Note: Descriptions are shown in the official language in which they were submitted.
1 284~23
In the drawings accompanying this application Fig. 1 is a
front view showing a radiator for a car. Fig. 2 is a graph
showing the distribution of average corrosition of radiator in a
corrosive environment.
The present invention relates to a heat exchanger fin and a
method of making it. In particular, the invention has made
thinning of the fin possibIe through an improvement in corrosion
resistance without decreasing heat transferability. The fin
according to the invention is particularly suitable for heat
exchangers used under intense conditions in corrosive environments
as in automobiles etc.
For the radiating fins used for shell and tube type heat
exchangers, strength and corrosion resistance are required
together with heat transferability. For instance, heat exchangers
for cars use a radiator for cooling the engine and a heater for
air-conditioning. In all cases, a copper core fitted with fins
between a plurality of tubes, through which the heat exchange
medium circulates, is used and tanks are installed at both ends of
the core through washer plates. In the radiator of Fig. 1, core
(3) is constructed by fitting corrugated fins (2) between a
plurality of tubes (1) through which the heat exchange medium
circulates, washer plates (4a) and (4b) being provided at both
ends of tubes (1) in core (3), and tanks (5a) and (Sb) are
installed onto washer plates (4a) and (4b). Numerals (6) and (7)
indicate the entrance and exit respectively for refluxing of the
heat exchange medium and numerals (8) and (9) indicate injection
and ejection ports for the heat exchange medium, respectively.
For the Cu-based core of the radiator, brass tubes and Ru or
Cu alloy corrugated fins are generally used and the fins are
fitted between the tubes by a soldering method called core
burning. For the fin, Cu or Cu alloy strip having a thickness of
0.025 to 0.060 mm is used, and in order to improve the strength
and the heat resistance, small amounts of Sn, Ag, Cd, P, Zr, Mg,
A
lXl~4~Z3
etc. are added within a range which does not lower heat
transferability. Moreover, on the radiator including the Cu core,
black paint is coated for the purpose of preventing corrosion, but
this treatment is confined only to the outer surfaces of the
radiator and the thickness is also confined to less than 10 ~m,
since thicker films are harmful to the radiation of the fin
section.
In recent years, a large quantity of chlorides such as NaCl
etc. is used on roads for the purpose of melting snow and ice, and
corrosion of the body of a car by these chlorides is serious. The
corrosion of the fin is extensive also with heat exchangers such
as radiators, air conditioners, etc. for automobiles, and lowering
of the radiation ability has become a disadvantage. For this
reason, the use of corrosion-resistant alloys such as Cu-Ni-based
alloys etc. was investigated but, because of low heat
transferability, thickening became necessary to achieve a
predetermined performance, which led to high prices and increase
in weight. Moreover, with conventional materials, the thickening
having made allowance for a margin to corrosion and the painting
for the prevention from corrosion also brought about similar
results making it impossible to fit for practical use.
On the other hand, decrease in weight for automobiles is
desired from the point of view of energy conservation. Decrease
in weight i5 desired also for the heat exchangers for
automobiles. However, it has been difficult technically to
æatisfy both the measures against salt damage aforementioned and
the requirement of weight decrease simultaneously.
SUNNARY OF THE INVENTION
As a result of various investigations in view of this
situation, a fin material for heat exchangers which has excellent
corrosion resistance standing up to severe environments over a
long period of time and having sufficient heat transferability,
and which is not easily corroded and worn out even if thinned for
decrease in weight and which exhibts radiation ability for a long
tlme, has been developed as well as a method of making such fin.
A 2
.
1.284923
Namely, the heat exchanger fin of the invention is
characterized in that a Cu-Zn diffused alloy layer with a Zn
content of not less than 1 wt % is formed on the surface of a
Cu-based substrate.
Moreover, the method of making the fin of the invention is
characterized in that Zn is allowed to diffuse thermally after the
surface of the Cu-based substrate is coated with Zn or Zn alloy or
an alloy layer with a Zn content of not less than 1 wt % is formed
on the surface by carrying out rolling and tempering after thermal
diffusion.
