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. For that reason,
in the field of automobiles, the switching from a lead-tin alloy plated steel sheet,
15 which has been a main ingredient of fuel tanks, to a material not containing lead has
been promoted. Accordingly, to a unique required performance of the fuel tanks,
which is high inner corrosion resistance with respect to an enclosed fuel is enclosed
(hereinafter, also referred to as fuel corrosion resistance), many suggestions are given
involving using zinc-based plated steel sheets instead of lead-tin-based plated steel
20 sheets as i1mer and outer sheets 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
25 incompatible with the recent market needs for not using harmful metals. For this
reason, as described in the following Patent Literatures 4 to 9, a chromate-fi"ee zincbased
plated steel sheet that does not contain chromium for a fuel tank is disclosed.
30 Patent Literature
[0004]
Citation List
5
10
Patent Literature 1:
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/JP2016/052912
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JP H5-106058A
JP H9-324279A
JP H9-324281A
JP 2004-169122A
JP 2007-186745A
JP 2013-133527A
JP 2013-227646A
JP 2011-38139A
W02007 /011008
Summary oflnvention
However, compared to the fuel corrosion resistance of the conventional
15 zinc-hased plated steel sheet subjected to the chromate treatment, the fuel corrosion
resistance of the chromate-free zinc-based plated steel sheets for fuel tanks disclosed
in the Patent Literatures 4 to 9 are not sufficient in fuel corrosion resistance under
severer conditions, for example, corrosion resistance over a longer time period, and
corrosion resistance in the case where damage in the coating film has occurred
20 during processing. Therefore, the chromate-free zinc-based plated steel sheets for
fuel tanks require further improvement.
[0006]
For example, Patent Literatures 4 and 5 each disclose a chromate-free
coating film mainly containing an organic resin. However, when such a chromate-
25 free coating film mainly containing an organic resin is exposed to a fuel environment
for a long time period, the chromate-free c0ating film has a defect that the organic
resin is swelled by the fuel, and adhesion between surfaces of plating decreases. It
is assumed that such swelling of the organic resin is a factor in insufficient corrosion
resistance.
30 [0007]
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Further, Patent Literatures 6 and 7 each disclose a phosphoric acid-based
inorganic chromate-free coating film. However, the phosphoric acid-based
inorganic chromate-free coating film has a defect that the phosphoric acid-based
inorganic chromate-free coating film does not necessarily have sufficient water
5 resistance, and lacks corrosion resistance in particular in the case where a fuel
contains dew condensation water.
[0008]
Patent Literature 8 describes that a chromate-free chemical converswn
treated layer is formed on a zinc-nickel alloy plated layer having a crack. However,
10 the chemical conversion treated layer mainly contains a urethane-based aqueous
resin, and is improved in corrosion resistance, but has a defect that the chemical
conversion treated layer is not imparted with the corrosion resistance with respect to
deteriorated gasoline. The reason therefor is, as will be described later, phosphoric
acid, vanadium, titanium, and zirconium contained in the chemical conversion
15 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
20 organosilicon compound. However, the steel sheet is improved in corrosion
resistance, but has a defect that the steel sheet is not sufficient in terms of corrosion
resistance with respect to deteriorated gasoline.
[0010]
Accordingly, the present invention has been made in view of the above
25 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
30 chromium.
Solution to Problem
[00 II]
4/33
PCT/JP2016/052912
The inventors of the present invention have investigated improvement in
fuel corrosion resistance in a chromate-free zinc-based plated steel sheet, and have
5 found that remarkable improvement can be obtained by making a zinc-based plated
layer to be a Zn-Ni alloy plated layer, forming a predetermined crack on the plated
layer, and providing a chromate-free chemical conversion coating film.
The gist of the present invention accomplished on the basis of the above
finding is as follows.
10 [0012]
(1)
A steel sheet for a fuel tank, the steel sheet including:
a Zn-Ni alloy plated layer which is placed on one surface or each of both
surfaces of a base metal; and
15 a 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
the Zn-Ni alloy plated layer and the chromate-free chemical conversion coating film
and reaching an interface between the Zn-Ni alloy plated layer and the steel sheet,
20 the chromate-free chemical conversion coating film consists of an
organosilicon compound consisting of a condensation polymer of a silane coupling
agent, a phosphoric acid compound and/or a phosphonic acid compound, a vanadium
compound, and a titanium compound and/or a zirconium compound, and
a concentration of a total of amounts in terms of metal, per surface of the
25 chromate-free chemical conversion coating film, of the phosphoric acid compound
and/or the phosphonic acid compound + the vanadium compound + the titanium
compound and/or the zirconium compound, is more than or equal to 5 mass% and
less than or equal to 20 mass%.
(2)
30 The steel sheet for a fuel tank according to (1 ), in which
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one surface of the base metal has the Zn-Ni alloy plated layer and the
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 chromate-free chemical
conversion coating film.
5 (3)
The steel sheet for a fuel tank according to (2), in which
a deposition amount of Zn and/or Ni on the surface not having the Zn-Ni
alloy plated layer and the chromate-free chemical conversion coating film is 0.01 to
0.5 g/m2
•
10 (4)
The steel sheet for a fuel tank according to any one of (I) to (3 ), 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 100 f.Lm is more than or equal to 5 and
less than or equal to 50.
