Note: Descriptions are shown in the official language in which they were submitted.
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description
COMPOSITION AND PROCESS FOR TREATING SURFACES OF LIGHT METALS AND
THEIR ALLOYS
FIELD AND BACKGROUND OF THE INVENTION
The invention relates to a novel water-based liquid composition, often called
a
"bath" hereinafter for brevity, even though it may be used with other methods
than
immersion for establishing contact between the composition and the surface to
be
s treated with it, and to processes using such compositions for treating the
surfaces of light
metals and light metal alloys for the purpose of imparting thereto an
excellent corrosion
resistance and an excellent adherence to paint films. This invention can be
applied in
a particularly advantageous manner to the surface treatment of aluminum fin
stock for
heat exchangers, aluminum alloy coil, aluminum alloy sheet, and magnesium and
~o magnesium alloy automotive and aerospace components and electronic devices
and
instruments.
The baths used to treat aluminum and aluminum alloy surfaces can be broadly
classified into chromate-type baths and non-chromate-type baths. Chromic acid
chro-
mate conversion baths and phosphoric acid chromate conversion baths are
typical ex-
is amples of the chromate-type treatment baths.
Chromic acid chromate conversion baths first reached practical application in
about 1950 and even now are widely used for the surface treatment of aluminum
(the
word "aluminum" and all of its grammatical variations being understood
hereinafter,
unless the context indicates otherwise, to apply to alloys that contain at
least, with
2o increasing preference in the order given, 45, 60, 75, 85, 90, 95, or 99 %
by weight of
aluminum) fin stock for heat exchangers and aluminum wheels, building
materials, and
aerospace materials. The main components in chromic acid chromate conversion
baths
are chromic acid and a fluoride-type reaction accelerator. This type of bath
produces a
conversion coating containing moderate amounts of hexavalent chromium on the
metal
z5 surface.
Phosphoric acid chromate conversion baths originated with the invention dis-
closed in United States Patent No. 2,438,877. The main components in
phosphoric acid
chromate conversion baths are chromic acid, phosphoric acid, and hydrofluoric
acid. A
conversion coating whose main component is hydrated chromium phosphate is
formed
so by this type of bath on the metal surface. Since the resulting conversion
coating does
not contain hexavaient chromium, this type of bath is in wide used at the
present time as
an underpaint treatment for the body stock and lid stock of beverage cans.
While the conversion coatings generated by these chromate-type surface treat-
ment baths exhibit an excellent corrosion resistance and an excellent
adherence to paint
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2
films, these treatment baths also contain toxic hexavalent chromium. The
associated en-
vironmental problems have made it desirable to use treatment baths that are
completely
free of hexavalent chromium.
The method disclosed in Japanese Patent Publication (Kokai or Unexamined)
s Number Sho 52-131937 (131,937/1977) exemplifies chromium-free surface
treatment
baths. Surface treatment baths of this type are acidic (pH = approximately 1.5
to 4.0)
aqueous coating solutions that contain phosphate, fluoride, and zirconium or
titanium or
a mixture thereof. The treatment of metal surfaces with such a treatment bath
results
in the formation on the metal surface of a conversion coating whose main
component
~o is an oxide of zirconium or titanium. Non-chromate-type surface treatment
baths offer
the advantage of not containing hexavalent chromium and for this reason are
widely
used at present for treating aluminum DI can surfaces. Unfortunately, the
coatings
produced by non-chromate-type surface treatment baths are less corrosion
resistant than
chromate coatings.
~s The treatment method disclosed in Japanese Laid Open (Kokai or Unexamined)
Patent Application Number Sho 57-41376 (41,376/1982) involves treating the
surface
of aluminum, magnesium, or an alloy thereof with an aqueous solution
containing at least
one selection from titanium salts and zirconium salts and at least one
selection from
imidazole derivatives.
zo The coatings produced in the working examples of Japanese Laid Open Patent
Application Number Sho 57-41376 have an anticorrosion performance
corresponding
to no rusting at 48 hours according to Japanese Industrial Standard
(hereinafter usually
abbreviated as "JIS") Z-2371. This performance, while satisfactory 15 years
ago, is not
unequivocally satisfactory at present. This patent application also describes
the
2s supplementary addition, at from 0.01 to 100 g/L as the compound, of an
oxidizer such
as nitric acid, hydrogen peroxide, or potassium permanganate, but does not
provide a
working example supporting the use of a potassium permanganate oxidizer.
Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 8-
144063 (144,063/1996) teaches a surface treatment method for the formation of
conver-
3o sion coatings on the surface of aluminum stock. This method uses an aqueous
solution
that contains potassium permanganate or potassium manganate or both in
addition to
a coating-forming accelerator such as a mineral acid (HN03, H2S04, HF), an
alkali (KOH,
NaOH, NH40H), a neutral fluoride (KF, NaF), an acidic fluoride (NH4HF2, NaHF2,
KHFZ),
or a fluorosilicate (MnSiFs, MgSiF6). However, the conversion coating formed
by this
35 treatment bath has not been found to have a corrosion resistance in long-
term corrosion-
resistance testing equal to or greater than that of chromate coatings.
Thus, as described above, the use of the aforementioned prior-art non-chromate-
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3
type surface treatment baths remains associated with problems with the
corrosion
resistance of the produced conversion coatings and pollution abatement of the
effluent
from the surface treatment bath. It is for these reasons that at present non-
chromate-
type surface treatment baths are little used on surface treatment lines where
a
s particularly good corrosion resistance is required, for example, for
aluminum fin stock for
heat exchangers and aluminiferous metal coil and sheet stock.
In summary, then, there has yet to be established a bath, for treating
aluminum
and aluminum alloy surfaces, that does not contain hexavalent chromium, that
requires
little or no pollution abatement, and that has the ability to form highly
corrosion-resistant,
~o highly paint-adherent conversion coatings.
The surface treatment methods already mentioned above suffer from a number
of practical and economic problems, such as (1 ) the use of high treatment
bath concen-
trations, (2) the use of high treatment temperatures, and (3) the use of long
treatment
times, and improvements in each of these areas would be desirable.
~s Chromate treatments as typified by JIS H-8651 and U. S. Military Standard
("MIL") M-3i 71 are in use for treating magnesium and magnesium alloy surfaces
(the
word "magnesium" and all of its grammatical variations being understood
hereinafter,
unless the context indicates othervvise, to apply to alloys that contain at
least, with
increasing preference in the order given, 45, 60, 75, 85, 90, 95, or 99 % by
weight of
2o magnesium). The conversion coatings generated by these chromate-type
surface
treatment baths exhibit an excellent corrosion resistance and an excellent
adherence to
paint films, but these treatment baths also contain highly toxic hexavalent
chromium.
The associated environmental problems have made it desirable to use treatment
baths
that are entirely free of hexavalent chromium.
2s The method disclosed in Japanese laid Open (Kokai or Unexamined) Patent
Application Number Hei 3-6994 (6,994/1991 ) is an invention typical of the
chromium-free
non-chromate-type surface treatment baths for magnesium and its alloys. This
is a
phosphate surface treatment method, and while it does not employ hexavalent
chromium, it also does not have the ability to generate high-level properties.
More
so specifically, this treatment method requires the execution of a silicate
treatment after the
phosphate treatment and the execution of a silicone treatment after the
silicate
treatment. The phosphate treatment coating by itself provides a low level of
corrosion
resistance and paint adherence when used as an underpaint treatment for
magnesium
and magnesium alloy surfaces. This treatment method also requires a multistage
35 treatment process, uses high treatment temperatures, and requires long
treatment times.
The known phosphate-based surface treatment methods include methods that
employ treatment baths based on zinc phosphate, iron phosphate, calcium
phosphate,
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or zirconium phosphate. However, these methods essentially cannot provide a
corrosion
resistance that is satisfactory at the level of practice.
A manganese phosphate treatment is disclosed in category 7 of JIS H-8651.
However, this treatment bath contains chromium, requires high treatment
temperatures
s of 80 °C to 90 °C, and requires long treatment times of 30 to
60 minutes and thus is not
acceptable from a practical standpoint.
Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 8-
35073 (35,073/1996) teaches a surface treatment method for the formation of
conversion
coatings on the surface of magnesium stock. This method uses an aqueous
solution
~o that contains permanganic acid or manganic acid or both in addition to a
coating-forming
accelerator such as a mineral acid (HN03, HZS04, HF), an alkali (KOH, NaOH,
NH,OH),
or a neutral fluoride (MnSiF6, MgSiF6). However, the conversion coating formed
by this
treatment bath has not been found to have a corrosion resistance in long-term
corrosion-
resistance testing equal to or greater than that of chromate coatings.
~s Thus, as described above, the use of the aforementioned prior-art non-
chromate-
type surface treatment baths for magnesium and its alloys remains associated
with prob-
lems with the corrosion resistance of the produced conversion coatings and
with
requiring treatment conditions unsuitable from a practical standpoint. It is
for these
reasons that at present non-chromate-type surface treatment baths are little
used on
zo surface treatment lines where a particularly good corrosion resistance and
paint
adherence are required, for example, for magnesium alloy automotive materials,
aerospace materials, and materials for electronic devices and instruments.
