DESCRIPTION
Title of Invention
Chromate-Free Type Precoated steel sheet Excellent in
5 Edge Red Rusting Resistance
Technical Field
[0001 ] The present invention relates to a chromatefree
type precoated steel sheet excellent in edge red
10 rusting resistance. More particularly, the present
invention relates to a chromate-free type precoated steel
sheet which is used in the fields of electrical products,
buildings, etc. which are used in environments which
become wet with water, in particular outdoor environments
15 which are exposed to rainwater etc., that is, a chromatefree
type precoated steel sheet which is excellent in
edge red rusting resistance.
Background Art
20 [0002] Precoated steel sheet (also called "PCM") is a
metal sheet which is shipped out formed in advance with a
coating which users demand for their product. This
enables coating and related work at the users to be
eliminated and makes facilities for such work
25 unnecessary, so is being increasingly utilized in various
fields. In the initial precoated steel sheet, as the base
steel sheet to be coated, chromate-treated steel sheet
treated to prevent rust by chromate has been used. After
this, due to the problem of the toxicity of the
30 hexavalent chromium which can be eluted from the
chromate-treated coating, instead of a chromate-treated
precoated steel sheet, a chromate-free type precoated
steel sheet which uses a base steel sheet treated to
prevent rust without chromium has come under the
35 spotlight. In recent years, utilization has been
particularly increasing. In conventional chromate-free
type precoated steel sheet, the rust prevention pigment
- 2 -
which is comprised in the coating (in general, calcium
ion exchanged silica (also commonly referred to as
"calcium silicate"), aluminum tripolyphosphate, etc.) has
the rust-prevention action.
5 [0003 ] The mainstream of precoated steel sheets is
shifting from chromate-treated precoated steel sheet to
chromate-free type precoated steel sheet. Along with
this, in precoated steel sheet which is used in outdoor
environments exposed to rainwater etc., the problem has
10 arisen that red rust occurs at the edges and the
aesthetic appearance of products using the precoated
steel sheet is impaired. This problem does not occur with
chromate-treated precoated steel sheet and is a problem
unique to chromate-free type precoated steel sheet. Its
15 solution is being sought.
[0004 ] Several prior arts are known relating to the
improvement of the edge red rusting resistance of
chromate-free type precoated steel sheet.
For example, a chromate-free type precoated steel
20 sheet which can prevent the occurrence of edge red rust
which occurs by the effect of the low electrical
conductivity water without using a chromate-based rust
prevention pigment by having the coating at the outermost
layer which is formed at a plated steel sheet which has a
25 Zn-containing plating layer contain as a rust prevention
pigment a non-chromium compound (as an example, sodium
tripolyphoshate) which satisfies the conditions of (A) a
pH of 7 to 12 when dissolved in ion exchanged water by
0.1 masso concentration, (B) an electrical conductivity
30 of 500 μS/cm or more when dissolved in ion exchanged
water by 0.1 masso concentration, and (C) no thermal
decomposition up to 200°C, is known (PLT 1).
[0005 ] Further, a chromate-free type precoated steel
sheet which can suppress the occurrence of edge red rust
35 by having the coating at the outermost layer which is
formed at a plated steel sheet which has a Zn-containing
plating layer contain a non-chromium compound (as an
- 3 -
example, an alkali metal phosphate, chloride, and alkali
earth metal hypochlorite) which satisfies the conditions
of (A) an electrical conductivity of 500 μS/cm or more
when dissolved in ion exchanged water (electrical
5 conductivity: 4 μS/cm or less) by 0.1 masse concentration
and (B) no thermal decomposition up to 200°C, is known
(PLT 2).
[0006] Furthermore, a chromate-free type precoated
steel sheet excellent in edge corrosion resistance
10 wherein the primer coating on the non-chromium-based base
coating and/or topcoat coating of the steel sheet
comprises slow release rust-prevention particles
comprised of a composite of porous inorganic particles
(as an example, porous silica particles) and a metal
15 compound-based rust prevention pigment (as an example, Mg
or Ca compound) and which defines the elution speed of
the metal ions which are eluted from the rust prevention
pigment of the coating inscribed to 100 squares of 1 mm x
1 mm when immersed in a 40°C carbonate pH standard
20 solution is known (PLT 3).
[0007] Further, a chromate-free type precoated steel
sheet able to suppress the occurrence of edge red rust by
having a coating which comprises alkali earth metal
compound particles, zinc compound particles, and
25 phosphate particles and by defining a molar ratio of the
amounts of elution of an alkali earth metal, zinc, and
.phosphorus ingredients which are eluted from the coating
when immersed in ordinary temperature pure water is also
known (PLT 4).
30
Prior Art List
Patent Literature
PLT 1: Japanese Patent Publication (A) No. 2008-
229518
35 PLT 2: Japanese Patent Publication (A) No. 2009-
045923
4
PLT 3: Japanese Patent Publication (A) No. 2009-
078450
PLT 4: Japanese Patent Publication (A.) No. 2008-
189965
5
Summary of Invention
Technical Problem
[0008] The present invention has as its object the
provision of a new type of chromate-free type precoated
10 steel sheet excellent in edge red rusting resistance
which can be used without worry as to the occurrence of
the red rust which detracts from aesthetic appearance
even in an outdoor environment which is exposed to
rainwater etc. for promoting expansion of utilization of
15 chromate-free type precoated steel sheet free of elution
of harmful chromium.
Solution to Problem
[0009] The chromate-free type precoated steel sheet
20 excellent in edge red rusting resistance of the present
invention is a chromate-free type precoated steel sheet
comprising a zinc-based plated steel sheet, a chromatefree
type conversion treatment layer on the two surfaces
of the zinc-based plated steel sheet, and at least one
25 layer of a coating formed on the conversion treatment
layer, characterized in that the outermost layer of
coating among at least the coatings of the back surface
of the precoated steel sheet comprises a tungstate or
silicate with a high edge residence. The "high edge
30 residence" which is used in the present Specification
means the property judged as "good" or "fair" by the edge
residence evaluation which is explained below.
