U.S. patent application number 09/029574 was filed with the patent office on 2001-05-31 for manufacturing process on chromate-coated lead-containing galvanized steel sheet with anti-black patina property and anti-white rust property.
This patent application is currently assigned to Tatsuya Miyoshi. Invention is credited to KUBOTA, TAKAHIRO, MIYOSHI, TATSUY, SAGIYAMA, MASARU, SUGIMOTO, YOSHIHARU, YAMASHITA, MASAAKI.
Application Number | 20010001965 09/029574 |
Document ID | / |
Family ID | 27475535 |
Filed Date | 2001-05-31 |
United States Patent
Application |
20010001965 |
Kind Code |
A1 |
MIYOSHI, TATSUY ; et
al. |
May 31, 2001 |
MANUFACTURING PROCESS ON CHROMATE-COATED LEAD-CONTAINING GALVANIZED
STEEL SHEET WITH ANTI-BLACK PATINA PROPERTY AND ANTI-WHITE RUST
PROPERTY
Abstract
In order to manufacture a chromate-coated lead-containing
galvanized steel sheet having beautiful spangles and excellent
anti-black patina and anti-white rust properties without relying
upon any flushing treatment with Ni, etc., a lead-containing
galvanized steel sheet obtained by plating in a galvanizing bath
containing appropriate amounts of Pb and Al, and having beautiful
spangles is coated with a chromate coating solution containing
trivalent and hexavalent chromium ions in a molar ratio of 1/9 to
1/1, and nitrate ions in a molar ratio of 0.1 to 1.6 to the total
of the chromium ions, preferably after treatment with an aqueous
alkali solution having a pH of 9 or above, and washing with water,
and the sheet is dried at an appropriate sheet temperature without
being washed with water, whereby a chromate film having a coating
weight of 5 to 50 mg/m.sup.2 is formed thereon. The lead and
aluminum with which the galvanized surface is enriched are, as a
result, removed effectively, thereby enabling excellent anti-black
patina and anti-white rust properties to be obtained.
Inventors: |
MIYOSHI, TATSUY; (TOKYO,
JP) ; KUBOTA, TAKAHIRO; (TOKYO, JP) ;
SAGIYAMA, MASARU; (TOKYO, JP) ; SUGIMOTO,
YOSHIHARU; (TOKYO, JP) ; YAMASHITA, MASAAKI;
(TOKYO, JP) |
Correspondence
Address: |
NIELDS LEMACK & DINGMAN
176 E MAIN STREET
SUITE 8
WESTBORO
MA
01581
|
Assignee: |
Tatsuya Miyoshi
|
Family ID: |
27475535 |
Appl. No.: |
09/029574 |
Filed: |
March 23, 1998 |
PCT Filed: |
June 30, 1997 |
PCT NO: |
PCT/JP97/02261 |
Current U.S.
Class: |
148/264 |
Current CPC
Class: |
C23C 2/26 20130101; C23C
22/30 20130101 |
Class at
Publication: |
148/264 |
International
Class: |
C23C 022/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 2, 1996 |
JP |
8-191364 |
Jul 2, 1996 |
JP |
8-191365 |
Dec 27, 1996 |
JP |
8-357954 |
Dec 27, 1996 |
JP |
8-357955 |
Claims
1. A process for manufacturing a chromate-coated lead-containing
galvanized steel sheet having excellent anti-black patina and
anti-white rust properties, characterized in that a lead-containing
galvanized steel sheet obtained by plating a steel sheet in a
galvanizing bath containing 0.05 to 0.3% by weight of Pb and 0.1 to
0.3% by weight of Al is coated with a chromate coating solution
containing hexavalent and trivalent chromium ions and nitrate ions
in such proportions that the trivalent chromium ions have a molar
ratio of 1/9 to 1/1 to the hexavalent chromium ions, while the
nitrate ions have a molar ratio of 0.1 to 1.6 to the total of the
chromium ions, and that the sheet is dried at a sheet temperature
of 40-250.degree. C. without being washed with water, whereby a
chromate film having a coating weight of 5 to 50 mg/m.sup.2 in
terms of metallic chromium is formed thereon.
2. A process for manufacturing a chromate-coated lead-containing
galvanized steel sheet having excellent anti-black patina and
anti-white rust properties as set forth in claim 1, wherein the
lead-containing galvanized steel sheet is obtained by employing a
temperature of 440-500.degree. C. for the galvanizing bath and a
temperature of 440-520.degree. C. for the sheet to be immersed into
the bath.
3. A process for manufacturing a chromate-coated lead-containing
galvanized steel sheet having excellent anti-black patina and
anti-white rust properties as set forth in claim 1 or 2, wherein
the chromate coating solution contains one or more kinds of metal
ions selected from among cobalt, nickel, strontium and barium ions,
and having a total molar ratio of 0.04 to 0.2 to the total of the
chromium ions.
4. A process for manufacturing a chromate-coated lead-containing
galvanized steel sheet having excellent anti-black patina and
anti-white rust properties as set forth in claim 1 or 2, wherein
the chromate coating solution contains cobalt ions having a molar
ratio of 0.04 to 0.2 to the total of the chromium ions.
5. A process for manufacturing a chromate-coated lead-containing
galvanized steel sheet having excellent anti-black patina and
anti-white rust properties, characterized in that a lead-containing
galvanized steel sheet obtained by plating a steel sheet in a
galvanizing bath containing 0.05 to 0.3% by weight of Pb and 0.1 to
0.3% by weight of Al is treated with an aqueous alkali solution
having a pH of 9 or above, and that after it is washed with water,
it is coated with a chromate coating solution containing hexavalent
and trivalent chromium ions and nitrate ions in such proportions
that the trivalent chromium ions have a molar ratio of 1/9 to 1/1
to the hexavalent chromium ions, while the nitrate ions have a
molar ratio of 0.1 to 1.6 to the total of the chromium ions, and
that the sheet is dried at a sheet temperature of 40-250.degree. C.
without being washed with water, whereby a chromate film having a
coating weight of 5 to 50 mg/m.sup.2 in terms of metallic chromium
is formed thereon.
6. A process for manufacturing a chromate-coated lead-containing
galvanized steel sheet having excellent anti-black patina and
anti-white rust properties as set forth in claim 5, wherein the
lead-containing galvanized steel sheet is obtained by employing a
temperature of 440-500.degree. C. for the galvanizing bath and a
temperature of 440-520.degree. C. for the sheet to be immersed into
the bath.
