U.S. patent application number 12/310289 was filed with the patent office on 2009-10-01 for phosphate-treated galvanized steel sheet and method for making the same.
Invention is credited to Satoru Ando, Nobue Fujibayashi, Akiro Matsuzaki, Hiroki Nakamaru, Kenichi Sasaki, Chiyoko Tada.
Application Number | 20090242080 12/310289 |
Document ID | / |
Family ID | 39344351 |
Filed Date | 2009-10-01 |
United States Patent
Application |
20090242080 |
Kind Code |
A1 |
Ando; Satoru ; et
al. |
October 1, 2009 |
PHOSPHATE-TREATED GALVANIZED STEEL SHEET AND METHOD FOR MAKING THE
SAME
Abstract
A method for making a phosphate-treated galvanized steel sheet,
including forming a phosphate film on the surface of a galvanized
layer of a galvanized steel sheet using a phosphate treatment
solution containing Zn.sup.2+ and Mg.sup.2+ so as to satisfy
2.0<Zn.sup.2+.ltoreq.5.0 g/L, 2.0.ltoreq.Mg.sup.2+.ltoreq.5.0
g/L, and 0.4.ltoreq.Mg.sup.2+/Zn.sup.2+.ltoreq.2.5, and satisfying
0.020.ltoreq.free acidity/total acidity<0.10. The making method
allows the quick formation of a uniform phosphate film, whereby a
phosphate-treated galvanized steel sheet having excellent corrosion
resistance and blackening resistance is obtained.
Inventors: |
Ando; Satoru; (Fukuyama,
JP) ; Tada; Chiyoko; (Chiba, JP) ; Nakamaru;
Hiroki; (Chiba, JP) ; Sasaki; Kenichi; (Chiba,
JP) ; Fujibayashi; Nobue; (Kawasaki, JP) ;
Matsuzaki; Akiro; (Kawasaki, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
220 Fifth Avenue, 16TH Floor
NEW YORK
NY
10001-7708
US
|
Family ID: |
39344351 |
Appl. No.: |
12/310289 |
Filed: |
October 31, 2007 |
PCT Filed: |
October 31, 2007 |
PCT NO: |
PCT/JP2007/071590 |
371 Date: |
February 19, 2009 |
Current U.S.
Class: |
148/262 ;
148/320 |
Current CPC
Class: |
C23C 22/22 20130101 |
Class at
Publication: |
148/262 ;
148/320 |
International
Class: |
C23C 22/07 20060101
C23C022/07; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 31, 2006 |
JP |
2006-295589 |
Claims
1. A method for making a phosphate-treated galvanized steel sheet,
comprising forming a phosphate film on the surface of a galvanized
layer of a galvanized steel sheet using a phosphate treatment
solution containing Zn.sup.2+ and Mg.sup.2+ so as to satisfy
2.0<Zn.sup.2+.ltoreq.5.0 g/L, 2.0.ltoreq.Mg.sup.2+.ltoreq.5.0
g/L, and 0.4.ltoreq.Mg.sup.2+/Zn.sup.2+.ltoreq.2.5, and satisfying
0.020.ltoreq.free acidity/total acidity<0.10.
2. The making method of claim 1, wherein the phosphate film is
formed by contacting the galvanized layer surface with the
phosphate treatment solution for 3 to 15 seconds.
3. A phosphate-treated galvanized steel sheet made by the making
method of claim 1, the galvanized steel sheet having thereon a
phosphate film containing Mg in an amount of 0.2.ltoreq.Mg<2.0%
by mass at a coating weight of 0.2 to 3.0 g/m.sup.2.
4. A method for making a phosphate-treated galvanized steel sheet
comprising treating a galvanized steel sheet with a phosphate
treatment solution to form a phosphate film on the surface of the
galvanized steel sheet, wherein the phosphate treatment solution
contains Zn.sup.2+ in an amount of more than 2.0 g/L and 5.0 g/L or
less, Mg.sup.2+ in an amount of from 2.0 to 5.0 g/L, the
concentration ratio of the Mg.sup.2+ to Zn.sup.2+
(Mg.sup.2+/Zn.sup.2+) is from 0.4 to 2.5, and the ratio of the free
acidity to the total acidity in the treatment solution is 0.020 or
more and less than 0.10.
5. A phosphate-treated galvanized steel sheet made by the making
method of claim 2, the galvanized steel sheet having thereon a
phosphate film containing Mg in an amount of 0.2.ltoreq.Mg<2.0%
by mass at a coating weight of 0.2 to 3.0 g/m.sup.2.
