Note: Descriptions are shown in the official language in which they were submitted.
HIGH-RATE CHROMIUM ALLOY PLATING
Back~round of the Invention
Extensive use of relatively scarce materi-
als, such as nickel and chromium, in corrosive envi-
ronments may be reduced by an acceptable plating pro-
5 cess which may form a corrosion- resistant coating of,
say, 25 um of a chromium alloy, on an inexpensive
substrate~ such as steel or brass. A bright, decora~
tive coating of chromium alloy is also valued in some
uses.
In the past, most commercial plating of
bright chromium has been carried out from solutions oE
hexavalent chromium, such as chromic acid. Unfortu~
nately, these baths, where chromium is complexed as an
anion, are historically ineffective for plating al~
15 loys. Efforts at plating from divalent and/or tri
valent chromium solutions have allowed the production
of some alloy plate, but at low deposi~ion rates and
typically at current densities below abo~t 1 A/in2 (15
A/dm2), and often much lower.
Moreover, in electrodepositing an alloy from
a solution containing metal ions, it is well known that
a less active metal will deposit in preference to a more
active metal~ Considering chromium alloys containing
iron and/or nickel, the relative nickel, iron and
25 chromium reduction potentials would be expectecl to
result in deposits which are rich in nickel and iron.
The chromium is clearly more active with a potential of
about --0.74 volts for the Cr+3 to Cr reduction.
Summary of the Invention
_
It is an object of the present invention to
provide a method of plating chromium alloy with iron
and, optionally nickel and/or cobalt.
It is al~o an object to plate such alloy
composition ~hich may substantially approximate the
35 metal ratio in the electrolyte, in spite of the differ-
ence in activity of the metals~
It is also an object to provide a high-rate
plating process for chromium alloy.
It is further an object to provide such
electrodeposition process for producing chromium alloy
from solutions comprising divalent and trivalent chro-
mium.
It is finally an object that such process be
controllable to yield a ~hick, dense chromium alloy
deposit.
In accordance with the objectives, the in-
vention is a method for high rate plating of chromium
alloy from an electrolyte solution containing divalent
and trivalent chromium ions, ions of iron and, option-
ally, ions of nickel and/or cobalt as additional al-
:Loying constituents. The high-rate plating is carried
out at a current density of at least about 75 A/dm2
(preferably at least about 150 A/dm2), a pH of between
about 0~5 and 2.0 and with relative motion between the
cathode and the plating solution of at least about l
20 m/sec (preferably 1-8 m/sec).
Deposits of composition 5-80% (by weight)
chromium, 20-95~ iron and 0--50~ nickel are preferably
formed by electrolyzing an electrolyte solution having
metal ion concentrations of 20 g/l to saturation diva--
25 lent/trivalent chromium~ l-S0 g/l iron and 0-50 g/l
nickel. Complexing anions of sulfuric, sulEarnic, hy-
drochloricl phosphoric and boric acids are preferred in
the electrolyte. When using insoluble anodes, a porous
barrier is typically positioned around the cathode to
30 prevent migration of anode oxidation reaction products
to the cathode where they would otherwise oxidize the
divalent/trivalent chromium to the hexavalent state.
Within the general conditions stated above,
the inventor has also discovered that the best deposits
35 of chromium alloy may be obtained by strictly main-
tainir.g the free acid of the electrolyte within a
narrow range corresponding to a pH of about 1.7 to 1.8.
- ^ ~
Very accurate metering must be used to monitor pH or a
titration may be necessary to establish the amount of
free-acid in the bath.
The invention also comprises the novel aque-
5 ous electroplating solution which comprises of from 20
g/l to saturation divalent/trivalent chromium ions,
1-50 g/l iron ions and 0 50 g/l total nickel ions, with
a pH adjusted to between about 1.7 and 1.8. Complexing
anions of mineral acids may be used in the elec~rolyte
10 solution.