[0010] The typical chromate-free type precoated steel
sheet of the present invention has a primer coating layer
35 and a top coating layer on the chromate-free type
conversion treatment layer at the two surfaces, and the
top coating layer of the back surface comprises a
tungstate.
[0011] in addition to the top coating layer at the
back surface, the primer coating layer at the back
surface may also comprise a tungstate or silicate with a
5 high edge residence.
[0012] Preferable examples of tungstates include
sodium tungstate, calcium tungstate, ammonium tungstate,
lithium tungstate, and magnesium tungstate. Sodium
tungstate and magnesium tungstate are particularly
10 preferred. Two or more types of tungstates may also be
jointly used.
[0013] The content of the tungstate in the coating is
preferably 6 to 50 wt%.
[0014] Preferable examples of silicates are sodium
15 silicate, potassium silicate, and lithium silicate, while
sodium silicate and potassium silicate are particularly
preferred. Two or more types of silicates may also be
jointly used. t
[0015 ] The content of the silicate in the coating is
20 preferably 5 to 50 wt%.
[0016 ] The precoated steel sheet (PCM) of the final
product is generally supplied to a user in a form
provided on one surface with a coating of the performance
which is sought by the user which works it into the final
25 product. In the present invention, the surface which is
provided with the coating of the performance which is
sought by the user is viewed as the "front surface" of
the precoated steel sheet, while the surface at the
opposite side to that surface is called the "back
30 surface". The "front surface" is the surface which is
positioned at the outside in the final product which is
produced at the user side and is readily visible, while
the "back surface" is not readily visible at the final
product. Therefore, a coating at the back surface does
35 not require the performance which is sought for a coating
at the front surface. Due to this, the distinction
between the "front surface" and "back surface" of the
6
precoated steel sheet becomes clear at a glance from the
appearance. Comparing the two, the "back surface" is
particularly inferior from the aesthetic viewpoint, so
the two can be easily differentiated.
5
Advantageous Effects of Invention
[0017] According to the present invention, the
tungstate or silicate with a high edge residence which is
comprised in at least the outermost layer of coating
10 among the coatings of the back surface of a chromate-free
type precoated steel sheet contributes to the expression
of edge red rusting resistance and effectively suppresses
the occurrence of edge red rust in outdoor environments
which are exposed to rainwater etc. Due to this, it is
15 possible to greatly contribute to the expansion of
utilization of a chromate-free type precoated steel sheet
free of elution of toxic chromium.
Description of Embodiments
20 [0018] Here, first, the history leading up to where
the inventors perfected the invention of the new type of
chromate-free type precoated steel sheet excellent in
edge red rusting resistance of the present application
will be explained.
25 [0019] In the chromate-treated precoated steel sheet
which was broadly used in the past before chromate-free
type precoated steel sheet, the "edge corrosion
resistance" indicates the characteristic of prevention of
blistering of the coating near the edge. Therefore, how
30 to prevent zinc from being eluted from a zinc coating
layer of a precoated steel sheet so as to prevent a
coating from blistering from the edge parts even if used
in a salt fog region such as a region along the seashore
has been the target of development. The reason is that in
35 the age of chromate-treated precoated steel sheet, due to
the excellent corrosion resistance of chromate, there was
almost no occurrence of red rust at the edges due to just
a simple wet environment such as being wet by rainwater,
so attention was not paid to the corrosion resistance
under a simple wet environment. The focus was on the
blistering of the coating due to the dissolution of zinc.
5 Accordingly, in development of new products of chromatetreated
precoated steel sheet, salt spray tests (SST) in
the laboratory were stressed. In the galvanic environment
of a salt spray test, due to the galvanic corrosion
prevention effect of the zinc plating of a precoated
10 steel sheet, even if white rust occurred, this seldom led
to red rust until the zinc ended up being consumed.
[0020] However, the instant that use of chromate-free
type precoated steel sheet, which is inferior in
corrosion resistance level to chromate-treated precoated
15 steel sheet, spread, the problem of red rust occurring at
the edges in a short time in a non-galvanic atmosphere
surfaced all at once. For example, the occurrence of red
rust at the air-conditioner outdoor unit became a major
problem. In particular, the rapid occurrence of red rust
20 became viewed as a problem in fitting parts with the side
panels of the drawn top panels, knockout parts
(structural parts formed by making cuts into steel sheets
and shifting about 1 mm or so), and other parts where
water easily pools due to capillary action and the metal
25 sheet edge continues to be wet over a long period (in
some cases, several days). Further, the regions in which
red rust mainly occurs were mostly not along the seashore
or in other salt fog environments, but at the mountainous
areas. This is because no corrosion cell between a metal
30 sheet and a zinc plate is formed by continued wetting of
the edges by water not containing an electrolyte, so when
at the time of a high humidity environment (that is,
particularly in the rainy season etc.), the zinc does not
galvanically dissolve and the exposed iron edges end up
35 being corroded.
[0021] Such red rust does not easily occur in
chromate-treated precoated steel sheet because the
8
chromate ingredient (strontium chromate) is eluted into
the water from the edges (nearby chromate-treatment
coating) and forms a protective layer at the iron edges.
As opposed to this, the rust prevention pigment (calcium
5 ion exchanged silica, aluminum tripolyphosphate, etc.)
which is comprised in the coating of ordinary chromatefree
type precoated steel sheet is low in water
solubility. Even if the ingredients are eluted into
water, the rust prevention ability is inferior to a
10 chromate ingredient, so is insufficient for preventing
the occurrence of red rust in wet environments in which
the galvanic corrosion of zinc does not effectively
function.
[0022 ] In view of the above points, the inventors
15 thought that to obtain a sufficient edge red rust
suppression effect in the development of the new type of
chromate-free type precoated steel sheet of the present
invention, all of the following conditions (1) to (3)
have to be satisfied:
20 (1) The elution of a sufficient amount of an ingredient
from inside the coating when a precoated steel sheet edge
is wet by water over a long period,
(2) The effective suppression, by the eluted ingredient,
of the occurrence of red rust or red rust and white rust,
25 and
(3) The continued residence of the eluted ingredient
near the edge.
