Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to a composition and
process for forming colored chromate coatings on
zinc-nickel alloy surfaces an~, more particularly,
relates to composit;ons and proccsses for forming
colored chromate coatings on electrodeposited
zinc-nicke1 alloy surfaces to provide a composite
coating having improved corrosion resistanse as
compared to that obtained with zinc electrodeposits.
The use of zinc electrodeposits on iron or
steel substrates to provide improved corrosion
protection has long been practiced. Although such zinc
1~ electrodeposits greatly enhance the corrosion
resistance oF the iron or steel substrate, the zinc
itself forms white "rust" or corrosion ~hich,
ultimately, can result in the corrosion of the iron or
; steel substrate itself with the consequent ~ormation of
red rust. To minimize this and impro~e the corrosion
;~
--1--
'
resistance of the zinc electroplated substrates, it has
commonly been the practice to treat such surfaces ~lith
an acidic, hexavalent chromium containing solution to
form a visible or colored chromate passivating film on
the surface. Typical compositions and process for
forming such chromate passivating films are disclosed
in U.S. patents 2~021,592; 27106,904; 2,288,007;
2,376,1~8; 2,9399664; 29610,133; 2,7~0,891; 3,090,710;
3,404,046; and 3,895,969.
Recently, considerable work has been done to
improve the corrosion resistance of zinc electroplated
substrates by the substitution of zinc alloy
electrodeposits for the substantially pure zinc
electrodeposits which have heretofor-e been used.
Although various different metals have been used with
the zinc in such alloy electrodeposits, particularly
good results, in terms of the improvements in corrosion
resistance and the brightness or gloss of the surface,
have been obtained with zinc-nickel alloy
electrodeposits. Depending upon the nickel content of
such zinc-nickel alloy electrodeposits~ the time for
; the formation of red rust in salt spray testing can be
as much as five to ten times greater than the time for
such red rust formation with zinc electrodeposits.- In
spite of this, the formation of white rust or corrosion
'
. ~
4~
on such zinc-nickel alloy deposits is still a problem
which must be minimized by the application of a
chromate or other passivating film.
It has been found, however, that the
formation of a colored or visible chromate film having
high corrosion resistance on such z;nc-n;ckel alloy
electrodeposits is much more difficult than on zinc
; electrodeposits. In the various patents disclosed
hereinabove~ there is no specific disclosure of the
treatment of zinc-nickel alloy electrodeposits,
although the treatment of 7inc alloy deposits, broadly,
i mentioned. Generally, the chromating compositions
of these patents have not been effective in producing a
satisfactory corrosion resistant chromate film on zinc-
nickel alloy electrodeposits. For example, in U.S.
patent 2,106,904, there is disclosed a chromic
acid/sulfuric acid solution which contains from about
13 to 104 g/l hexavalent chromium and from about 1.8 to
144 gtl S04, with a weight ratio of hexavlaent
chromium:S04 of 0.09-Sg:1. This solution is
specified as having a pH which is not in excess of 1.0,
and when used for the treatment of zinc-nickel alloy
electrodeposits, the resulting chromate film has
relatively poor corrosion resistance. This difflculty
..,~
.
. ,
~2~ 7
in forming colored chromate film with high corrosion
resistance on the zinc-nickel alloy electrodeposits is
one reason why such deposits have not been more widely
adopted as a replacement for zinc electrodeposits for
the production of decorative and corrosion resistant
coatings on iron and steel sùbstrates.
The present invention provides an improved
chromating composition which will produce excellent
colored, corrosion resistant coatings on zinc-nickel
alloy electrodeposits~
The present invention also provides an impro-
ved process for the forma-tion of colored, corrosion
resis-tant chromate films on zinc-nickel alloy electro-
deposits.
.
: ~ :
- ,
According -to the present invention there is provided an
aqueous acidic chroma-ting solution suitable for forming
colored chromate Eilms on zinc-nickel alloy electrodeposi-ts
containing up to about 15~ by weight nickel, which solution
has a pH of Erom about 1.3 to about 2.7 and comprises hexa-
valent chromium in an amount of at least 0.5 grams per liter
and sulEate in a weight ratio of S04-2:Cr 6 of abou-t 0.025-
1.5:1.
