Note: Descriptions are shown in the official language in which they were submitted.
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NC-96,346/347 PATENT
PROCESS FOR PREPARING CHROMIUM CONVERSION COATINGS FOR
IRON AND IRON ALLOYS
ORIGIN OF INVENTION
The invention described herein was made by employee(s) of the United States
Government and may be manufactured and used by or for the Government for
governmental purposes without the payment of any royalties thereon or
therefor.
CONTINUATION APPLICATION
This Application is a Continuation-In-Part of copending Application Serial No.
NC-96,347, Filed: , 2005.
BACKGROUND OF THE INVENTION
FIELD OF T'HE INVENTION
T'his invention relates to a process for preparing zirconium-chromium
conversion
coatings on iron and iron alloys. The process comprises pretreating iron and
its alloys
with effective amounts of an acidic aqueous solution containing trivalent
chromium
compounds, at least one hexafluorozirconate and optionally, tetrafluoroborates
and/or
hexafluorosilicates, zinc compounds, surfactants, wetting agents and/or
thickeners.
More specifically, this invention relates to a process for pretreating iron
and iron alloys to
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improve the alloys adhesion bonding and corrosion resistant properties. The
process
comprises treating iron and its alloys with an acidic aqueous solution
containing effective
amounts of at least one water-soluble trivalent chromium compound, a water
soluble
alkali metal hexafluorozirconate, at least one water soluble alkali metal
tetrafluoroborate
and/or hexafluorosilicate, at least one water soluble divalent zinc compound,
and
effective amounts of water soluble thickeners and/or water soluble
surfactants.
Current surface preparation of iron alloys such as steel include a variety of
mechanical and chemical processes, depending on the application. Phosphate
coatings
have been used historically to improve the adhesion of various outer coatings
such as
paint and corrosion resistance of steel. For example, grit blasting has been
used to
improve the adhesion when phosphate coatings were not practical to apply. In
this case,
the desired paint primer is applied directly to the grit-blasted steel which
is referred to as
the "direct-to-metal" process. Many organizations rely on the direct-to-metal
technique
as the use of phosphate coatings decline due to their reliance on a hexavalent
chromium
"rinse" for optimum adhesion and corrosion resistance. While direct-to-metal
coating is
straightforward and does not rely on a chemically-produced coating, it does
not, however,
provide an optimum-performing coating system. The absence of a chemical
pretreatment
lowers the overall corrosion resistance of the system. This is partially due
to the near
ubiquitous use of non-chromated primers for painting iron alloys such as
steel. In
addition, while grit- blasting imparts a sound mechanical base for paint
adhesion it is
labor intensive and requires application of the grit over all the steel
surface, generating
large quantities of spent grit that must be collected and then recycled or
disposed.
Further, where there is no corrosion-resistant conversion coating on the iron
alloys
such as steel there is the problem of " flash rusting" that occurs after the
steel parts are
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grit blasted or cleaned, but before the paint can be applied. The requirement
to keep flash
rust from occurring places a tremendous burden on scheduling and does not
allow for
components to sit for more than a few hours before being painted. This
prevents the
parts or components from being allowed to sit overnight or over the weekend
and
therefore prevents the work from being completed by the end of a shift where
the parts
cannot be painted before the end of a particular shift. This renders
production less
productive than it ordinarily could be. In addition, parts or components that
are
processed, but not yet painted that need to be shipped or moved before
painting requires
an elaborate and costly protective coating that must then be subsequently
stripped before
application of the paint. A chemical pretreatment for iron alloys e.g. steel
that is
analogous to an aluminum pretreatment is not available but would be desirable.
Such a
coating would impart good paint adhesion to the steel substrate without
needing a grit-
blasted surface and thereby increase the corrosion resistance of the painted
steel. A
pretreatment of the alloy would also serve to prevent flash-rusting and allow
for expanded
handling times of the steel parts before they need to be painted. Such a
pretreatment
could be produced by merely immersing the components or steel parts in a tank
of the
pretreating solution, or by spraying the solution onto the alloy, or by wiping
the solution
on to the alloy.