7. A process for manufacturing a chromate-coated lead-containing
galvanized steel sheet having excellent anti-black patina and
anti-white rust properties as set forth in claim 5 or 6, wherein
the chromate coating solution contains one or more kinds of metal
ions selected from among cobalt, nickel, strontium and barium ions,
and having a total molar ratio of 0.04 to 0.2 to the total of the
chromium ions.
8. A process for manufacturing a chromate-coated lead-containing
galvanized steel sheet having excellent anti-black patina and
anti-white rust properties as set forth in claim 5 or 6, wherein
the chromate coating solution contains cobalt ions having a molar
ratio of 0.04 to 0.2 to the total of the chromium ions.
Description
TECHNICAL FIELD
[0001] This invention relates to a process for manufacturing a
chromate-coated lead-containing galvanized steel sheet having
excellent anti-black patina and anti-white rust properties, and
more particularly, to a process which is useful for the chromate
coating of a lead-containing galvanized steel sheet having
beautiful spangles.
BACKGROUND ART
[0002] As the galvanizing of steel sheet is the most effective and
economical means for protecting it from corrosion by a galvanic
action, ten millions of tons of crude steel, which correspond to
about 10% of the yearly crude steel production of Japan (about 100
millions of tons), are used for the manufacture of galvanized steel
sheets which are used in a wide variety of fields including their
use as building materials, and as materials for automobiles and
electric appliances. Zinc manifests a galvanic action as described
below. The two metals, zinc and iron, contacting each other form a
cell in which zinc, which is the baser metal, forms the anode,
while iron becomes the cathode. Thus, it restrains any anodic
dissolution by a local cell as formed by iron alone, and thereby
prevents its corrosion. This rust-preventing action ends upon loss
of all of zinc contacting iron, and in order to sustain the action
for a long period of time, it is necessary to restrain the
corrosion of the zinc layer, and it is, therefore, common practice
to coat the zinc layer with chromate.
[0003] The chromate coating of galvanized steel sheet has, however,
the drawback of having a black patina formed on the sheet during
its storage or transport and impairing its appearance seriously,
though it may drastically improve its corrosion resistance
(anti-white rust property). It is known that a black patina is
likely to appear on, among others, galvanized steel sheet subjected
to skin pass after galvanizing, or produced in a galvanizing bath
containing lead, or coated with a zinc layer containing several
percent of aluminum.
[0004] The black patina is characterized by a grayish black color
presented by spangles formed in a galvanized surface and having a
specific crystalline orientation, and it is, therefore, effective
to minimize the spangles by a known method to restrain the
appearance of a black patina to some extent. It is also known that
the use of a galvanizing bath containing only a very small amount
of lead (Pb not exceeding 0.01% by weight) makes a black patina
less likely to appear, since the spangles in which a black patina
appears contain lead particles forming the active sites which
promote the appearance of a black patina. The addition of lead to a
galvanizing bath is, however, unavoidable, since many users of
galvanized steel sheets like spangles.
[0005] According to pages 939 to 946 of TETSU & HAGANE, Vol. 77
(1991), published by The Japan Iron and Steel Association, the
spangles are classified into seven types, i.e. fern I, fern II,
mirror, frost, half fern, feather, and triangle types, and the
frost type spangles are particularly likely to be enriched with Pb
and Al. The users who like spangles generally prefer a surface
having many frost type spangles as presenting a beautiful
appearance, but a black patina is particularly likely to appear in
frost type spangles, since they are enriched with Pb and Al, as
stated above.
[0006] Japanese Patent Application Laid-Open No. Sho 59-177381
proposed flashing treatment with an aqueous solution containing Ni
or Co ions (for the chemical deposition of a very small amount of
metal) as a method of preventing a black patina from appearing
after chromate treatment, and the flashing treatment has recently
come to be considered as an effective method of preventing a black
patina from appearing after chromate treatment. According to the
disclosure of Japanese Patent Application Laid-Open No. Sho
59-177381, the surface of a zinc or zinc-alloy plated steel sheet
is given flashing treatment with an aqueous solution having a pH of
1 to 4, or 11 to 13, and containing Ni or Co ions, or both prior to
its chromate treatment, whereby the metal ions are deposited in
metallic or oxide form on the sheet surface, and after it is washed
with water, a chromate film is formed thereon.
[0007] There is not yet any definite opinion about the mechanism
which enables the flashing treatment of a zinc or zinc-alloy plated
steel sheet with Ni or Co to prevent a black patina from appearing
thereon after its chromate treatment, but according to the
statement on pages 150 and 151 of the Preprint for the 60th
Scientific Lecture Meeting of the Association of Metal Surface
Technology, the metal as deposited by flashing is mostly found in
the grain boundary of zinc crystals, and the chromium compound as
deposited by the subsequent chromate coating treatment is likewise
distributed in the grain boundary, and it can, therefore, be
presumed that some interaction occurs between the metal as
deposited by flashing and the chromium compound, and causes the
latter to be adsorbed and fixed to the former.
[0008] The black patina formed on a zinc or zinc-alloy plated steel
sheet looks black, apparently because the basic zinc carbonate of
which it is composed, and which is represented as
(ZnCO3)x.multidot.[Zn(OH)2]y, like white rust, has a particle
diameter falling within the visible light wavelength range of 400
to 700 nm, and is, therefore, very likely to scatter and absorb
light. The black patina is considered as a product of corrosion
formed in an environment lacking oxygen, and particularly with the
progress of corrosion from the grain boundary. Accordingly, it is
considered that the chromium compound with which the grain boundary
is enriched by the metal deposited by flashing restrains the
corrosion from the grain boundary and thereby contributes to
preventing the formation of a black patina. Thus, the flashing
treatment of a zinc or zinc-alloy plated steel sheet with Ni, Co,
etc. prior to its chromate treatment can be an effective means for
preventing the formation of any black patina thereon.
[0009] It has, however, been found that the flashing treatment
carried out prior to chromate treatment makes white rust more
likely to form, though it may restrain the formation of a black
patina. This is apparently due to the fact that Ni or Co as
deposited on the galvanized surface by the flashing treatment forms
a local cell with zinc.
[0010] It is, therefore, an object of this invention to provide a
process which can manufacture a chromate-coated lead-containing
galvanized steel sheet having excellent anti-black patina and
anti-white rust properties without relying upon any flashing
treatment with Ni, Co, etc., and more particularly, a
chromate-coated lead-containing galvanized steel sheet which hardly
has any black patina formed thereon, even if it may have many
spangles of the frost type which is liked by many users.
