 
phosphorous acid) is added to the bath initially in order to facilitate 
the provision of relatively smooth and bright platings. Such baths are 
usually operated at as low an anode current density as possible, typically 
of about 50 amperes per square foot, or less. Upon extended plating 
utilizing such baths, it has been found that a number of deleterious 
effects occur in the bath over time. in particular, the platings obtained 
from the bath degrade in quality over time, in that they are less 
resistant to corrosion by ferric chloride or concentrated nitric acid. A 
typical lifetime of the bath before it need be replaced to avoid such 
quality degradation is about 30-50 ampere-hours per liter. During this 
lifetime, the cathode efficiency gradually increases from about 40% to 
about 70%. 
According to the present invention it has been found that the major 
contributor of the deleterious effects on the bath has been the ever 
increasing concentration of free acid in the bath. A substantial 
proportion of this free acid is phosphoric acid (H.sub.3 PO.sub.4), which 
is believed to result from the oxidation of phosphorous acid (H.sub.3 
PO.sub.3) at the anode. It has been further found according to the present 
invention that at low anode current densities this oxidation reaction is 
substantial, whereas at high anode current densities it is much less 
substantial, and in fact almost non-existent. Therefore, according to the 
present invention it has been found that it is possible to provide a bath 
for plating nickel and/or cobalt phosphorus in amorphous form that shows 
no significant deleterious effects after 250 ampere hours/liter operation 
where the anode current density is controlled so as to maintain the 
phosphoric acid concentration of the bath substantially constant, and so 
that it does not ever reach a value sufficient to cause deleterious 
effects. Preferably the phosphoric acid concentration is kept below 0.5 
molar. However, it has been found that good plating can be obtained even 
if the phosphoric acid concentration is up to 4.6 molar, as long as the 
acid titer is properly controlled. The cathode efficiency of the bath 
according to the invention retains a value of about 40-50% throughout its 
life. 
While the manifestation of the deleterious effects on the bath is an ever 
increasing concentration of phosphoric acid, it is believed that the high 
concentration of phosphoric acid per se is not what results in the 
deterioration, but rather a condition of overall excessive bath acidity. 
The desired free acid range in baths according to the invention is so 
acidic that pH meters are unreliable. Consequently, the free acid 
concentration is conveniently measured by acid titer. The acid titer is 
the volume (in milliliters) of deci-normal solidum hydroxide required, 
when titrating one milliliter of bath, to reach the methyl orange endpoint 
(which is a pH of about 4.2). The recommended acid titer range is about 9 
to 14, representing 0.9 to 1.4 moles/liter of excess acid. The bath is 
generally maintained approximately 10 mls. acid titer. 
At acid titer below 9, the cathode efficiency decreases, undesirably, to 
below 30%. In the range of about 9 to 13 cathode efficiency is about 
40-60%. Above acid titer 14, cathode efficiency increases to the range of 
70-80%, but the corrosion resistance of the plating deteriorates, 
presumably due to a reduced phosphorus content in the plating. The acid 
titer is lowered by additions of nickel carbonate and increased by 
additions of phosphorous acid. 
There are alternative ways of measuring the free acid level, such as by 
measuring the PO.sub.4.sup.-3, HPO.sub.3.sup.-2, Cl.sup.-, and Ni.sup.+2 
levels and deriving the acidity. However the acid titer method is usually 
easier in practice. 
In a presently preferred bath, which yields a more ductile plating and is 
set forth in said co-pending application Ser. No. 923,270 the disclosure 
of which is hereby incorporated by reference, the desired acid titer level 
is between 20 and 30. 
Preferably, the anode current density is maintained so that it is always 
greater than about 200 amperes per square foot. At levels significantly 
below about 200 amperes per square foot, the desired control of the 
phosphoric acid buildup and/or free acid concentration does not occur. In 
fact, anode current densities of at least about 500 amperes per square 
foot for nickel phosphorus coating baths are preferred. Anode current 
densities as high as 1250 amperes per square foot are useful, and 
apparently the upper limits on anode current density are determined by 
non-electrochemical constraints, such as I.sup.2 R corrosion of accessory 
electrical components (such as bus bars) at higher voltages, etc. 