15 (5)
The steel sheet for a fuel tank according to any one of (1) to ( 4 ), in which
the number X of the cracks in a visual field of 1 00 f.Lm obtained by
observing the cross section of the Zn-Ni alloy plated layer and a concentration Y (%)
ofac total of amounts in terms of metal of P+ V + Ti+ Zr in the chromate-free chemical
20 conversion coating film satisfy a relationship represented by the following formula
(I),
Y::C-0.06X +6.8 (I).
(6)
The steel sheet for a fuel tank according to ( 5), in which
25 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 f.Lm is more than or equal to 5 and
less than 50, and a maximum width of a crack is less than 0.5 f.Lm.
Advantageous Effects of Invention
30 [0013]
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According to the present invention, there can be provided a steel sheet for a
fuel tank, which exhibits 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
5 an organic acid, without using environmental burden substances such as lead and
chromium.
Description of Embodiments
[0014]
10 Hereinafter, preferred embodiments of the present invention will be
described in detail.
[0015]
The present invention relates to a steel sheet for a fuel tank that uses a zincbased
plated steel sheet which is excellent in corrosion resistance to various fuels,
15 does not use lead or chromate treatment, and is environmentally friendly. The
present invention also relates to a steel sheet used for an automobile, a motorcycle,
industrial machinery, and construction machinery, and in addition, used for a tank in
which a fuel is enclosed and a part of the tank.
20
[0016]
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 a predetermined chromate-free chemical
conversion coating film (hereinafter, also simply referred to as "chemical conversion
coating film") which is placed over the Zn-Ni alloy plated layer (hereinafter, also
25 simply referred to as "plated layer").
[0017]
In the case where the steel sheet for a fuel tank according to the present
invention is used for a fuel tank, a surface having the Zn-Ni alloy plated layer and the
chromate-free chemical conversion coating film which is placed over the Zn-Ni alloy
30 plated layer is a fuel tank-inner surface (hereinafter, referred to as inner surface). In
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that case, the surface opposite to the inner surface IS a fuel tauk -outer surface
(hereinafter, referred to as outer surface).
[0018]
In the steel sheet according to the present invention, the Zn-Ni alloy plated
5 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
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
10 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
har~d, in the case where the steel sheet is used for the case where the outer surface of
the fuel tauk 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
15 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.
[0019]
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
20 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
example, grinding with a brush). Further, the above-mentioned methods can be
25 used in combination.
[0020]
Owing to the above methods, the non-plated surface (outer surface) of the
steel sheet according to the present invention can be completely prevented from
having Zn and/or Ni being deposited thereon. In that case, the deposition amount of
30 Zn and/or Ni is 0. As described above, this is for improving the weldability and the
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coating property of the steel sheet. It is desirable that the deposition amount be 0.01
to 0.5 g/m2
, preferably 0.01 to 0.3 g/m2
, more preferably 0.01 to 0.1 g/m2
.
[0021]
Here, a base material of the steel sheet for a fuel tank according to an
5 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
material of a zinc-based plated steel sheet.
[0022]
The Zn-Ni alloy plated layer formed on at least one surface of the steel sheet
10 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
method.
[0023]
The Zn-Ni alloy plated layer is characterized in having a crack starting from
15 a surface layer of the plated layer (in other words, an interface between the Zn-Ni
alloy plated layer and the chromate-free chemical conversion coating film) 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). Note that the presence of the
crack can be confirmed by observing a cross section using a scanning electron
20 microscope (SEM).
[0024]
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
25 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
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.
30 [0025]
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On the other hand, in a chromate-free chemical conversion coating film that
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
5 that there is no strong corrosion inhibitor such as Cr6
+, which exists in the chromate
coating film.
[0026]
In order to exhibit fuel corrosiOn resistance, m particular, satisfactory
corrosion resistance under a fuel environment including an organic acid such as
10 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
necessary that, in addition to that a crack exist in the Zn-Ni alloy plated layer, a
specific chromate-free chemical conversion coating film described later be formed.
[0027]
15 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
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 processing as described above, a corrosion inhibiting effect at the
20 damaged parts in the chemical conversion coating film and the plated layer during
processmg.
[0028]
The corrosion inhibiting effect of the steel sheet according to the present
invention will be described.
25 In the case where a steel sheet that does not have a crack in a Zn-Ni alloy
plated layer is damaged at a coating film and a plated layer at the surface due to press
working or the like, a base metal at which a flaw occurs is exposed, the anticorrosion
function with a sacrifice of the plated layer alone is not sufficient, and the corrosion
progresses. On the other hand, in the case where the steel sheet according to the
30 present invention is damaged at the coating film and the plated layer at the surface
due to press working or the like, the base metal at which a flaw occurs is exposed,
PCT/JP2016/052912
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however, since the chemical conversion coating film that has entered the crack in the
plated layer is present in the vicinity of the exposed base metal and since a specific
substance that has a corrosion inhibiting function is eluted from the chemical
conversion coating film according to the present invention, as will be described later,
5 the corrosion can be remarkably inhibited.
[0029]
Next, the chromate-free chemical conversion coating film according to the
present embodiment will be described.
The chromate-free chemical conversiOn coating fihn consists of: an
10 organosilicon compound consisting of a condensation polymer of a silane coupling
agent; (i) a phosphoric acid compound and/or a phosphonic acid compound; (ii) a
vanadium compound; and (iii) a titanium compound and/or a zirconium compound.
Here, it is necessary that a concentration of a total of amounts in terms of metal, per
surface of the chromate-free chemical conversion ·coating film, of P+ V + Ti+ Zr (in
15 more detail, the total mass in terms of metal, per surface, of the above-mentioned
compounds (i) to (iii) with respect to the total solid content of the chemical
conversion coating film) be more than or equal to 5 mass% and less than or equal to
20 mass%.
