U.S. patent application number 09/802087 was filed with the patent office on 2002-11-21 for composition and method for inhibiting corrosion of aluminum and aluminum alloys using mercapto substituted silanes.
Invention is credited to Crotty, David, Girard, Justin, Nahlawi, Tarek.
Application Number | 20020172776 09/802087 |
Document ID | / |
Family ID | 25182808 |
Filed Date | 2002-11-21 |
United States Patent
Application |
20020172776 |
Kind Code |
A1 |
Crotty, David ; et
al. |
November 21, 2002 |
Composition and method for inhibiting corrosion of aluminum and
aluminum alloys using mercapto substituted silanes
Abstract
Corrosion of metal surfaces, particularly aluminum or aluminum
alloy surfaces, is inhibited by contacting the metal surfaces with
a solution comprising a mercapto-substituted silane and then
preferably baking the metal to cure the silane coating.
Multifunctional organo-mercapto-substit- uted silanes are preferred
in the practice of the process. Preferably, the treatment solution
is an aqueous mixture of the mercapto-substituted silane and
selected solvents.
Inventors: |
Crotty, David; (Pleasant
Ridge, MI) ; Girard, Justin; (South Lyon, MI)
; Nahlawi, Tarek; (Yipsilanti, MI) |
Correspondence
Address: |
John L. Cordani
Carmody & Torrance LLP
P.O. Box 1110
50 Leavenworth Street
Waterbury
CT
06721-1110
US
|
Family ID: |
25182808 |
Appl. No.: |
09/802087 |
Filed: |
March 8, 2001 |
Current U.S.
Class: |
427/387 ;
106/14.11 |
Current CPC
Class: |
C23C 22/68 20130101;
C09D 4/00 20130101; C23C 22/56 20130101; C23C 2222/20 20130101;
C09D 4/00 20130101; C08G 77/28 20130101 |
Class at
Publication: |
427/387 ;
106/14.11 |
International
Class: |
B05D 003/02; C09D
005/08 |
Claims
We claim:
1. A method of treating a metal to improve corrosion resistance,
comprising contacting the metal with a treatment solution, which
treatment solution comprises a mercapto-substituted silane.
2. A method according to claim 1 wherein the mercapto-substituted
silane is a multifunctional mercapto-substituted silane having the
following chemical structure: 3Wherein R.sub.1, R.sub.2 and R.sub.3
are independently selected from the group comprising of alkoxy
groups, alkyl groups and hydrogen, and wherein n is an integer from
0 to 10.
3. A method according to claim 1 wherein the metal is selected from
the group consisting of aluminum and aluminum alloys.
4. A method according to claim 1 wherein the metal is baked, after
contact with the treatment solution, at from 80.degree. C. to
250.degree. C.
5. A method according to claim 1 wherein the metal is subjected to
a process selected from the group consisting of cleaning,
deoxidizing, etching and a combination of the foregoing, prior to
contacting the metal with the treatment solution.
6. A method according to claim 1 wherein the treatment solution
comprises an aqueous solution of a mercapto-substituted silane and
a solvent, effective to solubilize the mercapto-substituted silane
in aqueous solution, and wherein the pH of the treatment solution
is less than 7.
7. A method according to claim 2 wherein the multifunctional
mercapto-substituted silane is selected from the group consisting
of gamma-mercaptopropyltrimethoxy silane, (3-mercaptopropyl)
trimethoxy silane, (3-mercaptopropyl) methyl dimethoxy silane, and
mixtures of the foregoing.
8. A method according to claim 2 wherein the metal is selected from
the group consisting of aluminum and aluminum alloys.
9. A method according to claim 2 wherein the metal is baked, after
contact with the treatment solution, at from 80.degree. C. to
250.degree. C.
10. A method according to claim 2 wherein the metal is subjected to
a process selected from the group consisting of cleaning,
deoxidizing, etching and a combination of the foregoing, prior to
contacting the metal with the treatment solution.
11. A method according to claim 2 wherein the treatment solution
comprises an aqueous solution of a mercapto-substituted silane and
a solvent effective to solubilize the mercapto-substituted silane
in aqueous solution and wherein the pH of the treatment solution is
less than 7.