Typically, such zinc-nickel alloy electrodeposits are
immersed in this aqueous acidic chromating solution for a
period of time sufficient to form the desired corrosion re-
sistant chromate fllm on the alloy surface. Alternatively,
the improved corrosion resistant colored chroma-te film may
be formed by electrolysis of the zinc-nickel alloy electro-
deposit in the chromating solution, using a rela-tively weak
electric current with the zinc-nickel alloy surface as the
anode prior to the immersion treatment.
- 5 -
~ ."
. . ,
~l2~9~
The thus-formed chromated coatings are found
to provide excellent corro5ion resistance to both white
rust and red rust on zinc-nickel alloy electroplated
substrates. Additionally, it has been found that the
heat resistance to color fading of such films has also
been increased.
The improved chromating solutions of the
present invention are aqueous acidic solutions having a
pH of from about 1.3 to about 2.7 and comprising
hexavalent chromium (Cr+6) in an amount of at least
0.5 g/l and sulfate (S04~ in a weight ratio of
S04oCr~6 of . about 0 . 025-1 . 5 :1 . Although the
maximum concentration of the hexavalent chromium in
this solution has not been found to be critical~
amounts up to the maximum saturation of hexavalent
chromium in the solution being suitable, where high
hexavalent chromium concentraitons are used, the cost
of the process is increased and additional treatment of
the waste or rinse water is often necessary in order to
meet environmental standards. In general, it has been
found that the use of hexavalent chromium
concentrations in excess o~ about 100 g/l does not
provide any significant and further improvement in
the corrosion resistance of the chromate ilm.
Accordingly, ~rom the standpoint o practical
considerations, hexavalent chromium concentrations in
the present chromating solutions of from about 0.5 to
about 100 g/l are pre~erred. Where hexavalent chromium
concentrations below about 0.5 g/l are used, the
resulting chromate film becomes relatively thin and
sufficient corrosion resistance is, generally, not
obtained.
It has been found to be important in the
improved chromating solutions o the present invention,
to maintain the weight ratio o S04:Cr+6 within the
specified range of from 0.025-1.5:1, preerably 0005-l.0:1.
Where the sulfate to hexavalent chromium ratio is below
about 0.025, the desired colored chromate film is
not completely formed on the zinc-nickel alloy
electrodeposit. Moreover, where the ral-io of sulfate
to hexavalent chromium is in excess o about 1.5/
the resulting chromate film is relatively thin and
its corrosion resistance is undesirably low.
Similarly, it is important to maintain the pH of
the present chromating solution within the prescribed range
of about 1.3 to 2.7 preferably 1.4 to 2.2. It has been
-- 7 --
` ~L2~47
found that where the pH of the solution is below about
1.3, the resulting chromate film contains a relatively
small amount of adherent chromium and, thus, has
undesirably low corrosion resistance and an
unsat;sfactory color. This finding is somewhat
surprising in that with the treatment of zinc
electrodeposits, rather than zinc-nickel alloy
electrodeposits, using a conventional chromate solution
containing 100 g/l CrO3 and 5 g/l H2S0~ with a pH
of 0.5, a colored chromate film having good appearance
and corrosion resistance is formed. It has further
been found that when the pH of the present chromating
solutions is in excess of about 2.73 the reactivity of
the chromating solution is reduced and a colored
chromate film having good appearance and corrosion
resistance is not formed on the zinc-nickel alloy
electrodeposit.
