Note: Descriptions are shown in the official language in which they were submitted.
CA 02056159 2000-07-26
NON-CHROMATED COBALT CONVERSION CQATING
BACKGROUND OF THE INV_~NTION
1) Field-of the Invention_
This environmental-quality invention is in the field of
chemical conversion coatings formed onr-me_tal substrates,r,for
example, on aluminum substrates. More particularly, one~aspect of
the invention is a new type of oxide coating (which I refer to as
a "cobalt conversion coating") which is chemically formed on metal
substrates. The invention enhances the quality of the environment
of mankind by contributing to the maintenance of air and water
quality.
2) Description of the Related Art
In general, chemical conversion coatings are formed chemically
by causing the surface of the metal to be "converted" into a
tightly adherent coating, all or part of which consists of an
oxidized form of the substrate metal. Chemical conversion
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coatings can provide high corrosion resistance as well~as strong
bonding affinity for paint. The industrial application of paint
(organic finishes) to metals generally requires the use of a
chemical conversion coating, particularly when the performance
demands are high.
Although aluminum protects itself against corrosion by
forming a natural oxide coating, the protection is not
complete. In the presence of moisture and electrolytes,
aluminum alloys, particularly the high-copper 2000-series
aluminum alloys, such as alloy 2024-T3, corrode much more
rapidly than pure aluminum.
In general, there are two types of processes for treating
aluminum to form a beneficial conversion coating. The first
is by anodic oxidation (anodization) in which the
aluminum component is immersed in a chemical bath, such as a
chromic or sulfuric acid bath, and an electric current is
passed through the aluminum component and the chemical bath.
The resulting conversion coating on the surface of the
aluminum component offers resistance to corrosion and a
bonding surface for organic finishes.
The second type of process is by chemically producing a
conversion coating, which is commonly referred to as a
chemical conversion coating, by subjecting the aluminum
component to a chemical solution, such as a chromic acid
solution, but Without using an electric current in the
process. The chemical solution may be applied by immersion
application, by manual application, or by spray application.
The resulting conversion coating on the surface of the
aluminum component offers resistance to corrosion and a
bonding -surface for organic finishes. The present invention
relates to this second type of process for producing chemical
conversion coatings. The chemical solution may be applied by
immersion application, by various types of manual
application, or by spray application.
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One widely-used chromic acid process for forming chemical
conversion coatings on aluminum substrates is described in
various embodiments in Ostrander et al. U.S. Patent 2,796,370
and Ostrander et al. U.S. Patent 2,796,371; in military
process specification MIL-C-5591, and in Boeing Process
Specification BAC 5719. These chromic acid chemical
conversion baths contain hexavalent chromium, fluorides: and
cyanides, all of Which present significant environmental as
well as health and safety problems. The constituents of a
typical chromic acid conversion bath; such as ALODINE~1200,
are as follows: Cr03 - "chromic acid" (hexavalent chromium);
NaF - sodium-fluoride: KBF~ -wpotassium tetrafluoroborate;
K2ZrF6 - potassium hexafluorozirconate: K3Fe(CN)6 - potassium
ferricyanide; and, HN03 - nitric acid (for pH control).
Many aluminum structural parts, as well as Cd plated, Zn
plated, Zn-Ni plated, and steel parts, throughout the
aircraft and aerospace industry are currently being treated
using this chromic aria process technology. Chromic acid
conversion films, as formed on aluminum substrates, meet a
168 hours corrosion resistance criterion, but they primarily
serve as a surface substrate for paint adhesion. Because of
their relative thinness and low coating weights (40-150
milligrams/ft2): chromic acid conversion coatings do not
cause a fatigue life reduction in the aluminum structure.
However, environmental regulations in the United States
particularly in California, and in other countries are
drastically reducing the allowed levels of hexavalent
chromium compounds in effluents and emissions from metal
finishing processes. Accordingly, chemical conversion
processes employing hexavalent chromium compounds must be
replaced. The present invention, which does not employ
hexavalent chromium compounds, is intended to replace the
previously used chromic acid process for forming conversion
coatings on aluminum substrates.
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1.
SUMMARY OF THE INVENTION
(A.) In one aspect, the invention is a process for
forming a cobalt conversion coating on a metal substrate,
thereby imparting corrosion resistance and paint adhesion
properties. The invention was developed as a replacement for
the prior art chromic acid process.
In a broad sense, the invention is a process for forming
a cobalt conversion coating on a metal substrate, the process
comprising the steps of:
(a) providing a cobalt conversion solution comprising an
aqueous solution of a soluble cobalt-III hexacoordinated
complex (hereafter referred to as cobalt-III complex), the
concentration of the cobalt-III hexacoordinated complex being
from about 0.1 mole per gallon of solution up to the
saturation limit of said cobalt-III hexacoordinated complex;
and
(b) contacting the metal substrate with the solution for
a sufficient amount of time, whereby the cobalt conversion
coating is formed.
The substrate may be aluminum or aluminum alloy, as well
as magnesium and its alloys, Cd plated substrates, and ~n
plated substrates. The cobalt-III hexacoordinated complex
may be present in the form of Me3[Co(N02)6] wherein Me
corresponds to Na, K, or Li.
