Note: Descriptions are shown in the official language in which they were submitted.
:3~08~7
The present invention relates to an aqueous composition,
process for forming a protective coating on zinc or an alloy
thereof employing such a composition, and a coated article
produced thereby.
Zinc has conventionally been employed mainly for
imparting corrosion resistance to steel by means of the hot
galvanized or electrolytic plating method. However, under a
high humidity atmosphere, zinc metal develops white stains on
the surface which are detrimental to the appearance and corrosion
resistance thereof and which reduce markedly the adhesion of
paints applied thereon.
Chromating processes employing mainly chromic acid,
bichromic acid or a salt thereof have been employed to prevent
the development of such white stains. The chromating processes
are inexpensive and exhibit good corrosion resistance, however,
the toxic effects of chromates and their destructive effect on
the natural environment have come closely into question. Thus
standards for controlling the use of chromic acids and chromates
have become more and more severe. At the same time, there have
arisen various problems such as the process control for chro-
mating, waste water treatment of chromating operation,
secondary contamination by the chromated articles and the like.
It has been well known to treat metal surfaces with
tannic acid as disclosed, for example, in Japanese Patent Kokai
Nos. 47224/74 and 8317/73.
However, the corrosion resistance imparted to sur-
faces by such conventional processes is not sufficient. In
addition, when used as a base for paints, the resistance
against scratches is insufficient and blistering of paint film
tends to occur under a high-humidity atmosphere.
The surface treatment of the metal emplo~ing mainly
silica or silicates has been also disclosed in Japanese Patent
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Publication Nos. 19686/69 and 5130/70. However, the coating
free from chromates and obtained by such processes provides
insufficient corrosion resistance and has been used as an
electrically insulating rather than corrosion resistant coating
in many cases as disclosed in Japanes~ Patent Publication Nos.
7~49/68 and 13562/69.
It has now been found that by combining tannin or
tannic acid and an aqueous suspension containing one or more
refracting oxides such as silica, silica sol and alumina sol,
a protective coating can be formed on the surface of zinc or
an alloy thereof which improves the corrosion resistance
markedly as compared with those achieved by either component
alone and which can prevent humidity blistering of paint film
applied thereto. Thus this invention has been completed.
According to the invention there is provided an
aqueous composition comprising a tannin or tannic acid and a
refractory oxide in a weight ratio of 100:1 to 1:100, wherein
the concentration of tannin or tannic acid is 0.01 to 20 wt.%
and the concentration of refractory oxide is 0.1 to 20 wt.%.
According to another aspect of the invention there is
provided a coated article comprising a substrate having a zinc
or zinc alloy surface and a protective coating on said surface
comprising a tannin or tannic acid and a refractory oxide in
a weight ratio of 100:1 to 1:100.
According to yet another aspect of the invention there
is provided a process for forming a protective coating on a zinc
or zinc alloy surface comprising contacting the surface with an
aqueous compo~ition of the invention and thereafter drying the
~urface. In a particular embodiment the surface is contacted
with the composition for at least 0.1 seconds at a temperature
of at least 20C.
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In the process of the invention for forming the pro- :
tective coating on zinc or an alloy thereof, the coating may
suitably be formed by treating the latter with an aqueous solution
or suspension containing at least one refractory oxide, for
example, silica, silica sol and alumina sol in a content ranging
from 0.1 to 2~/o by weight and tannin or tannic acid in a content
ranging from O.Ql to 2~o by weight. ~.