[0030]
20 The coating film that mainly contains an organosilicon compound consisting
of a condensation polymer of a silane coupling agent is assumed to have, although
the mechanism therefor is not clear, satisfactory wettability with the Zn-Ni alloy
plated layer, and enters a small crack in the plated layer to exhibit the abovementioned
effects. With that, the coating film that mainly contains an organosilicon
25 compound consisting of a condensation polymer of a silane coupling agent is
excellent in water resistance and swelling resistance to a hydrocarbon, and exhibits
satisfactory fuel corrosion resistance.
[0031]
The phosphoric acid compound and/or the phosphonic acid compound, the
30 vanadium compound, and the titanium compound and/or the zirconium compound
elute P, V, and Ti and/or Zr, respectively, under a corrosive environment, function as
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corrosion inhibitors, and consequently exhibit the above-mentioned effects. The
corrosion inhibiting effect is exhibited particularly in the case where all P, V, and Ti
(and/or Zr) are present. In the case where the concentration of the total of amounts
in terms of metal ofP+V+Ti+Zr in the chemical conversion coating film is less than
5 5%, the corrosion resistance with respect to deteriorated gasoline is insufficient. On
the other hand, in the case where the concentration of the total of amounts in terms of
metal of P+V+ Ti+Zr in the chemical conversion coating film exceeds 20%, general
corrosion resistance such as corrosion resistance with respect to a brine environment
decreases. Here, the concentration of the total of amounts in terms of metal of
10 P+V+ Ti+Zr in the chemical conversion coating film is preferably more than or equal
to 5 mass% and less than or equal to 15 mass%. It is more preferably more than or
equal to 7 mass% and less than or equal to 15 mass%, and still more preferably more
than or equal to 7.4 mass% and less than or equal to 13.5 mass%.
15
[0032]
Note that the method of measuring the concentration of the total of amounts
in terms of metal of P+V+ Ti+Zr in the chemical conversion coating film is not
particularly limited. The measurement can be performed thorough a known
measurement method, and can be performed through an X-ray fluorescence method,
for example. The X-ray fluorescence method is a method involving creating in
20 advance a calibration curve with a deposition amow1t -known sample by means of the
gravimetric method or the like for each of elements of interest, and calculating a
deposition amount from an X-ray fluorescence intensity of a sample of interest.
[0033]
Specific examples of the silane coupling agent which is a base of the
25 organosilicon compound include vinyl trimethoxy silane, vinyl triethoxy silane, 3-
glycidoxypropyl trimethoxy silane, 3-glycidoxypropyl triethoxy silane, 3-
glycidoxypropyl methyl dimethoxy silane, 3-glycidoxypropyl methyl diethoxy silane,
2-(3,4 epoxycyclohexyl)ethyl trimethoxy silane, 3-methacryloxypropyl methyl
diethoxy silane, 3-methacryloxypropyl trimethoxy silane, 3-methacryloxypropyl
30 methyl diethoxy silane, 3-methacryloxypropyl triethoxy silane, 3-acryloxypropyl
trimethoxy silane, 3-aminopropyl trimethoxy silane, 3-aminopropyl triethoxy silane,
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N-2(aminoethyl)3-aminopropyl trimethoxy silane, N-2(aminoethyl)3-aminopropyl
triethoxy silane, N-2(aminoethyl)3-aminopropyl methyl diethoxy silane,
bis(trimethoxysilyl propyl)amine, 3-triethoxysilyl-N-(1,3-dimethylbutylidene)
propyl amine, N-phenyl-3-aminopropyl trimethoxy silane, 3-ureidopropyl
5 triethoxy silane, 3-mercaptopropyl methyl diethoxy silane, 3-mercaptopropyl
trimethoxy silane, 3-mercaptopropyl triethoxy silane, 3-isocyanatepropyl triethoxy
silane, and bis(trimethoxysilyl)hexane. Those silane coupling agents may be used
alone or two or more may be used in combination. The silane coupling agent( s)
may be dissolved or dispersed in water, the resultant is stirred at predetermined
10 temperature for a predetermined period of time to be subjected to polycondensation,
and thus, an organosilicon compound can be obtained.
[0034]
The phosphoric acid compound according to the present embodiment is not
particularly limited, and examples thereof include phosphoric acid, metaphosphoric
15 acid, pyrophosphoric acid, polyphosphoric acid, primary, secondary, or tertiary
phosphoric acid alkali salt, primary, secondary, or tertiary phosphoric acid
ammomurn salt, and primary phosphate of polyvalent metals represented by
magnesturn pnmary phosphate, aluminum primary phosphate, and manganese
primary phosphate.
20 [0035]
The phosphonic acid compound according to the present embodiment is not
particularly limited, and examples thereof include 1-hydroxyethylidene, 1,1 'diphosphonic
acid, aminotrimethylene phosphonic acid, ethylenediamine
tetramethylene phosphonic acid, hexamethylenediamine tetramethylene phosphonic
25 acid, diethylenetriamine pentamethylene phosphonic acid, and salts thereof.