DISCLOSURE OF THE INVENTION
[0011] As a result of a wide range of experiments and study which
we have made to explore the possibility of improving the anti-black
patina and anti-white rust properties of a chromate-coated
lead-containing zinc or zinc-alloy plated steel sheet having
beautiful spangles, we, the inventors of this invention, have found
that it is possible to manufacture a chromate-coated
lead-containing galvanized steel sheet having beautiful spangles
and yet showing excellent anti-black patina and anti-white rust
properties on an industrially steady basis if a lead-containing
zinc or zinc-alloy plated steel sheet produced in a galvanizing
bath having a specific composition is treated with a chromate
coating solution having a specific composition and containing
nitrate ions.
[0012] We have, moreover, found that it is possible to manufacture
a chromate-coated lead-containing galvanized steel sheet having an
outstandingly high corrosion resistance by pre-treating the sheet
with an aqueous alkali solution before its treatment with the
chromate coating solution having a specific composition as
mentioned above, or by giving it two steps of treatment consisting
of its pre-treatment with the aqueous alkali solution and its
treatment with the chromate coating solution having a specific
composition and containing nitrate ions.
[0013] This invention is based on what we have found as stated
above, and its salient features are as follows:
[0014] [1] A process for manufacturing a chromate-coated
lead-containing galvanized steel sheet having excellent anti-black
patina and anti-white rust properties, characterized in that a
lead-containing galvanized steel sheet obtained by plating a steel
sheet in a galvanizing bath containing 0.05 to 0.3% by weight of Pb
and 0.1 to 0.3% by weight of Al is coated with a chromate coating
solution containing hexavalent and trivalent chromium ions and
nitrate ions in such proportions that the trivalent chromium ions
have a molar ratio of 1/9 to 1/1 to the hexavalent chromium ions,
while the nitrate ions have a molar ratio of 0.1 to 1.6 to the
total of the chromium ions, and that the sheet is dried at a sheet
temperature of 40-250.degree. C. without being washed with water,
whereby a chromate film having a coating weight of 5 to 50
mg/m.sup.2 in terms of metallic chromium is formed thereon.
[0015] [2] A process for manufacturing a chromate-coated
lead-containing galvanized steel sheet having excellent anti-black
patina and anti-white rust properties as set forth at [1] above,
wherein the lead-containing galvanized steel sheet is obtained by
employing a temperature of 440-500.degree. C. for the galvanizing
bath and a temperature of 440-520.degree. C. for the sheet to be
immersed into the bath.
[0016] [3] A process for manufacturing a chromate-coated
lead-containing galvanized steel sheet having excellent anti-black
patina and anti-white rust properties as set forth at [1] or [2]
above, wherein the chromate coating solution contains one or more
kinds of metal ions selected from among cobalt, nickel, strontium
and barium ions, and having a total molar ratio of 0.04 to 0.2 to
the total of the chromium ions.
[0017] [4] A process for manufacturing a chromate-coated
lead-containing galvanized steel sheet having excellent anti-black
patina and anti-white rust properties as set forth at [1] or [2]
above, wherein the chromate coating solution contains cobalt ions
having a molar ratio of 0.04 to 0.2 to the total of the chromium
ions.
[0018] [5] A process for manufacturing a chromate-coated
lead-containing galvanized steel sheet having excellent anti-black
patina and anti-white rust properties, characterized in that a
lead-containing galvanized steel sheet obtained by plating a steel
sheet in a galvanizing bath containing 0.05 to 0.3% by weight of Pb
and 0.1 to 0.3% by weight of Al is treated with an aqueous alkali
solution having a pH of 9 or above, and that after it is washed
with water, it is coated with a chromate coating solution
containing hexavalent and trivalent chromium ions and nitrate ions
in such proportions that the trivalent chromium ions have a molar
ratio of 1/9 to 1/1 to the hexavalent chromium ions, while the
nitrate ions have a molar ratio of 0.1 to 1.6 to the total of the
chromium ions, and that the sheet is dried at a sheet temperature
of 40-250.degree. C. without being washed with water, whereby a
chromate film having a coating weight of 5 to 50 mg/m.sup.2 in
terms of metallic chromium is formed thereon.
[0019] [6] A process for manufacturing a chromate-coated
lead-containing galvanized steel sheet having excellent anti-black
patina and anti-white rust properties as set forth at [5] above,
wherein the lead-containing galvanized steel sheet is obtained by
employing a temperature of 440-500.degree. C. for the galvanizing
bath and a temperature of 440-520.degree. C. for the sheet to be
immersed into the bath.
[0020] [7] A process for manufacturing a chromate-coated
lead-containing galvanized steel sheet having excellent anti-black
patina and anti-white rust properties as set forth at [5] or [6]
above, wherein the chromate coating solution contains one or more
kinds of metal ions selected from among cobalt, nickel, strontium
and barium ions, and having a total molar ratio of 0.04 to 0.2 to
the total of the chromium ions.
[0021] [8] A process for manufacturing a chromate-coated
lead-containing galvanized steel sheet having excellent anti-black
patina and anti-white rust properties as set forth at [5] or [6]
above, wherein the chromate coating solution contains cobalt ions
having a molar ratio of 0.04 to 0.2 to the total of the chromium
ions.
[0022] According to this invention, a chromate-coated
lead-containing galvanized steel sheet which is excellent in all of
its film appearance, and anti-black patina and anti-white rust
properties can be manufactured steadily from a lead-containing
galvanized steel sheet having beautiful spangles without relying
upon any flashing treatment thereof with Ni, Co, etc.
BEST MODE OF CARRYING OUT THE INVENTION
[0023] This invention is a process for the chromium coating of a
lead-containing galvanized steel sheet obtained by plating in a
galvanizing bath containing 0.05 to 0.3% by weight of Pb and 0.1 to
0.3% by weight of Al. The lead and aluminum in the galvanizing film
on the lead-containing galvanized steel sheet are the constituents
necessary for forming a beautiful pattern of spangles, and
improving the adhesion of the film, respectively. The galvanizing
film has a surface enriched with such lead and aluminum, but its
surface enriched with lead makes the film lack electrochemical
uniformity, and thereby promotes the formation of a black patina.
The enrichment of the film surface with lead is likely to occur
particularly to the surface having frost type spangles as obtained
by galvanizing under specific conditions (i.e. a specific
temperature of the galvanizing bath and a specific temperature of
the steel sheet upon immersion into the bath). The aluminum stays
in most of the cases as a stable oxide in the skin of the
galvanizing film, but as the aluminum oxide does not easily react
with a chromate coating solution, it makes it difficult to form a
uniform chromate film and thereby achieve an improved corrosion
resistance.
