JPH0252995A - Heat exchanger core with excellent corrosion resistance - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a core of an aluminum heat exchanger such as a radiator, a car air conditioner condenser, or an evaporator, which is manufactured by a brazing method using a carbide flux. The present invention relates to a heat exchanger core that has excellent corrosion resistance of the entire core and can withstand long-term use by providing the core with fins that have a sufficient sacrificial anode effect on the liquid passage pipe and have excellent pitting corrosion resistance. .

(Prior technology) Aluminum radiators, car air conditioner condensers, evaporative heat exchangers, and other heat exchangers were made possible by the development of brazing methods and the good formability of aluminum. It is now manufactured using a brazing method that uses a fluoride-based flux.

That is, the heat exchanger has a core as a main structural part, which is made up of a liquid passage tube for carrying a working fluid or a refrigerant, etc., and fins for promoting heat exchange. As a pipe material, it has 2 high strength and corrosion resistance.
A 3000 series alloy (A/-Mn series alloy) with good formability is widely used, and as the material for the fins, an alloy made by adding 1.5 wt% Zn to A 3003 alloy is widely used as it has a proven track record. There is.

Compared to the conventional brazing method using chloride-based flux, which connects the fluid pipe and the fins to each other, the reaction products remaining on the surface of the wax body are less corrosive. This has the advantage that the step of cleaning the wax body covered after brazing can be omitted. However, the brazing method using this carbide-based flux is performed by replacing the atmosphere in the brazing furnace with an inert gas with a low dew point, but this is because the flux reacts with the moisture in the furnace atmosphere during the brazing process. This is to reduce the ability to remove aluminum oxide on the surface of the covered wax body and prevent joint defects from occurring, and for this reason, it is necessary to adequately control the moisture in the furnace atmosphere, and it requires a lot of effort. There is a sense of tediousness that requires a lot of effort.

By the way, when a vehicle passes through coastal areas or roads where antifreeze containing chloride has been sprayed, the aluminum heat exchanger installed in the vehicle is subject to severe corrosion.
The surface of the liquid passage pipe corrodes, eventually forming a through hole and causing the internal fluid to leak out.

For this reason, anti-corrosion measures have been taken for aluminum heat exchangers.

In other words, the anti-corrosion measure is to coat the surface of the iJL liquid pipe with zinc, but this method alone cannot be said to be permanent, so improvements in the material of the fins have led to this anti-corrosion measure. . This anti-corrosion measure is to cathodically protect the liquid passage pipe by making the natural potential of the fins less noble than the natural potential of the liquid passage pipe, and for this purpose, as mentioned above, the material constituting the fin contains zinc, and the fin It lowers the potential and prevents corrosion of the liquid pipe.

To explain this in more detail, in the case of corrosion caused by contact between the fin and the liquid pipe, the magnitude of the corrosion or the form of the corrosion is not determined solely by the magnitude of the potential difference (the polarization behavior of each is taken into account). It is necessary to do so.

That is, the polarization behavior of these alloys is schematically shown in FIG. 1 using an alloy in which 5 wt % of Zn is added to the A 3003 alloy conventionally used for fins and liquid passage pipes, and the A 3003 alloy. The anodic polarization curve of the fin is a curve 1 having a bending point 3 as shown by a solid line, and the cathodic polarization curve of the liquid passage tube is a curve 2 as shown by a dotted line. If the polarization behavior of the fins and liquid passage tubes has this relationship,
Potential at intersection 4 of curves 1 and 2 (hereinafter referred to as corrosion potential)
is nobler than the potential at bending point 3 (hereinafter referred to as pitting corrosion potential), so the corrosion type of the fin becomes a pitting corrosion type and part of the fin falls off, shortening the life of the fin. If the corrosion potential is more base than the corrosion potential, the corrosion type of the fin will be a front corrosion type and the life will be longer.

Also, the current density at this intersection 4 (hereinafter referred to as corrosion current density)
It is known that the corrosion-protected area of the liquid pipe is determined by the size of the pipe.

In this way, in order to prevent corrosion of the liquid passage pipe through the sacrificial anode action of the fins, it is possible to achieve this by combining the liquid passage pipe and the fin, and it is possible to prevent corrosion of the liquid passage pipe by using a combination of the liquid passage pipe and the fin. The required characteristics are: (1) the pitting potential of the fin is nobler than the corrosion potential, and (2) the corrosion current density is high.