The improved chromating solutions of the
present invention may be formulated using any bath
soluble hexavalent chromium and sulphate compounds, thP
anions or cations of which are not detrimental to the
chromating solution or the chromate ~ilm subsequently
forrned on the zinc-nickel alloy surface. Typical of
2~ the compounds which may be used are chromic acid,
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'
sulfuric acid, the alkali metal chromates and
dichromates, metallic sulfates, such as zinc sulphate
and chromium sulphate, and the like. In this latter
regard, it has been found that in some instances, the
inclusion of trivalent chromium, typically added as
chromium sulphate, may be beneficial in the formation
of the desired corrosion resistant chromate film. Even
where trivalent chromium is not added to the bath as it
is initially made up, it will typically be formed in
the bath by reduction of the hexavalent chromium during
use~
To adjust the pH of the present chromating
solutions, acids, such as chromic acid or sulfuric
acid, are typically used where a reduction in the pH is
desired. Where the pH is to be raised, alkaline
compounds are typically added~ such as the alkali metal
hydroxides, zinc oxide, zinc carbonate, zinc hydroxide,
nickel carbonate, nickel hydroxide, and the like.
- -
In addition to the components which have been
noted hereinabove, the chromating baths of the present
invention may also contain other components which are
typically present in such chromating solutions~ as is
known in the art. Exemplary of such other components
are phosphates, which may be added as phosphoric acid,
or in ~he form of the alkali metal phosphates and acid
phosphates and lower carboxylic acids, such as acetic acid
or its bath soluble salts. It has been found, however,
that the presence of nitrate in the chromating bath or
on the surface of the zinc-nickel ailoys to be treated
tends to inhibit the formation of the desired chromate
film on such alloy electrodeposit surfaces.
Accordingly, in the practice of the present invention~
nitrate-containing compounds are not used in
formulating the chromating solutions and the substrates
on which the zinc-nickel alloy electrdeposit is formed
should not be treated with a nitric acid solution prior
to the present chromate treatment.
- The present chromating solutions may be
applied to the zinc-nickel alloy electrodeposits
containing up to about 15% by weight nickel.
Generally, such alloy deposits will contain at least
about 1% nickel, with nickel contents of from about
5-12% by weight being particularly preferred.
Typically, the chromating solutions are applied by
immersion of ~he zinc-nickel alloy electrodeposit in
the solution, although other application techniques,
such as spraying, flooding, and the like, may also be
-1 O-
used. When using such application techniques, the
zinc-nickel alloy electrodeposit is maintained in
contact with the chromating solution for a period of
time sufficient to form the desired chromate film on
the surface. In many instances, contact times of from
about 10 to about 30 seconds are typical 7 although this
time may be varied in each instance and both shorter
and longer contact times may be used to obtain the
desired chromate ~ilm. The temperature of the
chromating bath may be from room temperature, e.gO,
- about 20C, up to temperatures which approach the
boiling point of the solution. Typically, the solution
is used at a temperature of from about 25 to about
60C.
It has been found that the surface of the
zinc nickel alloy electrodeposit may become inactive or
inert if it is exposed to the atmosphere for any
significant period of time before being subjected to
the chromating treatment. In such instances, it has
been found desirable to utilize the chromating
solutions at higher temperatures, such as about 35 to
55C, in order to form the desired chromate film.
Where the zinc-nickel alloy electrodeposit is treated
with the chromating solutions immediately after
electrodeposition of the alloy, the lower solution
t~mperatures of 25 to 35~C may be used to
satisfactorily obtain the chromate coating.
In this regard, it has further been found
that activation of the zinc-nickel alloy electrodeposit
surface may also be accomplished by making the
zinc-nickel alloy substrate the anode and electrolyzing
the substrate in the chromate solution at a relatively
low current density. Typically, the electrolysis, with
the zinc-nickel alloy electrodeposit surface as the
anode, is carried out at a current density of from
about 0.01 to about 0.2 A/dm2 for a period of up to
about ten (10) seconds. Thereafter, the zinc-nickel
alloy's substrate is retained in the chromatiny
so!ution, without the application of current, until.the
desired chromate film is formed on the surface.
.
--1 2--
~` ` `.
., .