Description
TECHNICAL FIELD
[0001] The present invention relates to a surface-treated steel
sheet used mainly for building and home appliance applications, and
specifically to a phosphate-treated galvanized steel sheet suitable
as a steel substrate to be coated, and a method for making the
same.
BACKGROUND ART
[0002] Galvanized steel sheets coated with zinc or zinc alloys are
used in areas requiring corrosion resistance for building and home
appliance applications. Such galvanized steel sheet is rarely used
as it is. In usual cases, a coating is applied onto the galvanized
layer of the sheet. Further, before the application of a coating,
the sheet is usually subjected to chemical treatment such as
phosphate treatment or chromate treatment.
[0003] The phosphate treatment is carried out by contacting an
acidic solution containing phosphate ions with a galvanized steel
sheet to allow them to react, thereby forming a crystalline film
composed mainly of zinc phosphate on the coating surface. The
phosphate treatment improves coating adhesion, whereby primary
coating properties stable to various coatings are made available.
Therefore, galvanized steel sheets treated with phosphate are
widely used as steel substrates to be coated for building and home
appliance applications. Further, in recent years, in order to
improve the corrosion resistance of phosphate films, techniques for
forming a zinc phosphate film containing Mg have been disclosed in
many patent documents.
[0004] For example, Japanese Unexamined Patent Application
Publication No. 2002-285346 discloses a zinc phosphate-treated
galvanized steel sheet with excellent corrosion resistance and
color tone, the steel sheet having a zinc phosphate film containing
2.0% or more of Mg and 0.01 to 1% of at least one element selected
from Ni, Co, and Cu at a coating weight of 0.7 g/m.sup.2 or
more.
[0005] However, under the technique, the zinc phosphate film
contains a large amount of Mg, so that the surface of the steel
sheet coated with the phosphate film may be discolored black, or
blackened when exposed to high temperatures and humidity. There is
another problem that the color tone of the zinc phosphate film is
dark because the film contains Ni, Co, and/or Cu at high
concentrations.
[0006] Japanese Patent No. 2680618 discloses a technique for
preventing the formation of spots of phosphate crystals through the
treatment of a galvanized steel or an aluminum-zinc coated steel
sheet with a magnesium zinc phosphate-based aqueous solution
containing 0.4 to 2.0 g/L of Zn, 0.4 to 5.0 g/L of Mg, and 0.05 to
2.0 g/L of Ni, and 8.0 to 20.0 g/L of P.sub.2O.sub.5, wherein the
ratio of the free acid content to the total acid content (free
acidity/total acidity) in the solution is from 0.02 to 0.15.
[0007] Under the technique, in order to densely form phosphate
crystals, the treatment requires a relatively long period of time
of 20 seconds to 10 minutes. When the treatment is followed by
after treatment such as electroplating, the above treatment time is
preferably as short as possible from the viewpoint of production
efficiency, but phosphate crystals tend to be incompletely formed
with a short treatment such as several seconds, which may result in
local vacancies of phosphate crystals.
[0008] Japanese Patent No. 2770860 discloses a technique for
quickly forming a phosphate film with a white color tone through
the treatment with a phosphate aqueous solution containing 0.5 to
5.0 g/L of Zn, 0.3 to 3.0 g/L of Mg, and 3.0 to 20.0 g/L of
P.sub.2O.sub.5, wherein the ratio of the free acid content to the
total acid content (free acidity/total acidity) in the solution is
from 0.1 to 0.4.
[0009] Under the technique, the free acid concentration is
increased thereby enhancing the etching effect on zinc in a
galvanized steel sheet. However, continuous treatment of a steel
sheet tends to result in development of streaks, depending on the
surface state of the galvanized steel sheet. This is likely due to
the fact that the difference between the levels of local reactivity
of the zinc surface layer becomes obvious through the treatment
with a high etching effect, which results in the development of
macroscopic flaws.
[0010] The object of the invention is to provide a method for
making a phosphate-treated galvanized steel sheet which allows the
quick formation of a uniform phosphate film, and a
phosphate-treated galvanized steel sheet having excellent corrosion
resistance and blackening resistance made by the method.
DISCLOSURE OF INVENTION
[0011] An aspect of the present invention is a method for making a
phosphate-treated galvanized steel sheet, including forming a
phosphate film on the surface of a galvanized layer of a galvanized
steel sheet using a phosphate treatment solution containing
Zn.sup.2+ and Mg.sup.2+ so as to satisfy
2.0<Zn.sup.2+.ltoreq.5.0 g/L, 2.0.ltoreq.Mg.sup.2+.ltoreq.5.0
g/L, and 0.4.ltoreq.Mg.sup.2+/Zn.sup.2+.ltoreq.2.5, and satisfying
0.020.ltoreq.free acidity/total acidity<0.10.