[0036]
30
The vanadium compound according to the present embodiment is not
particularly limited, and examples thereof include vanadium pentoxide, metavanadic
acid, armnonium metavanadate, sodium metavanadate, vanadium oxytrichloride,
vanadium trioxide, vanadium
oxyacetylacetonate, vanadium
dioxide, vanadium oxysulfate, vanadium
acetylacetonate, vanadium trichloride. and
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phosphovanadomolybdic acid. In addition, it is also usable that the resultant
obtained by reducing a pentavalent vanadium compound to a tetravalent, trivalent, or
divalent compound using an organic compound having at least one functional group
selected from the group consisting of a hydroxyl group, a carbonyl group, a carboxyl
5 group, a primary amino group, a secondary amino group, a tertiary amino group, an
amide group, a phosphoric acid group, and a phosphonic acid group.
[0037]
The titanium compound according to the present embodiment is not
particularly limited, and examples thereof include hexafluorotitanic acid or salts
10 thereof, tetramethoxy titanium, tetraethoxy titanium, tetra-i-propoxy titanium, tetran-
propoxy titanium, tetra-n-butoxy titanium, tetra-i-butoxy titanium, tetra-sec-butoxy
titanium, tetra-t-butoxy titanium, tetraacetylacetonato titanium, diisopropoxy
bis( acetylacetonato )titanium, isopropoxy(2-ethyl-1,3-hexanediolato )titanium,
15
diisopropoxy bis(triethanolaminato )titanium,
bis(triethanolaminato )titanium, and hydroxybis(lactato )titanium.
[0038]
di-n-butoxy
The zucomum compound according to the present embodiment IS not
particularly limited, and examples thereof include hexafluorozirconic acid or salts
thereof, tetramethoxy zirconium, tetraethoxy zirconium, tetraci-propoxy zirconium,
20 tetra-n-propoxy zirconium, tetra-n-butoxy zirconium, tetra-i-butoxy zirconium, tetrasec-
butoxy zirconium, tetra-t-butoxy zirconium, tetraacetylacetonato zirconium,
diisopropoxy bis( acetylacetonato )zirconium, isopropoxy(2-ethyl-1 ,3-
hexanediolato )zirconium, diisopropoxy bis(triethanolaminato )zirconium, di-n-butoxy
bis(triethanolarninato )zirconium, hydroxybis(lactato )zirconium, and oxycarbonate
25 z1rcomum.
[0039]
The deposition amount of the 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 of the
30 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
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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
conversion coating film per surface is not particularly limited. The measurement
5 can be performed thorough a known measurement method, and can be performed
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.
10 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.
15
[0040]
Here, the 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 known method such as bar coater and roll coater. Then, the
20 obtained applied film may be heated and dried at predetermined heating temperature.
[0041]
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
25 resin, performing cross section vertical polishing, and observing a cross section using
a SEM. In this case, observing a visual field of 100 !liD at approximately 1000-fold
magnification, it is more preferred that the number of cracks starting from the plated
surface layer and reaching the base metal be more than or equal to 5 and less than or
equal to 50. In the case where the number of cracks is small, for example, in the
30 case where the number of cracks in the visual field of 100 !liD is less than 5, the fuel
corrosion resistance tends to decrease. On the other hand, in the case where the
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number of cracks is too large, for example, in the case where the number of cracks in
the visual field of 100 fill exceeds 50, general corrosion 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 number of cracks in the visual
5 field of 100 fim is more preferably more than or equal to 10 and less than or equal to
40.
[0042]
Moreover, in the case where cracks are observed in the above-mentioned
method and a crack having the largest opening width among the cracks starting from
10 the plated surface 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 fiill. In the case where the maximum width is more
than or equal to 0.5 fill, general corrosion resistance snch as corrosion resistance
with respect to a brine environment tends to decrease.
15 [0043]
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
cracks and a concentration of a total of amounts in terms of metal of P+V+ Ti+Zr,
which are elnted components for corrosion inhibition. To be specific, in the case
20 where the number of cracks is small, it is important that the concentration of the
eluted components be kept higher, and on the other hand, in the case where the
number of cracks is large, satisfactory corrosion resistance with respect to
deteriorated gasoline can be obtained in a concentration of the elnted components of
a relatively wide range. As a result of conducting a detailed investigation, to be
25 specific, the best corrosion resistance with respect to deteriorated gasoline can be
obtained if the number X of cracks (in a visna! field of 100 fllll) and the
concentration Y(%) of the total of amounts in terms of metal of P+V+ Ti+Zr satisfy
the following relationship of Expression (I).
[0044]
30 Y2:-0.06X +6.8 (I)
[0045]
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In order to obtain the best characteristics in the corrosion resistance with
respect to deteriorated gasoline and general corrosion resistance such as corrosion
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
5 base metal more than or equal to 5 and less than 50 (in the visual field of 100 11m),
and to make the maximum width of the crack less than 0.5 11m.
[0046]
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
10 deposition amount of the Zn-Ni alloy plated layer per surface is less than 5, 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
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
15 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
limited, and is preferably 9 to 14 mass%, the range in which the corrosion resistance
with respect to deteriorated gasoline becomes satisfactory. The Zn-Ni alloy plated
20 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.
[0047]
Note that the deposition amount of the Zn-Ni alloy plated layer per surface
25 can be controlled through a quantity of electricity (amount of coulomb) in the case of
using a.'1 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
thorough a known measurement method, for example, through a gravimetric method
30 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
PCT/JP2016/052912
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only the plated layer in hydrochloric acid, and determining a deposition amonnt from
the difference with a weight after the dissolving. 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,
5 and calculating a deposition amount from an X-ray fluorescence intensity of a
sample of interest.