~L2(~ 7
By means of -the process described hereinabove,
zinc-nickel alloy electrodeposi-ts containing up to about 15%
by weight nickel are provided with a chromate film having a
-thickness of at leas-t about lO0 mg/m2. These films impart a
pleasing color -to the alloy surface and provide good corro-
sion resis-tance of a magnitude which has heretofore not been
possible on zinc-nickel alloy electrodeposi-ts. Moreover,
the zinc-nickel alloy electrodeposits containing these chro-
mate films are found to have a corrosion resistance which is
superior to that of zinc electrodeposits which have been
subject to conventional chromating treatments.
: 1 :
- 13 -
~2~ 7
Specific Examples of the Invention
In order that those skilled in the art may
better understand the present invention and the manner
in which it may be practiced, the following specific
examples are given:
Example I
An aqueous chromating bath was formulated by
dissolving the following components in water in the
amounts indicated:
Na2Cr207 2~20 lO g/l ~Cr+~ = 3.43 g/l)
~2S04 2 g/l (S04-2 = l.96 g/l)
(SO4~2/Cr+6 = 0.56)
pH 1.8 . . .
A steel sheetg electroplated to a thickness of 3 ~m
with a zinc-nickel alloy containing 8% by weight nickel
was chromated with this solution by immersing the sheet
in the solution For fifteen (15) seconds at a
temperature of 35~C.
-l4-
4~
Example II
An aqueous chromating bath was formulated by
dissolving the following components in water in the
amounts indicated:
Na2Cr207;2H20 20 9/1 (Cr+6 = 6.g8 9/1)
Cr2(S04)2-aqueous solution T 9/1 3 S04-2 = 0-75 9/1)
(Cr2 (504)3 content 40% (S04-2/Cr+6 = 0.11)
pH 2.1 (adjusted with CrO3)
A steel sheet, electroplated to a thickness of lJum
with a zinc-nickel alloy containing 8% by weight nickel
was chromated in thîs solution by immersing the sheet
in the solution for 2~ seconds at 50C.
Example I II
A chromating bath was fcrmulated by
dissolving the following components in water in the
amounts indicated:
CrO3 2 9/1 (Cr+6 = 1.04 g/l )
H2S04 0.1 9/l (S04-2 = 0.098 9/l )
(S04~2/Cr+5 = 0.094)
pH 1.8
~ 1 5 - '
lZ~ 7
5 A steel sheet, electroplated to a thickness of 3 ~m
with a zinc-nickel alloy containing 8% by weight nickel
was chromated by immersing the sheet in the solution
for fifteen (15) seconds at a temperature of 40C.
Example IV
An aqueous chromating bath was formulated by
dissolving the following components in water in the
amounts indicated:
Na2Cr207 2H20 30 g/l (Cr+6 = ll.82 g/l)
Cr2(S04)3aqueous solut;on T 9ll (S04-2 = 0-96 g/l)
(Crz(s04)340% content) (S04~Z/Cr+6 = O~O~TJ
pH 2~0
~ A steel sheet, electroplated to a thickness of 2 ~m
: \ with a zinc-nickel alloy containing 12% by weight
nickel ~as chromated by immersing the sheet in the
25 solution while the solution-was electrolyzed at a
current density of 0.1 A/dm2 for 5 (five) seconds
::
:.
~ -l6-
`, :
' ' ' -: :
~ ,
: ~ :
with the sheet as the anode. After five (5) seconds,
the passage of electric current was discontinued and
the sheet was retained in the solution for an
additional twenty (20) seconds. The temperature of the
5 chromating solution during the entire treatment was
50-C.
Example V
An aqueous chromating solution was formulated
by dissolving the following components in water in the
amounts indicated:
CrO3 50 g71~(Cr+6 = 26~0 9/1)
lS H2S04 (S04~2/Cr+6 = 0.38)
Na2HP04 2 9/1
pH T.~ (adjusted with NaOH)
A steel sheet, electroplated to a thickness of 6 ~m
with a zinc-nicke1 alloy having a nickel content of 10X
by weight was chromated by immersing the sheet in the
chromating bath for ten (10) seconds at a bath
temperature of 30C.
