U.S. patent application number 14/967470 was filed with the patent office on 2016-04-07 for heat exchanger tube precursor and method of producing the same.
This patent application is currently assigned to MITSUBISHI ALUMINUM CO., LTD.. The applicant listed for this patent is MITSUBISHI ALUMINUM CO., LTD.. Invention is credited to Yasunori Hyogo, Masaya KATSUMATA.
Application Number | 20160097607 14/967470 |
Document ID | / |
Family ID | 51017494 |
Filed Date | 2016-04-07 |
United States Patent
Application |
20160097607 |
Kind Code |
A1 |
KATSUMATA; Masaya ; et
al. |
April 7, 2016 |
HEAT EXCHANGER TUBE PRECURSOR AND METHOD OF PRODUCING THE SAME
Abstract
A heat exchanger tube precursor that allows manufacturing a heat
exchanger having high corrosion resistance after brazing treatment
is provided. The heat exchanger tube precursor includes: an Al
alloy tube; and a flux layer including a Si powder, a Zn-containing
flux, a Zn-free flux, and a binder, the flux layer being formed on
an outer surface of the Al alloy tube.
Inventors: |
KATSUMATA; Masaya;
(Susono-shi, JP) ; Hyogo; Yasunori; (Izu-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MITSUBISHI ALUMINUM CO., LTD. |
Minato-ku |
|
JP |
|
|
Assignee: |
MITSUBISHI ALUMINUM CO.,
LTD.
Minato-ku
JP
|
Family ID: |
51017494 |
Appl. No.: |
14/967470 |
Filed: |
December 14, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14142371 |
Dec 27, 2013 |
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14967470 |
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12690685 |
Jan 20, 2010 |
8640766 |
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14142371 |
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11218595 |
Sep 6, 2005 |
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12690685 |
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10823563 |
Apr 14, 2004 |
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11218595 |
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Current U.S.
Class: |
428/586 |
Current CPC
Class: |
B23K 35/0222 20130101;
Y10T 428/13 20150115; B23K 1/008 20130101; B23K 35/24 20130101;
B23K 35/286 20130101; B23K 35/362 20130101; B23K 35/3603 20130101;
F28F 2255/16 20130101; B23K 2103/10 20180801; B23K 35/36 20130101;
B23K 1/203 20130101; C23C 4/12 20130101; B23K 35/3605 20130101;
F28F 21/084 20130101; B23K 1/0012 20130101; B23K 35/282 20130101;
B23K 35/0244 20130101; B23K 35/3612 20130101; B23K 2101/14
20180801; B23K 2101/34 20180801; B23K 35/0238 20130101; B23K
2101/06 20180801; B23K 35/28 20130101; B32B 15/017 20130101; B23K
1/012 20130101; B23K 35/3613 20130101; F28F 19/02 20130101; F28F
19/06 20130101 |
International
Class: |
F28F 21/08 20060101
F28F021/08; B32B 15/01 20060101 B32B015/01 |
Foreign Application Data
Date |
Code |
Application Number |
May 6, 2003 |
JP |
2003-128170 |
Claims
1. A heat exchanger tube precursor comprising: an Al alloy tube;
and a flux layer including a Si powder, a Zn-containing flux, a
Zn-free flux, and a binder, the flux layer being formed on an outer
surface of the Al alloy tube.
2-10. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is continuation in part of application Ser.
No. 12/690,685, filed Jan. 20, 2010, which is continuation in part
of application Ser. No. 11/218,595, filed on Sep. 6, 2005, which is
a continuation of application Ser. No. 10/823,563, filed on Apr.
14, 2004, which claims priority to Japanese Patent Application No.
JP 2003-128170, filed on May 6, 2003, the entire contents of each
of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The present invention relates to a heat exchanger tube
precursor and, more particularly, relates to a heat exchanger tube
precursor, which allows manufacturing a heat exchanger having high
corrosion resistance after brazing treatment. The present invention
also relates to a method of producing the heat exchanger tube
precursor.
[0003] As shown in FIG. 2, a heat exchanger generally comprises a
pair of right and left pipe bodies called header pipes 5, a
multitude of tubes 1 made of an aluminum alloy installed in
parallel at intervals from each other between the header pipes 5,
and fins 6 installed between the tubes 1, 1. The inner space of
each of the tubes 1 and the inner space of the header pipes 5
communicate with each other, so as to circulate a medium through
the inner space of the header pipes 5 and the inner space of each
of the tubes 1, thereby achieving efficient heat exchange via the
fins 6.
[0004] It is known to constitute the tubes 1 of the heat exchanger
from heat exchanger tube precursors 11 made by coating the surface
of an Al alloy extruded tube 3, that has flattened cross section
and a plurality of holes 4 for passing the medium as shown in
perspective view of FIG. 1, with a flux containing a brazing
material powder so as to form a flux layer 2. It is also known to
make the Al alloy extruded tube 3 from material (JIS1050) that has
high workability for extrusion forming process, and to use a Si
powder, an Al--Si alloy powder or an Al--Si--Zn alloy powder as the
brazing material contained in the flux layer 2.
[0005] A heat exchanger is manufactured using the conventional heat
exchanger tube precursor 11 described above in a process such as:
the heat exchanger tube precursors 11 are installed at right angles
to the header pipes 5 that are disposed in parallel at a distance
from each other, ends of the heat exchanger tube precursors 11 are
inserted into openings (not shown) that are provided in the side
face of the header pipe 5, the fins 6 having corrugated shape are
assembled between the heat exchanger tube precursors 11, and the
assembly is heated in a heating furnace so that the header pipes 5
and the tubes 1 are fastened to each other by brazing with the
brazing material provided on the heat exchanger tube precursor 11
while the fins 6 of corrugated shape are fastened between the tubes
1, 1 by brazing.
[0006] Wall thickness of the tube 1 that constitutes the heat
exchanger is made smaller than that of the header pipe 5 in order
to achieve high efficiency of heat exchange. As a result, in the
case in which the tube and the header pipe are corroded at
comparable rates, it is likely that a penetrating hole will be
formed by corrosion first in the tube thereby allowing the medium
to leak therethrough. Thus, it has been a major concern in the heat
exchanger to prevent corrosion of the tubes.
[0007] In order to improve the corrosion resistance of the heat
exchanger tube 11, a sacrificial anode layer containing Zn as a
major component is formed on the surface of the tubes in the
conventional heat exchangers. As the process to form the
sacrificial anode layer, such processes are known as thermal
spraying of Zn and coating with a Zn-containing flux. Japanese
Patent Application Unexamined Publication No. 7-227695 discloses an
example that employs Zn-containing flux.
[0008] However, when forming the sacrificial anode layer by thermal
spraying, it is difficult to precisely control the amount of metal
applied by thermal spraying, thus leading to such a problem that
the sacrificial anode layer cannot be formed uniformly on the tube
surface, and the corrosion resistance of the tube cannot be
improved.