[0048]
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
10 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
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
15 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
the case where there is an attempt to suppress the crack width, the time for the
20 treatment may be made short. In the case where the treatment is performed at high
temperature for a short period of time, the munber 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
too large, but the crack width tends to increase.
25 [0049]
The configuration of the present invention described above relates to the
inner surface of a fuel tank, in which the fuel corrosion resistance becomes a problem.
The outer surface of the fuel tank is not particularly limited. The surface to be the
outer surface of the fuel tank does not necessarily have the Zn-Ni alloy plated layer,
30 may have the Zn-Ni alloy plated layer, and may have, in addition, the chromate-free
chemical conversion coating film over the Zn-Ni alloy plated layer.
[Examples]
[0050)
18/33
PCT/JP2016/052912
Next, the steel sheet for a fuel tank according to the present invention will
be described further specifically with reference to Examples and Comparative
5 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
according to the present invention is not limited to the following examples.
[0051)

10 (Examples 1 to 27 and Comparative Examples 1 to 6)
An extra-low carbon steel sheet was used as an original sheet, a sulfuric acid
acidic plating bath was used, electroplating was performed, and a Zn-Ni alloy plated
layer whose deposition amount per surface was 15 g/m 2 and containing 10 mass% of
Ni was formed. The resultant was held in the plating bath for three seconds in the
15 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 g/L of zinc sulfate heptahydrate, 3 80 g/L of nickel sulfate
hexahydrate, 80g/L of sodium sulfate, and I Og/L of sulfuric acid. After that, each
of the chromate-free chemical conversion coating films was formed in a
20 predetermined amount over the obtained Zn-Ni alloy plated layer, the chromate-free
chemical conversion coating films being obtained by making a base coating film
shown in Table 1 contain the phosphoric acid compound and/or the phosphonic acid
compound, the vanadium compound, and the titanium compound and/or the
zirconium compound in amounts shown in Table 2. The chromate-free chemical
25 conversion coating film was formed through application using a bar coater and
baking in a hot air drying furnace (ultimate sheet temperature: 100°C). Note that
the deposition amount of the chromate-free chemical conversion coating film and the
concentration of the total of amounts in terms of metal ofP+V+Ti+Zr were measured
through the X-ray fluorescence method described above.
5
[0052]
[Table 1]
----
No
a
b
c
~--------
[0053]
[Table 2]
PCT/JP2016/052912
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cdmposition
Condensation polymer of 3-glycidoxypropyl trimethoxy silane and 3-aminopropyl triethoxy silane in a ratio of 1:1
(molecular weight of approximately 3000)
Condensation polymer of 3-glycidoxypropyl trimethoxy silane and 3-aminopropyl triethoxy silane in a ratio of 3:2
(molecular weight of approximately 2500)
Mixture of Na neutralized ionomer resin, silica, and 3-glycidoxypropyl trimethoxy silane in a ratio of 100:30:3
-~--
PCT/JP2016/052912
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T. a-b-·le- -2
Coating film No. - Remarks
e1 ,, •3 •4 •5 •6 ,, ,, ,g •10 ·11 '" b1 b2 b3 b4 b5 b6 "1 "2 "3 "' "5 o1
~
Type of base ooati11g fllm ' ' ' ' ' ' ' I• ' ' ' ' b b b b b b ' ' ' ' ' 0
Phosphoric acid Phosphoric acid 2.5 2 2.7 3 3.4 5 5 J 5 3 1.6 2.5 2 2..7 3 3.4 5 3 3.5 1.5 5 2.5
compound .... """" .................. ·-·········-· ---- ...... ------ '" ---. ---- I· . ........ ··-·-··-·· ----- -- ----- --· ·····--·
_____ ,. ___ -------- ------ ------- -- ----· ••........
Concentration of P
" Magnesium primary phosphate 4.5
(mass%)
phosphonic acid ------- ........ ····-············- ---·----r----- r--- ... ---- ......... ·---·- - -·- ---- -----1-- - ---
compound 1-hydroxyethylidene-1, 1 '-diphosphonic acid 5 5 10 12
Vanadyl acetylacetonate 14 2.9 2 2.3 2.5 4 5 2 2 2.3 4 1.4 2.9 2 2.3 2.5 4 3 3.5 1 6 1.4 Concentration of V
Vanadium compound
Ammonium metavanadate 0.5 3 (mass%)
Titanium compound
Hexafluorotitanate 11 0.5 1.2 1.2 1.5 -'- 5 ~- 2.5 1.5 1.1 0.5 -'~ 1.2 1.5 2 2.5 2.5 1 2 ....... -···-·--·" ----·"- --· - r'-'- Concentration of Ti
oc Titanium diisopropoxy bis(acetylacetonate) 0.5 2
and Zr (mass%)
zirconium compound
He xafluNotircon ate 3 3 .
Concentration of P+V+ TI+Zr (mass%) 5 5.4 5.9 6.5 7.4 11 20 8.6 14 9 17 8.8 5 5.4 5.9 6.5 7.4 11 6 6 L 6 .~2. '-"- 5
PCT/JP2016/052912
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[0054]
(Comparative Examples 7 to 1 0)
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
5 electricity after the Zn-Ni alloy plating was not performed.
[0055]
[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.
10 A visual field of 100 rtm was observed at 1000-fold magnification, and the number
of cracks starting from the plated surface layer and reaching the base metal was
counted. Moreover, a crack having the largest opening width among the cracks
starting from the plated surface layer and reaching the base metal in the visual field
was observed using the SEM at approximately 10000-fold magnification to measure
15 the maximum width.