17-
,
Example VI
An aqueous chromating bath was formulated by
dissolving the following components in water in the
amounts indicated:
Na2Cr207~2H20~50 g/l (Cr+6 = 52.4 g/l)
~2S4 lO g/l (S04-2 = 9.8 g/l)
(S0~~2/Cr~6 = 0.~9
10 HCOONa l.5 g/l
pH - - 1.6
A steel sheet, electroplated to a thickness of 3 ~m
with a zinc-nickel alloy having a nickel content of 12%
by weight, to which had been applied a flash
electrodeposit of zinc having a thickness of 0.1 ~m was
chromated by immersing it in this solution for fifteen
(15) seconds at a bath temperature of 30C.
23 By way of comparison, the following Examples
VII through IX were run:
;
-l8-
,
, :
..... . '
.,
Example VII
!
An aqueous chromating bath was formulated by
dissolv;ng the following components in water in the
amounts indicated:
CrO3 .~00 9/l (Cr+6 = 52.0 g/l)
4 lO g/l (S04-2 9.8 g/l)
pH . 0.6 (S04~2tCr+6 = OI9)
A steel sheet, electroplated to a thickness of 3 ~m
with a zinc-nickel alloy having a nickel content of 10%
: by weight, was chromated by immersing the sheet in the
chromating bath for ten (10) seconds at a bath
temperature ~f 30C.
Example VIII
An aqueous chromating bath was formulated by
dissolving the following components in water in the
amounts indicated: .
_l g_
~i!LZ~ 7
CrO3 To g/l (Cr+6 5.2 g/1)
H2S04 l g/l (S04~Z/Cr+6 = 0.98 9/l)
pH l.2 (S04~2/Cr+6 = 0.19)
A steel sheet, electroplated to a thickness of 3 ym
with a zinc-nickel alloy having a nickel con-tent of 10%
by weight, was chromated by immersing the sheet in the
chromating solution for 30 seconds at a bath
temperature of 30DC.
:
I
:: ; ~
::; :
: :: -20- : ~
,
: ~
The chromated samples prepared in accordance
; with the preceding Examples I through IX were analyzed
to determine the chromium content of the chromate
coating and were tested for corrosion resistance using
the 5% neutral salt spray test and for heat resistance
of the chromate coating. The analytical methods and
test methods used were as follows:
(1~ Analysis of Cr in the chromate film:
Each test specimen was immersed in an
aqueous acidic solution containing 100 gll HCl. The
chromate film was oompletely dissolved from the test
specimen and the eluted Cr portion was quantitatively
analyzed by atomic absorption spectroscopy. The amount
of chrom;um determined was expressed in terms of the
surface area of the test specimen (miligrams per square
meter).
(2) Corrosion resistance test:
Each test specimen was subjected to the
5% neutral salt spray test in accordance with the
procedure ASTM-8117 and the time was noted for the
-21-
.
development of white corrosion products on the surface
of the specimen (white rust) and for the development of
red corrosion products (red rust) in accordance with
; this procedure.
(3) Heat resistance test:
. Each test specimen was placed in a
. thermostated oven and maintained in the oven for
twenty-four hours at fixed temperatures of 100p 150,
200, and 250C. Thereafter, each test speciment was
removed from the oven and the failure or fading of the
chromate film due to the heat was visually observed.
Using thes~ procedures, the test results as shown in
the following Table I were obtained:
....
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al E V
LL ~ O O O
t t~ O
.~ 3~ C~J ~
aJ ~ ~ I = = = = =
~o ~__ O o o
s_ C o L~) o
~s~ ~ ~ ~
~)
I1
_ _ , ,
O Q O N C~J
_ a) ~ u- O ~ Is~
~ r- aJ ~ s o = _ = = = o In
~ ~ ~ C~J ~
Q ..
ro
3 ~ O
~ aJ ~ ~u s co ~ = = 0 = O ~D
._ E aJ ~ c ~ ~ ~t
LLI ~ ~3
~ _ . ~ _ .