For the Cu-based substrates, thin copper alloy plates as for
example Cu-Zn, Cu-Cr, Cu-Ag, Cu-Sn, Cu-Cd, Cu-Pb-Sn, Cu-In, Cu-Te,
which are highly electroconductive (highly heat-transferable) and
can be improved in strength through the alloy effect, for example,
high electroconductive alloy plates having an electroconductivity
of not less than 85% IACS, preferably 90 to 98% IACS are used
besides pure Cu. On these substrates, Zn or Zn alloys as for
example pure Zn or Zn-Cu, Zn-Ag, Zn-Sn, Zn-Cd, Zn-Ni, Zn-Fe,
Zn-Pb, Zn-Bi-Pb, Zn-Ni-Co., Zn-As, Zn-Sb, are applied by means of
electroplating, PVD, etc. by heating above the diffusion
temperature of Zn to allow Zn to diffuse from the surface of the
substrates.
The method by which Zn or Zn alloy is applied at high
temperature and sufficient diffusion is allowed to proceed
simultaneously may be useful from the viewpoint of shortening the
process. The temperature is preferably higher than 350C from a
practical point of view and the hot-dip and the metallization
methods are advantageously used.
After the manufacturing processes described above, rolling and
tempering such as annealing etc. are carried out, if necessary, to
finish to a desired size and an alloy layer with a Zn content of
not less than 1 wt%, preferably of not less than 10 wt%, is formed
on the surface, the thickness of the alloy layer being preferably
not less than 1 ~m and not more than one fourth of the thickness
of the fin plate.
'~ 3
1 ~4~23
From the fact that the fin material is used usu~lly as a strip
material with a thickness of 0.05 to 0.025 mm, it may be desirable
to form the aforementioned diffused layer on the surface of the
substrate with a thickness of about 1.0 mm and, therafter, to
carry out rolling and tempering such as annealing etc. to finish
to the desired size.
With the fin of the invention, such treatment as the Cu-Zn
aforementioned diffused layer formed on a portion of the surface,
in particular, within a range not more than 10 mm from the edge of
the fin exposed to the outer circumference of the heat exchanger,
is as effective as treatment of the whole surface. Besides
partial covering-diffusion treatment of the fin material,
covering-diffusion treatment can also be made after construction
of the heat exchanger.
The fin material of the invention has made both protection
against salt damage and decrease in weight possible by improving
corrosion resistance under the conditions of salt damage, through
the formation of an alloy layer with a Zn content of at least 1
wt~ on the surface of a Cu-based substrate and by making highly
electroconductive (highly heat-transferable) through the core
portion comprising an alloy with a Zn content of not more than 1
wt~.
It has been know experimentally that the addition of Zn to Cu
is effective for the prevention of corrosion by salt. Pure Zn is
a metal apt to be corroded under conditions of salt damage,
whereas, excellent corrosion resistance is not exhibited until
alloyed with Cu. Moreover, the Zn diffused layer has a
distribution of concentration of Zn decreasing continuously from
the surface to the interface with the core material. For this
reason, the surface becomes anodic against the inner portion and
the inner portion becomes cathodic over the whole period of
corrosion resulting in the prevention of corrosion. Thus the
general corrosion is suppressed and averaged over the whole
-7r
lZ~4~
surface, so that rapid deterioration in the strength of fin due to
corrosion in the form of corrosion pits, observed conventionally
with fins made from Cu only or Cu alloy, can be suppressed to a
great extent.
When adding Zn to Cu, the electroconductivity decreases to,
for example, 80 to 85~ IACS by the addition of 1 wt% of Zn, about
70~ IACS by the addition of 3 wt~, about 44~ IACS by the addition
of 10 wt% and about 25~ IACS by the additon of 30 wt~. Therefore,
if the desired corrosion resistance is achieved simply by the
addition of Zn, the electroconductivity (heat transferability) is
unsuitably lowered so, in accordance with the invention, an alloy
layer with a Zn content of not less than 1 wt~, preferably of not
less than 10 wt~, is formed in a thickness of not less than 1 ~m
on the surface of a Cu-based substrate to improve corrosion
resistance against salt damage as aforementioned and the alloy
layer with a high Zn content is confined to the surface to prevent
lowering of the electroconductivity.
Usually, by making the thickness of the surface layer not more
than one fourth of that of the fin plate, an electroconductivity
more than 70~ IACS can be displayed in most cases.
In the Zn-Cu diffused layer according to the invention, Zn or
Zn alloy surface layer unreacted with the surface layer may be
left behind. Although this is corroded relatively fast at the
beginning, the Cu-Zn diffused layer underneath serves to prevent
corrosion at the next ~tep.