According to the present invention, the anode current density is preferably 
controlled utilizing a particular anode construction vis-a-vis the cathode 
construction. Typically, the cathode of the bath is provided by the 
workpiece being coated, such as fluid jet orifice plate, cookware, 
cutlery, etc. The cathode-workpiece is immersed in the bath. Disposed 
adjacent to, but spaced from, the cathode, the anode is immersed in the 
bath. The anode configuration is selected so that the anode's effective 
surface area is small enough that the current density is in the desired 
range. 
According to one embodiment of the invention the anode comprises a 
plurality of spaced strips of anode material, and a section of anode may 
be provided adjacent each major face of the cathode. For example, an anode 
may be constructed from 125 individually suspended segments of platinum 
wire, each having a diameter of about 0.01 inches, and each being about 
3.23 inches long. It has been found that platinum and rhodium strips (e.g. 
wires) are more effective over time than other conventional anode 
materials, such as iridium, gold, palladium, rhenium, and ruthenium. 
Platinized titanium prevents the oxidation of phosphorous acid, but spalls 
and in time becomes unusable. 
An anode can be configured of platinum connected to a titanium bus bar by 
connecting (e.g. welding) the ends of the platinum wire to the titanium 
bus, wrapping the wire helically around the bus between its ends, and then 
covering the welds with an insulating material such as a plastic, glass, 
or ceramic. The insulating material must be able to withstand the bath 
conditions without significant breakdown or pollution of the bath. The 
insulating cover may be plastic tubes shrink fit over the welds. In use, 
the exposed titanium quickly develops a protective oxide covering, while 
the platinum wire effectively serves as an anode. Since there is a very 
small anode area, but the bus can carry a significant current, the current 
density of the anode will be at least 200 (preferbly over 500) amperes per 
square foot. As will be apparent, an insulating covering is a desirable 
feature at the location of the titanium/platinum weld, regardless of the 
specific anode configuration. 
According to another aspect of the present invention an anode constructed 
of platinum and titanium that does not spall is formed by making a thin 
wire or bar titanium bus, and shrink fitting a platinum tube over the bus. 
The platinum tube is heated so that it expands, it is placed over the bus, 
and then cooled, shrinking so that the tube makes a mechanical bond with 
the bus. 
It is the primary object of the present invention to provide an improved 
apparatus for the production of nickel and/or cobalt phosphorus 
electrolytically plated articles utilizing a bath having long life, and 
anode configurations particularly suited for that purpose. This and other 
objects of the invention will become clear from an inspection of the 
detailed description of the invention, and from the appended claims.

DETAILED DESCRIPTION OF THE DRAWINGS 
According to the invention, it has been found that if the anode current 
density is maintained at a high enough level, the oxidation of phosphorous 
acid to phosphoric acid within the plating bath is controlled such that 
there is essentially no increase in the level of phosphoric acid within 
the bath, so that deleterious effects that result from an increasing 
concentration of H.sub.3 PO.sub.4 are avoided, and/or the free acid 
concentration is controllable so that it is in an acid titer range of 
about 9-14 (or 20-30, depending on the bath formulation used). The bath 
can have an indefinite life as long as phosphorous acid and sources of 
nickel and/or cobalt are added. These sources initially are preferably in 
the form of NiCl.sub.2 and/or CoCl.sub.2, to promote conductivity, 
together with lesser amounts of NiCO.sub.3 and/or CoCO.sub.3. Makeup 
sources during plating preferably are NiCO.sub.3 and/or CoCO.sub.3, to 
avoid chloride buildup in the bath, while evolving CO.sub.2. Preferably 
according to the method of the present invention the anode current density 
is maintained at a minimum level of about 200 amperes per square foot, 
with a preferred anode current density, particularly for nickel phosphorus 
plating, of a minimum of about 500 amperes per square foot. The desired 
high anode current density may be achieved according to the present 
invention by utilizing an anode of small effective area, utilizing various 
anode configurations. 