[0012] In the making method, the phosphate film is preferably
formed by contacting the galvanized layer surface with the
phosphate treatment solution for 3 to 15 seconds.
[0013] Another aspect of the present invention is a
phosphate-treated galvanized steel sheet made by any of the above
making methods, the galvanized steel sheet having thereon a
phosphate film containing Mg in an amount of 0.2.ltoreq.Mg<2.0%
by mass at a coating weight of 0.2 to 3.0 g/m.sup.2.
[0014] Yet another aspect of the present invention is a method for
making a phosphate-treated galvanized steel sheet including
treating a galvanized steel sheet with a phosphate treatment
solution to form a phosphate film on the surface of the galvanized
steel sheet, wherein the phosphate treatment solution contains
Zn.sup.2+ in an amount of more than 2.0 g/L and 5.0 g/L or less,
Mg.sup.2+ in an amount of from 2.0 to 5.0 g/L, the concentration
ratio of the Mg.sup.2+ to Zn.sup.2+ (Mg.sup.2+/Zn.sup.2+) is from
0.4 to 2.5, and the ratio of the free acidity to the total acidity
in the treatment solution is 0.020 or more and less than 0.10.
BEST MODE FOR CARRYING OUT THE INVENTION
[0015] As a result of dedicated research to solve the above
problems, the inventors have found that a uniform phosphate film is
quickly formed on a galvanized steel sheet through the use of a
phosphate treatment solution containing a zinc ion and a magnesium
ion, wherein the zinc ion level, the magnesium ion level, and the
concentration ratio of the magnesium ion to the zinc ion are within
specific ranges, and the ratio of the free acidity to the total
acidity is optimum. They have also found that the resultant
phosphate-treated galvanized steel sheet has excellent corrosion
resistance and blackening resistance. The present invention has
been accomplished on the basis of the findings.
[0016] The structure of the present invention and the reason for
the numerical limitation of each essential feature are described
below.
[0017] The phosphate-treated galvanized steel sheet obtained by the
method of the present invention is composed of a galvanized steel
sheet having thereon a phosphate film containing 0.2% or more and
less than 2.0% by mass of Mg, at a coating weight of 0.2 to 3.0
g/m.sup.2.
[0018] (Galvanization)
[0019] The galvanized steel sheet as the steel substrate for the
steel sheet of the present invention may be any galvanized steel
sheet, for example, a hot dip galvanized steel sheet, an
electrogalvanized steel sheet, a galvannealed steel sheet, an
aluminum-zinc alloy-coated steel sheet (for example, a molten
zinc-55% by mass aluminum alloy-coated steel sheet, or a molten
zinc-5% by mass aluminum alloy-coated steel sheet), an iron-zinc
alloy-coated steel sheet, a nickel-zinc alloy-coated steel sheet,
or a nickel-zinc alloy-coated steel sheet after blackening
treatment. The steel sheet as the substrate is not particularly
limited as long as it is suitable for use as a galvanized steel
sheet, and may be appropriately selected according to the intended
use. The coating weight of the galvanized layer may be
appropriately selected according to the intended use, and is
preferably from 1 to 100 g/m.sup.2. When the coating weight is 1
g/m.sup.2 or more, sufficient corrosion resistance is achieved.
However, a coating weight of more than 100 g/m.sup.2 is wasteful,
in terms of cost. The coating weight is more preferably from 5 to
70 g/m.sup.2.
[0020] (Phosphate Film)
[0021] The galvanized steel sheet has on at least one side thereof
a phosphate film containing 0.2% by mass or more and less than 2.0%
by mass of Mg, at a coating weight of 0.2 to 3.0 g/m.sup.2.
[0022] The phosphate film is formed mainly for improving the
adhesion between the galvanized layer and coating, and more
preferably improves corrosion resistance as well as the adhesion.
The Mg content of the phosphate film is preferably 0.2% by mass or
more and less than 2.0% by mass. When the content is 0.2% by mass
or more, sufficient corrosion resistance is achieved, and when the
content is less than 2.0% by mass, excellent blackening resistance
is achieved. The Mg content is more preferably from 0.5 to 1.0% by
mass. The phosphate film may contain unavoidable impurities such as
Ni, Mn, and Co within a range of 0.01 to 0.4% by mass.
[0023] The coating weight of the phosphate film is preferably from
0.2 to 3.0 g/m.sup.2. When the coating weight is 0.2 g/m.sup.2 or
more, sufficient corrosion resistance is achieved, and when the
coating weight is 3.0 g/m.sup.2 or less, coarsening of the
phosphate crystals in the phosphate film is rather inhibited, which
results in the improvement of the coating adhesion.