[0024] In connection with these problems, we have found that, if a
lead-containing galvanized steel sheet is brought into contact with
a chromate coating solution containing nitric acid, it is possible
to remove lead from its galvanized surface and thereby restrain the
formation of a black patina effectively. Although even such
chromate treatment may, of course, not be able to remove all of the
lead from the galvanized surface, but may allow some lead to remain
therein, we have found that such chromate treatment is remarkably
effective in restraining the formation of a black patina, and that
a chromate film thereby formed provides an improved corrosion
resistance, too, since a large amount of chromium covers the
surface of active lead remaining in the galvanized surface. The
effective removal of the lead enriching the galvanized surface is
achieved particularly by the chromate coating solution containing
nitric acid, and no satisfactory removal of such lead can be
obtained by any chromate coating solution containing any other
inorganic acid, such as phosphoric, sulfuric or fluoric acid.
[0025] No satisfactory removal of aluminum from the galvanized
surface can be achieved by its treatment with any chromate coating
solution containing chromic acid and an inorganic acid (e.g.
nitric, phosphoric, sulfuric or fluoric acid) as mentioned above.
As a result of further study, we have found that the treatment of
the galvanized surface with an aqueous alkali solution prior to its
chromate treatment makes it possible to remove any aluminum oxide
from it and thereby improve its corrosion resistance effectively.
We have, thus, ascertained that the lead and aluminum staying in
the galvanized surface in an enriching form, which can hardly be
removed by any known chromic acid, or partially reduced chromic
acid solution, can be removed adequately by two steps of treatment
consisting of its pre-treatment with an aqueous alkali solution and
its chromate treatment with a chromate coating solution containing
nitric acid as mentioned above to thereby improve its anti-black
patina property and corrosion resistance effectively.
[0026] We have further found that, if the chromate treatment with a
chromate coating solution containing nitric acid as stated above is
carried out by employing a chromate coating solution containing
hexavalent and trivalent chromium ions and nitrate ions, and having
a molar nitrate ion ratio of 0.1 to 1.6 to the total of the
chromium ions, it is possible to remove lead effectively by a very
small amount of etching, and that such a chromate coating solution
can be used for a long time without having any sludge formed by the
inclusion of zinc, or other impurity ions, but staying stable in
composition.
[0027] The following is a detailed description of the invention and
the reasons for the various limitations employed for defining
it.
[0028] The lead-containing galvanized steel sheet for which the
chromate treatment of this invention is intended is obtained by
plating a steel sheet in a galvanizing bath containing 0.05 to 0.3%
by weight of Pb and 0.1 to 0.3% by weight of Al, and is
particularly likely to present the problem of a black patina, as
stated before. If the lead content of the bath is lower than 0.05%
by weight, no satisfactorily beautiful form of regular spangles can
be produced on the galvanized steel sheet, but if it exceeds 0.3%
by weight, it is not only uneconomical, since it cannot be expected
to produce any better results, but it also makes the intergranular
corrosion of the galvanizing layer likely to occur easily. If the
aluminum content of the bath is lower than 0.1% by weight, no
satisfactory adhesion of the galvanizing layer can be obtained, but
if it exceeds 0.3% by weight, the chromate-coated galvanized
surface is undesirably likely to form a black patina when exposed
to an environment of high temperature and humidity.
[0029] The lead-containing galvanized steel sheet exhibits a
particularly beautiful form of regular spangles if it is obtained
by employing a galvanizing bath temperature of 440-500.degree. C.
and a temperature of 440-520.degree. C. for the sheet to be
immersed into the bath. It is, therefore, advisable to use a
lead-containing galvanized steel sheet obtained under the
conditions as mentioned if the appearance of its regular spangles
is of particular importance. If the galvanizing bath temperature is
lower than 440.degree. C., no beautiful form of regular spangles
can be obtained, but if it exceeds 500.degree. C., the alloying of
iron and zinc proceeds to an undesirable extent. If the temperature
of the sheet to be immersed in the bath is lower than 440.degree.
C., no beautiful form of regular spangles can be obtained, but if
it exceeds 520.degree. C., it is not only impossible to obtain any
beautiful form of regular spangles, but also the galvanizing layer
shows an undesirably low adhesion. The preferred galvanizing
conditions include a galvanizing bath temperature of
450-480.degree. C. and a sheet immersion temperature of
450-490.degree. C., and make it possible to obtain a particularly
beautiful form of spangles.
[0030] While the process of this invention includes the chromate
treatment of the lead-containing galvanized steel sheet which is
carried out by treating it with the chromate coating solution
having a specific composition as described above, it is preferable
to pre-treat the sheet with an aqueous alkali solution having a pH
of 9 or above, and more preferably a pH of 9 to 12, in order to
remove aluminum from the galvanized surface.
[0031] If the aqueous alkali solution used for the pre-treatment
has a pH below 9, the galvanized surface hardly has its aluminum
oxide dissolved, but has a lower reactivity with the chromate
coating solution, and thereby a lower corrosion resistance. If it
has a pH of over 12, an excessively large amount of zinc is etched,
and the chromate coating solution has a lower level of stability.
If such pre-treatment is employed, the galvanized steel sheet is
washed with water prior to its chromate treatment.
[0032] Its chromate treatment is carried out for coating it with a
chromate film formed from a chromate coating solution containing
hexavalent and trivalent chromium ions and nitrate ions, and having
a molar ratio of trivalent to hexavalent chromium ions and a molar
ratio of nitrate to total chromium ions which have both been
controlled within appropriate ranges.
[0033] The trivalent and hexavalent chromium ions in the chromate
coating solution have a molar ratio of from 1/9 to 1/1, and
preferably from 1/4 to 2/3 (trivalent/hexavalent). If the molar
ratio of the trivalent/hexavalent chromium ions is smaller than
1/9, the dissolution of chromium in a corrosive environment is too
excessive to sustain corrosion resistance for a long time, and is
also undesirable, as it brings about environmental pollution. If
their molar ratio exceeds 1/1, no satisfactorily improved corrosion
resistance can be expected from hexavalent chromium ions.
[0034] Referring to the mechanism which enables a chromate film to
prevent white rust, it is generally understood that hexavalent
chromium ions have an inhibitive effect for restraining the
corrosion of zinc, and it is also considered that its self-healing
effect exhibited by the hexavalent chromium ions eluted from the
chromate film and healing any damaged part thereof contributes to
providing an improved anti-white rust property. If a chromate film
is formed from a solution containing only hexavalent chromium ions,
however, the excessive elution of hexavalent chromium ions in the
presence of water causes white rust to form, and it is, therefore,
common practice to add trivalent chromium ions in order to prevent
the excessive elution of hexavalent chromium ions. For the same
purpose, this invention also employs a chromate coating solution
containing trivalent and hexavalent chromium ions in the molar
ratio as mentioned before.
