JPS59205445A - Aluminium alloy clad material for heat exchanger - Google Patents

Abstract

PURPOSE:The titled Al-alloy clad material excellent in pitting corrosion resistance, obtained by using an Al-alloy containing Cu and Mn in a specific amount as a core material and high purity Al obtained by a smelt casting method as a skin material. CONSTITUTION:An Al-alloy containing, on a wt. basis, 0.2-0.8% Cu, 0.1-0.8% Mn and one or more of an element amount 0.1-0.6% Mg, 0.1-0.6% Si, 0.01- 0.3% Cr, 0.01-0.3% Zr and 0.01-0.3% Ti and comprising the remainder of Al and inevitable impurities is used as a core material while pure Al with purity of 99.5% or more obtained from a cast ingot by a smelt casting method is used as a skin material. The potential difference of the core material and the skin material is about 50mV or more in any case while the natural electrode potential of the skin material itself is about -750mV or less and a Al-alloy clad material for a heat exchanger excellent in pitting resistance is obtained by using said core material and said skin material.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an aluminum alloy cladding material for heat exchangers having excellent pitting corrosion resistance.

In general, heat exchangers such as automobile radiators and heaters, that is, heat exchangers whose working fluid is an aqueous solution whose temperature changes significantly from room temperature (0 to 25°C) to high temperature (boiling) and which inevitably contain corrosive substances. Aluminum was rarely used.

The reason for this is that under the above conditions, aluminum has insufficient corrosion resistance, particularly pitting corrosion resistance, and its lifespan is reduced to one-third or less compared to conventionally used brass. Aluminum exhibits a unique form of corrosion called pitting corrosion, in which a portion of aluminum can penetrate even for a short period of time (working fluid leaks and the aluminum loses its function as a heat exchanger).
It is almost impossible to repair it.

As a countermeasure, conventionally AA-Zn materials (e.g. A7
A method of protecting the core material by using the sacrificial anode effect of the skin material has been widely adopted.
It is effective even in neutral aqueous solutions at temperatures below room temperature.

However, when the corrosive medium is a neutral aqueous solution and the temperature rises to around 100°C, such as in an automobile radiator, the potential of the Al-Zn alloy layer is reduced by the oxide film formed on the aluminum surface. The sacrificial anode effect of the skin material is lost. Not only that, but there are also cases where the potential with the core material is reversed, and the exposed portion of the core material becomes a sacrificial anode, causing further corrosion of the core material.

As a countermeasure against this problem, a cladding material whose skin material is made of high-purity aluminum with a purity of 99.7% or more has been disclosed (Japanese Patent Laid-Open No. 56-44742).

However, clad materials made of high-purity aluminum are
When we actually conduct corrosion tests, there are variations in the data, and there are cases where pitting corrosion stops at the sacrificial anode layer and there is a clear effect, and cases where pitting corrosion progresses to the core material and there is no effect at all. It will be done. As a result of various investigations into these matters, if an effect is recognized, the potential difference between the core material and the skin material is 50 mV or more, and 5% NaC4 solution (
When the potential of the skin material was measured at 25°C, it was found that the natural electrode potential was less than -750 mV (vs. SCE).

In addition, if pitting corrosion has progressed to the core material and the effect as a cladding material is not recognized, the potential difference between the core material and the skin material is 50 m.
V or less, the natural electrode potential of the leather material is -750 to -700m
It was in the range of V.

Furthermore, even with about 99.5% pure aluminum, -750
Some showed natural electrodes below mV, and some showed sufficient sacrificial anode effects.

That is, conventional pure AA or A7! - Aluminum alloy clad materials with Zn alloy as a skin material are produced by casting Al ingot obtained by electrolytic smelting and then re-melting it into an ingot before use, or by removing alloy components. After adjustment, it is cast into an ingot (remelt casting method), and the ingot is hot-rolled using a common method to form a skin plate. This skin material is placed over the core material, and after heating. It is made into a clad material by hot rolling and then cold rolling.

