STEEL SHEET FOR A FUEL TANK
Technical Field
[0001]
The present invention relates to a steel sheet for a fuel tank.
Background Art
[0002]
With tightening environmental regulation in recent years, market needs for
materials that do not contain harmful metals have been increased. In the field of
automobiles, the switching from a lead-tin alloy plated steel sheet, which has been a
15 main ingredient of fuel tanks, to a material not containing lead has been promoted.
To a unique required performance of the fuel tanks, which is high inner corrosion
resistance under the enviromnent in which a fuel is enclosed (hereinafter, also
referred to as fuel corrosion resistance), many suggestions are given involving using
zinc-based plated steel sheets, which achieve steady success as inner and outer sheets
20 of automobiles (for example, see the following Patent Literatures 1 to 3).
[0003]
The technologies using zinc-based plated steel sheets described above each
have an assumption that chromate treatment is performed, and therefore are
incompatible with the recent market needs for not using harmful metals. For this
25 reason, as described in the following Patent Literatures 4 to 9, a chromate-free zincbased
plated steel sheet that does not contain chromium for a fuel tank is disclosed.
Citation List
Patent Literature
30 [0004]
Patent Literature 1: JP H5-106058A
5
10
Patent Literature 2:
Patent Literature 3:
Patent Literature 4:
Patent Literature 5:
Patent Literature 6:
Patent Literature 7:
Patent Literature 8:
Patent Literature 9:
Technical Problem
[0005]
PCT/JP2016/052911
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JP H9-324279A
JP H9-324281A
JP 2004-169122A
JP 2007-186745A
JP 2013-133527A
JP 2013-227646A
JP 2011-38139A
W02007/011008
Summary of Invention
However, compared to the fuel corrosion resistance of the conventional
zinc-based plated steel sheet subjected to the chromate treatment, the fuel corrosion
15 resistance of the chromate-free zinc-based plated steel sheets disclosed in the Patent
Literatures 4 to 9 are not sufficient in fuel corrosion resistance under a severer
conditions (for example, corrosion resistance over a longer time period, and
corrosion resistance in the case where damage in the coating fihn has occurred
during processing), and requires further improvement.
20 [0006]
For example, Patent Literatures 4 and 5 each disclose a chromate-free
coating fihn mainly containing an organic resin. However, when such a chromatefree
coating film mainly containing an organic resin is exposed to a fuel environment
for a long time period, the organic resin is swelled by the fuel, and adhesion between
25 surfaces of plating decreases. It is assumed that such swelling of the organic resin
is a factor in insufficient corrosion resistance.
[0007]
Patent Literatures 6 and 7 each disclose a phosphoric acid-based inorganic
chromate-free coating film. However, the phosphoric acid-based inorganic
30 chromate-free coating film does not necessarily have sufficient water resistance, and
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lacks corrosion resistance m particular m the case where a fuel contains dew
condensation water.
[0008]
Patent Literature 8 describes that a chromate-free chemical conversiOn
5 treated layer is formed on a zinc-nickel alloy plated layer having a crack. However,
the chemical conversion treated layer mainly contains a urethane-based aqueous
resin, and is improved in corrosion resistance, but is not imparted with the corrosion
resistance with respect to deteriorated gasoline. Further, as will be described later,
phosphoric acid, vanadium, titanium, and zirconium contained in the chemical
10 conversion treated film are not sufficient for imparting the corrosion resistance with
respect to deteriorated gasoline.
[0009]
Patent Literature 9 discloses a steel sheet obtained by coating a zinc-based
plated steel sheet with an aqueous metal surface treatment agent containing an
15 organosilicon compound. However, the steel sheet is improved in corrosion
resistance, but is not imparted with corrosion resistance with respect to deteriorated
gasoline.
[0010]
Accordingly, the present invention has been made in view of the above
20 problems, and the present invention aims to provide a steel sheet for a fuel tank,
which can exhibit excellent corrosion resistance under an environment in which
various fuels exist, the fuels including a fuel containing an organic acid such as
deteriorated gasoline and a fuel containing dew condensation water in addition to an
organic acid, without using environmental burden substances such as lead and
25 chromium.
Solution to Problem
[0011]
The inventors of the present invention have investigated improvement in
30 fuel corrosion resistance in a chromate-free zinc-based plated steel sheet, and have
found that remarkable improvement can be obtained by making a zinc-based plated
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layer to be a Zn-Ni alloy plated layer, forming a predetermined crack on the plated
layer, making a chromate-free chemical conversion coating film to mainly contain
inorganic substances, and imparting a surface of the coating film with water
repellency.
5 The gist of the present invention accomplished on the basis of the above
10
finding is as follows.
[0012]
(1)
A steel sheet for a fuel tank, including:
a Zn-Ni alloy plated layer which is placed on one surface or each of both
surfaces of a base metal and formed on at least one surface; and
an inorganic chromate-free chemical conversion coating film which IS
placed over the Zn-Ni alloy plated layer, in which
the Zn-Ni alloy plated layer has a crack starting from an interface between
15 the Zn-Ni alloy plated layer and the inorganic chromate-free chemical conversion
coating film and reaching an interface between the Zn-Ni alloy plated layer and the
steel sheet, and
a water contact angle on a surface of the inorganic chromate-free chemical
conversion coating film is more than or equal to 50 degrees.
20 (2)
The steel sheet for a fuel tank according to (1 ), in which
one surface of the base metal has the Zn-Ni alloy plated layer and the
inorganic chromate-free chemical conversion coating film, and a surface opposite to
the one surface does not have the Zn-Ni alloy plated layer and the inorganic
25 chromate-free chemical conversion coating film.
(3)
The steel sheet for a fuel tank according to (2), in which
a water contact angle on the surface not having the Zn-Ni alloy plated layer
and the inorganic chromate-free chemical conversion coating film is less than 10
30 degrees.
(4)
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The steel sheet for a fuel tank according to (2) or (3), in which
a deposition amount of Zn and/or Ni on the surface not having the Zn-Ni
alloy plated layer and the inorganic chromate-free chemical conversion coating film
is 0.01 to 0.5 g/m2
•
5 (5)
The steel sheet for a fuel tank according to any one of (1) to ( 4 ), in which
the inorganic chromate-free chemical conversion coating film contains one
or more selected from a silane coupling agent, a condensation polymer of a silane
coupling agent, silica, silicate, phosphoric acid, and phosphate, and a compound of
10 one or more metals selected from Ti, Zr, V, Mo, and W.
(6)
The steel sheet for a fuel tank according to any one of (1) to ( 5), in which
the inorganic chromate-free chemical conversion coating film contains a
water repellent.
