Note: Descriptions are shown in the official language in which they were submitted.
~L~71158
CE~ROMIU~--IRON ALLOY PLATII!~G FROM A SOLUI~IOl!l
COlilTAIWING BOTIl EIE~ V~LE:l IT AI~ID qlRIVALl~lT C~Rt)~IUM
Background and Summary of the Invention
This invention relates to the plating oE a chromium-
iron alloy on a substrate, and more particularly to a plating
process of this description where excellent current
efficiencies are experienced and sound coatings of uniform
coloration are produced.
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Chromium electroplating baths containing dissolved
chromium trioxide, with chromium ions in the bath solution
therefore being hexavalent chromium, are known in the art. A
common Eorm of such an electroplating ba-th contains, in
addition to chromium trioxide and water, a catalyst, such as
a sulfate catalyst, or a mixture oE catalyst, such as a
-~ sulfate and a silicofluoride.
While such baths or electrolyte solutions have
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~ performed satisfactoriIy in the electrodeposition of chromium
- on a subskrate, cerkain disadvantages have attended the use
of su~h plating solutions. For example, current efficiencies
have not been as high as would be desirable to obtain
efficient plating. Furthermore, such solutions generally
have not been employed in the electroplating of a chromium-
iron alloy directly on a substrate.
Studies that have been made o~ electroplating
processes utilizing chromic acid, i.e., hexavalent chromium
;~ ion baths, have indicated the appearance on the cathode,
i.e., the substrate being plated, of a viscose-type layer
which ~orms over the plating being produced. The layer has a
coloring di~ferent from the plating solution as a whole.
Studies made of this layer indicate that it has a composition
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different fro~ the plating solution as a whole, and that such
contains, in addition to hexavalent chromium ions, a significant
concentration of trivalent chromium ions.
Additional studies made of electroplating solutions have
indicated that trivalent chromium ions in the plating solution tend
to produce thereabout what might be thought of as a shield
apparently of an ionic nature. This shield has the effect of
inhibiting the acceptance of electrons by trivalent chromium ions
from the cathode, and the formation from such ions of metallic
chromium deposited on the cathode. These and other studies have
indicated that the production of trivalent chromium ions in the
vicinity of the cathode in many resp~cts is a parasitic type of
reaction effective to consume electricity in the production of the
ions from the hexavalent chromium ions in the solution, without
thereby producing appreciable deposits of chromium of the cathode.
This invention is based on the realization that
significantly improved electroplating results are obtainable from
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a hexavalent chromium bath solution where such solution in addition
to hexavalent chromium, contains significant amounts of trivalent
chromium introduced in the bath solution by the addition to the
solution of an inorganic trivalent chromium compound, such as
chromic sulfate, chromic chloride, and chromic carbonate.
This invention is further based on the realization that
; optimum results are obtained when the plating solution is prepared
to contain, in addition to trivalent and hexavalent inorganic
chromium compounds, a significant amount of an inorganic iron salt
or other iron-ion producing compound, such as ferrous sulfate. the
exact mechanics of what occurs with the inclusion in the plating
solution of iron ions is not entirely understood. It is believed,
however, that such
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results in an effective destruction or deactivating of the
shield above-described which is produced about th~ trivalent
chromium ions resul-ting in increased susceptibility of these
ions to electroplating on the cathode by the acceptance of
electrons from the cathode. Furthermore, of course~ the
advantage results that there is formed on the cathode
directly a chromium-iron alloy in the electroplating process.
The resulting alloy platings that are produced are of good
quality, uniform coloring and produced with high current
efficiencies experienced.
A general object of the invention, therefore, is to
provide an improved method for electrodepositing a chromium-
iron alloy on a substrate.
A more specific object is to provide a method of
electrodepositing a chromium-iron alloy on a substrate
comprising preparing an aqueous electrolyte solution through
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` mixing with water the following ingredients (a~ a water-
soluble inorganic trivalent chromium compound, (b) a water-
soluble inorganic hexavalent chromium compound, and ~c) a
~ 20 water-soluble inorganic iron compound, said electrolyte
solution being prepared with the exclusion of an organic
reducing agent and containing the metals of said compounds as
ions in the solution and the amounts of hexavalent and
trivalent compounds added being such that the concentration
of hexavalent chromium ions in this solution exceeds the
concentration o~ trivalent chromium ions, said solution
further being prepared through hydroxyl ion addition to have
a pH within the range of 0.5 to 2.0, immersing said substrate
in said electrolyte solution, and passing an electric current
through said solution to effect depositing of chromium and
iron on said substrateO
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These and other objects and advantages are attained
by the invention, which will become more fully apparent from
a reading of the following description and examples given in
conjunction therewith.
Describing in more particularity the
electrodepositing method of the invention, the electrolyte or
plating solutions are aqueous solutions, prepared through
mixing inorganic water soluble compounds with water, such
introducing into the aqueous solution the various chromium
and iron cations desired.
