Note: Descriptions are shown in the official language in which they were submitted.
K~IW(102ll)J~
~ 23~
NOVEL CHROMIUM PLATING PROCESS
Thls invention relates to novel compositlons and
to novel processes for the electrodeposition of chromium ~rom
an aqueous acidic chromium plating bath.
BACKGROUND OF THE INVENTION
In the present art of plating there is great hesitakion -
in chromium plating any basis metal containing aluminum~
~luminum alloys or even alumlnum-bearing alloys such as zinc-base
~ die castings with high-speed (fluoride containing) baths. In
,~ the plating of these metals, it ils usual to use chromium plating
baths containing only the sulfate ion as a catalyst because
fluoride or complex fluorlde ions cause more rapid dissolution
of the aluminum-containing metal. Increased etching from the
presence o~ fluorides or complex ~luorides in the chromium
plating bath has two notable e~fects~ (1) exposed metal may
show an etched or dull appearahce and (2) the chromium bath may
actually manifest in passive deposits, reduced plating efficiency
~and bad composition balance. The desirabllity of having
fluorideæ or complex fluorides present for high-speed plating
~in~hard and decorBtive plating and for good activation
stainless steels and some metals is well known.
Heretofore inclusion of ~luminum ln chromium plating
~baths~has been~considered undesirable because of the debilitating
~e~fects characterized as white streaks~ blotches in the high
~and intermediate ourrent density ranges and brown or iridescent
; 25 films in the low current density ranges as well as l!OSS in ~ :
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cathode efficiency. The concentration of aluminum in the chromium plating
bath continually varies during operation due to the addition of aluminum
impurities arising from the catalyst and basis metal decomposition products.
It is an object of this invention to provide improved electro-
plating compositions and processes. A further object of the invention is to
provide stabilized chromium plating bath compositions. Other objects of the
invention will be apparent to those skilled in the art upon inspection of
the following detailed description of the invention.
In accordance with one of its aspects, this invention relates to
a process for electroplating chromium plate onto a basis metal which com-
prises passing current from an anode to a cathode, at least a portion of
which contains a conductive metal layer through an aqueous acidic chromium
plating solution, at least one chromium compound providing hexavalent
chromium ions for electroplating chromium, and aluminum ions in combination
with fluoride ions and sulfate ions, maintaining a molar ratio of total
1uoride to total aluminum of from 0.5 to 6, for a time sufficient to deposit
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a chromium electroplate.
According to another aspect of the invention, there is provided
a composition suitable for electroplating chromium plate onto a basis metal,
the composition being an aqueous acidic chromium plating solution, contain~
ing at least one chromium compound providing hexavalent chromium ions for
electroplating chromium, and aluminum ions in combination with fluoride
ions and sulfate ions maintaining a molar ratio of total fluoride to total
aluminum of from 0.5 to less than 6.
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It has now been found that the addition of aluminum ions in
combination with fluoride and sulfate catalyst ions in an aqueous
acidic bath containing chromic acid results in a chromium electro-
~ plating composition which is stable over the life of the bath and
-~ maintains the activi~y of the bath over its useful life by regulating
the concentrations of the aluminum ions and fluoride ions as the ehromic
acid eoncentration is depleted and in the presence of typical impurities
` and decomposition products which occur during the chromium plating
` operation.
Preferred basis metals are stainless steels, aluminum alloys
and aluminum. Other metal articles which may be plated in accordance
with this invention include niekel, nieklsl-eobalt, and eopper plated
`` ~ine allo~ articles.
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The fluoride to be added to nullify excess aluminum
may be a simple fluorlde such as is obtained from hydrofluoric
acid, alkali or alkaline earth fluorides and other fluoride salts
or may be a complex fluoride such as silicofluoride and other
complex fluorides which will furnish fluoride ion in solution.
Other complex fluoride ions which may be utilized in the operation
of the bath may include ions of BF4-l, AlF6-3, TiF6-2, ZrF6~2,
etc.
The baths are characterized by the presence of
aluminum and fluoride in a ratio such that there are fewer than
six~atoms of fluoride present per atom of aluminum. Fluoride
means total soluble ~luoride, including that found in complexes.