[0056]
[Corrosion resistance with respect to deteriorated gasoline]
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
20 removing oil through grease removing treatment, a flaw that reaches up to the base
metal was made by a cutter on the inner surface base (which imitates a flaw caused
by press working). Then, test liquid that imitates the deteriorated gasoline (gasoline
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.
25 After that, rust was removed, a maximum reduction in sheet thickness (mm) due to
corrosion was measured.
[0057]
[SST corrosion resistance]
A back surface and an edge of each of the samples were sealed and a salt
30 spray test defined in JISZ23 71 was performed for 72 hours to measure an area
proportion(%) of white rust occurrence.
PCT/JP2016/052912
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[0058]
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
5 respect to deteriorated gasoline, and SST corrosion resistance. Note that the
evaluation results of the corrosion resistance with respect to deteriorated gasoline can
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
10 less than 5%.
[0059]
[Table 3]
PCT/JP2016/052912
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Table 3
Zn-Ni alloy plated layer Chemical conversion treated coating film
Corrosion
. . . . . resistance
with respect
SST
Type of corrosion
Maximum Deposition to Remarks
Number of chemical P+Y+Ti+Zr resistance
width amount deteriorated
cracks (%)
( J1 m)
conversion
(g/m2)
(mass%) gasoline
coating film (mm)
1 13 0.3 a1 0.7 5 0.04 0
2 13 0.3 a2 0.7 5.4 0.02 0
3 13 0.3 a3 0.7 5.9 0.01 0
4 13 0.3 a4 0.7 6.5 0 0
5 13 0.3 a5 0.7 7.4 0 0
6 13 0.3 a6 0.7 11 0 0
7 13 0.3 a7 0.7 20 0 2
8 13 0.3 aS 0.7 8.6 0 0
9 13 0.3 a9 0.7 13.5 0 0
10 13 0.3 a10 0.7 9 0 0
11 13 0.3 a11 0.7 16.8 0 1
12 13 0.3 a12 0.7 8.8 0 0
13 13 0.3 b1 0.7 5 0.02 0
Example 14 13 0.3 b2 0.7 5.4 0.01 0
15 13 0.3 b3 0.7 5.9 0.01 0
16 13 0.3 b4 0.7 6.5 0 0
17 13 0.3 b5 0.7 7.4 0 0
18 13 0.3 b6 0.7 11 0 0
19 13 0.3 a3 0.3 5.9 0 2
20 13 0.3 a3 1 5.9 0 0
21 13 0.3 a3 1.6 5.9 0 0
22 13 0.3 a4 0.1 6.5 0.04 3
23 13 0.3 a4 0.5 6.5 0 0
24 13 0.3 a4 2 6.5 0 0
25 13 0.3 b4 0.4 6.5 0 1
26 13 0.3 b4 12 6.5 0 0
27 13 0.3 b4 1.7 6.5 0 0
1 13 0.3 aa1 0.7 - 0.1 5
Ti and Zr not
included
2 13 0.3 aa2 0.7 0.12 8 V not included
3 13 0.3 aa3 0.7 - 0.15 7 P not included
4 13 0.3 aa4 0.7 3.5 0.13 0.1
Campara 5 13 0.3 aa5 0.7 25 0.06 50
tive Organic resin-
Example 6 13 0.3 o1 0.7 - 0.25 2 based coating film
7 0 - a3 0.7 5.9 0.12 0
8 0 - aa3 0.7 - 0.14 0 P not included
9 0 aa4 0.7 3.5 0.1 0
10 0 - " 0.7 - 0.19 0.1
Organic resinbased
coating film
[0060]
As is clear from Table 3, Examples of the present invention each showed
5 satisfactory characteristics.
On the other hand, Comparative Examples 1, 2, and 3 did not contain any
one of P, V, and Ti (and/or Zr), which are constituent elements of the chromate-free
PCT/JP2016/052912
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chemical conversion coating film according to the present invention, and had poor
corrosion resistance with respect to deteriorated gasoline and poor SST corrosion
resistance. Further, even if P, V, and Ti (and/or Zr) were contained, in the case
where the total concentration ofP, V, and Ti (and/or Zr) was less than the lower limit
5 of the present invention (Comparative Example 4), the corrosion resistance with
respect to deteriorated gasoline was poor, and, in the case where the total
concentration of P, V, and Ti (and/or Zr) exceeded the upper limit (Comparative
Example 5), the corrosion resistance with respect to deteriorated gasoline and also
the SST corrosion resistance were poor. The coating film that mainly contained an
10 organic resin (Comparative Example 6) also had poor corrosion resistance with
respect to deteriorated gasoline. Moreover, in the case where the Zn-Ni alloy plated
layer did not have a crack (Comparative Examples 7 to 1 0), the corrosion resistance
with respect to deteriorated gasoline was poor. Note that, among the examples that
did not satisfy the constituent elements of the chromate-free chemical conversion
15 coating film according to the present invention, the examples which had the Zn-Ni
alloy plated layers with cracks tended to have decreased corrosion resistance with
respect to deteriorated gasoline and decreased SST corrosion resistance (comparisons
between Comparative Examples 3 and 8, between 4 and 9, and between 6 and 10).
20
[0061]
As described above, it was found that satisfactory characteristics can be
obtained with a combination of a plated layer-crack and a predetermined chromatefree
chemical conversion coating film.