(Problem to be solved by the invention) When the inventors conducted a detailed study on the materials of fins that have been proposed in the past, they found that conventional fins shift the natural potential or pitting potential of the fins to the base side and reduce the potential of the liquid passage pipe. A variety of compositions have been proposed based on the concept of cathodic protection for liquid passage pipes by sufficiently protecting them from corrosion. As a result, as mentioned above, the corrosion of the fins becomes a pitting corrosion type, causing part of the fins to fall off, shortening the life of the core.

Based on this, the inventors believe that if they develop a fin that exhibits a sufficiently high value of pitting current density, the corrosion current density will increase and the range of corrosion protection for the liquid pipe will become wider. Moreover, the polarization behavior of the fins and the liquid passage tube has the relationship shown in Figure 2, and the corrosion form of the fins is a general corrosion type, which prevents the fins from falling off (based on the idea that they can withstand long-term use). As a result of considering the composition of the fin,
The present invention was completed by obtaining the knowledge that the pitting current of the fin increases in density with the addition of a small amount of Mg, and by integrating the effects of other compositions.The purpose of this invention is to: A, which is widely used in the fluoride-based flux brazing method, achieves good brazing even when the humidity in the furnace atmosphere is high.
The present invention provides a heat exchanger core equipped with a liquid passage pipe made of a 3000 series alloy, and long-life fins that cathodically protect the liquid passage pipe over a wide area and prevent pitting corrosion.

(Means for solving the problem) That is, the present invention... In a heat exchanger core assembled by brazing a liquid passage tube and a fin using a fluoride-based flux, the liquid passage tube is attached to an A 3000 series alloy with an Mg content of 0.
The fin is made of an alloy limited to less than 1 wt%, and the fin is
1. O~3.0wt%, Mn0.5~2.0wt%,
ho, 02 to less than 0.1 wt%, with the remainder impurities and A7! and 0.3 wt% of St as an impurity.
This is an aluminum heat exchanger core characterized in that it is made of an alloy containing less than 0.2 wt% of Cu.

(Function) The brazing method using carbide-based flux has already been put into practical use, and as a flux. In this method, a powder whose main component is aluminum fluoride alkali is used, and the powder is adhered to the fins and the liquid pipe, dried, and then brazed in an inert gas atmosphere.

In the present invention, this inert gas atmosphere allows reliable bonding even when the dew point is lowered to about -25°C by limiting the NG content of the liquid passage tube and fin to 0.1 wL%. can.

The A3000 series alloy is an AeMn series alloy containing about 1.5 wt% of Mn and has excellent high-temperature strength and formability. The liquid passage pipe according to the present invention is made of this A3000 series alloy, and improves the brazing method using carbide flux by limiting the Mg content in the alloy to less than 0.1 wt%.

Next, the composition and proportion of the alloy used for the fin will be explained.

Zn1.0-3.0wt% Zn is used to shift the potential of the fin to the base side and give the fin a sufficient sacrificial anode effect.If the content is below the lower limit, the sacrificial anode effect will not be sufficient. Moreover, if it exceeds the upper limit value, self-corrosion of the fins will occur violently, shortening the life of the fins, which is undesirable.

Mn0.5-2.0wt% Mn is used to improve the strength of the fins at high temperatures and after brazing, and to shift the pitting corrosion current to the high-density side to make the fins a full-scale corrosion type and prevent the fins from partially falling off. If the content is below the lower limit, the strength will be insufficient and it will be easily deformed.
It is also undesirable because it tends to cause pitting corrosion. In addition, if the content exceeds the upper limit, A e -Mn (
Fe)-based giant products are likely to be produced, which impairs processability.

MgO, less than 02~0.1 wt% Mg coexists with Zn to shift the pitting corrosion current of the fins to the higher density side, change the corrosion form to a general corrosion type, and prevent the fins from partially falling off. If the content is below the lower limit, the effect will be small, and if it is above the upper limit, the brazing method will be degraded when the humidity in the atmosphere in the brazing furnace becomes high, which is not preferable.

When the content of Si and Cu as impurities increases, the pitting current of the fin shifts to the lower density side, making the corrosion type pitting, so the upper limit of the content is set as SiO
, less than 3 wt%, CuO, 2 wt%, preferably each 0
．． The content shall be 15 wt% or less. The content of Fe is 0.7w
If it exceeds t%, A e -Fe(Mn)-based giant crystallized substances are likely to be formed, which impairs processability, which is not preferable.

Also, when casting this type of alloy, up to 0.2 wt% of Ti, which is usually added to refine the ingot structure, or up to 0.2 wt% of Ti and 0.02 wt% of B The inclusion of is rather preferable.

(Example) Fins having the composition shown in Table 1 were manufactured by the method shown below.