DescrLption of the Invention
_______
The invention is a method for electroplating
a chromium alloy containing iron and, optionally, nick-
el and/or cobalt. The alloy compositions preferably
15 fall in the range (by weight) of 5-30~ chromium, 2~-95~
iron and 0~50~ total nickel and/or cobalt. We have
found that alloys outside of this range may be pJated
according to the invention, but for the desired cor-
rosion-resistance of the coatings, at least about 5-10~
20 chromium is neces~sary. Chromium and nickel contents
above the preferred range unduly raise the cost of the
alloys and are, therefore not preferred. Chromium-
nickel-iron alloys are the preferred coating composi-
tions and, in particular, the 300 and 400 classes of
25 stainless steels are preferred. Type 304 stainless
(18~ Cr-8% Ni-~ Mn-balance Fe) is one desirable com-
posi~lon. ~lowever, examples and discussion regarding
chromium-irorl-nickel alloys are intended to include
alloys wherein cobalt may be substituted, as known in
30 the art, for all or a portion of the nickel. Other
impurities which may enter the deposit from the anode,
for example, may also be deposi~ed without harm. Man-
ganese, silicon and copper are examples.
The alloy coatin~ is formed on a conventional
35 cathode surface of, for example, steel, iron, aluminum,
brass or copper. Insoluble anodes, such as made from
lead, may be used, although soluble alloy anodes of
iron and chromium have been most useful in the inven-
tive process.
Plating Solution
The electrolyte is a divalent/trivalent
chromium salt solution preferably containing 20 g/l to
saturation of chromium ions, 1-50 9/l iron ions and a
total of 0-50 g/l of nickel and/or cobalt ions. The
trivalent chromium may be converted to the divalent
10 form and vice versa so that the exact ratio thereof was
not clearly identified. Therefore, the two species are
believed to both be present and necessary~ and the
refererlce to trivalent chromium is also intended to
include the lower ~e~e which coexists in the bath.
15 Excess divalent form can adversely affect nickel depo-
sition because it tends to reduce the nickel ions to the
metal, resulting in precipitation or plating on the
walls, etc. of the cell.
Some electrolyte solutions require a period
20 of stabilization before yielding superior product.
This may be due to a need to produce some particular
minimum quantity of divalent chromium in the bath.
Conventional complexing anions for chromium
plating are also necessary in the inventive method. In
25 particular, these include the anions from the mineral
acids: sulfuric, sulfamic, hydrochloric, phosphoric
and boric acids.
The pH oE the electrolyte has been found to
be a critical factor in depositing thiclc~ bright and
30 semi-bright coatings. Within the pH range of 0.5~2.0,
good chromium alloy coatings can be deposited which are
matte t~xtured, but which are still useful in some
applications of corrosion and wear resistance These
coatings are generally limited in thickness ~o about 12
35 to 25 ~m. Thicker coatings tend to crack or peel as a
result of increasing internal stresses.
It has been found, however, that when the
acidity of the electrolyte corresponds to a pH of
between about 1.7 and 1.8, bright and semi-bright
coatings can be obtained which are adherent, dense and
5 crack-free, even a~ thicknesses above 125 ~m. The
reason for this phenomenon is not understood at this
point, but the result is dramatic over this range.
The acidity range is so narrow that diffi-
culty May be encountered in accurately measuring and
10 maintaining it throughout the solution. Certainly,
sensitive instruments exist for measuring the pH, and
in practice a pH meter might be used for convenience.
However, for accuracy~ we prefer to determine the
acidity by measuring the amount of "free acid" by
15 titration against a standard basic solution. We define
the "free acid" content as the quantity of 0.1 N NaOH
solution needed to bring a 1.0 ml aliquot of electro-
lyte to pH 3.5. The preferred range of free acid usiny
this titration method is about 0.5 ml - 1.5 ml NaOH,
20 corresponding to the pH of about 1.8 - 1.7, respect-
ively.
The temperature of the plating solution is
preferably in the range of 25-75C.
Operatin~ Conditions
~long with acidity, the most critical oper-
ating parameters to obtaining craclc-free, adherent
coatings are the current density and the agitation or
solution flow. The acidity and solution flow particu-
larly affect the depositiorlrate and the density oE the
30 coatinc3, but acidity does not significantly affect
composition of the deposit except at very low pH where
nickel and iron plati~g reactions decrease in effi-
ciency. Compositio~ s~ m~rè particularly afected by
the current density and the electrolyte composition.