15 (7)
20
25
The steel sheet for a fuel tank according to ( 6), in which
the water repellent is one or more selected from polyolefin wax, a siliconbased
resin, and a fluorine-based resin.
(8)
The steel sheet for a fuel tank according to any one of (1) to (7), in which
in the case where a cross section of the Zn-Ni alloy plated layer is observed,
the number of the cracks in a visual field of l 00 J.Lm is more than or equal to 5 and
less than or equal to 50.
(9)
The steel sheet for a fuel tank according to (8), in which
the number X of the cracks in a visual field of l 00 J.!ffi obtained by
observing the cross section of the Zn-Ni alloy plated layer and the water contact
angle Y (degrees) satisfy a relationship represented by the following formula (I),
Y2::-0.l8X+56.5 (I).
30 (10)
The steel sheet for a fuel tank according to (9), in which
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in the case where a cross section of the Zn-Ni alloy plated layer is observed,
the number of the cracks in the visual field of 100 !!ill is more than or equal to 5 and
less than 50, and a maximum width of a crack is less than 0.5 !!ill.
5 Advantageous Effects of Invention
[0013]
As described above, according to the present invention, there can be
provided a steel sheet for a fuel tank, which exhibits excellent corrosion resistance
under an enviromuent in which various fuels exist, the fuels including a fuel
10 containing an organic acid such as deteriorated gasoline and a fuel containing dew
condensation water in addition to an organic acid, without using enviromuental
burden substances such as lead and chromium.
Brief Description of Drawings
15 [0014]
20
25
[FIG. 1] FIG. 1 is an explanatory view of a relationship of Expression (I) showing a
comparison between a case where the number of cracks is small (few cracks) and a
case where the number of cracks is large (many cracks).
Description of Embodiments
[0015]
Hereinafter, preferred embodiments of the present invention will be
described in detail.
[0016]
The present invention described below in detail relates to a steel sheet for a
fuel tank that uses a zinc-based plated steel sheet which is excellent in corrosion
resistance to various fuels, does not use lead or chromate treatment, and is
enviromuentally friendly. The present invention also relates to a steel sheet used
for an automobile, a motorcycle, industrial machinery, and construction machinery,
30 and in addition, used for a tank in which a fuel is enclosed and a part of the tank.
[0017]
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The steel sheet for a fuel tank according to an embodiment of the present
invention includes: a Zn-Ni alloy plated layer which is placed on one surface or each
of both surfaces of a base metal; and an inorganic chromate-free chemical conversion
coating film (hereinafter, also simply referred to as "chemical conversion coating
5 film") which is placed over the Zn-Ni alloy plated layer (hereinafter, also simply
referred to as "plated layer").
[0018]
In the case where the steel sheet for a fuel tank according to an embodiment
of the present invention is used for a steel sheet for a fuel tank, a surface (one
10 surface) having the Zn-Ni alloy plated layer and the inorganic chromate-free
chemical conversion coating film which is placed over the Zn-Ni alloy plated layer is
a fuel tank-inner surface (hereinafter, referred to as inner surface). In that case, the
surface opposite to the one surface is a fuel tank-outer surface (hereinafter, referred
to as outer surface).
15 [0019]
In the steel sheet according to the present invention, the Zn-Ni alloy plated
layer may be provided to each of the both surfaces. However, in order to make the
weldability of the steel sheet satisfactory, it is preferred that the surface to be the
outer surface of the fuel tank do not have the plated layer. Further, for example, in
20 the case where the steel sheet is used as a fuel tank for a motorcycle which places
importance on the visual quality of the coating of the outer surface of the fuel tank, it
is preferred that the outer surface of the fuel tank do not have the plated layer in
order to make the external appearance of the coating satisfactory. On the other
hand, in the case where the steel sheet is used for the case where the outer surface of
25 the fuel tank also requires high corrosion resistance, it is preferred that the outer
surface of the fuel tank have the plated layer. In the steel sheet according to the
present invention, the presence and absence of the plated layer on the outer surface
and the inner surface can be controlled in accordance with the use.
30
[0020]
In order to make the outer surface into a state that does not have the plated
layer, there can be employed a method not involving applying electric current to the
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outer side to be the non-plated surface during electroplating. Alternatively, there
can be employed a method involving performing plating, and then removing the
plating on the outer side to be the non-plated surface through an electrochemical
method (for example, anode electrolytic treatment) or a mechanical method (for
5 example, grinding with a brush). Further, the above-mentioned methods can be
used in combination.
[0021]
A contact angle on the non-plated surface (surface to be the outer surface)
formed through the above-mentioned method is, in terms of a water contact angle,
10 preferably less than 10 degrees and more preferably less than 5 degrees. In this way,
the coating property is improved. In order to ensure the above contact angle, it is
effective to suppress oxidation on the surface of the steel sheet, and in this point, it is
desirable that a trace amount of Zn and/or Ni is present also on the non-plated
surface side. It is desirable that the deposition amount be preferably 0.01 to 0.3
15 g/m2
, and more preferably 0.01 to 0.1 g/m2
.
[0022]
Here, a base material of the steel sheet for a fuel tank according to an
present embodiment is not particularly limited, and any known steel sheet can be
used appropriately as long as it is a steel sheet that is generally used as a base
20 material of a zinc-based plated steel sheet.
[0023]
The Zn-Ni alloy plated layer formed on at least one surface ofthe steel sheet
is an alloy plated layer containing at least an alloy of zinc and nickel. The Zn-Ni
alloy plated layer can be formed by a known plating method such as an electroplating
25 method.
[0024]
The Zn-Ni alloy plated layer is characterized in having a crack starting from
a surface layer of the plated layer (in other words, an interface between the Zn-Ni
alloy plated layer and the inorganic chromate-free chemical conversion coating film)
30 and reaching the base metal (in other words, an interface between the steel sheet,
which is the base material, and the Zn-Ni alloy plated layer). Further, the inorganic
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chromate-free chemical conversion coating film formed over the Zn-Ni alloy plated
layer is characterized in having a water contact angle on a surface of the coating film
of more than or equal to 50 degrees.
[0025]
5 It is a well-known phenomenon that the fuel corrosion resistance improves
in the case where there is a crack in the Zn-Ni alloy plated layer, on the assumption
that the chromate treatment is performed. Such a phenomenon can be explained
through: improvement in adhesion of the coating film owing to an anchor effect of a
chromate coating film entered in the crack; and an effect of preventing a new crack
10 from being generated during press working when there is a crack in the plated layer
in advance. It is presumed that such effects can also be seen in the steel sheet
described in Patent Literature 8.
