U.S. patent application number 12/272686 was filed with the patent office on 2009-03-12 for corrosion resistant conversion coatings.
This patent application is currently assigned to DEFT, INC.. Invention is credited to Eric L. Morris.
Application Number | 20090065101 12/272686 |
Document ID | / |
Family ID | 36081312 |
Filed Date | 2009-03-12 |
United States Patent
Application |
20090065101 |
Kind Code |
A1 |
Morris; Eric L. |
March 12, 2009 |
Corrosion Resistant Conversion Coatings
Abstract
A conversion coating composition for coating a metal substrate
is provided which imparts corrosion resistance to the underlying
metal substrate. The conversion coating composition comprises an
aqueous carrier and first and second rare earth element salts. A
complete coating system employing the conversion coating
composition is also provided as well as methods for conversion
coating a metal substrate with the rare earth element conversion
coating compositions of the present invention.
Inventors: |
Morris; Eric L.; (Irvine,
CA) |
Correspondence
Address: |
SHELDON MAK ROSE & ANDERSON PC
100 Corson Street, Third Floor
PASADENA
CA
91103-3842
US
|
Assignee: |
DEFT, INC.
Irvine
CA
|
Family ID: |
36081312 |
Appl. No.: |
12/272686 |
Filed: |
November 17, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11002741 |
Dec 1, 2004 |
7452427 |
|
|
12272686 |
|
|
|
|
Current U.S.
Class: |
148/400 ;
148/243; 148/26; 428/469 |
Current CPC
Class: |
C23C 22/83 20130101;
C09D 5/086 20130101; C23C 22/48 20130101 |
Class at
Publication: |
148/400 ; 148/26;
428/469; 148/243 |
International
Class: |
B32B 15/04 20060101
B32B015/04; C23C 22/05 20060101 C23C022/05 |
Claims
1. A composition for coating a metal substrate comprising an
aqueous carrier and first and second salts, each salt comprising an
anion and a cation, the anion of the first and second salts being
different, and the cation of the first and second salts being the
same or different, wherein each cation, individually, is a rare
earth element, and the first and second salts in combination are
present in the composition in an amount effective to form a
corrosion resistant coating on the metal substrate.
2. A composition according to claim 1 wherein at least one salt is
capable of reaction at or near the surface of a metal substrate to
form a coating on the metal substrate.
3. A composition according to claim 1 wherein the first and second
salts in combination are present in the composition in at least
about 0.04 weight percent.
4. A composition according to claim 3 wherein the first and second
salts in combination are present in the composition in at least
about 1.5 weight percent.
5. A composition according to claim 4 wherein the first and second
salts in combination are present in the composition in about 8
weight percent.
6. A composition according to claim 1 wherein at least one cation
is a Cerium (III) cation.
7. A composition according to claim 6 wherein at least two cations
are Cerium (III) cations.
8. A composition according to claim 6 wherein at least one of the
first and second salts is a Cerium (III) halide or Cerium (III)
nitrate.
9. A composition according to claim 1 further comprising an
oxidizing agent.
10. A composition according to claim 9 wherein the oxidizing agent
is hydrogen peroxide.
11. A composition according to claim 1 further comprising a
self-cleaning additive.
12. A composition according to claim 11 wherein the self-cleaning
additive is a surfactant or a detergent.
13. A composition according to claim 1 further comprising an
oxidizing agent, and wherein each cation of the first and second
salts is a different rare earth element cation.
14. A composition for coating a metal substrate comprising water,
an oxidizing agent, and first and second salts, wherein the first
and second salts in combination are present in the composition in
at least about 0.04 weight percent, and wherein each salt comprises
at least two different anions of the same rare earth element
cation.
15. A composition according to claim 14 wherein the first and
second salts in combination are present in the composition in an
amount effective to form a corrosion resistant coating on the metal
substrate.
16. A composition for coating a metal substrate comprising a
plating bath having the dissolution products from one or more rare
earth element oxides and an acid generating compound, wherein at
least one dissolution product from a rare earth element oxide is
present in the plating bath in an amount effective to form a
corrosion resistant coating on the metal substrate.
17. A composition according to claim 16 wherein the acid generating
compound is selected from the group consisting of gypsum,
anhydrite, celestite, and barite, in hydrous and anhydrous forms,
in naturally occurring mineral forms, and as precipitated
salts.
18. A composition according to claim 16 further comprising an
oxidant.
19. A composition according to claim 18 further comprising an
additive.
20. A metal substrate coating comprising: a) a conversion coating
prepared from a composition having first and second salts, each
salt comprising an anion and a cation, the anion of the first and
second salts being different, and the cation of the first and
second salts being the same or different, wherein each cation,
individually, is a rare earth element, and the first and second
salts in combination are present in the composition in an amount
effective to form a corrosion resistant coating on the metal
substrate; and b) a primer coat.
21. A metal substrate coating according to claim 20 wherein the
primer coat comprises a chromate based coating composition.
22. A metal substrate coating according to claim 20 wherein the
primer coat comprises a chromate-free, --coating composition.
23. A metal substrate coating according to claim 20 further
comprising a topcoat.
24. A metal substrate coating according to claim 23 wherein the
topcoat is an advanced performance topcoat.
25. A metal substrate coating comprising: a) a conversion coating
prepared from a composition having first and second salts, each
salt comprising an anion and a cation, the anion of the first and
second salts being different, and the cation of the first and
second salts being the same or different, wherein each cation,
individually, is a rare earth element, and the first and second
salts in combination are present in the composition in an amount
effective to form a corrosion resistant coating on the metal
substrate; and b) a self-priming top-coat or an enhanced
self-priming topcoat.
26. A process for coating a metal substrate comprising: a)
providing a metal substrate; and b) coating the metal substrate
with a composition according to claim 1.
27. A process according to claim 26 further comprising pre-treating
the metal substrate prior to placing the coating on the metal
substrate.
28. The process according to claim 26 wherein the pre-treating
comprises pre-cleaning the metal substrate prior to placing the
coating on the metal substrate.
29. The process according to claim 26 wherein the pre-treating
comprises deoxidizing the metal substrate prior to placing the
coating on the metal substrate.
30. A process for coating a metal substrate according to claim 26,
wherein the coating composition further comprises an oxidizing
agent and the first and second salts in combination are present in
the composition in at least about 0.04 weight percent.
