U.S. patent application number 10/106098 was filed with the patent office on 2002-11-28 for chemical conversion reagent for magnesium alloy, surface-treating method, and magnesium alloy substrate.
This patent application is currently assigned to NIPPON PAINT CO., LTD.. Invention is credited to Futsuhara, Masanobu, Miyamoto, Satoshi, Yamasoe, Katsuyoshi, Yasuhara, Kiyotada.
Application Number | 20020174915 10/106098 |
Document ID | / |
Family ID | 18948432 |
Filed Date | 2002-11-28 |
United States Patent
Application |
20020174915 |
Kind Code |
A1 |
Futsuhara, Masanobu ; et
al. |
November 28, 2002 |
Chemical conversion reagent for magnesium alloy, surface-treating
method, and magnesium alloy substrate
Abstract
The objective of the present invention is to impart coating film
adhesion, corrosion resistance and rust prevention to magnesium
alloys. A chemical conversion reagent for a magnesium alloy which
comprises a phosphate ion and a permanganate ion and has pH of 1.5
to 7.
Inventors: |
Futsuhara, Masanobu;
(Yokohama-shi, JP) ; Miyamoto, Satoshi;
(Toyonaka-shi, JP) ; Yamasoe, Katsuyoshi;
(Sakura-shi, JP) ; Yasuhara, Kiyotada;
(Kashiwa-shi, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 Pennsylvania Avenue, NW
Washington
DC
20037-3213
US
|
Assignee: |
NIPPON PAINT CO., LTD.
|
Family ID: |
18948432 |
Appl. No.: |
10/106098 |
Filed: |
March 27, 2002 |
Current U.S.
Class: |
148/254 |
Current CPC
Class: |
C23C 22/68 20130101;
C23G 1/10 20130101; C23C 22/12 20130101; C23C 22/18 20130101; C23G
1/12 20130101 |
Class at
Publication: |
148/254 |
International
Class: |
C23C 022/78 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 28, 2001 |
JP |
2001-094195 |
Claims
1. A chemical conversion reagent for a magnesium alloy which
comprises a phosphate ion and a permanganate ion and has pH of 1.5
to 7.
2. The chemical conversion reagent for a magnesium alloy according
to claim 1 wherein the concentration of a compound serving as a
source of the phosphate ion is 20 to 50 g/L and the concentration
of a compound serving as a source of the permanganate ion is 1 to
10 g/L.
3. A surface-treating method which comprises a step of bringing the
chemical conversion reagent according to claim 1 or 2 into contact
with a magnesium alloy substrate.
4. The surface-treating method according to claim 3 wherein the
magnesium alloy substrate is subjected to degreasing, pickling and
desmutting treatments in advance.
5. The surface-treating method according to claim 4 wherein the
pickling treatment is carried out with a reagent containing at
least one member selected from the group consisting of sulfuric
acid, nitric acid, phosphoric acid and a fluorine-containing
compound.
6. A magnesium alloy substrate obtainable by the surface-treating
method according to any of claims 3 to 5.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a chemical conversion
reagent for magnesium alloys which imparts high corrosion
resistance, rust prevention, and coating film adhesion, a
surface-treating method which comprises using said chemical
conversion reagent, and a magnesium alloy substrate obtained by the
method. The surface-treating method of the invention is suitable
for the surface treatment of magnesium alloys which are used in
applications requiring corrosion resistance, rust prevention,
coating film adhesion, and electrical conductivity, for example
electronic components such as portable telephones and personal
computer housings, and household electrical appliances such as
television receivers. In accordance with the surface-treating
method of the invention, the high oxidizing power of a permanganate
ion is utilized to encourage formation of a magnesium oxide coat
and, at the same time, the reaction of permanganate and magnesium
ions with a phosphate ion is utilized to form a coat comprising a
phosphorus-manganese compound and a phosphorus-magnesium compound,
whereby a chemical conversion film having excellent corrosion
resistance, rust prevention, and coating film adhesion can be
produced on the magnesium alloy.
