JPS6130673A - Formation of zinc film onto aluminum material - Google Patents

Abstract

PURPOSE:To form a thick Zn film having excellent adhesiveness by forming preliminarily the Zn film then activating the same and forming again the Zn film in the stage of forming the Zn film for corrosion prevention on the surface of an Al material for the tubes, fins, etc. of a heat exchanger. CONSTITUTION:ZnO is dissolved into an aq. NaOH soln. and the tubes and fins of the heat exchanger made of Al or Al alloy are dipped for 30-60sec into a basic Zn bath having 30-110g/l ZnO concn. and 300-500g/l NaOH concn. and kept at 20-50 deg.C to form the dense Zn film having the excellent adhesiveness on the surface of the Al material to 1-2g/m<2> thickness. The tubes or fins are then dipped for 10-30sec into an aq. NaOH soln. having 0.1-48wt% NaOH concn. and kept at 45-55 deg.C to activate the Zn plating surface. The tubes or fins are again dipped into the Zn-contg. NaOH liquid similar to the above-mentioned soln. by which the thick Zn film for corrosion prevention having the excellent adhesiveness is formed in a short period.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is directed to a material formed on aluminum or an aluminum alloy for cathodic corrosion protection of members such as tubes or fins of heat exchangers for air conditioners in automobiles, etc. This invention relates to a method for forming a zinc film.

[Prior art and its problems]

For example, members such as tubes and fins of heat exchangers for air conditioners in automobiles etc. are used in severely corrosive environments, so the aluminum or aluminum alloy (referred to as aluminum material) that is the tube or fin material has corrosion resistance, especially resistance to corrosion. Pitting corrosion resistance is required.

As a means to meet these requirements, it has been proposed to form a zinc film on the surface of aluminum materials.
For example, there is a substitution treatment method in which an aluminum material is immersed in a zincate solution in which zinc oxide is dissolved in caustic soda, and zinc is precipitated on the surface of the aluminum material through a substitution reaction with zinc as aluminum is eluted.

However, this general simple zinc substitution treatment method cannot produce a relatively thick film, is time consuming, and is extremely inefficient. Furthermore, the comparatively thick zinc film produced in this manner has the disadvantage of poor adhesion.

Therefore, in order to solve these drawbacks to some extent, after degreasing the surface of the aluminum material with an organic solvent, zinc is selectively precipitated on the surface that is active for zinc substitution under weak substitution conditions, and then zinc is selectively precipitated under weak substitution conditions on the surface that is active for zinc substitution. A method has been proposed in which zinc is deposited on the entire surface under stronger precipitation conditions than in the previous step (Japanese Patent Laid-Open No. 81961/1983).

Although it is possible to form a thick zinc film using this method, it takes time to process and there remains a problem in terms of economy.

[Disclosure of the invention]

As a result of continuing various studies to eliminate the above-mentioned drawbacks, the inventor of the present invention has found that after forming a zinc film on the surface of aluminum or aluminum alloy, the zinc film is subjected to an activation treatment and then a zinc film is formed again. It was discovered that a thick zinc film with excellent adhesion can be formed in a short time using this process.

However, in order to form a primary zinc film on the surface of the aluminum or aluminum alloy, it is desirable to use a zinc substitution treatment using a basic zinc bath.

This basic zinc bath preferably has a simple composition of zinc oxide dissolved in caustic soda;
It is more preferable that metal salts such as nickel and iron or cyanide are not included. For example, if ZnO is
Zn.
Preferably, the O concentration is about 300 to 500 g/L depending on the O concentration.

Similarly, it must be noted that the basic zinc bath contains metal salts such as copper, nickel, and iron, as well as cyanide ions, but organic amine compounds such as triethanolamine and EDTA, tartrates, K OH, Ca (O
There is no need to be so careful about the inclusion of basic compounds such as H)= (however, it is preferable that these compounds are not included as well).

That is, Na2ZnO□, NaOH, NaAtO2(
Aluminum is dissolved during zinc substitution, and N a A t
02) is highly desirable.

Similarly, if the ZnO and NaOH concentrations are too low or too high, problems may occur with the adhesion of the zinc film that is formed, so it is important to adjust the concentrations to the specified levels as described above. It is.

In addition, the bath temperature for zinc substitution treatment should be approximately 20 to 50°C, since if the bath temperature is too high, there will be problems with the adhesion of the zinc film, and if it is too low, the reaction will be too slow, causing problems in terms of time. , most preferably around 30°C.

In addition, the time required for zinc replacement treatment is relatively long in the case of zinc replacement treatment performed as a base treatment for subsequent electroplating of copper, nickel, tin, chromium, silver, etc. on the surface of aluminum materials. In these cases usually 30
~60 seconds is required, but in the case of the present zinc substitution treatment, it is relatively short, 5 to 30 seconds, preferably about 15 to 30 seconds.

That is, instead of continuing the zinc substitution reaction treatment from an initial intense state until it reaches a saturated state,
The zinc replacement treatment ends before reaching saturation.

Then, by the above treatment, for example, 1 to 2 g/
A dense and highly adhesive zinc film with a thickness of m is formed.

In addition, in order to activate the zinc film formed on the surface of the aluminum or aluminum alloy and having a thickness of, for example, 1 to 2 g/m, it is immersed in a basic solution such as an aqueous solution of caustic soda and subjected to a predetermined chemical reaction. It is desirable to carry out this process until H2 gas is generated from the surface of the zinc film through a dissolution reaction.