31. A method of coating a metal substrate comprising: a) providing
a metal substrate having a surface; b) contacting the metal
substrate with a composition, the composition comprising an
oxidizing agent and first and second salts, each salt comprising an
anion and a cation, the anion of the first and second salts being
different, and the cation of the first and second salts being the
same or different, wherein each cation, individually, is a rare
earth element, and the first and second salts, in combination, are
present in the composition in at least about 0.04 weight percent;
and c) oxidizing at least one rare earth element cation, thereby
resulting in the formation of a rare earth element oxide coating, a
rare earth element hydroxide coating, or mixture thereof, at or
near the surface of the metal substrate.
Description
BACKGROUND
[0001] The oxidation and degradation of metals used in aerospace,
commercial, and private industries are a serious and costly
problem. To prevent the oxidation and degradation of the metals
used in these applications, an inorganic protective coating can be
applied to the metal surface. This inorganic protective coating,
also referred to as a conversion coating, may be the only coating
applied to the metal, or the coating can be an intermediate coating
to which subsequent coatings are applied.
[0002] Chromate based coatings are currently used as inorganic
conversion coatings because they provide corrosion resistant
properties and adhesion for application of subsequent coatings.
However, due to environmental concerns over chromium based
compounds in the environment, there is a need for an
environmentally safer replacement for chromate based conversion
coatings. There is also a need for environmentally safer conversion
coatings that can provide corrosion resistance to an underlying
metal surface and adhesion to subsequently applied coatings.
[0003] Cerium and other rare earth element containing coatings have
been identified as potential replacements for chromate based
coatings in metal finishing. These coatings include cerium and
other rare earth element containing coatings that are formed by
various processes such as immersion, electroplating from a cerium
nitrate solution, plating from an acidic cerium chloride containing
solution and an oxidant (at elevated temperatures), as well as
multi-step processes, and electrolytic and non-electrolytic
processes having a sealing step. Further information on such
coatings can be found in: Hinton, B. R. W., et al., Materials
Forum, Vol. 9, No. 3, pp. 162-173, 1986; Hinton, B. R. W., et al.,
ATB Metallurgie, Vol XXXVII, No. 2, 1997; U.S. Pat. Nos. 5,582,654;
5,932,083; 6,022,425; 6,206,982; 6,068,711; 6,406,562; and
6,503,565; U.S. Patent Application Publication No. US 2004/0028820
A1; and PCT Application Publication No. WO 88/06639.
[0004] However, at least some of the coatings prepared using these
compositions and methods do not perform as well as those formed
using chromate treatments and/or can develop blisters on the
surface and exhibit poor adhesion. Further, at least some of the
prior art cerium and other rare earth element containing coatings
can also suffer from one or more of the following disadvantages:
(1) a tendency of the rare earth element to precipitate in solution
away from the metal surface in the form of a sludge-like material;
(2) difficulty in obtaining a uniform coating which does not tend
to over-coat and exhibit poor adhesion to the substrate; (3) the
necessity to use multiple steps and extensive periods of time to
deposit a coating; (4) the use of commercially unattractive steps,
such as sealing and/or the use of elevated temperature solutions;
and (5) the necessity to use specific pretreatments and solution
compositions in order to coat multiply alloys, especially aluminum
2024 alloys.
[0005] The ability to deposit a cerium-based conversion coating
composition on the surface of a high copper-containing aluminum
alloy, such as aluminum 2024, which is thick enough to provide
corrosion protection can be problematic. Known coating compositions
often exhibit poor adhesion or require the use of multiple steps
and/or elevated temperature solutions to deposit the coating
composition on the alloy. Specific deoxidizers have been used to
more uniformly coat the metal substrate. However, when industrially
accepted cleaners and deoxidizers are used on the alloy, the
surface of relatively high copper-containing aluminum alloys has a
tendency to pit and corrode as the cerium-based coating composition
is deposited on the alloy. The rate of the undesired pitting can be
more extensive than the ability of the cerium to deposit onto the
alloy, resulting in visual pits across the alloy surface.
[0006] Therefore, there is a need for a conversion coating that can
replace chromate based conversion coatings and that overcomes
several of the deficiencies, disadvantages and undesired parameters
of known replacements for chromate based conversion coatings.
SUMMARY
[0007] According to the present invention, a conversion coating
composition for coating a metal substrate comprising an aqueous
carrier and first and second salts, each salt comprising an anion
and a cation, the anion of the first and second salts being
different, and the cation of the first and second salts being the
same or different, wherein each cation, individually, is a rare
earth element. The first and second salts in combination are
present in the composition in an amount effective to form a
corrosion resistant coating on the metal substrate. The conversion
coating can additionally have an oxidizing agent such as hydrogen
peroxide, and/or a self-cleaning additive, such as a surfactant
and/or a detergent. Preferably, but not required, the coating
compositions have an oxidizing agent and the first and second salts
in combination are present in the composition in at least about
0.04 weight percent, and the first and second salts comprise at
least two different anions of the same metal cation, such as a
halide and a nitrate of a rare earth element cation and an
oxidizing agent.
[0008] In another embodiment, the invention is a conversion coating
composition for coating a metal substrate comprising a plating bath
having the dissolution products from one or more rare earth element
oxides and an acid generating compound. According to this
embodiment, the acid generating compound can be one or more of
gypsum, anhydrite, celestite, and barite, in hydrous and anhydrous
forms, in naturally occurring mineral forms, and as precipitated
salts. The plating bath composition can additionally have an
oxidant and/or other additives.
[0009] In another embodiment, the invention is a metal substrate
coating comprising a conversion coating prepared from a coating
composition and a primer coat, such as a chromate based coating
composition, or a chromate-free, rare earth element based coating
composition. The metal substrate coating can additionally have a
topcoat layer, such as an advanced performance topcoat, or the
metal substrate can have a self-priming top-coat or an enhanced
self-priming topcoat.
[0010] In another embodiment, the invention is a process for
coating a metal substrate comprising providing a metal substrate
and coating the metal substrate with a coating composition. The
metal substrate can be pre-treated prior to placing the conversion
coating composition on the metal substrate, such as by pre-cleaning
the metal substrate prior to placing the conversion coating
composition on the metal substrate, and/or by deoxidizing the metal
substrate prior to placing the conversion coating composition on
the metal substrate. Preferably, the coating composition also
includes an oxidizing agent and the first and second salts, in
combination, are present in the composition in at least about 0.04
weight percent. In another embodiment, the invention is a method of
coating a metal substrate comprising providing a metal substrate
having a surface and coating the metal substrate with a coating
composition. Then, at least one rare earth element cation is
oxidized to form a rare earth element oxide or a rare earth element
hydroxide coating at or near the surface of the metal
substrate.