[0023] These conditions will be explained in detail
next:
30 Regarding condition (1)
The occurrence of red rust at a precoated steel
sheet edge is a phenomenon of the occurrence of red rust
from the edges in an extremely short time in'the rainy
season or other high humidity time in an environment in
35 which the edge continues to be wet due to the small
amount of water contained in the electrolyte since the
galvanic corrosion prevention effect of the zinc cannot
- 9 -
be expressed and iron is eluted into the water. To
suppress the occurrence of such red rust, it is effective
to include an ingredient effective for rust prevention
into the coating, have the active ingredient be eluted
5 from the coating when the edge is wet by water, and
suppress the occurrence of red rust at the edge. If the
water solubility of the substance contained in the
coating is not sufficient, even if the substance has
rust-prevention ability, it will not be eluted into the
10 water, so the effect of suppression of red rust cannot be
obtained. Accordingly, the substance which is comprised
into the coating has to have sufficient water solubility.
[0024] Incidentally, the calcium ion exchanged silica
(product name Shieldex) or aluminum tripolyphosphate and
15 other rust prevention pigments which are being used in
current chromate-free type precoated steel sheet are
examples of substances which do not have sufficient water
solubility. 1
[0025 ] Regarding condition (2)
20 The ingredient which is eluted from the coating into
water must be a substance which can suppress the
occurrence of red rust or red rust and white rust. Among
the substances which have red rust suppression ability,
there are various rust prevention mechanisms. Further, it
25 is possible to use the rust prevention mechanisms to
classify substances which have a red rust suppressing
ability as follows.
(i) Substances which suppress the occurrence of red
rust by being eluted into water to form electrolytes and
30 raising the electrical conductivity to cause the
expression of the galvanic corrosion prevention effect by
zinc
(ii) Substances which suppress the elution of iron
itself by being adsorbed at the iron surface or forming a
35 passivation layer (substance working as an inhibitor
against dissolution of iron).
(iii) Substances which also work as inhibitors
- 10 -
against the dissolution of zinc in addition to the effect
of (ii).
[0026] If a water soluble salt, if dissolved in water,
it disassociates to form an electrolyte. If a substance
5 does not have the properties of the substance of the
above (ii) or (iii), it is considered that it should be
classified as a substance of the above (i). However, the
mechanism of suppression of red rust of the substance of
the above (i) is predicated on causing corrosion of zinc,
10 so the occurrence of white rust and the occurrence of
blistering under the coating cannot be avoided. If
corrosion of zinc progresses, the galvanic corrosion
prevention effect is lost and finally red rust starts to
occur.
15 [0027] On the other hand, with a substance of the
above (ii), the elution of iron itself is prevented
without relying on the galvanic corrosion prevention
effect of zinc, so dissolution of zinc is kept down to
the level of self dissolution in water and the occurrence
20 of white rust or blistering becomes smaller. With a
substance of the above (iii), even self dissolution of
zinc is suppressed, so occurrence of white rust or
occurrence of blistering becomes further rarer. From the
above viewpoint, in the present invention, as the
25 substance effective for suppression of the occurrence of
red rust of the chromate-free type precoated steel sheet
edge, not a substance of the above (i), but a substance
of the above (ii) or (iii) is used.
[0028 ] whether a specific substance is one of the
30 above (i) to (iii) can be judged by the following tests.
[0029] The substances under test were subjected to the
following two tests:
(Test'1) Immersion of cold rolled steel sheet in a
solution obtained by dissolving 100 mg of the substance
35 under test in 40 ml of distilled water (500 hours).
(Test 2) Immersion of ordinary chromate-free type
precoated steel sheet (one having a susceptibility to red
- 11 --
rust at the edge) which is formed with a coating which
comprises calcium ion exchanged silica or aluminum
tripolyphosphate as a rust prevention pigment in the same
aqueous solution (500 hours).
.5 [0030] The immersion time of 500 hours in the tests is
set envisioning the actual usage conditions of precoated
steel sheet assuming the ability to suppress the
occurrence of red rust at the edges even with continuous
500 hours of immersion is a necessary requirement for a
10 substance effective for suppression of the occurrence of
red rust at the edges.
[0031] The inventors investigated the occurrence of
red rust and white rust for the various steel sheets
which are taken out from the aqueous solution and judged
15 which of the substances of the above (i) to (iii) the
substances should be classified into by the following
criteria:
Substances classified as (i)
Red rust occurs in test 1, while red rust does not
20 occur and large white rust occurs in test 2.
Substances classified as (ii)
Red rust does not occur in test 1, while red rust
does not occur and slight white rust occurs in test 2.
Substances classified as (iii)
25 Red rust does not occur in test 1, while both of red
rust and white rust do not occur in test 2.
[0032] Regarding condition (3)
Even if the test substance satisfies the
requirements of the above conditions (1) and (2) (that
30 is, even if sufficiently being eluted from the coating to
the water and acting as an inhibitor against dissolution
of iron), if the substance ends up immediately being
washed away, it will not be able to act as an inhibitor.
Accordingly, to secure the edge red rusting resistance,
35 the substance of the inhibitor ingredient has to "reside"
at the edge or its vicinity continuously.
[0033] The "reside" referred to here includes various
- 12 -
meanings. For example, any of (a) adsorption of the
inhibitor ingredient or formation of a passivation
coating and coverage of the edge, (b) continued presence
of the inhibitor solution at the edge due to the
5 viscosity, surface tension, or other physical factors,
and (c) presence on the edge of a liquid film boundary
layer region free of fast movement of the inhibitor
concentration (due to zero flow rate for liquid which
adjoins the solid surface in the liquid which flows over
10 a solid surface), etc. all correspond to the "reside"
referred to here. When a test substance is in a state
which can exhibit an inhibitor effect with this "reside"
by any of these phenomena, the substance is expressed as
having an "edge residence".
15 [0034] The edge residence of specific substances can
be evaluated by the following method:
[0035 ] 1) Dissolving 10 mg of the substance in 40 ml
of distilled water, forming a coating comprising calcium
ion exchanged silica or aluminum tripolyphosphate as a
20 rust prevention pigment (inhibitor) on an ordinary
chromate-free type precoated steel sheet (one where red
rust tends to easily form at the edges), cutting this to
50 x 50 mm, and immersing it in an aqueous solution for
72 hours.