(B.) In another aspect, the invention is a chemical
conversion coating solution. Tn a broad sense, the invention
is a chemical conversion coating solution for producing a .
cobalt conversion coating on a metal substrate, the solution
comprising an aqueous solution of a soluble cobalt-III
hexacoordinated complex, the concentration of said cobalt-III
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hexacoordinated complex being Crom about 0.1 mole per gallon
of solution up to the saturation limit of said cobalt-III
hexacoordinated complex. The substrate may be aluminum or
aluminum alloy, as well as magnesium and its alloys, Cd
plated substrates, and Zn plated substrates. The cobalt-III
hexacoordinated complex may be present in the form of
Me3[Co(N02)6] wherein Me corresponds to Na, K, or Li.
The cobalt conversion solution may be prepared by a bath
makeup sequence including the steps of: (a) dissolving a
metal nitrite salt; (b) dissolving an accelerator such as
NaI; (c) dissolving a cobalt-II salt; and (d) then adding an
oxidizer such as H202.
(C.) In yet another aspect, the invention is a coated
article exhibiting corrosion resistance and paint adhesion
properties, the article including: (a) a metal substrate; and
(b) a cobalt conversion coating formed on the substrate, the
cobalt conversion coating including aluminum oxide A1203 as
the largest volume percent, and one or more cobalt oxides
from the group consisting of CoO, Co304, and Co203. The
substrate may be aluminum or aluminum alloy, as well as
magnesium and its alloys, Cd plated substrates, and Zn plated
substrates.
BRIEF DESCRIPTION OF THE DRAWINGS
The figures herein are photomicrographs of images
produced by a scanning electron microscope (SGM) of coatings
on aluminum alloy test panels. FIGS. 1-20 are
photomicrographs (scanning electron microscope_operated at 20
KV) of aluminum alloy 2024-T3 test panels with cobalt
conversion coatings made by the invention. FIGS. 1-16 show
surface views and fracture views of unsealed cobalt
conversion coatings. The photomicrographs of FIGS. 1-16
reveal a highly porous surface oxide {unsealed cobalt
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conversion coatings) with a thickness range of about 0.12 to
0.19 micron (1200 to 1900 Angstroms). FIGS. 1-9 show an
unsealed cobalt conversion coating formed by a 20 minute
immersion in a typical cobalt coating solution. FIGS. 5-a
show an unsealed cobalt conversion coating formed by a 30
minute immersion in a typical cobalt coating solution. FIGS.
9-12 show an unsealed cobalt conversion coating formed by a
50 minute immersion in a typical cobalt coating solution.
FIGS. 13-16 show an unsealed cobalt conversion coating formed
by a 60 minute immersion in a typical cobalt coating
solution. There were only minor differences in oxide coating
thickness between these immersion times. This suggests that
at any given bath operating temperature, the oxide structure
becomes self limiting. FIGS. 17-20 show surface views and
fracture views of a sealed cobalt conversion coating.
FIG. 1 is a photomicrograph at X10,000 magnification of a
test panel showing a cobalt conversion coating 130 of the
invention. The photomicrograph is a top view, from an
elevated angle, of the upper surface of oxide coating 130.
The top of oxide coating 130 is porous and looks like a layer
of chow mein noodles. The porosity of oxide coating 130
gives excellent paint adhesion results. This test panel was
immersed in a cobalt conversion coating solution for 20
minutes. The white bar is a length of 1 micron.
FIG. 2 is a photomicrograph at X50,000 magnification of
the test panel of FIG. 1. The photomicrograph is a top view,
from an elevated angle, of the upper surface of oxide coating
130. FIG. 2 is a close-up, at higher magnification, of a
small area of FIG. 1. The white bar is a length of 1 micron.
FIG. 3 is a photomicrograph at X10,000 magnification of a
test panel showing a side view of a fractured cross section
of a cobalt conversion coating 130 of the invention. The
fractured cross section of the aluminum substrate of the test
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panel is indicated by reference numeral 132. This test panel
was immersed in a coating bath for 20 minutes. To make the
photomicrograph, the test panel was bent and broken 6ff to
expose a cross section of oxide coating 130. The white bar
is a length of 1 micron.
FIG. 4 is a photomicrograph at X50,000 magnification of
the test panel of FIG. 3 showing a side view of a fractured
cross section of cobalt conversion coating 130 of the
invention. FIG. 9 is a close-up, at higher magnification, of
a small area of FIG. 3. The aluminum substrate of the test
panel is indicated by reference numeral 132. The white bar
is a length of 1 micron. Oxide coating 130 has a vertical
thickness of about 0.12-0.19 micron.
FIG. 5 is a photomicrograph at X10,000 magnification of
another test panel showing another cobalt conversion coating
150 of the invention. The photomicrograph is a top view,
from an elevated angle, of the upper surface of oxide coating
150. The top of oxide coating 150 is porous and looks like a
layer of chow mein noodles. This test panel was immersed in
a cobalt conversion coating solution for 30 minutes. The
white bar is a length of 1 micron.
FIG. 6 is a photomicrograph at X50,000 magnification of
the test panel of FIG. 5. The photomicrograph is a top view,
from an elevated angle, of the upper surface of oxide coating
150. FIG. 6 is a close-up, at higher magnification, of a
small area of FIG. 5. The white bar is a length of 1 micron.
FIG. 7 is a photomicrograph at X10,000 magnification of a
test panel showing a side view of a fractured cross section
of cobalt conversion coating 150 of the invention. The
aluminum substrate of the test panel is indicated by
reference numeral 152. This test panel was immersed in a
coating bath for.30 minutes. To make the photomicrograph,
CA 02056159 2001-O1-22
the test panel was bent and broken off to expose a cross
section of oxide coating 150. The white bar is a length of 1
micron.