Tannin or tannic acid useful in this invention may
be any hydrolyzable tannin or condensed tannin or partial
decomposition products thereof and includes, for example, :
depside tannin, Gallotannin, Chinese tannic acid, Turkey
tannic acid, hamamelitannic acid, tannic acid from Acer
Ginnala, cheblinic acid, sumac tannin, Chinese gallotannin
ellagitannin, catechine, catechine-tannic acid, quebracho-
tannin and the like in a concentration ranging from 0.01 to 2 ~/o
by weight, At a concentration of less than 0.01% by weight,
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P-10679 10 81 407
the effect of tannin becomes insignificant and a concentration
of higher than 20~ by weight is disadvantageous from the
workability and econom~,
Any particulate silica may be used in this invention
S including, for e~ample, Aerosil (a trademark for silica
available from Nihon Aerosil Co., Ltd~) and Siloid (a trademark
for silica available from Fuji Devison Co., Ltd~). As silica
gel, there may be used in this invention a solution obtained
by removing sodium ion from an aqueous solution of sodium
silicate through an ion exchange column or commercially
available silica sol containing SiO2 in an amount ranging from
20 to 30% such as Snowtex (a trademark product available from
Nissan Chemicals Co., Ltd,), There may also be used silica
sol wherein silica gel is dispersed evenly in an alkali solution,
such as that obtained by neutralizing sodium silicate with an
appropriate acid and isolating the resulting sol so as to be
free from Na ion, or that obtained by dispersing solid SiO2
such as Aerosil or the like in an aqueous medium. In every
silica sol, the SiO2 should be dispersed homogeneously in the
,20 colloidal state in the aqueous medium. There may also be
usable an aqueous solution or dispersion of silica or silica
sol containing lithium oxide as disclosed in Japanese Patent
Publication Nos. 19686/69 and 5130/70 to improve the corrosion
Xesistance~ Com~ercially available products include Lithium
`25 $i1icate (a trademark product available from Nissan Chemicals
Co " ~td~)~
Any alum~na sol is usable in this invention~
Commercially available alumina sol dispersed in water ~avail-
able from Nissan Chemicals Co., ~td,) is convenient for the
handling, This product contains A12O3 in an amount of higher
than 10~ and is stabilized at a pH value from 2,5 to 6 with an
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P-10679 ~0 81 407
inorganic acid or OrgalliC acid~
The precise composition of the aqueous solution or
suspension depends on the proce~s to be employed ~orming the
protectiVe coating, such as immersion, roll coating, flooding,
spr~ying, electrostatic coating, brushin~ or the like but is
generally an aqueous solution or suspension containing 0~01
to 20~ by weight of tannin or tannic acid and 0,1 to 20%
~frdcto 7,~
~-' by weight of at least one re-a6~4~ oxide, Preferably the
xefractor~ is selected from the group consisting of silica,
silica sol and alumina sol, the effective ratio of tannin or
tannic acid to silica and/or silica sol and/or alumina sol
ranging from 100:1 to 1:100 as the treating agent, If tannin
or tannic acid is present in a higher content than the specified
upper limit, blistering will tend to occur in painted specimens
by the corrosion test~ If silica and/or silica sol and/o~
alumina sol are present in a higher content than the specified
upper limit, the ~oxmed coating will tend to become too brittle~
~; Since the stable pH ranges depend on the kinds of
tannin, tannic acid, silica, silica sol, or alumina sol, an .
. . ~ . .
.20 inorganic acid such as phosphoric acid, nitric acid, sulfuric
acid, hydrofluoric acid, hydrochloric acid and salts thereof
~ or an organic acid such as oxalic acid, citric acid, malic :
.. acid, maleic acid, phthalic acid, acetic acid, lactic acid,
taxtaric acid, chloroacetic acid, acrylic acid and salts there-
o~ ma~ ~e used to adjust the pH value of the aqueous solutions .
; and suspensions~
An alkaline compound such as sodium hydroxide, potassium
h~dxoxide, lithium hydroxide, ammonium h~vdroxide or the like,
~ OX an amine such as ethyl amine, diethyl amine, triethyl
;`30 amine, ethanol amine or the like may be used for pH adjustment.
Since, however, the residual content of such water-
soluble salts in the coating facilitates the adsorption of
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moisture in the air and at the same time has a possibility to
enhance the corxosion, such ~Jater-soluble salts should be
limited to an am~unt as small as possible and preferably
excess soluble sal.ts are removed, if possible, by rinsing
with water.
The aqueous solution or suspension of this invention
may be added with a water-soluble, emulsifiable or dispersible
organic polymer resin to improve the adhesion of the coating
comprising mainly tannin, tannic acid, and silica, silica sol
or alumina sol onto the metal surface, to provide flexibility
to the coating and also to improve the corrosion resistance of
the coating.
By embodying the process of this invention, it is
possible to form a protective coating having an excellent
corXosion resistance and to provide a paint base having
excellent adhe~ion.
The invention will be illustrated by way of the
follow.ing examples.
; EXAMPL~ 1
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A hot galvanized steel plate which had been temporarily
treated with chromic acid and had a size of 100 x 300 x 0.3 mm
.. was polished thrice by the wet Scotch prvcess to remove the
chxomic acid adhered on the surface thereof. A treating
; suspension was-prepared by dispersing and dissolving 30 g of
Chi~nese gallatannin (available from Fuji Chemical Industries
, ~, Ltd~ as Tannic Acid AL) and 30 g of powdery silica (avail-
: able ~rom Nihon Aexosil Co, Ltd, under the trademark ~erosil
. 380~ into about 500 ml o~ deionized water and b~ making up to
; lOOO.g with pure water~ The pH value o~ the prepared suspension
3~ was about 3~5, The cleaned galvanized steel plate was coated
with the suspension by the bar coating process with a No. 6
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108~407
P-10679 :~
bar and dxied in a hot air circulating oven at 120C for one
minute to form the protective coating. The treated plate was
then subjected to the salt spray test according to JIS-Z-2371.