One desirable particular anode configuration according to the present 
invention is illustrated schematically, generally, by the reference 
numeral 10 in FIG. 1. The anode 10 consists of a large plurality of widely 
spaced, essentially parallel, strips (e.g. wires, or rectangular 
cross-section segments) 12 of anodic material. The strips are held in 
their widely spaced positions, as illustrated in FIG. 1, preferably by a 
pair of titanium bars 14, with one end of each of the strips 12 being 
sandwiched between the bars 14, and with screws 16, or like fasteners, 
clamping the strips between the bars 14, with a screw 16 disposed between 
each pair of strips 12. For best operation, the anodic material comprising 
the strips 12 is selected form the group consisting essentially of 
platinum and rhodium. Iridium, gold, palladium, rhenium, ruthenium, and 
other like conventional anodic materials, are much less desirable. 
The length, cross-sectional area, number, spacing, and like variables of 
the anode strips 12 may vary widely, so long as the general requirements 
of maintaining an anode current density of at least about 200 amperes per 
square foot (and preferably at least about 500 amperes per square foot) 
are met. In one example an anode 10 would comprise 125 strips 12 of 
platinum wire having a diameter of 0.01 inches, and each strip having a 
length of 3.23 inches. 
Another exemplary anode configuration is illustrated at 110 in FIG. 2, and 
comprises a piece of platinum or rhodium wire 112 which zig-zags back and 
forth between titanium screws 116 associated with a pair of titanium bus 
bars 114, to provide widely spaced portions. 
The anode configuration will vary depending upon the shape of the piece 
being plated, with the object being to have the anode equidistant to all 
parts of the piece being plated, to insure uniform plating. 
A typical bath according to the present invention is illustrated 
schematically and generally by reference numeral 20 in FIG. 3. The bath 20 
includes a container 22 of conventional construction and material, having 
the bath liquid 24 disposed therein. The bath liquid initially includes 
NiCl.sub.2 and/or CoCL.sub.2, a small amount of NiC0.sub.3, a relatively 
large amount of phosphorous acid, and a relatively small amount of 
phosphoric acid. Of course other bath constituents can be utilized 
depending on the particular workpieces being plated, and other conditions. 
In particular the bath of said U.S. application Ser. No. 923,270 is 
desirable. Bath additives that might affect electrical resistance of the 
workpieces being plated, or corrosion protection, include boric acid, 
acetic acid, surfactants of the alkoxylated linear alcoholic class, 
succinic acid, and the like. Typical constituents of an initial plating 
bath would be 1.25 molar H.sub.3 PO.sub.3, 0.30 molar H.sub.3 PO.sub.4, 
0.25 molar NiCO.sub.3, with NiCl.sub.2 and CoCl.sub.2 together comprising 
about 0.75 molar. Where no cobalt is provided in the final alloy, but the 
final alloy being coated is solely nickel phosphorus, as much as about 
0.90 molar NiCl.sub.2 may be desirable. 
In initially making up the bath, the nickel chloride, phosphorous acid, and 
phosphoric acid are added to the bath as liquids and nickel carbonate is 
added to adjust acid titer. As noted above, makeup of nickel ions as 
plating proceeds is preferably effected by addition of NiCO.sub.3 at the 
intervals. 
The bath 20 further comprises, immersed therein, one or more anode sections 
10. As illustrated schematically in FIG. 3, the anode sections 10 are 
disposed with respect to the bath container 22 so that most of the length 
of the strips 12 thereof is immersed in the bath, while the titanium buses 
14 remain above the level of the bath. For the bath 20 illustrated in FIG. 