The cladding material produced in this way was prepared as described above (A of the cladding material).
Even if the purity of A is 99.7% or more, the potential difference between the core material and the skin material is 50 mV or less, and the natural electrode potential of the skin material itself is -7
In some cases, the sacrificial anode effect of the skin material is insufficient, and the corrosion resistance as a cladding material may not be good.

In view of this situation, the present invention provides a heat exchanger with excellent pitting corrosion resistance, in which the potential difference between the core material and the skin material is 50 mV or more and the natural electrode potential of the skin material itself is -750 mV or less even when charring occurs. Developed aluminum alloy clad material with Cu0.2~0.8wt%, Mn 0.1~0.8w
t%, Mg 0.1-0.6 wt%, Si0.
1 to 0.6 wt%, CrO, 01 to 0.3 wt%
%, Zr O, 01-0.3 wt%, TIo, o I
99.5% purity obtained from an ingot using the smelt casting method, with an A1 alloy containing one or more of ~0.3wt% and the remainder consisting of Al and unavoidable impurities as a core material. This is an aluminum alloy clad material for a heat exchanger, characterized in that the above pure A7 is used as a skin material.

The smelt casting method is a method in which aluminum obtained by electrolytic smelting is directly cast from an electrolytic furnace into an ingot for rolling or extrusion, and is only used in very special cases.

The pure aluminum skin material of the present invention is made from an ingot obtained by the smelt casting method, and can be used in corrosive environments such as aqueous solutions, at low temperatures below room temperature, and at high temperatures around 100 degrees Celsius. Since the electrode potential is maintained at a low level, it always has a sacrificial anode effect and is useful for preventing corrosion of the core material.

The aluminum purity is limited to 99.5% or higher because if the purity is less than that, impurities, especially Fe and Si, will increase, making it impossible to maintain the natural electrode potential below -750 mV. .

In addition, the reason why we limited the ingot casting method for producing pure aluminum skin material to the smelt casting method is because skin materials made from ingots produced by the remelt casting method have large variations in natural electrode potential (the This is because the potential may not be stably maintained at a base state of 1750 mV or less.

CLI added to the core material is responsible for the natural electrode potential of the core material,
Moreover, it is useful for maintaining a stable potential value, and C
When u is less than 0.2%, it becomes difficult to maintain the potential difference with the skin material at 50 mV or more due to the potential.

Moreover, if it is added in excess of 0.8%, it will not only reduce the corrosion resistance of the core material alone (and significantly reduce the plastic workability), but 5°Mn will not impair the workability (it will improve the strength). If it is less than 0.1%, the strength may be improved, which is not preferable.

Mg and Si form Mg and Si and are useful for improving the strength of the core material.If both are less than 0.1%, there is no strength improvement effect, and if more than 0.6% is added, Mg2Si or grain boundaries It precipitates in large quantities, increasing susceptibility to intergranular corrosion.

Cr and Zr are useful for improving the corrosion resistance and strength of the core material, but if they are less than 0.01%, they will not have these effects, and if they are added in excess of 0.3%, they will form intermetallic compounds with huge particle sizes. This makes it unsuitable as a practical material as it reduces plastic workability.

Tl is effective in refining the material structure and improves moldability. If the diameter is less than 0.01 inch (0.01 in.), intermetallic compounds with a huge grain size will be formed due to the movement of refining, resulting in a decrease in plastic workability.

Mg, Si, Cr, Zr, and Ti may be appropriately selected and added to the core material depending on the required properties.

Even when the cladding material of the present invention is used as a cladding material for a heat exchanger having a structure in which the skin material is attached to both sides of the core material to form a corrugated fin and this is brazed to a tube, the skin material of the cladding material is attached to the core of the fin. Of course, other heat exchanger constituent materials such as brazed fillets, tube materials, and tube sheets (Al-Mg series, A7-Mg-8i series, Al
- IVIn - Corrosion-resistant aluminum alloys such as Mg series) Since the electrode potential is more base, it acts as a sacrificial anode material and prevents pitting corrosion that occurs in the core material and other components of the heat exchanger.