12. A method according to claim 6 wherein the solvent is selected
from the group consisting of n-methyl-2 pyrrolidone, butyrolactone,
diethylene glycol butyl ether, hexylene glycol, ethylene glycol
monobutyl ether, alcohols, and mixtures of the foregoing.
13. A method according to claim 7 wherein the metal is selected
from the group consisting of aluminum and aluminum alloys.
14. A method according to claim 7 wherein the metal is baked, after
contact with the treatment solution, at from 80.degree. C. to
250.degree. C.
15. A method according to claim 7 wherein the metal is subjected to
a process selected from the group consisting of cleaning,
deoxidizing, etching and combination of the foregoing, prior to
contacting the metal with the treatment solution.
16. A method according to claim 7 wherein the treatment solution
comprises an aqueous solution of a mercapto-substituted silane and
a solvent, effective to solubilize the mercapto-substituted silane
in aqueous solution, and wherein the pH of the treatment solution
is less than 7.
17. A method according to claim 7 wherein the concentration of the
mercapto-substituted silane in the treatment solution is from 0.5
to 10 percent by weight of the treatment solution.
18. A method according to claim 16 wherein the solvent is selected
from the group consisting of n-methyl-2-pyrrolidone, butyrolactone,
diethylene glycol butyl ether, hexylene glycol, ethylene glycol
monobutyl ether, alcohols, and mixtures of the foregoing.
19. A method according to claim 16 wherein the metal is selected
from the group consisting of aluminum and aluminum alloys.
20. A method according to claim 16 wherein the metal is baked,
after contact with the treatment solution, at from 80.degree. C. to
250.degree. C.
21. A method according to claim 16 wherein the concentration of
mercapto-substituted silane in the treatment solution is from 0.5
to 10 percent by weight of the treatment solution.
22. A method according to claim 19 wherein the metal is baked,
after contact with the treatment solution, at from 80.degree. C. to
250.degree. C.
23. A composition useful in improving the corrosion resistance of a
metal, said composition comprising: a) from 0.5 to 10 weight
percent of a mercapto-substituted silane; b) a solvent effective to
solubilize the mercapto-substituted silane in aqueous solution; c)
water. wherein the pH of the composition is less than 7.
24. A composition according to claim 23 wherein the solvent is
selected from the group consisting of a methyl-2-pyrrolidone,
butyrolactone, diethylene glycol butyl ether, hexylene glycol,
ethylene glycol monobutyl ether, alcohols, and mixtures of the
foregoing.
25. A composition according to claim 23 wherein the
mercapto-substituted silane is a multifunctional
mercapto-substituted silane having the following chemical
structure: 4Wherein R.sub.1, R.sub.2 and R.sub.3 are independently
selected from the group comprising of alkoxy groups, alkyl groups
and hydrogen, and wherein n is an integer from 0 to 10.
26. A composition according to claim 25 wherein the multifunctional
mercapto-substituted silane is selected from the group consisting
of gamma-mercaptopropyltrimethoxy silane, (3-mercaptopropyl)
triethoxy silane, (3-mercaptopropyl) methyl dimethoxy silane, and
mixtures of the foregoing.
27. A composition according to claim 25 wherein the pH of the
composition is from 3 to 5.
28. A composition according to claim 27 wherein the multifunctional
mercapto-substituted silane is selected from the group consisting
of gamma-mercaptopropyltrimethoxy silane, (3-mercaptopropyl)
triethoxy silane, (3-mercaptopropyl) methyl dimethoxy silane, and
mixtures of the foregoing.
29. A composition according to claim 27 wherein the solvent is
selected from the group consisting of n-methyl-2-pyrrolidone,
butyrolactone, diethylene glycol butyl ether, hexylene glycol,
ethylene glycol monobutyl ether, alcohols, and mixtures of the
foregoing.
30. A composition according to claim 28 wherein the solvent is
selected from the group consisting of n-methyl-2-pyrrolidone,
butyrolactone, diethylene glycol butyl ether, hexylene glycol,
ethylene glycol monobutyl ether, alcohols, and mixtures of the
foregoing.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a composition and method
for its use in preventing or inhibiting corrosion of metals,
particularly aluminum and aluminum alloys. Treatment of the metal
surfaces with the composition of this invention inhibits corrosion
of that surface and also improves adhesion of subsequent organic
coatings to the treated surface.