BACKGROUND OF THE INVENTION
[0002] The common method of chemical conversion treatment for a
magnesium alloy comprises dipping the magnesium alloy in a chemical
conversion reagent containing a hexavalent chromium as described in
JIS-H8651 and MIL-M-3171, for instance. However, hexavalent
chromium exerts harmful effects on the environment and human
physiology. Therefore, the working environment for the surface
treatment is unfavorable and a sophisticated effluent disposal
system is required for preventing discharge of the hexavalent
chromium into the environment. The use of hexavalent chromium has
the disadvantage that, in addition to said harmful effects on the
environment and human physiology, an additional capital expenditure
is required for the surface-treating equipment. Furthermore, it is
likely that its use will be controlled or prohibited by law in many
countries of the world in near future. Therefore, development of a
chemical conversion treatment technology for magnesium supplanting
chromating treatment, the so-called nonchromate chemical conversion
treatment technology, is awaited.
[0003] Regarding such nonchromate chemical conversion treatment
methods, some of them are described in Japanese Kokai Publication
Hei-07-126858. Thus, a treatment method using a magnesium phosphate
treatment as a base and a phosphate treatment method involving
additional use of a metal other than chromium, such as zirconium,
titanium or zinc, are mentioned as the prior art. However, as
pointed out in Japanese Kokai Publication Hei-07-126858, these
surface-treating methods are disadvantageous in that they are not
practical because of a time-consuming treatment step, involve a
long time for the treatment and/or cannot impart sufficient
corrosion resistance, rust prevention or coating film adhesion,
etc.
[0004] As a technology overcoming the above disadvantages, Japanese
Kokai Publication Hei-07-126858 discloses a manganese phosphate
treatment. This manganese phosphate treatment method is
characterized in that not only a bivalent manganese ion (Mn.sup.2+)
and a phosphate ion but also an aliphatic amine, an aromatic amine,
or a heterocyclic amine compound is formulated in a chemical
conversion bath. Furthermore, this solution is optionally
supplemented with one member selected from a nitrate ion, a sulfate
ion and a fluorine-containing compound. It is described that by
this treatment method, a coat having excellent corrosion
resistance, rust prevention and coating film adhesion can be formed
on a magnesium alloy. However, addition of an amine compound is not
an eco-friendly practice and, moreover, the composition of this
treating solution is so much complicated that bath control is
rendered difficult.
[0005] Japanese Kokai Publication Hei-08-35073 discloses a chemical
conversion treatment method employing a permanganate ion. It is
stated that by adding a promoter such as a mineral acid or a
fluoride to an aqueous permanganate solution, a chemical conversion
coat having satisfactory corrosion resistance can be formed.
However, the coat so formed is not a manganese phosphate coat but a
coat composed of manganese oxide and manganese hydroxide, not
containing a phosphate bond, and, therefore, is not fully
satisfactory in coating film adhesion.
SUMMARY OF THE INVENTION
[0006] The prior art mentioned above has several drawbacks, namely
(1) hexavalent chromium exerting harmful effects on human
physiology and the environment is contained, (2) a treatment step
is time-consuming and complicated, (3) a high temperature is
required for the treatment, (4) corrosion resistance, rust
prevention and coating film adhesion cannot be imparted as
satisfactory as for the chromated coat, and/or (5) bath composition
is complicated. The object of the present invention is to overcome
these disadvantages.
[0007] The present invention relates to a chemical conversion
reagent for a magnesium alloy
[0008] which comprises a phosphate ion and a permanganate ion
and
[0009] has pH of 1.5 to 7.
[0010] The concentration of a compound serving as a source of said
phosphate ion is preferably 20 to 50 g/L and the concentration of a
compound serving as a source of said permanganate ion is preferably
1 to 10 g/L.
[0011] The present invention further relates to a surface-treating
method
[0012] which comprises a step of bringing said chemical conversion
reagent into contact with a magnesium alloy substrate.