This zinc coating activation treatment process was discovered through a casual experiment by the present inventor, and was published in the above-mentioned Japanese Patent Application Laid-Open No.
While it is not possible to form a thick zinc film with good adhesion in a short time by simply performing a secondary zinc replacement treatment after the primary zinc replacement treatment in the conventional method such as in Publication No. 961, the present invention does not require this basic activation treatment. It is now possible to form a thick zinc film with good adhesion in a short time by simply adding an additional process.

Similarly, the concentration of caustic soda for performing such basic activation treatment is about 0.1 to 48 wt%, preferably about 5 to 1 wt%.
The bath temperature during the treatment is preferably about 45 to 55°C, and it is desirable to carry out the treatment for about 10 to 30 seconds until hydrogen gas is generated from the zinc film by subzinc substitution treatment.

And if the time after this basic activation treatment is too long,
In other words, if the hydrogen gas is left for a long time after generation, the adhesion of the zinc film resulting from the secondary zinc replacement treatment will decrease, and conversely, if the time is too short, the growth of the zinc film resulting from the secondary zinc replacement treatment will be slow.

Further, the formation of the zinc film after the activation treatment is preferably performed by the same treatment as the above-mentioned secondary zinc replacement treatment, that is, by secondary zinc replacement treatment.

That is, the treatment may be carried out for about 30 to 120 seconds using a treatment bath similar to that used for the subzinc substitution treatment.

Similarly, the bath temperature during this secondary zinc replacement treatment is desirably higher than the bath temperature during the primary zinc replacement treatment, and is particularly preferably 60° C. or higher, for example. In other words, the bath temperature during treatment is 60
If the temperature is lower than ℃, the formation of zinc film will not be sufficient.

In the same article, we explained the basic treatment process that is desirable for this trench.
Generally, in order to ensure the uniformity of the zinc deposited film, oxides are etched and dissolved after the surface oil is degreased, but in the case of the present invention, if there is no dirt, there is no need for pretreatment, and if there is no dirt, In some cases, it is only necessary to remove dirt with a neutral detergent or an organic solvent such as trichloroethylene, and it is preferable not to etch and dissolve the surface after the degreasing treatment before the zinc substitution treatment.

After the above-mentioned treatment, a predetermined heat treatment, for example, zinc diffusion treatment at a temperature of 600°C for 2 minutes, is performed to increase the zinc concentration on the aluminum surface to 3 to 4 parts and the diffusion depth to about 100 μm. The resulting product has extremely high corrosion resistance, and is extremely durable even when used as a tube or fin material for heat exchangers for air conditioners in automobiles, etc.

[Example 1] Pure aluminum (99,5mm) with a width of 32u and a height of 5mal
After degreasing a car cooler condenser flat tube made of 0% A/=, 0.04% Cu, 0.04% Mn) with trichlorethylene vapor, ZnOioog/z, NaOH35
A primary zinc substitution treatment is performed under the conditions of a bath composition of 0 g/l, a bath temperature of 45° C., and an immersion time of 30 seconds to form a zinc film on the surface.

Next, this flat tube was placed in a 10 wt% NaOH aqueous solution for 15 min.
Dip for a second to generate hydrogen gas by the dissolution reaction of the zinc film, and then wash with water.

Immediately after that, a bath with the same bath composition as the next zinc replacement treatment was applied.
Secondary zinc substitution treatment is performed at 0°C and immersion time is 60 seconds.

As a result, a total of 1o, og7. ? A zinc film with a uniform thickness and good adhesion was formed, and this was heated at a temperature of 600°C for 2
When zinc diffusion treatment was performed for 30 minutes, the surface zinc concentration was 3 to 4.
A diffusion depth of 150 μm was obtained.

Then, the condenser flat tube subjected to the above treatment was
When a 20-hour cast test was conducted, no pitting corrosion occurred and the corrosion resistance was extremely excellent.

[Example 2] A JIS A3003 car cooler condenser flat tube with a width of 32 mm and a height of 5 mm was soaked in a neutral detergent degreasing bath (temperature 6
Zn070g//-1N
Bath composition: aOH 380g/bath temperature 20°C, immersion time 30
Primary zinc substitution treatment is performed under conditions of 2 seconds to form a zinc film on the surface.

Next, this was added to a 5 wt% NaOH aqueous solution (temperature 45°C).
Soak for 20 seconds and rinse with water.

Thereafter, a secondary zinc replacement treatment is performed using the same bath composition as the secondary zinc replacement treatment at 60° C. and an immersion time of 60 seconds.

As a result, a uniform zinc film with a total thickness of 8.0 g/m with good adhesion was formed, and when this was subjected to zinc diffusion treatment at a temperature of 600°C for 2 minutes, the surface zinc concentration was 3 to 4%.
A diffusion depth of 150 μm was obtained.

Then, the condenser flat tube subjected to the above treatment was
When a 20-hour cast test was conducted, no pitting corrosion occurred and the corrosion resistance was extremely excellent.

[Comparative Examples 1 and 2] The treatment with the NaOH aqueous solution in Examples 1 and 2 was omitted, and the condenser flat tube was subjected to the same degreasing treatment, secondary zinc substitution treatment, secondary zinc substitution treatment, and zinc diffusion treatment, and When a time cast test was performed, pitting corrosion was observed, and the corrosion resistance was poor.

Claims (2)

[Claims] (1) A method for forming a zinc film on an aluminum material, which comprises forming a zinc film on the surface of aluminum or an aluminum alloy, subjecting the zinc film to activation treatment, and then forming a zinc film again. (2) In the method for forming a zinc film on an aluminum material according to claim 1, after forming a zinc film on the surface of aluminum or an aluminum alloy by a zinc substitution treatment using a basic zinc bath, After the coating is activated with a basic solution, a zinc coating is formed again using a zinc replacement treatment using a basic zinc bath.