In summary, then, there has yet to be established a bath for treating
magnesium
and magnesium alloy surfaces that does not contain hexavalent chromium, that
has
2s excellent process characteristics, and that has the ability to form highly
corrosion-
resistant, highly paint-adherent conversion coatings.
PROBLEMS TO BE SOLVED BY THE INVENTION
The object of the present invention is to provide a surface treatment bath
that can
coat light metal and light metal alloy surfaces with a film that imparts an
excellent
so corrosion resistance and excellent paint adherence to the treated surface.
SUMMARY OF THE INVENTION
It has been found that a highly corrosion-resistant, highly paint-adherent
conversion coating can be formed on the surfaces of light metals and light
metal alloys
as by the use of a surface treatment bath that has a pH of 1.0 to 7.0 and that
contains
specific amounts of permanganic acid or salt thereof and specific amounts of
at least
one compound selected from water-soluble titanium compounds and water-soluble
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ziroonium compounds.
DETAILED DESCRIPTION OF THE INVENTION AND ITS PREFERRED
EMBODIMENTS
A surface treatment bath according to the present invention has a pH from 1.0
s to 7.0 and comprises, preferably consists essentially of, or more preferably
consists of:
0.01 to 50 grams of permanganic acid and/or salt thereof per liter of the
total treatment
bath, this concentration unit being freely applied hereinafter to any
constituent of the
surface treatment bath and being hereinafter usually abbreviated as "g/L"; and
0.01 to
20 g/L of at least one compound selected from water-soluble titanium compounds
and
io water-soluble zirconium compounds; and, optionally, one or more of a pH
adjustment
agent, a sequestering agent, a supplemental oxidizing agent, and a component
of water-
soluble manganese compounds other than permanganic acid and its salts. Such a
bath
is believed to form a manganese-containing composite coating comprising
compounds
of at least two of the heavy metal elements, i.e., manganese + titanium, or
manganese
is + zirconium, or manganese + titanium + zirconium, and a conversion coating
of this type
preferably is formed in a process according to this invention, because this
type of
conversion coating exhibits the desired improved corrosion resistance.
Furthermore,
independently of other preferences, a composition according to this invention
does not
contain more than, with increasing preference in the order given, 1.0, 0.5,
0.2, 0.10,
20 0.070, 0.030, 0.010, 0.007, 0.003, 0.001, 0.0007, 0.0003, or 0.0001 percent
by weight
of chromium in any chemical form. This preferred exclusion, being motivated by
avoidance of pollution and/or pollution abatement expense, applies only to
chromium
containing materials deliberately added to the compositions and not to any
chromium
containing materials that might be eluted into the compositions from their
contact with
2s substrates containing chromium as an alloying element.
A single selection or several selections from the group consisting of
permanganic
acid and its salts can be used to furnish permanganic acid or permanganate
salt to the
surface treatment bath according to the present invention. The particular
species used
is not crucial. The concentration of permanganic acid and/or salts) thereof in
a surface
so treatment bath according to the invention preferably is at least, with
increasing
preference in the order given, 0.01, 0.05, 0.15, 0.25, 0.35, or 0.45 g/L, and
unless the
surface treatment bath also comprises a concentration of other manganese
containing
solutes that is at least as great as the concentration of permanganic acid
andlor its salts
more preferably is at least, with increasing preference in the order given,
0.55, 0.65,
35 0.75, 0.85, 0.95, 1.5, 2.0, 3.0, 4.0, or 4.5 g/L. Independently, the
concentration of
permanganic acid and/or its salts) preferably is not more than, with
increasing
preference in the order given, 50, 35, 20, 15, 10, or 6.0 g/L. While the use
of lower
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6
concentrations of permanganic acid and/or salts) thereof still results in the
formation
of a conversion coating, such a coating may exhibit a poor resistance to
corrosion and
a poor paint adherence. At the other end of the range, good-quality conversion
coatings
are in fact obtained with concentrations above 50 g/L, but higher
concentrations than this
and even the moderately preferred concentrations lower than 50 g/L are almost
always
uneconomical, because insufficient increases in corrosion resistance or paint
adhesion
result to offset the higher costs of the treatment bath.
One or more selections from, for example, the sulfates, oxysulfates, acetates,
ammonium salts, and fluorides of titanium and zirconium can be used to furnish
the
io water-soluble titanium compound or water-soluble zirconium compound to the
surface
treatment bath according to the present invention. The particular species used
is not
crucial as long as it is a water-soluble compound. This component preferably
is present
in a treatment bath according to the invention at a concentration that is at
least, with
increasing preference in the order given, 0.01, 0.03, 0.05, 0.07, 0.075,
0.080, 0.085,
~s 0.090, 0.095, or 0.100 g/L and independently preferably is not more than,
with increasing
preference in the order given, 20, 10, 5, 3, 2.0, 1.5, 1.2, 1.0, 0.80, 0.70,
0.60, or 0.55 g/L.