Generally, the opposite side of the skin material is A7-8i or A4.
It is preferable to use the -81-Mg brazing material in the form of a clad plating sheet, and it can be widely used in water pipes (tubes) and tube plates (header plates), for example in automobile radiators and heaters.

The present invention will be explained below with reference to Examples.

Examples Various materials shown in Table 1 were selected as skin materials for aluminum alloy cladding materials for heat exchangers.

Table 1 In Table 1, the natural electrode potential of the skin material was measured in a 5% NaCl solution at 25° C. after the following 13 types of treatments were performed.

■ As is as a raw plate ■ 600℃10 equivalent to vacuum plating process
Heating in vacuum for 1 minute Heating in air at 610°C for 10 minutes, which corresponds to the waxing process As is clear from Table 1, skin materials L1 to L used in the present invention
No. 5 has a natural electrode potential of -770 mV or less not only as a material but also after heating in vacuum and after heating in the air, and the natural electrode potential of the core material is -710 to -680 mV, so the skin material of the present invention was used. If used, the potential difference with the core material will be 50 mV or more regardless of the process, indicating that a stable sacrificial anode effect can be expected.

On the other hand, comparative skin material L6 with AA purity of 99.4% obtained from an ingot by smelt casting has a natural electrode potential of 740 m
V and the potential difference with the core material may be less than 50 mV,
It can be seen that the sacrificial anode effect may not be exhibited. Also, the purity of skin materials L7 and L8 made by remelt casting is 99.7.
% or more, its natural electrode potential is -730 mV, which is nobler than L1 to L5, and no sacrificial anode effect can be expected.

Furthermore, znt, 1. %
It can be seen that the conventional skin material L9 containing Zn is quite base in material, but when heated under vacuum, Zn evaporates, the natural electrode potential becomes high and noble, and the sacrificial anode effect is lost.

Next, alloys having various compositions shown in Table 2 were used as core materials, and were combined with skin materials shown in Table 1 to produce clad materials shown in Table 3.

Table 2 Table 3 This cladding material (plate thickness 0.35mm, including skin material 0.C1
35), after vacuum heating (600'Cf1O min,
5 × 10 Torr) ASTM artificial water (100p
pmcrt-1so4-1Hc6. , - ) to 101)
I) Add m Cu”+ and heat at 90°C for 8 hours at 25°C.
An immersion corrosion test was conducted for 2 months with a temperature cycle of 16 hours. In addition, the natural electrode potential of this counterfeit food solution at 25°C and 90°C was measured. These results are also listed in Table 3. The results of the corrosion test were expressed as the maximum pitting depth.

As is clear from Table 3, in Examples 1 to 15 of the present invention, the potential difference between the skin material and the core material was 50 mV or more, and the maximum pitting depth after the corrosion test was 35 μm or less, in the skin layer. The sacrificial anode effect is obvious.

Comparative examples Nn 16-21, Nu 26
．． In 27, pitting corrosion progressed through the skin layer to the core material, and in N123 and Nχ24, the potential difference between the core material and the skin material was 50 mV.
The depth of pitting corrosion is 40μ, which is relatively hot, but the pit 23 is M
n? Due to moxibustion, %24 contains a large amount of Mg2Si, so intergranular corrosion is observed in the core material, and its effectiveness as a cladding material cannot be expected. Nα22 and Nα25 had a maximum pitting depth of 35 μm or less and had excellent pitting corrosion resistance, but the clad materials with this composition had poor plastic workability and were impossible to mass produce.

As mentioned above, the inventive crat material has excellent pitting corrosion resistance in high-temperature corrosive liquids, which was insufficient with conventional materials. It is extremely useful as an exchanger material and has significant industrial effects.

255-

Claims (1)

[Claims] Cu 0.2-0.8 wt%, Mn 0.1-0.8
wt% and Mg0.1-0.6 wt%, Si
O, 1-0.5 wt%, Cr O, 01-0.3
wt%, Zr O, 01-0.3 wt%, Ti
Purity 99 obtained from an ingot using the smelt casting method, with the core material being an A6 alloy containing one or more of O. An aluminum alloy clad material for a heat exchanger, characterized in that the skin material is made of .5% or more pure Al.