3, the cathode-workpiece is in the form of a fluid jet orifice plate 26 
which has a pair of opposite major side faces thereof, one of the side 
faces 27 being seen in FIG. 3, which major side faces have significantly 
more area than the other portions of the plate 26. The plate 26 is 
typically clamped by clamps 30 at the ends thereof so that it is immersed 
within the bath, and an anode section 10 is disposed on either side of the 
plate 26 so that each of the anode sections 10 is parallel to and adjacent 
(but spaced from) one of the faces (e.g. face 27). A typical spacing 
between the anode 10 adjacent the face 27 and the other face 27 is 8.5 
inches, although the spacing may be varied widely depending upon the type 
of cathode-workpiece 26, and other conditions. 
The apparatus 20 according to the invention includes as the final major 
component a battery 32, or like source of electrical power, which is 
operatively electrically connected to the anode sections 10, and to the 
cathode-workpiece 26. 
In the practice of the present invention, the cathode current density will 
widely vary depending upon the particular geometry of the 
cathode-workpiece, and other variables. A typical cathode current density 
would be about 50 amperes per square foot, regardless of the cathode area. 
Typical variations in cathode area, and like parameters, in exemplary 
manners of practice of the invention are indicated by the following table 
I': 
TABLE I 
______________________________________ 
CATHODE ANODE 
AREA AM- WIRE ANODE 
PER PERES/ DIA- CURRENT 
SIDE NODE METER DENSITY VOLTAGE 
______________________________________ 
.5 sq. ft. 
25 .01" 280 ASF 
1 sq. ft. 
50 .01" 570 ASF 5.6-5.9 volts 
1.76 sq. ft. 
88 .008" 1250 ASF 
1.76 sq. ft. 
88 .01" 1000 ASF 7.9-8.0 volts 
______________________________________ 
A typical example of the practice of plating using an anode configuration 
according to the present invention is as follows: 
EXAMPLE 
An initial bath formulation comprising 1.25 molar H.sub.3 PO.sub.3, 0.3 
molar H.sub.3 PO.sub.4, 0.90 molar NiCl.sub.2, and 0.25 molar NiCO.sub.3, 
was provided. Two anodes 10 having platinum strips (portions) 12, as 
illustrated in FIGS. 1 and 2, were provided, and the cathode-workpiece 26 
being plated was a 1.8 meter long plate. A number of plates 26 were 
consecutively plated, with sufficient NiCO.sub.3 and phosphorous acid 
being added at intervals to replenish the nickel and phosphorus components 
of the bath. H.sub.3 PO.sub.4 concentration readings were taken at various 
points of time, and were 0.31, 0.31, 0.28, and 0.30 molar respectively. 
Nickel phosphorus coatings produced were amorphous, with a high 
concentration (viz. about 20+ atomic percent) of phosphorus. The anode 
current density was about 1,000 amperes per square foot, with an anode 
amperage of 88 amperes. 
In the FIG. 4 embodiment of the invention, a titanium bus bar 214 connected 
to a power supply 232 supports a platinum or rhodium electrode. For 
example, a platinum wire 212 having spaced portions (ends) 40, 41, is 
connected to the bus 214 at those spaced portions 40, 41. Preferably the 
connection is by welding--see welds 46, 47. It has been found that during 
plating with such an anode a small leakage current passes through the 
titanium to the bath at the weld between titanium and platinum. As a 
result the titanium corrodes in the region of the weld, weakening the 
connection and allowing the platinum anode to become severed from the 
titanium bus. According to the invention this is avoided by providing an 
insulative covering on the bus bar/anode connections--that is over the 
welds. The insulation may be a plastic material, such as a vinyl-like pvc, 
polytetrafluoroethylene, or polyethylene; or a glass, or ceramic. It 
essentially may be any material which is suitably electrically resistive 
and chemically inert in the highly corrosive bath environment, and which 
will not pollute the bath. In the embodiment illustrated in FIG. 4, the 
insulative covering is provided by a pair of plastic tubes 44, 45, which 
are shrink fit over the welds 46, 47. A plastic tube, such as a vinyl 
tube, is heated to expand, and then slipped over the portion of the bus 
bar covering the weld to the platinum wire. Note that the tube 45 has an 
end cap 49, covering the end of the bus 214. 
