U.S. patent application number 10/601247 was filed with the patent office on 2004-12-23 for corrosion resistant, chromate-free conversion coating for magnesium alloys.
Invention is credited to Hammerschmidt, Kenneth, Jaworowski, Mark, Tang, Xia.
Application Number | 20040256030 10/601247 |
Document ID | / |
Family ID | 33418585 |
Filed Date | 2004-12-23 |
United States Patent
Application |
20040256030 |
Kind Code |
A1 |
Tang, Xia ; et al. |
December 23, 2004 |
Corrosion resistant, chromate-free conversion coating for magnesium
alloys
Abstract
A chromate-free, phosphate-fluoride conversion coating, with or
without vanadate, formed on a magnesium or a magnesium alloy
substrate, includes an active corrosion inhibitor selected from the
group consisting of organo-phosphonic acids. The phosphonic acid
group reacts with the magnesium metal of the substrate to form an
insoluble salt.
Inventors: |
Tang, Xia; (W. Hartford,
CT) ; Jaworowski, Mark; (Glastonbury, CT) ;
Hammerschmidt, Kenneth; (St. Hubert, CA) |
Correspondence
Address: |
BACHMAN & LAPOINTE, P.C.
900 CHAPEL STREET
SUITE 1201
NEW HAVEN
CT
06510
US
|
Family ID: |
33418585 |
Appl. No.: |
10/601247 |
Filed: |
June 20, 2003 |
Current U.S.
Class: |
148/250 |
Current CPC
Class: |
C23C 22/36 20130101;
C23C 22/44 20130101 |
Class at
Publication: |
148/250 |
International
Class: |
C23C 022/00 |
Claims
1. A solution for forming a chromate-free, corrosion resistant
coating on a product formed from magnesium or a magnesium alloy,
comprising: the solution having phosphate and fluoride ions; and an
active corrosion inhibitor selected from the group consisting of
organo-phosphonic acids.
2. A solution according to claim 1, wherein the organo-phosphonic
acid is selected from the group consisting of straight chained
amino-alkyl phosphonic acids, branched amino-alkyl phosphonic
acids, straight chained alkyl phosphonic acids, branched alkyl
phosphonic acids, triphosphonic acids, and mixtures thereof.
3. A solution according to claim 2, wherein the triphosphonic acids
comprise nitrilotris (methylene) triphosphonic acid (NTMP).
4. A solution according to claim 1, wherein the solution includes
vanadate.
5. A solution according to claim 1, wherein the solution comprises
1 ppm to 1 wt % of the corrosion inhibitor, preferably 10 ppm to
0.5 wt %.
6. A solution according to claim 5, wherein phosphate ions are
present in an amount of between 1 g/L to 50 g/L, preferably between
10 g/L to 25 g/L, and the fluoride ions are present in an amount of
1 g/L to 10 g/L, preferably 3 g/L to 5 g/L.
7. A process for preparing a corrosion-resistant, chromate free,
coating on magnesium or a magnesium alloy substrate comprises
treating the substrate with the solution of claims 1 through 6,
wherein the phosphonic acid group reacts with magnesium metal
forming an insoluble salt.
8. An article comprising the magnesium or the magnesium alloy
substrate having a corrosion coating prepared in accordance with
the process of claim 7.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a corrosion resistant,
chromate-free, phosphate-fluoride conversion coating, with or
without vanadate, for a product formed from magnesium or a
magnesium alloy and to a coating solution for use in a coating
process.
[0002] Magnesium alloys are light and strong, but very vulnerable
to corrosion due to the reactive nature of magnesium. Magnesium
alloys are protected from corrosion in all practical applications.
A commonly used, low cost, corrosion resistant treatment for
magnesium alloys is a dichromate based conversion coating. While
dichromate based conversion coatings provide good corrosion
protection, they are based on a chemical compound (hexavalent
chromium) that has many occupational exposure risks. A
non-chromate, corrosion resistant magnesium conversion coating is
required to meet industry demands.
[0003] Another treatment for protecting magnesium or magnesium
alloy products is shown in U.S. Pat. No. 5,683,522 to Joesten,
which is hereby incorporated by reference herein. In this
treatment, a paint adherent and corrosion resistant coating of
magnesium phosphate and magnesium fluoride is applied to a product
formed from a magnesium alloy. The process for applying the coating
involves immersing the magnesium alloy product in a solution having
phosphate and fluoride ions. This treatment while providing a
barrier film and very good paint adhesion, does not include
electrochemically active ingredients to suppress corrosion.
Co-pending application Ser. No. 10/073,688, filed Feb. 11, 2002,
and incorporated herein by reference, discloses an improved
phosphate-fluoride corrosion coating for magnesium and process for
applying same.
[0004] It is an object of the present invention to provide an
improved chromate-free corrosion resistant conversion coating for
magnesium and magnesium alloy products.