[0026]
On the other hand, in a chromate-free chemical conversion coating film that
15 has recently been studied, no remarkable fuel corrosion resistance-improvement
effect can be expected even if there is a crack in the plated layer, and, on the contrary,
the fuel corrosion resistance deteriorates. It is considered that the reason therefor is
that there is no strong corrosion inhibitor such as Cr6
+, which exists in the chromate
coating film.
20 [0027]
In order to exhibit fuel corrosiOn resistance, in particular, satisfactory
corrosion resistance under a fuel environment including an organic acid such as
deteriorated gasoline and in addition dew condensation water, and also under a
condition in which a part of the coating film is damaged during press working, it is
25 necessary that all of the following be satisfied: a crack exists in the Zn-Ni alloy
plated layer; the chromate-free chemical conversion coating film is inorganic; and
the water contact angle on the surface of the chemical conversion coating film is
more than or equal to 50 degrees.
30
[0028]
The crack in the Zn-Ni alloy plated layer according to the present
embodiment refers to a crack starting from the plated layer surface and reaching the
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base metal. It is assumed that function mechanisms of the crack are achieved from,
in addition to the anchor effect and the prevention of a new crack from being
generated during press working as described above, effects of increasing a surface
area of the water-repellent inorganic chemical conversion coating film, and, owing to
5 the increase in the surface area, protecting the plated layer and the base metal from
corrosion factors in a fuel, particularly from hydrophilic corrosion factors. Note
that the presence of the crack can be confirmed by observing a cross section using a
scanning electron microscope (SEM).
10
[0029]
It is necessary that the chromate-free chemical conversion coating film
according to the present embodiment be an inorganic coating film. The inorganic
coating film does not represent a coating film mainly containing an organic resin.
Although the inorganic coating film does not exclude the containing of an organic
resin, the inorganic coating film represents a coating film having the content of the
15 organic resin (content with respect to the total solid content of the coating film) of
less than 50%, preferably less than or equal to 30%, and more preferably less than or
equal to 10% (including zero). In the case where the chromate-free chemical
conversion coating film mainly contains an organic resin (that is, in the case where
the chromate-free chemical conversion coating film is an organic coating film), the
20 organic resin is swelled by a hydrocarbon in the fuel, the adhesion between the plated
layer and the chemical conversion coating film decreases, and the corrosion starting
from a crack in the plated layer also progresses. With the inorganic coating film
like the chemical conversion coating film according to the present embodiment, such
a swelling phenomenon can be suppressed.
25 [0030]
It is necessary that the water contact angle on the surface of the chemical
conversion coating film according to the present embodiment be more than or equal
to 50 degrees. With the combined effect of the water contact angle being more than
or equal to 50 degrees and the effect of a crack in the plated layer, the plated layer
30 and the base metal can be protected from corrosion factors in a fuel, particularly from
hydrophilic corrosion factors, and thus, satisfactory corrosion resistance can be
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obtained. The water contact angle on the surface of the chemical converston
coating film according to the present embodiment is preferably more than or equal to
55 degrees, and more preferably more than or equal to 60 degrees. Note that the
upper limit of the water contact angle is not particularly defined, and, although it is
5 preferred that the water contact angle be a value near 180 degrees which is the
theoretical limit, the limit that the water contact angle can reach on the plated steel
sheet according to the present embodiment is approximately 120 degrees. The
water contact angle can be measured by a !mown method using a contact angle meter.
[0031]
10 The inorganic chromate-free chemical conversion coating film according to
the present embodiment desirably contains one or more selected from a silane
coupling agent, a condensation polymer of a silane coupling agent, silica, silicate,
phosphoric acid, and phosphate, and a compound of one or more metals selected
from Ti, Zr, V, Mo, and W. Further, the inorganic chromate-free chemical
15 conversion coating film according to the present embodiment desirably contains a
water repellent.
[0032]
Here, the water repellent according to the present embodiment represents a
substance that has an action of increasing the water contact angle by being added to
20 an inorganic chemical conversion coating film. Specific examples of the water
repellent include organic resins each having a C-H bond or a C-F bond. Further
preferable specific examples of the water repellent include polyolefin wax, a siliconbased
resin, and a fluorine-based resin. The predetermined water contact angle can
be obtained with a small amount of addition of those water repellents (for example,
25 less than or equal to 10% with respect to the total solid content of the chemical
conversion coating film), and therefore, there is no risk that the chemical conversion
coating film according to the present embodiment is no longer an inorganic coating
film.
30
[0033]
It is important that, in the inorganic chromate-free chemical conversiOn
coating film according to the present embodiment, the above-mentioned water
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repellents be adjusted such that the water contact angle on the surface of the
chemical conversion coating film be more than or equal to 50 degrees.
[0034]
To be specific, in the case where the chemical conversion coating film
5 mainly contains one or more selected from a silane coupling agent and a
condensation polymer of a silane coupling agent, since those components each have
a C-H bond, a relatively high water contact angle can be easily obtained without
adding a water repellent, but it is more preferred to add a water repellent. The
water repellent to be added in this case is one or more selected from polyolefin wax,
10 a silicon-based resin, and a fluorine-based resin, and it is preferred that the water
repellent be added in an amount of more than or equal to 0.1% with respect to the
total solid content.
[0035]
Further, in the case where the chemical conversion coating fihn mainly
15 contains one or more selected from silica, silicate, phosphoric acid, and phosphate,
since it is generally difficult to obtain the predetermined water contact angle unless a
water repellent is added, it is desirable to add a water repellent. The water repellent
to be added in this case is one or more selected from polyolefin wax, a silicon-based
resin, and a fluorine-based resin, and it is preferred that the water repellent be added
20 in an amount of more than or equal to 1% with respect to the total solid content.
[0036]
The deposition amount of the inorganic chromate-free chemical conversion
coating film according to the present embodiment per surface is preferably 0.1 to 2
g/m2
, and more preferably 0.3 to 1 g/m2
. In the case where the deposition amount
25 of the chemical conversion coating film per surface is less than 0.1 g/m2
, corrosion
resistance with respect to deteriorated gasoline may decrease, and in the case where
the deposition amount of the chemical conversion coating film per surface exceeds 2
g/m2
, the cost increases and weldability may deteriorate depending on a welding
condition. The method of measuring the deposition amount of the chemical
30 conversion coating film per surface is not particularly limited. The measurement
can be performed thorough a known measurement method, and can be performed
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through a gravimetric method or an X-ray fluorescence method, for example. The
gravimetric method used here is a method involving measuring a weight of a sample
whose area is defined, then forming the chemical conversion coating film, and
determining a deposition amount from the difference with a weight of the product.