[0009] When the Zn-containing flux described in Japanese Patent
Application Unexamined Publication No. 7-227695 as mentioned above
is used, it may be believed that corrosion resistance of the tube
can be improved since the flux and Zn are supplied simultaneously
onto the tube surface. In actuality, however, it is difficult to
achieve a stable coating condition with ordinary coating methods
such as immersion coating and roll coating, and therefore it has
been difficult to uniformly apply the Zn-containing flux. As a
result, Zn distribution in the sacrificial anode layer becomes
uneven, thus leading to insufficient corrosion resistance of the
tubes with preferential corrosion occurring in a portion that has
higher concentration of Zn.
SUMMARY OF THE INVENTION
[0010] The present invention, which has been completed in view of
the background described above, has an object of providing a heat
exchanger tube precursor, which allows manufacturing a heat
exchanger that has higher corrosion resistance.
[0011] In order to achieve the object described above, the present
invention employs the following constitution.
[0012] The first aspect of the present invention is a heat
exchanger tube precursor of the present invention includes: an Al
alloy tube; and a flux layer including a Si powder, a Zn-containing
flux, a Zn-free flux, and a binder, the flux layer being formed on
an outer surface of the Al alloy tube.
[0013] In the first aspect of the present invention, an amount of
the Si powder applied on the Al alloy tube may be in a range of 1
g/m.sup.2 to 5 g/m.sup.2, and an amount of the Zn-containing flux
applied on the Al alloy tube is in a range of 3 g/m.sup.2 to 20
g/m.sup.2.
[0014] Also, in the first aspect of the present invention, the Si
powder may have a particle diameter distribution such that 99%
particle diameter (D.sub.99) is 5 .mu.m or more and 20 .mu.m or
less, and an amount of coarse particles having diameters of not
smaller than 5 times (D.sub.99) is less than 1 ppm by volume,
wherein (D.sub.99) denotes a critical diameter defined such that
cumulative volume of particles not larger than (D.sub.99)
constitute 99% by volume of all the particles.
[0015] Also, in the first aspect of the present invention, 50%
particle diameter (D.sub.50) of the Si powder may be
(D.sub.99).times.0.05 or more and (D.sub.99).times.0.7 or less,
wherein (D.sub.99) denotes a critical diameter defined such that
cumulative volume of particles not larger than (D.sub.99)
constitute 99% by volume of all the particles, and (D.sub.50)
denotes a critical diameter defined such that cumulative volume of
particles not larger than (D.sub.50) constitute 50% by volume of
all the particles.
[0016] Also, in the first aspect of the present invention,
Zn-containing flux may contain at least one selected from
ZnF.sub.2, ZnCl.sub.2 and KZnF.sub.3.
[0017] Also, in the first aspect of the present invention, the
Zn-free flux may contain at least one selected from LiF, KF,
CaF.sub.2, AlF.sub.3, SiF.sub.4, KAlF.sub.4, and KAlF.sub.3.
[0018] When such a heat exchanger tube precursor is used, since a
mixture of the Si powder and the Zn-containing flux is applied, the
Si powder melts and turns into a brazing liquid during a brazing
process, and Zn contained in the flux is diffused uniformly in the
brazing liquid and is distributed uniformly over the tube surface.
Since the diffusion velocity of Zn in a liquid phase such as the
brazing liquid is significantly higher than the diffusion velocity
in the solid phase, Zn concentration in the tube surface becomes
substantially uniform, thus making it possible to form a uniform
sacrificial anode layer and improve the corrosion resistance of the
heat exchanger tube.
[0019] It is preferable for maximum particle size of the Si powder
to be 30 .mu.m or less. Maximum particle size larger than 30 .mu.m
results in an increase in the erosion depth of the tube and is
therefore not desirable. When maximum particle size of the Si
powder is less than 0.1 .mu.m, Si particles aggregate, and the
erosion depth of the tube increases also in this case. Therefore,
the maximum particle size is preferably not less than 0.1
.mu.m.
[0020] Also, in the first aspect of the present invention, the Al
alloy tube may be constituted of an alloy containing Si of 0.05% or
more and 1.0% or less by weight, Mn of 0.05% or more and 1.2% or
less by weight, and the balance being consisting of Al and
inevitable impurities.
[0021] The second aspect of the present invention is a method of
producing a heat exchanger tube precursor, the method including the
steps of: classifying a pre-classification Si powder to obtain a
post-classification Si powder in which 99% particle diameter
(D.sub.99) is 5 .mu.m or more and 20 .mu.m or less, an amount of
coarse particles having diameters of not smaller than 5 times
(D.sub.99) is less than 1 ppm by volume, and 50% particle diameter
(D.sub.50) of the Si powder is (D.sub.99).times.0.05 or more and
(D.sub.99).times.0.7 or less; preparing a coating material
including the post-classification Si powder, a Zn-containing flux,
a Zn-free flux, and a binder; and applying the coating material on
an Al alloy tube, wherein an amount of the Si powder applied on the
Al alloy tube is in a range of 1 g/m.sup.2 to 5 g/m.sup.2, an
amount of the Zn-containing flux applied on the Al alloy tube is in
a range of 3 g/m.sup.2 to 20 g/m.sup.2, (D.sub.99) denotes a
critical diameter defined such that cumulative volume of particles
not larger than (D.sub.99) constitute 99% by volume of all the
particles, and (D.sub.50) denotes a critical diameter defined such
that cumulative volume of particles not larger than (D.sub.50)
constitute 50% by volume of all the particles.
[0022] In the second aspect of the present invention, the
Zn-containing flux may contain at least one selected from
ZnF.sub.2, ZnCl.sub.2, and KZnF.sub.3.
[0023] In the second aspect of the present invention, the Zn-free
flux may contain at least one selected from LiF, KF, CaF.sub.2,
AlF.sub.3, SiF.sub.4, KAlF.sub.4, and KAlF.sub.3.
[0024] In the second aspect of the present invention, the step of
classifying may be performed by passing the pre-classification Si
powder through a sieve. Alternatively, a cyclone may be used for
classification in the step of classifying.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a perspective view of a heat exchanger tube
precursor of the prior art.
[0026] FIG. 2 is a perspective view of a heat exchanger of the
prior art.
[0027] FIG. 3 is photographic images of heat exchangers showing
time degradation of the coating material.
DETAILED DESCRIPTION OF THE INVENTION
[0028] Next, preferred embodiments of the present invention will be
described in detail.
[0029] The heat exchanger tube precursor of the present invention
is made by forming the external surface of an Al alloy extruded
tube with a flux layer containing a Si powder and a Zn-containing
flux.
[0030] The Al alloy extruded tube which constitutes the heat
exchanger tube precursor is made of an Al alloy containing Si and
Mn, with the balance being Al and inevitable impurities, where a Si
content is 0.5% by weight or more and 1.0% by weight or less, and a
Mn content is 0.05% by weight or more and 1.2% by weight or
less.
[0031] The reason for restricting the composition of the Al alloy
extruded tube will be described below. Si has an effect in that a
large amount of Si forms a solid solution in the Al alloy extruded
tube, thus resulting in noble potential of the Al alloy extruded
tube, and causes preferential corrosion to occur in the header
pipes and the fins that are brazed with the tubes when assembling
the heat exchanger, thereby suppressing deep pitting corrosion from
occurring in the Al alloy extruded tube, while improving the
brazing characteristic and forming good joint thereby to improve
the strength after brazing. Si content of less than 0.5% cannot
achieve the desired effect, and is therefore not desirable. Si
content higher than 1.0%, on the other hand, lowers the melting
point of the alloy resulting in excessive melting during brazing
and poor extrusion forming characteristic, and is not desirable.