DESCRIPTION
[0011] According to one embodiment of the present invention, there
is provided a conversion coating composition for coating a metal
surface, also referred to as a metal substrate. The composition
comprises an aqueous carrier and first and second salts, each salt
comprising an anion and a cation, the anion of the first and second
salts being different, and the cation of the first and second salts
being the same or different, wherein each cation, individually, is
a rare earth element.
[0012] The conversion coating composition minimizes or overcomes
problems of known conversion coating compositions, such as
excessive over-coating, which can lead to poor adhesion, the need
for sealing the coatings, undesired excessive corrosion of the
substrate during coating deposition, and the need for multiple step
processing. The conversion coating compositions also exhibit good
adhesion to metal substrates, minimize the tendency to over-coat,
can be used to treat multiple aluminum alloys of low to relatively
high copper content without the use of multiple steps or specific
deoxidizers, and can be used as part of a complete chromate-free
coating system. Another advantage of the conversion coating
composition is the ability of the coating composition to be used in
conjunction with a paint system, such as with a primer and topcoat
that provides corrosion resistance comparable to known chromate
containing systems.
[0013] As used herein, the following terms have the following
meanings.
[0014] The term "salt" means an ionically bonded inorganic compound
and/or the ionized anion and cation of one or more inorganic
compounds in solution.
[0015] The term "substrate" means a material having a surface. In
reference to applying a conversion coating, the term "substrate"
refers to a metal substrate such as aluminum, iron, copper, zinc,
nickel, magnesium, and alloys thereof.
[0016] The term "conversion coating", also referred to as a
"conversion treatment" or "pretreatment" means a treatment for a
metal substrate that causes the metal surface to be converted to a
different material. The meaning of the terms "conversion treatment"
and "conversion coating" also include a treatment for a metal
surface where a metal substrate is contacted with an aqueous
solution having a metal that is a different element than the metal
contained in the substrate. An aqueous solution having a metal
element in contact with a metal substrate of a different element,
where the substrate dissolves, leading to precipitation of a
coating (optionally using an external driving force to deposit the
coating on the metal substrate), is also within the meaning of the
terms "conversion coating" and "conversion treatment".
[0017] The term "rare earth element" means an element in Group IIIB
of the periodic table of the elements, that is, elements 57-71 and
Yttrium.
[0018] As used in this disclosure, the term "comprise" and
variations of the term, such as "comprising" and "comprises," are
not intended to exclude other additives, components, integers
ingredients or steps.
[0019] All amounts disclosed herein are given in weight percent of
the total weight of the composition at 25.degree. C. and one
atmosphere pressure, unless otherwise indicated.
[0020] In one embodiment, the present invention is a composition
for coating a metal substrate. The composition comprises an aqueous
carrier and first and second salts, each salt comprising an anion
and a cation, the anion of the first and second salts being
different, and the cation of the first and second salts being the
same or different, wherein each cation, individually, is a rare
earth element, and the first and second salts in combination are
present in the composition in an amount effective to form a
corrosion resistant coating on the metal substrate.
[0021] According to the present invention, the first and second
salts are rare earth element salts, such as praseodymium, cerium,
neodymium, samarium, and terbium salts. It has been found that
conversion coating compositions having rare earth elements salts,
where the salts are mixtures of multiple anions of one or more rare
earth element cations, incorporated into the same coating solution,
are significantly influential on the deposition parameters of the
resulting rare earth element coating, the resulting coating's
morphology, and the resulting coating's performance. Further, these
compositions are capable of affecting the deposition environment of
the rare earth element metal cations as they precipitate onto the
metal substrate, that is, the compositions are capable of reaction
at or near the surface of the metal substrate (during local changes
in pH at the surface of a metal or metal alloy, such as an aluminum
7075 or 2024 alloy), to form a coating on the substrate. The
specific nature of the induced environment created by the coating
composition and coating methods allow for formation of a more
uniform coating. The formation of pits that can be formed during
the coating process the tendency of the rare earth deposits to
overcoat, which can result in poor adhesion are minimized with the
coating compositions according to the present invention. In
addition, the specific rare earth element metal cations used in the
coating compositions may, under certain conditions, deposit onto a
metal substrate without undergoing a change in oxidation state
during deposition, allowing for regions of the metal substrate that
may not normally be coated to have a rare earth element deposit
form.
[0022] It is preferable, but not required that at least one rare
earth element salt is a Cerium(III) salt, present in the conversion
coating composition in an amount from about 0.04 to about 70 weight
percent. More preferably, in combination, the rare earth element
salts are present in the composition in at least about 1.5 weight
percent, and most preferably, the rare earth element salts are
present in the composition in about 8 weight percent. In one
embodiment, the first rare earth element salt is Cerium halide
present in the conversion coating composition, initially in a
trivalent oxidation state, in an amount from about 0.01 to about 24
weight percent, and the second rare earth element salt is Cerium
nitrate present in the conversion coating composition, initially in
a trivalent oxidation state, in an amount from about 0.03 to about
60 weight percent. However, use of such salts other than cerium can
be used according to the present invention and can improve the
stability of the plating solutions at broader pH ranges in the
presence of an oxidizer, as will be understood by those of skill in
the art with reference to this disclosure.
[0023] According to the present invention, optionally, the
conversion coating composition can additionally contain an
oxidizing agent, such as peroxides, persulfates, perchlorates,
sparged oxygen, bromates, peroxi-benzoates, and ozone. It is
preferable, but not required that the oxidizing agent is hydrogen
peroxide, present in the conversion coating composition in an
amount from about 0.1 to about 15 weight percent of a 30 weight
percent solution. However, the conversion coating composition of
the present invention can comprise other oxidizing agents, as will
be understood by those of skill in the art with reference to this
disclosure.
[0024] The pH of the bulk conversion coating composition may vary
depending upon the rare earth components used, as well as the
nature of the desired properties of the final coating. An increase
in pH near or on the metal surface facilitates precipitation of the
rare earth species. An increase in local pH in the vicinity of the
metal substrate may be generated in several ways, such as
generation of local cathodes across surface, pre-seeding metal
surface with a hydroxyl species, and the like. The bulk solution pH
of a typical conversion coating composition has a pH range of
between about 1.5 and about 8. A preferred pH range is between
about 1.5 and about 5. In certain embodiments, however, the pH
range is between about 1.5 and about 12. Examples of embodiments
where the pH range is about 12 includes a conversion coating using
or combining a pre-cleaning process, such as a basic pre-treatment
cleaner, or the incorporation of the cleaning and conversion
coating in a single step.