FIG. 8 is a photomicrograph at X50,000 magnification of
the test panel of FIG. 7 showing a side view of a fractured
cross section of cobalt conversion coating 150 of the
invention. FIG. 8 is a close-up, at higher magnification, ~of
a small area of FIG. 7. The aluminum substrate of the test
panel is indicated by reference numeral 152. The white bar
is a length of 1 micron. Oxide coating 150 has a vertical
thickness of about 0.12-0.19 micron.
FIG. 9 is a photomicrograph at X10,000 magnification of a
test panel showing a cobalt conversion coating 190 of the
invention. The photomicrograph is a top view, from an
elevated angle, of the upper surface of oxide coating 190.
The top of oxide coating 190 is porous and looks like a layer
of chow mein noodles. This test panel was immersed in a
cobalt conversion coating solution for 50 minutes. The
oblong object indicated by reference numeral 192 is an
impurity, believed to be a piece of oxidized material, on top
of oxide coating 190. The white bar is a length of 1 micron.
FIG. 10 is a photomicrograph at X50,000 magnification of
the test panel of FIG. 9. The photomicrograph is a top view,
from an elevated angle, of the upper surface of oxide coating
190. FIG. 10 is a close-up, at higher magnification, of a
small area of FIG. 9. The roundish object indicated by
reference numeral 192a is an unidentified impurity on top of
oxide coating 190. The white bar is a length of 1 micron.
FIG. 11 is a photomicrograph at X10,000 magnification of
a test panel showing a side view of a fractured cross section
of a cobalt conversion coating 190 of the invention. The
fractured cross section of the aluminum substrate of the test
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CA 02056159 2001-O1-22
panel is indicated by reference numeral 194. This test panel
was immersed in a coating bath for 50 minutes. To make the
photomicrograph; the test panel was bent and broken off to
expose a cross section of oxide coating 190. The white bar
is a length of 1 micron'.
FIG. 12 is a photomicrograph at X50,000 magnification of
the test panel of FIG. 11 showing a side view of a fractured
cross section of cobalt conversion coating 190 of the
invention. FIG. 12 is a close-up, at higher magnification,
of a small area of FIG. 11. The aluminum substrate of the
test panel is indicated by reference numeral 199. The white
bar is a length of 1 micron. Oxide coating 190 has a
vertical thickness of about 0.12-0.14 micron.
FIG. 13 is a photomicrograph at X10,000 magnification of
another test panel showing a cobalt conversion coating 230 of
the invention. The photomicrograph is a top view, from an
elevated angle, of the upper surface of oxide coating 230.
The top of oxide coating 230 is porous and looks like a layer
of chow mein noodles. This test panel was immersed in a
cobalt cor~version coating solution for 60 minutes. The white
bar is a length of 1 micron.
FIG. 14 is a photomicrograph at X50,000 magnification of
the test panel of FIG. 13. The photomicrograph is a top
view, from an elevated angle, of the upper surface of oxide
coating 230. FIG. 14 is a close-up, at higher magnification,
of a small area of FIG. 13. The white bar is a length of 1
micron.
FIG. 15 is a photomicrograph at X10,000 magnification of
a test panel showing a side view of a fractured cross section
of cobalt conversion coating 230 of the invention. The
aluminum substrate of the test panel is indicated by
reference numeral 232. This test panel was immersed in the
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coating bath for 60 minutes. To make the photomicrograph,
the test panel was bent and broken off to expose a cross
section of oxide coating 230. The white bar is a length of 1
micron.
FIG. 16 is a photomicrograph at X50,000 magnification of
the test panel of FIG. 15 showing a side view of a fractured
cross section of cobalt conversion coating 230 of the
invention. FIG. 16 is a close-up, at higher magnification,
of a small area of FIG. 15. The white bar is a length of 1
micron. Oxide coating 150 has a vertical thickness of about
0.12-0.14 micron.
FIG. 17 is a photomicrograph at X10,000 magnification of
another test panel showing a sealed cobalt conversion coating
270 of the invention. The photomicrograph is a top view,
from an elevated angle, of the upper surface of sealed oxide
coating 270. This test panel was immersed in a sealing
solution for 20 minutes. Sealed oxide coating 270 is not as
porous as an unsealed oxide coating, the pores of the oxide
coating being partially filled by hydration as a result of
immersion in a sealing solution. The partial sealing of the
oxide coating gives reduced paint adhesion results, but
excellent corrosion resistance performance. The whitish
areas identified by reference numeral 274 are believed to be
impurities from the sealing solution. The white bar is a
length of 1 micron.
FIG. 18 is a photomicrograph at X50,000 magnification of
the test panel of FIG. 17. The photomicrograph is a top
view, from an elevated angle, of the upper surface of sealed
oxide coating 270. FIG. 18 is a close-up, at higher
magnification, of a small area of FIG. 17. Sealed oxide
coating 270 is not as porous as an unsealed oxide coating,
the pores of the oxide coating being partially filled by
hydration as a result of immersion in a sealing solution.
The white bar is a length of 1 micron.
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FIG. 19 is a photomicrograph at X10,000 magnification of
a test panel showing a side view of a fractured cross section
of sealed cobalt conversion coating 270 of the invention.
The aluminum substrate of the test panel is indicated by
reference numeral 272. This test panel was immersed in the
sealing bath for 20 minutes. To make the photomicrograph,
the test panel was bent and broken off to expose a cross
section of oxide coating 270. 'the white bar is a length of 1
micron.