Therefore, Si concentration in the Al alloy extruded tube is set in
a range from 0.5 to 1.0%. More preferable range of Si concentration
is from 0.6% to 0.8%.
[0032] Mn has the effect of turning the Al alloy extruded tube to
noble potential and, because of less likelihood of diffusing in the
brazing material, allows higher potential difference with the fin
or the header pipe so as to make the corrosion preventing effect of
the fin or the header pipe more effective, thereby improving the
external corrosion resistance and the strength after brazing. Mn
content of less than 0.05% cannot achieve sufficient effect of
turning the Al alloy extruded tube to noble potential, and is
therefore not desirable. Mn content higher than 1.2%, on the other
hand, results in poor extrusion forming characteristic, and is not
desirable.
[0033] Therefore, Mn concentration in the Al alloy extruded tube is
set in a range from 0.05 to 1.2%.
[0034] The flux layer formed on the tube surface contains the
Zn-containing flux and the Si powder, so that a molten brazing
material layer is formed over the entire surface of the tube after
brazing. Since the brazing material layer contains Zn uniformly
distributed therein, the brazing material layer has similar effect
as that of the sacrificial anode layer so that the brazing material
layer is subject to preferential planar (general) corrosion.
Therefore deep pitting corrosion can be suppressed and corrosion
resistance can be improved.
[0035] The amount of a Si powder applied to the heat exchanger tube
precursor is preferably not less than 1 g/m.sup.2 and not more than
5 g/m.sup.2. When the amount is less than 1 g/m.sup.2, sufficient
brazing strength cannot be achieved because of insufficient amount
of the brazing material, and sufficient diffusion of Zn cannot be
achieved. When the amount is more than 5 g/m.sup.2, Si
concentration in the tube surface increases and the rate of
corrosion increases, and is therefore not desirable.
[0036] The flux layer contains at least the Zn-containing flux. In
addition to the Zn-containing flux, a flux which does not contain
Zn may also be contained.
[0037] The Zn-containing flux preferably contains at least one Zn
compound selected from ZnF.sub.2, ZnCl.sub.2 and KZnF.sub.3. The
flux which does not contain Zn preferably contains at least one
fluoride such as LiF, KF, CaF.sub.2, AlF.sub.3 or SiF.sub.4 or a
complex compound of the fluoride such as KAlF.sub.4 or
KAlF.sub.3.
[0038] As the Zn-containing flux is contained in the flux layer of
the heat exchanger tube precursor, a Zn-diffused layer (brazing
material layer) is formed on the tube surface after brazing, so
that the Zn-diffused layer functions as a sacrificial anode layer,
thereby improving the anti-corrosion effect of the tube.
[0039] Also, because a mixture of the Si powder and the
Zn-containing flux is applied, the Si powder melts and turns into a
brazing liquid during a brazing process, Zn contained in the flux
is diffused uniformly in the brazing liquid and is distributed
uniformly over the tube surface. Since diffusion velocity of Zn in
a liquid phase such as the brazing liquid is significantly faster
than the diffusion velocity in a solid phase, Zn concentration in
the tube surface becomes substantially uniform, thus making it
possible to form a uniform Zn-diffused layer and improve the
corrosion resistance of the heat exchanger tube.
[0040] The amount of the Zn-containing flux applied to the heat
exchanger tube precursor is not less than 3 g/m.sup.2 and not more
than 20 g/m.sup.2. An amount of less than 3 g/m.sup.2 results in
insufficient formation of a Zn-diffused layer that does not have
sufficient anti-corrosion effect, and is therefore not desirable.
An amount of more than 20 g/m.sup.2 causes excessive Zn to be
concentrated in a fillet that is the joint of the fin with other
components which results in higher rate of corrosion in the joint,
and is therefore not desirable.
[0041] The heat exchanger can be constituted by brazing the heat
exchanger header pipes and the fins to the heat exchanger tube
precursor described above.
[0042] That is, the heat exchanger is constituted from the heat
exchanger tube precursor of the present invention, the heat
exchanger header pipes and the fins that are joined with each
other. Similarly to the heat exchanger described in conjunction
with the prior art, the heat exchanger comprises a pair of right
and left pipe bodies called "heat exchanger header pipes", a
plurality of heat exchanger tube precursors installed in parallel
at intervals from each other between the heat exchanger header
pipes, and fins installed between the heat exchanger tube
precursors. The inner space of the heat exchanger tube precursor
and the inner space of the heat exchanger header pipe are
communicated with each other, so as to circulate a medium through
the inner space of the heat exchanger header pipe and the inner
space of the heat exchanger tube precursor, thereby to achieve
efficient heat exchange via the fins.
[0043] The flux layer may be applied to the Al alloy tube with a
roller coating apparatus. In this case, multiple Al alloy tubes are
passed by the roller that is applied with a coating material (flux
layer) in advance. In this way, the flux layer can be applied on an
Al alloy tubes at once continuously. Alternatively, the flux layer
can be applied on the Al alloy tube by thermal spraying. In
addition to the methods described above, any conventional method
for applying the coating material on a solid object can be used in
the application process of the flux layer.
[0044] In order to obtain Si powder that has an intended
specification, particularly in terms of their sizes, the Si powder
is subjected to a classification process before preparing the
coating material. For example, a commercially available Si powder
(pre-classification Si powder) is subjected to a classification
process in order to obtain the Si powder required to prepare the
flux layer used in the first aspect of the present invention
(post-classification Si powder).
[0045] The classification process includes the wet-type
classification method, in which powder suspended in a liquid is
subjected to classification, and dry-type classification method, in
which a powder in a dried-form is subjected to the classification.
In term of apparatus for the classification of Si powder, a single
or multiple filters can be used for example. Alternatively, a
cyclone apparatus can be used for classification. In addition to
the methods described above, any conventional method can be used
for classifying Si powder to prepare the post-classification Si
powder.
Second Embodiment
[0046] In the following, another embodiment according to the
present invention is explained.
[0047] A heat exchanger tube precursor of this embodiment is also
constituted by forming a flux layer which includes Si powder and
Zn-containing flux on an outer surface of an Al-alloy extruded tube
(tube main body). In this embodiment, the flux layer further
contains binder.
[0048] A main body of the heat exchanger tube precursor may be
constituted of aluminum extruded tube or of aluminum alloy extruded
tube.
[0049] The component of the Al alloy extruded tube may contains Si,
Mn, and the balance being Al and inevitable impurities, where an
amount of Si may be 0.05% by weight or more and 1.0% by weight or
less, an amount of Mn may be 0.05% by weight or more and 1.2% by
weight or less.
[0050] In the production process of a heat exchanger, it is
preferable to constitute fins of a material (fin member) that has
electric potential lower (less noble) than that of the tube.
[0051] In the following, reasons for limiting component elements of
the heat exchanger tube precursor are explained.