That is, the molten metal is directly water-cooled and continuously cast to obtain an ingot with a thickness of 120 + nm, and this ingot is held at a temperature of 590°C for 3 hours to homogenize it, and then hot-rolled into a hot-rolled ingot with a thickness of 5 mm. A plate was obtained, and then this plate was cold rolled to a thickness of 0.1611 at 400°C.
This was held at a temperature of 1 hour for intermediate annealing treatment, and was finally cold rolled into a fin material with a thickness of 0.115 ml, which was used as a test material.

A commercially available liquid pipe coated with a brazing material (A4343 alloy) and having the composition shown in Table 2 was used.

The following margin (L%) Table 2 In order to investigate the corrosion forms of the 13 types of sample materials A-M, the pitting corrosion potential (saturated calomel standard) of each sample material and the pitting corrosion current density at that potential were determined under the following conditions. It was measured with

That is, the final cold-rolled fin material with a thickness of 0.1151 was held for 3 minutes at the temperature heated in the brazing process, that is, 600°C, and then cooled at a cooling rate of 50°C for 1 minute. It was cooled down. The sample thus obtained was placed in 3.5 wt% saline solution that had been thoroughly degassed with nitrogen gas, and the potential sweep rate was set to 2.
It was measured at 0 mν/min.

The results are shown in Table 3.

Next, to investigate the brazing method for 13 types of sample materials A-M.
Each test material and liquid passage pipe were brazed using a fluoride-based flux under the following conditions.

That is, the above-mentioned fin material having a thickness of 0.115.0 obtained by final cold rolling is formed into a corrugated shape to obtain a corrugated fin, and this fin and the brazing material coated with the composition shown in Table 2 are formed. The liquid passage pipes were assembled into the core shape of a heat exchanger, and powder of KA/F and 3A N F4 having a eutectic composition ° (melting point approximately 562 °C) was applied to this core (5 g/m).
After that, a brazing material treatment was carried out by holding at a temperature of 600° C. for 3 minutes in a nitrogen atmosphere with a dew point of −25° C., and the bonding state of the fin and the liquid passage pipe was inspected. The results are shown in Table 3.

Next, in order to examine the corrosion resistance of the core after brazing, an alternate immersion test was conducted under the following conditions.

That is, the core brazed as described above was heated to P113 with acetic acid.
A 1-hour cycle of immersing it in 5 wt% saline at 40°C for 10 minutes, holding it at room temperature for 50 minutes, and drying it.
This was repeated for 1000 hours, and the corrosion morphology of the fins and liquid passage tubes was observed. The results are shown in Table 3.

From the results in Table 3, it can be seen that the pitting current density of the fins according to the present invention has shifted significantly to the high density side. Furthermore, it can be seen that the fins according to the present invention are well suited for brazing. Further, even in the alternate immersion test of the core after brazing, no corrosion occurred in the liquid passage pipe, and the fins were completely corroded, so it was found that the fins did not partially fall off. On the other hand, the comparative examples were found to be inferior in either pitting current density, brazing method, or corrosion resistance in alternate dipping tests.

(Effects of the Invention) As described above, the heat exchanger core according to the present invention has a good brazing method even under high humidity conditions, so there is no need to strictly control the humidity in the furnace atmosphere. Since the pitting corrosion current of the fins is on the low density side, the corrosion current density is also high, and the liquid passage pipe can be cathodic protected over a wide area.
Further, since there is no corrosion in the liquid passage pipe and the fins are corroded all over, the fins have a long service life and can be used for a long period of time with good corrosion resistance.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically representing a decentralization curve of a conventional material for a fin and a liquid passage pipe, and FIG. 2 is a diagram schematically representing a polarization curve for a fin material according to the present invention and a conventional liquid passage pipe. This is a diagram.

Claims (2)

[Claims] 1. In the heat exchanger core assembled by joining the liquid passage pipes and fins using the fluoride flux brazing method, the liquid passage pipes are M
It is made of A3000 series alloy with a g content of less than 0.1 wt%, and the fins are made of Zn1.0 to 3.0 wt%, Mn0
．． An aluminum alloy containing 5 to 2.0 wt%, Mg 0.02 to less than 0.1 wt%, the balance consisting of impurities and Al, and containing less than 0.3 wt% of Si and less than 0.2 wt% of Cu as impurities. Heat exchanger core with excellent corrosion resistance. 2. The heat exchanger core with excellent corrosion resistance according to claim 1, wherein the fin has a Si content of 0.15 wt% or less and a Cu content of 0.15 wt% or less.