[0002] Many metals are susceptible to corrosion. In this regard,
atmospheric corrosion is of particular concern. Such corrosion may
affect the performance and/or appearance of the metals affected,
and the products produced therefrom. In addition, when polymer
coatings such as paints, adhesives or sealants are applied to the
metal, corrosion of the underlying metal may cause a loss of
adhesion between the polymer coating and the base metal. A loss of
adhesion between the polymer coating and the base metal may
similarly lead to corrosion of the metal. Aluminum and aluminum
alloys frequently require corrosion protection and improvements in
adhesion between the base aluminum (or aluminum alloys) and
subsequent polymer coatings. Aluminum alloys, in particular, can be
susceptible to corrosion since the alloying elements used to
improve the metal's mechanical properties may decrease corrosion
resistance.
[0003] Specifications for testing the effectiveness of the
corrosion inhibition and adhesion promotion of various treatments
have been established. Examples of the foregoing include ASTM
standard D3359-87, Military specification MIL-C-5541D and ASTM
standard B117.
[0004] Prior art techniques for improving corrosion resistance of
metals widely employ the use of chromate conversation coatings to
passivate the surface. Such chromate treatments are undesirable,
however, because the chromium used is highly toxic, carcinogenic,
and environmentally undesirable. Phosphate conversion coatings are
also used, but generally provide substantially less corrosion
protection unless used in conjunction with a chromate.
[0005] Recently, various techniques for eliminating the use of
chromates in corrosion inhibition and adhesion promotion treatments
have been proposed. U.S. Pat. No. 5,108,793 discusses treating the
metal with an inorganic silicate followed by treating the silicate
coating with a silane. U.S. Pat. No. 5,292,549 teaches the
treatment of metals with a solution of a silane and a crosslinking
agent in order to form a siloxane film for temporary corrosion
protection.
[0006] U.S. Pat. No. 5,200,275 discusses treating a lead or tin
coated steel surface with a silicate and a metal salt followed by a
further optional treatment with a silane. U.S. Pat. No. 5,759,629
discusses the use of a hydrolyzed vinyl silane in a method for
inhibiting corrosion on metal sheet. U.S. Pat. No. 5,322,713
discusses the treatment of metal sheet with an alkaline aluminate
coating followed by a rinse with a hydrolyzed organofunctional
silane.
[0007] U.S. Pat. No. 5,759,629 discusses treatment of a metal sheet
with a hydrolyzed vinyl silane for corrosion inhibition. U.S. Pat.
No. 5,750,197 discusses treatment of metals with a solution
containing a multifunctional silane having at least two
trisubstituted silyl groups, wherein the substituents are either
alkoxy or acetoxy. An optional second treatment solution containing
an organofunctional silane may also be employed, particularly if
the metal is to be painted.
[0008] Many of the foregoing proposed techniques, however, have
been proven to be ineffective, or to require time consuming, energy
inefficient, multi-step processes. Thus, there remains a need for a
simple, low cost, effective technique for inhibiting corrosion of
metals, particularly for aluminum and aluminum alloys.
[0009] It is an object of this invention to provide an improved
method of inhibiting corrosion of metals, especially aluminum and
aluminum alloys, which is simple to apply, cost effective and
environmentally friendly. It is another object of this invention to
provide a treatment for metals which improves the adhesion of
subsequent organic coatings to the metal while at the same time
improving the corrosion resistance of the metal.
SUMMARY OF THE INVENTION
[0010] The foregoing objectives can be accomplished by treating a
metal, particularly aluminum or aluminum alloys, with a treatment
composition comprising a mercapto-substituted silane. The treatment
composition is applied directly to the metal surface, preferably
without any intervening treatment other than cleaning, deoxidizing
or etching, by spray, flood or other means of direct contact. The
treatment composition is preferably applied at room temperature and
preferably has a pH between 3 and 5.
[0011] Preferably, the metal surface is cleaned, deoxidized and/or
etched prior to treatment with the mercapto-substituted silane. A
variety of known cleaners, deoxidizers and etchants may be employed
for this purpose, with the appropriate choice being made with the
specific metal surface to be prepared in mind.