[0005] It is a further object of the present invention to provide a
coating solution for forming the chromate-free corrosion resistant
coating.
SUMMARY OF THE INVENTION
[0006] The foregoing objects are attained by the present
invention.
[0007] In accordance with the present invention, a chromate-free,
phosphate-fluoride conversion coating, with or without vanadate,
formed on a magnesium or a magnesium alloy substrate, includes an
active corrosion inhibitor selected from the group consisting of
organo-phosphonic acids. The phosphonic acid group reacts with the
magnesium metal of the substrate to form an insoluble salt. The
preferred organo-phosphonic acids used as corrosion inhibitors in
accordance with the present invention are selected from the group
consisting of straight chain or branched amino alkyl phosphonic
acids, straight chain or branched alkyl phoshonic acids, and
triphosphonic acids, particularly, nitrilotris (methylene)
triphosphonic acid (NTMP). When the corrosion inhibitor includes
amino alkyl phosphonic acids, the amine group can interact with
vanadate ions in the coating solution to increase the vanadate
incorporation into the conversion coating.
[0008] Other details of the magnesium-magnesium alloy conversion
coating of the present invention, as well as objects and advantages
attended thereto, are set forth in the following detailed
description.
DETAILED DESCRIPTION
[0009] In accordance with the present invention, a solution for
forming a chromate-free, corrosion resistant coating on a magnesium
or magnesium alloy substrate comprises a solution having phosphate
and fluoride ions and, an active corrosion inhibitor selected from
the group consisting of organo-phosphonic acids.
[0010] The solution of the present invention may optionally include
vanadate anions. The active corrosion inhibitor is selected from
the group consisting of straight chained or branched amino-alkyl
phosphonic acids, straight chained or branched alkyl phosphonic
acids, triphosphonic acids, and mixtures thereof. A particular
useful triphosphonic acid comprises nitrilotris (methylene)
triphosphonic acid (NTMP).
[0011] As noted above, the chromate-free solution include phosphate
and fluoride ions. Phosphate and fluoride ions are present in an
amount of between about 1 g/L to 50 g/L and 1 g/L to 10 g/L,
respectively, preferably between 10 g/L to 25 g/L and 3 g/L to 5
g/L, respectively. It is important in the present invention to
control the pH of the solution and this is achieved by the amount
of phosphate ions and fluoride ions in the solution. The pH of the
solution is preferably in the range of 5 to 7. The particular
phosphate and fluoride compounds employed in forming the solution
having the appropriate pH is disclosed in detail in co-pending
application Ser. No. 10/073,688, filed Feb. 11, 2002, and this
co-pending application and the disclosure therein is incorporated
herein by reference.
[0012] In accordance with the present invention, the corrosion
inhibitor is present in the solution in an amount of between about
1 ppm to 1 wt %, preferably 10 ppm to 0.5 wt %. The corrosion
inhibitor is in the form of an organo-phosphonic acid. Preferred
organo-phosphonic acids include straight chained and branched
amino-alkyl phosphonic acids, straight chained or branched alkyl
phosphonic acids and triphosphonic acids. A particularly useful
triphosphonic acid is nitrilotris (methylene) triphosphonic acid
(NTMP).
[0013] In accordance with the present invention, the solution may
optionally include vanadate anions. Sodium vanadate is a suitable
source of vanadate ions for the solution of the present invention.
In accordance with the present invention, the vanadate ions are
present in an amount of between about 1.0 g/l to 5.0 g/l. It has
been found that the vanadate anions interact with the amine group
of the amino alkyl phosphonic acids increase incorporation of the
vanadate into the conversion coating thereby improving the coating
performance. In addition, the phosphonic acid groups of the
corrosion inhibitor react with the magnesium metal substrate to
form insoluble salts which improve corrosion performance.
[0014] The preferred chromate-free solution comprises phosphate
ions in an amount of between 10 to 25 g/L, fluoride ions in an
amount of between 3 g/L to 5 g/L, vanadate anions in an amount of
between 1 g/L to 5 g/L, and the corrosion inhibitor in an amount of
between 10 ppm to 0.5 wt %.
[0015] A magnesium or magnesium alloy substrate having a conversion
coating in accordance with the present invention comprises mostly
magnesium phosphate and magnesium fluoride. It is believed that the
amino alkyl phosphonic acid will form insoluble magnesium salt as
part of the conversion coating. This component will increase the
interaction of vanadium with conversion coating, therefore increase
vanadium content in the coating, should vanadium is used as
corrosion inhibitor as well.
[0016] It is to be understood that the invention is not limited to
the illustrations described and shown herein, which are deemed to
be merely illustrative of the best modes of carrying out the
invention, and which are susceptible of modification of form, size,
arrangement of parts and details of operation. The invention rather
is intended to encompass all such modifications which are within
its spirit and scope as defined by the claims.
* * * * *