Aluminum is total soluble aluminum. These can be determined
by usual analytical methods, e.gO, aluminum by atomic absorption
and fluoride by distlllation and titration. ~hus, where M
i~ the moles~litre Or aluminum and MF is the moles/litre of
fluorlde, it is required thats
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Al ~ -
and prefer:
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In addition the chromium plating baths must conform to the
usual specifications on ratio of chromic acid to catalyst to
be operative.
(CrO3)
(S0-4 ~ (F-)
Where y is a number depending on the complex e.g. 2 for SiF-6
and l/4 for simple F-.
The dissolution of aluminum causes a decatalyzing
effect, i~e., khe presence of aluminum in solution effectively
reduces the available catalyst, especially the available fluoride.
Thus, dissolving aluminum in the bath causes the deposits to be
blotchyj gray, and even form ~ilms characteristic o~ a bath of
too high ratio, i.e., low or reduced catalyst content. (F-) is
defined as available fluoride. ~[F/MAl is less than six (6),
: r as used for oomplex fluorides is appropriate, i.e., r =2.
The operable ratio may be defined as:
- CrO3
R = ~
(S0~ + (FAv3/2
~ - .
Useful and operative baths of this invention ~all within the
approximate 11mits of:
; 50 ~ R ~ 160
and pre~erably:
;~ ~ ~ 70 ~ r - 130
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The available fluoride may be determlned in either of two ways.
A series of standards are prepared containing known quantities
of hydrofluoric acid or its simple non-complexing salts such as
sodium fluoride or potassium fluoride. These standards may be
prepared in solutions of chromic acid and at appropriate
temperatures. The effect of this available- fluoride is measured
in either of two ways:
(1) A selective ion electrode reversible to fluoride
ion may be inserted into the baths and the emf produced recorded.
A graph of available fluoride ion concentration vs. emf is thus
prepared with di~erent curves ~or dif~erent chromic acid
concentrations and different temperatures. Subsequent emf
measurements with the electrode in chromium plating baths then
uniquely determine the available fluoride ion concentration in
the bath.
~2) Standard curves can also be produced by measuring
weight loss of a known area o~ aluminum immersed in a given
amount o~ chromic acid solution for a given period at a given
r ~ I temperature.
The determinatlon o~ the active fluoride content of an
.
~acid solution containing fluoride may be effected by immersing
aluminum metal in said acid solution, maintaining said aluminum
metal in said solution for a predetermined time during which a
,~ ; portion o~ said aluminum metal dissolves in proportion to the
~ 25 active fluoride content of said solution, withdrawing said
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aluminum metal from said solution, determining the weight loss
from said aluminum metal during said immersion, and converting
said weight loss into values showing the concentration of active
fluoride.
The determination of the active fluoride content of
such a chromium plating bath may be effected by preferably
isolating an aliquot test portion of the bath. Typically the
aliquot portion may be lO0-lO00 ml.~ say 500 ml. Preferably
the analytical test may be effected at 43C.~lC.; and
accordingly the aliquot may be placed within a constant
temperature water reservoir malntained at this temperature for
at least one hour until water reservoir temperature is reached.
` In practice a piece of aluminum may be employed. This
piece may be pure aluminum or aluminum alloy in the form of a
strip or sheet, typically 0.2 to 2, say 1 mm. thick, kypically
5 mm. to 25 mm., say lO mm. wide~ and 50 mm. to 150 mm., say
75 mm~ long. The strlp may be cleaned typically by wiping lt
wlth a cloth and bent into convenient U-shape which facilitates
placlng the strip in the aliquot and maintaining it in exposed
relation to the allquot~with minimum area of contact with the
~container. The so-treated, cleaned strlp may then be weighed
to the nea~rest tenth of a milllgram. Satis~actory results
may normally be obtained by use of a str-ip which typically may
w-18ht ~ro 1 g. to 5 g., typloal1y about 2 g,
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The so-treated aluminum strlp may then be placed in
central portion of the container in which the aliquot solution
is maintained, so that the strip is standing up in the center
of the container. The strip may be maintained therein for an
exact predetermined period of time, typically 10 minutes to 60
minutes, and preferably for exactly 30 minutes. During this
time, the solution is maintained preferably at the hereinbefore
noted temperature of about 43C. i.e. 43C.+1C., and no agitation
is provided other than that generated by the reaction of the
aluminum with the solution. ~
~ The chromium plating bath which may be employed in
practice of this invention may be an aqueous solution containing
10 g./1.-500 g./l., typically 15t) g./1.-300 g./l., say 225 g.il.