FIG. 20 is a photomicrograph at X50,000 magnification of
the test panel of FIG. 19 showing a side view of a fractured
cross section of sealed cobalt conversion coating 270 of the
invention. FIG. 20 is a close-up, at higher magnification,
of a small area of FIG. 19. The white bar is a length of 1
micron. Sealed oxide coating 270 has a vertical thickness of
about 0.12-0.14 micron.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
I have invented two classes of cobalt conversion
coatings. The first class is a cobalt conversion coating
consisting of an oxide structure in unsealed condition and
suitable for use in service where paint adhesion is
especially important. The second class is a cobalt
conversion coating consisting of an oxide structure in sealed
condition and suitable for use in service where bare metal
corrosion resistance performance is desired.
A considerable amount of empirical research was conducted
in order to arrive at the present invention. A variety of
multivalent compounds was investigated, used either by
themselves or in combination with alkalies, acids, or
fluorides. Among these compounds were vanadates, molybdates,
cerates, ferrates and a variety of borates. While film
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deposition of compounds containing these elements on aluminum
alloy substrates has been achieved, none afforded any
appreciable corrosion protection nor paint adhesion.
A significant increase in corrosion protection was
observed, however, when aluminum substrates were immersed in
aqueous solutions of simple cobalt-II (Co2+) salts heated to
180°F. This led to an investigation of a number of cobalt-II
and cobalt-III (Co3+) reactions, in particular as described
in copending application Serial No. 07/525,800 filed May 17,
1990.
When 2-valent cobalt salts are dissolved into an aqueous
solution of MeN02 (where Me = Na, K, or Li) (alkali metal
ions), in the presence of an oxidizing agent, such as H202,
then 3-valent cobalt nitrite complexes are formed:
(1) 2 CoX2 + 12 MeN02 + H202 ~ 2 Me3[Co(N02)6] + 9 MeX +
2 MeOH
where X2 = (N03)2, C12, (CH3C00)2 (acetate), 504, Br2, (CN)2,
(SCN)2, C03. From an environmental standpoint, the cyanide
and thiocyanate salts are not preferred. In particular, the
following bath chemistries were prepared and tested:
(2) 2 Co(N03)2~6H20 + 12 NaN02 + H202 ~ 2 Na3[Co(NOZ)6]
+ 4 NaN03 + 2 NaOH
(3) 2 Co(N03)2~6H20 + 12 KN02 + H202 ~ 2 K3[Co(N02)6]
+ 4 KN03 + 2 KOH
(4) 2_Co(CH3C00)2~4H20 + 12 NaN02 + H202
2 Na3[Co(N02)6] + 4 CH3COONa + 2 NaOH
(5) 2 CoCl2~6H20 + 12 NaN02 + H202 ~ 2 Na3[Co(N02)6] +
4 NaCl + -2 NaOH
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These 3-valent cobalt nitrite complexes were found to
produce brightlyiridescent oxide coatings on aluminum
substrates.
The chemistry described in United States patent
number 5,298,092 involved the chemistry of an aqueous
solution containing a cobalt-II salt such as CoX2 (where X2 -
C12. Br2. (N03)2. (CN)2. (SCN)2.,2/ap04~SOq. (CH3C00)2, C03)
and the corresponding ammonium salt NHqX in the presence of
ammonium hydroxide (ammonia) to form a cobalt-III hexammine
coordination complex, for example:-
(6) 4 CoX2 + 4 NHqX + 20 NH3 -
4 ICo(NH3)6]X3 + water
It should 'be noted that the above cobalt hexammine
chemistry in equation (6) involves avcobalt coordination
complex where the portion of the complex which includes the
ligand (the bracketed portion in equation (6)) is positively
charged; i:e., ~.
(7) [Co(NH3)6]3+
In the cobalt hexanitrite chemistry subsequently
dweloped and described herein; cobalt coordination complexe s
are formed where the portion of the complex which includes
the ligand (the bracketed portion in equations (1)-(5)) is
negatively charged, i.e.,
(8) [Co(N02)6]3-
and the complete complex is
(9) Me3[Co(N02)6]
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where Me corresponds to Na, K, or Li (alkali metal ions).
This cobalt nitrite complex bath chemistry (equation (1))
has a distinct advantage over the previously described cobalt
hexammine complex chemistry (equation (6)) in that pH control
of the cobalt hexanitrite complex bath is not required.
It was discovered that when aluminum alloy substrates
(such as alloy 2024-T3) are immersed in an aqueous solution
containing the cobalt-III nitrite complexes above, bright
iridescent coatings are formed on the aluminum alloy, which
give excellent corrosion resistance properties.
It is surprising that cobalt-III hexanitrite complexes
are capable of forming oxide structures on aluminum
substrates. The oxidizing ability of the cobalt-III
hexanitrite complex is believed to be responsible for the
formation of the observed oxide films (which I refer to as
"cobalt conversion coatings") on aluminum substrates. The
formation of oxide structures has been confirmed by
instrumental analysis (Auger analysis and electron
microscopy) of the coating. The photomicrographs in FIGS.
1-20 illustrate the appearance of the cobalt conversion
coating of the invention.
Initial bath formulations were made up using
Co(N03)2~6H20 and NaN02. Reaction quantities were used in
accordance with stoichiometric amounts as shown in equation
(2) above.