[0013] Said magnesium alloy substrate is preferably subjected to
degreasing, pickling, and desmutting treatments in advance and,
more preferably, said pickling treatment is carried out with a
reagent containing at least one member selected from the group
consisting of sulfuric acid, nitric acid, phosphoric acid, and a
fluorine-containing compound.
[0014] The present invention is further directed to a magnesium
alloy substrate obtainable by said surface-treating method.
[0015] The present invention is now described in further
detail.
DETAILED DESCRIPTION OF THE INVENTION
[0016] The chemical conversion reagent for a magnesium alloy
according to the invention comprises a phosphate ion and a
permanganate ion. The role of said phosphate ion is that of
imparting corrosion resistance and improving coating film adhesion
through formation of a phosphoric compound. The source of said
phosphate ion includes, for example, phosphoric acid salts such as
primary sodium phosphate, secondary sodium phosphate, primary
ammonium phosphate, secondary ammonium phosphate, primary potassium
phosphate, secondary potassium phosphate, etc. and orthophosphoric
acid. The concentration of the phosphoric acid compound in the
chemical conversion bath may range from 5 g/L to the solubility
limit of the compound to be used but is more preferably 20 g/L to
50 g/L. If it is less than 5 g/L, no sufficient coating film
adhesion will be obtained. If it exceeds 50 g/L, substantially no
further improvement in performance will be obtained but rather will
result in a loss.
[0017] The role of said permanganate ion is that of promoting
formation of an oxide film on the surface of a magnesium alloy and
causing formation of a phosphorus-manganese compound/manganese
oxide film which is excellent in corrosion resistance. The
permanganate ion is supplied from a permanganate salt compound.
Specifically, a permanganate ion can be generated by dissolving
potassium permanganate, sodium permanganate, ammonium permanganate,
or the like in water. The concentration of the permanganate salt
compound in the chemical conversion bath may range from 1 g/L to
the solubility limit of the compound to be used but is suitably 1
g/L to 10 g/L. If it is less than 1 g/L, deposition of the
manganese compound coat will be insufficient so that no adequate
corrosion resistance may be obtained. If the concentration exceeds
10 g/L, no further improvement in performance will be obtained.
[0018] The chemical conversion reagent for a magnesium alloy
according to the invention has pH of 1.5 to 7. In accordance with
the invention, excellent corrosion resistance and coating film
adhesion can be obtained over a broad pH range of 1.5 to 7. The pH
can be mainly controlled by adding an alkaline solution, such as an
aqueous sodium hydroxide solution, or an acidic solution, such as a
solution of orthophosphoric acid, but can also be controlled by
modulating the concentration ratio of primary sodium phosphate and
secondary sodium phosphate. If the pH is below 1.5, dissolution of
the magnesium alloy will be so vigorous as to damage the surface
thereby no sufficient corrosion resistance will be obtained. If the
pH exceeds 7, neither sufficient corrosion resistance nor
sufficient coating film adhesion will be obtained, hence it is not
suitable. This is probably because the deposited amount of the coat
is drastically decreased and the oxidizing power of the
permanganate ion is attenuated.
[0019] The chemical conversion reagent of the present invention can
suitably be applied to magnesium alloys. The surface-treating
method which comprises a step of bringing said chemical conversion
reagent into contact with a magnesium alloy substrate is also
another aspect of the present invention.
[0020] The magnesium alloys mentioned above are mainly magnesium
alloys prepared by a die-casting technique or a thixomolding
technique, and as preferred species, AM50D, AM60D, and AZ91D can be
mentioned. As other metals which is used in the preparation of
magnesium alloys, there can be mentioned, for example, aluminum,
manganese, zinc, silver, and rare earth elements, etc.
