Note: Descriptions are shown in the official language in which they were submitted.
~1347Z5
BACKGROUND OF THE INVENTION
The formation of chromate conversion coatings on
surfaces of various metals, such as zinc and cadmium, is presently
the most common technique of imparting increased brightness and
S corrosion resistance to the metal. In a typical process, the
metal work pieces are immersed in an acidic solution containing
hexavalent chromium compounds, which react with the metal causing
the precipitation of a complex gel-like coating or film of
trivalent chromium and entrapped soluble he~avalent chromium
compounds onto the metal surface. The coated work pieces are
then rinsed and dried under controlled conditions.
There are several serious disadvantages common to all
chromate conversion coating processes. One of these is the
relatively short life of the process bath expressed in terms ofunit
surface area coated per unit volume of bath. The main reason for
the short life if the continuous build-up in the bath of dissolved
347'ZS
trivalent chromium resulting from the oxidation-reduction
:
reactions that occur between the metal and the hexavalent
, chromium. Trivalent chromium is a contaminant in the process
affecting the coating efficiency. Thus, when reduced coating
activity is noted, or when the contaminants have built up to a
certain predetermined level, a process solution of this type is
at least partially replaced with freshly prepared solution, and
ultimately completely discarded in favor of a fresh bath.
The disposal of the spent process solution is wasteful,
as the solution still contains considerable quantities of
hexavalent chromium. Not only does the loss of these values
contribute significantly to the overall cost of the coating
process, but disposal also adds to this coat in that the
solution present a substantial waste treatment problem. Hexavalent
chromium is highly toxic and must be reduced to the trivalent
form, e.g. by reaction with sodium hydrosulfite or sodium
bisulfite, and is thereafter precipitated from solution by
addition of alkalies, such as sodium carbonate or lime. After
dewatering of the precipitate by settling or filteration, the
concentrated sludge of trivalent chromium hydroxide must be
disposed of in specially designated areas, since trivalent chromium
is still too toxic to be used as landfill. Substantial waste
treatment requirements of spent rinse waters are also created
due to dragout of toxic chemicals from the process bath into
Z5 subsequent rinse waters. Although there are integrated processes
for the reoxidation and regeneration of spent chromate solutions
and rinse water, the small processor usually finds that the refined
and sophisticated techniques involved are neither practical nor
economically feasible for solving his waste treatment problems.
Applicant has developed a non-toxic conversion
coating solution which is comprised of sulfuric acid,
hydrogen peroxide and a soluble silicate and optionally
--2--
.
Ij 113~'7'Z5
¦Icontaining additives, e.g. certain or~anophos~horus compounds for
further enhancement of corrosion resistance of metal surfaces
Itreated with the solution.
Ii Although the acidic silicate 'solution" may or may not
S ¦Ibe a true solution but rather in the form of a hydrosol, for the
ii ¦purpose of this application, the term "solution" is intended to
cover a hydrosol as well as a true solution.
In addition to the formation of conversion coatings of
excellent properties, there are many other important advantages
~ of the invention described in the aforementioned application.
One of these is the extremely long life of the conve~sion coating
Isolution before it is discarded in favor of a fresh ~olution. It
has been found that the solutions are capa~le of treating up to
!approximately 185 m2 of surface area per liter, which is far
superior to the typical value of approximately 20 m2/1 obtained
llwith conventional chromate conversion coating baths.
I! Another and related advantage is that, apart from some
build-u~ of dissolved metal in the solution, there are no detri-
mental by-products forming and accumulating therein during use,
!las is the case with conventional chromate conversion coating
;solutions, in which trivalent chromium rapidly builds up.
The most important advantage, however, is the non-toxic
, nature of the system, which greatly facilitates waste disposal of
! spent solutions from the conversion coating process. Rinse waters
can usually be disposed of without any treatment reouired. SPent
conversion coating baths are merel~ treated with lime for neutral-
ization and removal of diss~lved metal ions and phosphorus (when
, organophosphorus nromoters are used) as a precipitate. After
!:
settling or other separation, the liquid phase may be disposed of
11 ~afely in co~mon sewers, while the dewatered sludge mainly com-
l¦posed of silicate can be dumped in munici~al landfill areas.