It became apparent during experimentation-with this
initial formulation that a number of parameters are important
from the standpoint of bath chemistry and uniform Formation
of oxide coating films. These parameters are: chemical
reactant selection; chemical reactant concentrations; bath
makeup sequence; temperature; and immersion time. It should
be noted that pH control is not a factor.
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Chemical Reac- nt Sele ion
Regarding reactant selection, a wide variety of cobalt salts
and metal nitrite salts are operable for cobalt complexing.
Among the cobalt-II salts which are operable in water solution
are: cobalt nitrate, Co(N03)2~6H20; cobalt chloride, CoCl2~6H20;
cobaltous sulfate, CoSOq; cobaltous acetate, Co(CH3C00)2~4H20;
and cobaltous basic carbonate, 2CoC03~Co(OH)2~H20. Each of the
foregoing cobalt-II salts may be reacted with a nitrite salt
such as NaN02, KN02, or LiN02.
Furthermore, other cobalt-II salts may be used if they
possess a minimum solubility in water or in a water solution
containing a metal nitrite salt. The minimum solubility needed
is 25 grams per 100 ml of water at 20°C (68°F) or 25 grams per
100 ml of water solution containing a metal nitrite salt at 20°C
(68°F).
It may also be noted that for aluminum and aluminum alloys
the preferred reactants are Co(N03)2~6H20 and NaN02, since
cobalt nitrite complexes formed with potassium or lithium
nitrite are of limited solubility and will eventually drop out
of an aqueous solution.
A preferred chemical additive is an oxidizer, preferably
hydrogen peroxide, H202. The function of the oxidizer is to
oxidize the cobalt-II ions in solution to cobalt-III ions. Care
must be taken that an excess amount of chemical oxidizer is not
used because an excess would have the undesired effect of
oxidizing the nitrite ions in solution to nitrate ions. The
stream of__air flowing,into the tank functions as an oxidizer, so
the presence of hydrogen peroxide is not essential for
operability. The hydrogen peroxide increases the rate of
oxidation of the cobalt-II ions in solution to cobalt-III ions
and therefore is useful for commercial practice of the invention
in that the solution becomes operational in a shorter period of
time.
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20~61~~
Furthermore, it may also be noted that a reaction
accelerator chemical such as sodium bromide (NaBr) or sodium
iodide (NaI) may be added to the solution. (NaI is preferred.)
The reaction accelerator was found to have the effect of
accelerating the formation of the oxide conversion coatings on
aluminum alloy substrates as compared to solutions of cobalt-III
hexanitrite complexes which dad not contain this additive. The
presence of the accelerator is not essential for operability.
The accelerator increases the rate of formation of the oxide
conversion coatings on aluminum alloys and therefore is useful
for commercial practice of the invention.
Thus the preferred chemical reactants and additives are:
Cobalt nitrate Co(N03)2~6H20
Sodium nitrite NaN02
Hydrogen peroxide (oxidizer) H2p2
Sodium iodide (accelerator) NaI
Chemical Reactant Concentration, pH Temperature And Immersion
Time
With respect to chemical reactant concentrations, the
concentration of dissolved cobalt--II salt used may be from about
O.i moles per gallon of final solution up to the saturation
limit of the cobalt-II salt employed. The concentration of
dissolved metal nitrite salt may be from about 0.6 to 12 moles
per gallon of final solution. The concentration of oxidizer,
such as hydrogen peroxide, may be from complete omission up to
about 0.5 moles per gallon of final solution. As stated above,
an excess amount of hydrogen peroxide has undesired effects.
The concentration of accelerator salt, such as NaI, may be from
complete omission up to the solubility limit of the accelerator
in the solution. The pH of the bath may be from about 7.0 to
7.2. The temperature of the bath may be from about 68°F to
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150°F; below 100°F coating formation is very slow; above
150°F
gradual decomposition of the cobalt-III hexanitrite complex
occurs. The immersion time may be from about 3 minutes to 60
minutes.
Preferred Bath Preparation Seguence
1. A stainless steel tank fitted with air agitation
plumbing and heating coils is filled to 3/4 with deionized water
at a temperature of 68°F to 90°F. Air agitation is commenced to
a gentle bubble. (The tank may be equipped with a filter unit
to remove any solid impurities (dust, aluminum silt, etc.)
during processing.)
2. A quantity of nitrite salt (NaN02 is preferred) is added
and completely dissolved. Stainless steel baskets may be used
to hold the nitrite salt granules suspended in the water while
dissolving. The preferred concentration of nitrite salt is
about 3.6 moles per gallon of final solution. The amount used
is based on the mole ratio of nitrite salt to cobalt salt which
will produce an oxide coating exhibiting high paint adhesion
properties. The preferred molar ratio of nitrite salt to cobalt
salt is about 12 to 1.
3. A quantity of sodium iodide (the conversion coating
reaction accelerator) may now be added. The concentration of
this additive may be from no addition up to the solubility
limit, however the preferred quantity is 80-100 gm per gallon of
final solution.
9... The cobalt-II salt is now added and dissolved. The
preferred'-concentration is about 0.3 moles per gallon of final
solution. This concentration of the cobalt salt, when added to
a solution already containing 3.6 moles per gallon of nitrite
salt, achieves the preferred molar ratio of nitrite salt to
cobalt salt of 12 to 1. Moderate air agitation is maintained.