Conversion coatings can be formed when this component is present at a
concentration
of less than 0.01 g/L, but such coatings usually have a poor corrosion
resistance. At the
other end of the range, good-quality conversion coatings are in fact obtained
above 20
2o g/L, but quantities in excess of this value and the higher among even the
moderately
preferred concentrations are usually uneconomical because the performance
improve-
ment obtained with them is insufficient to offset the higher costs of a
concentrated treat-
ment bath.
The pH of the surface treatment bath according to the present invention must
be
2s from 1.0 to 7Ø When used to treat aluminum or aluminum alloy surfaces,
the bath
preferably has a pH that is at least, with increasing preference in the order
given, 1.5,
2.0, 2.2, or 2.4 and independently preferably is not more than, with
increasing preference
in the order given, 6.0, 5.5, 5.0, or 4.6. When used to treat magnesium or
magnesium
alloy surfaces, the pH is strongly preferred to be at least 2Ø The metal
substrate being
so treated normally will undergo excessive etching when the pH is below the
given lower
limit, so that an uneven appearance will be produced. A pH above the given
upper limit
can produce various problems, all of which are undesirable. These problems
include the
inability to obtain a highly corrosion-resistant conversion film and bath
stability problems
due to the facile production of precipitate from the metal ion present in the
treatment
bath. The pH of the surface treatment bath according to the present invention
can be
adjusted to a preferred value by using a suitable selection as known to those
skilled in
the art, from acids such as nitric acid, sulfuric acid, phosphoric acid,
hydrofluoric acid,
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and fluorosilicic acid, and bases such as sodium hydroxide, sodium carbonate,
potassium hydroxide, and ammonium hydroxide.
W hen the metal exposed to the treatment bath according to the present
invention
is an aluminum alloy containing copper, iron, or magnesium, the treatment bath
may
s suffer from a substantial reduction in stability due to rations of the
alloying components)
that elute into the surface treatment bath. A sequestering agent may be added
in such
cases in order to chelate these metal alloy components. This sequestering
agent can
be, for example, an organic acid such as gluconic acid, heptogluconic acid,
oxalic acid,
tartaric acid, an organophosphonic acid, and ethylenediaminetetraacetic acid
or an alkali
~o metal salt thereof. These sequestering agents can also advantageously be
used in the
case of magnesium alloys containing, for example, aluminum or zinc.
A supplemental oxidizer other than permanganic acid and its salts may also be
used in the present invention in order to accelerate formation of the
conversion coating.
This supplemental oxidizer can be exemplified by tungstic acid, molybdic acid,
and their
~s salts and by water-soluble organoperoxides such as tart-butyl
hydroperoxide.
A conversion coating formed by the hereinabove described method preferably
comprises manganese and at least one selection from titanium and zirconium.
The
Mn/(Ti + Zr) weight ratio in such a conversion coating is preferably from 0.05
to 100.
When the substrate is aluminum or aluminum alloy, this weight ratio is more
preferably,
2o with increasing preference in the order given, from 0.1 to 20.0, from 0.2
to 5.0, or from
0.2 to 1.5. When the substrate is magnesium or a magnesium alloy, the
aforementioned
weight ratio is more preferably from 0.1 to 20 and most preferably is from 0.2
to 20. The
corrosion resistance becomes increasingly poor as this weight ratio declines
below the
specified lower limit, while the long-term corrosion resistance becomes
increasingly poor
2s as this weight ratio increases above the specified upper limit.
Independently, the total coating weight for Mn, Ti, and Zr is preferably at
least,
with increasing preference in the order given, 5, 10, 20, or 30 milligrams of
the metals
per square meter of substrate surface treated, this unit of coating weight
being
hereinafter usually abbreviated as "mg/m2", and independently preferably is
not more
so than, with increasing preference in the order given, 500, 300, 270, or 240
mg/m2. A
coating weight below 5 mg/m2 can cause an inadequate corrosion resistance and
paint
adherence. Good-quality conversion coatings are obtained at coating weights in
excess
of 500 mg/m2, but coating weights in excess of this value and even coating
weights
among the higher moderately preferred values are usually uneconomical because
any
ss additional performance produced by the additional coating weight is
insufficient to offset
the higher costs of the treatment bath. In addition, the paint adherence
becomes
increasingly impaired as the coating weight increases above 500 mg/m2, and
coating
7
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8
weights above 500 mg/m2 also result in substantial variations in the
appearance.
The chemical characteristics of the metal in the coating, e.g., occurrence as
the
oxide or phosphate, etc., are not particularly critical for the manganese,
titanium, and
zirconium constituent components of the conversion coating according to the
present
s invention.