[0024] The phosphate film is formed by contacting the surface of
the galvanized layer with the below-described phosphate treatment
solution. The contact method is not particularly limited, and may
be an ordinary method such as spraying or immersion.
[0025] The treatment time with the phosphate treatment solution is
preferably from 3 to 15 seconds. When the treatment time is 3
seconds or more, the phosphate film is readily formed, and when the
treatment time is 15 seconds or less, etching by the phosphate
treatment solution is rather inhibited, which facilitates the
formation of a more uniform phosphate film.
[0026] Before the formation of the phosphate film, it is preferable
that the galvanized layer be subjected to surface conditioning
treatment using a colloidal titanium active treatment agent.
Examples of the colloidal titanium active treatment agent include
"PREPALENE ZN" manufactured by Nihon Parkerizing Co., Ltd. The
surface conditioning treatment may be carried out by spraying the
treatment agent on the surface of the galvanized layer.
[0027] The method of the present invention for making a
phosphate-treated galvanized steel sheet includes forming a
phosphate film on the surface of a galvanized layer of a galvanized
steel sheet using a phosphate treatment solution containing
Zn.sup.2+ and Mg.sup.2+ so as to satisfy
2.0<Zn.sup.2+.ltoreq.5.0 g/L, 2.0.ltoreq.Mg.sup.2+.ltoreq.5.0
g/L, and 0.4.ltoreq.Mg.sup.2+/Zn.sup.2+.ltoreq.2.5, and satisfying
0.020.ltoreq.free acidity/total acidity<0.10. In the present
description, the liter unit is expressed as "L".
2.0<Zn.sup.2+.ltoreq.5.0 g/L
[0028] Zn.sup.2+ is an essential component for forming phosphate
crystals, so that the Zn.sup.2+ concentration in the phosphate
treatment solution must be more than 2.0 g/L and 5.0 g/L or less,
and is more preferably from 3.0 to 5.0 g/L. If the concentration is
2.0 g/L or less, the phosphate insufficiently deposits, which
results in the formation of a nonuniform phosphate film locally
devoid of phosphate crystals, and if more than 5.0 g/L, the
phosphate crystals are coarsened, which results in the failure to
achieve sufficient corrosion resistance of the phosphate film.
2.0.ltoreq.Mg.sup.2+.ltoreq.5.0 g/L
[0029] Mg.sup.2+ is an essential component for improving the
corrosion resistance of the phosphate film, so that the Mg.sup.2+
concentration in the phosphate treatment solution must be from 2.0
to 5.0 g/L, and is more preferably from 2.5 to 5.0 g/L. If the
concentration is less than 2.0 g/L, inclusion of the magnesium
component is so low that the corrosion resistance of the zinc
phosphate film deteriorates, and if more than 5.0 g/L, the content
of the magnesium components is so high that the blackening
resistance of the zinc phosphate film deteriorates. The Mg.sup.2+
concentration varies depending on the concentration ratio of
Mg.sup.2+ to Zn.sup.2+ (Mg.sup.2+/Zn.sup.2+) in the below-described
phosphate aqueous solution, so that the Mg.sup.2+ concentration
must be adjusted within an appropriate range of
Mg.sup.2+/Zn.sup.2+.
0.4.ltoreq.Mg.sup.2+/Zn.sup.2+.ltoreq.2.5
[0030] In order to form a phosphate film containing an appropriate
amount of Mg, in the present invention, the concentration ratio of
the magnesium ion to the zinc ion in the phosphate treatment
solution (Mg.sup.2+/Zn.sup.2+) is defined as from 0.4 to 2.5, and
more preferably from 0.8 to 1.2. If Mg.sup.2+/Zn.sup.2+ is less
than 0.4, the Mg.sup.2+ concentration in the treatment solution is
less than 2.0 g/L, so that Zn is preferentially taken into the
phosphate film of the product, which results in a decrease of the
ratio of Mg to Zn that deteriorates the corrosion resistance of the
zinc phosphate film. On the other hand, if Mg.sup.2+/Zn.sup.2+ is
more than 2.5, the Mg.sup.2+ concentration in the treatment
solution is more than 5.0 g/L, the ratio of Mg to Zn in the
phosphate film of the product is out of the appropriate range, and
the blackening resistance of the zinc phosphate film
deteriorates.
[0031] In addition to the above-described conditions, the phosphate
treatment solution preferably has a temperature of from 30 to
70.degree. C., and a pH of from 1.0 to 2.5. The reasons for these
ranges are as follows.
[0032] Firstly, under the conditions, the Mg salt readily dissolves
in the phosphate treatment solution, which facilitates optimization
of the Mg.sup.2+ concentration in the solution.