[0012] Once the mercapto-substituted silane is applied to the metal
surface the treated metal should be baked in order to dry and cure
the coating. After curing, the treatment with the
mercapto-substituted silane may be repeated and baked again, as
desired.
[0013] The compositions and methods of this invention are
particularly suitable for treating aluminum and aluminum alloys.
The inventors have found that treating aluminum or aluminum alloys
with the mercapto-substituted silane containing composition of this
invention provides both increased corrosion resistance and enhanced
adhesion of subsequent organic coatings to the treated surface.
DETAILED DESCRIPTION OF THE INVENTION
[0014] The inventors have found that the corrosion resistance of
metal, particularly aluminum and aluminum alloys, can be greatly
enhanced by applying a treatment solution comprising a
mercapto-substituted silane, preferably followed by a bake dry and
cure. The mercapto-substituted silane of this invention is
preferably a multifunctional mercapto-substituted organic silane of
the following chemical formula: 1
[0015] Wherein R.sub.1, R.sub.2 and R.sub.3 are independently
selected from the group consisting of alkoxy groups (e.g., methoxy,
ethoxy, or similar alkoxy groups), alkyl groups and hydrogen
groups, and wherein n is an integer from 0 to 10, but is preferably
an integer from 1 to 4. A particularly preferred
mercapto-substituted silane is gamma-mercaptopropyltrimethoxy
silane which has the following structure: 2
[0016] The inventors have found gamma-mercaptopropyltrimethoxy
silane to be the most preferred mercapto-substituted silane for use
in this invention. Gamma-mercaptopropyltrimethoxy silane is
available from the Witco Corporation of Greenwich, Conn. under the
tradename SILQUEST.RTM. A-189 Silane. Other useful
mercapto-substituted silanes include (3-mercaptopropyl) methyl
dimethoxy silane available from the Fluka Chemie AG of Buchs,
Switzerland. The inventors have unexpectedly discovered that the
mercapto group on these mercapto-substituted silanes provides
significantly better corrosion protection than is provided by other
silanes which do not have the mercapto substituted group. Thus, the
inventors believe that the mercapto-substitution on the silane
provides significant and unexpected improvements in corrosion
protection and adhesion promotion in relation to
non-mercapto-substituted silanes.
[0017] The mercapto-substituted silane is preferably solubilized,
with a suitable solvent, into a treatment solution. The
concentration of mercapto-substituted silane in the treatment
solution may be from 0.5 to 10% by weight but is preferably from 2
to 4% by weight. Mercapto-substituted silanes, such as
gamma-mercaptopropyltrimethoxy silane, may not be soluble in water
alone to the extent necessary to practice this invention
effectively. The inventors have discovered that a combination of
water with an appropriate solvent are required to effectively
solubilize these mercapto-substituted silanes in a water matrix.
Appropriate solvents for this purpose include
N-methyl-2-pyrrolidone, butyrolactone, diethylene glycol butyl
ether, hexylene glycol, ethylene glycol monobutyl ether and
alcohols. Most preferred as a solubilization matrix for the
mercapto-substituted silanes is a combination of water and ethylene
glycol monobutyl ether. Preferably, the mercapto-substituted silane
is added to the solvent (e.g., ethylene glycol monobutyl ether) and
then that mixture is added to the water, with stirring to form the
treatment solution. The inventors have found that an aqueous
solution of ethylene glycol monobutyl ether does an excellent job
of solubilizing the mercapto-substituted silane and leaves behind a
spot free finish as it evaporates from the treated surface. The
concentration of solvent in the water must be adjusted in order to
properly solubilize and/or maintain the mercapto-substituted silane
in solution. The inventors have found that ratios of water to
ethylene glycol monobutyl ether concentration, based on volume
percentages of from 65/35 to 94/6 were suitable for
mercapto-substituted silane concentrations of from 5 to 0.5 volume
percent. The aqueous solution of the mercapto-substituted silane
will hydrolyze the silane and will improve its effectiveness in
treating the metal surfaces.