The explanation will now tum to methods for treating light metals and light
metal
alloys using the surface treatment bath according to the present invention.
A surface treatment bath according to the present invention, in a preferred
embodiment, is used as part of the following sequence of process operations:
~o (1 ) surface cleaning: degreasing (acidic, basic, or solvent degreasers can
be used);
(2) water rinse;
(3) surface treatment using the surface treatment bath according to the
present
invention;
(4) water rinse;
~s (5) rinse with deionized water;
(6) drying.
A surtace treatment bath according to the present invention is preferably
brought
into contact with the surface of the light metal or light metal alloy at a
temperature that
is at least, with increasing preference in the order given, 10, 15, or 20
°C and independ-
zo ently preferably is not more than 80, 75, or 70 °C. Independently,
the time of contact be-
tween the treatment bath according to this invention and the substrate being
treated in
a process according to the invention preferably is at least, with increasing
preference in
the order given, 1, 3, 5, 7, 9, 13, 16, 20, 25, or 28 seconds and
independently preferably
is not more than, with increasing preference in the order given, 300, 250,
220, 200, 180,
2s 160, 140, or 120 seconds. The reactivity between the treatment bath and
metal surface
is usually inadequate at a contact temperature below 10 °C, so that a
good-quality con-
version coating will not usually be formed at such temperatures. A conversion
coating
is still formed at a contact temperature above 80 °C, but the increased
energy costs as-
sociated with such temperatures make them uneconomical. Sufficient reaction to
form
so a conversion coating that will exhibit a high level of corrosion resistance
does not usually
occur in a treatment time of less than 1 second. Times in excess of 300
seconds provide
no additional improvement in the corrosion resistance or paint adherence of
the resulting
conversion coating.
Techniques such as immersion and spraying can be used to effect contact with
35 the surface treatment bath according to the present invention.
The method used for the present invention preferably provides a conversion
coating add-on on the light metal or light metal alloy surface of from 5 to
300 mg/m2 as
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manganese and from 3 to 100 mg/m2 as titanium and/or zirconium. The conversion
coating will usually exhibit an inadequate corrosion resistance and paint
adherence at
a manganese add-on below 5 mg/m2, while substantial irregularities in the
external
appearance of the conversion coating usually occur at values in excess of 300
mg/m2.
s The conversion coating will often suffer from an inadequate corrosion
resistance at a
titanium or zirconium add-on below 3 mg/m2. Highly corrosion-resistant
conversion
coatings are formed at a titanium or zirconium add-on above 100 mg/mz, but
such values
are uneconomical since no additional performance is produced by the additional
add-on.
The aluminum and aluminum alloys that may be subjected to surface treatment
io according to the present invention encompass pure aluminum and its alloys.
Examples
of the latter are alloys such as AI-Cu, AI-Mn, AI-Mg, and AI-Si. Similarly,
the magnesium
and magnesium alloys that may be subjected to surface treatment according to
the
present invention encompass materials of magnesium and magnesium alloy metals.
The latter can be exemplified by Mg-AI-Zn, Mg-Zn, and Mg-AI-Zn-Mn.
~s The shape and dimensions of the light metal or light metal alloy used in
the
present invention are not critical, and, for example, the present invention
encompasses
both sheet stock and various formed products.
Working and comparative examples are provided below in order to more
specifically describe the effects of the surface treatment bath according to
the present
2o invention. The materials used, together with indicia used to identify them
in subsequent
tables, are shown in Table 1 below.
TREATMENT CONDITIONS FOR SUBSTRATE A
Substrate A was treated using the following processes in the sequence (1 ) --
(2)
- (3) ~ (4) - (5) - (6) to give the surface-treated sheet.
2s (1 ) Degreasing (60 °C, 60 seconds, immersion)
A 3 % aqueous solution of a commercial alkaline degreaser
(FINECLEANER~ 315 from Nihon Parkerizing Company, Limited) was used.
(2) Water rinse (ambient temperature, 30 seconds, spray)
(3) Conversion treatment (immersion)
3o Surface treatment was carried out using the composition and treatment
conditions reported in Tables 1 and 2. The reagent weights reported in the
Table 1
Material Indicium and Description
suBSmuTE sHeFr tRUl.s Zs~
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Substrate A: Al-Mn alloy sheet (JIS 3004),
150
millimeters x 70 millimeters
by 0.2
millimeters thick.