SUMMARY OF THE INVENTION
This invention relates to a process for preparing conversion coatings on iron
and
its alloys at ambient temperatures or higher e.g. ranging up to about 120 F.
More
specifically, this invention relates to a process of preparing conversion
coatings on iron
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alloys such as steel to improve its corrosion resistance and adhesion bonding
properties.
The trivalent chromium process (TCP) of this invention comprises an acidic
aqueous
solution having a pH ranging from about 2.5 to 5.5 and preferably 3.7 to 4.0,
and per liter
of said acidic solution, from about 0.01 to 22 grams of a water soluble
trivalent chromium
compound, about 0.01 to 12 grams of an alkali metal hexafluorozirconate, from
0.0 to 12
grams of at least one fluorocompound selected from the group consisting of
tetrafluoroborates, hexafluorosilicates and various combinations thereof in
any ratio,
from 0.0 to 10 grams of at least one water soluble divalent zinc compound,
from 0.0 to 10
grams and preferable 0.5 to 1.5 grams of at least one water-soluble thickener,
and/or from
0.0 to 10 and preferably 0.5 to 1.5 grams of at least one water soluble non-
ionic, cationic
or anionic surfactant or wetting agent.
It is therefore an object of this invention to provide an acidic aqueous
solution
comprising a trivalent chromium compound, hexafluorozirconates, and
tetrafluoroborates
and/or hexafluorosilicates for pretreating iron alloys to improve its adhesion
and
corrosion-resistance properties.
It is another object of this invention to provide a stable acidic aqueous
solution
having a pH ranging from about 2.5 to 5.5 which comprises a trivalent chromium
salt and
hexafluorozirconates for pretreating iron alloys such as steel.
It is another object of this invention to provide a pretreatment process for
coating
steel that has a practical color change and imparts good adhesion without a
grit-blasted
surface. The pretreatment process of this invention also provides improved
corrosion
resistance of painted steel compared to steel that is painted by the direct-to-
metal method.
It is a further object of this invention to provide a stable acidic aqueous
solution
having a pH ranging from about 3.7 to 4.0 comprising a trivalent chromium
compound
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and hexafluorozirconates for treating iron alloys at about room temperature or
higher
wherein said acidic solution contains substantially no hexavalent chromium.
These and other object of the invention will become apparent by reference to
the
detailed description when considered in conjunction with the accompanying
FIG's. 1-6,
(photos).
DESCRIPTION OF THE DRAWINGS
Fig. 1 is a photo of M1L-C-53022 primer on steel, with no grit blast.
Fig. 2 is a photo of MIL-C-53022 primer on steel with TCP-P coating, with no
grit
blast.
Fig. 3 is a photo of MIL-C-53022 primer on steel with Aqua Zen treatment, with
no grit blast.
Fig. 4 is a photo of MIL-C-53022 primer on steel with grit blast treatment.
Fig. 5 is a photo of MII.-C-53022 primer on steel with TCP-P coating with grit
blast treatrnent.
Fig. 6 is a photo of MIL-C-53022 primer on steel with Aqua Zen treatment with
grit blast treatment.
DETAILED DESCRIPTION OF THE INVENTION
This invention relates to the process of using an acidic aqueous solution
having a
pH ranging from about 2.5 to 5.5, and preferably from about 2.5 to 4.5 or 3.7
to 4.0 for
preparing a zirconium-chromium conversion coating on iron and its alloys such
as steel to
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improve adhesion bonding and the corrosion-resistance properties of the alloy.