5 Further, the X-ray fluorescence method is a method involving creating in advance a
calibration curve with a deposition amount-known sample by means of the
gravimetric method or the like, and calculating a deposition amount from an X -ray
fluorescence intensity of a sample of interest.
[0037]
10 Here, the inorganic chromate-free chemical conversiOn coating film
according to the present embodiment can be formed through a known method. For
example, the method involves preparing application liquid containing predetermined
component(s) described above, and applying the prepared application liquid over the
Zn-Ni alloy plated layer through a lmown method such as bar coater and roll coater.
15 Then, the obtained applied film may be heated and dried at predetermined heating
temperature.
20
[0038]
Regarding a crack in the Zn-Ni alloy plated layer according to the present
embodiment, a more preferred embodiment will be described below.
The presence of the crack can be confirmed by embedding a sample in a
resin, performing cross section vertical polishing, and observing a cross section using
a SEM. In this case, observing a visual field of 100 J.llll at approximately 1000-fold
magnification, it is more preferred that the number of cracks starting from the surface
layer of the plated layer and reaching the base metal be more than or equal to 5 and
25 less than or equal to 50. In the case where the number of cracks is small, for
example, in the case where the number of cracks in the visual field of 100 J!m is less
than 5, the fuel corrosion resistance tends to decrease. On the other hand, in the
case where the number of cracks is too large, for example, in the case where the
number of cracks in the visual field of l 00 J!m exceeds 50, general corrosion
30 resistance such as corrosion resistance with respect to a brine environment tends to
decrease. In the Zn-Ni alloy plated layer according to the present embodiment, the
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number of cracks in the visual field of 100 J.!m is more preferably more than or equal
to 10 and less than or equal to 40.
[0039]
Moreover, in the case where cracks are observed in the above-mentioned
5 method and a crack having the largest opening width among the cracks starting from
the surface layer of the plated layer and reaching the base metal is observed at
approximately 10000-fold magnification to determine the maximum width, the
obtained maximum width is desirably less than 0.5 J.!m. In the case where the
maximum width of the crack is more than or equal to 0.5 J.!m, general corrosion
10 resistance such as corrosion resistance with respect to a brine environment tends to
decrease.
[0040]
In order to obtain the best corrosion resistance with respect to deteriorated
gasoline, it is important to take into account the relationship between the number of
15 cracks and the water contact angle on the surface of the coating fihn. To be specific,
in the case where the number of cracks is small, it is important that the water contact
angle be kept higher, and on the other hand, in the case where the number of cracks
is large, satisfactory fuel corrosion resistance can be obtained in a water contact
angle of a relatively wide range. As a result of conducting a detailed investigation,
20 to be specific, the best corrosion resistance with respect to deteriorated gasoline can
be obtained if the number X of cracks (in a visual field of 100 J.!ffi) and the water
contact angle Y (degree(s)) on the surface of the coating fihn satisfy the following
relationship of Expression (I).
25
[0041]
Y2:-0.18X +56.5 (I)
[0042]
Referring to FIG. 1, the relationship shown in Expression (I) will be
described. As shown in FIG. 1, in the case where the number X of cracks is small,
during processing of the steel sheet, a large number of new cracks (newly generated
30 cracks due to processing) are generated in the Zn-Ni alloy plated layer, and,
accompanied by the damage to the Zn-Ni alloy plated layer, the inorganic chromatePCT/
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free chemical conversion coating film is also damaged. Therefore, in order to
obtain satisfaetory fuel corrosion resistance, it is necessary that the water contact
angle Y on the surface of the coating film be high.
On the other hand, in the case where the number X of cracks is large, during
5 processing of the steel sheet, the pre-existent cracks reduce stress, and hence, new
cracks are unlikely to be generated. Therefore, the inorganic chromate-free
chemical conversion coating film is hardly damaged, and the reduction in the fuel
corrosion resistance is small. Moreover, in the case where the inorganic chromatefree
chemical conversion coating film is formed so as to cover the inner surfaces of
10 the cracks, the surface area of the inorganic chromate-free chemical conversion
coating film increases, and the fuel corrosion resistance further improves. It is
assumed that the reason therefor is that, since the surface area of the inorganic
chromate-free chemical conversion coating film increases with the increase in the
number X of cracks, the effect of protecting the plated layer and the base metal from
15 corrosion factors in a fuel, particularly from hydrophilic corrosion factors increases,
even with a coating film having a relatively small contact angle Y.
[0043]
In order to obtain the best characteristics in the corrosion resistance with
respect to deteriorated gasoline and general corrosion resistance such as corrosion
20 resistance with respect to a brine environment, it is advisable to satisfy Expression (I),
to make the number of cracks starting from the plated surface layer and reaching the
base metal more than or equal to 5 and less than 50 (in the visual field of 100 f!Ill),
and to make the maximum width of the crack less than 0.5 fLm.
25
[0044]
The deposition amount of the Zn-Ni alloy plated layer according to the
present embodiment is preferably 5 to 40 g/m2 per surface. In the case where the
deposition amount of the Zn-Ni alloy plated layer per surface is less than 5 g/m2
, the
corrosion resistance with respect to deteriorated gasoline tends to be insufficient.
Further, in the case where the deposition amount of the Zn-Ni alloy plated layer per
30 surface exceeds 40 g/m2
, although the corrosion resistance with respect to
deteriorated gasoline is imparted, but it is disadvantageous in terms of cost, therefore
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it is not preferred. Note that, from the viewpoint of a coating property, the
deposition amount of the Zn-Ni alloy plated layer per surface is preferably 0.01 to
0.5 g/m2
. Note that the Ni content in the Zn-Ni alloy plated layer is not particularly
lintited, and is preferably 9 to 14 mass%, the range in which the corrosion resistance
5 with respect to deteriorated gasoline becomes satisfactory. Further, the Zn-Ni alloy
plated layer may contain a known third component, for example, metals such as Fe,
Co, Sn, and Cr, and the underneath the Zn-Ni alloy plated layer may have pre-plating
of Fe and Ni, for example.
10
[0045]
Note that the deposition amount of the Zn-Ni alloy plated layer per surface
can be controlled through a quantity of electricity (amount of coulomb) in the case of
using an electroplating method, for example, and can also be measured afterward.
The measurement method of the deposition amount of the Zn-Ni alloy plated layer
per surface is not particularly limited, and the measurement can be performed
15 thorough a known measurement method, for example, through a gravimetric method
or an X-ray fluorescence method. The gravimetric method used here is a method
involving measuring a weight of a plated sample whose area is defined, dissolving
only the plated layer in hydrochloric acid, and determining a deposition amount from
the difference with a weight after the dissolving. Further, the X-ray fluorescence
20 method is a method involving creating in advance a calibration curve with a
deposition amount-known sample by means of the gravimetric method or the like,
and calculating a deposition amount from an X-ray fluorescence intensity of a
sample of interest.