B: Die-cast sheet of the AZ91D
magnesium
alloy specified in JIS H2222,
150
millimeters x 70 millimeters
by 0.2
millimeters thick.
a: manganese sulfate (MnSO,~HzO)
Water-soluble manganese compound b: potassium manganate (KZMnO,)
c: otassium erman anate (KMnO
)
Water-soluble titanium compound Av 40 lo fluorotitanic acid
(H2TiF6)
B: 24 % titanium sulfate Ti(SO
Water-soluble zirconium compound (1)v20 % fluorozirconic acid
(HZZrFb)
ii ammonium fluorozirconate
: (NH ) ZrF
(i):67.5 % nitric acid (HN03)
pH adjuster (In:40 % fluorosilicic acid
(HZSiFb)
Bl):25 % a ueous ammonia ( OH)
"Conversion Treatment Bath Composition" columns in Table 2 are values that
refer to
the pure reagent.
The surface treatment conditions for Comparative Examples 5 to 7 are
reported further below.
s (4) Water rinse (ambient temperature, 30 seconds, spray)
(5) De-ionized water rinse (ambient temperature, 30 seconds, spray)
(6) Thermal drying (80 °C, 3 minutes, forced convection oven)
TREATMENT CONDITIONS FOR SUBSTRATE B
Substrate B was treated using the following processes in the sequence (1 ) -
(2)
10 -- (3) -- (4) ~ (5) -- (6) to give the surface-treated sheet.
(1 ) Degreasing (60 °C, 60 seconds, immersion)
The 3 % aqueous solution of a commercial alkaline degreaser
(FINECLEANER~ 315 from Nihon Parkerizing Company, Limited) was used.
(2) Water rinse (ambient temperature, 30 seconds, spray)
,s (3) Surface treatment (immersion)
Surface treatment was carried out using the composition and treatment
conditions reported in Tables 1 and 3. The reagent weights reported in the
"Con-
version Treatment Bath Composition" columns for Examples 6 to 10 and Com-
parative Examples 8 to 11 are values that refer to the pure reagent.
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11
Table 2
Working
example Conversion
Treatment
Bath
Composition
in
g/L
or
Compa
Mn Ti Zr pH
l Compound(s) Compounds) Compound(s) Adjuster
E and
e and and Amounts)
xamp Amounts) Amounts)
Number
WORKING
EXAMPLES
1 a 1.0 - - (i) 0.3 (III)
b 10.0
c 0.5
2 b 3.0 A 0.5 - - -
c 3.0
3 c 5.0 B 0.8 - - Q)
4 c 5.0 - - (i) 0.1 (II)
a 1.0 A 0.1 (ii) 0.1 (III)
c 1.0
COMPARATIVE
EXAMPLES
1 c 10.0 - - - - (I)
_ - _ _ {i) 0.3 -
3 b 10.0 A 0.008 - - (I),
(II)
4 a 0.03 A 1.0 - - (III)
- _ _ _ _ _ _
- _ _ _ _ _ -
_ _ _ _ - _ _
(Table 2 is continued on the next page)
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12
Table 2 Continued from previous page
Treatment Coating
Conditions Weight
in mg/m2
of:
pH Temper- Time, Mn Ti Zr _ Cr
ature Seconds
C
WORKING
EXAMPLES
1 4.5 60 60 5 - 30 -
2 2.6 35 180 110 80 - -
3 2.4 50 30 28 12 - -
4 3.5 60 60 68 - 8 -
3.5 70 10 50 12 45 -
COMPARATIVE EXAMPLES
1 2.0 60 60 55 - - -
2 3.0 60 60 - - 45 -
3 3.5 60 60 48 3 - -
4 3.0 60 60 3 38 - -
5 2.7 40 30 - - 15 -
6 1.6 40 50 - - - 170
7 1.3 50 20 - - - 70
(7-able 2 is continued on bye next page)
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Table 2 Continued from previous page
Weight Ratio,Total CoatingCorrosion Adherence,
Mn/(Ti + Zr) Weight, Resistance, Number
Mn + Ti + Sait Spray of
Zr, Test, Grid
mg/mz 1,000 hours Squares
remaining
WORKING EXAMPLES
1 0.17 35 +++ 100
2 1.38 190 +++ 98
3 2.33 40 + + 98
4 8.50 76 + + 99
0.88 107 + + + 100
COM PARATIVE EXAMPLES
1 + 98
x 75
3 16.00 51 + 99
4 0.08 41 x 82
g x 100
6 +++ 99
7 + + 100
The surface treatment conditions for Comparative Examples 12 and 13
are reported further below.
(4) Water rinse (ambient temperature, 30 seconds, spray)
(5) De-ionized water rinse (ambient temperature, 30 seconds, spray)
(6) Thermal drying (80 °C, 3 minutes, forced convection oven)
Comparative Example 1 is a comparative example that used a permanganate salt
by itself as the treatment bath component to form a manganese-containing
coating.