It took about 200 to 240 hours prior to the development of
white stains, thus the coating showed excellent corrosion
resistance as compared with those obtained for the comparative
test plates prepared according to Comparative Examples 1, 2, 3
and 4,
Comparative Example 1 .
A hot galvanized steel plate identical to that of
Example 1 was polished thrice by the wet Scotch process and
the cleaned galvanized steel plate free from chromic acid was
employed as the test plate.
i. . .. .
Comparative Example 2
15 A cleaned galvanized steel plate identical to that
o~ Example 1 was txeated to as in Comparative Example 1 and
wa~ coatèd with an aqueous solution of 1% by weight chromic ~ '''
acid in a manner similar to Example l and dried.
Camparative Example 3
,20 A cleaned galvanized-steel plate identical.to that
o~ Example 1 was treated as in Comparative Example 1 and was
coated with an aqueous solution prepared by dissolving 30 g
, of Chinese gallotannin in about 500 ml of deionized water and
~ade up to 1000 g by adding with deionized water in a manner ,.-
si~m~lar to Example 1 and d~ied~
~omparati~e Example 4
cleaned galvanized steel plate identical to that
. o~ Example 1 was treated as in Comparative Example 1 and was
coated with an aqueous suspension prepared by dispersing 30 g
,30 of powdery silica (Aerosil 380) in about 500 ml of deionized
water by means o~ a high speed agitator in a manner similar to
Example 1 and dried~ -
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P-10~79
EXAMPLE 2
The test plates prepared according to Example 1 and
Comparative Examples 1, 2, 3 and 4 were coa~ed with a thermo~
settlng alkyd paint (,a paint for coating the back surface of
S colored galvanized steel plates) by the bar coating process
with a No~ 12 bar and dried and baked in a hot air circulating
,,,' oven at 280C for 50 seconds to form coatings having a thickness
of 6 microns. The coated plates were scribed through the
painted layer to the base metal by means of a razor blade
edge and then subjected to salt spray test according to JIS-Z-
2371 for 240 hours, Transparent tape was then applied to the
' coated surfaces and then peeled rapidly, On the test plate
prepared in Example 1, the extent of corrosion developed from
the scribe was a width of less than 3 mm on both of the sides
' 15 and no blistering was observed on the smooth surface of the
p~int film.
When the film surface was scratched with a copper
coin, it was difficult to penetrate to the galvanized layer.
' ~ When the test plates prep-ared in Comparative ~xamples
~20 1, 2, 3 and 4 were coated with the paint and subjected to
, the corrosion test in the manner of Example 2, the results
- were inferior to those in Example 2, These results are shown
, in Table 1.
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P-10679 ~0 8 1 407
- TAble 1
Corrosion re- Corrosion Resis~ Scratch
sistance of tance of painted resistance
Unpainted test pl.ate 1) by coin
test plate test 2)
by salt
spray test
Width Blister
.. . . . . . . . . . . . ..
Examples 1 and 2 200 - 240 hr 3 mm No blisters Good
Comparative 2 hr, More Blisters Inferior
Example 1 than ovex all
10 mm surface :.--
10Comparative 120 -240 hr 3 mm Slightly
Example 2 blistered Infexior
- Comparative
. Example 3 48 - 72 hx More Blisters Inferior
than over all .:
10 mm - surface
Comparative
Example 4 24 hr More Blisters
than over all Good
10 mm surface
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I Note 1): Test for 240 hours according to the salt spray
, test according to JIS-Z-2371,
Note 2): Scratch resistance by copper coin.
-EXAMPLE 3
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A hot galvanized steeI plate cleaned as in Example 1
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w~s coated.as in Example 1 with a suspension which was prepared
by dissolving 30 g of Chinese gallotannin in about 500 ml of
. deionized water, dispersing 100 g of alumina sol (available
from Nissan Chemicals Co. Ltd, under the trade,mark Alumina Sol
100) and making up to 1000 g by adding deionized water,j When
a.aorxosion test was carried out a~ in Example l, it.took 200
` to 2~0 hours prior to the development o~ white stains.
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