25
[0046]
The method of forming a crack in the Zn-Ni alloy plated layer according to
the present embodiment is not limited, and treatment in an acidic aqueous solution
after the plating is preferably used. In particular, in the case where the Zn-Ni alloy
plated layer is formed by performing electroplating in an acidic plating solution, a
method is appropriately used, involving cutting electricity after the plating and
30 immersing the resultant in a plating solution in the state with no electricity. The
number of cracks and the maximum width can be adjusted by a concentration of a
PCT/JP2016/052911
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treatment bath, temperature, or a treatment time. In the case where treatment is
carried out using an acidic plating bath, the number of cracks has particularly high
dependency on temperature, and, the higher the temperature, the number tends to
mcrease. On the other hand, the crack width has high dependency on time, and, in
5 the case where there is an attempt to suppress the crack width, the time for the
treatment may be made short. In the case where the treatment is performed at high
temperature for a short period of time, the number of cracks is large and the crack
width is not too large. On the other hand, in the case where the treatment is
performed at low temperature for a long period of time, the number of cracks is not
10 too large, but the crack width tends to increase.
[0047]
The configuration of the present invention described above relates to the
surface to be the inner surface of a fuel tank, in which the fuel corrosion resistance
becomes a problem. The surface to be the outer surface of the fuel tank is not
15 particularly limited. The surface to be the outer surface of the fuel tank does not
necessarily have the Zn-Ni alloy plated layer, may have the Zn-Ni alloy plated layer,
and may have, in addition, the chromate-free chemical conversion coating fihn over
the Zn-Ni alloy plated layer.
[Examples]
20 [0048]
Next, the steel sheet for a fuel tank according to the present invention will
be described further specifically with reference to Examples and Comparative
Examples. Note that Examples shown below are merely examples of the steel sheet
for a fuel tank according to the present invention, and the steel sheet for a fuel tank
25 according to the present invention is not limited to the following examples.
[0049]

(Examples 1 to 28 and Comparative Examples 1 and 2)
An extra-low carbon steel sheet was used as an original sheet, a sulfuric acid
30 acidic plating bath was used, electroplating was performed, and a Zn-Ni alloy plated
layer whose deposition amount per surface was 20 g/m2 and containing 10 mass% of
PCT/JP2016/052911
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Ni was formed. The resultant was held in the plating bath for three seconds in the
state that the electricity was cut, and a predetermined plated layer-crack was formed.
Note that the sulfuric acid acidic plating bath that was used was a bath of 50°C
containing 200 giL of zinc sulfate heptahydrate, 3 80 g/L of nickel sulfate
5 hexahydrate, 80g/L of sodium sulfate, and 1 Og/L of sulfuric acid. After that, each
of the chromate-free chemical conversion coating films having different
compositions shown in Table 1 was formed in a predetermined amount over the
obtained Zn-Ni alloy plated layer. The chromate-free chemical conversion coating
film was formed through application using a bar coater and drying in a hot air drying
10 furnace (ultimate sheet temperature: 100°C). Note that the deposition amount of
the chromate-free chemical conversion coating film was measured through the X -ray
fluorescence method described above.
[0050]
[Table 1]
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~ Solid content concentration (mass%)
m1 m2 m3 m4 m5 m6 m7 m8 m9 m10 m11 m12 m13 m14 m15 m16 m17 m18 m19 m20 y1
Silane coupling agent *1 4.7 3.7 3.1 4 4.5 4.5 4.2 4.7 2 1 3 3 2
Silane coupling agent oligomer *2 88 85 62 75 85 85.5 87 87 85 80 80
Silica 3.9 5 4 5 20 5 26
Lithium silicate 80 80 70
Phosphoric acid 5 4 3.5 4.3 4.8 5 4.9 5 10 1
Aluminum primary phosphate 60 59 58.2 57 54 40
Ammonium phosphate 1 3 1 1
Phosphonic acid *3 2 30 29 29.1 28.5 27 30 2
Ammonium hexafluorotitanate 2 1 1.4 1.7 1.7 2 1.9 2.3 2 3 4 2.91 2.85 2.7
Zirconium oxycarbonate 1 5 5 2 5 3 5
Vanadyl acetylacetonate 5 3 5 4 4.85 4.75 4.5 3 4 5 2 3 1
Ammonium molybdate 1 2 2 1
T ungstic acid 2 2 2 1.94 1.9 1.8 1
Urethane resin *4 30 10
lonomer resin *5 5 65
Polyethylene wax 0.3 0.3 5 4 3 2 1 0.1 3 1 3 5 10 5 5 3 5 5
Fluorine-based resin *6 1 1
Total 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
[0051]
20/33
Here, the numbers in Table 1 represent the following.
* 1: 3-glycidoxypropyl trimethoxy silane
PCT/JP2016/052911
*2: condensation polymer of 3-glycidoxypropyl trimethoxy silane and 3-
5 aminopropyltriethoxysilane in a ratio of 3:2 (molecular weight of approximately
3000)
10
[0052]
*3: 1-hydroxy-ethylidene-1, I '-diphosphonic acid
*4: ester-based urethane resin
*5: Na neutralized ionomer resin
*6: tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer
(Comparative Example 3)
Preparation was performed in the same manner as described above, except
that the chemical conversion coating film was not formed.
15 [0053]
(Comparative Examples 4 to 7)
Each preparation was performed in the same manner as described above,
except that the treatment of being immersed in a plating bath in the state with no
electricity after the Zn-Ni alloy plating was not performed.
20 [0054]
(Comparative Example 8)
Preparation was performed in the same manner as described above, except
that electrogalvanizing was used instead of Zn-Ni alloy plating.
[0055]
25 [Observation on plated layer-crack]
Each of the prepared samples was embedded in a resm, cross section
vertical polishing was performed, and the cross section was observed using a SEM.
A visual field of 100 11m was observed at 1000-fold magnification, and the number
of cracks starting from the surface layer of the plated layer and reaching the base
30 metal was counted. Moreover, a crack having the largest opening width among the
cracks starting from the surface layer of the plated layer and reaching the base metal
5
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m 1he visual field was observed usmg the SEM at approximately 10000-fold
magnification to measure the maximum width.
[0056]
[Water contact angle]
A contact angle meter (DM-901 manufactured by Kyowa Interface Science
Co., Ltd) was used, 3 fll of ion exchanged water were dripped under 25°C
atmosphere, and a static contact angle after 60 seconds was measured.