A plating tank, vessel or cell containing the bath
solution is provided with one or more anodes, which may be of - ~-
lead, and one or more cathodes, which constitute the
substrate to be plated. A current is established between the
anode and cathode structures. The plating process may
typically be carried out at temperatures ranging from 20 to
80C, with the preferred temperature utilized being within
the range of 35 to 45C. With the passage of time, chromium
and iron ions in the bath solution become deposited as a
chromium-iron alloy on the cathode structure.
Chromium plating may be carried out on various
types of cathode substrates. Such include nickel substrates,
low-carbon steel substrates, iron substrates, copper
substrates, etc. As far as has been observed, the present
~ inventlon is applicable to all of such commonly employed
; substrates.
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Current efficiency, as the term is used herein, is
calculated by determining the mass of the material deposited
on the cathodic substrate during the plating process (which
may be calculated by determining the weight gain in the
plated article), and dividing this quantity by the
theoretical mass that would be electrodeposited under 100
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percent efficient conditions. This assumes that all the
electrons transferred by the current employed in the plating
are effective to reduce metal ions in the electrolyte of the
bath solution to metal atoms deposited on the cathode
structure.
Inorganic trivalent chromium compounds that may be
used with advantage in practicing the invention comprise such
materials as chromic carbonate, chromic chloride, and chromic
sulfate, the latter compound being preferred. The inorganic
hexavalent chromium compound in the usual instance comprises
chromic trioxide. The inorganic iron compound or salt may
; comprise ferric chloride, ferrous chloride, ferric sulEate
and ferrous sulfate~ with good results observed when the
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- ferrous sulfate salt is utilized.
Plating solutions prepared as contemplated, and
without pH adiustment, are highly acidic, and typically may
have, for example, negative pH values. It has been observed
that optimum results are obtained when the pH of the plating
solution is adjusted to be within the range of 0.5 to 2Ø
Such pH adjustment may be produced by introducing into the
solution the-required amounts of a strong inorganic base,
such as sodium hydroxide or ammonium hydroxide.
The presence of sulfate ions in the solution
appears to have a catalytic affect. However, optimum plating
- efficiencies appear to result when sulfate ion concentration
` ; in the plating solution is limited. This may be performed by
including in the plating solution a certain amount of barium
carbonate. With barium carbonate introduced to the solution
a precipitate of barium sulfate in produced which effectively
- 30 removes sulfate ions from the solution. Barium sulfate
production is accompanied with the evolution oE carbon
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dioxide.
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Ordinarily, plating solutions prepared as
contemplated contain a greater concentration of hexavalent
chromium ions than trivalent chromium ions. Thus, in most
instances, the solutions are prepared with the mole ratio of
hexavalent chromium compound to trivalent chromium compound
introduced to the solution being 2:1, or greater. -
In the following examples, electroplating was
performed in apparatus including a vessel containing 25 mls.
of plating bath solution. The substrate plated, or cathode
specimen, was a rectangular piece of sheet material having a
combined surface area on opposite sides thereof, of 2 cms.2.
One hollow cylindrical lead anode was placed in a position
surrounding the cathode specimen, which had a surface area ~-
submerged in the bath solution of 24 cms.2 Plating was
performed on a nickel substrate for a period of one hour.
The temperature of the bath solution was maintained at 40C.
A current density with direct current was maintained at 0.25
A (amperes)/cms.2. Quantities of materials indicated are on
the basis of additions made to one liter of water to produce
the bath solution.
~; Example l
A plating bath solution was prepared utilizing
~; 250 grams CrO3, 90 grams FeSO4-7H2O, and 120 grams of
~ ~ Cr2~SO4)3 5H2O. Without p~ adjustment, the resulting
; ~ ~ solution had a negative pH value. The pH of the solution was
adjusted by the addition of sodium hydroxide (about 100
grams) t~ raise the pH to 1.5.
Electroplating was performed with a bath solution
so prepared on a number of cathode substrate specimens.
Chromium-iron alloy platings resulted on the substrates, that
were sound and uniformly gray. Observed current efficiencies
in making of the platings were in the range of 40-50 percent.
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Example 2
Electroplatings were performed utili~ing a plating
solution prepared as in Example 1, which further included, in
the case of one series of platings, 80 grams of barium
carbonate, and in the case of another series o platings,
120 grams oE barium carbonate. Again, as in Example 1, sound
chromium-iron alloy platings were produced of uniform gray
coloring and good guality. In the case of the series of
platings produced with the addition of 80 grams of barium
carbonate, average current efficiency observed was close to
53 percent. In the case of platings performed with a bath
solution containing 120 grams of barium carbonate, average
current efficiency observed was somewhat greater than
54 percent.
The above set forth examples have been included for
the purpose of illustration and not limitation. It is
desired to cover all modifications and variations of the
invention as would be apparent to one skilled in the art.
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