[0052] Content of Si is important for forming moderate diffusion
profile, and forming an uniform sacrificial anode layer. Where the
amount of Si is less than 0.05% by weight, diffusion profile of Si
has a large gradient, and it is difficult to form an uniform
sacrificial anode layer. Where the amount of Si is more than 1.0%
by weight, melting point of the alloy constituting the tube
decreases, and extrudability of the alloy decreases. Therefore, in
the present embodiment, the amount of Si contained in the alloy of
the tube main body is controlled to be 0.05 to 1.0% by weight. More
preferable Si content of the alloy is 0.1 to 0.6% by weight.
[0053] Mn forms an intermetallic compound with diffusing Si and is
effective in forming an uniform sacrificial anode layer. In
addition, Mn is also effective in improving corrosion resistance of
the tube, improving mechanical strength of the tube, and improving
extrudability of the alloy during forming the tube by
extrusion.
[0054] Where the amount of Mn in the alloy is less than 0.05% by
weight, diffusion profile of the Si has large gradient, and it is
difficult to form an uniform sacrificial anode layer. Where the
amount of Mn exceeds 1.2% by weight, extrudability of the alloy is
decreased because of increasing extrusion pressure. Therefore, in
the present embodiment, the amount of Mn contained in the alloy for
constituting the tube is controlled to be 0.05 to 1.2% by weight.
More preferable Mn content is 0.1 to 0.6% by weight.
[0055] Further, the Al alloy constituting the tube may further
includes one or more selected from the group consisting of Fe: 0.1
to 0.6% by weight, Ti: 0.005 to 0.2% by weight, and Cu: 0.1% by
weight or less.
[0056] Fe forms an intermetallic compound with diffusing Si and is
effective in forming an uniform sacrificial anode layer. Where the
amount of Fe is less than 0.1% by weight, diffusion profile of Si
has a large gradient, and it is difficult to form a uniform
sacrificial anode layer. Where the amount of Fe exceeds 0.6% by
weight, intermetallic compound increases in the alloy constituting
the tube, thereby tending to deteriorate the extrudability,
resulting in short tool life of an extrusion die. More preferable
Fe content is 0.15 to 0.5% by weight.
[0057] Ti forms fine intermetallic compound that does not inhibit
corrosion resistance, and contributes to enhancement of strength of
the tube 3. Where the amount of Ti is less than 0.005% by weight,
no effective result can be obtained by the addition of Ti. Where
the amount of Ti exceeds 0.2% by weight, extrudability of the tube
alloy is deteriorated because of increasing extrusion pressure.
More preferable Ti content is 0.005 to 0.1% by weight.
[0058] Cu is effective in raising the electric potential of the
tube and maintaining the effect of the sacrificial anode layer for
a long time. However, where the amount of Cu exceeds 0.1% by
weight, corrosion rate increases, and the effect of the sacrificial
anode layer is lost in the short time period. More preferable Cu
content is less than 0.05% by weight.
[0059] The tube main body may be formed by extrusion of aluminum or
aluminum alloy of the above-described composition. The extruded
tube constituting the tube main body may be formed as a multi bore
(port) extruded tube having a plurality of refrigerant
passageways.
[0060] The extruded tube may have a surface constituted of two
substantially flat main surfaces, two side surfaces, and two end
surfaces in which opening of the refrigerant passageways are
formed. The flux layer may be formed on the two main surfaces and
the two side surfaces, or only on the two main surfaces.
[0061] Preferably, surface roughness of the extruded tube (tube
main body) is less than 20 .mu.m in R.sub.max.
[0062] The flux layer formed on the surface of the tube includes
Zn-containing flux, Si powder, and binder.
[0063] Preferably, the Zn containing flux contains one or more Zn
compound selected from the group consisting of ZnF.sub.2,
ZnCl.sub.2, and KZnF.sub.3. The Zn containing flux may be
consisting of the above-described Zn compound. Alternatively, the
Zn-containing flux may be a mixture of the above-described Zn
compound and another flux. For example, one or two or more selected
from the group consisting of LiF, KF, CaF.sub.2, AlF.sub.3,
SiF.sub.4, K.sub.1-3AlF4-6, Cs.sub.1-3AlF4-6,
Cs.sub.0.02K.sub.1-2AlF4-5, K.sub.2SiF6 may be included in the Zn
containing flux. Preferably, the Zn containing flux may have a
particle diameter of 1 to 6 .mu.m in mean particle diameter
(D.sub.50).
[0064] Where the Zn containing flux is formed as a mixture of Zn
compound and Zn-free flux, it is preferable to control the amount
of Zn compound applied on the tube surface to be 3 g/m.sup.2 or
more.
[0065] Acrylic based resin may be used as the binder. Where the
acrylic based resin is used as the binder, the resin has an effect
of adhering the components of the flux layer such as Si powder and
fluoride based flux to the surface of the tube, and preventing
delamination of the flux layer from the tube during the pre-brazing
process such as assembling process of the heat exchanger. In
addition, since the acrylic based resin is easily decomposed and
evaporated during heating in the process of brazing, brazeability
and corrosion resistance of the tube are not deteriorated.
Therefore, the acrylic based resin is preferred as the binder.
[0066] In the present embodiment, 99% particle size (D.sub.99) of
the Si powder contained in the flux layer is preferably 5 .mu.m or
more and 20 .mu.m or less, where 99% particle diameter denotes a
critical diameter defined such that cumulative volume of particles
having the diameters of not greater than the diameter constitute
99% of the volume of the whole powder. Further, amount of coarse
particles having a particle diameter (hereafter referred to as
D.sub.coarse) of not smaller than (D.sub.99).times.5 is preferably
less than 1 ppm by volume.
[0067] Where the Si powder has a particle size of 20 .mu.m or less
in 99% particle diameter (D.sub.99), it is possible to form an
uniform sacrificial anode layer. On the other hand, where 99%
particle diameter exceeds 20 .mu.m, localized deep erosion is
generated, and it is difficult to form a uniform sacrificial anode
layer. Therefore, particle diameter of Si powder is preferably 20
.mu.m or less in 99% particle diameter (D.sub.99). Where the Si
powder has a particle diameter of less than 5 .mu.m in 99% particle
diameter (D.sub.99), Si powder as a whole has fine particle
diameter. Since the fine Si particle tends to accumulate with each
other, granulation or aggregation of powder particles easily occurs
in the powdered brazing composition formed by mixing the Si powder,
flux, and binder.
[0068] More preferably, (D.sub.99) of the Si powder is 5 .mu.m or
more and 15 .mu.m or less.
[0069] Based on the above-described reason, the Si powder
preferably has a particle diameter distribution such that 99%
particle diameter (D.sub.99) is 5 .mu.m or more and 20 g m or less,
and an amount of coarse Si particles of not smaller than 5 times
the 99% particle diameter (D.sub.99) is less than 1 ppm.
[0070] More preferably amount of the coarse Si particles of not
smaller than 5 times the 99% particle diameter (D.sub.99) is less
than 0.5 ppm, more preferably less than 0.1 ppm.