The construction of FIG. 4 is very desirable in that the anode area is kept 
to a minimum (only the exposed portions of the platinum wire--that is 
those portions outside the coverings 44, 45) while still carrying a great 
deal of current to the electrode. The titanium bus 214 carries a large 
current without excessive heating despite being in air, which is a poor 
heat sink, while the platinum electrode 212 provides the necessary small 
anode area so that the anode current density is at least 200 amperes per 
square foot and preferably greater than 500 amperes per square foot. The 
platinum, despite the fact that it has a small volume, does not overheat 
since the bath serves as a coolant. In use the portion 51 of the titanium 
bus bar within the bath that is exposed to the bath quickly oxides when 
voltage is applied, providing a resistive coating so that the current 
passes through the surface of the platinum, and not the titanium (for the 
most part). 
In the construction of the anode of FIG. 4, initially the bare titanium 
metal is cleaned in a fluoride containing acid, such as hydrofluoric acid. 
After welding one end 40 at weld 46 to the bus bar, the electrode wire 212 
is helically wrapped around the bus bar 214 and the other end 41 is welded 
at 47 to the bus to 214. Then the shrink fit tubes 44, 45 are applied over 
the welds 46, 47. The tubes 44, 45 not only provide a protective function 
for the titanium bus at the welds, but also other portions that they 
cover. As will be apparent, the notion of applying the insulative coating 
to anode/bus bar welds is applicable to other geometries, including those 
of FIGS. 1 and 2. 
FIGS. 5 and 6 illustrate another embodiment of anode configuration 
according to the invention, and FIG. 7 schematically illustrates a method 
of construction of the anode of FIGS. 5 and 6. The anode of FIGS. 5 and 6 
comprises a thin wire or bar titanium bus 314, having a tube of platinum 
or rhodium, 312, disposed thereover, and protecting it. The connection of 
the tube 312 to the bus 314 is provided by heating the tube 312 so that it 
expands (the tube 312 initially having an interior diameter the same as, 
or only very slightly greater than, the outside diameter of the bus 314); 
then inserting the bus 314 into the tube 312 (moving the tube 312 over the 
bus 314); and then allowing the system to cool so that the tube 312 
shrinks to fit over the bus 314, making a mechanical bond therewith. The 
bus 314 is connected up to a power supply 332. In use in a commercial 
bath, of course a large number of the electrodes 312, 314 would be 
provided. They would be arranged in a uniform manner within the bath to 
provide appropriate uniform electroplating, and the size of the anodes, 
and the number thereof, would be such so that the anode current density 
was at least 200 amperes per square foot, and preferably at least 500 
amperes per square foot. 
Other techniques may also be developed for covering, plating, or coating 
titanium rods or bars with platinum so as to provide an effective anode 
construction. 
The use of the insulation to protect the welds of anodes and bus bars is 
particularly suitable to the nickel-phosphorus and cobalt-phosphorus 
platings described hereinabove, because of the desirability of maintaining 
the anode area at a minimum to increase the anode current density. 
However, this aspect of the invention is also useable in other plating 
regimes and is desirable, in particular, wherever anode current densities 
are high, such as the plating onto the interior surface of tubes with a 
wire anode concentric with the tube-cathode. 
It will thus be seen that according to the present invention a bath with 
particular anode construction, and particular anode configurations, have 
been provided which are advantageous, particularly in the plating of a 
substrate with nickel and/or cobalt phosphorus. While the invention has 
been herein shown and described in what is presently conceived to be the 
most practical embodiment thereof, it will be apparent to those of 
ordinary skill in the art that many modifications may be made thereof 
within the scope of the invention, which scope is to be accorded the 
broadest interpretation of the appended claims so as to encompass all 
equivalent structures and devices.