[0035] It is generally necessary for an aqueous solution containing
only trivalent and hexavalent chromium ions to have a molar ratio
of trivalent/hexavalent chromium ions not exceeding 2/3, so that
the trivalent chromium ions may not settle, but may remain as ions
in the solution. The color tone of its surface is often considered
as an important feature of a lead-containing galvanized steel sheet
having a beautiful pattern of spangles, and as the presence of a
large amount of chromium coating produces a yellowish color having
an adverse effect on its color tone, there is no alternative but to
restrict the amount of chromium coating even at some sacrifice of
corrosion resistance. The chromate coating solution employed for
the purpose of this invention, however, makes it possible to
prevent any such undesirable coloring without having the amount of
chromium coating reduced, since it contains nitric acid as an acid
constituent other than chromic acid, and has, therefore, a smaller
proportion of hexavalent chromium ions having an adverse effect on
the color tone of the galvanized surface, and thereby a molar ratio
of trivalent/hexavalent chromium ions which is higher than 2/3 (but
not higher than 1/1, as stated before).
[0036] An aqueous solution of chromic anhydride which has been
partially reduced by a known reducing agent can be used as a source
of chromium ions for the chromate coating solution, while chromium
nitrate can be used as a source of trivalent chromium ions.
[0037] The nitrate ions in the chromate coating solution have a
molar ratio of from 0.1 to 1.6, and preferably from 0.4 to 1.2, to
the total chromium ions. If their molar ratio is smaller than 0.1,
no satisfactory result can be obtained in restraining any black
patina, and if it exceeds 1.6, the chromate coating solution has so
high an etching force that a sludge is formed by the inclusion of
impurity ions, such as Zn and Al, and makes it impossible to form a
layer having a constantly high corrosion resistance.
[0038] Nitric acid, chromium nitrate, cobalt nitrate, or zinc
nitrate can, for example, be used as a source of nitrate ions for
the chromate coating solution.
[0039] The chromate coating solution may further contain one or
more kinds of metal ions selected from among cobalt, nickel,
strontium and barium ions, and having a molar ratio of 0.04 to 0.2
to the total chromium ions. These metal ions and chromate ions form
an insoluble compound which is believed to improve the barrier
property of the chromate film and thereby the corrosion resistance
of the steel. If their molar ratio is smaller than 0.04, hardly any
improved corrosion resistance can be obtained, while if it exceeds
0.2, the chromate coating solution easily forms a sediment and
becomes lower in stability.
[0040] The chromate coating solution is likely to contain Zn, Al,
Pb, or other metal ions as unavoidable inclusions by its etching
action from the galvanizing layer, but they do not adversely affect
the advantages of this invention.
[0041] While several kinds of metal ions have been mentioned, the
most remarkably improved anti-white rust property can be obtained
when cobalt ions are added. We have experimentally compared a
chromate film formed from a chromate coating solution containing
cobalt ions with a chromate film formed from a solution not
containing cobalt ions, and found that the former contains a larger
amount of hexavalent chromium in its outermost layer than the
latter does, though they are substantially equal in their
hexavalent chromium content. It is, therefore, obvious that the
chromate film containing cobalt ions exhibits a greater
self-healing effect owing to the hexavalent chromium ions than the
chromate film not containing cobalt ions does, and thereby a
remarkably improved anti-white rust property. Thus, the cobalt ions
are the most preferable metal ions to be added to the chromate
coating solution.
[0042] A basic carbonate, carbonate, or nitrate of a metal can, for
example, be used as a source of its ions for the chromate coating
solution.
[0043] The chromate coating solution may further contain a silica
gel, fumed silica, or other colloidal silica, an aqueous resin,
etc., as required.
[0044] After the chromate coating solution has been coated on the
surface of the galvanized steel sheet, it is not washed with water,
but is dried until a maximum sheet temperature of 40-250.degree. C.
is reached. If the sheet temperature is lower than 40.degree. C.,
the remaining water makes an easily dissolvable chromate film, and
if it exceeds 250.degree. C., the hexavalent chromium ions which
are effective for corrosion resistance are reduced to trivalent
chromium ions, and a chromate film formed as a high molecule
becomes a low molecule, resulting in an undesirable lowering of
corrosion resistance.
[0045] The chromate film as formed by coating and drying has a
coating weight of from 5 to 50 mg/m.sup.2, and preferably from 10
to 30 mg/m.sup.2, in terms of metallic chromium. If its coating
weight is less than 5 mg/m.sup.2 in terms of metallic chromium, no
satisfactorily high corrosion resistance can be obtained, while if
it exceeds 50 mg/m.sup.2, the film is distinctly colored and
damages the beautiful surface appearance of the lead-containing
galvanized steel sheet.
[0046] Any known method, such as spray or dip coating followed by
roll or air squeezing, or roll coating, can be employed for coating
the galvanized surface with the chromate coating solution.
EXAMPLES
[0047] Chromate treatment was given to lead-containing galvanized
steel sheets as shown at (A) to (D) below after a part of them had
been pre-treated with an aqueous alkali solution, while no such
pre-treatment had been given to the rest thereof. The pre-treatment
was carried out under the conditions as stated at (a) or (b) below,
and was followed by washing with a spray of tap water (continued
for 10 seconds), and air drying. The chromate treatment was carried
out by roll coating the sheets with chromate coating solutions
having the compositions shown in Tables 1 to 5 (and basing the
control of the chromium coating weight on wet weight), and drying
them in a hot-air drying furnace having a temperature of
300.degree. C. and an air flow rate of 2 m/sec. until a maximum
sheet temperature of 40-270.degree. C. was reached, whereby
chromate-coated lead-containing galvanized steel sheets were
prepared as samples.
[0048] The samples were evaluated for their galvanized surface
appearance, film appearance, anti-black patina property, and
corrosion resistance (anti-white rust property). The evaluation was
made of both of samples as obtained soon after chromate treatment
had been started, and samples as obtained after the progress of
chromate treatment to some extent (i.e. after the continuous
treatment of 20 m.sup.2 of samples with one liter of coating
solution), and by treatment with the solution containing dissolved
zinc, as far as the products of this invention were concerned,
while two such kinds of samples were evaluated only partly, as far
as the comparative cases were concerned. The results are shown in
Tables 1 to 5 with the composition of the chromate coating
solution, the chromium coating weight, etc.