[0025] According to the present invention, optionally, the
conversion coating composition can additionally contain an additive
to provide desired aesthetic or functional effects. An additive, if
used, can constitute from about 0.01 weight percent up to about 80
weight percent of the total weight of the conversion coating
composition. These optional additives are chosen as a function of
the conversion coating system and application. Suitable additives
can include a solid or liquid component admixed with the conversion
coating composition for the purpose of affecting one or more
properties of the composition. Examples of additives include a
surfactant, which can assist in wetting the metal substrate, and
other additives that can assist in the development of a particular
surface property, such as a rough or smooth surface. Other examples
of suitable additives include flow control agents, thixotropic
agents such as bentonite clay, gelatins, cellulose, anti-gassing
agents, degreasing agents, anti-foaming agents, organic
co-solvents, catalysts, dyes, amino acids, urea based compounds,
complexing agents, valence stabilizers, and the like, as well as
other customary auxiliaries. However, other suitable additives are
known in the art of formulated surface coatings and can be used in
the conversion coating compositions according to the present
invention, as will be understood by those of skill in the art with
reference to this disclosure.
[0026] Preferably, the conversion coating composition is an aqueous
coating composition. In one embodiment, the conversion coating
composition comprises an aqueous carrier, which optionally contains
one or more organic solvents. Suitable solvents include propylene
glycol, ethylene glycol, glycerol, low molecular weight alcohols,
and the like.
[0027] In a preferred, but not required embodiment, the conversion
coating composition additionally comprises a media, which is a
surfactant, mixture of surfactants, or detergent-type aqueous
solution, present in the conversion coating solution in an amount
from about 0.02 weight percent.
[0028] In one embodiment, the conversion coating composition having
a surfactant, mixture of surfactants, or detergent-type aqueous
solution is utilized to combine a metal substrate cleaning step and
a conversion coating step in one process. In another embodiment,
the conversion coating composition having a surfactant, mixture of
surfactants, or detergent-type aqueous solution can additionally
contain an oxidizing agent, as previously described herein.
[0029] In another embodiment, a conversion coating is applied as a
dissolution product obtained from a slurry of rare earth oxides in
the presence of a mineral, an oxidizing agent, and an optional
additive, to conversion coat a metal surface. According to this
embodiment, a rare earth oxide, such as cerium (III) hydrated
oxide, cerium (IV) oxide, praseodymium (III) oxide, praseodymium
(IV) oxide, praseodymium (III,IV) oxide, samarium (III) oxide,
neodymium (III) oxide, terbium (III) oxide, terbium (IV) oxide,
terbium (III,IV) oxide, lanthanum (III) oxide, ytterbium (III)
oxide, yttrium (III) oxide, and/or mixtures thereof, are combined
with an acid generating compound, such as gypsum, anhydrite,
celestite, barite, and the like, in either hydrous or anhydrous
forms, and/or as their naturally occurring minerals, and/or as
precipitated salts, an oxidizing agent, and an optional
additive.
[0030] The rare earth element conversion coating compositions may
be prepared in several ways, varying the order and nature of
component addition. In general, the rare earth element conversion
coating solutions are prepared by first dissolving the appropriate
amount of rare earth halide in the appropriate amount of
distilled/de-ionized water. The other rare earth salt or salts are
then dissolved into the rare earth halide solution. The pH of the
bulk solution may be lowered to about 1.5 using an appropriate
acid, such as but not limited to nitric, sulfuric, hydrochloric, or
increased to a pH of about 12 using an appropriate base, such as
but not limited to sodium hydroxide, surfactants, detergents,
soaps, and the like. Additives and/or solvents, if any, are then
incorporated into the rare earth-containing solution in the
appropriate manner. The appropriate amount of hydrogen peroxide is
then added about five minutes to the solution prior to use.
[0031] According to another embodiment, the present invention is a
process for coating a metal substrate. According to this
embodiment, a metal substrate is provided. Then, the metal
substrate is contacted with a conversion coating composition
according to the present invention.
[0032] According to one embodiment, the metal substrate is
pre-treated prior to contacting the metal substrate with the
conversion coating. The term pre-treating refers to a surface
modification of the substrate that enhances the substrate for
subsequent processing. Such surface modification can include one or
more operations, including, but not limited to cleaning (to remove
impurities and/or dirt from the surface), deoxidizing, and/or
application of one or more solutions or coatings, as is known in
the art. Pretreatment has many benefits, such as generation of a
more uniform starting metal surface, improved adhesion of a
subsequent coating to the pretreated substrate, or modification of
the starting surface in such a way as to facilitate the deposition
of the subsequent conversion coating.
[0033] According to a preferred process, the metal substrate is
prepared by first solvent rinsing the substrate to assist in
removal of inks and oils that may be on the metal surface. The
metal substrate is then degreased and/or deoxidized.
[0034] In one embodiment, the metal substrate is pre-treated by
mechanically deoxidizing the metal prior to placing the conversion
coating composition on the metal substrate. An example of a typical
mechanical deoxidizer is uniform roughening of the surface using a
Scotch-Brite pad.
[0035] In another embodiment, the metal substrate is pre-treated by
cleaning with an alkaline cleaner prior to application of the
conversion coating composition. A preferred pre-cleaner is a basic
(alkaline) pretreatment cleaner, where the surface of the metal
substrate is treated with a sodium hydroxide based cleaner, which
can also have one or more corrosion inhibitors to "seed" the
surface of the metal substrate during the cleaning process with the
corrosion inhibitor to minimize metal surface attack, and/or
facilitate subsequent conversion coating. Suitable pre-cleaners
include degreasing in an alkaline cleaner, such as Turco 4215-NCLT,
available from Telford Industries, Kewdale, Western Australia.
[0036] In another embodiment, the metal substrate is deoxidized to
further remove contaminants on the metal's surface, as well as to
remove the native oxide layer, thus allowing for a more uniform
surface deposit a coating onto the metal surface. Suitable
deoxidizers include industrially acceptable deoxiders, such as
Amchem 7/17 deoxidizers, available from Henkel Technologies,
Madison Heights, Mich. A preferred deoxidizing agent is a
phosphoric acid-based deoxidizer, such as Deft Inc., product code
number 88X2, available from Deft Inc., Irvine, Calif.