25 2) Taking the precoated steel sheet (50 x 50 mm) out
from the aqueous solution of the above 1) (at this point
of time, no red rust occurring), lightly rinsing it by
distilled water, then quickly immersing it in distilled
water and allowing it to stand for 350 hours and 500
30 hours.
[0036] In the above 2), the edge residence was
evaluated as follows: Samples with red rust occurring
within 350 hours were evaluated as "poor", samples with
red rust not occurring by immersion for 350 hours, but
35 occurring by immersion for 500 hours were evaluated as
"fair", and samples with red rust not occurring at all
even with immersion for 500 hours were evaluated as
- 13 -
"good". When red rust did not occur after standing for
350 hours ("fair" or "good"), the substance was judged to
have "high edge residence". The inhibitor aqueous
solution immersion time of 72 hours of the precoated
5 steel sheet in the above 1) was set as a model condition
for causing manifestation of the difference of the edge
residence of the substance. This is a condition at the
top panel fitting part of an air-conditioner outside unit
where a large amount of red rust occurs at the time of
10 the rainy season and a condition which the results
generally match. The immersion times of 350 hours and 500
hours in the above 2 ) were set for the same reasons as
the immersion time in the test for judging the type of
substance which has the above red rust suppression
15 ability.
[0037] Based on the above assumption, the inventors
investigated various substances to determine how they act
as inhibitors against dissolution of iron and discovered
several promising substances. On top of that, they tested
20 these substances for edge residence. Part of the results
of evaluation was summarized in Table 1.
14 -
[0038 ] Table 1. Red Rust Suppression Ability and Edge
Residence of Various Test Substances
Immersion in
inhibitor aqueous "Edge
solution residence"
cold After 72
rolled Current Class of
hours
steel PCM red rust
immersion in
Test substance sheet supprestest
i
solution
s on
substance 350, 500
hours
Test 1 Test 2 immersion in
distilled
water
Red rust Red rust Red rust
No inhibitor
(distilled water as is)
Poor Poor - Poor
Ca silica (Shieldex) Poor Poor - Poor
Aluminum
tripolyphosphate
Poor Poor - Poor
Strontium chromate Good Good ii or iii Good
Sodium hydrogen
carbonate poor Poor - Poor
Potassium chloride Poor Good i Poor
Sodium chloride Poor Good i Poor
Sodium tungstate Good Good ii Good
Calcium tungstate Good Good ii Fair
Ammonium tungstate Good Good ii Fair
Lithium tungstate Good Good ii Fair
Sodium phosphotungstate Poor Poor - Poor
Magnesium tungstate Good Good ii Good
Barium tungstate Good Good ii Poor
Sodium molybdenate Good Good ii Poor
Sodium silicate Good Good ii or iii Good
Magnesium silicate Good Good ii or iii Poor
Potassiumsilicate Good Good ii or ii 1. Good
Calcium silicate Poor Poor - Poor
Lithium silicate Good Good ii or iii Fair
[0039] As a result, while by which of the above-
5 mentioned "residence" mechanisms a high edge residence is
expressed is unclear, the inventors determined that, in
the tungstate group, a tungstate such as sodium
tungstate, calcium tungstate, ammonium tungstate, lithium
tungstate, and magnesium tungstate is a useful substance
10 which acts as an inhibitor against dissolution of iron
and gives edge red rusting resistance to a chromate-free
type precoated steel sheet.
[0040] In the tungstate group, it was learned that
- 15 -
sodium tungstate and magnesium tungstate are particularly
preferred. From Table 1, it was confirmed that the edge
red rusting resistance which is exhibited by these
tungstates is equal to the strontium chromate of the rust
5 prevention ingredient in chromate-treated precoated steel
sheet. Further, while a tungstate, barium tungstate was
judged "good" by the evaluation of test 1 and test 2, but
was judged "poor" by the edge residence test. Sodium
molybdenate, which was known in the past as a rust
10 prevention substance, was also judged "good" by the
evaluation of test 1 and test 2, but was judged "poor" by
the edge residence test.
[0041 ] The mechanism by which sodium tungstate or
another tungstate gives an edge red rust suppression
15 effect, as explained above, has not been fully
elucidated, but it is guessed that this causes the
formation of a passive state of the dense iron oxides at
the iron surface of the precoated steel sheet edge and
thereby acts as an inhibitor against the dissolution of
20 iron. The formation of some white rust is observed in the
above test 2 which judges the type of the substance, so
tungstate is believed to have a low effect in suppressing
the elution of zinc of the plating layer. However,
conversely, tungstate does not suppress the elution of
25 zinc (self dissolution), so zinc is suitably eluted and a
galvanic corrosion prevention effect is given. Under
repeated drying and wetting conditions, that is, in an
actual exposure environment (environment of actual use of
a final product including worked precoated steel sheet),
30 an edge protective coating is formed. it is believed that
this acts more advantageously against the suppression of
the occurrence of red rust. Further, by jointly using the
calcium ion exchanged silica (product name Shieldex),
aluminum tripolyphosphate, or other rust prevention
35 pigment which is being used for current chromate-free
type precoated steel sheets, it is expected to be
possible to synergistically improve the edge red rusting
- 16 -
resistance.
[0042 ] In the silicate group, a silicate such as
sodium silicate, potassium silicate, and lithium silicate
is an inhibitor of dissolution of iron and simultaneously
5 acts as an inhibitor of dissolution of zinc and therefore
is a useful substance imparting edge red rusting
resistance to chromate-free type precoated steel sheet.
Among these as well, it was learned that sodium silicate
and potassium silicate are particularly preferable. From
10 Table 1, it was confirmed that the edge red rusting
resistance which is shown by the silicate of the present
invention is equal to that of the rust-prevention
ingredient of chromate-treated precoated steel sheet,
that is, strontium chromate. Further, while a silicate,
15 it was judged that calcium silicate does not have the
evaluated rust prevention effect in either test 1 or test
2. This appears to be due to the fact that calcium
silicate is an insoluble salt. Further, magnesium
silicate was judged as "good" in the evaluation of test 1
20 and test 2, but was judged as "poor" in the edge
residence test.