!l One disadvantage, however, iY that the resulting con~
j~version coatings, although brigh~ and corrosion resistant, lack
¦Ithe decorative color, usually blue, which is characteristiC of
l~chromate conversion coated parts. The incornoratiOn into the
!¦silicate conversion coating solution of conventional dyes
; 1134725
- recommended and used in the industry for coloring chromate con-
version coatings failed to impart any lasting color to the coated
surfaces, even at very high dye concentrations and prolon~ed
treatment times. Attempts to use these dves in a water solution
as a post-dip treatment after formation of the conversion coatings
on the work pieces also failed to impart any color.
In addition, it was found that many of the conventional
metal dyes were unstable in the sulfuric acid-hydrogen peroxide-
silicate environment resulting in excessive hydrogen peroxide
consumption, complete loss of color or a drastic color change of
the bath solution.
It is therefore an object of the present invention to
iprovide an improved, stable conversion coating solution which
imparts a lasting color to the coated surface.
Another object of the invention is a process for the
formation of bright, corrosion resistant, colored conversion
coatings onto metallic surfaces.
Still another object is to provide decorative bright
corrosion resistant work pieces.
THE INVENTION
We have now discovered that certain dyes which heretofore
E)redominantly have been used in the dyeing of natural fibers such
as paper, cotton, wool, silk, etc., when incorporated into a Sll]-
furic acid-hydrot~en peroxide-silicate conversion coating solution,
unexpectedly impart pleasing and lasting colors to the coated
work ~ieces without detrimentally affecting the corrosion resis-
tance ~uality of the coating or the stability of the coating
solution. The dyes, which are ùseful in the present invention are
cationic triarylmethane dyes.
Thus in accordance with the present invention there is
rrovided a dyeable conversion coating solution which comprises an
; aqueous solution of from about 0.2 g/l to about 45 g/l of free
sulfuric acid, from about 1.5 ~/1 to about 58 g/l of H2O2, from
about 3 ~/1 to ahout 33 g/l of SiO2 and an effective amount of at
least one cationic triarylmethane dye.
4'~25
The SiO2 component is conveniently Provide~ in the form
of a soluble silicate, e.g. sodium silicate or potassium silicate,
of predetermined contents of SiO2 and Na2O or K2O. The mole
ratios of SiO2 to either ~a2O or K2O generallv range between 1 and
l4, and it is preferred to use those silicates wherein the mole
ratio is at least about 1.8 and most preferably at least about 2.2.
IAMmonium or lithium silicates are also useful in providing the
;ISiO2 component.
The triarylmethane dyes used in this invention are well
known in the art and are recognized as a separate generic group
of dyes having a Colour Index (C.I.) in the range from 92,000 to
44,999. They are commercially available in a wide variety of
,colors both in solid form or as aqueous solution concentrates with
solids contents ty~ically in the 40 - 50~ range. The amount of
dye to ~e added to the conversion coating solution depends
obviously on the desired depth of color.
The solution is easily prepared, e.g. by first adding
sufficient sulfuric acid to at least a major portion of the
makeup water under agitation to provide the desired free H2SO4
~o content and ta~ing into account that some o the free acid will be
; subsequently neutralized by the Na2O or K2O ~ortions introduced
'; ~ with the silicate. The silicate is added under agitation to the
; cooled acidic solution until it is completely dispersed. The
peroxide is added and then the dye, prefera~ly in the form of a
dilute solution in a minor portion of the water used in the ~rep-
aration of the conversion coating solution. The se~uence of
addition can be changed, however, without any detrimental effect,
rovided that the silicate is acidified with sulfuric acid prior
to mixing with the hydroyen peroxide, or peroxide decompositiOn
will occur.