[0033] Secondly, the phosphate treatment solution is more reactive
at a liquid temperature of 30.degree. C. or higher, which
facilitates quick formation of a uniform film. In addition, when
the liquid temperature is 70.degree. C. or lower, etching is rather
inhibited and the phosphate readily deposits, which markedly
facilitates the control of the treatment time. Further, when the pH
is 1.0 or more, etching rarely occurs and the film readily
deposits, which facilitates the control of the treatment time as
described above. In addition, when the pH is 2.5 or less, the
treatment solution is stable.
[0034] The inventors also studied the selection of the anion
countering Mg.sup.2+ in the treatment solution. The anion is
preferably a nitrate ion. The anion may be a hydroxide ion, a
carbonate ion, or a sulfate ion, but Mg salts of these ions have
rather inferior solubility. When a chloride ion is used as the
anion, the Mg salt has sufficient solubility, but chlorine ions may
be included in the phosphate treatment solution concurrently with
Mg.sup.2+ to cause a deleterious effect. On the other hand, a
nitrate ion has an oxidative effect and is less likely to remain in
the film components than other anions, and thus further improves
the performance of the phosphate film. Accordingly, the anion is
preferably a nitrate ion, and the Mg ion source in the treatment
solution is preferably magnesium nitrate. The phosphate treatment
solution used in the present invention is preferably a commercial
treatment solution containing a zinc ion, a phosphate ion, and
other additives such as a promoter, and examples of the treatment
solution include "PB3312M" (trade name) manufactured by Nihon
Parkerizing Co., Ltd. mixed with a specified amount of the nitrate
ion.
0.020.ltoreq.free acidity/total acidity<0.10
[0035] The phosphate film is formed as follows: the pH at the
solid-liquid interface of the treatment solution is increased by
the etching action of the free orthophosphoric acid (free acid) in
the treatment solution on the plated surface, and the concentration
equilibrium between zinc dihydrogenphosphate
(Zn(H.sub.2PO.sub.4).sub.2) and orthophosphoric acid
(H.sub.3PO.sub.4) in the treatment solution changes, so that the
zinc dihydrogenphosphate deposits as zinc phosphate crystals
containing magnesium. Accordingly, in the formation of the
phosphate film, the free acid plays a very important role.
Accordingly, the inventors focused attention on the etching action
of the free acid, and eagerly studied a method for forming a
uniform phosphate film through short treatment (about 3 to 15
seconds).
[0036] As a result of this, they have found that (i) the increase
of the free acid concentration enhances the etching effect on zinc
plating, and the surface state becomes nonuniform by the degreasing
and surface conditioning processes conducted as pretreatment before
the phosphate treatment, so that an uneven phosphate film is
formed, and that (ii) the increase of the free acid concentration
hinders the deposition of zinc phosphate crystals, so that no
phosphate film is formed in some areas with short treatment for
several seconds. As a result of further research, they have also
found that the optimization of the ratio of the free acidity to the
total acidity in a lower range than in the prior art allows the
deposition of phosphate crystals on the same level as in the prior
art while controlling the etching effect, whereby a uniform
phosphate film is quickly formed.
[0037] The free acid (orthophosphoric acid) concentration is
preferably from 0.5 to 3.4 in terms of free acidity, and more
preferably from 1.0 to 3.0. The total acidity is preferably from 20
to 26, which must include the described free acidity.
[0038] The ratio of the free acidity to the total acidity (free
acidity/total acidity) must be 0.020 or more and less than 0.10,
and is more preferably from 0.035 to 0.096. If the ratio is less
than 0.020, the free acid concentration is so low that the etching
effect on zinc is poor, and reaction necessary for deposition of
phosphate crystals is rather hindered, which results in the failure
to form a sufficient phosphate film. In addition, stability of the
phosphate treatment solution deteriorates, and zinc and solids,
which are likely phosphate compounds containing iron occurring as
an impurity, deposit and disperse in the treatment solution. On the
other hand, if the concentration is 0.10 or more, after short
treatment for few seconds, the phosphate film may have flaws due to
the nonuniform surface state of zinc.
[0039] The term free acidity is determined as follows: several
drops of bromophenol blue as an indicator are added to 10 ml of the
phosphate treatment solution, the treatment solution is titrated
with 0.1 N caustic soda, and the amount of 0.1 N caustic soda (ml)
used for the neutralization is expressed as an absolute number. In
the same manner, the total acidity is determined as follows:
several drops of phenolphthalein as the indicator are added to 10
ml of the phosphate treatment solution, the treatment solution is
titrated with 0.1 N caustic soda, and the amount of 0.1 N caustic
soda (ml) used for the neutralization is expressed as an absolute
number.