[0037] Additional optional steps for preparing the metal substrate
include the use of a surface brightener, such as an acid pickle or
light acid etch, a smut remover, as well as immersion in an
alkaline solution per one of the embodiments of this disclosure.
The metal substrate is typically rinsed with either tap water, or
distilled/de-ionized water between processing steps, and is rinsed
well with distilled/de-ionized water prior to contact with the
conversion coating composition.
[0038] Once the metal substrate has been appropriately pretreated,
cleaned and/or deoxidized, the conversion coating composition is
then allowed to come in contact with at least a portion of the
metal's surface. The metal substrate is contacted with the
conversion coating composition using any conventional technique,
such as dip immersion, spraying, or spread using a brush, roller,
or the like, and so forth. With regard to application via spraying,
conventional (automatic or manual) spray techniques and equipment
used for air spraying and electrostatic spraying can be used. In
other embodiments, the coating can be an electrolytic-coating
system or the coating can be applied in paste or gel form. The
conversion coating compositions may be applied in any suitable
thickness, depending on the application requirements. In a
preferred but not required embodiment, the final coating thickness
is between about 100 to about 600 nm. During application, the
conversion coating composition is maintained at a temperature
between about 10 degrees C. and the boiling temperature of the
composition, which varies depending upon the nature of the
composition. A preferred temperature range is between about 25
degrees C. and about 45 degrees C., and more preferably, about 25
degrees C.
[0039] When the metal substrate is coated by immersion, the
immersion times may vary from a few seconds to multiple hours based
upon the nature and thickness of conversion coating desired. When
the metal substrate is coated using a spray application, the
conversion coating solution is brought into contact with at least a
portion of the substrate using conventional spray application
methods. The dwell time in which the conversion coating solution
remains in contact with the metal substrate may vary based upon the
nature and thickness of conversion coating desired. Typical dwell
times range from a few seconds to multiple hours. When the metal
substrate is treated using a gel application, the conversion
coating gel is brought into contact with at least a portion of the
metal substrate using either conventional spray application methods
or manual swabbing. The dwell time in which the conversion coating
gel remains in contact with the metal substrate may vary based upon
the nature and thickness of conversion coating desired. Typical
dwell times range from a few seconds to multiple hours. The
conversion coating may also be applied using other techniques known
in the art, such as application via swabbing, where an appropriate
media, such as cloth, is used to soak up the conversion coating
solution and bring it into contact with at least a portion of a
metal substrate's surface. Again, the dwell time in which the
conversion coating solution remains in contact with the metal
substrate may vary based upon the nature and thickness of
conversion coating desired. Typical dwell times range from a few
seconds to multiple hours. If an externally driven electrolytic
application process is desired, such as electroplating, care should
be given to the concentration level of halides present in the
conversion coating plating bath, such as to not generate harmful
species, such as chlorine gas, or other harmful by-products.
[0040] According to another embodiment, the present invention is a
metal substrate coating system containing a conversion coating
composition and a primer coat. The conversion coating compositions
according to the present invention are compatible with currently
used chromate-based primers and advanced performance topcoats. The
primer coat can be a conventional chromate based primer coat, such
as the Deft Inc. primer coat, product code 44GN072, available from
Deft Inc., Irvine, Calif. Alternately, the primer coat can be a
chromate-free primer coat, such as the coating compositions
described in U.S. patent application Ser. No. 10/758,973, titled
"CORROSION RESISTANT COATINGS CONTAINING CARBON", and U.S. patent
application Ser. Nos. 10/758,972, and 10/758,972, both titled
"CORROSION RESISTANT COATINGS", all of which are incorporated
herein by reference, and other chrome-free primers that are known
in the art, and which can pass the military requirement of
MIL-PRF-85582 Class N or MIL-PRF-23377 Class N may also be used
with the current invention. Preferred primer coats are available
from Deft Inc., Irvine, Calif., product code numbers Deft 02GN083
or Deft 02GN084.
[0041] The metal substrate coating system can additionally contain
a topcoat. The term "topcoat" refers to a mixture of binder(s),
which can be an organic or inorganic based polymer or a blend of
polymers, typically at least one pigment, can optionally contain at
least one solvent or mixture of solvents, and can optionally
contain at least one curing agent. A topcoat is typically the
coating layer in a single or multi-layer coating system whose outer
surface is exposed to the atmosphere or environment, and its inner
surface is in contact with another coating layer or polymeric
substrate. Examples of suitable topcoats include those conforming
to MIL-PRF-85285D, such as Deft Inc. product code numbers Deft
03W127A and Deft 03GY292, available from Deft Inc., Irvine, Calif.
A preferred topcoats is an advanced performance topcoat, such as
Deft Inc. product code numbers Defthane.RTM. ELT.TM. 99GY001 and
99WO09, available from Deft Inc., Irvine, Calif. However, other
topcoats and advanced performance topcoats can be used in the
coating system according to the present invention as will be
understood by those of skill in the art with reference to this
disclosure.
[0042] In an alternate embodiment, the present invention is a metal
substrate coating system containing a conversion coating according
to the present invention and a self-priming topcoat, or an enhanced
self-priming topcoat. The term "self-priming topcoat", also
referred to as a "direct to substrate" or "direct to metal"
coating, refers to a mixture of a binder(s), which can be an
organic or inorganic based polymer or blend of polymers, typically
at least one pigment, can optionally contain at least one solvent
or mixture of solvents, and can optionally contain at least one
curing agent. The term "enhanced self-priming topcoat", also
referred to as an "enhanced direct to substrate coating" refers to
a mixture of functionalized fluorinated binders, such as a
fluoroethylene-alkyl vinyl ether in whole or in part with other
binder(s), which can be an organic or inorganic based polymer or
blend of polymers, typically at least one pigment, can optionally
contain at least one solvent or mixture of solvents, and can
optionally contain at least one curing agent. Examples of
self-priming topcoats include those that conform to TT-P-2756A.
Preferred self-priming topcoats are Deft product code numbers
03W169 and 03GY369, available from Deft Inc., Irvine, Calif.
Examples of enhanced self-priming topcoats include Defthane.RTM.