[0071] Preferably, mean diameter (D.sub.50) of the Si powder is in
the range of not smaller than 0.05 times the 99% particle diameter
(D.sub.99) and not larger than 0.7 times the 99% particle diameter
(D.sub.99), where the mean diameter (D.sub.50) denotes a critical
particle diameter defined by the fact that cumulative volume of Si
powder of not larger than that diameter constitute 50% by volume of
the whole Si powder. Where the Si powder applied on the surface of
the tube has a mean diameter (D.sub.50) of not smaller than 0.05
times the 99% particle diameter (D.sub.99) and not larger than 0.7
times the 99% particle diameter (D.sub.99), uniform sacrificial
anode layer is formed after the heating in the brazing process. On
the other hand, where the Si powder applied on the surface of the
tube has a mean diameter (D.sub.50) larger than 0.7 times the 99%,
relatively coarse particles are dispersed on the surface of the
tube. As a result, the sacrificial anode layer formed after the
brazing tends to have spot like distribution of deep (thick)
portions. As a result, area in which Zn is not diffused is
distributed between the spot like sacrificial anode layers. In such
a case, by a large potential difference between the area of
sacrificial anode layer and the area in which Zn is not diffused,
there is a possibility of occurrence of localized deep
corrosion.
[0072] Where the Si powder has a small mean diameter less than 0.05
times (D.sub.99), granulation tends to occur by accumulation of the
fine particles.
[0073] Based on the above-explained reason, mean diameter
(D.sub.50) of Si powder is preferably not smaller than 0.05 times
the 99% particle diameter (D.sub.99) and not larger than 0.7 times
the 99% particle diameter (D.sub.99).
[0074] Amount of Si powder applied on the surface of the tube (heat
exchanger tube precursor) is preferably 1 g/m.sup.2 or more and 5
g/m.sup.2 or less. Where the amount of Si powder is less than 1
g/m.sup.2, brazing filler is not formed sufficiently, and it is
difficult to form an uniform sacrificial anode layer. On the other
hand, where the amount of Si powder exceeds 5 g/m.sup.2, noble
cathode layer is formed on the surface of the sacrificial anode
layer. As a result, effect of the sacrificial anode layer is lost
in a short time period. Therefore, amount of the Si powder in the
coating (flux layer) is preferably 1 to 5 g/m.sup.2.
[0075] The amount of Zn-containing flux applied on the surface of
the heat exchanger tube precursor is preferably in the range of 3
g/m.sup.2 or more and 20 g/m.sup.2 or less. Where the amount of the
Zn-containing fluoride based flux in the coating (flux layer) is
less than 3 g/m.sup.2, it is difficult for the sacrificial anode
layer to exert the sacrificial anode effect because of small
potential difference. In addition, decomposition and removal of
surface oxide layer of the member to be brazed (tube) tends to be
insufficient. On the other hand, where the amount of the
Zn-containing fluoride based flux in the coating exceeds 20
g/m.sup.2, corrosion rate of the sacrificial anode layer is
increased because of large potential difference. As a result,
corrosion protection effect by the presence of the sacrificial
anode layer is lost in a short time.
[0076] More preferable amount of the Zn-containing flux is 4
g/m.sup.2 or more and 15 g/m.sup.2 or less.
[0077] In addition to the Si powder and the Zn-containing fluoride
based flux, the coating composition includes binder. For example,
acrylic base resin may be used as the binder. The binder has an
effect of adhering the Si powder and the Zn containing flux
(necessary materials in the formation of the sacrificial anode
layer) to the surface of the tube. Where the amount of binder in
the coating is less than 0.2 g/m.sup.2, there is a possibility of
failing formation of an uniform sacrificial anode layer by dropping
out of the Si powder and/or Zn-containing flux from the tube during
the brazing process. On the other hand, where the amount of binder
exceeds 8.3 g/m.sup.2 by weight, brazeability is deteriorated by
the presence of residue of the binder, and it is difficult to form
an uniform sacrificial anode layer. Therefore, it is preferable to
control the amount of the binder in the coating to be in the range
of 0.2 to 8.3 g/m.sup.2. In usual, the binder is evaporated by
heating during the brazing process.
[0078] More preferable amount of the binder coated on the surface
of the tube is 0.3 g/m.sup.2 or more and 7 g/m.sup.2 or less.
Preferably, the amount of the binder is controlled to be in the
range of 5% to 25% or the total amount of the Si powder and the Zn
containing flux and binder in the coating.
[0079] A heat exchanger may be constituted by brazing headers and
fins for heat exchanger to the above-described heat exchanger tube
precursor.
[0080] A heat exchanger is constituted by joining the
above-described tubes, header pipes, and fins. A pair of pipes so
called header pipes is arranged with a clearance in between in
vertical or lateral (horizontal) direction. A plurality of tubes is
arranged between the header pipes. Openings corresponding to the
numbers of the tubes are opened in one side of each header pipe.
For example, the tubes and header pipes are assembled by inserting
the end portions of the tubes into the openings of the header
pipes. Plate members so called fins are arranged between each pair
of the adjacent tubes. The fin may be a corrugated fins made by
working a plate into a corrugated (wavy) shape.
[0081] After assembling the tubes, fins, and header pipes, the
assembled body is heated at a predetermined temperature, for
example, at 580 to 615.degree. C. During the heating, melting of
flux is followed by melting of the Si powder, and brazing liquid is
formed. The brazing liquid includes Zn of the flux, Si of the
silicon powder, and alloy components of the surface portion of the
tube main body eutectically molten with the Si powder. The brazing
liquid is solidified by cooling and forms a brazing filler
layer.
[0082] Zn is uniformly distributed by diffusion in the brazing
liquid, and also diffuses into the Al alloy of the tube main body.
As a result, an uniform Zn diffused layer is formed on the surface
of the tube.
[0083] In the process of heat exchange, medium is circulated in the
inner space of each header pipes and tubes, and heat exchange can
be performed effectively via the fins having large contact surfaces
with the outer environment.
[0084] The flux layer may be applied to the Al alloy tube with a
roller coating apparatus. In this case, an Al alloy tube is passed
by the roller that is applied with a coating material (flux layer)
in advance. In this way, the flux layer can be applied on numbers
of Al alloy tubes at once continuously. Alternatively, the flux
layer can be applied on the Al alloy tube by thermal spraying. In
addition to the methods described above, any conventional method
for applying the coating material on a solid object can be used in
the application process of the flux layer.
[0085] In order to obtain Si powder that has an intended
specification, particularly in terms of their sizes, the Si powder
is subjected to a classification process before preparing the
coating material. For example, a commercially available Si powder
(pre-classification Si powder) is subjected to a classification
process in order to obtain the Si powder required to prepare the
flux layer used in the first aspect of the present invention
(post-classification Si powder).
[0086] The classification process includes the wet-type
classification method, in which powder suspended in a liquid is
subjected to classification, and dry-type classification method, in
which a powder in a dried-form is subjected to the classification.
In term of apparatus for the classification of Si powder, a single
or multiple filters can be used for example. Alternatively, a
cyclone apparatus can be used for classification. In addition to
the methods described above, any conventional method can be used
for classifying Si powder to prepare the post-classification Si
powder.
Third Embodiment
[0087] In the following, another embodiment according to the
present invention is explained.
[0088] The third embodiment differs from the first and the second
embodiments in having both the Zn-containing flux and the Zn-free
flux at the same time in the flux layer applied on the Al alloy
tube. Any other configurations described in the first and the
second embodiments can be adopted in the third embodiment of the
present invention.