[0021] These alloys are sometimes severely contaminated by emulsion
oils, known as mold release agents, which are used in casting
operations, and depending on casting conditions of alloys,
segregation layers may occur on surfaces of alloys. For insuring a
normal chemical conversion treatment in such cases, the surface of
the magnesium alloy needs to be subjected to a suitable
pretreatment. The surface treatment is generally carried out in the
sequence of degreasing, washing with water, pickling, washing with
water, desmutting, washing with water, chemical conversion
treatment, washing with water, and drying.
[0022] The degreasing treatment mentioned above is carried out to
remove oil from the surface. This degreasing step alone is
sufficient when the degree of contamination is low. The degreasing
agent to be used in the above degreasing treatment can be roughly
classified into an alkaline degreasing agent and an acidic
degreasing agent. The degreasing method for use in the practice of
the present invention is not particularly restricted but since
magnesium dissolves vigorously in an acidic aqueous solution,
selective dissolution of the magnesium in the magnesium alloy may
possibly take place to damage the surface. Therefore, it is
preferable to use an alkaline degreasing agent.
[0023] The pickling treatment mentioned above is carried out to
remove the segregation layer composed of fine crystallites which
have segregated on the surface and the mold release agent which has
penetrated into the segregation layer. In accordance with the
method of the invention, it is preferable to use an aqueous
solution of orthophosphoric acid supplemented with a
fluorine-containing compound such as hydrosilicofluoric acid or an
aqueous solution of an inorganic acid such as sulfuric acid or
nitric acid. The concentration range of such an acid is preferably
0.3 to 20 g/L, more preferably about 0.3 to 5 g/L. If it is less
than 0.3 g/L, repeated use will diminish a cleaning potency life,
thus necessitating frequent supplementation or exchange of the
solution. If the concentration exceeds 20 g/L, dissolution of the
magnesium alloy will be so vigorous that the alloy surface will be
damaged and a considerable amount of smut is produced, hence it is
not preferable. The temperature of the pickling bath can be
controlled within the range of room temperature to not higher than
the boiling point of the aqueous acidic solution but for conserving
the working environment and avoiding the surface damage due to
excessive etching or the excessive formation of smut, the range of
room temperature to about 50.degree. C. is preferred. When an
aqueous solution of a carboxyl group-containing organic acid is
used, a sufficient cleaning effect may not be obtained owing to
formation of a compound coat on the surface of the magnesium alloy,
which may possibly cause a problem on coating film adhesion.
Moreover, an aqueous solution of either orthophosphoric acid or
phosphorous acid alone in a concentration of about 1 g/L or higher
may phosphorylate the surface of magnesium with the consequent
failure to express a sufficient cleaning effect or insure the
necessary coating film adhesion.
[0024] The acid to be used is preferably an acid inert to the
magnesium alloy, such as sulfuric acid or nitric acid.
[0025] The desmutting treatment mentioned above is carried out to
remove the foulant (smut) on the surface of magnesium alloy, and
the washing is generally carried out with an aqueous alkaline
solution, such as an aqueous sodium hydroxide solution.
[0026] After each of said degreasing, pickling, and desmutting
treatments, washing with water is carried out in the per se known
manner. The above drying step can also be carried out in the
conventional manner.
[0027] Since the surface-treating method of the present invention
imparts excellent corrosion resistance, rust prevention and coating
film adhesion to magnesium alloy substrates over a broad pH range
of 1.5 to 7, it is expected to find application in a broad range of
fields for example electronic components such as portable
telephones and personal computer housings, and household electrical
appliances such as television receivers. The magnesium alloy
substrate obtainable by this manner also constitutes another aspect
of the present invention.
[0028] The chemical conversion reagent for a magnesium alloy
according to the present invention can impart coating film adhesion
and corrosion resistance to magnesium alloys with high
reproducibility over a broad pH range of pH 1.5 to 7 so that it
facilitates bath control and handling. This is a major industrial
merit.
[0029] The surface-treating method of the present invention can
impart excellent corrosion resistance and coating film adhesion to
magnesium alloys, even to magnesium alloys prepared by a casting
technique and hence contaminated with a mold release agent. The
method, therefore, can be used in a broad range of applications
such as personal computer and portable telephone housings and other
shaped articles, parts having intricate profiles, automotive parts,
and so forth.