It is well understood that the least active
metal will deposit in preference to a more active
metal. But in the inventive method, using high current
density and solution flow, the composition of the
deposit can be made to more closely approximate the
electrolyte composition than in prior plating methods,
especially for the iron-chromium binary alloy from sul-
famate solutions, even for high-chromium depositsO
Current densities for the inventive method
lO are at least 75 amps/dm2, but preferably within the
range of about 150-400 amps/dm2. The higher current
densities favor deposition of chromium over the iron or
other metals and are necessary for obtaining the high-
chromium alloys from the trivalent chromium solutions.
At such high current, the chromium, iron and
particularly the nickel or cobalt, would be hard to
plate in dense, adherent deposits were it not for high
agitation or solution flow rates in conjunction there-
with. I'urbulent action near the cathode, resulting
20 from cathode motion or solution flow, creates a trans-
port mechanism for replacing depleted electroly~e with
cation-rich solution. Relative motion of at least l
m/sec bet~een the cathode surface and the plating
solution is generally sufficient to create the ~.urbu-
25 lent conditions necessary for good deposits~ Typi-
cally, velocities of l-~0 m/sec could be usedr but 1-8
m/sec i9 preferred.
With the agitation and other means Eor mi-
gration of anode products to the region of the cathode,
30 :it may be necessary to erect a barrier between an
insoluble anode and the cathode to prevent the anode
products from oxidizing the divalent and trivalent
chromium near the cathodeO Conventional porous mem-
branes (ceramic cups) may be used around the cathode
3S for this purposeO
Examples of the Preferred Embodiments
Example 1 - Iron-Chromium Alloy
Composition Compa~able to Bath Composition
According to the invention, an alloy may be
5 deposited having a composition ratio virtually the same
as the metal ratio in the electrolyte, despite the
difference in reduction potentials of the chromium and
iron plating reactions.
In samples identified as 43F and 52A, an iron
10 and chromium sulfamate electrolyte was made by dis-
solving the metals in an acid solution of sulfamic
acid. The concentrations were 0.25 molar chromium (13
g~l Cr) and 0.75 molar iron (42 g/l Fe). The current
density was 160 amps/dm2 and the rod-shaped steel
15 cathode was rotated with a 2.5 m/sec surface velocity.
A lead anode was utilized and was isolated Erom the
cathode by a porous alumina diaphragm. Temperatures
were between about 37 and 49C.
Sample 43F used a 10 minute deposition at pH
20 1.6 while sample 52 plated for 5 minu~es at pH 1.7. In
both cases the alloy composition weight ratio was
substantially ~he same as the electrolyte, 72 Fe - 28
Cr and 75 Fe - 25 Cr (~3%) respectively. Cathode
efficiencies were about 26 ~ 27%.
At the end of the deposition, the lead anode
showed signs of dissolving ~n the sulEamate bath. To
avoid this in longer deposi~iolls, a platinum or graph-
ite anode or, preEerably, a soluble anode could be
us~ .
Example 2 - pH Effects
Recognition of the importance of pH occurred
when plating several 47 mm~diameter rings (as cathodes)
in succession in a bath containing chromium sulfate
(0.9 moles), iron sulfate (0O4 moles~ and sulfuric
35 "free acid" ~1.3 ml)~ pH was measured at 1~75. A
~ 8--
lustrous deposit having a few matte spots was plated at
160 amps/dm2 and 3 m/sec cathode surface velocity.
Deposits on successive carbon-steel rings improved to
almost full bright plate and then began getting more
5 matte textured as the pH increased ~o about 1.8 (free
acid of 0.5 ml). Sulfuric acid was added ~o bring the
free acid to about 1.1 ml and adherent, bright plates
were again deposited.