As a method of increasing the heat transferability (or
elctroconductivity) of the fin of the invention, Zn covering is
made only on the fin portion corresponding to the outer
circumference of the heat exchanger where the corrosion is most
intense. Salt adheres in large amounts to the outer
circumferential portion, but the adherence is confined within a
distance not more than 10 mm from the edge of the fin according to
many experiences in heat exchangers for cars. Fig. 2 is an
.,w~,
.
1 ~4~23
example thereof, which shows the distribution of the corrosion of
a radiator (fin: Cu-0.15 Sn alloy, 0.046 mm thickness x 30 mm
width) having operated for a mileage of 1,000 km in a corrosive
environment i.e. a seashore area. As evident from the diagram,
the distribution is effectively biased toward 10 mm from the front
and 7 mm from the rear.
Moreover, with the fin material of the invention, the Zn
diffused layer can be formed on the surface through coating the
surface by means of electroplating, hot dip, PVD, mechanical
cladding methods, etc. followed by thermal diffusion. In
particular, by means of electroplating, the coating of Zn or Zn
alloy accurate as to thickness and uniformity is possible.
Moreover, in order to form the alloy layer with a predetermined
thickness, the heat treatment may be carried out at a temperature
of 250 to 700C or higher than this. Furthermore, by passing the
Cu-based substrate through Zn vapor at higher than 500C, coating
with Zn and diffusion thereof can be done simultanously.
Example 1
Zn was electroplated on heat-resistant Cu strips
(electroconductivity 95.9~ IACS) having a thickness of 0.07 mm and
containing 0.06 wt~ of Cd, in a bath described below to the
thicknesses shown in Table 1 and, after diffusion treatment under
the conditions shown in Table 1, the strips were submitted to
rolling to convert to fin materials with a thickness of 0.038 mm.
The electroconductivity of these fins was measured, while the
cross-section was analyzed by the use of an X-ray microanalyzer to
determine the Zn contents on the surface and at depths of 1 and
S ym below the surface. Moreover, the corrosion test described
below was carried out to determine the average amount of corrosion
by the weight method and, further, a tensile strength test was
carried out on the fin before and after corrosion to determine the
1 2~4~'~3
reduction in strength. These results are shown in Table 1 in
comparison with those of heat-resistant Cu strip plated only with
Zn and heat-resi$tant Cu strip without any treatment.
Plating bath
NaCN 50 g/~
Zn(CH)2 70 g/~
NaOH 100 g/.
Bath temperature 30 C
Current density 3 A/dm2
Corrosion test
After saline was sprayed for 1 hour according to JIS Z2371,
the strip wa6 kept for 23 hours in a conditioning oven regulated
to 60C and 95% RH. This procedure was repeated 30 times.
As evident from Table 1, in the cases of Zn-plated fin No. 4
and untreated fin No. 5, the amount of corrosion reached 8 to 9 ~m
~one side) on average and the reduction in strength was about 85%,
the state of the strips having become almost crumbly. Whereas, it
can be seen that, in the cases of fins Nos. 1 and 2 formed with an
alloy layer with a Zn content of not less than 1 wt% on the
surface, the deterioration by corrosion was only slight. In
particular, the reason why the amount of corrosion and the
reduction in strength are small is due to the fact that pit
corrosion causing deterioration in strength is prevented through
the diffusion of Zn on the surface layer. On the other hand, in
the case of fin No. 3, the Zn content in the alloy layer at a
depth of 5 ~m from the surface layer being less than 1 wt%, the
amount of corrosion and the reduction in strength are greater than
for fins No. 1 and 2 described above, suggesting that the
improvement is insufficient under severe conditions.
Example 2
Employing plating baths described below in place of the Zn
plating in Example 1, Zn-5 wt % Ni alloy and Zn-10 wt % Cd alloy
were electroplated to the thicknesses shown in Table 2 and, after
~' ' - ' .
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~ ~ ~ 8~- 5~
E~ ~ E~ ,~ ~.X X X X ~
8 ~ o
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a~ x x x . .
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- 8
1284~3Z3
diffusion treatment under the conditions shown in Table 2, the
strips were submitted to processing by rolling to convert to fin
materials with a thickness of 0.038 mm. Using these fins, tests
similar to those of Example 1 were carried out and the results
were compared with tho~e obtained using the fin materials plated
simply with Zn-5 wS% Ni alloy and Zn-10 wt% Cd alloy.