ELT.TM./ESPT, available from Deft Inc., Irvine, Calif. An example
of a preferred self-priming topcoat is Deft Inc. product code
number 97GY121, available from Deft Inc., Irvine, Calif. However,
other self-priming topcoats and enhanced self-priming topcoats can
be used in the coating system according to the present invention as
will be understood by those of skill in the art with reference to
this disclosure.
[0043] The self-priming topcoat and enhanced self-priming topcoat
is typically applied directly to the conversion coated substrate.
The self-priming topcoat and enhanced self-priming topcoat can
optionally be applied to an organic or inorganic polymeric coating,
such as a primer or paint film. The self-priming topcoat layer and
enhanced self-priming topcoat is typically the coating layer in a
single or multi-layer coating system where the outer surface of the
coating is exposed to the atmosphere or environment, and the inner
surface of the coating is typically in contact with the conversion
coated substrate or optional polymer coating or primer.
[0044] The topcoat, self-priming topcoat, and enhanced self-priming
topcoat can be applied to the conversion coated substrate, in
either a wet or "not fully cured" condition that dries or cures
over time, that is, solvent evaporates and/or there is a chemical
reaction. The coatings can dry or cure either naturally or by
accelerated means for example, an ultraviolet light cured system to
form a film or "cured" paint. The coatings can also be applied in a
semi or fully cured state, such as an adhesive.
[0045] In another embodiment, the present invention is a method of
coating a metal substrate. According to this embodiment, the method
comprises providing a metal substrate having a surface. Then, at
least a portion of the metal substrate is contacted with a coating
composition. In a preferred embodiment, the method comprises
coating the metal substrate with a coating composition that
additionally comprises an oxidizing agent, where the first and
second salts in combination are present in the composition in at
least about 0.04 weight percent. Preferably, a conversion coating
composition is placed in contact with at least a portion of the
metal substrate, the coating composition having two different
anions of the same or different rare earth element cation and an
oxidizing agent. Next, at least one rare earth element cation is
oxidized at or in the vicinity of the metal surface, and
precipitate as either a hydroxide, oxide, complexed salt, or
combinations thereof coating onto the metal surface. Other rare
earth element cations present in the composition also can be
oxidized on the metal surface, or may require a greater oxidation
potential and/or precipitation pH. This potentially minimizes the
extent of over coating, minimizes the formation of undesired
sludge, may allow for a more protective coating to form over
various regions, and may not require a change in valence to
precipitate onto the metal surface. The use of multiple anions also
facilitates this process by modifying the environment close to the
metal surface where these reactions take place.
[0046] The invention will be further described by reference to the
following non-limiting examples, which are offered to further
illustrate various embodiments of the present invention. It should
be understood, however, that many variations and modifications be
made while remaining within the scope of the present invention.
EXAMPLE 1
Preparation of Rare Earth Element Conversion Coatings
[0047] The following example demonstrates the general procedures
for preparation of the rare earth element conversion coating
compositions, metal substrate preparation, and application of the
coating compositions to the metal substrate. However, other
formulations and modifications to the following procedures can be
used according to the present invention as will be understood by
those of skill in the art with reference to this disclosure.
Coating Composition Preparation:
[0048] The rare earth element conversion coating composition was
prepared with the amounts of ingredients shown in Table 1 for panel
1A. The coating composition was prepared by first dissolving 90 g
of cerium chloride in 860 g of distilled/de-ionized water.
Ce(NO.sub.3).sub.3 (40 g), was then dissolved into the cerium
chloride solution. Hydrogen peroxide (10 g, 30% solution) was then
added to the solution about five minutes prior application to the
metal substrate.
Metal Substrate (Panel) Preparation:
[0049] The metal substrate, panel 1A, was prepared as follows.
Panel 1A, a bare 2024-T3 aluminum alloy panel, was first solvent
rinsed and then deoxidized for about three to five minutes using
Deft product code 88X2 formulation (Deft, Inc., Irvine, Calif.).
The panel was then rinsed well with de-ionized water prior to
contact with the conversion coating solution.
Application Procedure:
[0050] The conversion coating solution, prepared as described
above, was applied to the metal substrate using a dip/immersion
process. After application of the conversion coating, the coated
substrate was rinsed well with de-ionized water.
TABLE-US-00001 TABLE 1 Solution Compositions Used to Prepare Coated
Panels: Panel Panel Panel Panel Panel Panel Panel Panel Panel Panel
Panel Component 1 2A 2B 2C 3A 3B 4B 7A/7B 8A/8B 9A/9B 10B/11A
Nd(NO.sub.3).sub.3 -- -- -- 5 g -- -- -- -- -- -- --
Ce(NO.sub.3).sub.3 40 g 13 g -- -- 5 g 8 g 5 g 5 g 19 g 96 g
CeCl.sub.3 90 g -- 13 g 8 g 13 g 8 g 16 g 67 g 10 g -- 44 g
H.sub.2O.sub.2 (30%) 10 g 14 g 14 g 14 g 14 g 14 g 15 g 100 g 15 g
27 g 16 g De-Ionized 860 g 973 g 973 g 973 g 973 g 973 g 961 g 828
g 970 g 954 g 844 g Water Total 1000 g 1000 g 1000 g 1000 g 1000 g
1000 g 1000 g 1000 g 1000 g 1000 g 1000 g
EXAMPLE 2
Comparison of Rare Earth Element Based Coating Compositions Applied
Via Immersion
[0051] Three bare 2024-T3 aluminum alloy panels, panels 2A, 2B, and
2C, were prepared using the coating composition preparation
procedure described in Example 1 with the formulations shown in
Table 1.
[0052] The coating compositions were applied by immersion at a
deposition time of about five 20 minutes each.
[0053] As shown in Table 1, panel 2A was coated with a
Ce(NO.sub.3).sub.3 based coating composition, panel 2B was coated
with a CeCl.sub.3 based coating composition, and panel 2C was
coated with a CeCl.sub.3/Nd(NO.sub.3).sub.3 based coating
composition.
[0054] Visual observation of panel 2A showed that little or no
coating was formed on the panel using the rare earth nitrate based
coating composition. Visual observation of panel 2B showed
excessive corrosion and surface pitting with the coating that was
formed using the rare earth chloride based coating composition.
However, visual observation of Panel 2C showed that a dense coating
with no excessive pitting was formed using the
CeCl.sub.3/Nd(NO.sub.3).sub.3 based coating composition, i.e., a
coating composition having a combination of two different anions of
different rare earth cations.