The
process comprises preparing the pretreatment coating by using an acidic
aqueous solution
at temperatures ranging up to about 120 F or higher which comprises from about
0.01 to
22 grams and preferably from about 0.01 to 10 grams e.g. 5.0 to 7.0 grams of
at least one
water soluble trivalent chromium compound e.g. chromium sulfate, about 0.01 to
12
grams and preferably about 1.0 to 8.0 grams e.g. 6.0 to 8.0 grams of at least
one alkali
metal hexafluorozirconate, about 0.0 to 12 grams and preferably from about
0.01 to 1.2
grains e.g. 0.12 to 0.24 grams of at least one fluorocompound selected from
the group
consisting of alkali metal tetrafluoroborates, alkali metal
hexafluorosilicates and various
mixtures or combinations thereof in any ratio, and from about 0.0 to 10 grams
and
preferably 0.001 to 10 or 0.1 to 5.0 grams of at least one divalent zinc
compound such as
zinc sulfate.
After cleaning and deoxidizing or pickling the iron alloy e.g. steel substrate
via
conventional mechanical or chemical techniques, the solution is applied at
about room
temperature to the substrate via immersion, spray or wipe-on techniques
similar to the
process used for aluminum pretreatments. Solution dwell time ranges from about
1.0 to
10 minutes. With this solution, the 1.0 to 10 minute dwell time yields an
optimum film
for color change, paint adhesion, and corrosion resistance. More importantly
for this
invention, the 1.0 to 10 minute dwell time yields appreciable color change to
the as-
deposited coating that ranges from royal blue to blue-gray depending
prirnarily on the
chemical composition of the aqueous solution. The remaining unreacted solution
is
subsequently rinsed from the substrate with tap or deionized water. No
additional post-
treatments of the alloy are necessary. The pretreatment coating is allowed to
dry
thoroughly before subsequent painting.
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In some processes, depending on the physical characteristics of the iron alloy
i.e.
steel substrate such as the physical size of the substrate, an unique feature
is the addition
of a thickener to the solution that aids in optimum film formation during
spray and wipe-
on applications by slowing down solution evaporation. This also mitigates the
formation
of powdery deposits that degrade paint adhesion. The addition of thickeners,
also aids in
proper fihn formation during large area applications and mitigates the diluent
effect of
rinse water that remains on the substrate during processing from previous
steps. This
feature of the process yields films that have no streaks and are an
improvement in
coloration and corrosion protection. Water-soluble thickeners such as the
cellulose
compounas are present in the acidic aqueous solution in amounts ranging from
about 0.0
to 10 grams per liter and preferably 0.5 to 1.5 e.g., or about 1.0 gram per
liter of the
aqueous solution.
Further, depending on the characteristics of the iron alloy, an effective but
small
amount of at least one water-soluble surfactant or wetting agent can be added
to the acidic
solution in amounts ranging from about 0.0 to 10 grams and preferably from 0.5
to 1.5
grams e.g. 1.0 gram per liter of the acidic solution. There are many water
soluble
surfactants known in the prior art and therefore for purpose of this invention
the
surfactants can be selected from the group consisting of non-ionic, cationic
and anionic
surfactants.
The trivalent chromium is added to the solution as a water-soluble trivalent
chromium compound, preferably as a trivalent chromium salt. Specifically, in
formulating the acidic aqueous solutions of this invention, the chromium salt
can be
added conveniently to the solution in its water soluble form wherein the
valence of the
chromium is plus 3. For eacample, some of the preferred chromium compounds are
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incorporated in the solution in the form of Cr2(S04)3, (NH4)Cr(S04)2 or
KCr(S04)2 and
any mixtures of these compounds. A preferred trivalent chromium salt
concentration is
within the range of about 5.0 to 7.0 grams per liter of the aqueous solution.
It has been
found that particularly good results are obtained from these processes when
the trivalent
chromium compound is present in solution in the preferred ranges.