[0049] [Lead-containing galvanized steel sheet]
[0050] (A) A regular-spangled sheet made by dipping a sheet having
a temperature of 480.degree. C. in a galvanizing bath containing
0.2% Al and 0.1% Pb, both by weight, and having a temperature of
470.degree. C. (and having a coating weight of 90 g/m.sup.2);
[0051] (B) A regular-spangled sheet made by dipping a sheet having
a temperature of 440.degree. C. in a galvanizing bath containing
0.2% Al and 0.1% Pb, both by weight, and having a temperature of
440.degree. C. (and having a coating weight of 90 g/m.sup.2);
[0052] (C) A regular-spangled sheet made by dipping a sheet having
a temperature of 530.degree. C. in a galvanizing bath containing
0.2% Al and 0.1% Pb, both by weight, and having a temperature of
510.degree. C. (and having a coating weight of 90 g/m.sup.2);
and
[0053] (D) A regular-spangled sheet made by dipping a sheet having
a temperature of 430.degree. C. in a galvanizing bath containing
0.2% Al and 0.1% Pb, both by weight, and having a temperature of
430.degree. C. (and having a coating weight of 90 g/m.sup.2).
[0054] [Conditions of pre-treatment with an aqueous alkali
solution]
[0055] (a) Pre-treatment with an aqueous alkali solution having a
pH of 9 (spraying at a temperature of 60.degree. C. for 30
seconds);
[0056] and
[0057] (b) Pre-treatment with an aqueous alkali solution having a
pH of 13 (spraying at a temperature of 50.degree. C. for 3
seconds).
[0058] [Evaluation for properties]
[0059] (1) Appearance of galvanized surface (of spangles)
[0060] The spangles formed on the galvanized surface were visually
examined to determine the ratio by area of the frost type spangles,
and the galvanized surface was evaluated for its appearance in
accordance with the following criteria:
[0061] .circleincircle.: The ratio by area is 60% or more;
[0062] .smallcircle.: From 40%, inclusive, to 60%, exclusive;
[0063] .DELTA.: From 20%, inclusive, to 40%, exclusive;
[0064] X: Less than 20%.
[0065] (2) Appearance of film
[0066] The value of *b between a sample and an untreated material
(b value of sample-b value of untreated material) was determined by
a color difference meter, and the film was evaluated for its
appearance in accordance with the following criteria:
[0067] .circleincircle.: .DELTA.b is less than 1;
[0068] .smallcircle.: .DELTA.b is from 1, inclusive, to 3,
exclusive;
[0069] .DELTA.: .DELTA.b is from 3, inclusive, to 5, exclusive;
[0070] X: .DELTA.b is 5 or more.
[0071] (3) Anti-black patina property
[0072] A plurality of testpieces each measuring 70 mm by 150 mm
were cut out from each sample, and 5 to 10 pairs of testpieces so
prepared that the surfaces to be tested of the testpieces in each
pair might face each other, were laid one pair upon another, and
packed in a sheet of vinyl-coated paper. Each package was held
between two stainless steel sheets each having a thickness of 10 mm
and carrying an acrylic sheet bonded to its inner surface, and
after the stainless steel sheets had been fastened together by
bolts at their four corners, a load of 0.67 kgf.cm.sup.2 was
applied to each package in a humidity test cabinet having a
temperature of 50.degree. C. and a relative humidity of 95%, and
after 240 hours, it was removed from the cabinet and each testpiece
was visually inspected for any black patina on its surface to be
tested. The criteria for evaluation were as follows:
[0073] .circleincircle.: No black patina was found;
[0074] .smallcircle.: A very light gray color was recognized;
[0075] .DELTA.: A black patina was found;
[0076] X: A serious black patina was found.
[0077] (4) Corrosion resistance
[0078] A plurality of testpieces each measuring 70 mm by 150 mm
were cut out from each sample, and a salt spray test conforming to
JIS Z 2371 was conducted on each testpiece, and each testpiece was
visually examined for the area covered by white rust after 96 and
120 hours if it was of any sample prepared without any
pre-treatment, or after 120 and 200 hours if it was of any sample
prepared after pre-treatment. The criteria for evaluation were as
follows:
[0079] .circleincircle.: No white rust was found;
[0080] .smallcircle.: White rust was found in an area of less than
5%;
[0081] .DELTA.: White rust was found in an area of from 5%,
inclusive, to 25%, exclusive;
[0082] X: White rust was found in an area of 25% or more.
[0083] Tables 1 to 5 confirm that the chromate-coated
lead-containing galvanized steel sheets manufactured by the process
of this invention has a galvanized surface with beautiful spangles,
and a chromate film appearance, and anti-black patina and
anti-white rust properties which are all excellent. The samples
according to this invention also confirm that the restrained
etching effect on galvanized steel sheets makes the process highly
suitable for continuous operation. On the other hand, the
comparative samples are inferior in any of galvanized surface, or
chromate film appearance, and anti-black patina and anti-white rust
properties.
1 TABLE 1 [3] [1] [2] Cr.sup.3+/Cr.sup.6+ NO.sup.3-/T-Cr [4]
Zn.sup.2+ [6] [7] [11] [12] No. *1 *2 *3 *3 /T-Cr *3 [5] (g/l)
(.degree. C.) *4 [8] [9] [10] *5 *6 1a A Not given 1/4 0.8 -- -- 0
60 15 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 1b A Not given 1/4 0.8 -- -- 0.4
60 17 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 2a B Not given 1/4 0.8 -- -- 0 60
16 .largecircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 2b B Not given 1/4 0.8 -- -- 0.4
60 15 .largecircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 3a C Not given 1/4 0.8 -- -- 0 60
16 X .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 3b C Not given 1/4 0.8 -- -- 0.4
60 14 X .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 4a D Not given 1/4 0.8 -- -- 0 60
16 .DELTA. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 4b D Not given 1/4 0.8 -- -- 0.4
60 18 .DELTA. .circleincircle. .circleincircle.
.circleincircle./.largeci- rcle. I 5a A Not given 1/4 0.8 Co:0.04
-- 0 60 17 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.circleincircle. I 5b A Not given 1/4 0.8 Co:0.04
-- 0.4 60 15 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.circleincircle. I 6a A Not given 1/1 1.6 Co:0.20
-- 0 150 47 .circleincircle. .largecircle. .circleincircle.
.circleincircle./.circleincircle. I 6b A Not given 1/1 1.6 Co:0.20
-- 0.8 150 49 .circleincircle. .largecircle. .circleincircle.