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5. A quantity of hydrogen peroxide H202 is now slowly
added. The preferred amount is 0.3 to 0.5 moles of ti202 (30 to
50 ml of H202 (30 volume o)) per gallon of final solution. The
tank is filled to the final volume with deionized water. Air
agitation of this
solution is maintained fox 12-16 hours at a temperature of
68-90°F and then the solution is heated to preferably 120 ~
5°F. The use of H202 is preferred for fast and consistent
cobalt-III hexanitrite complex formation. The solution is now
ready for operation.
6. Optionally, a second stainless steel tank (to be used
for a coating seal step) is prepared with air agitation plumbing
and heating coils and is filled 3/4 with deionized water. This
post-cobalt conversion coating step serves as an oxide coating
sealer to promote corrosion resistance performance. The tank is
heated to 180 ~ 5°F with air agitation.
7. A quantity of ammonium nitrate, NH4N03, is added to the
seal tank and dissolved. The preferred amount is 119 gm
(1.42 moles) per gallon of final solution. Stir as necessary to
dissolve.
8. A quantity of nickel sulfate, NiS04~6H20, and a quantity
of manganese acetate, Mn(CH3C00)2~4H20, are added to the seal
tank and dissolved. The preferred amount of nickel sulfate is
152 gm (0.58 moles) per gallon of final solution. The preferred
amount of manganese acetate is 76 gm (0.31 moles) per gallon of
final solution. Stir as necessary to dissolve.
9. The seal tank is then filled to final volume with
deionized water. No further air agitation is needed.
Preferred Overall Processing Sequence
The preferred overall processing sequences may be summarized
as follows:
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2o~sm9
PRO ESS SEQ~ NCE FLOW CHART
FOR MAXIMUM PAINT ADHESION
(1) PRECLEAN IF REQUIRED '
(2) MASK AND RACK AS REQUIRED
(3) ALKALINE CLEAN AND RINSE
(4) DEOXIDIZE AND RINSE
(5) FORM OXIDE COATING - 15 MINUTES AT 125 ~ 5°F
(6) IMMERSION RINSE - 140°F, 5 MINUTES MINIMUM
(7) DRY - 140°F MAXIMUM
PROCESS SEQUENCE FLOW ~C'HART
FOR MAXIMUM ORROSION RESISTANCE
(1) PRECLEAN IF REQUIRED
(2) MASK AND RACK AS REQUIRED
(3) ALKALINE CLEAN AND RINSE
(4) DEOXIDIZE AND RINSE
(5) FORM OXIDE COATING - 30 MINUTES AT 125 _+ 5°F
(6) IMMERSION RINSE - 140°F, 5 MINUTES MINIMUM
(7) SEAL AS REQUIRED
(8) RINSE - ROOM TEMPERATURE, 3 MINUTES MINIMUM
(9) DRY - 140°F MAXIMUM
General Notes With Respect To The Above Process Flow Charts
The cobalt conversion coating should be applied after all
trimming and fabrication have been completed. Parts, where
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solution entrapment is possible, should not be subjected to
immersion alkaline cleaning or immersion deoxidizing; manual
cleaning and manual deoxidizing procedures should be used to
obtain water break-free surfaces before applying cobalt conversion
treatment. A water break-free surface is a surface which
maintains a continuous water film for a period of at least 30
seconds after having been sprayed or immersion rinsed in clean
water at a temperature below 100°F.
Thorough rinsing and draining throughout processing is
necessary as each solution should be completely removed to avoid
interference with the performance of the next solution in the
sequence. Parts should be processed from one step to the next
without delay and without allowing the parts to dry. When it is
necessary to handle wet parts, wear clean latex rubber gloves.
After conversion coating, handle dry parts only with clean fabric
gloves. For processing systems which require part clamping, the
number and size of contact points should be kept to a minimum as
necessary for adequate mechanical support.
Precleaning
Vapor degrease may be performed in accordance with Boeing
Process Specification eAC 5908, emulsion clean in accordance with
Boeing Process Specification BAC 5763, or solvent clean in
accordance with Boeing Process Specification BAC 5750 if parts are
greasy or oily. Parts with open faying surfaces or spot-welded
joints where solution entrapment is possible should be immersed in
cold water (or in hot and cold water) for 2 minutes after
precleaning.
Masking And Rackinv
Areas which do not require cobalt conversion coatings should
be masked with maskants. Dissimilar metal inserts {except
chromium, nickel or cobalt alloy or plating, CRES, or titanium)
and non-aluminum coated plasma flame sprayed area should be masked
off.
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CA 02056159 2001-O1-22
Alkaline ClP~anir~4
Alkaline clean and rinse may be performed in accordance with
Boeing Process Specification DAC 5799 or Boeing Process
Specification BAC 5749 except for parts with open faying surfaces
or spot welded joints, in which case, rinsing should be for at
least 10 minutes using agitation with multiple immersions (a
minimum of four times) followed by manual spray rinsing as
required to prevent solution entrapment.
Deoxidizing
Deoxidize and rinse may be performed in accordance with Boeing
Process Specification BAC 5765 except for parts where solution
entrapment is possible, which parts may be rinsed using the method
described above under "Alkaline Cleaning". Castings may be
deoxidized by either of the following methods:
a. Deoxidize in accordance with Boeing Process Specification
BAC 5765, Solution 37, 38 or 39.
b. Dry abrasive blast castings in accordance with Boeing
Process Specification BAC 5748, Type II, Class 1 and
rinse.
Examples
Examples of specific solution formulations within the scope of
the invention are as follows:
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Example 1.