The kright plates were tested and found to be
10 extremely adherent, corrosion resistant to nitric ac.id
and resistant to high-temperature oxidation. Hardness
was on the order of 410 ~Knoop) with a 100 gram load,
equiva:Lent to Vickers DPH=360 or Rockwell C ~ 39.
Example 3 - Preferred Alloy Composi~ions in Chloride
and Sulfate Baths
A number of coatings were applied to 12.5
mm-diameter steel rods, 25 mm long, from sulfate and
mixed sulfate/chloride solutions having the following
compositions:
~0 Concentration
Chem_cal Added As: (g/l)
1 2 3 4
Chromium Chromium chloride 39 49 0 0
Chromium Chromium sulfate 0 0 ~6 39
25 Iron Ferrous sulfate 3 1 6.6
Nickel Nickelous sulEate 45 1]. 11 5.6
Boric Acid Boric acid 35 35 23 35
Ammonia Ammonium sulfate 14 14 8.5 13
Manganese Manganous sulfate 0 0 2.2 3.3
A Type 304 stainless steel alloy anode was
used. Operating parameters are given in Table 1~
- - 9 -
Manganese content in the alloy samples was less than 1%
and is, therefore, not reported.
Table_
_ _ _
Deposition Conditions Alloy
Composition
~ _ . .
Sample pH C.D. Temp. Agita- Solu~ Cr Ni Fe
No. (~/dm2) (C) tion tion (w/o) ~/o) (w/o)
(rn/sec) No.
__ _
203/4-13A 0.45 160 62 2 1 53O7 1.6 44.7
-14C 1.4 310 62 2 2 26.5 1.9 71.6
l9E 1.8 160 62 2 3 21 22 57
-18D 1.5 160 62 2 3 4 41 55
-18F 1.65 220 62 2 3 16 36 48
-19L 1.8 160 65 2 3 8 48 44
-20~ 160 50 2 4 31 7 62
The chromium content in the al:loy cleposit is
dependent on several operating conditions, including
current densi-ty, a~itation, pH, ratio of metal ions in
solution and type of anion used to complex the metal
~0 ionC~. Comparing samples 13A and 14C, the difference in
p~1 is the ma~or variable and the chromium content is
higher when the pH was lower (higher acid content)O
This is reasonable because the coulombic (cathode)
efficiency for plating both iron and nickel is known to
25 be poor at the lower pH values.
Samples 18D and 18F were plated under similar
conditions with the exception of current density. The
~s~
-10-
results show ~hat the higher current density used for
sample 18F resulted in a higher chromium content.
Temperature also affects the percentage of
chromium in a deposit. Comparing samples l9E and l9L,
5 the temperature was increased ~rom 62 to 65C and the
chromium content in the deposit was reduced from 21 to
8 percent~ In general, the temperature does not appear
to be quite this critical, but higher temperatures do
not favor the chromium deposition.
It is evident-that by making several changes
in the plating parameters, for example, lower temper-
atures, higher pEI, higher concentration of chromium and
lower concentrations of both nickel and iron, the alloy
deposit may be pushed to a highe~ chromium and a lower
15 ni.ckel content.
Generally, good bright and semi-bright coat-
ings were obtained in the deposits plated between about
pH 1.7 and pH 1.8 while the others were matte textured
and sub~ect to cracking in thicker deposits.
Example 4 - Cr-Fe-Ni Alloy in
Mixed Chloride/SulEate Bath
Sample 202/98--14E was plated in a conven-
tional cell using a soluble Type 304 stainless steel
anode and a solution oE:
25 Chromium (chlori.de salt) 4808 g/l
Iron (sulfate salt) 1.0 g/l
~ickel (sulfate salt)11.2 g/l
~ori.c acid 35.0 g/l
Ammonium Sulfate 55.0 g/1
30 The temperature was 62C and the pH was 1.4.
With a ca~hode surface velocity of 2 m~sec
and a current density of 155 amps/dm2~ a 125 ~m coatlng
was applied in 30 minutesO The relatively dense coat-
ing was matte textured on the surface but otherwise
5 generally crack free and had a compositi.on of 16 Cr-21
Ni-63 Fe.