~chromic acid CrO3. The bath may also contain catalysts, as
herein defined, the ratio of chromic acid to catalyst typically
being from 10:1 to 150:1, pre~erably 50:1 to 100:1, say 80:1.
The ratio as the term is used in this appllcation, refers to the
ratio of ~-
CrO
~ ~ - ( SO I, ) + ~i7
i~ 20~ wherein each of the quantities i6 specified in g./l. ~he symbol
~F- may refer to the fluorlde-con~ainlng catalyst which may be,
;~ e.g. fluoride se or silioofluoride SiF6=, or other ~luoride-
- contalning ions as set forth infra. ~ is an integer which may be
~ two ror fluoride complexes,~e.g. SiF6= and 0.25 for fluoride ion
:! 25 ~e. Typically the concentration of catalyst ln the bath may be
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~57231
0.7-50.0 g./l., preferably 1.5-6.0 g./l. The catalyst may
include sulfate S04-~ typically provided as sulfuric acid,
strontium sulfate, etc. The sulfate ion may typically comprise
5%-85%, preferably 10%-50%, say 30% of the total catalyst
concentration. It is a particular feature of this invention
that the chromium plating bath contain fluoride 3 typically
supplied as fluoride ion F- or as a complex fluoride. Typically
complex fluorides may include fluoroaluminates, fluorozirconates~
fluorotitanates, fluoroborates, etc. The preferred fluoride
ion may be silicofluoride SiF6-, preferably provided as
potassium silicofluoride. The preferred catalyst may comprise
fluoride in amount of 15%-95%, typically 50%-90%, say 70% o~
the total catalyst.
In the practice of this invention, the temperature
o~ plating may be 27C.-90C., typically 50C.-70C., say
60C.
; The chromium electroplating bath compositions o~
the invention may contain about lon-500 ~ . of chromlc acid
(expressed as CrO3) and preferably about 200-350 e-/l f ;
~20 ~ chromic acid. The fluoride ions may be added to the chromium
~electroplating bath in the form of compounds su~h as sodium
sllicofluoride (Na2SiF6), fluosilicic acid (H2SiF6), strontlum
silicofluoride (SrSiF6), ammonium silicofluoride [(NH4)~SiF6],
magneslum silicofluoride~(MgSiF6), calcium ailicofluoride (CaSiF6)
¦¦ cto. Obhe luoride compound~ whioh may be y~ed in the bath
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according to the invention include sodium flu~ride~ potassium
fluo.ride, calcium fluoride, hydrofluoric acid, ceric fluoride,
cerous fluoride, etc. Other fluoride lons (including complex
fluoride ions) which may be employed include, for exampleg
fluoalum,nates, fluoborates, fluotitanates, and fluozirconates.
The sulfate ions may be added to the chromium -~
electroplating bath in the form of suitable sulfate compounds
such as strontium sulfate (SrSO4); sulfuric acid (H2SO4);
lithium sulfate (Li2SO4); ammonium sulfate [(NH4) 2S4 ]; .
calcium sulfate (CaS04); etc.
The chromium electroplating process may use temperatures
of 30-70C. with a chromic acid bath containing 100-600 g./l. of
chromic acid (as CrO3). The ratio of chromic acid to sul*ate
ion (CrO9:SO~~) may be malntained at 100-550:1, typically
;~ 15 150-300;1 and preferably about 200:1. :
The:Pollowing examples are submitted for the purpose
o~ illustration only and are not to be ~onstrued as limiting the
~scope of the invention in any way. - ~ .