.circleincircle./.circleincircle. I 7a A Not given 1/2 1.1 Co:0.10
-- 0 80 20 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.circleincircle. I 7b A Not given 1/2 1.1 Co:0.10
-- 0.6 80 18 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.circleincircle. I 8a A Not given 1/9 0.1 Ni:0.06
-- 0 100 21 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 8b A Not given 1/9 0.1 Ni:0.06 --
0.8 100 22 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 9a A Not given 1/5 1.0 Co:0.10 --
0 80 8 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 9b A Not given 1/5 1.0 Co:0.10 --
0.6 80 9 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I *1 Lead-containing galvanized
steel sheet as set forth at any of (A) to (D) above in the
specification *2 Conditions of pre-treatment as set forth at (a) or
(b) above *3 Molar ratio *4 Coating weight in terms of metallic
chromium (mg/m.sup.2) *5 96 hours/120 hours *6 I: Sample of the
invention; C: Comparative sample [1]: Steel sheet [2]:
Pre-treatment [3]: Composition of chromate coating solution [4]:
Metal ion [5]: Others [6]: Sheet temp. for drying [7]: Cr coating
weight [8]: Galvanized surface appearance [9]: Film appearance
[10]: Anti-black patina property [11]: Anti-white rust property
[12]: Division
[0084]
2 TABLE 2 [3] [1] [2] Cr.sup.3+/Cr.sup.6+ NO.sup.3-/T-Cr [4]
Zn.sup.2+ [6] [7] [11] [12] No. *1 *2 *3 *3 /T-Cr *3 [5] (g/l)
(.degree. C.) *4 [8] [9] [10] *5 *6 10a A Not given 1/5 1.0 St:0.10
-- 0 80 25 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 10b A Not given 1/5 1.0 St:0.10 --
0.4 80 22 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 11a A Not given 1/5 1.0 Ba:0.10 --
0 80 15 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 11b A Not given 1/5 1.0 Ba:0.10
0.3 80 17 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 12a A Not given 1/8 0.2 Co:0.05
SiO.sub.2:4 *7 0 60 20 .circleincircle. .circleincircle.
.circleincircle. .circleincircle./.largecircle. I 12b A Not given
1/8 0.2 Co:0.05 SiO.sub.2:4 *7 0.2 60 22 .circleincircle.
.circleincircle. .circleincircle. .circleincircle./.largecircle. I
13a A Not given 1/4 0.8 Co:0.08 resin:5 *8 0 230 32
.circleincircle. .circleincircle. .circleincircle.
.circleincircle./.circleincircle. I 13b A Not given 1/4 0.8 Co:0.08
resin:5 *8 0.4 230 31 .circleincircle. .circleincircle.
.circleincircle. .circleincircle./.circleincircle. I 14 A Not given
1/19 0.8 Co:0.05 -- 0 40 19 .circleincircle. .circleincircle.
.largecircle. X/X C 15 A Not given 1/4 -- -- -- 0 60 25
.circleincircle. .largecircle. X X/X C 16a A Not given 1/1 2.0
Co:0.05 -- 0 150 30 .circleincircle. .circleincircle.
.circleincircle. X/X C 16b A Not given 1/1 2.0 Co:0.05 -- 3.5 150
28 .circleincircle. .circleincircle. .circleincircle. X/X C 17 A
Not given 1/3 0.4 Co:0.08 -- 0 270 20 .circleincircle.
.largecircle. .circleincircle. X/X C 18 A Not given 1/2 -- Co:0.10
PO.sub.4.sup.3-:0.3 *9 0 80 18 .circleincircle. .circleincircle. X
.circleincircle./.circleincircle. C 19 A Not given 1/2 -- Co:0.10
SO.sub.4.sup.2-:0.3 *9 0 80 48 .circleincircle. X X
.circleincircle./.circleincircle. C 20 A Not given 1/2 -- Co:0.05
F.sup.-:0.2 *9 0 80 30 .circleincircle. .largecircle. X
.circleincircle./.circleincircle. C 21 A Not given 1/1 2.0 Co:0.05
-- 0 80 72 .circleincircle. .DELTA. .circleincircle.
.DELTA./.circleincircle. C *1 Lead-containing galvanized steel
sheet as set forth at any of (A) to (D) above in the specification
*2 Conditions of pre-treatment as set forth at (a) or (b) above *3
Molar ratio *4 Coating weight in terms of metallic chromium
(mg/m.sup.2) *5 96 hours/120 hours *6 I: Sample of the invention;
C: Comparative sample *7 Weight ratio of SiO.sub.2(colloidal
silica)/total Cr ions *8 Weight ratio of acrylic emulsion (solid
resin content)/total Cr ions *9 Molar ratio to total Cr ions [1]:
Steel sheet [2]: Pre-treatment [3]: Composition of chromate coating
solution [4]: Metal ion [5]: Others [6]: Sheet temp. for drying
[7]: Cr coating weight [8]: Galvanized surface appearance [9]: Film
appearance [10]: Anti-black patina property [11]: Anti-white rust
property [12]: Division
[0085]
3 TABLE 3 [3] [1] [2] Cr.sup.3+/Cr.sup.6+ NO.sup.3-/T-Cr [4]
Zn.sup.2+ [6] [7] [11] [12] No. *1 *2 *3 *3 /T-Cr *3 [5] (g/l)
(.degree. C.) *4 [8] [9] [10] *5 *6 22a A a 1/4 0.8 -- -- 0 40 18
.circleincircle. .circleincircle. .circleincircle.
.largecircle./.largecircle. I 22b A a 1/4 0.8 -- -- 0.3 40 17
.circleincircle. .circleincircle. .circleincircle.
.largecircle./.largecircle. I 23a B a 1/4 0.8 -- -- 0 40 19
.largecircle. .circleincircle. .circleincircle.
.largecircle./.largecircle. I 23b B a 1/4 0.8 -- -- 0.3 40 17
.largecircle. .circleincircle. .circleincircle.
.largecircle./.largecircl- e. I 24a C a 1/4 0.8 -- -- 0 40 20 X
.circleincircle. .circleincircle. .largecircle./.largecircle. I 24b
C a 1/4 0.8 -- -- 0.3 40 18 X .circleincircle. .circleincircle.
.largecircle./.largecirc- le. I 25a D a 1/4 0.8 -- -- 0 40 19
.DELTA. .circleincircle. .circleincircle.
.largecircle./.largecircle. I 25b D a 1/4 0.8 -- -- 0.3 40 20
.DELTA. .circleincircle. .circleincircle.
.largecircle./.largecircle. I 26a A a 1/4 0.8 Co:0.04 -- 0 60 21
.circleincircle. .circleincircle. .circleincircle.