[0062]

25 (Examples 28 to 80)
An extra-low carbon steel sheet was used as an original sheet, a sulfuric acid
acidic plating bath was used, electroplating was performed, and a Zn-Ni alloy plated
layer whose deposition amount per surface was 15 g/m2 and containing 10 mass% of
Ni was formed. A predetermined plated layer-crack was formed by varying
30 temperatme and time during imtnersion 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
PCT/JP2016/052912
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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 2 was formed in a predetermined amonnt in the same manner as in
5 Experiment 1. Further, the deposition amount of the chromate-free chemical
conversion coating film and the concentration of the total of amounts in terms of
metal of P+V+Ti+Zr were measured through the X-ray fluorescence method
described above.
[0063]
10 Performance evaluation was carried out in the same manner as Experiment 1
described above.
[0064]
Table 4 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
15 amount of a chemical conversion, corrosion resistance with respect to deteriorated
gasoline, and SST corrosion resistance.
[0065]
[Table 4]
PCT/JP2016/052912
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Table 4
I . Zn-Ni alloy plated layer Chemical conversion treated coating film Corrosion
resistance
Satisfy with
SST
Type of Deposition Expression
corrosion
Number of
Maximum
chemical P+Y+Ti+Zr
respect to
resistance
width amount (I)? deteriorated
(%) · .. cracks
( /lm)
conversion
(g/m2)
(mass%) gasoline
coating film (mm)
28 5 0.15 ,, 0.8 5 No 0.04 0
29 5 0.15 ,, 0.8 5.4 No 0.03 0
30 5 0.15 e3 0.8 5.9 No O.Q2 0
31 5 0.15 e4 0.8 6.5 Vee 0 0
32 5 0.15 e5 0.8 7.4 Vee 0 0
33 5 0.15 ,, 0.8 11 Vee 0 0
34 5 0.15 e8 0.8 8.6 Vee 0 0
35 5 0.15 ,, 0.8 13.5 Vee 0 0
36 5 0.15 ,,, 0.8 16.8 Vee 0 0
37 49 0.4 ,, 0.8 5 Vee 0 0
38 49 0.4 ,, 0.8 5.4 Vee 0 0
39 49 0.4 e3 0.8 5.9 Vee 0 0
40 49 0.4 e4 0.8 6.5 Vee 0 0
41 49 0.4 e5 0.8 7.4 Vee 0 0
42 49 0.4 e6 0.8 11 Vee 0 0
43 49 0.4 e8 0.8 8.6 Vee 0 0
44 49 0.4 ,, 0.8 13.5 Vee 0 0.3
45 49 0.4 ,,, 0.8 16.8 Vee 0 0.8
46 40 0.4 ,, 0.8 5 Vee 0 0
47 40 0.4 e2 0.8 5.4 Vee 0 0
48 40 0.4 e3 0.8 5.9 Vee 0 0
49 40 0.4 a4 0.8 8.5 Vee 0 0
50 40 0.4 a5 0.8 7.4 Vee 0 0
51 40 0.4 e6 0.8 11 Vee 0 0
52 40 0.4 e8 0.8 8.6 Vee 0 0
~ 40 0.4 ,, 0.8 13.5 Vee 0 0
Example 54 40 0.4 ,,, 0.8 16.8 Vee 0 0
55 16 0.3 a1 0.8 5 No 0.04 0
56 16 0.3 a2 0.8 5.4 No 0.02 0
57 16 0.3 a3 0.8 5.9 Vee 0 0
58 16 0.3 a4 0.8 6.5 Vee 0 0
59 16 0.3 aS 0.8 7.4 Vee 0 0
60 16 0.3 e6 0.8 11 Vee 0 0
61 16 0.3 e8 0.8 8.6 Vee 0 0
62 16 0.3 ,, 0.8 13.5 Vee 0 0
63 16 0.3 e11 0.8 16.8 Vee 0 0
64 28 0.25 el 0.8 5 No 0.03 0
65 28 0.25 a2 0.8 5.4 Vee 0 0
66 28 0.25 a3 0.8 5.9 Vee 0 0
67 28 0.25 a4 0.8 6.5 Vee 0 0
68 28 0.25 a5 0.8 7.4 Vee 0 0
69 28 0.25 ,, 0.8 II Vee 0 0
70 28 0.25 a8 0.8 8.6 Vee 0 0
71 28 0.25 ,, 0.8 13.5 Vee 0 0
72 28 0.25 ,,, 0.8 16.8 Vee 0 0
73 31 0.5 ,, 0.8 5 Yes 0 0.5
74 31 0.5 a2 0.8 5.4 Vee 0 1
75 31 0.5 e3 0.8 5.9 Vee 0 2
76 31 0.5 e4 0.8 6.5 Vee 0 2
77 39 0.7 ,, 0.8 5 Vee 0 2
7B 39 0.7 e2 0.8 5.4 Yee 0 3
79 39 0.7 e3 0.8 5.9 Yes 0 3
80 39 0.7 e4 0.8 6.5 Yes 0 3
PCT/JP2016/052912
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[0066]
As is clear from Table 4, 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, which exhibited particularly excellent corrosion resistance with
respect to deteriorated gasoline. Further, each of Examples having a crack width of
less than 0.5 1-lm had a rate of white mst occurrence in the SST corrosion resistance
10 of less than 1%, which exhibited excellent SST corrosion resistance. In particular,
in the case where the crack width was less than 0.5 1-lm and the number of cracks was
less than 50, the rate of white mst occurrence in the SST corrosion resistance was
zero under any condition, which was particularly excellent
15
[0067]
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.