EXAMPLES
[0030] The following Examples illustrate the present invention in
further detail. However, these are by no means limitative to the
scope of the invention.
Example 1
[0031] In a sequence of degreasing, washing with water, pickling,
washing with water, desmutting, washing with water, manganese
phosphate treatment, washing with water, and drying, each carried
out under the following conditions, a chemical conversion coat was
formed on a magnesium alloy. Magnesium alloy: AZ91D test pieces
(size: 100 mm.times.50 mm.times.3 mm)
[0032] Degreasing: A 1 weight % aqueous solution of "Surf Fine 100"
(an alkaline degreasing agent: product of Nippon Paint Co.); bath
temperature 50.degree. C., treating time 2 min. Pickling:
Orthophosphoric acid 0.4 g/L, hydrosilicofluoric acid 0.03 g/L, tap
water as balance; bath temperature 50.degree. C., treating time 2
min
[0033] Desmutting: Sodium hydroxide 20 g/L, sodium gluconate 3.1
g/L; bath temperature 60.degree. C., treating time 5 min. Chemical
conversion treatment: KMnO.sub.4 concentration 5.5 g/L, primary
phosphate concentration 45 g/L, orthophosphoric acid concentration
1.8 g/L, tap water as balance; pH 2.8, bath temperature 50.degree.
C., treating time 2 min Drying condition: Drying in an oven at
100.degree. C. for 10 min
[0034] The uncoated corrosion resistance, coated corrosion
resistance, coating film adhesion, and coat appearance were
evaluated and the surface electrical resistivity was measured. The
evaluations of parameters other than coated corrosion resistance
were made on the above magnesium alloy test piece obtained and the
evaluation of coated corrosion resistance was made on the test
piece prepared by further applying a powder coating on each of the
above magnesium alloy test pieces obtained. The results are
presented in Table 1.
[0035] (1) Evaluation of Corrosion Resistance
[0036] The uncoated corrosion resistance (the corrosion resistance
of a magnesium alloy after chemical conversion treatment) was
evaluated by the salt spray test (SST). SST is a test in which the
test piece is sprayed with a 5 weight % aqueous sodium chloride
solution for a predetermined time in a tester controlled at
35.degree. C. and the incidence of corrosion is then evaluated. The
evaluation of the corrosion resistance was made by visual
assessment of uncorroded area after 48 hours of SST exposure.
[0037] (2) Coated Corrosion Resistance
[0038] Coated corrosion resistance was evaluated by SST. The
coating film was cross-cut and set in an SST tester for a
predetermined time (96 h). A cellophane tape was pressed against
the surface of the cross-cut portion and, then, peeled off as
defined and the maximum peel width from the cut portion was
measured. The smaller the peel width is, the better the coated
corrosion resistance is. As the coating, a gray epoxy powder
coating (Magdyne PD-E, product of Nippon Paint Co.) was used. The
curing conditions of coating films were 160.degree. C. and 20
minutes. The dry film thickness of the coating film was 40
.mu.m.
[0039] (3) Coating Film Adhesion
[0040] The coating film adhesion was evaluated according to a
warm-water immersion test. The warm-water immersion test is a
method in which a sample is immersed in warm water at 50.degree. C.
for a predetermined time (24 h, 96 h) and the coating film adhesion
is then evaluated. The coating film adhesion in this occasion was
evaluated by the cross-hatch test method which comprises
cross-hatching the coating film into 100 squares, 1 mm.times.1 mm
each, pressing a cellophane tape against the cross-hatched surface,
peeling the tape off as defined, and counting the remaining coating
film squares.
[0041] (4) Measurement of Surface Electrical Resistivity
[0042] The surface electrical resistivity was measured by the
two-terminal method. The measurement was made in 9 points per
sample and, among the values found, 7 values to the exclusion of
the maximum and minimum values were averaged to find surface
resistivity value. For measurements, a surface electrical
resistivity tester (EP-T360, manufactured by Keyence) was used.