.circleincircle./.circleincircle. I 26b A a 1/4 0.8 Co:0.04 -- 0.4
60 19 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.circleincircle. I 27a A a 1/1 1.6 Co:0.20 -- 0
150 32 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.circleincircle. I 27b A a 1/1 1.6 Co:0.20 -- 0.8
150 35 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.circleincircle. I *1 Lead-containing galvanized
steel sheet as set forth at any of (A) to (D) above in the
specification *2 Conditions of pre-treatment as set forth at (a) or
(b) above *3 Molar ratio *4 Coating weight in terms of metallic
chromium (mg/m.sup.2) *5 120 hours/200 hours *6 I: Sample of the
invention; C: Comparative sample [1]: Steel sheet [2]:
Pre-treatment [3]: Composition of chromate coating solution [4]:
Metal ion [5]: Others [6]: Sheet temp. for drying [7]: Cr coating
weight [8]: Galvanized surface appearance [9]: Film appearance
[10]: Anti-black patina property [11]: Anti-white rust property
[12]: Division
[0086]
4 TABLE 4 [3] [1] [2] Cr.sup.3+/Cr.sup.6+ NO.sup.3-/T-Cr [4]
Zn.sup.2+ [6] [7] [11] [12] No. *1 *2 *3 *3 /T-Cr *3 [5] (g/l)
(.degree. C.) *4 [8] [9] [10] *5 *6 28a A a 1/2 1.1 Co:0.10 -- 0 80
18 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.circleincircle. I 28b A a 1/2 1.1 Co:0.10 -- 0.6
80 19 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.circleincircle. I 29a A a 1/9 0.1 Ni:0.06 -- 0
100 20 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 29b A a 1/9 0.1 Ni:0.06 -- 0.8 100
22 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 30a A a 1/5 1.0 Co:0.10 -- 0 80 8
.circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 30b A a 1/5 1.0 Co:0.10 -- 0.6 80
9 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 31a A a 1/5 1.0 Sr:0.10 -- 0 80 22
.circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 31b A a 1/5 1.0 Sr:0.10 -- 0.6 80
21 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 32a A a 1/5 1.0 Ba:0.10 -- 0 80 18
.circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 32b A a 1/5 1.0 Ba:0.10 -- 0.6 80
19 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.largecircle. I 33a A a 1/8 0.2 Co:0.05
SiO.sub.2:4 *7 0 60 10 .circleincircle. .circleincircle.
.circleincircle. .circleincircle./.largecircle. I 33b A a 1/8 0.2
Co:0.05 SiO.sub.2:4 *7 0.2 60 11 .circleincircle. .circleincircle.
.circleincircle. .circleincircle./.largecircle. I *1
Lead-containing galvanized steel sheet as set forth at any of (A)
to (D) above in the specification *2 Conditions of pre-treatment as
set forth at (a) or (b) above *3 Molar ratio *4 Coating weight in
terms of metallic chromium (mg/m.sup.2) *5 120 hours/200 hours *6
I: Sample of the invention; C: Comparative sample *7 Weight ratio
of SiO.sub.2(colloidal silica)/total Cr ions [1]: Steel sheet [2]:
Pre-treatment [3]: Composition of chromate coating solution [4]:
Metal ion [5]: Others [6]: Sheet temp. for drying [7]: Cr coating
weight [8]: Galvanized surface appearance [9]: Film appearance
[10]: Anti-black patina property [11]: Anti-white rust property
[12]: Division
[0087]
5 TABLE 2 [3] [1] [2] Cr.sup.3+/Cr.sup.6+ NO.sup.3-/T-Cr [4]
Zn.sup.2+ [6] [7] [11] [12] No. *1 *2 *3 *3 /T-Cr *3 [5] (g/l)
(.degree. C.) *4 [8] [9] [10] *5 *6 34a A a 1/4 0.8 Co:0.08 resin:5
*8 0 230 41 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.circleincircle. I 34b A a 1/4 0.8 Co:0.08 resin:5
*8 0.4 230 42 .circleincircle. .circleincircle. .circleincircle.
.circleincircle./.circleincircle. I 35 A a 1/19 0.8 Co:0.05 -- 0 40
21 .circleincircle. .circleincircle. .largecircle. X/X C 36 A a 1/4
-- -- -- 0 60 24 .circleincircle. .largecircle. X X/X C 37a A b 1/1
2.0 Co:0.05 -- 0 150 29 .circleincircle. .circleincircle.
.circleincircle. X/X C 37b A b 1/1 2.0 Co:0.05 -- 3.5 150 27
.circleincircle. .circleincircle. .circleincircle. X/X C 38 A a 1/3
0.4 Co:0.08 -- 0 270 20 .circleincircle. .largecircle.
.circleincircle. X/X C 39 A a 1/2 -- Co:0.10 PO.sub.4.sup.3-:0.3 *9
0 80 18 .circleincircle. .circleincircle. X
.circleincircle./.largecir- cle. C 40 A a 1/2 -- Co:0.10
SO.sub.4.sup.2-:0.3 *9 0 80 48 .circleincircle. X X
.circleincircle./.largecircle. C 41 A a 1/2 -- Co:0.05 F.sup.-:0.2
*9 0 80 30 .circleincircle. .largecircle. X
.circleincircle./.largecircle. C 42 A a 1/1 2.0 Co:0.05 -- 0 80 72
.circleincircle. .DELTA. .circleincircle. .circleincircle./.DELTA.
C *1 Lead-containing galvanized steel sheet as set forth at any of
(A) to (D) above in the specification *2 Conditions of
pre-treatment as set forth at (a) or (b) above *3 Molar ratio *4
Coating weight in terms of metallic chromium (mg/m.sup.2) *5 120
hours/200 hours *6 I: Sample of the invention; C: Comparative
sample *7 Weight ratio of SiO.sub.2(colloidal silica)/total Cr ions
*8 Weight ratio of acrylic emulsion (solid resin content)/total Cr
ions *9 Molar ratio to total Cr ions [1]: Steel sheet [2]:
Pre-treatment [3]: Composition of chromate coating solution [4]:
Metal ion [5]: Others [6]: Sheet temp. for drying [7]: Cr coating
weight [8]: Galvanized surface appearance [9]: Film appearance
[10]: Anti-black patina property [11]: Anti-white rust property
[12]: Division
INDUSTRIAL UTILITY
[0088] This invention enables the manufacture of chromate-coated
lead-containing galvanized steel sheets which are suitable as
building materials, or materials for electric appliances.
* * * * *