Component Make-Up Control
Per Gallon Of Limits
Final Solution
Cobalt(ous) nitrate, 85 gm 75-95 gm/gal
Co(N03)26H20 (about 0.29 mole)
(hexahydrate)
Sodium nitrite, NaN02242 gm 227-246 gm/gal
(about 3.51 moles)
Sodium Iodide, NaI 90 gm 83-99 gm/gal
(about 0.60 moles)
Hydrogen peroxide, 30-50 ml
H202 {30 vol. %) (about 0.3-0.5 moles
of H202)
Water balance
Temperature 120 + 5° F
pH 7.0 - 7.2
The formulation of Example 1, with a molar ratio of nitrite
salt to cobalt salt of about 12 to 1, is useful for producing
oxide coatings exhibiting high paint adhesion in unsealed
condition.
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Example 2.
Component Make-Up Control
Per Gallon Of Limits
Final Solu ion
Cobalt(ous) chloride, CoC12~6H20 69 gm
(hexahydrate) (about 0.29 mole)
Sodium nitrite, NaN02 242 gm
(about 3.51 moles)
Sodium iodide, NaI 90 gm
(about 0.60 moles)
Hydrogen peroxide, H202 (30 vol. %) 30-50 ml
(about 0.3-0.5 moles of
H202)
Water balance
Temperature 120-150°F
pH 7.0 - 7.2
The formulation of Example 2, also having a molar ratio of
nitrite salt to cobalt salt of about 12 to 1, is useful for
producing oxide coatings possessing high paint adhesion
properties in unsealed condition.
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Example 3.
Component Make-Up Control
Per Gallon Of Limits
Final Solution
Cobalt acetate, 73 gm
Co(CH3C00)2~4H20 (about 0.29 moles)
Sodium nitrite, NaN02 242 gm
(about 3.51 moles)
Sodium iodide, NaI 90 gm
(accelerator) (about 0.60 moles)
Hydrogen peroxide, 30-50 ml
H202 (30 vol. %) (about 0.3-0.5 moles of
H202)
Water balance
Temperature 120-150°F
pH 7.0 - 7.2
It should be noted that in all of the above examples hydrogen
peroxide H202 (30 vol. %) is employed to convert the 2-valent cobalt
salt into the 3-valent cobalt hexanitrite complex. While air
bubbling (aeration) of the solution alone will convert a sufficient
quantity of cobalt-II salt to cobalt-III complex, the procedure will
be time consuming and complete conversion may never be obtained.
In principle, any 2-valent soluble cobalt salt may be reacted
with any soluble nitrite salt to form 3-valent cobalt hexanitrite
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CA 02056159 2001-O1-22
complexes. Furthermore, this type of complexing, as shown in
formula (8) above, is not restricted to nitrites only. For research
purposes, cyanide salts were used (i.e., sodium cyanide, NaCN) to
form hexacyano complexes of the type shown below
(10) Me3[Co(CN)6J
and have yielded satisfactory conversion coatings on aluminum
alloys. However, cyanide complexes will not be used because of
environmental considerations.
As mentioned above, in order to produce cobalt conversion
coatings with maximum corrosion resistance performance (168 hrs.
salt spray corrosion resistance, when tested in accordance with ASTM
H117) it is necessary to subject the cobalt conversion coating to a
sealing step. For this purpose, a number of sealing solutions were
found to be useful, however, the sealing solution formulation below
is preferred.
Example 4.
Componen t Make-Up Control
Per Gallon Of Limits
Final Solution
Nickel sulfate, 152 gm 144-159 gm
NiSOq~6H20 (about 0.58 moles)
(hexahydrate)
Ammonium nitrate, 119 gm 105-121 gm
NH4N03 (about 1.42 moles)
Manganese acetate, 76 gm 68-89 gm
Mn(CH3C00)2~9H20 (about 0.31 moles)
Operating temperature 185 + 5° F
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2056159
The immersion time in the sealing solution may be about
10-30 minutes, with 15 minutes being preferred. The sealing
solution is believed to seal the cobalt conversion coating by a
hydration mechanism. FIGS. 17-20, particularly FIG. 18, show a
sealed cobalt conversion coating 270. Other sealing solutions
which may be employed are as follows:
Solution 1: Boric acid, H3B03, 50 gm/gal.
Cobalt sulfate, CoS04~7H20, 25 gm/gal.
Ammonium acetate, CH3COONH4, 25 gm/gal.
Solution 2: Boric acid, H3B03, 30 gm/gal.
Sodium borate, Na2B407, 30 gm/gal.
Sodium nitrite, NaN02, 30 gm/gal.
Ammonium vanadate, NH4V03, 5 gm/gal.
Solution 3: Cobalt sulfate, CoS04~7H20, 25 gm/gal.
Ammonium vanadate, NH4V03, 5 gm/gal.
Boric acid, H3B03, 50 gm/gal.
Solutions 1-3 are not preferred because they lose their
effectiveness over a period of time, whereas the solution in
Example 4 has a long life.
Cobalt Conversion Coatinw Solution Temperature And Immersion Time
The two process parameters of solution temperature and immersion
time have been found to be important as relating to cobalt
conversion coating performance.