[0089] As explained above, the flux layer formed on the surface of
the tube includes a Zn-free flux, in addition to the Si powder, the
Zn-containing flux, and binder in the present embodiment. The
Zn-free flux is a chemical composition included in the flux layer
to destroy the oxide film formed on the surface being soldered. The
Zn-free flux does not contain Zn. During process, the Zn-free flux
destroys the oxidized film formed on the soldering surface together
with the Zn-containing flux, improving solderability at the
joint.
[0090] Therefore, two different kinds of flux, the Zn-free and
Zn-containing fluxes, are used at the same time. In terms of the
Zn-containing flux, those named in the explanations in the first
and second embodiments can be used.
[0091] The Zn-free flux contains at least fluoride such as LiF, KF,
CaF.sub.2, AlF.sub.3 or SiF.sub.4, or a complex compound of the
fluoride such as KAlF.sub.4 or KAlF.sub.3. Alternatively, the
Zn-free flux contains one or more selected from the group
consisting LiF, KF, CaF.sub.2, AlF.sub.3, SiF.sub.4,
K.sub.1-3AlF.sub.4-6, Cs.sub.1-3AlF.sub.4-6,
Cs.sub.0.02K.sub.1-2AlF.sub.4-5, and K.sub.2SiF.sub.6.
[0092] By using the two different kinds of flux, the Zn-free and
Zn-containing fluxes, Zn content in the coating material (flux
layer) can be controlled. By controlling the Zn content in the
coating material, the preferential corrosion in the fillet part of
the joint can be prevented, for example. The preferential corrosion
occurs when the fillet part includes a much higher Zn content
compared to other coated part.
[0093] In addition to the technical effect explained above, time
degradation of the coating material can be suppressed by having the
Zn-free and the Zn-containing fluxes in the flux layer at the same
time.
[0094] When the Al alloy tube applied with the coating material
(flux layer) is kept under a higher temperature than normal room
temperature, such as 40.degree. C. or higher, and a higher
humidity, such as 98% or higher, for a certain period of time, such
as 7 days or longer, the performance of the flux layer
deteriorates, if the flux layer only includes the Zn-containing
flux.
[0095] Specifically, when the Al alloy tube applied a flux layer
including only the Zn-containing flux as a flux is kept under the
condition described above, a larger amount of the Si powder is not
melted during brazing process causing poor soldering and a higher
susceptibility to corrosion.
[0096] However, such a time degradation of the flux layer under the
circumstance can be suppressed by including the both Zn-free and
Zn-containing fluxes in the flux layer.
[0097] The amount of Zn-containing flux applied on the surface of
the heat exchanger tube precursor is preferably in the range of 3
g/m.sup.2 or more and 20 g/m.sup.2 or less. Where the amount of the
Zn-containing fluoride based flux in the coating (flux layer) is
less than 3 g/m.sup.2, it is difficult for the sacrificial anode
layer to exert the sacrificial anode effect because of small
potential difference. In addition, decomposition and removal of
surface oxide layer of the member to be brazed (tube) tends to be
insufficient. On the other hand, where the amount of the
Zn-containing fluoride based flux in the coating exceeds 20
g/m.sup.2, corrosion rate of the sacrificial anode layer is
increased because of large potential difference. As a result,
corrosion protection effect by the presence of the sacrificial
anode layer is lost in a short time.
[0098] More preferable amount of the Zn-containing flux is 4
g/m.sup.2 or more and 15 g/m.sup.2 or less.
[0099] The amount of Zn-free flux applied on the surface of the
heat exchanger tube precursor is preferably in the range of 1.2
g/m.sup.2 or more and 26 g/m.sup.2 or less. Where the amount of the
Zn-free flux in the coating (flux layer) is less than 1.2
g/m.sup.2, degradation of the heat exchanger due to humidity is
accelerated. In addition, the Zn content is increased at the fillet
part making the fin part prone to be separated after the brazing
process. On the other hand, where the amount of the Zn-free flux in
the coating exceeds 26 g/m.sup.2, it is wasteful since there is no
improvement on the corrosion resistance.
[0100] More preferable amount of the Zn-free flux is 2 g/m.sup.2 or
more and 20 g/m.sup.2 or less.
[0101] It is preferable that the amount ratio between Zn-containing
flux and Zn-free flux is in the range of 10:4 to 10:13 as the mass
per area unit (g/m.sup.2).
[0102] The flux layer may be applied to the Al alloy tube with a
roller coating apparatus. In this case, multiple Al alloy tubes are
passed by the roller that is applied with a coating material (flux
layer) in advance. In this way, the flux layer can be applied on
numbers of Al alloy tubes at once continuously. Alternatively, the
flux layer can be applied on the Al alloy tube by thermal spraying.
In addition to the methods described above, any conventional method
for applying the coating material on a solid object can be used in
the application process of the flux layer.
[0103] In order to obtain Si powder that has an intended
specification, particularly in terms of their sizes, the Si powder
is subjected to a classification process before preparing the
coating material. For example, a commercially available Si powder
(pre-classification Si powder) is subjected to a classification
process in order to obtain the Si powder required to prepare the
flux layer used in the first aspect of the present invention
(post-classification Si powder).
[0104] The classification process includes the wet-type
classification method, in which powder suspended in a liquid is
subjected to classification, and dry-type classification method, in
which a powder in a dried-form is subjected to the classification.
In term of apparatus for the classification of Si powder, a single
or multiple filters can be used for example. Alternatively, a
cyclone apparatus can be used for classification. In addition to
the methods described above, any conventional method can be used
for classifying Si powder to prepare the post-classification Si
powder.
EXAMPLES
Examples A
[0105] Al alloy extruded tubes having 10 cooling medium passing
holes and cross section measuring 20 mm in width, 2 mm in height
and wall thickness of 0.20 mm were produced, by extrusion forming
of billets made of an Al alloy containing 0.7% by weight of Si and
0.5% by weight of Mn.
[0106] Then a flux mixture was prepared by mixing the Zn-containing
flux to Si powder. The flux mixture was applied by spraying onto
the outer surface of the Al alloy extruded tube that was produced
in advance, thereby forming a flux layer. The amounts of the Si
powder and the flux mixture applied to the Al alloy extruded tube
are shown in Table 1. Thus the heat exchanger tube precursors of
Examples 1 to 6 and Comparative Examples 1 to 4 were produced.
[0107] Then fins made of cladding material (JIS3003 or
JIS3003/JIS4045) were assembled on the heat exchanger tube
precursors of Examples 1 to 6 and Comparative Examples 1 to 4, and
the assemblies were kept at 600.degree. C. in a nitrogen atmosphere
for three minutes so as to carry out brazing. The tubes with the
fins brazed thereon were subjected to corrosion tests (SWAAT, 20
days) to measure the maximum corrosion depth of the tubes. The test
results are shown in Table 1.