[0043] (5) Evaluation of Coat Appearance
[0044] The appearance was evaluated visually.
1 TABLE 1 Coated Uncoated corrosion corrosion resistance Coating
film adhesion resistance Blister Number of remaining Surface Coat
Uncorroded width coating film squares resistivity appearance area
(%) (mm) 24 h 96 h (.OMEGA.) (color) Ex. 1 70% 0 100 100 0.4 Brown
Ex. 2 80% 0 100 100 10 Reddish brown Ex. 3 80% 0 100 100 0.4 Pale
brown Compar. Ex. 1 80% 0 0 -- -- Very pale brown Compar. Ex. 2 40%
0 0 -- -- White Ex. 4 80% 0 100 100 0.1 Pale yellow Ex. 5 80% 0 100
100 0.1 Pale brown Compar. Ex. 3 20% 0 100 100 0.4 Pale gray
Compar. Ex. 4 30% 7 10 5 0.1 Pale yellow Compar. Ex. 5 50% 20 0 0
0.08 White
[0045] Except that 75 weight % orthophosphoric acid was added to
the same chemical conversion bath as used in Example 1 while
monitoring with a pH meter to bring the pH to 1.8, the procedure of
Example 1 was otherwise repeated to prepare test pieces and
evaluation and measurement were carried out.
Example 3
[0046] Except that 20 weight % aqueous sodium hydroxide solution
was added to the same chemical conversion bath as used in Example 1
to bring the pH to 6.9, the procedure of Example 1 was otherwise
repeated to prepare test pieces and evaluation and measurement were
carried out.
Comparative Example 1
[0047] Except that 20 weight % aqueous sodium hydroxide solution
was added to the same chemical conversion bath as used in Example 1
to bring the pH to 9, the procedure of Example 1 was otherwise
repeated to prepare test pieces and evaluation and measurement were
carried out.
Comparative Example 2
[0048] Except that 20 weight % aqueous sodium hydroxide solution
was added to the same chemical conversion bath as used in Example 1
to bring the pH to 12, the procedure of Example 1 was otherwise
repeated to prepare test pieces and evaluation and measurement were
carried out.
Example 4
[0049] Except that, in the pickling step of Example 3, 1 g/L
aqueous sulfuric acid solution was used as the pickling bath, the
procedure of Example 3 was otherwise repeated to prepare test
pieces and evaluation and measurement were carried out.
Example 5
[0050] Except that, in the pickling step of Example 3, 1 g/L
aqueous nitric acid solution was used as the pickling bath, the
procedure of Example 3 was otherwise repeated to prepare test
pieces and evaluation and measurement were carried out.
Comparative Example 3
[0051] Except that, in the chemical conversion treatment step of
Example 3, 20 weight % aqueous sodium hydroxide solution was added
to 45 g/L aqueous primary sodium phosphate solution while
monitoring pH value with a pH meter to bring the pH of the chemical
conversion bath to 6.8 and the resulting bath was used, the
procedure of Example 3 was otherwise repeated to prepare test
pieces and evaluation and measurement were carried out. Here, a
chemical conversion reagent not containing a permanganate ion was
used and this was intended to verify the effect of a permanganate
ion.
Comparative Example 4
[0052] Except that, in the chemical conversion treatment step of
Example 3, a reagent comprising 5.5 g/L of the permanganate ion was
used in the absence of a phosphate ion, the procedure of Example 3
was otherwise repeated to prepare test pieces and evaluation and
measurement were carried out. This was intended to verify the
effect of a phosphate ion.
Comparative Example 5
[0053] Except that the chemical conversion treatment and subsequent
step of washing with water were not performed among the steps
described in Example 1, the procedure of Example 1 was otherwise
repeated to prepare test pieces and evaluation and measurement were
carried out.
* * * * *