~: : ~ The unexpectedly advantageous properties obtained by
20~ ;using the aluminum ion in combination with fluorldes (including
~complex Pluorldes) may be shown by reference to the examples
~;: summarised in Table ~I. In~each experiment, a sparingly soluble
;~ ~fluorlde salt was Pormed and enough oP the compound was used
~n the bath to provide an excess of undissoIved compound. Thus,
: 25 ~ an~alumlnum ~luorlde-oontalning precipi~ate remained undissolved
~ in each solution and acted as a reservoir to control the :
:~ concentratlon of:~the catalytic 10n7
. In each o~ the bath compositions described in Table I,
excellent chromium deposlts were obkalned,
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1057Z31
EXAMPLE 10
.
Test panels were plated in a plating cell and increments
o~ fluosiliclc acid were added to a bath originally containing
200 g/l CrO3, o.8 g/l S04= and 7.5 g/l A1~3 (0.278 Moles).
Up to 6 g/l of added SiF6= the test panels all showed
haze, blotch and brown films, the usual symptoms o~ a bath
contaminated with aluminum. At 7.2 g/l fluosilicate the panels
were acceptable with some high current density haze which was
lost at 11 13 g/l. Excellent deposits were thenceforth plated
with some loss in coverage first being noted at about 17 g/l
and no untoward high current density effects were obtained until
about 24 g/l atarted to show serious over-catalyzation.
Thus under these conditions 7 g/l (0.049 Moles) to
24 g/l (0.169 Moles) fluosilicate was acceptable with 11 g/l
(0.077 Moles) to 15 g/1 (Ox0~105 Moles) being preferred. Note
that the operative range o~ mole~ular ratios of ~luosilicate
to aluminum have a range o~ 1.1 to 2.3. ~his is in contrast
~wlth usual fluoride-containing baths when as little as 1-2 g/l
~of~ excess alumlnum cause the b~th to be 1noperative.
In;a bath containing 200 g/1 CrO3~ o.s g/l S042 ab
20~ ~ 490C~ A satis~actory deposits were obtained on stainless ~teel~
~when the available~fluoride concentration was from about
.9 3,7 g/1. ~
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EXAMPLE ll.
A bath containing 600 g/l CrO3, 2.84 g/l Na2 S04,
4.20 g/l Na3 Al F6 and 3.12 g/l Al (OH)3 by first making a dry
mix of the above then dissolving in distilled water and diluting
to one liter; MF 0.12 Moles, MAl o.o6 Moles, 3.2 g/l estimated
(F-Av), 2.0 g/l MF/MAl, R 170, resulted in a deposit with
passivlty patches.
EXAMPLE 12 (Control)
Hull Cell panels were also plated using the following
chromium plating bath composition:
ComponentConcentration
Chromic acid ~ 600 g/1
tcrO3)
Sulfate 1.92 g/1
(SO.,=). ,
` Fluoride 3.2 g/l
(F-)
MF/MAl 2.0 g/1
. After a plating time of 3 minutes at 5 amperes the
resultine deposit had passivity patches.
EXAMPLE 13.
The prooess of Example 12 was followed except that
the ratio of fluoride to aluminum was adjusted to 2.9. A total
plating time of 5 minutes was required to obtain an acceptable
.~ ¦¦ deposlt wi a Fhrom~um co~erage of 75 mm.
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1057231
EXAMPLE 14.
The process of Example 12 was again followed except
that a bath with the following composition was used:
600 g/l CrO3, 1.92 g/l S04, MF 0.17 Moles, MAl 0.06 Moles,
4.0 g~l tF-AV) (estimated), 2.9 g/l MF/MAl~ R. 153, which
resulted in a deposit with very good chromium coverage of 75 mm. ;~
600 g/l CrO3, 1.92 g/l SO", MF 0.22 Moles, MAl 0.06 Moles,
4-8 g/1 (F AV)~ 3-7 g/1 MF/MAl, R- 95, resulted in an acceptable
deposlt w1th chromium coverage of 48 mm.
Make a dry mix of 150 g/l CrO3, 1,48 g/l Na2 S04 ~
1.27 g/l NaBF4. Add distilled water, dissolve, dilute to 1 liter.