TABLE-US-00001 TABLE 1 Si powder Maximum Flux Maximum Amount of
particle size Amount of corrosion depth coating (g/m.sup.2) (.mu.m)
Composition coating (g/m.sup.2) Fin (.mu.m) Remark Example 1 1 10
KZnF.sub.3 5 JIS3003 75 -- Example 2 3 10 KZnF.sub.3 10 JIS3003 70
-- Example 3 5 10 KZnF.sub.3 15 JIS3003 80 -- Example 4 5 10
KZnF.sub.3 20 JIS3003 75 -- Example 5 3 10 ZnCl.sub.2 + 10 + 10
JIS3003 95 -- KAlF.sub.4 Example 6 3 35 KZnF.sub.3 10 JIS3003 80
somewhat deep erosion Comparative 3 10 KAlF.sub.4 10 JIS3003 350 --
Example 1 Comparative 3 10 KZnF.sub.3 + 2 + 10 JIS3003 300 --
Example 2 KAlF.sub.4 Comparative -- -- ZnF.sub.2 10 JIS3003/ 175 --
Example 3 JIS4045 Comparative -- -- KZnF.sub.3 20 JIS3003/ 200 --
Example 4 JIS4045
[0108] As shown in Table 1, maximum corrosion depth was less than
100 it m in any of the finned tubes of Examples 1 to 6, indicating
that corrosion of the tubes was suppressed. Example 6 showed a
little deeper erosion because of larger maximum particle size of
the Si powder.
[0109] Extent of corrosion was larger in the comparative examples,
presumably because Zn was not added to the flux in Comparative
Example 1, smaller amount (2 g/m.sup.2) of the Zn-containing flux
(KZnF.sub.3) was added in Comparative Example 2, and Zn was
distributed unevenly since Si powder was not added in the
comparative examples 3 and 4.
Examples B
[0110] Multi-bore (Multi-port) extruded tubes each having a
dimension of 1.5 mm in height, 14 mm in width, 500 mm in length,
and 0.4 mm in wall thickness were produced by extrusion of aluminum
alloy having a composition of Al-0.3% Si--0.3% Fe-0.3% Mn-0.05% Ti.
39 tubes were prepared. Coating in an amount of 9.5 g/m.sup.2 of
powdered brazing composition containing a Si powder, a KZnF.sub.3
flux, an acrylic resin binder was applied on the surface of each
tube, where fraction of each component was controlled such that Si
powder:KZnF.sub.3 flux:acrylic resin=2.6:5.7:1.2 (g/m.sup.2). Each
tube was subjected to heating for brazing at 600.degree. C. for 2.5
minutes in a furnace of nitrogen atmosphere.
[0111] Particle diameter distribution of Si powder was controlled
as shown in Table 2 by measuring the diameter distribution using
the below explained laser diffraction particle diameter analyzer,
and grading the particle diameter by sieving.
Method of Controlling the Particle Diameter Distribution of Si
Powder.
[0112] 1. Firstly commercially available Si powder of a
predetermined grade is prepared. In this stage, it is impossible to
analyze the distribution of coarse grains by the laser diffraction
particle diameter analyzer. Commercially available powders are
provided with data of particle diameter distribution. However,
those powders include substantial amounts of coarse particles not
described in the data. 2. The Si powder is sieved using a sieve
having a mesh aperture size of (D.sub.99).times.5. 3. The powder is
subjected to grading of particle diameter by sieving. 4. After the
sieving, particles remaining on the sieve are mainly constituted of
coarse particles. Therefore, it is possible to analyze particle
diameter distribution using the laser diffraction type particle
diameter analyzer. 5. By adding a necessary amount of the
above-described coarse Si particles to the Si powder having a
particle diameter of a certain diameter or less, Si powder
containing a desired amount of coarse Si particles is prepared. 6.
By the above-described process, it is possible to control the
amount of coarse Si powder to be a calculated value.
[0113] Si powders of the thus controlled particle diameter
distribution were used in the testing.
[0114] The 39 tubes subjected to heating process equivalent to
brazing at 600.degree. C. for 2.5 minutes were subjected to SWAAT
test for 11 days, and maximum depth of corrosion generated in the
coated portion was measured by focal depth method.
[0115] With respect to the thus obtained Examples B1 to B36 and
Comparative Examples B1 to B3, amount of Si powder in the coating
(flux layer), 99% particle diameter of the Si powder, amount of
coarse Si particles having a diameter of not smaller than 5 times
(D.sub.99), ratio of (D.sub.30)/(D.sub.99) of the Si powder, amount
of KZnF.sub.3 flux in the coating, maximum depth of corrosion, and
numbers of corroded portions having a depth of 120 .mu.m or more
measured as a result of corrosion test of each tube are shown in
the below Tables 2 and 3.
TABLE-US-00002 TABLE 2 Number of Si powder corrosion Amount having
a of Flux Maximum depth of Amount D.sub.99 D.sub.course Amount
Amount corrosion 120 .mu.m (g/m.sup.2) (.mu.m) (ppm)
D.sub.50/D.sub.99 Composition (g/m.sup.2) Fin depth (.mu.m) or more
Example B1 2 5 0.02 0.3 KZnF.sub.3 15 JIS3003 85 0 Example B2 3 10
0.05 0.35 KZnF.sub.3 5 JIS3003 90 0 Example B3 3 20 0.02 0.4
KZnF.sub.3 3 JIS3003 95 0 Example B4 4 5 0.02 0.3 KZnF.sub.3 6
JIS3003 91 0 Example B5 2.5 10 0.05 0.35 KZnF.sub.3 15 JIS3003 93 0
Example B6 4 20 0.02 0.4 KZnF.sub.3 15 JIS3003 98 0 Example B7 3 10
0.01 0.35 KZnF.sub.3 20 JIS3003 74 0 Example B8 2.5 10 0.1 0.35
KZnF.sub.3 3 JIS3003 130 1 Example B9 3 10 0.5 0.35 KZnF.sub.3 6
JIS3003 125 4 Example B10 3 10 0.9 0.35 KZnF.sub.3 4 JIS3003 125 8
Example B11 1 20 0.05 0.04 KZnF.sub.3 5 JIS3003 125 20 Example B12
2.5 20 0.02 0.05 KZnF.sub.3 10 JIS3003 95 0 Example B13 3.5 20 0.02
0.5 KZnF.sub.3 5 JIS3003 84 0 Example B14 3.5 20 0.01 0.7
KZnF.sub.3 15 JIS3003 92 0 Example B15 3 20 0.05 0.8 KZnF.sub.3 8
JIS3003 131 0 Example B16 1.5 5 0.02 0.3 KZnF.sub.3 3 JIS3003 115 0
Example B17 1.5 10 0.05 0.35 KZnF.sub.3 5 JIS3003 100 0 Example B18
1.5 15 0.03 0.4 KZnF.sub.3 15 JIS3003 90 0 Example B19 1.5 20 0.02
0.4 KZnF.sub.3 10 JIS3003 92 0
Remark:
[0116] D.sub.course denotes particles having a diameter of 5 times
(D.sub.99) or more.