Comparative Example 2 is a comparative example that used a Zr compound by
itself as the treatment bath component to form a zirconium-containing coating.
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Table 3
Woriung
Example Conversion
Treatment
Bath
Composition
in
g/L
or
Compa
rison Mn Ti Zr pH
l Compound(s) Compounds) Compound(s) Adjuster
E and
xamp and and Amounts)
e Amounts) Amounts)
Number
WO RKING LES
EXAMP
6 a 1.0 - - (i) 0.3 (III)
b 10.0
c 0.5
7 b 3.0 A 0.5 - - -
c 3.0
8 c 5.0 B 0.8 - - (ITI)
9 c 5.0 - - (i) 0.1 (II)
a 1.0 A 0.1 (ii) 0.1 (III)
c 1.0
COMPARATIVE MPLES
EXA
8 c 10.0 - _ _ _ (I)
g _ _ _ _ (i 0.3 -
10 a 0.05 A 0.008 - - (III)
11 b 10.0 - - i 0.008 (III)
12 - _ _ _ _ _ _
13 - _ _ _ _ _ _
(Table 3 is conb~nued on the next page)
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Table 3 Continued from previous page
Treatment Coating
Conditions Weight
in mg/m~
of:
pH Temper- Time, Mn Ti Zr Cr
ature Seconds
C
WORKING AMPLES
EX
6 4.5 60 30 26 - 70 -
7 2.6 20 120 220 12 -
8 6.0 50 30 25 3 - -
9 3.2 40 120 15 - 80 -
10 5.0 70 3 10 4 7 -
COMPARATIVE EXAMPLES
8 4.0 50 120 40 - - -
9 3.0 40 60 - - 33 -
10 3.0 70 20 1 135 - -
11 7.5 40 120 29 - 0.2 -
12 - 30 60 - - - 20
13 - 95 1800 - - - 300
(Table 7 is con~nued on the next page)
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Table 3 Continued from previous page
Weight Ral3o,Total CoatingCorrosion Adherence,
Mn/(Ti + Zr) Weight, Resistance, Number
Mn + Ti + Salt Spray of
Zr, Test, Grid
mg/mZ 1,000 hours Squares
remaining
WORKING EXAMPLES
6 0.37 96 + + + 100
7 18.33 232 + + + 98
8 8.33 28 + + 99
9 0.19 95 + + + 99
0.91 21 + + 100
COMPARATIVE
EXAMPLES
8 + 96
9 x 85
10 0.007 136 + 76
11 145.09 29.2 + 98
12 + + 99
13 +++ 100
Comparative Example 3 produced a very low-Ti (Mn/Ti) coating, while
Comparative Example 4 produced a very low-Mn (Mn/Ti) coating.
Comparative Example 5
This comparative example used a 7 % aqueous solution of a commercial zircon-
s ium phosphate-based surface treatment agent (ALCHROME~ 713 from Nihon
Parkeriz-
ing Company, Limited) for conversion treatment. This bath was used to treat
the AI-Mn
alloy sheet specified above using the following treatment conditions: 40
°C, 60 seconds,
immersion. Treatment was followed by evaluation of the corrosion resistance
and paint
adherence.
~o Comparative Example 6
This comparative example used a 7 % aqueous solution of a commercial chromic
acid chromate conversion treatment agent (ALCHROME(~ 713 from Nihon
Parkerizing
Company, Limited) for surtace treatment. This bath was used to treat the AI-Mn
alloy
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sheet specified above using the following treatment conditions: 40 °C,
60 seconds,
immersion. Treatment was followed by evaluation of the corrosion resistance
and paint
adherence.
Comparative Example 7
s This comparative example used a 3 % aqueous solution of a commercial phos-
phoric acid chromate conversion treatment agent (mixed aqueous solution of 4
ALCHROME~ K702SL and 0.3 % ALCHROME~ K702AC, both from Nihon Parkerizing
Company, limited) for the conversion treatment. This bath was used to treat
the AI-Mn
alloy sheet specified above using the following treatment conditions: 50
°C, 20 seconds,
~o spray. Treatment was followed by evaluation of the corrosion resistance and
paint
adherence.
Comparative Example 8
This comparative example used permanganate salt by itself as the treatment
bath
component to produce a manganese-containing coating.
is Comparative Example 9
This comparative example used a Zr compound by itself as the treatment bath
component to form a zirconium-containing coating.
Comparative Example 10
This produced a very low-Mn (Mn/Ti) film.
Zo Comparative Example 11
This produced a very low-Zr (Mn2r) film.