[0018] The pH of the treatment solution should be less than 7 and
is preferably between about 3 and 5. The inventors have found that
using an aqueous solution of the foregoing solvents and the
mercapto-substituted silane, the treatment solution will naturally
make up at a pH of about 4. Acetic acid can be used to make any
needed adjustments to pH. The concentration of mercapto-substituted
silane may range from 0.5 to 10 percent by volume, but is
preferably from 2 to 5 percent by volume of the treatment solution.
The concentration of mercapto-substituted silane in the treatment
solution will affect the corrosion protection achieved, with better
corrosion protection achieved at higher concentrations.
[0019] The process of this invention has been found to increase the
corrosion resistance of metals generally, not including zinc and
zinc plated surfaces, but is particularly effective on aluminum and
aluminum alloys. Thus, the inventors have found this process to be
especially useful on aluminum, and aluminum alloys such as 6061-T6
aluminum alloy, 2024-T3 aluminum alloy and 356 aluminum alloys.
[0020] The treatment solution is applied to the metal surfaces by
immersion, spraying, flooding or other similar means of contact.
Preferably, the contact occurs at room temperature and lasts for
from 15 seconds to several minutes.
[0021] The inventors have found that it is important to dry and
bake the treated metal after application of the
mercapto-substituted silane in order to properly cure the silane
coating on the metal surface. Baking may occur at from 60.degree.
C. to 300.degree. C. for from several minutes to several hours,
preferably at from 80.degree. C. to 250.degree. C. for from 20
minutes to 2 hours. However, the inventors have found that baking
at a temperature of about 200.degree. C. for about 1 hour provides
optimum performance. Baking temperatures greater than or less than
about 200.degree. C. have been found to decrease the amount of
corrosion protection achieved with the process.
[0022] The silane treatment solution of this invention can become
unstable over time. As the silane becomes hydrolyzed by the water
it will have a tendency to precipitate and the solution will begin
to take on a white cloudy appearance. It is believed that this
instability may also arise from the polymerization of the silane
molecules in the solution to form 2 or 3 member chains. This
potential instability can be controlled by increasing the
concentration of solvent in the treatment solution before, or at
the first sign of cloudiness. As a result of this potential
instability, it may be useful to prepare a concentrated solution of
the mercapto-substituted silane without water that is storage
stable and is then added to water to prepare the treatment
solution. In this regard, a storage stable concentrate of
mercapto-substituted silane can be prepared by dissolving the
silane in a solvent, such as ethylene glycol monobutyl ether,
without water. For example, a concentrate of 90% by volume ethylene
glycol monobutyl ether and 10% by volume gamma-mercaptopropyltri-
methoxy silane can be prepared and is storage stable. A 20% by
volume solution of this concentrate in water can then used as the
treatment solution.
[0023] The metal is preferably cleaned, deoxidized and/or etched
prior to contacting it with the treatment solution of this
invention. Prior preparation of the metal surface generally
increases the activity of that surface towards subsequent
treatments, in this case, towards the mercapto-substituted silane
treatment. As a result, prior preparation of the surface will
generally yield better corrosion resistance results after treatment
with the process of this invention. However, preferably the surface
is not treated with any non-mercapto-substituted silanes,
silicates, chromates or other conversion coatings prior to
treatment with the mercapto-substituted silane. Preferably, the
metal surfaces are cleaned and etched prior to treatment with the
process of this invention. A variety of cleaners and etchants are
known for preparing a variety of metal surfaces. For cleaning the
surfaces of aluminum and aluminum alloys, both acid and alkali
based cleaners are available. One preferred alkali based cleaner is
ISOPREP 49L available from MacDermid, Incorporated of Waterbury,
Conn. ISOPREP 49L is typically used at 10% v/v concentration with
water at 150.degree. F. for 2-5 minutes. A preferred acid based
cleaner is ISOPREP 160 also available from MacDermid, Incorporated.
ISOPREP 160 is typically used at 10% v/v concentration with water
at 160.degree. F. for 2-5 minutes. These aluminum cleaners are
typically relatively weakly basic or acidic in comparison to the
etchants.