TABLE-US-00003 TABLE 3 Number of Si powder Maximum corrosion Amount
Amount Flux corrosion having a depth Amount D.sub.99 of
D.sub.course of Amount depth of 120 .mu.m or (g/m.sup.2) (.mu.m)
(ppm) D.sub.50/D.sub.99 Composition (g/m.sup.2) Fin (.mu.m) more
Example B20 1.5 20 0.02 0.4 KZnF.sub.3 10 JIS3003 92 0 Example B21
1.5 10 0.05 0.35 KZnF.sub.3 20 JIS3003 80 0 Example B22 2.5 15 0.03
0.4 KZnF.sub.3 20 JIS3003 78 0 Example B23 2.5 5 0.02 0.3
KZnF.sub.3 5 JIS3003 110 0 Example B24 3 5 0.02 0.3 KZnF.sub.3 15
JIS3003 97 0 Example B25 4.5 5 0.02 0.3 KZnF.sub.3 3 JIS3003 110 0
Example B26 4.5 10 0.05 0.35 KZnF.sub.3 5 JIS3003 88 0 Example B27
4.5 15 0.03 0.4 KZnF.sub.3 10 JIS3003 90 0 Example B28 4.5 20 0.02
0.4 KZnF.sub.3 15 JIS3003 80 0 Example B29 4.5 15 0.03 0.4
KZnF.sub.3 20 JIS3003 85 0 Example B30 2 15 0.03 0.4 KZnF.sub.3 20
JIS3003 80 0 Example B31 2 20 0.02 0.4 KZnF.sub.3 10 JIS3003 90 0
Example B32 3 15 0.03 0.4 KZnF.sub.3 14 JIS3003 87 0 Example B33
3.5 5 0.02 0.3 KZnF.sub.3 20 JIS3003 81 0 Example B34 3.5 10 0.05
0.35 KZnF.sub.3 10 JIS3003 88 0 Example B35 3.5 15 0.03 0.4
KZnF.sub.3 3 JIS3003 115 0 Example B36 4 10 0.05 0.35 KZnF.sub.3 3
JIS3003 110 0 Comparative B1 3 2 0.05 0.3 KZnF.sub.3 2 JIS3003 140
not measured Comparative B2 5 30 0.01 0.4 KZnF.sub.3 1.7 JIS3003
185 not measured Comparative B3 2 10 1 0.35 KZnF.sub.3 4 JIS3003
160 not measured
[0117] As shown in Table 2, 3, in each of the tubes of Examples B1
to B36 brazed with the fins, numbers of generation of corrosion
pits having a depth of 120 .mu.m or more were 20 or less. That is,
practical corrosion protection effect was ensured.
[0118] In the Comparative Examples B1 to B3, maximum depth of
corrosion was 140 .mu.m or more. Since the unsatisfactory corrosion
protection was obvious, numbers corrosion pits having a depth of
120 .mu.m or more were not measured in the Comparative Examples B1
to B3.
[0119] In the Comparative Example B1, granulation occurred because
of small (D.sub.99). In addition, amount of Zn containing flux was
small. As a result, maximum corrosion depth had a large value of
140 .mu.m.
[0120] In the Comparative Example B2, aggregation occurred because
of large (D.sub.99). In addition, amount of Zn containing flux was
small. As a result, maximum corrosion depth had a large value of
185 .mu.m.
[0121] As described in detail above, in the heat exchanger tube
precursor of the present invention, since the mixture of the Si
powder and the Zn-containing flux is applied, the Si powder melts
and turns into a brazing liquid during a brazing process, while Zn
contained in the flux is diffused uniformly in the brazing liquid
and is distributed uniformly over the tube surface. Since diffusion
velocity of Zn in a liquid phase such as the brazing liquid is
significantly higher than diffusion velocity in a solid phase, Zn
concentration in the tube surface becomes substantially uniform,
thus making it possible to form a uniform sacrificial anode layer
and improve the corrosion resistance of the heat exchanger
tube.
[0122] Since the amount of the Zn-containing flux is in a range not
less than 5 g/m.sup.2 and not more than 20 g/m.sup.2, Zn can be
distributed uniformly over the tube surface.
[0123] In addition, by controlling the particle diameter
distribution of the Si powder, it is possible to form uniform
distribution of Zn on the tube surface even when the amount of Zn
containing flux is 3 g/m.sup.2 or more and 20 g/m.sup.2 or
less.
Examples C
[0124] Multi-bore (Multi-port) extruded tubes each having a
dimension of 1.5 mm in height, 14 mm in width, 500 mm in length,
and 0.4 mm in wall thickness were produced by extrusion of aluminum
alloy having a composition of Al-0.3% Si--0.3% Fe-0.3% Mn-0.05% Ti.
39 tubes were prepared. Coating in an amount of 9.5 g/m.sup.2 of
powdered brazing composition containing a Si powder, a KZnF.sub.3
flux, a Zn-free flux, such as KAlF.sub.4, KAlF.sub.6,
K.sub.2AlF.sub.5.H.sub.2O, AlF.sub.3, and the like, an acrylic
resin binder was applied on the surface of each tube, where
fraction of each component was controlled such that Si
powder:KZnF.sub.3 flux:acrylic resin=2.6:5.7:1.2 (g/m.sup.2).
[0125] After application of the coating material on the Al alloy
tube, the heat exchanger tube precursors were kept in two different
conditions. One condition is the normal condition, and they were
kept in normal room temperature (about 25.degree. C.) and normal
humidity (30 to 60%) for 7 days. Other is the stringent condition,
and they were kept in a condition where the temperature is
40.degree. C. and humidity is 98% for 7 days. After the storage
period, each tube was assembled with head and fin portions and
subjected to heating for brazing at 600.degree. C. for 2.5 minutes
in a furnace of nitrogen atmosphere.
[0126] After brazing treatment, the header pipe and fin portions
were disassembled from the heat exchange tube in order to observe
the top surface of the tubes.
[0127] FIG. 3 is photographic images of heat exchangers showing
time degradation of the coating material. The coating material was
applied on the Al alloy tube. Then, the heat exchanger tube
precursors were left in conditions indicated on top. Then, heat
exchangers were assembled and subjected brazing treatment. Header
pipes and fin were disassembled to observe the top surface of the
brazed tubes. The bright area indicates the area the Si powder is
melted properly and the dark area indicates the area that the
coating material was deteriorated. In the area, the Si powder was
not melted. The fine vertical lines correspond to the part fin
portions being attached.
[0128] In the heat exchanger tubes stored in the normal condition,
the Si powder was properly melted, Zn was diffused properly, and
corrosion-resistance was obtained in the entire surface of the
tubes regardless of the Zn-free flux contents. However, when the
heat exchanger tubes were stored in the stringent condition, time
degradation of the coating materials was observed.
[0129] Dark areas not observed in the normal condition appeared on
some of the heat exchanger tubes stored in the stringent condition.
The largest dark area was observed in the heat exchanger tube
applied with a coating material not including the Zn-free flux
content (compare two images on the top row).
[0130] The dark area decreased when the Zn-free flux was added.
When the Zn-free flux content was increased to 5 g/m.sup.2, the
surface covered by the localized dark area is decreased.
[0131] The result indicates that time degradation of the coating
material can be suppressed by including both the Zn-free flux and
the Zn-containing flux in the coating material even if the tubes
were stored under a stringent condition (a higher temperature and a
higher humidity).
[0132] While preferred embodiments of the invention have been
described and illustrated above, it should be understood that these
are exemplary of the invention and are not to be considered as
limiting. Additions, omissions, substitutions, and other
modifications can be made without departing from the spirit or
scope of the present invention. Accordingly, the invention is not
to be considered as being limited by the foregoing description, and
is only limited by the scope of the appended claims.
* * * * *