[0043] The mechanism by which sodium silicate or
another silicate gives an edge red rust suppression
effect, as explained above, has not been fully
25 elucidated, but it is believed that the substance resides
at the iron surface and zinc surface at the precoated
steel sheet edge and acts as an inhibitor against
dissolution of the two metals. If the immersion time
exceeds 500 hours in the tests of 2) and 3) for
30 evaluation of the edge residence explained above, a state
is recognized in which red rust starts to occur, so it is
believed that even if the silicate is adsorbed at the
edge, the force of that is weak and there may be a strong
possibility of residence due to presence of a liquid film
35 boundary layer.
[0044] It is known that sodium phosphate, magnesium
hydrogen phosphate, and another phosphate-based
- 17 -
substances have the feature of being adsorbed at the iron
surface, so a "high edge residence" was expected, but
these were poor in the results in the above edge
residence tests. These examples show that substances with
5 a high adsorption ability at the iron surface cannot
necessarily be said to be have high edge residence.
[0045] In the present invention, a tungstate or
silicate with a high edge residence which imparts an edge
red rusting resistance to a chromate-free type precoated
10 steel sheet is preferably present in a coating at the
back surface side of the precoated steel sheet. When
there are plurality of coatings at the back surface side,
the tungstate or silicate with a high edge residence may
be present in all coatings or may be present in only part
15 of the coatings. This being said, to enable a tungstate
or silicate with a high edge residence to effectively
exhibit edge red rusting resistance, the tungstate or
silicate is preferably present in at least the coating at
the outermost side. For a tungstate or silicate with a
20 high edge residence to exhibit edge red rusting
resistance, it must dissolve in the water wetting the
precoated steel sheet. The outermost layer of coating
which is exposed at the back surface of the precoated
steel sheet can provide the area required for this (area
25 wet by water). The coatings other than the outermost
layer are only exposed at the edge parts. The exposed
area are slight, so with just the coating other than the
outermost layer, it is considered this is insufficient
for guaranteeing the elution of an effective amount of a
30 tungstate or silicate with a high edge residence.
Including a tungstate or silicate with a high edge
residence in a coating at the front surface side .s also
possible, but in this case , the tungstate or'silicate
should be mixed in by a range not detracting from the
35 requirements of the user sought for the coating of the
front surface side.
[0046 ] The tungstate with a high edge residence may be
- 18 -
mixed into the outermost layer coating of the precoated
steel sheet back surface in an amount of about 6 to 50
wti, while the silicate with a high edge residence may be
mixed into the outermost layer coating of the precoated
5 steel sheet back surface in an amount of 5 to 50 wt-. If
smaller than this amount, the expression of the edge red
rusting resistance is insufficient, while if greater than
50 wt%, when the formed coating is exposed to a moist
environment, the tungstate or silicate with a high edge
10 residence will sometimes excessively be eluted and
blistering or peeling of the coating will occur. The
tungstate or silicate with a high edge residence
concentration in the outermost layer coating of the back
surface is more preferably 7 to 40 wt*-., most preferably
15 10 to 30 wt%.
[0047 ] When coatings of the precoated steel sheet
other than the outermost layer coating of the back
surface comprises a tungstate or silicate with a high
edge residence, from the viewpoint of facilitating
20 elution into water, it is preferable to employ an amount
close to the upper limit in the range of concentration in
the outermost layer coating.
[0048 ] The tungstate or silicate with a high edge
residence which is used in the present invention is a
25 powder at ordinary temperature. Considering the thickness
of the coating to be mixed with, it is preferably used
dispersed in the paint until achieving a maximum particle
size of 5 to 20 μm, more preferably 5 to 10 μm.
[0049] A tungstate or silicate powder with a high edge
30 residence can be mixed into the target coating use paint
by any method. For example, the powder may be added to
paint ready for coating use (including the required
ingredients and solvent other than the tungstate or
silicate with a high edge residence) or may be mixed with
35 at least one of the ingredients of the paint in advance
and then mixed with the other ingredients of the paint to
prepare the paint.
- 19 -
[0050 ] The coating of the chromate-free type precoated
steel sheet of the present invention may be formed by a
paint which is used for the production of an ordinary.
chromate-free type precoated steel sheet except for
5 addition of a tungstate or silicate with a high edge
residence which is required for imparting edge red
rusting resistance. As one example, the front surface may
be formed with a primer coating comprised of a polyesterbased
resin, melamine resin, isocyanate, calcium ion
10 exchanged silica, aluminum tripolyphosphate, titanium
oxide, silica, or other additive, and surface coatings
comprised of a polyester-based resin, melamine resin,
isocyanate, calcium ion exchanged silica, aluminum
tripolyphosphate, titanium oxide, silica, various types
15 of coloring pigments, waxes, other additives etc. The
back surface may be formed with a primer coating
comprised of a polyester-based resin, melamine resin,
isocyanate, calcium ion exchanged silica, aluminum
tripolyphosphate, titanium oxide, silica, or other
20 additive etc., and surface coatings comprised of a
polyester-based resin, melamine resin, isocyanate,
calcium ion exchanged silica, aluminum tripolyphosphate,
titanium oxide, silica, various types of colored
pigments, wax, or other additive etc. Among these
25 coatings, at least the outermost side coating at the back
surface comprises a tungstate or silicate with a high
edge residence according to the present invention.
[0051] As the steel sheet forming the base of these
coatings, a zinc-based plated steel sheet may be used. As
30 the zinc-based plated steel sheet, for example, any of a
hot dip galvanized steel sheet,'electrogalvanized steel
sheet, Al-galvanized steel sheet, Al-Mg-Si-galvanized
steel sheet, etc. may be used, but when using an Al-Mg-
Si-galvanized steel sheet, the improvement of the red
35 rust resistance is particularly remarkable.