The preferred concentrations of the components in the
aqueous solution are from about 1.8 g/l to about 18 g/l of free
O4~ from about 7 g/l tO about 29 g/l of ll2O2, from about 8 g/l
, to about 18 g~l of SiO2 and from about 0.05 to about 0.3 g/l of
~the triarylmethane dye or mixture of dyes.
5_
l~ 1134725
The solution is useful for forming conversion coatings
on various metallic surfaces, such as those of zinc, cadmium,
silver, copper, aluminum, magnesium, and zinc alloys.
The rnost common application is, however, in the forma-
tion of conversion coatings on zinc plated articles such as zinc
,plated steel articles. The zinc plate provides the steel with
! cathodic protection against corrosion, and the conversion coating
further improves the corrosion resistance, reduces the suscepti-
~lbility to finger markings and enhances the appearance by chemical
polishing of the article and by the color imparted by the dye.
IIt is important that the zinc plate deposit is relatively smooth
and fine-grained prior to coating, and that the thickness of the
l'plate deposit is at least 0.005 mm since some metal removal occurs
,when the fil~ is formed. The preferred plate thickness is between
jjabout 0.005 ~n and about 0.02 mm.
I' Usually the formation of the conversion coating follows
¦limmediately after the last rinse in the plating cycle. Thus, the '
'freshly plated articles are immerse~ for a period of from about
5 seconds to about 300 seconds into the solution which is main-
tained at ambient temperatures. For best results, the immersion
treatment is carried out for a duration of from about 20 seconds
to about 50 seconds in a bath maintained at temperatures not less
than about 20C and not more than about 35C. The coated articles
are suksequently rinsed, first in cold water and then briefly in
~5 warm water to aid drying of tlle films. The hot water rinse
ty~ically has a tem~erature in the range of from about 60 to about
70C. The final step of the coating process is a drying step,
which is carried out by any means that will neither a~rade the
soft and then rather fragile film, nor expose it to excessive
temperatures, i.e. temperatures higher than about 70C. The use
of circulating warm air or an airblast are examples of suitable
means in the drying operation. After drying, the conversion
~' coatings are quite resistant to damage from abrasion and generally
Il do not require the 12-24 hour a~in~ necessary with conventional
1, chromate conversion coatings.
il - 6 -
1134725
The resulting conversion coatings have very good
resistance to corrosion as determined by the accepted accelerated
~orrosion test AsTr~ 117-64. By the use of one or more of
certain organic promoters as additives to the solution of sulfuric
;acid-hydrogen peroxide-silicate the corrosion resistance of the
coatings can be further enhanced. The organophosphorus compounds
sE~ecified hereinafter have been found to be especially useful in
this respect.
i These promoters are organic phosphorus compounds having
the general formula:
lX(Rl)m]n [R2]p lX(Rl)m]q
',wherein
X is a group of the formula Z1 ~ P ~ Z2 in which
O
Zl and Z2 independent from each other are hydrogen,
sodium or potassium;
m is either O or l;
p is either O or l;
n -~ q is either (a) 1 when p = O, or
2() (b) equal to the number of available bonds
provided by R2 when p = 1;
Rl is a (a) Cl-C4 alkyl or a Cl-C4 hydroxy-substituted
alkyl and p = O; and
(b) Cl-C4 alkylene or a Cl-C4 hydroxy-substituted
alkylene and p = l;
R2 is selected from (a) N- , m = 1
(b) =N(CH2)rN= , m = 1 and r is an
integer from 2 to 6
(c) =N(CH2)2 N (CH2)2
(Rl)
X
~ .
--` 11347Z5
and
(d) a Cl-C4 alkylene or a Cl-C4
hydroxy-substituted alkylene,
m = 0 or l.