In some treatments, the alkali metal tetrafluoroborates and/or
hexafluorosilicates
can be added to the acidic solutions in amounts as low as 0.01 grams per liter
up to the
solubility limits of the compounds. For example, about 50% weight percent of
the
fluorosilicate is added based on the weight of the fluorozirconate. In other
words, for 8.0
grams per liter of the fluorozirconate salt, about 4.0 grams per liter of
fluorosilicate is
added to the solution. An alternative is to add about 0.01 to 100 weight
percent of the
fluoroborate salt based on the weight of the fluorozirconate salt. For
example, about 1.0
to 10 weight percent of the fluoroborate salt can be added based on the weight
of the
fluorozirconate salt. A specific example comprises from about 6.0 to 8.0 grams
per liter
of potassium hexafluorozirconate, about 5.0 to 7.0 grams per liter of chromium
III sulfate
basic, about 0.1 to 5.0 grams per liter of divalent zinc sulfate and about
0.12 to 0.24
grams per liter of potassium tetrafluoroborate and/or hexafluorosilicate. An
important
result of the addition of the stabilizing additives i.e. the fluoroborates
and/or
fluorosilicates is that the solution is stable while the pH is maintained
between about 2.5
and 5.5. However, in some examples the solutions may require small adjustments
to the
pH by the addition of effective amounts of a dilute acid or base to maintain
the pH in the
range of about 2.5 to 5.5 and preferably from 2.5 to 4.5 or 3.7 to 4Ø
The solution may contain at least one divalent zinc compound to improve the
color and corrosion protection of the alloy when compared to other treatment
or
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compositions that do not contain zinc. The amount of the zinc compounds can be
varied
to adjust the color imparted to the coating, from as little as about 0.001
grams per liter up
to 10 grams per liter e.g. 0.1 to 5.0 grams of Zinc2+cation. The divalent zinc
can be
supplied by any chemical compound e.g. salt that dissolves in water and is
compatible
with the other components in the acid solution. Divalent zinc compounds that
are water
soluble at the required concentrations preferably include, for example, zinc
acetate, zinc
telluride, zinc tetrafluoroborate, zinc molybdate, zinc hexafluorosilicate,
zinc sulfate and
the like or any combination thereof in any ratio.
The pretreatment of the iron alloys can be carried out at various temperatures
including the temperature of the solution which ranges from ambient e.g. from
about
room temperature up to about 120 F or higher up to about 200 F. Room
temperature is
preferred, however, in that this eliminates the necessity for heating
equipment. The
coating may be air dried by any of the methods known in the art including, for
example,
oven drying, forced-air drying, exposure to infra-red lamps, and the like. For
purposes of
this invention, the term "iron alloys" includes any iron alloy such as steel
containing
small but effective amounts of various other metals and non-metals such as
carbon.
The following Examples illustrate the stable solutions of this invention, and
the
method of using the solutions in providing color recognition, improved
adhesion bonding
and corrosion-resistant coatings for iron and its alloys.
EXAMPLE 1
A stable acidic aqueous solution having a pH ranging from about 3.4 to 4.0 for
pretreating steel to provide a corrosion-resistant and a color recognized
coating thereon
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comprises, per liter of solution, about 3.0 grams of trivalent chromium
sulfate basic,
about 4.0 grams of potassium hexafluorozirconate and about 1.0 gram of zinc
sulfate.
EXAMPLE 2
A stable acidic aqueous solution for treating steel to form a corrosion-
resistant
coating thereon comprises, per liter of solution, about 3.0 grams of trivalent
chromium
sulfate basic, about 4.0 grams of potassium hexafluorozirconate, and from
about 0.0 to
0.12 grams of potassium tetrafluoroborate.
EXAMPLE 3
A stable acidic aqueous solution for treating steel to provide a corrosion-
resistant
and a color recognized coating thereon comprises, per liter of solution, about
3.0 grams of
trivalent chromium sulfate basic, about 4.0 grams of potassium
hexafluorozirconate,
about 0.12 grams of potassium tetrafluoroborate and about 2.0 grams of
divalent zinc
sulfate.
The following Examples and data in Tables 1 and 2 show the paint adhesion
performance of coatings using Mil-P-53022 epoxy primer and 4130 steel test
coupons.