[0040] The above-described embodiment is only an example of the
embodiments of the present invention, and various modifications
thereof may be made.
EXAMPLES
[0041] Examples of the present invention are described below.
Examples 1 to 16 and Comparative Examples 1 to 9
[0042] A cold rolled steel sheet having a thickness of 1.0 mm was
subjected to, as pretreatment, electrolytic degreasing for 30
seconds at a current density of 5 A/dm.sup.2 in an alkali
degreasing liquid (liquid temperature: 70.degree. C.) containing
sodium orthosilicate (60 g/L), with stainless steel as the counter
electrode. The steel sheet was washed with water, immersed in a 30
g/L sulfuric acid aqueous solution (liquid temperature: 30.degree.
C.) for 5 seconds for pickling, and then washed with water. The
pretreated steel sheet was subjected to electrogalvanizing
treatment thereby forming a galvanized layer on one side of the
steel sheet at a coating weight of 20 g/m.sup.2. For the
electrogalvanizing treatment, a galvanizing bath filled with a zinc
plating solution containing 440 g/L of zinc sulfate heptahydrate
was used. The pH of the zinc plating solution was adjusted to 11.5
with sulfuric acid. The temperature of the galvanizing bath was
50.degree. C. In the electrogalvanizing bath, the counter electrode
was iridium oxide-coated Ti plate electrode, which was disposed in
parallel with the test plate at a distance of 10 mm. A current was
passed at a current density of 70 A/dm.sup.2 with the plating
solution circulated between the electrodes at a flow rate of 1.5
m/s.
[0043] As described above, a galvanized layer was formed on the
steel sheet surface, washed with water, and then subjected to
phosphate treatment.
[0044] As pretreatment before the phosphate treatment, the
galvanized layer surface was treated with a surface conditioner
(trade name "PREPALENE Z", manufactured by Nihon Parkerizing Co.,
Ltd.). The galvanized layer was then sprayed with a phosphate
treatment solution (a mixture of "PB3312M" manufactured by Nihon
Parkerizing Co., Ltd. and magnesium nitrate) with the spraying time
varied as appropriate, washed with water, and dried to form a
phosphate film. The phosphate treatment solution had a temperature
of 60.degree. C., and a pH of 2.1 to 2.7, which differed among
examples and comparative examples. All the treatment solutions
contained Ni in an amount of 0.1 to 0.4 g/L.
[0045] The Zn.sup.2+ concentration, Mg.sup.2+ concentration, and
free acidity and total acidity in the phosphate treatment solution
were varied as follows. The free acidity and total acidity in the
examples and comparative examples were varied by controlling the
concentration of "PB3312M" and adding as necessary a sodium
hydroxide aqueous solution, orthophosphoric acid, and nitric acid.
The Zn.sup.2+ concentration was varied by changing the initial
concentration of "PB3312M", and the Mg.sup.2 concentration was
varied by changing the content of magnesium nitrate.
[0046] The Mg content of the phosphate film was measured by
dissolving the phosphate treated layer with an ammonium dichromate
aqueous solution, and analyzing the solution by ICP
(inductively-coupled plasma atomic emission spectrometry). The
phosphate film coating weight was varied by changing the period of
contact with the phosphate treatment solution. The phosphate film
coating weight was measured by a gravimetric method using a
solution of the film dissolved with an ammonium dichromate aqueous
solution.
[0047] Table 1 lists the Zn.sup.2+ concentration, Mg.sup.2+
concentration, Mg.sup.2+/Zn.sup.2+ ratio, free acidity, total
acidity, and free acidity/total acidity ratio in the phosphate
treatment solution in each of the examples and comparative
examples, and the Mg content and coating weight of the phosphate
film on each of the phosphate-treated galvanized steel sheets.
[0048] The phosphate-treated galvanized steel sheets obtained as
described above were subjected to various tests. The criteria for
the tests conducted in the examples are described below.