[0057]
[Corrosion resistance with respect to deteriorated gasoline]
10 Each of the prepared samples was molded in a cylinder shape having an
inside diameter of 50 mm and a depth of 35 mm, which imitates a fuel tank. After
removing oil through grease removing treatment, a flaw 1hat reaches up to 1he base
metal was made by a cutter on the inner surface base (which imitates a flaw caused
by press working). Then, test liquid 1hat imitates the deteriorated gasoline (gasoline
15 containing 100 ppm of formic acid, 300 ppm of acetic acid, 100 ppm of chloride ions,
1.0 capacity % of water) was enclosed, and was retained at 40°C for two months.
After that, rust was removed, a maximum reduction in sheet thickness (mm) due to
corrosion was measured.
20
[0058]
Table 2 shows evaluation results for each sample of a state of a plated layercrack,
a type of a chemical conversion coating fihn that was used, a deposition
amount of a chemical conversion, a water contact angle, and corrosion resistance
with respect to deteriorated gasoline. Note that, in Table 2, the evaluation results of
the corrosion resistance with respect to deteriorated gasoline can be determined as
25 satisfactory in the case where the maximum reduction in sheet thickness was less
than 0.05 mm.
[0059]
[Table 2]
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I~ Chemical conversion treated Corrosion
Zn-Ni alloy plated layer
coating film resistance
with
Maximum
Type of Deposition Water respect to Remarks
Number of
width chemical amount contact deteriorated
cracks (J1m)
conversion {glm2) angle gasoline
coating film (degrees) (mm)
1 12 0.3 m1 0.7 53 0.01
2 12 0.3 m2 0.7 57 0
3 12 0.3 m3 0.7 60 0.01
4 12 0.3 m4 0.7 71 0
5 12 0.3 m5 0.7 70 0
6 12 0.3 m6 0.7 68 0
7 12 0.3 m7 0.7 64 0
B 12 0.3 mB 0.7 59 0
9 12 0.3 m9 0.7 50 0.04
10 12 0.3 m10 0.7 69 0
11 12 0.3 m12 0.7 59 0
12 12 0.3 m13 0.7 55 0
13 12 0.3 m14 0.7 56 0
14 12 0.3 m15 0.7 58 0
Example
15 12 0.3 m16 0.7 61 0
16 12 0.3 m17 0.7 62 0
17 12 0.3 m18 0.7 63 0
18 12 0.3 m19 0.7 60 0
19 12 0.3 m20 0.7 91 0
20 12 0.3 m1 0.5 53 0.01
21 12 0.3 m1 1 53 0
22 12 0.3 m1 1.5 53 0
23 12 0.3 m5 0.3 70 0
24 12 0.3 m5 1.2 70 0
25 12 0.3 m5 2 70 0
26 12 0.3 m16 0.4 61 0
27 12 0.3 m16 0.9 61 0
28 12 0.3 m16 1.8 61 0
1 12 0.3 m11 0.7 20 0.2
2 12 0.3 y1 0.7 78 0.17 Organic chromate-free coating film
3 12 0.3 none - 5 0.36
Comparative 4 0 - m1 0.7 53 0.1
Example 5 0 - m5 0.7 70 0.09
6 0 - mll 0.7 20 0.14
7 0 - y1 0.7 78 0.09 Organic chromate-free coating film
B - - m5 0.7 69 0.19 Bectrogalvanizing
[0060]
As is clear from Table 2, since Comparative Example 1 had a small water
5 contact angle and was out of the range of the present invention, the corrosion
resistance with respect to deteriorated gasoline was poor. Comparative Example 2
had a high water contact angle, but the coating film was organic, which was out of
the range of the present invention, and therefore, the corrosion resistance with
respect to deteriorated gasoline was poor. In each of Comparative Example 4 and
10 Comparative Example 5, although the water contact angle was within the range of
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the present invention, the plated layer did not have a crack, and therefore, the
corrosion resistance with respect to deteriorated gasoline was poor.
[0061]
Comparing Comparative Example 1 and Comparative Example 6 with each
5 other, it was found that, in coating fihns each having a small water contact angle
which was out of the range of the present invention, the corrosion resistance
deteriorated with the formation of the plated layer-crack. In the same manner,
comparing Comparative Example 2 and Comparative Example 7 with each other, it
was found that, in organic coating films which were out of the range of the present
10 invention, the corrosion resistance deteriorated with the formation of the plated
layer-crack.
[0062]
As above, it was found that Examples of the present invention each having a
combination that satisfied all plated layer-crack, inorganic coating fihn, and water
15 contact angle could obtain satisfactory characteristics.
[0063]

(Examples 29 to 81)
An extra-low carbon steel sheet was used as an original sheet, a sulfuric acid
20 acidic plating bath was used, electroplating was performed, and a Zn-Ni alloy plated
layer whose deposition amount per surface was 20 g/m2 and containing 10 mass% of
Ni was formed. A predetermined plated layer-crack was formed by varying
temperature and time during immersion in the plating bath in the state that the
electricity was cut. The plating bath that was used was the same as the plating bath
25 used in Experiment 1, and the temperature was varied between 50 to 65°C, and the
immersion time was varied between 1 to 5 seconds. After that, each of the
chromate-free chemical conversion coating films having different compositions
shown in Table l was formed in a predetermined amount in the same manner as in
Experiment 1. Further, the deposition amount of the chromate-free chemical
30 conversion coating film was measured through the X-ray fluorescence method
described above.
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[0064]
The evaluations were performed in the same manner as in Experiment 1,
except that in this Experiment, SST corrosion resistance was additionally evaluated.
[0065]
5 [SST corrosion resistance]
10
A back surface and an edge of each of the prepared samples were sealed and
a salt spray test defined in JISZ23 71 was performed for 72 hours to measure an area
proportion (%) of white rust occurrence.
[0066]
Table 3 shows evaluation results for each sample of a state of a plated layercrack,
a type of a chemical conversion coating film that was used, a deposition
amount of a chemical conversion, a water contact angle, corrosion resistance with
respect to deteriorated gasoline, and SST corrosion resistance. Note that the
evaluation results of the corrosion resistance with respect to deteriorated gasoline can
15 be determined as satisfactory in the case where the maximum reduction in sheet
thickness was less than 0.05 mm. Further, the SST corrosion resistance can be
determined as satisfactory in the case where the rate of white rust occurrence was
less than 5%.