[0068]
20
(Examples 81 to 88)
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
a fuel tank was varied. In Example 81, the outer surface was not provided with a
25 coating film. In each of Examples 82 and 83, the plating on the surface to be the
outer surface was completely removed through grinding with a brush. After that, in
Example 82, a predetermined coating film was applied. In each of Examples 84 to
88, on the outer surface from which the plating was completely removed through the
above method, predetermined amounts of Zn and Ni were deposited again through
30 electroplating.
[0069]
28/33
Perfonnance evaluation was carried out as follows.
(Coating property)
PCT/JP20 16/052912
The coating property of the surface to be the outer surface of a fuel tank was
evaluated.
5 Usually, pre-coating treatment (zinc phosphate treatment) and electrodeposition were
performed for the outer surface of the fuel tank for an automobile or a motorcycle,
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 [0070]
(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!mand 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 enviromnent
for one month, the same coating as in the coating property evaluation was performed,
and the evaluation was performed using the same criteria
[0071]
20 (Weldability)
The surfaces to be the inner surfaces 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.
- Electrode: a disc-shaped electrode made of a Cu-Cr alloy, in which a cross section
25 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 rnm
- Welding method: piling of two sheets, lap seam welding
-Welding pressure: 400 kgf (note that 1 kgf is approximately 9.8 N)
-Welding time: 2/50 sec on, 1/50 sec off
30 - Cooling: inner part water cooled, and outer part water cooled
-Welding speed: 2.5 rnlmin
PCT/JP2016/052912
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[0072]
The results are shown in Table 5. As is clear from Table 5, Examples of
the present invention each showed satisfactory characteristics. Also, it was found
that the coating property, the coating property after storage, and the weldability of
5 the steel sheet particularly improved in the case where the deposition amount of Zn
and Ni was 0.01 to 0.5 g!m2
10---
[0073]
[Table 5]
Example
[0074]
81
82
83
84
85
86
87
88
Deposition amount
on outer surface side
Zn Ni
13.5 1.5
0 0
0 0
0.5 0
0 0.1
0.2 0.1
0 0.01
1.0 0
Coating film on outer
Outer sutface performance
surface side
Weldability
Type
Oeposftion Coating Coating property
amount property after storage
- - Satisfactorv Satisfactory 32 kA
a1 0.7 Satisfactory Satisfactory 3.5 kA
- - Satisfactory Satisfactory 3.7 kA
- - Excellent Excellent 3.7 kA
- - Excellent Excellent 3.7 kA
- - Excellent ExceUent 3.7 kA
- - Excellent Excellent 3.7 kA
- - _Satisfactory Satisfactory 3.7 kA
The preferred examples of the present invention have been described above,
whilst the present invention is not limited to the above examples. A person skilled
15 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
technical scope of the present invention.
Industrial Applicability
20 [0075]
The present invention provides the steel sheet which shows excellent
corrosion resistance under a severer fuel environment containing dew condensation
water in addition to deteriorated gasoline, by forming, over the Zn-Ni alloy plated
layer, the chromate-free chemical conversion coating film in which the phosphoric
25 acid compound and/or the phosphonic acid compound + the vanadium compound +
PCT/JP2016/052912
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the titanium compound and/or the zirconium compound are blended at a specific
proportion in terms of metal.
[0076]
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 [0077]
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 I
A steel sheet for a fuel tank, the steel sheet comprising:
a Zn-Ni alloy plated layer which is placed on one surface or each of both
5 surfaces of a base metal; and
a chromate-free chemical conversion coating film which is 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 chromate-free chemical conversion coating film
10 and reaching an interface between the Zn-Ni alloy plated layer and the steel sheet,
15
20
the chromate-free chemical conversion coating film consists of an
organosilicon compound consisting of a condensation polymer of a silane coupling
agent, a phosphoric acid compound and/or a phosphonic acid compound, a vanadium
compound, and a titanium compound and/or a zirconium compound, and
a concentration of a total of amounts in tenns of metal, per surface of the
chromate-free chemical conversion coating film, of the phosphoric acid compound
and/or the phosphonic acid compound + the vanadium compound + the titanium
compound and/or the zirconium compound, is more than or equal to 5 mass% and
less than or equal to 20 mass%.
Claim 2
The steel sheet for a fuel tank according to claim I, wherein
one surface of the base metal has the Zn-Ni alloy plated layer and the
chromate-free chemical conversion coating film, and a surface opposite to the one
25 surface does not have the Zn-Ni alloy plated layer and the chromate-free chemical
conversion coating film.
Claim 3
The steel sheet for a fuel tank according to claim 2, wherein
PCT/JP2016/052912
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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 chromate-free chemical conversion coating film is 0.01 to
0.5 g/m2
.
5 Claim4
10
The steel sheet for a fuel tank according to any one of claims 1 to 3, wherein
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 100 f.UI1 is more than or equal to 5 and
less than or equal to 50.
Claim 5
The steel sheet for a fuel tank according to any one of claims 1 to 4, wherein
the number X of the cracks in a visual field of 100 f.UI1 obtained by
observing the cross section of the Zn-Ni alloy plated layer and a concentration Y(%)
15 of a total of amounts in terms of metal ofP+V+Ti+Zr in the chromate-free chemical
conversion coating film satisfy a relationship represented by the following formula
(I),
Y?.-0.06X+6.8 (I).
20 Claim 6
25
The steel sheet for a fuel tank according to claim 5, 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 f.UI1 is more than or equal to 5 and
less than 50, and a maximum width of a crack is less than 0.5 J.tm.