[0049] (1) Appearance Uniformity
[0050] The surface appearance after the phosphate treatment was
visually observed, and the uniformity after the phosphate treatment
was evaluated on the basis of the following criteria:
[0051] .largecircle.: uniform appearance
[0052] x: nonuniform appearance
[0053] (2) Crystallization Condition
[0054] Crystallization condition was evaluated on the basis of the
presence or absence of local vacancies of phosphate crystals in the
phosphate film observed with SEM. Randomly chosen ten areas (100
.mu.m.times.100 .mu.m) in the central visual field on the
150.times.70 mm.sup.2 specimen excluding the fringe areas of 20 mm
from the edge of the specimen were observed with an electron
microscope at a magnification of 1000, and the number of points
having no phosphate crystal with a diameter of 20 .mu.m was counted
in each area. The average number of the points having no phosphate
crystal counted in the ten areas was evaluated on the basis of the
following criteria:
[0055] .largecircle.: less than 3
[0056] .DELTA.: 3 or more and less than 10
[0057] x: 10 or more
[0058] (3) Corrosion Resistance
[0059] Corrosion resistance was evaluated as follows: a specimen
(size: 100.times.50 mm) was cut out from each of the
phosphate-treated galvanized steel sheets made above, and the edges
and back side of the specimen were sealed with tape, and then
subjected to the salt spray test according to JIS Z 2371-2000. The
top surface of the specimen was periodically observed, and the time
until the ratio of the white rust area became 5% with reference to
the total measuring area on the specimen (white rust formation
time) was measured, and evaluated on the basis of the following
criteria:
[0060] .circle-w/dot.: 24 hours or more
[0061] .largecircle.: 8 hours or more and less than 24 hours
[0062] .DELTA.: 4 hours or more and less than 8 hours
[0063] x: less than 4 hours
[0064] (4) Blackening Resistance
[0065] Blackening resistance was evaluated as follows: a specimen
(size: 100.times.50 mm) was cut out from each of the
phosphate-treated galvanized steel sheets made above, and the
initial L value (lightness) of the specimen was measured using a
spectroscopic color-difference meter SQ2000 (manufactured by Nippon
Denshoku Industries Co., Ltd.). Subsequently, the specimen was
allowed to stand for 24 hours in a constant temperature and
humidity bath at a temperature of 80.degree. C. and a relative
humidity of 95%. After standing, the L value of the specimen was
measured in the same manner, and the amount of change .DELTA.L from
the initial L value (L value after standing--initial L value) was
calculated and evaluated on the basis of the following
criteria:
[0066] .circle-w/dot.: .DELTA.L.gtoreq.-1
[0067] .largecircle.: -1>.DELTA.L.gtoreq.-2
[0068] .DELTA.: -2>.DELTA.L.gtoreq.-4
[0069] x: .DELTA.L<-4
[0070] (5) Coating Adhesion
[0071] Coating adhesion was evaluated as follows: a specimen
(70.times.150 mm) was coated with an alkyd melamine-based paint
(DELICON #700 manufactured by Dai Nippon Toryo Co., Ltd., dried at
130.degree. C. for 30 minutes, film thickness: 28.+-.5 .mu.m)
without pretreatment such as degreasing, incised with a cutter to
make cross cuts (10.times.10 grid at intervals of 1 mm), and then
subjected to Erichsen extrusion at a height of 5 mm. A piece of
cellophane adhesive tape (type C LP-18) manufactured by Nichiban
Co., Ltd. was affixed to the crosscut area after the Erichsen
extrusion, and tightly contacted thereon using a spatula.
Thereafter, the tape was removed, and the coating residual rate was
measured and evaluated on the basis of the following criteria:
[0072] .largecircle.: 100%
[0073] .DELTA.: 90% or more and less than 100%
[0074] x: less than 90%
[0075] Table 1 shows the results of the evaluation results in the
above tests.
[0076] These results indicate that the phosphate-treated galvanized
steel sheets of Examples 1 to 16 had favorable appearance
uniformity, crystallization condition, corrosion resistance,
blackening resistance, and coating adhesion, and that the quickly
formed phosphate films also had sufficient performance.