[0067]
20 [Table 3]
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~ Chemical conversion treated Corrosion
Zn-Ni alloy plated layer
coating film resistance SST Satisfy with
corrosion
Maximum
Type of
Deposition Water Expression respect to
Number of chemical contact (I) deteriorated
resistance
width amount cracks conversion (%) (g/mz) (J.tm) angle gasoline
coating film (degrees) (mm)
29 5 0.2 ml 0.8 53 No 0.02 0
30 5 0.2 m5 0.8 70 y., 0 0
31 5 0.2 m6 0.8 68 y., 0 0
32 5 0.2 m7 0.8 64 y., 0 0
33 5 0.2 mB 0.8 59 y., 0 0
34 5 0.2 m9 0.8 50 No 0.04 0
35 5 0.2 m13 0.8 55 No 0.01 0
36 5 0.2 m14 0.8 56 v., 0 0
37 5 0.2 m17 0.8 62 v., 0 0
38 50 0.4 ml 0.8 53 y., 0 0.5
39 50 0.4 m5 0.8 70 v., 0 0
40 50 0.4 m6 0.8 68 v., 0 0
41 50 0.4 m7 0.8 64 y., 0 0
42 50 0.4 mB 0.8 59 v., 0 0
43 50 0.4 m9 0.8 50 y., 0 0.5
44 50 0.4 m13 0.8 55 v., 0 0
45 50 0.4 m14 0.8 56 y., 0 0
46 50 0.4 m17 0.8 62 v., 0 0
47 38 0.4 ml 0.8 53 v., 0 0
48 38 0.4 m5 0.8 70 v., 0 0
49 38 0.4 m6 0.8 68 v., 0 0
50 38 0.4 m7 0.8 64 v., 0 0
51 38 0.4 m8 0.8 59 v., 0 0
52 38 0.4 m9 0.8 50 v., 0 0
53 38 0.4 m13 0.8 55 v., 0 0
54 38 0.4 m14 0.8 56 v., 0 0
Example 55 38 0.4 m17 0.8 62 v., 0 0
56 15 0.3 ml 0.8 53 No 0.01 0
57 15 0.3 m5 0.8 70 v., 0 0
58 15 0.3 m6 0.8 68 v., 0 0
59 15 0.3 m7 0.8 64 v., 0 0
60 15 0.3 mB 0.8 59 v., 0 0
61 15 0.3 m9 0.8 50 No 0.03 0
62 15 0.3 m13 0.8 55 v., 0 0
63 15 0.3 m14 0.8 56 y., 0 0
64 15 0.3 m17 0.8 62 v., 0 0
65 29 0.3 ml 0.8 53 v., 0 0
66 29 0.3 m5 0.8 70 v., 0 0
67 29 0.3 m6 0.8 68 v., 0 0
68 29 0.3 m7 0.8 64 v., 0 0
69 29 0.3 mB 0.8 59 y., 0 0
70 29 0.3 m9 0.8 50 No 0.02 0
71 29 0.3 m13 0.8 55 v., 0 0
72 29 0.3 m14 0.8 56 v., 0 0
73 29 0.3 m17 0.8 62 v., 0 0
74 30 0.5 ml 0.8 53 v., 0 4
75 30 0.5 m5 0.8 70 v., 0 2
76 30 0.5 m6 0.8 68 v., 0 2
77 30 0.5 m7 0.8 64 v., 0 2
18 40 0.7 ml 0.8 53 v., 0 4.5
79 40 0.7 m5 0.8 70 v., 0 2
80 40 0.7 m6 0.8 68 v., 0 3
81 40 0.7 m7 0.8 64 v., 0 3
PCT/JP2016/0529ll
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[0068]
As is clear from Table 3, Examples of the present invention each showed
satisfactory characteristics.
5 In particular, regarding the corrosion resistance with respect to deteriorated
gasoline, each of Examples that satisfies Expression (I) had a reduction in sheet
thickness of 0 in the test of corrosion resistance with respect to deteriorated gasoline,
which exhibited particularly excellent corrosion resistance with respect to
deteriorated gasoline. Further, each of Examples having a crack width of less than
10 0.5 1-lm had a rate of white rust occurrence in the SST corrosion resistance of less
than 1%, which exhibited excellent SST corrosion resistance. In particular, in the
case where the crack width was less than 0.5 flm and the number of cracks was less
than 50, the rate of white rust occurrence in the SST corrosion resistance was zero
under any condition, which was particularly excellent.
15 [0069]
As described above, each of Examples that satisfies Expression (I), has the
number of cracks of more than or equal to 5 and less than 50, and has the crack width
ofless than 0.5 flm was particularly excellent in the corrosion resistance with respect
to deteriorated gasoline and the SST corrosion resistance.
20 [0070]

(Examples 82 to 89)
Each of the surfaces to be the inner surface of a fuel tank was prepared in
the same manner as in Example 1, but each of the surfaces to be the outer surface of
25 a fuel tank was varied. In Example 82, the outer surface was not provided with a
coating film. In each of Examples 83 and 84, the plating on the surface to be the
outer surface was completely removed through grinding with a brush. After that, in
Example 83, a predetermined coating film was applied. In each of Examples 85 to
89, on the outer surface from which the plating was completely removed through the
30 above method, predetermined amounts of Zn and Ni were deposited again through
electroplating.
PCT/JP2016/052911
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[0071]
Performance evaluation was carried out as follows.
(Coating property)
The coating property of the surface to be the outer surface of a fuel tank was
5 evaluated. In the same manner as the ordinary outer surface of the fuel tank for an
automobile or a motorcycle, pre-coating treatment (zinc phosphate treatment) and
electrodeposition were performed, and the external appearance was evaluated by
visual observation. Example of an acceptable level was evaluated as "Satisfactory",
and Example of remarkably uniformly fine was evaluated as "Excellent".
10 [0072]
(Coating property after storage)
To the prepared sample, rust-resistant oil (NOX-RUST 530, manufactured
by Parker Industries, Inc.) was applied extremely slightly (approximately 0.1 g/m2
),
and then the resultant was packed in a manner that the surface to be the inner surface
15 and the surface to be the outer surface came into contact with each other so that the
state looked like a coil. Using a sample after stored in 50°C 98%RH environment
for one month, the same coating as in the coating property evaluation was performed,
and the evaluation was performed using the same criteria
[0073]
20 (Weldability)
The surfaces to be the inner surfaces of fuel tanks were placed together and
seam welding was performed. A range of current in which an appropriate nugget
can be obtained was determined by changing the welding current. The conditions
are as follows.