Mil-P-53022 primers are commonly used on steel in a variety of Department of
Defense
systems. It is evident from the data in the Tables that the coatings provide a
substantial
increase in paint adhesion compared to the control coatings. The compositions
or
solutions, though typically used at ambient conditions may be applied at
elevated
temperatures to increase the reaction rate.
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EXAMPLE A
Process data and Examples for TCP/TCP-CC as a Conversion Coating for iron
Alloys. Conversion coatings were applied to 4130 steel as follows:
Test coupons made from 4130 steel were cleaned in a standard alkaline cleaner
(Turco HTC) at 140-160 F for 10 minutes. Coupons were then rinsed and immersed
directly into the test solution. The solution comprised an aqueous solution of
about 6.0
grams per liter of chromium sulfate basic and 8.0 grams per liter of potassium
hexafluorozirconate (TCP-P). Coupons were allowed to dwell in the the TCP-P
for
approximately 10 minutes, removed, and then thoroughly rinsed in de-ionized
water.
Coupons were then allowed to dry in a rack at ambient conditions. The
resulting coatings
were a deep royal blue in color, easily visible from across the laboratory.
This is a critical
indication for quality control during processing so that the processors have a
way of
telling that the coating is complete.
In addition to the TCP-P coated 4130 steel, control sets of 4130 were prepared
by
cleaning (Turco HTC) in the above-identified solution. A subset of these steel
panels or
coupons were grit blasted with alumina grit media to produce an average
profile of 1.0 to
1.5 mils, a standard for a direct-to-metal painting. These steel coupons were
then painted
with MII.-C-53022 primer to approximately 1.0 mil thick. One sub-set of
coupons was
painted with Aqua Zen "wash primer" before the primer. The Aqua Zen is an
industry
standard coating used to promote adhesion and improve corrosion performance of
the
coating system. The painted coupons sat for 14 days at ambient conditions to
allow the
paint to cure. After curing, the coupons were subjected to painted adhesion
and painted
corrosion tests. Table 1 describes the paint adhesion results. Ratings of 4
and 5 are
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considered passing, and 0 to 3 are failing. As shown by the data, TCP-P
coating provides
excellent paint adhesion to the 4130 steel whether or not the steel was grit
blasted. The
TCP-P coating also shows better overall performance in comparison to the two
controls
panels which are commonly used on DoD equipment. In this test, the TCP-P
coating is a
better alternative to the Aqua Zen treatment and shows excellent performance
with the
MIL-C-53022 primer without the grit blast, where paint adhesion is very poor
with only
the primer. Fig's. 1 through 6 (photos) show examples of 7-day wet-tape
adhesion for
each coating system described in Table 1.
TABLE 1
COATINGS AND PAINT ADHESION RESULTS
Grit Blast Coating Paint Adhesion Results
System Dry 1-day Wet 4-day Wet 7-day Wet
No MIL-C-53022
primer only 4 4 0 0
No Aqua Zen
plus .MIL-C-
53022 primer 5 4 3 3
No TCP-P plus
MIL-C-53022
primer 5 4 4 4
Yes 1VM-C-53022
primer only 5 5 5 3
Yes Aqua Zen
plus IVIII..-C-
53022 primer 4 4 3 1
Yes TCP-P plus
NIIL-C-53022
primer 5 5 5 4
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EXAMPLE B
Conversion coatings were applied to 4130 steel as follows:
Test coupons were prepared by the same process as in Example A. The
composition (TCP-CC) solution comprised 3.0 grams per liter of chromium
sulfate basic,
6.0 grams per liter of potassium hexafluorozirconate, 0.18 grams per liter of
potassium
tetrafluoroborate, and 2.0 grams per liter of zinc sulfate.
Table 2 sets-forth the paint adhesion results from these coated coupons that
are
comparable to the results in Table 1. As seen from the data, the coatings
formed from
this TCP i.e. color change composition (TCP-CC) performs as well as TCP-P as
an
adhesion promoter.