Comparative Example 12
This comparative example used a 7 % aqueous solution of a commercial chromic
acid chromate conversion treatment agent (ALCHROME~ 713 from Nihon Parkerizing
2s Company, Limited) for surface treatment. This bath was used to treat the
magnesium
alloy sheet specified above using the following treatment conditions: 30
°C, 60 seconds,
immersion. Treatment was followed by evaluation of the corrosion resistance
and paint
adherence.
Comparative Example 13
so This comparative example used a treatment bath formulated according to MIL-
M-
3171 C (TYPE III) for the surface treatment. The main component in this bath
was
sodium dichromate. This bath was used to treat the magnesium alloy sheet
specified
above using the following treatment conditions: 95 °C, 30 minutes,
immersion.
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EVALUATION METHODS
(1 ) Add-ons
The Mn, Ti, Zr, and Cr coating weights were measured using a fluorescent x-ray
analyzer.
s (2) Corrosion resistance
The corrosion resistance was evaluated using the salt-spray test specified in
JIS
Z-2371. The status of corrosion development on the surface-treated sheet was
visually
evaluated after exposure to salt spray for 1,000 hours and was reported using
the
following scale:
io + + + . area of corrosion less than 10
+ + . area of corrosion from 10 % up to 50 % (exclusive)
+ . area of corrosion from 50 % up to 90 % (exclusive)
x . area of corrosion at least 90
(3) Evaluation of paint adherence for Substrate A
15 This procedure was applied to the AI-Mn alloy sheet afforded by surface
treatment under the conditions of Examples 1 to 5 and Comparative Examples 1
to 7.
An epoxy-phenol paint for application to can lids was applied on the sheet
surface to a
paint film thickness of 5 Nm followed by baking for 3 minutes at 220
°C. A crosshatched
grid of 100 squares (width = 2 millimeters) was then executed in the center of
the painted
zo sheet using a cutter and the resulting specimen was immersed for 60 minutes
in boiling
de-ionized water. After the painted sheet had been air-dried, a cellophane
tape peel test
was carried out on the grid. The paint adherence was evaluated based on the
number
of unpeeled squares in the grid.
A larger number of unpeeled squares in this procedure is indicative of a
better
2s paint adherence, and a score of 98 or more unpeeled squares corresponds to
a satis-
factory performance at the level of practice.
The results of this evaluation are reported in Table 2.
(4) Evaluation of paint adherence for Substrate B
This procedure was applied to the magnesium alloy sheet afforded by surface
so treatment under the conditions of Examples 6 to 10 and Comparative Examples
8 to 13.
An epoxy resin paint was applied on the sheet surface to a dried paint film
thickness of
Nm followed by baking for 10 minutes at 200 °C. A crosshatched grid of
100 squares
(width = 2 millimeters) was then executed in the center of the painted sheet
using a
cutter and the resulting specimen was immersed for 60 minutes in boiling de-
ionized
ss water. After the painted sheet had been air-dried, a cellophane tape peel
test was
carried out on the grid. The paint adherence was evaluated based on the number
of
unpeeled squares in the grid.
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A larger number of unpeeled squares in this procedure is indicative of a
better
paint adherence, and a score of 98 or more unpeeled squares corresponds to a
satisfac-
tory performance at the level of practice.
The results of this evaluation are reported in Table 3.
s Tables 2 and 3 demonstrate that the conversion coatings afforded by the
treatment bath according to the present invention exhibit the same corrosion
resistance
as commercial chromic acid chromate and phosphoric acid chromate treatments.
These
tables also confirm that highly corrosion-resistant coatings can be realized
by the forma-
tion of composite coatings in which suitable amounts of Mn and Ti2r are both
present.
~o As the preceding explanation makes clear, a hexavalent chromium-free,
highly
corrosion-resistant, and highly paint-adherent conversion coating is produced
by the
application of the surface treatment bath according to the present invention
to light
metals and light metal alloys.
This performance makes the surface treatment bath according to the present
~s invention very useful at a practical level for application to light metals
and light metal
alloys.
The materials used for the casings and shells of, for example, computers and
portable phones, have recently shifted from plastics to magnesium alloys based
on
considerations of recyclability, thermal radiation, and relative strength per
unit weight.
2o At the same time, the electromagnetic radiation generated by electronic
devices, known
as noise, can cause other computer devices to malfunction with the production
of numer-
ous problems. Within the sphere of electronic devices that employ magnesium
alloy
materials, this noise problem has created desire for the appearance of a
surface treat-
ment method that provides an excellent electromagnetic shielding performance
in addi-
2s tion to an excellent corrosion resistance and excellent paint adherence.
The coating
formed by the surface treatment bath according to the present invention does
not contain
toxic chromium, exhibits excellent corrosion resistance and excellent paint
adherence,
and also has a low surface resistance and thereby can also provide an
excellent electro-
magnetic shielding performance.
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