[0052] On the surface of the zinc-based plated steel
sheet, a usual chromate-free type conversion treatment
- 20 -
layer may be provided as the pretreated layer for coating
the above coating-use paint. As one example, for the
conversion treatment layer, a coating comprising at least
two of silica, a silane coupling agent, tannin or tannic
5 acid, a zirconium compound, and a titanium compound and a
resin may be used. The conversion treatment layer can be
formed by dip coating, roll coater coating, ringer roll
coating, brush coating, spray coating, etc. of a chemical
conversion solution.
10 [0053] The chromate-free type precoated steel sheet of
the present invention can be produced by the usual
production facilities and production methods as with
chromate-free type precoated steel sheets except for
using a paint in which a tungstate or silicate with a
15 high edge residence is added so as to form a coating at
the outermost side of at least the back surface. These
are not explained in detail here.
Examples
20 [0054] Examples will be used to further explain the
present invention. Needless to say, the present invention
is not limited to the examples presented here.
[0055] A. Sample Material Sheet
The following sheets GI and SD were used to prepare
25 sample materials.
(1) GI: Hot dip galvanized steel sheet (0.6 mm
thickness, plating deposition 80 g/m2 per side)
(2) SD: 11%A1-3%Mg-0.2%Si-galvanized steel sheet
(0.6 mm thickness, plating deposition 80 g/m2 per side)
30 B. Chemical Conversion of Sheets
Silane coupling agent, tannic acid, silica, and
polyester resin mixed treatment solutions were used to
treat sheets by chromate-free chemical treatment (100
mg/m2) .
35 C. Painting of Sample Material
Paints comprised of the following paint resins in
which rust prevention pigment ingredients shown in Table
21
2 were blended were used to coat sheets, chemically
converted in advance, to form primer coatings and topcoat
coatings on their surfaces.
(1) Front surface/back surface primer paint resin
5 Polyester/melamine+isocyanate joint use curing type
(FLC687 paint resin made by Nippon Fine Coatings)
(2) Front surface topcoat paint resin
High molecular weight polyester/melamine curing type
(FLC7000 paint resin made by Nippon Fine Coatings)
10 (3) Back surface topcoat paint resin
Polyester/melamine curing type (FLC100HQ paint resin
made by Nippon Fine Coatings)
[00567 The ingredients of the rust prevention pigment
blended into the paint of Table 2 were as follows:
15 A: calcium ion exchanged silica
B: aluminum tripolyphosphate
C: sodium tungstate
D: calcium tungstate
E: ammonium tungstate
20 F: lithium tungstate
G: sodium phosphotungstate (comparative)
H: sodium hydrogen carbonate (comparative)
I: potassium chloride (comparative)
J: magnesium tungstate
25 K: barium tungstate (comparative)
L: sodium molybdenate (comparative)
- 22 -
[0057] Table 2
Front surface Back surface
Sheet
Primer (5 Ftm)
Top
(15 μm)
Primer (5 μm) Top (5 μm)
Comp. Ex. 1 GI A(20)+B(20) - A(20)+B(20) A(20)+B(20)
Comp. Ex. 2 GI A(20)+B (20) - A(30)+B(30) A(30)+B(30)
Comp, Ex. 3 GI A(20)+B(20) - A(20)+B(20)+C(4) A(20)+B(20)+C(4)
Comp. Ex. 4 GI A(20)+B(20) - A(20)+B(20)+C(5) A(20)+B(20)+C(5)
Comp. Ex. 5 GI A(20)+B(20) - A(20)+B(20)+C(5) -
Comp. Ex. 6 GI A(20)+B(20) - A(20)+B(20)+C(S) - (15μm)
Ex. 1 GI A(20)+B(20) - A(20)+ B(20)+C (6) A(20)+B(20)+C(6)
Ex. 2 GI A( 20)+B(20 ) - A(20)+B(20)+C(10) A(20)+B(20)+C(10)
Ex. 3 GI A(20)+B(20) - A(20)+B(20)+C(20) A(20)+B(20)+C(20)
Comp. Ex. 7 GI A(20)+B(20) - A(20)+8(20)+C(20) -
Ex. 4 GI A(20)+B(20) - A(20)+B(20)+C(20) - (15μm)
Ex. 5 GI A(20)+B(20) - A(20)+B(20) A(20)+B(20)+C(20)
Ex. 6 GI A(20)+B(20)+C(5) - A(20)+B(20)+C(20) A(20)+B(20)+C(20)
Ex. 7 GI A( 20)+B(20 )+C(5) C(5) A(20)+B ( 20)+C(20 ) A(20)+B(20)+C(20)
Ex. 8 GI A(20)+B(20) - A(5)+B(5)+C(50) A(5)+B(5)+C(50)
Comp. Ex. 8 GI A(20)+B(20) - A(5)+B(5)+C(55) A(5)+B(5)+C(55)
Ex. 9 GI A(20)+B(20) - A(20)+B(20)+D(20) A(20)+B(20)+D(20)
Ex. 10 GI A(20)+B(20) - A(20)+B(20)+E(20) A(20)+B(20)+E(20)
Ex. 11 GI A(20)+B(20) - A(20)+B(20)+F(20) A(20)+B(20)+F(20)
Comp. Ex. 9^ GI A(20)+B(20) - A ( 20)+B(20 )+G(20) A( 20)+B(20)+G(20)
Comp. Ex. 10 GI A(20)+B(20) - A(20)+B(20)+H(20) A(20)+B(20)+H(20)
Comp. Ex. it GI A(20)+B(20) - A(20)+B(20)+I(20) A(20)+B(20)+I(20)
Ex. 12 GI A(20)+B(20) - A(20)+B(20)+J(20) A(20)+B(20)+J(20)
Comp. Ex. 12 GI A(20)+B(20) - A(20)+B(20)+K(20) A(20)+B(20)+K(20)
Comp. Ex. 13 GI A(20)+ B(20) - A ( 20)+B(20 )+L(20) A(20)+B(20)+L(20)
Ex. 13 GI A( 20)+B(20 ) - A(20)+B(20)+C(10)+E(10) A(20)+B(20)+C(10)+E(10)
Comp. Ex. 14 SD A(20)+B(20) - A(20)+B(20) A(20)+B(20)
Comp. Ex. 15 SD A(20)+ B(20) - A (30)+B(30) A(30)+B(30)
Comp. Ex. 16 SD A(20)+B(20) - A( 20)+B(20 )+C(4) A(20)+B(20)+C(4)
Ex. 14 SD A(20)+B(20) - A(20)+B(20)+C(6) A(20)+B(20)+C(6)
Ex. 15 SD A(20)+B(20) - A(20)+B(20)+C(10) A(20)+B (20)+C(10)
Ex. 16 SD A(20)+B(20) - A(20)+B(20)+C(20) A(20)+B(20)+C(20)
Ex, 17 SD A(20)+B(20) - A(20)+B(20) A(20)+B(20)+C(20)
[0058] In Table 2, the figures in parentheses after
the symbols of the rust prevention pigments show the
5 weight concentration percent of the rust prevention
pigment to the total solid content of the paint. (for
example, "A.(20)" shows the paint comprises the pigment A
in 20 wt%). Table 2 shows only the rust prevention
pigment ingredients blended into the paint, while the
10 colored pigment ingredients are not shown.