,',
,' Examples of these organophosphorus compounds include
IICl-C4 alkyl phosphonic acids, Cl-C4 hydroxyalkalenephosphonic
¦iacids, amino tri-Cl-C4 alkylene phosphonic acids, C2-C8 alkylene
diamine-tetra (Cl-C4 alkylene phosphonic acid), diethylenetri-
llamine-Penta (Cl-C4 alkylene phosphonic acid) as well as the acid
!or neutral sodium or potassium salts of any of the above-listed
¦~phosphonic acids. l-hydroxyethylidene-l,l-diphosphonic acid is a
preferred compound.
The organophosphorus compound or mixture of such com-
lipounds is added to the conversion coating solution to provide a
Iconcentration therein of from about 0.15 g/l to about lO g/l,
!,
preferably from about 0.5 g/l to about 2 g/l.
During the course of the coating process, the coating
solution becomes depleted in both free sulfuric acid and hydrogen
peroxide values and must be replenished. lherefore, monitorin~
of these values should be carried out on a re~ular basis to
assure ~ha~ the respective concen~rations have not fallen below
their minima and to assess the arnounts needed for replenishment.
Free sulfuric acid can be determined by conventional titration
methods using sodium hydroxide or ~y pl{ determinations. In order
to maintain the free sulfuric acid within the broad ranges of
about 0.2 to about 45 g/l the pH should be controlled between
about 0.5 and about 3.5 and preferably between about l.0 and about
3.0 which approximately corresponds to a free sulfuric acid con-
centration of from about 1.8 to about 18 g/l. The hydrogen perox-,
~ ide concentration levels are advantageously monitored by conven-
j tional titration with ceric ammonium sulfate. The silicate (SiO
,jconsumption is relatively small compared to the consumptions of
either the free sulfuric acid or the hydrogen peroxide, and
--` 113~7Z5
generally neither monitoring (which can be carried out using e.~.
colorimetric ~rinciples involving the reaction of silicate with
ammonium molybdate to form a yellow-colored molybdo silicate
l,solution) nor replenishment is required during the practical life
llof the conversion coating bath. The rate of consumption (i.e.
Ilpercent decrease in concentration per unit time) of organophos-
jlphorus additives has been found to ~e approximately of the same
¦,order as that of the hydrogen peroxide consumption. Therefore,
l¦replenishments of the solutions with these additives are suitably
!Icarried out at the time of hydrogen peroxide replenishment in
j;amounts proportional to the h~drogen peroxide addition. The dye
! generally does not need to be replenished during the practical
j,lifetime of the conversion coating bath. Monitoring of the color
',depth quality of the coating is easily carried out by visual
l,inspection of the coated article and comparison against a refer-
ence color.
1'he following examples are provided to illustrate but
not to limit the invention.
lhe general procedures used in t~le examples for pre-
paring the conversion coating solutions, test specimens and
forming the conversion coatings are described below.
The aqueous conversion coating solutions were each
prepared to contain 2.4 g/l free ll2SO4 , 16.2 g/l SiO2 , 11.7 ~/l
~22 I'he SiO2 ingredient was added in the form of sodiuM
silicate (SiO2 = 33.2~ w/w; Na2O = 13.85~ w/w) and a sufficient
excess of sulfuric acid was provided to result in the indicated
free ll2SO4 content after neutralization of the Na2O in the
sodiuM silicate.
I Standard Hull cell steel panels (10 cm x 6.8 cm x
0.03 cm) were p]ated with ~inc using a cyanide electrolyte. After
thorouyh rinsing and drying, the samples were then immersed for
20 seconds (unless otherwise noted) in the conversion coating
, solution maintained at room temperature. The treated samples were
then rinsed in water and then dried with a hot air gun.
, I _ g _
~` ii3~'725
COMPARATIVE EXAMPLES 1-8
A number of conversion coating solutions containing
various blue dyes were prepared and tested for color and hydrogen
peroxide stability after 24 and 90 hours storage. Table 1 below
identifies the dyes, shows the dye concentrations and the results
of the stability testing. Of the seven dyes tested in this series
, only those of Examples 4 and 8 did not appear to promote peroxide
consumption of the bath nor undergo an undesired color change.