25 - Electrode: a disc-shaped electrode made of a Cu-Cr alloy, in which a cross section
at a central part had a radius of 15 mm and a width of 4.5 mm and a cross section at
an end part had a radius of 4 mm and a width of 8 mm
- Welding method: piling of two sheets, lap seam welding
-Welding pressure: 400 kgf(note that l kgfis approximately 9.8 N)
30 -Welding time: 2/50 sec on, 1150 sec off
- Cooling: inner part water cooled, and outer part water cooled
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-Welding speed: 2.5 m/min
[0074]
The results are shown in Table 4. As is clear from Table 4, Examples of
the present invention each showed satisfactory characteristics. Also, it was found
5 that the coating property, the coating property after storage, and the weldability of
the steel sheet particularly improved in the case where the deposition amount of Zn
and Ni was 0.01 to 0.5 g/m2 (Examples 85 to 88).
[0075]
[Table 4]
10
Deposition
Coating film on outer surface
amopnt on outer
side
Outer surface performance
surface side
Weldability
Deposition
Water
Coating
Coating
Zn Ni Type contact property after
amount
angle
property
storage
Example 82 18 2 - - Less than 5 Satisfactory Satisfactory 3.0 kA
Example 83 0 0 m1 0.7 53 Satisfactory Satisfactory 3.5 kA
Example 84 0 0 - - 10 Satisfactory Satisfactory 3.7 kA
Example 85 0.5 0 - - Less than 5 Excellent Excellent 3.7 kA
Example 86 0 0.1 - - Less than 5 Excellent Excellent 3.7 kA
Example 87 0.2 0.1 - - Less than 5 Excellent Excellent 3.7 kA
Example 88 0 O.Ql - - Less than 5 Excellent Excellent 3.7 kA
Example 89 ·l.o 0 - - Less than 5 Satisfactory Satisfactory 3.7 kA
[0076]
The preferred examples of the present invention have been described above,
15 whilst the present invention is not limited to the above examples. A person skilled
in the art may find various alterations and modifications within the scope of the
appended claims, and it should be understood that they will naturally come under the
teclmical scope of the present invention.
20 Industrial Applicability
[0077]
The present invention is industrially useful, because the present invention
can provide the steel sheet for a fuel tank under an environment in which various
PCT/JP2016/052911
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fuels exist, the fuels including a fuel containing an organic acid such as deteriorated
gasoline and a fuel containing dew condensation water in addition to an organic acid.
[0078]
The present invention provides the steel sheet having different surface states
5 between the inner and outer surfaces, by making the surface to be the outer surface of
the fuel tank to be a base metal and making the surface to be the inner surface of the
fuel tank to be the chromate-free chemical conversion coating film. Such a steel
sheet according to the present invention has every performance necessary from the
production of the fuel tank to the using of the fuel tank: (i) excellent workability that
10 is necessary during processing and welding of the steel sheet to make the steel sheet
into a tank shape; (ii) excellent coating property that is necessary during coating of
the processed tank to enhance the visual quality of the external appearance; and (iii)
excellent corrosion resistance that is necessary for using the produced tank for a long
time.
15 [0079]
The steel sheet according to the present invention also has an effect that is
not conventionally achieved in the point that the surface states that are different
between the inner and outer surfaces interact with each other and differences in
performances between the inner and outer surfaces can be further exhibited. To be
20 specific, since a surplus of the coating (oil) applied to the base metal on the outer
surface of the fuel tank is adsorbed on the chromate-free chemical conversion
coating film on the inner surface of the fuel tank, the coating property of the outer
surface improves.

CLAIMS
Claim 1
A steel sheet for a fuel tank, comprising:
a Zn-Ni alloy plated layer which is placed on one surface or each of both
5 surfaces of a base metal and formed on at least one surface; and
an inorganic chromate-free chemical conversion coating film which 1s
placed over the Zn-Ni alloy plated layer, wherein
the Zn-Ni alloy plated layer has a crack starting from an interface between
the Zn-Ni alloy plated layer and the inorganic chromate-free chemical conversion
10 coating fihn and reaching an interface between the Zn-Ni alloy plated layer and the
steel sheet, and
a water contact angle on a surface of the inorganic chromate-free chemical
conversion coating film is more than or equal to 50 degrees.
15 Clain12
The steel sheet for a fuel tank according to claim 1, wherein
one surface of the base metal has the Zn-Ni alloy plated layer and the
inorganic chromate-free chemical conversion coating film, and a surface opposite to
the one surface does not have the Zn-Ni alloy plated layer and the inorganic
20 chromate-free chemical conversion coating film.
Clain13
The steel sheet for a fuel tank according to clainl 2, wherein
a water contact angle on the surface not having the Zn-Ni alloy plated layer
25 and the inorganic chromate-free chemical conversion coating film is less th~ 10
degrees.
Claim4
The steel sheet for a fuel tank according to claim 2 or 3, wherein
PCT/JP2016/0529ll
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a deposition amount of Zn and/or Ni on the surface not having the Zn-Ni
alloy plated layer and the inorganic chromate-free chemical conversion coating film
is O.oi to 0.5 g/m2
.
5 Claim 5
The steel sheet for a fuel tank according to any one of claims 1 to 4, wherein
the inorganic chromate-free chemical conversion coating film contains one
or more selected from a silane coupling agent, a condensation polymer of a silane
coupling agent, silica, silicate, phosphoric acid, and phosphate, and a compound of
10 one or more metals selected from Ti, Zr, V, Mo, and W.
Claim 6
The steel sheet for a fuel tank according to any one of claims 1 to 5, wherein
the inorganic chromate-free chemical conversion coating film contains a
15 water repellent.
Claim 7
The steel sheet for a fuel tank according to claim 6, wherein
the water repellent is one or more selected from polyolefin wax, a silicon-
20 based resin, and a fluorine-based resin.
Claim 8
The steel sheet for a fuel tank according to any one of claims 1 to 7, wherein
in the case where a cross section of the Zn-Ni alloy plated layer is observed,
25 the number of the cracks in a visual field of 100 !J-ill is more than or equal to 5 and
less than or equal to 50.
Claim 9
The steel sheet for a fuel tank according to claim 8, wherein
5
PCT/JP2016/052911
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the n]lllber X of the cracks in a visl!tal field of J 00 !ffi1 obtained by ' . .
observing the cross section of the Zn-Ni alloy plated layer and the water. contact
'
angle Y (degrees) satisfY a relationship represented by the following formula (I),
Y2::-0.18X +56.5 (I).
Claim 10
The steel sheet for a fuel tank according to claim 9, wherein
in the case where a cross section of the Zn-Ni alloy plated layer is observed;
the number of the cracks in the visual field of 100. !ffi1 is more than or equal to 5 and ·
10 less than 50, and a m:L'Cimum width of a crack is less than 0.5 j.tm.