EXAMPLE 3
Comparison of Panels Treated with One Or Two Anions of a Rare Earth
Element Metal Cation Via Spray Application
[0055] Panels 3A and 3B, bare 2024-T3 aluminum alloy panels, were
prepared using the coating composition preparation procedure
described in Example 1 with the formulations shown in Table 1. As
shown in Table 1, panel 3A was coated with a CeCl.sub.3 based
coating composition and panel 3B was coated with a
CeCl.sub.3/Ce(NO.sub.3).sub.3 based coating composition. The
coating compositions were spray applied.
[0056] Visual observation of panel 3A showed that when the panel
was coated with a composition containing one anion of a rare earth
element metal cation, the metal surface was extensively pitted.
However, visual observation of panel 3B showed that when the panel
was coated with a composition containing two anions of a rare earth
element metal cation, not only did the coating not extensively pit
the substrate, but the coating did not excessively over coat the
substrate and exhibited good adhesion.
EXAMPLE 4
Comparison of Untreated And Treated Aluminum Alloy Panels
[0057] Panel 4A, an untreated bare 2024-T3 aluminum alloy panel,
was prepared by solvent rinsing the panel using methyl ethyl ketone
and then deoxidizing the panel for three minutes using Deft 88X2
solution (Deft Inc., Irvine, Calif.). Panel 4A was then rinsed well
using de-ionized water and was not further treated with a coating
composition. Panel 4A was exposed for 24 hours to an ASTM B-117
neutral salt spray test.
[0058] Panel 4B, a bare 2024-T3 aluminum alloy panel, was prepared
using the coating composition preparation procedure described in
Example 1 with the coating composition formulation shown in Table
1. As shown in Table 1, panel 4B was treated with a
CeCl.sub.3/Ce(NO.sub.3).sub.3 based coating composition. Panel 4B
was then exposed for 72 hours to an ASTM B-117 neutral salt spray
test.
[0059] Visual observation of panel 4A, the untreated aluminum
2024-T3 panel, showed that the panel was severely corroded after 24
hours of the salt spray test, as compared to panel 4B. Visual
observation of panel 4B, which was treated with a
CeCl.sub.3/Ce(NO.sub.3).sub.3 based coating composition, showed
marked corrosion resistance after 72 hours of the salt spray
test.
EXAMPLE 5
Comparison of Alloy Panels After Solution Immersion
[0060] Panels 5A and 5B, bare 2024-T3 aluminum alloy panels, were
prepared by first solvent rinsing the panels using methyl ethyl
ketone and then deoxidizing the panels for three minutes using Deft
88X2 solution (Deft Inc., Irvine, Calif.). The panels were then
rinsed well using de-ionized water.
[0061] Panel 5A was then immersed in a 2.5 weight percent sodium
chloride solution for 8 hours.
[0062] Panel 5B was immersed in a solution containing 2.5 weight
percent sodium chloride, and the dissolution products from a slurry
containing 4.5 weight percent praseodymium (III) oxide and 4.5
weight percent gypsum for 8 hours.
[0063] Visual observation of Panel 5A, the aluminum 2024-T3 panel
immersed in the sodium chloride solution for eight hours, showed
excessive pitting and the formation of aluminum corrosion products
on the metal surface. However, visual observation of panel 5B, the
aluminum 2024-T3 specimen immersed in the sodium chloride solution
in conjunction with the dissolution products of a rare earth oxide,
an acid generating mineral, and sodium chloride, showed that the
metal surface was free from extensive pitting after 8 hours of
immersion. Thus, panels 5A and 5B demonstrated that the dissolution
product from rare earth oxides in the presence of an acid
generating compound were able to protect a relatively high
copper-containing aluminum 2024-T3 alloy from corrosion in a
solution of sodium chloride.
EXAMPLE 6
Comparison of Alloy Panels after Immersion in a Rare Earth Element
Oxide Solution Having Additives
[0064] Panels 6A and 6B, aluminum 2024-T3 alloy specimens, were
prepared as follows. Panel 6A was prepared by immersing the panel
in a solution containing the dissolution products of a slurry
containing 4.5 weight percent praseodymium (III) oxide, 4.5 weight
percent gypsum, and 9.5 weight percent hydrogen peroxide (30 weight
percent solution) for 20 minutes. Panel 6B was prepared by
immersing the panel in a solution containing the dissolution
products of a slurry containing 4.5 weight percent praseodymium
(III) oxide, 4.5 weight percent gypsum, and 9.5 weight percent
hydrogen peroxide (30 weight percent solution), and 10 weight
percent Joy.TM., liquid washing solution, for 20 minutes.
[0065] Visual observation of panel 6A showed that rare earth
element containing conversion coatings were obtained over an
aluminum 2024-T3 panel after being immersed in the solution
containing the dissolution products from rare earth oxides in the
presence of an acid generating mineral and an oxidant, with no
additives present in the bath. Visual observation of panel 6B
showed that rare earth element containing conversion coatings were
obtained over an aluminum 2024-T3 panel after being immersed in a
solution containing the dissolution products from rare earth oxides
in the presence of an acid generating mineral, an oxidant, and
additives present in the plating bath. Thus, panels 6A and 6B
demonstrate the ability of the dissolution products from rare earth
oxides in the presence of an acid generating mineral, along with an
oxidant and optional additive, to be able to conversion coat a high
copper-containing aluminum 2024-T3 alloy specimen.
EXAMPLE 7
Pre-Cleaned Panels Treated With Rare Earth Element Based Coating
Compositions
[0066] Panels 7A and 7B, clad and unclad copper-containing 2024-T3
alloy panels, were pre-cleaned and treated with a rare earth
element based conversion coating as follows.
[0067] Both panels, panel 7A (an aluminum 2024-T3 alloy 1230 clad
surface, a relatively low copper containing surface (less than 0.1
wt % copper)) and panel 7B (a bare 2024-T3 not clad surface, which
is relatively high in copper (about 4.0 wt %)) were treated with a
basic pretreatment cleaner having the formulation shown in Table
2.
TABLE-US-00002 TABLE 2 Basic Pretreatment Cleaner: Amount Component
(Premix I): Sodium Hydroxide 0.4 g De-Ionized Water 993.0 g Total
993.4 g Component (Premix II): PrCl.sub.3 0.2 g CeCl.sub.3 0.4 g
Distilled/De-Ionized Water 6.0 g Total 6.6 g (Premix I and Premix
II) Total: 1000 g
[0068] The cleaner was prepared by mixing Premix I and II together
prior to use. Panels 7A and 7B were pre-treated with the alkaline
cleaner prior to the application of the conversion coating
composition. The panels were not rinsed after being cleaned. Panels
7A and 7B were then treated with the rare earth element based
conversion coating spray prepared according to the general
procedure described in Example 1, using the formulations shown in
Table 1.