[0024] Aluminum and aluminum alloys can be effectively etched using
strongly acidic or strongly basic solutions. Two preferred etchants
for aluminum and aluminum alloys include ISOPREP 161 and ISOPREP
35, both available from MacDermid, Incorporated. ISOPREP 161 is a
strongly acidic etchant typically used at 10% v/v concentration
with water at 140.degree. F. for 30 seconds to 2 minutes. ISOPREP
35 is a strongly basic etchant typically used at 4 oz/gal
concentration in water at 160.degree. F. for 30 seconds for 2
minutes.
[0025] Although the inventors have found that treatment of the
metal with a cleaner and an etchant prior to treatment with the
mercapto-substituted silane of this invention generally improves
the corrosion resistance provided by the process, the best choice
of cleaners and etchants may depend upon the metal being treated.
For instance, the inventors have found that strongly acid etchants
work better on 6061-T6 aluminum alloy while strongly basic etchants
work better on 2024-T3 aluminum alloy.
[0026] It is preferred that the metal surface be treated with the
mercapto-substituted silane directly after the cleaning,
deoxidizing, and/or etching process, without any intervening
treatment with another type of silane or conversion coating. The
inventors have surprisingly found that other silanes (i.e.,
non-mercapto-substituted silanes) do not provide nearly the same
corrosion protection as the mercapto-substituted silanes of this
invention and if the other silanes are applied prior to the
mercapto-substituted silane, they may interfere with the
mercapto-substituted silane's ability to effectively react with the
surface and provide corrosion protection.
[0027] The corrosion resistance of a treated metal part is
typically measured by exposing the treated metal part to a 5% by
weight sodium chloride solution in a salt spray chamber. Corrosion
resistance is measured as the maximum number of hours the part can
endure before corrosion becomes apparent in a specified manner on
its surface. Military specification MIL-C-5541D provides specific
procedures for measuring the corrosion resistance of a treated
part. ASTM B117 is another widely used specification for
determining the corrosion resistance of a metal treated part with a
salt spray procedure. Similar specifications, such as ASTM
D-3359-87, have been prepared to test the adhesion of organic
coatings to treated metal surfaces. These adhesion procedures
usually include a cross-hatch tape test whereby the organic coating
is cut in a specified cross-cut pattern and tape is applied and
removed to determine the quantity, if any, of the organic coating
that is removed.
[0028] The inventors have found that salt spray corrosion
resistance in excess of 1,000 hours on 6061-T6 aluminum alloy and
in excess of 350 hours on 2024-T3 aluminum alloy can be achieved
with the process of this invention. In addition, excellent adhesion
of organic coatings to the treated metal surfaces can be achieved
with the process of this invention.
[0029] This invention is further described by the following
examples which should be taken as illustrative only and not
limiting in any manner:
EXAMPLE I
[0030] Samples of aluminum alloys 6061-T6 and 2024-T3 were
processed through the following cycle by immersion:
1 Treatment Time (min) Temperature 1. 10% v/v ISOPREP 160.sup.1 5
160.degree. F. 2. Water Rinse 1 room temp. 3. 10% v/v ISOPREP
49L.sup.1 5 150.degree. F. 4. Water Rinse 1 room temp. 5. 50% v/v
Nitric Acid 1 room temp. 6. Water Rinse 1 room temp. 7.
Mercapto-Substituted Silane Treatment 1 room temp. Solution 8. Bake
60 200.degree. C. .sup.1ISOPREP 160 and ISOPREP 49L are proprietary
cleaning solutions for aluminum and aluminum alloys available from
MacDermid, Inc. 245 of Waterbury, CT.
[0031] Each sample was then processed in a salt spray chamber
according to the procedures set forth in ASTM B117 standard in
order to determine corrosion resistance. The composition of the
mercapto-substituted silane treatment solution and the corrosion
resistance results are given below in Table I.
2TABLE I Ethylene Glycol Salt Spray Salt Spray Monobutyl Hours on
Hours on Silane.sup.2 Ether Water 6061-T6 2024-T3 (%/vol.) (%/vol.)
(%/vol.) Aluminum Aluminum 5 30 65 >1000 384 4 25 71 1000 216 2
24 74 360 192 1.5 22 76.5 168 144 1.0 15 84 72 not attempted 0.5 6
93.5 <24 not attempted .sup.2Gamma-Mercapto-propyltrimethoxy
silane
* * * * *