TABLE 2
PAINT ADHESION RESULTS FOR TCP-CC
Grit Blast Coating Paint Adhesion Results
System Dry 1-day Wet 4-day Wet 7-day Wet
No TCP-CC plus NA
MIL-C-53022
primer 5 5 4
Yes TCP-CC plus NA
MIL-C-53022
primer 5 5 5
In preparing the acidic solutions of this invention, the water soluble
surfactants
can be added to the trivalent chromium solutions in amounts ranging from about
0 to 10
grams per liter and preferably about 0.5 to 1.5 grams per liter. The
surfactants are added
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to the aqueous solution to provide better wetting properties by lowering the
surface
tension thereby insuring complete coverage, and a more uniform film on the
iron alloy
substrates. The surfactants include at least one water soluble compound
selected from the
group consisting of non-ionic, anionic, and cationic surfactants. Some of the
better
known water soluble surfactants include the monocarboxyl imidoazoline,
alkylsulfate
sodium salts (DUPONOL ), tridecyloxy poly(alkyleneoxy ethanol), ethoxylated or
propoxylated alkylphenol (IGEPAL ), alkylsulfonamides, alkaryl sulfonates,
palmiticalkanol amides (CENTROL ), octylphenyl polyethoxy ethanol (TRITON ),
sorbitan monopalmitate (SPAN ), dodecylphenyl polyethyleneglycol ether (e.g.
TERGITROL ), alkyl pyrrolidone, polyalkoxylated fatty acid esters,
alkylbenzene
sulfonates and mixtures thereof. Other known water soluble surfactants include
the
alkylphenol alkoxylates, preferably the nonylphenol ethoxylates, and adducts
of ethylene
oxide with fatty amines; also see the publication: "Surfactants and Detersive
Systems",
published by John Wiley & Sops in Kirk-Othmer's Encyclopedia of Chemical
Technology, 3' Ed.
When large surfaces do not permit immersion or where vertical surfaces are to
be
sprayed, thickening agents can be added to retain the aqueous solution on the
surface for
sufficient contact time. The thickeners employed are known inorganic and
preferably the
organic water soluble thickeners added to the trivalent chromium solutions in
effective
amounts e.g. at sufficient concentrations ranging from about 0 to 10 grams per
liter and
preferably 0.5 to 1.5 grams per liter of the acidic solution. Specific
examples of some
preferred thickeners include the cellulose compounds, e.g. hydroxypropyl
cellulose (e.g.
Klucel), ethyl cellulose, hydroxyethyl cellulose, hydroxymethyl cellulose, or
methyl
cellulose and mixtures thereof. Other water soluble inorganic thickeners
include colloidal
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silica, clays such as bentonite, starches, gum arabic, tragacanth, agar and
various
combinations.
After preparing the iron alloy surface to be coated via conventional
techniques,
the solution can be applied via immersion, spray or wipe-on techniques. The
TCP
solutions can be used at elevated temperatures ranging up to 65 C or higher
and
optimally applied via immersion to further improve the corrosion resistance of
the
coatings. Solution dwell time ranges from about 1 to 60 minutes, and
preferably 5 to 15
minutes at about 80 F. After dwelling, the remaining solution is then
thoroughly rinsed
from the substrate with tap or deionized water. No additional chemical
manipulations of
the deposited films are necessary for excellent performance. However, an
application of
a strong oxidizing solution can yield a film having additional corrosion
resistance. The
additional corrosion resistance is presumed to be due to the formation of
hexavalent
chromium in the film derived from the trivalent chromium. The aqueous
solutions may
be sprayed from a spray tank apparatus designed to replace immersion tanks.
This
concept also reduces active chemical volume from about 1,000 gallons to about
30 to 50
gallons. Another feature of this invention is that this process provides
coatings with
corrosion resistance that is better or at least equivalent to other known
coatings produced
by sulfixric, chromic, or boric-sulfuric compositions.
While this invention has been described by a number of specific examples, it
is
obvious that there are other variations and modifications which can be made
without
departing from the spirit and scope of the invention as particularly set forth
in the
appended claims.