A continuous operating temperature range of the cobalt
conversion tank of 120-190°F yields optimum results with respect to
coating performance on aluminum alloy substrates. Optimum paint
adhesion is obtained when the tank is operated at or near 120°F,
while optimum corrosion resistance performance is given at 140°F in
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CA 02056159 2001-O1-22
combination with the subsequent seal process. Immersion times in
the cobalt conversion tank have an effect on the oxide coating
thickness as measured by the coating weight (in unsealed condition)
ranging from 40 to 60 mg/ft2. An optimum immersion time for maximum
paint adhesion is 15 minutes and for maximum corrosion resistance
performance is 30 minutes.
Corrosion Resistance
Salt spray corrosion resistance of cobalt conversion coatings
produced by the above processes varies over a wide range, depending
on reactant selection, immersion times, and bath operating
temperatures. Preferred results are obtained when the formulation
of Example 1 is utilized at immersion times of 30 minutes. In this
way, sealed oxide coatings have been produced with 168 hrs. of salt
spray corrosion resistance when sealed with the seal solution as
described herein and tested in accordance with ASTM B117.
Paint Adhesion
Paint adhesion tests were conducted using aircraft paints
qualified to Boeing Material Specification BMS 10-11 (a highly
crosslinked epoxy primer) and BMS 10-60 (a highly crosslinked
urethane topcoat). General trends observed with the present cobalt
conversion coatings are consistent with conventional chromic acid
conversion coatings, i.e., corrosion resistance and paint adhesion
performance properties have an inverse relationship. In general,
where corrosion resistance is at a maximum, paint adhesion is at a
minimum, and vice versa.
However, the optional post-conversion step-consisting of
immersion into a heated solution (at 185 ~ 5°F) of
NiS04/NH4N03/Mn-acetate minimizes this problem by maintaining
sufficient paint adhesion values while maintaining high corrosion
resistance properties.
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CA 02056159 2001-O1-22
Qxide Coating Analyses
ESCA surface analysis, using a Perkin-Elmer Model 550
surface analyzer, and Auger oxide profiles, using the same
machine (in a different operating mode), have been performed in
order to characterize the cobalt conversion coatings of the
invention. (ESCA = electron spectroscopy for chemical analysis
(also known as XPS or X-ray photoelectron spectroscopy).)
These analyses show that the cobalt conversion coating
consists of a mixture of oxides, namely, aluminum oxide, A1203,
as the largest volume percent, and cobalt oxides, CoO, Co30q,
and Co203. The term "largest volume percent" means that the
volume of this oxide exceeds the volume of any other oxide
which is present, but the term "largest volume percent" does
not necessarily imply that the volume of this oxide is more
than 50 volume percent.
The data further shows that in the lower portion of the
oxide coating (that is. next to the aluminum substrate), the
largest volume percent is A1203. The middle portion of the
oxide coating is a mixture of CoO, Co30q, Co203, and A1203.
And the data shows that in the top portion of the oxide
coating, the largest volume percent is a mixture of Co30q and
Co203.
Additional characterization of the cobalt conversion
coatings of the invention may be found above in the "Brief
Description Of The Drawings", in FIGS. 1-20, and in the
descriptions of FIGS. 1-20. FIGS. 1-4 show a cobalt conversion
coating 130 (in the unsealed condition) formed'by a 20 minute
immersion--in a typical cobalt conversion coating solution.
FIGS. 5-8 show a cobalt conversion coating 150 (in the unsealed
condition) formed by a 30 minute immersion in a typical cobalt
conversion coating solution. FIGS. 9-12 show a cobalt
conversion coating 190 (in the unsealed condition) formed by a
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CA 02056159 2001-O1-22
50 minute immersion in a typical cobalt conversion coating
solution. FIGS. 13-16 show a cobalt conversion coating 230 (in
the unsealed condition) formed by a 60 minute immersion in a
typical cobalt conversion coating solution. Comparing FIGS.
1-9, FIGS. 5-d, FIGS. 9-12, and FIGS. 13-16, there does not
appear to be any significant structural difference between
coating 130, coating 150, coating 190, and coating 230. This
suggests that at any given bath operating temperature, the
oxide coating becomes self limiting. The top surface of the
cobalt conversion coating, as shown in FIGS. 1, 2, 5, 6, 9, 10,
13, and 14 is porous and bears a resemblance to chow mein
noodles. This oxide structure provides appreciable surface
area and porosity for good paint adhesion.
FIGS. 17-20 show sealed cobalt conversion coating 270. The
cobalt conversion coating was formed on the substrate and then
the coating was partially sealed by immersion in a sealing
solution. In particular, FIG. 18 shows the partially sealed
structure of coating 270. Sealed oxide coating 270 is not as
porous as an unsealed oxide coating, the pores of the oxide
coating being partially filled by hydration as a result of
immersion in a sealing solution. The partial sealing of the
oxide coating gives reduced paint adhesion results, but
excellent corrosion resistance performance.
Other M _thnr~s ~f Application
The above examples illustrate producing cobalt conversion
coatings by immersion application. The same principles apply
to producing the conversion coating by manual application and
by spray application. -
The patents, specifications, and other publications
referenced above are incorporated herein by reference.
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205619
' As will be apparent to those skilled in the art to which
the invention is addressed, the present invention may be
embodied in forms other than those specifically disclosed
above, without departing from the spirit or essential
characteristics of the invention. The particular embodiments
of the invention described above and the particular details of
the processes described are therefore to be considered in all
respects as illustrative and not restrictive. The scope of the
present invention is as set forth in the appended claims rather
than being limited to the examples set forth in the foregoing
description. Any and all equivalents are intended to be
embraced by the claims.
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