TABLE-US-00001 TABLE 1 Phosphate treatment solution Treatment
Zn.sup.2+ concentration Mg.sup.2+ concentration Mg.sup.2+/Zn.sup.2+
Free Total Free acidity time Test No. (g/L) (g/L) Concentration
ratio acidity acidity Total acidity (second) Example 1 3.5 2.5 0.71
2.1 22.0 0.095 4.5 Example 2 3.3 3.0 0.91 1.6 20.7 0.077 4.2
Example 3 3.1 4.1 1.32 1.9 20.1 0.095 4.8 Example 4 3.0 2.4 0.80
1.3 21.0 0.062 5.3 Example 5 3.1 2.0 0.65 2.1 25.1 0.084 10.5
Example 6 3.5 2.8 0.80 2.4 26.3 0.091 11.2 Example 7 4.5 3.6 0.80
2.4 25.0 0.096 10.4 Example 8 3.0 4.5 1.50 2.1 23.0 0.091 5.2
Example 9 2.5 2.8 1.12 0.9 26.0 0.035 4.5 Example 10 3.1 4.5 1.45
2.2 24.0 0.092 4.5 Example 11 4.5 2.1 0.47 2.2 25.0 0.088 3.0
Example 12 2.5 3.3 1.32 2.5 26.0 0.096 3.8 Example 13 2.1 4.1 1.95
2.1 23.0 0.091 3.0 Example 14 3.1 4.1 1.32 1.9 20.1 0.095 16.0
Example 15 3.1 4.1 1.32 1.9 20.1 0.095 30.0 Example 16 3.1 4.1 1.32
1.9 20.1 0.095 2.5 Comparative 1.8 2.5 1.39 1.3 20.0 0.065 2.9
Example 1 Comparative 5.1 5.2 1.02 3.1 24.0 0.129 10.8 Example 2
Comparative 3.1 2.4 0.77 2.8 22.0 0.127 4.5 Example 3 Comparative
3.2 0.1 0.03 2.1 22.0 0.095 10.5 Example 4 Comparative 2.1 2.0 0.95
0.5 26.0 0.019 19.0 Example 5 Comparative 2.1 5.2 2.48 2.2 23.0
0.096 10.5 Example 6 Comparative 5.1 3.2 0.63 2.1 28.0 0.075 25.0
Example 7 Comparative 3.2 2.6 0.81 4.3 29.0 0.148 30.0 Example 8
Comparative 2.1 6.2 2.95 2.2 23.0 0.096 15.0 Example 9
Crystalization Phosphate film condition Mg content Coverage
Appearance (SEM Corrosion Blackening Coating Test No. (mass %)
(g/m.sup.2) uniformity observation) resistance resistance adhesion
Example 1 0.8 1.8 .largecircle. .largecircle. .circle-w/dot.
.circle-w/dot. .largecircle. Example 2 0.9 1.9 .largecircle.
.largecircle. .circle-w/dot. .circle-w/dot. .largecircle. Example 3
0.9 1.8 .largecircle. .largecircle. .circle-w/dot. .circle-w/dot.
.largecircle. Example 4 0.8 1.7 .largecircle. .largecircle.
.circle-w/dot. .circle-w/dot. .largecircle. Example 5 0.6 1.6
.largecircle. .largecircle. .circle-w/dot. .circle-w/dot.
.largecircle. Example 6 0.9 2.1 .largecircle. .largecircle.
.circle-w/dot. .circle-w/dot. .largecircle. Example 7 0.9 1.9
.largecircle. .largecircle. .circle-w/dot. .circle-w/dot.
.largecircle. Example 8 1.1 1.7 .largecircle. .largecircle.
.circle-w/dot. .largecircle. .largecircle. Example 9 0.8 1.6
.largecircle. .largecircle. .circle-w/dot. .circle-w/dot.
.largecircle. Example 10 1.8 1.9 .largecircle. .largecircle.
.circle-w/dot. .largecircle. .largecircle. Example 11 0.3 1.5
.largecircle. .largecircle. .largecircle. .circle-w/dot.
.largecircle. Example 12 1.3 1.9 .largecircle. .largecircle.
.circle-w/dot. .largecircle. .largecircle. Example 13 1.9 1.2
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. Example 14 0.9 2.5 .largecircle. .largecircle.
.circle-w/dot. .DELTA. .largecircle. Example 15 0.9 2.6
.largecircle. .largecircle. .circle-w/dot. .DELTA. .DELTA. Example
16 0.9 1.5 .largecircle. .DELTA. .largecircle. .largecircle.
.largecircle. Comparative 0.8 0.9 X .DELTA. X .DELTA. .DELTA.
Example 1 Comparative 1.1 1.9 X X .DELTA. .largecircle. .DELTA.
Example 2 Comparative 0.9 1.9 .largecircle. X .DELTA. .largecircle.
.largecircle. Example 3 Comparative 0.2 1.8 .largecircle.
.largecircle. X .largecircle. .largecircle. Example 4 Comparative
0.5 1.9 X X X .largecircle. .DELTA. Example 5 Comparative 1.9 2.1 X
X .largecircle. .largecircle. .DELTA. Example 6 Comparative 0.8 1.9
X X .DELTA. .largecircle. .DELTA. Example 7 Comparative 0.8 1.6 X X
.circle-w/dot. .circle-w/dot. .largecircle. Example 8 Comparative
2.2 2.1 X X .largecircle. .largecircle. .DELTA. Example 9
INDUSTRIAL APPLICABILITY
[0077] According to the making method of the present invention, a
uniform phosphate film is quickly formed, and thus a
phosphate-treated galvanized steel sheet superior to known
anticorrosive coated steel materials in corrosion resistance and
blackening resistance is obtained. The phosphate-treated galvanized
steel sheet is widely useful as a steel substrate to be coated for
building and home appliance applications, and thus markedly
contributes to the industry.
* * * * *