~^ .
C t ~ O ~ O O . I~ Ln
) ~ o~ t = r~ r
O_ ~) N
a;~_ .
_ _ .~ ~ . .
c E s_ c O
a~ r~ ~ O a) a~ ~ s_
E ~.~ ~ o ~_ ~
Q
a~ . . ~ I s ~I) Vl -- C
Q ~ a~ ~ 3 ~ > ~ ^ ^ ,, ~
Vl ~ ~1 (L) -- O ~ V) - = = O C ~ = = 3 a~ C ~ =
E x Q ~ o c 5 1~ cr) o _, o ' ~ ra
Q ~ c a) o n:~ .; ~ ,_ o a) ~
vl 5_ cl aJ ~s c O V) ~ ~ O X Q
_ ~ ~ l~ ~ l~
.
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., .,- ~
2 ~ = ~ ~ c~ D ~ ~ ~
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-23-
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< o o o o
oooo
~ C C C
O ~ ~ ~ a
O I S S S S
~'~
~J ~ ~I) a~ a) a
E v~
E E E E
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,-
C
Q>
,~
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s. O s ~
1~ (1) 3 o
a) ~ O I:
c a~ o o u~
aJ aJ ~ O U~
s U~ E ~n
V
q)
~ .
q~
+~ cn
.r~ 3 ~n ~)
Q O ~_ c C
1~ 1~
V) ~ _C S
O W ~ t~
C '-
a~ O o u. u
Cl ~ ~r-'~
~n ~ ~ E ~ ~. ~ 3
3 ~ ~ ~ ~ ~ s_
L~ a~
.,_ .,
aJ c ~ ~ ~.
H ~' ~ ~ c,--.
q~ E ~ ~ +~ ~ c
O O
,~
S ~ =~' ~ r~ .
V~
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a
>~
~ ~ ~ o
qJ ~ ~ __
us l -
=~ . o o o o
+, ~ o ~n o u,
O S ~ ~ ~ N N
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E V~ X
o
c
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-24-
~.`. . : `:
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~xample X
.
A concentrate composition was formulated
containing the following components in the amounts
indicated:
sodium dichromate (Na2Cr~07.2H20) 300 971 .
zinc sulfate (ZnS0~7H?aT 15 9/1
chromium sulfate ~Cr2(~04)3 . . . 40% sol.) 30 ml~l
chromic acid ~CrO3) 40 9/1
water balance
.
A chromating solution was formulated by add;ng the
foregoing concentrate solution to water in an amount of
SO mililiters per liter. The pH ol this chromating
soluton was 1.8 and steel sheets9 electroplated to a
thic~ness of 4 ~m with a zinc-nickel alloy containing
B.4X by weight nickel, were chromated by immersing the
sheets in the chromating solution for 15 seconds while
maintaining the bath temperature within the range of 45
- 55C. The results of the testing of the chromate
coating produced in accordance with this procedure are
found to correspond to the results obtained for Example
III.
_z5,
t
'`
~ ,
During the operation of ~he process of this
Example, the chromating solution is replenished using a
replenishing composition containing the following
components in the amounts indicated:
chromic acid (CrO3) 300 g/l
! sulfuric acid (H2S04 50 wt. ~) . lQO ml/l
Sp. Gr. = 1.4
water balance
The pH and CrO3 concentration of the chromating bath
is periodically measured and the above replenishing
solution is added in amounts necessary to reestablish
the pH and CrO3 content of the bath at the levels
originally formulated~
. . .
From the foregoing results, it is apparent
that the chromating solutions of the present invention
provide a colored chromate coating of high quality on
2a 2inc-nickel alloy electrodeposits, which chromate
coatings have excellent corrosion resistance, as well
as heat r`esistance to color fading. Moreover, the
corrosion resistance of the substrates plated with the
: zinc-nickel alloy electrodepGsits and chromated in
accordance with the present invention is signi~icantly
greater than that of substrates plated with zinc
electrodeposits which had been chromated using
conventional processes.
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.