These dyes were therefore used for conversion coating trials to
determine if they would impart a desired blue color to zinc plates
-~ treated with the respective solutions. Results of 20 second
immersion in each of the two baths were that no permanent color
was imparted to the surface of the test panels.
: - ~! ol 113~725
O ~:
~a) a~ o~ ~D O~ )
2 al
1~ ~ d
I ~
I o ~ I
~ o 3 o ~
o ~ ~ ~ U~ o
¢ S ~ 3 :~ h ~ m
m ~ o ~ ~ ~
o
;' ~
O ~ u~
u~ ~ o~
. 0~
~c
~:r
~ o ~ m
a~ ~ ~
~ ~C
I m ~ ~ h
m c~
~1 o o z
~ u
~ ~ o ~ ~ m u O
~J -- ::~ h ~ h :; h CJ
, a~ n~ h ~
I P~ m E~ u m a m a u ~ ~
11 ~ ~ ~
V~ o
-- O O O O O o o U O
_ ~ h
~: \ N~1
!! U C~ ~~O~ ~
P~
~ -- O
_ ,~ O
L~ ^ ~h
O
3 ~ al --
I l _ Q~
~ u~ . a
I ~ . ~ o
~ ~ ~ m
Q ::1 h ~O
E~ O
~ o o
I ~ ~
I ~ ~ o o o
I G) O O
I t~ K Z
O .C ,C _1 ~1 0
m u~
.
0 o o
Z ~ o
O ~ ~ N ~ ~ u
.) X
11;~4'~5
- 11
il EXAMPI.ES 9~
i' ~
In Exam~le 9 the procedures of the t~revious comparison
examples were followed exactly except that the dye was Basic
Violet 3, which is a cationic triarylmethane dye having a Colour
¦Index of 42555. The particular dye used in this example was
¦Paper Blue R solution obtained from E.I. DuPont de N~mours pro-
vided in the form of an aqueous acetic acid solution o~ a~out
1.115 sp.~r. and a solids content of about 50 wt. ~. When 0.5 ml/?
of the dye solution was added to the bath there resulted a dark
blue color which after 90 hour~ of storage did not change. The
peroxide concentration was not significantly affecte~ afte~ con- !
clusion of the testing (92% retention vs 94% without anY dye).
Results of 20 second immersion coatin~ test8 in the dyed
bath of zinc plated te~t panelg showed a permanent reddish blue
¦color to the surface. ~epeatinq the im~ersion coating tests with
¦varYing concentration8 of the dye from 0.1 to 0.5 ml/l showed that
any desired ~epth of color could be imparted to the surface merely
b~ changing the dye concentration.
Examples 10 and ll, in which the triarylmethane dves
were respectively a Basic Blue 7 (C.I. 42595) and Basic ~,reen 4
(C.I. 42000), showed the same successful coloration in the col-ce~
tration range used in Example 9.
EXAMPLE 12
ll In this example the conversion coating solution con-
1I tained 0.85 ~/1 (dry basis) of l-hydroxyethylidene-l,l-diphos-
Il phonic acid as a further promoter for corrosion resistance. The
¦I dyes used were a mixture of Ba6ic Blue 7 and Basic Violet 3
2 ml/l ~uPont Victoria Pure Blue BOP Solution, and 0.l ml/1
il DuPont Paper Blue R Liquid~. ~ull cell panels plated in a small
¦I scale as well as commercially plated clamps and elbow brackets
I ~ I
~ -12-
11347;~
,1
served as zinc-plated specimens for conversion coating, which was
, carried out for 20 seconds.
;I Visual examination of the coated specimens showed a
, ¦Idesirable blue color of excellent uniformity and shade, closely
S Ijnlatching those obtained with conventlonal blue c~lromate treatment.
1~
1~ ,
,
,,
11
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