Test panels - 4 amps. 3 min. 35C.
A. Original test panel, no aluminum, 51 mm chromium
coverage.
B. Dissolved 4 grams of Al therein ~by adding Al (OH) 3] .
Test panel showed extensive passivation.
150 g/l CrO3, 1.0 g/l so4-2, .148 MF/MAl,
0.80 g/l (F-AV) (estimated), MF/MAl = 0.31,
R. 107, plate passive.
`~ 20 CO Added 4 g/l TiF6.
Test panel good bright chromium.
150 g/l CrO3 ~ 1. O g/l S04 2, .146 MF,
.148 MAl~ 2-1 g/l (F-A~T) (estimated~,
ME ll~l - I.O, R. 71, chron1um coverage to 61 n
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EXAMPLE 16.
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Although chromic acid concentrations of less than 100-600
g/l may be used, the preferred range is 100-600 g/l~ and the
most preferred range is 200-400 g/l.
In general, as the aluminum concentration increases the
equilibrium solubility of the alumino fluoride remains
relatively constant. There are at least two results from this
fact. The bath parameters may conveniently be changed in two
ways: the addition of fluoride would eventually cause
precipitatlon of cryolite or the addition of aluminum could
directly precipitate aluminum fluoride (likely hydrated).
Effective regulations in aluminum fluoride ratios can be
obtained by changes in f'luoride concentration with or withou
corresponding modifications of aluminum concentration. The -
concentration of the fluoride ion may thus be suppressed by
the use of an excess of aluminum ion to directly precipitate
an alumino fluoride. This was verified by a solubility study
on aluminum fluoride under various conditions of chromic acid
oonoentration and temperature. The data is summarized in
Table II.
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ABLE II
Concentration Temperature Solubility of
Chromic Acid Aluminum Trifluoride
gi~l C . ' g/l . ' '
200. 38 12.ll
200. 49 14.7
200. 60 17.5
400. 49 14.7
100 4~ 12.l~
Thus it has been shown that aluminum fluoride has limited
solubility in chromic acid solutions and in a useful range for
chromium plating.
EXAMPL~ 17.
Hull Cell was used with a bath of the following
composition:
ComponentCon -ntration . .
Chromic acid 200 g/l
:' (CrO3)
Sulfate:0.8 g/l
Fluorideo.8 g/l - 3.25 g/l
(F-)
Aluminum0 g/l _ 4.5 g/l
(Al)
~ 9.2 grams per liter of hydrofluoric acid and 4.5 grams per liter
o~ aluminum were added incrementally to the bath to plate 304
stainless steel Hull Cell panels. The coverage on 304 stainless
; ~ steel psne1s lmproved from 55 to 83 mm. as the added aluminum
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content was increased from zero to .35 g/l. The fluoride
concentration test showed a concomitant decrease in fluoride
concentration from 3.25 g/l to 1.2 g/l. At 4.5 g/l excess
added aluminum the fluoride decreased to o.8 g/l and the test
panel showed passivity. ~hus it has been shown, unexpectedly,
that a self-regulating bath can be based on the limited
solubllity of AlF3 and that Al+3 can be used as a suppressant.
Further examples of the use of aluminum as a
suppressant were conducted where chromium coverages of 90 mm.
on stainless were obtained from an Al~3-saturated bath with
an excess aluminum addition of only 0. o8 Moles. Neutralization
is not significant at this level.
,
According to this invention, the aqueous acid
chromium plating bath compositions of the invention may be
employed at temperatures of about 30-70C., to prepare improved
chromium plated art~cles by passing current ~rom an anode to a
basis metal cathode through said aqueous acidic chromium plating
solution at a temperature of 30-70C., for a time sufficient to
deposit a chromium electroplate. Current densities which may be
employed may be l.0-90 amperes per square decimeter (asd) and
preferably about 3-50 asd. The bath compositions of the
invention may be used with lead anodes and/or lead alloy anodes.
Although this invention has been illustrated by
reference to speoific embodiments, modifications thereo~ which
7~ 25 are clearly wlthin the scope of the invention will be apparent
to those skilled in the art.
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