[0059 ] The prepared sample materials were subjected to
the following performance tests.
[0060 ] 1. Field exposure test
A field exposure test was performed for'the purpose
15 of directly evaluating the resistance to red rust which
occurs in a short period in a non-galvanic moist
environment.
A sample material was worked to reproduce the shape
- 23 -
of a top panel fitting part of an air-conditioner outdoor
unit. The worked material and the unworked flat panel
were used as test materials.
The test materials were set in a field area (inland
5 area of Kimitsu city, Chiba prefecture) and visually
observed after one month right after the start of the
rainy season.
The red rust of the top panel fitting part was
evaluated as follows: Samples where no red rust occurred
10 at all were evaluated as "Very Good", samples where
slight red rust occurred (red rust: less than 1%) were
evaluated as "Good", samples where small red rust
occurred (red rust: 1 to 50) were evaluated as "Fair",
and samples where large red rust occurred (red rust: over
15 5%) were evaluated as "Poor".
The blistering of the front flat part of the sheet
was evaluated as follows: Samples where no blistering at
all occurred were evaluated as "Very good", samples where
blistering was slight (ASTM D714 8F level or higher) were
20 evaluated as "Good", and samples where blistering was
small to great (less than 8F level) were evaluated as
"Poor".
[0061] 2. Salt Fog Exposure Test
A salt fog exposure test was performed for the
25 purpose of evaluating the corrosion resistance by the
conventional viewpoint of long term resistance under an
galvanic environment.
_ A sample material was worked to reproduce the shape
of a top panel fitting part of an air-conditioner outdoor
30 unit. The worked material and the unworked flat panel
were used as test materials.
The test materials were set in a salt fog area
(seaside area in Okinawa prefecture) and visually
observed after two years.
35 The red rust of the top panel fitting part was
evaluated as follows: Samples where no red rust occurred
at all were evaluated as "Very good", samples where
- 24 -
slight red rust occurred (red rust: less than 1%) were
evaluated as "Good", samples where small red rust
occurred (red rust: 1 to 5%) were evaluated as "fair",
and samples where large red rust occurred (red rust: over
5 5%) were evaluated as "Poor".
The edge blistering of the front surface of the flat
panel was evaluated as "Good" for samples with a maximum
value of blistering from the edge of 2 mm or less and as
"Poor" for samples with a maximum value of over 2 mm.
10 [0062] 3. Laboratory tests
Laboratory tests (distilled water immersion test,
40°C HCT test, and boiling water two-hour immersion test)
were performed for the purpose of simply evaluating the
resistance to red rust which occurs in a short time under
15 a non-galvanic moist environment. However, for the
present invention, the primary emphasis is on good
results of exposure tests in the field. The laboratory
tests are deemed reference tests.
The time which the aqueous solution which comprises
20 the substance which is eluted from the coating resides
near the edge is extremely long in the distilled water
immersion test, while it is conversely short in the 40°C
HCT test. The actual environmental conditions are
believed to be intermediate conditions of these
25 laboratory tests. Accordingly, the fact that good results
are obtained under the conditions of these laboratory
tests is believed to mean that good results are given in
an actual environment.
[0063] In the distilled water immersion test, a 50 x
30 50 mm sample material was immersed in distilled water 40
ml for 500 hours, then was evaluated for red rust at the
edge. Samples where no red rust occurred at all were
evaluated as "Very good, samples where slight red rust
occurred (red rust: less than 1%) were evaluated as
35 "Good", samples where small red rust occurred (red rust:
1 to 10%) were evaluated as "Fair", and samples where
large red rust occurred (red rust: over 10%) were
- 25 -
evaluated as "Poor".
[0064] In the 40°C HCT test, 50 x 100 mm sample
material was set in a constant temperature, high humidity
tank (40°C, 98%RH) suspended in the vertical length and
5 was evaluated for red rust at the edge after 240 hours.
Samples where no red rust occurred at all were evaluated
as "Very good", samples where slight red rust occurred
(red rust: less than 5%) were evaluated as "Good",
samples where small red rust occurred (red rust: 5 to
10 10%) were evaluated as "Fair", and samples where large
red rust occurred (red rust: over 10%) were evaluated as
"Poor".
[00651 in the boiling water two-hour immersion test, a
flat panel was immersed in boiling water for 2 hours and
15 evaluated for blistering at the front flat part and back
flat part. Samples with no blistering at all were
evaluated as "Very good", samples with slight blistering
(by ASTM D 714, 8F leveil or higher) were evaluated as
"Good", and samples with small to large blistering (less
20 than 8F level) were evaluated as "Poor".
[0066] The results of the above tests are shown in
Table 3.
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- 9E -
-- 27 -
the rust prevention pigments shown next and similar tests
were run. Table 4 shows the ingredients blended, while
Table 5 shows the test results.