Plating bath of Zn-5 wt% Ni alloy
ZnS04 75 g/~
NiS04 60 g/
CH3COONa ~0 g/,~
H3B03 15 g/l_
pH 3
Bath temperature 45 C
Current density7.5 A/dm2
Plating bath of Zn-10 wt X Cd alloy
Zn(CN)2 76 g/~
CdO 4 gt~
NaCN 45 g/R
NaOH 80 g/~Q
Bath temperature 35 C
Current density2 A/dm2
~. . . .
~2~4~Z3
As i5 evident from Table 2, in the cases of fins Nos. 6 and 7
of the invention formed with an alloy layer having a Zn content of
not le~s than 1 wt~ on the surface by carrying out diffusion
treatment after plating with Zn-5 wt% Ni alloy and Zn-10 wt% Cd
alloy, the deterioration by corrosion was only slight. On the
contrary, in the case of fin No. 8, the Zn content at S ym portion
being less than 1 wt% even though that on the surface being
greater than 1 wt%, the improvement in the corrosion resistance is
inferior to that of Nos. 6 and 7, showing insufficiency under the
severe conditions of use.
Example 3
Using a heat-resistant Cu strip (electroconductivity 98% IACS)
having a thickness of 0.06 mm and containing 0.09 wt % of Ag,
diffusion treatment of Zn combined with intermediate annealing was
carried out by exposing the strip for 15 seconds into a Zn bath
fused at 590C in an atmosphere of H2. This was submitted to
rolling to a thickness of 0.035 mm to convert to the fin
material. Tests similar to those of Example 1 were carried out.
The results are shown in Table 3 compared with those for an
untreated fin as above.
Table 3
Electro- ~n cor~LLation (wt~ A~t of Reduction rate
Fin conduct vity- Surface --~ corrosion in str~th
E~entthe 89.0 18 13 1.2 3.6 21
Fin withalt 97.0 O _ _ 8.8 90
treatm~nt
~ .
~ 2~49Z3
It is obvious from Table 3 that the corrosion resistance of
the fin of the invention is remarkably improved compared with that
of the untreated fin.
Example 4
In the example above, after hot-dipping for 4 seconds into the
Zn bath, the strip was wiped and cooled. ~olling wa~ carried out
similarly to finish. Results of similar tests are shown in Table
4. As evident from the table, the corrosion resistance is
improved drastically.
Table 4
Electro- Zn coxYntration (wt Z) h~nt of R~i~tion rate
l l cox~t~tyl sbrea e ~ ~ (~ in str~h
~ entknn. 1 34 18 0.9 2.4 18
FiD WQ~YUt 97.0 1 0 _ _ 8.8 90
Example 5
A radiator, as shown in Fig. 1, fitted with corrugated fins
consisting of Cu-0.15 Sn-O.OlP alloy and having a thickness of
0.040 mm and a width of 32 mm, was assembled in conventional
manner. Besides, this radiator was provided with two rows of
tubes to the width of the fin.
Under the plating conditions of Exa~ple 1, one side each of
the radiator was dipped partially while Zn was plated to a
thickness of 0.9 ~m at distances of 3 and 9 mm from the edge of
the fin. These were heated for 3 hours at 280C.
11 '
12~4~3
Using the articles of the invention thus obtained and the
conventional untreated article, exposure to saline (JIS Z2371) was
conducted for 10 minutes and further dampening exposure under 60C
x 90% RH was made for 23 hours, was repeated 60 times. Also, in
order to simulate the practical operation of a car, the
aforementioned test was conducted in a wind channel and the saline
was sprayed onto the radiator at a speed corresponding to the
operation of a car at 60 km/hr. From the results shown in Table
5, the deterioration of the articles of the invention can be seen
to be lessened significantly.
Table 5
Electro- I Zn cox~n~at~n (wt Z) R~h~tion rate
Fin cox~ctivity in stn~th
(~ ) rface 1 pm Dbpth 5 ~m Depth (%)
Ari~le of the 80 39 21 0.8 45
invention 3 mm
inventkn 9 m~ 82 36 16 0.9 36
A~i~le wi~xut 88 _ _ _ 75
trea~xnt
As described, the fin of the invention has excellent corrosion
re~istance and heat transferability, never loses its function as a
fin for a long period of time even under a severe environment and
makes thinning and decrease in weight possible. Particularly,
when used in a heat exchanger for a car, it renders not only
decrease in weight but also improvement in the life possible.
Therefore, it exerts remarkable effects industrially.
12