Example 5 - Fe-Cr Alloy Coatings
from Chloride Bath
Iron-chromium alloy coatings were deposited
10 from an electrolyte solution of the chromium ~56 g/l)
and iron (52 g/l) chloride salts at about 30C. I'he
apparatus of example 1 was used (with the exception of
a soluble 30/70 chromium-iron anode) to plate the alloy
coatings shown in Table 2. Cathode efficiency is
15 conventionally defined as the percentage of the applied
current used to deposit the chromium a].loy.
Table 2
Deposition Conditions Alloy
Composition
20 ~ample Eiciency pH C.D. Agitation Cr Fe
No. (A/dm2) m/sec
97A 5 0.83 0 2 98
96C 22 0.640 2.4 ~ 94
96B 36 0.680 2O4 8 92
25 96D 40 0.7160 2.4 18 82
-12-
These samples were made prior to our recog-
nition of the importance of pH and they are within our
broad range, but outside of our preferred p~ range.
Nevertheless, the effect of current density and agita-
5 tion upon efficiency and the final alloy compositionwas clearly shown~ wherein the chromium content and
efficiency of the deposit were proportional to the
current density. The importance of using high current
densities and agitations can be seén by observing
10 sample ~7~ wherein the efficiency and percent chromium
in -the deposit were both low because of low current
d~nsity and low agitation. The deposit was also
limited to a very thin section hecause of poor adher-
ence and cracking in tllicker deposits. Because the
15 coating was thin and low in chromium it had poor
corrosion resistance.
Samples 96B, 96C and 96D were marginally
cracked but were otherwise suitab]e coatings similar
to conventional hard chromium plates deposited in
20 catalyzed chromic acid solutions. These cracks in the
deposits may not be detrimental where wear resistance
in the main property desired in a coating.
~xample 6 - Fe-Cr Alloy Deposits
from Sulfate Baths
A 30/70 chromium-iron anode was again used
in a sul~ate solution to plate alloy coatings on a
copper-coated, steel-ring cathode. I'he plating solu-
.ion compositions were as follows:
Concentration, (g/l)
1 2 3
Iron metal ions 2~ 14 21
Chromium metal ions 26 39 ~7
_13-
Alloy coatings were deposited at 50C as shown
in Table 3.
Table 3
. ~ ..__
... . . _ .............. _ . ... _
Deposition Conditions Alloy
Composition
_ . . _ , . . _ _
Sample pH C~D. Agitation Solution Cr Fe
No. (A/dm2) (m/sec)No.(w/o) (w/o~
.
61~ 1.5 160 2 1 20 80
62P 2.0 160 2 1 26 74
10 80D 1.9 230 3 2 24 76
80E 1.~ 310 3 2 48 52
80F 1.8 390 3 2 42 58
67C 1.7 160 3 2 65 35
11~ 1.8 160 3 3 29 71
15 llA 1.6:L 160 3 3 26 74
12A 1.745 160 3 3 32 68
12C ~.. 7~5 390 3 3 62 28
~ ~._ _ _. . . = _, , _ _~ ==== =
Some early results (Samples 61-80) were tak-
en before the importance of pH was ascertained. Hence,
20a pH meter without extreme accuracy was used. Later,
the meter was replaced by a more accurate instrument.
Nevertheless, thin, matte coat:ings were obtained out-
side of the preferred pH range using the divalent/-
trivalent chromium electrolyte. These coatings may be
2smade with high chromium contents by use of the hiyh
_14-
current densities and agitation.
Results were not always consistent when us-
ing the less accurate pH meter as can be seen in the
Table 3, however, we attribute this to the lack of
5 sufficient accuracy in measuring pH and in maintaining
that pH throughout the bathD When ~sing the more
accurate meter and when within the preferred pH range
it may be seen that good control of the process can be
had. For example, in samples 12A and 12C, the pH was
10 within the preferred range and the chromium content of
the deposit was increased greatly with increasing cur-
rent density, eg. from 32 to 62% Cr with an increase in
current density from 160 to 390 amps/dm2.