[0069] The ingredients of the rust prevention pigments
5 blended into the paint of Table 4 were as follows:
a: calcium ion exchanged silica
b: aluminum tripolyphosphate
c: sodium silicate
d: magnesium silicate (comparative)
10 e: potassium silicate
f: calcium silicate (comparative)
g: sodium hydrogen carbonate (comparative)
h: potassium chloride (comparative)
is lithium silicate
15 [0070] Table 4
Front surface Back surface
Sheet
primer (5 μm)
Top (15
m)
Primer (5 μm) Top (5 μm)
Comp. Ex. 17 GI a(20)+b(20) - a(20)+b(20) a(20)+b(20)
Comp. Ex. 18 GI a(20)+b(20) a(30)+b(30) a(30)+b(30)
Comp. Ex. 19 GI a(20)+b(20) - a(20)+b(20)+c(4) a(20)+b(20)+c(4)
Ex. 18 CI a(20)+b(20) - a(20)+b(20)+c(5) a(20)+b(20)+c(5)
Ex. 99 GI a(20)+b(20) - a(20)+b(20)+c(10) a(20)+b(20)+c(10)
Ex. 20 GI a(20)+b(20) - a(20)+b(20)+c(20) a(20)+b(20)+c(20)
Comp. Ex. 20 GI a(20)+b(20) - a(20)+b(20)+c(20) -
Comp. Ex. 21 GI a(20)+b(20) - a(20)+b(20)+c(20) -.(15μm)
Ex. 21 GI a(20)+b(20) - a(20)+b(20) a(20)+b(20)+c(20)
Ex. 22 GI a(20)+b(20)+c(5) - a(20)+b(20)+c(20) a(20)+b(20)+c(20)
Ex. 23 GI a(20)+b(20)+c(5) c(5) a(20)+b(20)+c(20) a(20)+b(20)+c(20)
Ex. 24 GI a(20)+b(20) - a(5)+b(5)+c(50) a(5)+b(5)+c(50)
Comp. Ex. 22 GI a(20)+b(20) - a(5)+b(5)+c(55) a(5)+b(5)+c(55)
Comp. Ex. 23 GI a(20)+b(20) - a(20)+b(20)+d(20) a(20)+b(20)+d(20)
Ex. 25 GI a(20)+b(20) - a(20)+b(20)+e(20) a(20)+b(20)+e(20)
Comp. Ex. 24 GI a(20)+b(20) - a(20)+b(2o)+f(20) a(20)+b(20)+f(20)
Comp. Ex. 25 GI a(20)+b(20) - a(20)+b(20)+g(20) a(20)+b(20)+g(20)
Comp. Ex. 26 GI a(20)+b(20) - a(20)+b(20)+h(20) a(20)+b(20)+h(20)
Ex. 26 GI a(20)+b(20) a(20)+b(2o)+i(20) a(20)+b(20)+i(20)
Ex. 27 GI a(20)+b(20) - a(20)+b(20)+c(10)+e(10) a(20)+b(20)+c(10)+e(10)
Comp. Ex. 27 SD a(20)+b(20) - a(20)+b(20) a(20)+b(20)
Comp. EX. 28 SD a(20 )+b(20) - a(30)+b(30) a(30)+b(30)
Comp. Ex. 29 SD a(20)+b(20) - a(20)+b(20)+c(4) a(20)+b(20)+c(4)
Ex. 28 SD a(20)+b (20) - a(20)+b(20)+c(5) a(20)+b(20)+c(5)
Ex, 29 SD a(20)+b(20) - a(20)+b(20)+c(10) a(20)+b(20)+c(10)
Ex. 30 SD a(20)+b(20) - a(20)+b(20)+c(20) a(20)+b(20)+c(20)
Ex. 31 SD a(20)+b(20) - a(20)+b(20) a(20)+b(20)+c(20)
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CLAIMS
[Claim 1]
A chromate-free type precoated steel sheet excellent
in edge red rusting resistance comprising a zinc-based
5 plated steel sheet, a chromate-free type conversion
treatment layer on the two surfaces of the zinc-based
plated steel sheet, and at least one layer of a coating
formed on the conversion treatment layer, characterized
in that the outermost layer of coating among at least the
10 coatings of the back surface of the precoated steel sheet
comprises a tungstate or silicate. with a high edge
residence.
[Claim 2]
The chromate-free type precoated steel sheet
15 excellent in edge red rusting resistance as set forth in
claim 1 characterized in that the precoated steel sheet
has a primer coating layer and a top coating layer on the
chromate-free type conversion treatment layer at the two
surfaces and in that the top coating layer of the back
20 surface comprises a tungstate or silicate with a high
edge residence.
[Claim 3]
The chromate-free type precoated steel sheet
excellent in edge red rusting resistance as set forth in
25 claim 2 characterized in that the primer coating layer of
the back surface further comprises a tungstate or
silicate with a high edge residence.
[Claim 4]
The chromate-free type precoated steel sheet
30 excellent in edge red rusting resistance as set forth in
any one of claims 1 to 3 characterized in that said
tungstate with a high edge residence is sodium tuiigstate,
calcium tungstate, ammonium tungstate, lithium tungstate,
magnesium tungstate, or a mixture thereof.
35 [Claim 5]
The chromate-free type precoated steel sheet
excellent in edge red rusting resistance as set forth in
- 30 -
any one of claims 1 to 3 characterized in that said
silicate with a high edge residence is sodium silicate,
potassium silicate, lithium silicate, or a mixture of the
same.
5 [Claim 61
The chromate-free type precoated steel sheet
excellent in edge red rusting resistance as set forth in
claim 4 characterized in that said tungstate with a high
edge residence is sodium tungstate or magnesium
10 tungstate.
[Claim 7]
The chromate-free type precoated steel sheet
excellent in edge red rusting resistance as seL forth in
claim ,5 characterized in that said silicate with a high
15 edge residence is sodium silicate or potassium silicate.
[Claim 8]
The chromate-free type precoated steel sheet
excellent in edge red rusting resistance as set forth in
any one of claims 1 to '5 characterized in that a content
20 of said tungstate with a high edge residence in said
coating is 6 to 50 wto. [Claim 9]
The chromate-free type precoated steel sheet
excellent in edge red rusting resistance as set forth in
25 any one of claims 1 to 5 characterized in that. a content
of said silicate with a high edge residence in said
coating is 5 to 50 wt-'..