[0069] Visual observation of panels 7A and 7B showed that
pre-cleaning the panels prior to treatment with the rare earth
element conversion coating improved the deposition rate of the
coating composition. Visual observation of panels 7A and 7B also
showed uniform application of the conversion coating to both the
relatively low and high copper-containing alloy panel surfaces,
panels 7A and 7B respectively, that were immersed in the basic
pretreatment cleaner prior to application of the conversion
coating.
EXAMPLE 8
Rare Earth Element Based Coating Compositions Applied Over
Deoxidized Panels
[0070] Panels 8A and 8B were deoxidized and treated with a rare
earth element based coating composition as follows.
[0071] Panel 8A, a bare aluminum 2024-T3 alloy panel, was
chemically deoxidized by immersion in Deft 88X2 solution (Deft
Inc., Irvine, Calif.) for about three to five minutes. Panel 8B, a
bare aluminum 2024-T3 alloy panel, was mechanically deoxidized by
wet scuffing the metal surface using a 7447 Scotch Brite.TM. pad.
Both panels were then rinsed with de-ionized water and immersed in
the rare earth element based conversion coating, prepared according
to the procedure described in Example 1, using the formulation
shown in Table 1, for about five minutes.
[0072] Visual observation of panel 8A showed that the conversion
coating coated the chemically deoxidized metal surface. Visual
observation of panel 8B showed that the conversion coating
composition coated the mechanically deoxidized metal surface. Thus,
panels 8A and 8B demonstrate the compatibility of the conversion
coating compositions with various commercially accepted deoxidizing
methods.
EXAMPLE 9
Comparison of Halide-Free Rare Earth Element Based Coating
Compositions Applied Over Deoxidized Panels and Pre-Cleaned
Panels
[0073] Panels 9A and 9B, a deoxidized bare 2024-T3 aluminum alloy
panel and a pre-cleaned bare 2024-T3 aluminum alloy panel,
respectively, were treated with a halide-free rare earth element
based coating composition as follows.
[0074] Panel 9A, a bare aluminum 2024-T3 alloy panel, was
deoxidized by immersion in Deft 88X2 solution (Deft Inc., Irvine,
Calif.) for about three to five minutes. Panel 9B, a bare aluminum
2024-T3 alloy panel, was pre-cleaned by a basic pretreatment
cleaner, with the formulation shown in Table 2, for about five
minutes. Both panels were then immersed in the rare earth element
based conversion coating, prepared according to Example 1, using
the formula shown in Table 1 for about 120 minutes.
[0075] Visual observation of panel 9A showed that the halide-free
conversion coating composition coated the chemically deoxidized
metal surface. Visual observation of panel 9B showed that the
halide-free conversion coating composition coated the metal surface
that was pre-cleaned using the basic pretreatment cleaner. Thus,
panels 8A and 8B demonstrate that the halide-free conversion
coating compositions are compatible with various commercially
accepted deoxidizing and pre-treatment methods.
EXAMPLE 10
Comparison of Panels Treated with Chromate-Based and Rare Earth
Element Based Coating Compositions
[0076] Panels 10A and 10B, both bare aluminum 2024-T3 alloy panels,
were treated with a chromate-based conversion coating, and a rare
earth element based, chromate-free, conversion coating as
follows.
[0077] Panel 10A was treated with a standard chromate-based
conversion coating prepared per MIL-C-5541 using an Alodine 1200
type chromating process. Panel 10B was treated with the rare earth
element based, chromate-free, conversion coating prepared according
to the procedure described in Example 1, using the formulation
shown in Table 1. The panel was coated with the conversion coating
by swabbing.
[0078] Both panels 10A and 10B were then treated with the
chromate-based primer MIL-P-85582, Deft product code 44GN072 (Deft
Inc., Irvine, Calif.). The chromate-based primer had a dry film
thickness of about one mil, and was allowed to cure for four hours
before application of the topcoat. Defthane.RTM. ELT.TM. 99GY001
(Deft Inc., Irvine, Calif.) was used as the topcoat and had a dry
film thickness of about two mils. The topcoated panels were allowed
to cure for two weeks prior to testing. After curing, both panels
were then exposed to 3000 hours of an ASTM B-117 neutral salt spray
test.
[0079] Visual observation of panels 10A and 10B showed that the
corrosion resistance provided by the chromate-based conversion
coating/chromate-based primer/advanced performance topcoat system
used to coat panel 10A was comparable to that of the chromate-free
conversion coating/chromate-based primer/advanced performance
topcoat system used to coat panel 10B. Thus, panels 10A and 10B
demonstrate that the rare earth element conversion coating
compositions are compatible with various commercially accepted
chromated primer systems.
EXAMPLE 11
Aluminum Alloy Panel Treated with a Complete Chromate-Free Coating
System
[0080] Panel 11A was prepared by treating a bare aluminum 2024-T3
alloy panel with a complete rare earth element based,
chromate-free, coating system as follows. Panel 11A was coated with
a chromate-free conversion coating prepared according to the
procedure described in Example 1, using the formulation shown in
Table 1. The panel was coated by swabbing.
[0081] Panel 11A was then treated with the chromate-free primer,
Deft product code number O.sub.2GN083 (Deft Inc., Irvine, Calif.).
The primer coating had a one mill dry film thickness and allowed to
cure for one week. After curing, the panel was scribed and exposed
to 1000 hours of an ASTM B-117 neutral salt spray test. After
testing, the primer coating was removed from the bottom half of the
panel to allow for visual inspection of the underlying metal
substrate.
[0082] Visual observation of panel 11A showed that the exposed,
scribed region of the metal substrate was protected, allowing for
only minor corrosion to occur in the scribe. This suggests a
throwing power, or passivation ability of the chrome-free coating
system to protect the exposed metal. In the bottom half of panel
11A, where the primer coating was removed, the underlying coated
metal substrate was protected from corrosion by the chrome free
coating system.
[0083] Although the present invention has been discussed in
considerable detail with reference to certain preferred
embodiments, other embodiments are possible. Therefore, the scope
of the appended claims should not be limited to the description of
preferred embodiments contained herein.
* * * * *